JP6941589B2 - Switching power supply - Google Patents

Description

The present invention relates to a switching power supply device for converting a DC voltage into a desired voltage and supplying it to an electronic device. In particular, the value of the current flowing through the switching element is converted into a digital value by an AD converter and used in a control circuit unit. Regarding the switching power supply device.

(Previous technique 1)
Patent Document 1 discloses a method of converting a value of a current flowing through a switching element into a digital value by an AD converter and acquiring the value in a switching power supply device.

In Patent Document 1, the drain current Id flowing through the FET serving as the switching element is converted into a voltage by the current detection resistor and input to the terminal of the power supply IC. The terminal of the power supply IC has an AD converter inside, and the current flowing through the switching element is converted into a digital value and output. The CPU performs predetermined control on the switching power supply device based on this digital value.

However, if the technique disclosed in Patent Document 1 is used for a switching power supply device having a high switching frequency, there is a problem that an expensive AD converter is required. The reason will be described below.

The AD converter has a function for converting the input voltage (analog value) into a digital value, but a predetermined time is required to perform the conversion, and the AD converter is input during the conversion operation. When the voltage changes, the conversion result becomes an abnormal value.

Here, in the switching power supply device, when the switching element is on, the drain current flows through the current detection resistor to generate a voltage, but when the switching element is off, the drain current does not flow through the current detection resistor, so that a voltage is generated. do not.

From the above, when converting the current flowing through the switching element to a digital value with the AD converter, the AD converter must complete the conversion operation when the switching element is on (the period during which the voltage is generated in the current detection resistor). , It can be seen that the conversion result (digital value) becomes an abnormal value.

For this reason, in a switching power supply device having a high switching frequency, the on-time of the switching element is shortened, so that an AD converter capable of performing a high-speed conversion operation is required. Since an AD converter capable of performing a high-speed conversion operation is expensive, applying the technique of Patent Document 1 to a switching power supply device having a high switching frequency causes a problem that the switching power supply device becomes expensive.

Further, in a switching power supply device capable of varying the output voltage, this problem also occurs in a switching power supply device having a low switching frequency. A switching power supply has the characteristic that the output voltage increases when the switching element is on for a long time, and the output voltage decreases when the switching element is on for a short time. Therefore, switching is performed when the output voltage of the switching power supply is set low. The on-time of the element is shortened. Therefore, even in a switching power supply device having a low switching frequency, an AD converter that can perform a high-speed conversion operation is required.

Further, this problem also occurs in a switching power supply device provided with an overcurrent protection circuit of a type in which the output voltage is dropped. In the overcurrent protection circuit of the method of dropping the output voltage, when the output voltage is dropped, the control is performed so as to shorten the on-time of the switching element. For example, when the output of the switching power supply device is in a state close to a short circuit, the on-time of the switching element is controlled to be extremely short. A very high-speed AD converter is required to acquire the value of the current flowing through the switching element using the AD converter even when the overcurrent protection function is operating.

Japanese Unexamined Patent Publication No. 2013-188081 JP-A-2017-112977

In order to solve the problem that a high-speed AD converter is required in the switching power supply device as described above, another technique is disclosed in Patent Document 2.

(Previous technique 2)
Patent Document 2 discloses another technique for obtaining a digital value by converting a current flowing through a switching element into a digital value in a switching power supply device.

In Patent Document 2, the drain current flowing through the FET serving as a switching element is converted into a voltage by a current detection resistor, and the voltage averaged by using an integrating circuit composed of the current detection resistor and the capacitor is sent to the terminal of the digital control unit. It has been entered.

The terminal of the digital control unit has an AD converter inside, and the current flowing through the switching element is converted into a digital value and output. The CPU performs predetermined control on the switching power supply device based on this digital value.

In the technique of Patent Document 2, if the same current resolution as that of Patent Document 1 shown as the prior art 1 is to be obtained, the loss of the current detection resistor is increased, or the AD converter is expensive. Has the problem of needing it. The reason will be described below.

The AD converter has a predetermined resolution, and a digital value based on the resolution is output. For example, in an AD converter having a resolution of 1024, the converted digital value is an integer value from 0 to 1023. Further, a reference voltage is given to the AD converter, and the reference voltage is compared with the voltage to be converted (for example, the voltage applied to the terminal of the digital control unit in Patent Document 2), and the digital value is based on the resolution. Is converted to.

For example, in an AD converter with a resolution of 1024 where 5V is given as a reference voltage, 0 is output as a digital value when the voltage to be converted is 0V, and digital when the voltage to be converted is 0.5V. 102 is output as a value, and 204 is output as a digital value when the voltage to be converted is 1V.

Here, the switching power supply device of Patent Document 1 has a configuration in which a voltage generated in a current detection resistor when the switching element is on is input to an AD converter to obtain a digital value. It is assumed that the on-duty of the switching element is operating at 50%, the voltage generated in the current detection resistor is 0V when the output current of the switching element is 0%, and the voltage generated in the current detection resistor when the output current is 100%. It is assumed that the resistance value of the current detection resistor is set so that is 1 V.

Now, consider a case where a current flowing through a switching element is converted into a digital value by using an AD converter having a resolution of 1024, which is given a reference voltage of 5 V. Since the voltage generated in the current detection resistor is directly input to the AD converter in this switching power supply device, the voltage generated in the current detection resistor when the switching element is on is converted into a digital value by the AD converter. Therefore, when the output current of the switching element is 0%, the digital value converted by the AD converter is 0, and when the output current of the switching element is 100%, the digital value converted by the AD converter is 204. Approximately 0.5%, which is the value obtained by dividing 100% of the output current by 204, is the resolution when converting the current flowing through the switching element into a digital value.

Next, the switching power supply device of Patent Document 2 has a configuration in which a voltage obtained by averaging the voltage generated in the current detection resistor when the switching element is on is input to the AD converter by an integrating circuit to obtain a digital value.

As in the case of Patent Document 1, it is assumed that the on-duty of the switching element is operating at 50%, and when the output current of the switching element is 0%, the voltage generated in the current detection resistor is 0V and the output current is 100%. It is assumed that the resistance value of the current detection resistor is set so that the voltage generated in the current detection resistor becomes 1V at this time.

Similar to Patent Document 1, consider a case where a current flowing through a switching element is converted into a digital value by using an AD converter having a resolution of 1024, which is given a reference voltage of 5 V.

Since this switching power supply has a configuration in which the voltage generated in the current detection resistor is averaged and input to the AD converter, the digital value converted by the AD converter when the output current of the switching element is 0% is a patent document. It becomes 0 like 1, but when the output current of the switching element is 100%, the voltage input to the AD converter is 0.5V, which is the product of the voltage generated in the current detection resistor and the on-duty 50% of the switching element. , The digital value converted by the AD converter is 102. Therefore, about 1%, which is the value obtained by dividing 100% of the output current of the switching power supply by 102, is the resolution when converting the current flowing through the switching element into a digital value.

As described above, since the switching power supply device of Patent Document 2 has a configuration in which the voltage generated in the current detection resistor is averaged and input to the AD converter, the current flowing through the switching element is compared with the switching power supply device of Patent Document 1. It can be seen that the resolution when converting to a digital value is low.

Therefore, in order to obtain the same resolution as the switching power supply device of Patent Document 1 in the switching power supply device of Patent Document 2, a method of increasing the generated voltage of the current detection resistor can be considered. However, in this case, there arises a problem that the loss of the current detection resistor increases and the conversion efficiency of the switching power supply device decreases. As another method, a method using an AD converter having a high resolution can be considered, but an AD converter having a high resolution is expensive, and there arises a problem that the switching power supply device becomes expensive.

INDUSTRIAL APPLICABILITY The present invention reduces the loss of current-voltage conversion when the current flowing intermittently by turning on / off the switching element is converted into a digital value by the AD converter and acquired, and the cost due to the use of the low-speed AD converter can be reduced. It is an object of the present invention to provide a switching power supply device.

(Switching power supply device of the second invention)
In the present invention , in a switching power supply device including a power conversion unit, a current detection unit, and a control circuit unit,
The power conversion unit converts the input voltage supplied by the input power supply into an intermittent voltage by turning the switching element on and off by the switching element drive signal, and then rectifies and smoothes it to generate a DC voltage.
The current detector is composed of a current-voltage conversion element, a clamp element, a clamp capacitor, and a bias circuit.
The current-voltage conversion element has one end connected to the switching element and the other end connected to the ground of the input power supply to convert the current passing through the switching element into a current conversion signal which is a voltage signal.
The clamp element is a switch element that is turned on and off by a clamp element drive signal, and one end is connected to the connection point between the other end of the current-voltage conversion element and the ground of the input power supply, and the other end is connected to one end of the clamp capacitor. When the clamp element is on, the current-voltage conversion element and the clamp capacitor are connected to charge the current conversion signal converted by the current-voltage conversion element to the clamp capacitor. When the clamp element is off, the current-voltage conversion element and the clamp capacitor are charged. Is to prevent the discharge of the charge of the current conversion signal charged in the clamp capacitor and clamp it.
One end of the clamp capacitor is connected to the input of the AD converter in the control circuit unit via a bias circuit that generates a predetermined bias voltage, and the other end is connected to the ground of the control circuit unit and the connection point of one end of the current-voltage conversion element. Being done
The control circuit unit is equipped with an AD converter and operates with the connection point between one end of the current-voltage conversion element and the other end of the clamp capacitor as ground, and the voltage signal obtained by subtracting the current conversion signal clamped by the clamp capacitor from the bias voltage is generated. It is input to the AD converter and outputs the switching element drive signal and the clamp element drive signal so that the switching element and the clamp element are turned on and off in synchronization.
It is characterized by that.

(Clamp element with parasitic diode)
A switch element having a parasitic diode is used as the clamp element, and the clamp element is connected so that the forward direction of the parasitic diode is the direction from the clamp capacitor to the current-voltage conversion element.

(Reverse connection of clamp element with parasitic diode)
A switch element having a parasitic diode is used as the clamp element, and the parasitic diode is connected so that the forward direction is the direction from the current-voltage conversion element to the clamp capacitor, and the voltage value of the current conversion signal generated in the current-voltage conversion element. Is set to be less than or equal to the forward voltage drop of the parasitic diode.

(Drive of clamp element by switching element drive signal)
The control circuit unit drives the clamp element with a switching element drive signal that drives the switching element instead of the clamp element drive signal.

(Control that turns off the clamp element earlier than turning off the switching element)
The control circuit unit controls the timing at which the clamp element is turned off to be earlier than the timing at which the switching element is turned off.

(Off delay of switching element drive signal)
The control circuit unit includes an off-delay circuit that delays the off-timing of the switching element drive signal in the signal line that outputs the switching element drive signal to the switching element and the clamp element and outputs the switching element drive signal to the switching element.

(Effect of the first invention)
The present invention relates to a switching power supply device including a power conversion unit, a current detection unit, and a control circuit unit. After converting to, the current detector is rectified and smoothed to generate a DC voltage. The current detector is composed of a current-voltage conversion element, a clamp element, and a clamp capacitor. One end of the current-voltage conversion element is connected to a switching element, and the other end is connected. It is connected to the connection point between the input power supply and the ground of the control circuit, and converts the current passing through the switching element into a current conversion signal, which is a voltage signal. The clamp element is a switch element that is turned on and off by the clamp element drive signal. One end is connected to the connection point between one end of the current-voltage conversion element and the switching element, the other end is connected to one end of the clamp capacitor, and when the clamp element is on, the current-voltage conversion element and the clamp capacitor are connected. The current conversion signal converted by the current-voltage conversion element is charged to the clamp capacitor, and when the clamp element is off, the current-voltage conversion element and the clamp capacitor are separated to prevent the current conversion signal charged in the clamp capacitor from being discharged and clamped. One end of the clamp capacitor is connected to the input of the AD converter in the control circuit section, the other end is connected to the connection point between the other end of the current-voltage conversion element and the ground of the control circuit section, and the control circuit section is , AD converter is provided, and the connection point between the other end of the current-voltage conversion element and the input power supply is used as ground, and the switching element drive signal and the clamp element drive signal are output so that the switching element and the clamp element are turned on and off in synchronization. Therefore, when the switching element is turned on and the passing current flows to the current-voltage conversion element to generate the voltage of the current conversion signal, the clamp element is also turned on and the clamp capacitor is charged by the voltage of the current conversion signal. Then, the clamp voltage becomes the voltage of the current conversion signal, and when the switching element is turned off and no voltage is generated in the current-voltage conversion element, the clamp element is turned off and there is no gap between the current-voltage conversion element and the clamp capacitor. It is in a conductive state, and even if the generated voltage of the current-voltage conversion element disappears, the clamp voltage of the clamp capacitor is held without discharging the charge, and the switching element is turned on in the clamp capacitor regardless of whether the switching element is on or off. The state in which the voltage generated in the current-voltage conversion element is held by the passing current due to Therefore, even if an AD converter with a slow conversion operation is used, the current flowing through the switching element can be normally converted to a digital value, which is a high-speed and expensive problem in the prior art. It is not necessary to use an AD converter, and it is possible to solve the problem that the switching power supply device becomes expensive when detecting the current flowing through the switching element by using the AD converter in the switching power supply device having a high switching frequency.

In addition, in a switching power supply with a low switching frequency, even if the on-time of the switching element is shortened when the output voltage is set low by the output voltage variable function or when the output voltage is dropped by the overcurrent protection function. Solves the problem that the voltage of the current conversion signal of the current flowing when the switching element is turned on is held in the clamp capacitor, there is no need to use a high-speed and expensive AD converter, and the switching power supply becomes expensive. can.

Furthermore, since the voltage generated in the current-voltage conversion element when converting the current flowing by turning on the switching element into a voltage is input to the AD converter without being averaged, a resistor with a small value can be used as a current-voltage detection element. Even if the loss is reduced by using it, a sufficient voltage can be input to the AD converter, so that the problem that the conversion efficiency of the switching power supply device is lowered can be solved, and it is also necessary to use an AD converter with high resolution. It is also possible to solve the problem that the switching power supply becomes expensive because it is eliminated.

(Effect when using a clamp element with a parasitic diode)
In addition, a switch element having a parasitic diode is used for the clamp element, and the clamp element is connected so that the forward direction of the parasitic diode is the direction from the current-voltage conversion element to the clamp capacitor, so that the clamp element is turned off. At this time, the charge stored in the clamp capacitor is prevented from being released to the current-voltage conversion element via the parasitic diode, and the voltage generated in the current-voltage conversion element due to the passing current due to the switching element being turned on is surely applied to the clamp capacitor. It can be accurately converted to a digital value by an AD converter.

(Effect when a clamp element with a parasitic diode is connected in the opposite direction)
Further, a switch element having a parasitic diode is used for the clamp element, and the clamp element is connected so that the forward direction of the parasitic diode is the direction from the clamp capacitor to the current-voltage conversion element, and the current generated in the current-voltage conversion element. By setting the voltage value of the conversion signal to be less than or equal to the forward drop voltage of the parasitic diode, the charge stored in the clamp capacitor is not released regardless of the direction of the parasitic diode, and therefore, Since the clamp element can be connected so that the forward direction of the parasitic diode is the direction from the clamp capacitor to the current-voltage conversion element, it is possible to control the clamp element with reference to the potential of the clamp capacitor. Therefore, the clamp element can be turned on stably.

(Effect of the second invention)
In the present invention , in the switching power supply device including the power conversion unit, the current detection unit, and the control circuit unit, the power conversion unit is the input voltage supplied by the input power supply by turning on / off the switching element by the switching element drive signal. Is converted to an intermittent voltage and then rectified and smoothed to generate a DC voltage. The current detector is composed of a current-voltage conversion element, a clamp element, a clamp capacitor, and a bias circuit. One end of the current-voltage conversion element is a switching element. It is connected, the other end is connected to the ground of the input power supply, and the current passing through the switching element is converted into a current conversion signal which is a voltage signal. The clamp element is a switch element that is turned on and off by the clamp element drive signal. Yes, one end is connected to the connection point between the other end of the current-voltage conversion element and the ground of the input power supply, the other end is connected to one end of the clamp capacitor, and when the clamp element is on, the current-voltage conversion element and the clamp capacitor are connected. The current conversion signal converted by the current-voltage conversion element is charged to the clamp capacitor by connecting, and when the clamp element is off, the current-voltage conversion element and the clamp capacitor are separated to prevent the current conversion signal charged in the clamp capacitor from being discharged. One end of the clamp capacitor is connected to the input of the AD converter in the control circuit section via a bias circuit that generates a predetermined bias voltage, and the other end is connected to the ground of the control circuit section and current-voltage conversion. Connected to the connection point at one end of the element, the control circuit unit is equipped with an AD converter, operates with the connection point between one end of the current-voltage conversion element and the other end of the clamp capacitor as ground, and is clamped to the clamp capacitor from the bias voltage. The voltage signal obtained by subtracting the current conversion signal is input to the AD converter, and the switching element drive signal and the clamp element drive signal are output so that the switching element and the clamp element are turned on and off in synchronization. In addition to the above effect, even if an element with a large current rating, for example , an element having an input capacitance between the gate and source such as an N-channel MOS-FET, is used as the switching element, the current for turning on the switching element by the switching element drive signal. Does not flow to the current-voltage conversion element, and it is possible to solve the problem that the error becomes large when the current value that flows when the switching element is turned on by the AD converter is acquired.

(Effect when using a clamp element with a parasitic diode)
Further, the clamp element uses a switch element having a parasitic diode, and is connected so that the forward direction of the parasitic diode is the direction from the clamp capacitor to the current-voltage conversion element, so that when the clamp element is turned off. , Prevents the charge stored in the clamp capacitor from being released to the current-voltage conversion element via the parasitic diode, and securely holds the voltage generated in the current-voltage conversion element by the passing current due to the switching element being turned on. It can be accurately converted to a digital value by an AD converter.

(Effect of reverse connection of clamp element with parasitic diode)
Further, a switch element having a parasitic diode is used as the clamp element, and the current conversion signal generated in the current-voltage conversion element is connected so that the forward direction of the parasitic diode is the direction from the current-voltage conversion element to the clamp capacitor. By setting the voltage value to be less than or equal to the forward voltage drop of the parasitic diode, the charge stored in the clamp capacitor is not released regardless of the orientation of the parasitic diode, and therefore the parasitic diode The clamp element can be connected so that the forward direction is the direction from the clamp capacitor to the current-voltage conversion element.

(Effect of driving the clamp element by the switching element drive signal)
Further, since the control circuit unit drives the clamp element with a switching element drive signal that drives the switching element instead of the clamp element drive signal, it is necessary to provide a drive circuit for generating the clamp element drive signal in the control circuit unit. This eliminates the problem, and the circuit configuration can be simplified and the cost can be reduced.

(Effect of control that turns off the clamp element faster than turning off the switching element)
The control circuit unit controls the timing at which the clamp element is turned off to be earlier than the timing at which the switching element is turned off. For example, the control circuit unit outputs a switching element drive signal to the switching element and the clamp element, and the switching element. Since an off-delay circuit that delays the off-timing of the switching element drive signal is provided in the signal line that outputs the switching element drive signal, when a switch element that cannot respond quickly to the clamp element drive signal is used as the clamp element. It is possible to solve the problem that the acquisition error of the current value converted by the AD converter becomes large.

Circuit block diagram showing the basic configuration of the first embodiment of the switching power supply device A circuit block diagram showing a specific configuration of a first embodiment of a switching power supply device. Time chart showing signal waveforms of each part in the switching power supply device of FIG. A circuit block diagram showing a modified example of the first embodiment of a switching device that turns on and off a clamp element by a switching element drive signal. Circuit block diagram showing a second embodiment of a switching power supply device Clamp element A time chart showing the signal waveforms of each part in a switching power supply that uses a clamp element that cannot respond quickly to the drive signal. Circuit block diagram showing a third embodiment of a switching power supply device Time chart showing signal waveforms of each part in the switching power supply device of FIG.

[First Embodiment of Switching Power Supply Device]
FIG. 1 is a circuit block diagram showing a basic configuration of a first embodiment of a switching power supply device, and FIG. 2 is a circuit block diagram showing a specific configuration of a first embodiment of a switching power supply device.

(Outline of circuit configuration)
As shown in FIG. 1, the switching power supply device 10 of the present embodiment includes a power conversion unit 12, a control circuit unit 14, and a current detection unit 16. The power conversion unit 12 includes a switching element 18, a transformer 20, rectifying elements 22, 24, an output inductor 26, and an output capacitor 28.

The control circuit unit 14 includes a switching element drive circuit 32, a clamp element drive circuit 34, and an AD converter 36. The current detection unit 16 includes a current-voltage conversion element 38, a clamp element 40, and a clamp capacitor 42.

Here, in FIG. 1, the clamp element 40 is described as a switch element, but in FIG. 2, an N-channel MOS-FET is used as a specific example of the clamp element 40. Hereinafter, description will be given according to FIG.

(Power conversion unit)
As shown in FIG. 2, the power conversion unit 12 includes a switching element 18 using an N-channel MOS-FET, and a switching element in which the input voltage Vin of the input power supply 11 is connected to the primary winding 20a of the transformer 20. It is converted into an intermittent voltage by turning 18 on and off, transmitted to the secondary winding 20b of the transformer 20, rectified by the rectifying elements 22 and 24 connected to the secondary winding 20b, and then smoothed by the output inductor 26 and the output capacitor 28. The generated DC voltage Vo is supplied to the load 30.

Here, the power conversion unit 12 uses as an example a single-ended forward converter composed of a switching element 18, a transformer 20, rectifying elements 22, 24, an output inductor 26, and an output capacitor 28.

(Current detector)
As shown in FIG. 2, the current detection unit 16 includes a current-voltage conversion element 38 using a current detection resistor, a clamp element 40 using an N-channel MOS-FET, and a clamp capacitor 42.

One end of the current-voltage conversion element 38 is connected to the switching element 18, and the other end is connected to the connection point between the negative side (ground) of the input power supply 11 and the ground of the control circuit unit 14. One end of the clamp element 40, which is the source terminal S, is connected to one end of the current-voltage conversion element 38 and the connection point of the switching element 18, and the other end of the clamp element 40, which is the drain terminal D, is connected to one end of the clamp capacitor 42. There is.

One end of the clamp capacitor 42 is connected to the input of the AD converter 36 in the control circuit unit 14, and the other end of the clamp capacitor 42 is connected to the ground of the control circuit unit 14.

The current-voltage conversion element 38 converts the switching element passing current IM into current conversion signals VRs which are voltage signals. Assuming that the resistance value of the current-voltage conversion element 38 is Rs, the voltage of the current conversion signal VRs, which is a voltage signal, is VRs = IM × Rs.

The clamp element 40 is a switch element using an N-channel MOS-FET that is turned on and off by a clamp element drive signal VQ output from the clamp element drive circuit 34, and the clamp element 40 is turned on by the H level of the clamp element drive signal VQ. In the case of, the current-voltage conversion element 38 and the clamp capacitor 42 are connected, and when the clamp element 40 is off by the L bell of the clamp element drive signal VQ, the current-voltage conversion element 38 and the clamp capacitor 42 are separated to be in a non-conducting state. ..

(Control circuit section)
The control circuit unit 14 shown in FIG. 2 is composed of a switching element drive circuit 32, a clamp element drive circuit 34, and an AD converter 36. The control circuit unit 14 includes the other end of the current-voltage conversion element 38 and the input power supply 11. It operates with the connection point with the minus side (ground) as the ground. Therefore, the AD converter 36 in the control circuit unit 14 operates with the negative side of the input power supply 11 as ground.

Further, the control circuit unit 14 includes a CPU, a memory, etc. (not shown), and controls the duty of the switching element 18 based on the digital value of the switching element passing current IM that is the output of the AD converter 36, and a switching power supply. It controls the entire device.

The switching element drive circuit 32 outputs a switching element drive signal VTR toward the switching element 18. In FIG. 2, since an N-channel MOS-FET is used for the switching element 18, the switching element 18 is turned on when the switching element drive signal VTR is H level, and the switching element is turned on when the switching element drive signal VTR is L level. 18 turns off.

The clamp element drive circuit 34 outputs a clamp element drive signal VQ toward the clamp element 40. In FIG. 2, since the N-channel MOS-FET is used for the clamp element 40, the clamp element 40 is turned on when the clamp element drive signal VQ is H level, and the clamp is clamped when the clamp element drive signal VQ is L level. The element 40 turns off. Further, the clamp element 40 is controlled to be turned on and off in synchronization with the switching element 18.

The AD converter 36 receives the clamp signals VCs, which are analog voltages output by the clamp capacitor 42, and outputs the value converted into a digital value. As the AD converter 36, a low-speed AD converter having a conversion time longer than the on-time of the switching element 18 can be used.

(Circuit operation)
3A and 3B are time charts showing signal waveforms of each part in the switching power supply device of FIG. 2, FIG. 3A shows a switching element drive signal VTR, and FIG. 3B shows a switching element passing current IM. FIG. 3C shows current conversion signals VRs which are voltage signals of the current-voltage conversion element 38, FIG. 3D shows a clamp element drive signal VQ, and FIG. 3E shows clamp signals VCs of the clamp capacitor 42. Is shown.

(Power conversion operation)
As shown in FIG. 3A, the switching element drive circuit 32 outputs the switching element drive signal VTR toward the switching element 18. When the switching element drive signal VTR is H level, the switching element 18 is turned on, and when the switching element drive signal VTR is L level, the switching element 18 is turned off.

By turning the switching element 18 on and off, the input voltage Vin supplied from the input power supply 11 is converted into an intermittent voltage. The intermittent voltage is transformed, rectified, and smoothed by the power conversion unit 12, so that an output voltage Vo is generated and supplied to the load 30. The control circuit unit 14 controls the on-duty of the switching element 18 so that the output voltage Vo becomes a predetermined value.

As shown in FIG. 3B, when the switching element 18 is on, the switching element passing current IM flows, and when the switching element 18 is off, the switching element passing current IM does not flow. The switching power supply device 10 is a value obtained by multiplying all of the switching element passing current IM, the on-duty of the switching element 18, the input voltage Vin, and the conversion efficiency of the switching power supply device 10, and the output output from the power conversion unit 12 to the load 30. The value obtained by multiplying the current Io by the output voltage Vo becomes equal. Therefore, when the output current Io increases, the switching element passing current IM increases, and when the output current Io decreases, the switching element passing current IM decreases.

(Current detection operation)
The switching element passing current IM flows through the current-voltage conversion element 38, which is a current detection resistor. The current-voltage conversion element 38 generates current conversion signals VRs, which are voltage signals proportional to the switching element passing current IM. Therefore, as shown in FIG. 3C, when the switching element 18 is on, the current conversion signal VRs is the current as a voltage signal obtained by multiplying the switching element passing current IM by the resistance value of the current-voltage conversion element 38. When the conversion signal VRs (= IM × Rs) is generated and the switching element 18 is off, the current conversion signal VRs becomes zero.

(Clamp operation)
As shown in FIG. 3D, the clamp element drive circuit 34 outputs the clamp element drive signal VQ toward the clamp element 40. The clamp element drive signal VQ is controlled so that the clamp element 40 is turned on when the switching element 18 is on and the clamp element 40 is turned off when the switching element 18 is off.

When the clamp element 40 is on, the switching element 18 is also on, so the switching element passing current IM is flowing through the current-voltage conversion element 38, and the resistance value Rs of the current-voltage conversion element 38 is set to the switching element passing current IM. The voltage required by multiplying is generated.

Here, since the clamp element 40 is turned on, the current-voltage conversion element 38 and the clamp capacitor 42 are in a conductive state, and the clamp capacitor 42 is charged by the voltage of the current conversion signals VRs generated in the current-voltage conversion element 38. NS. Therefore, the clamp signal VCs, which is the voltage generated by the clamp capacitor 42, is the voltage of the current conversion signal VRs obtained by multiplying the switching element passing current IM by the resistance value of the current-voltage conversion element 38.

When the clamp element 40 is off, the switching element 18 is also off, so that the switching element passing current IM does not flow through the current-voltage conversion element 38, and no voltage is generated in the current-voltage conversion element 38.

Here, since the clamp element 40 is off, the current-voltage conversion element 38 and the clamp capacitor 42 are in a non-conducting state, and the charge of the clamp capacitor 42 is not discharged via the current-voltage conversion element 38. Even if the generated voltage of the current-voltage conversion element 38 disappears, the clamp signal VCs of the clamp capacitor 42 retains the voltage of the current conversion signal VRs obtained by multiplying the switching element passing current IM by the resistance value of the current-voltage conversion element 38. It will be in the state of being.

(AD converter operation)
The clamp signals VCs held in the clamp capacitor 42 are input to the AD converter 36 and converted into digital values. The clamp signal VCs of the clamp capacitor 42 holds the voltage of the current conversion signal VRs obtained by multiplying the switching element passing current IM by the resistance value of the current-voltage conversion element 38 regardless of whether the switching element 18 is on or off. Since it is in the state, the current flowing through the switching element 18 can be normally converted into a digital value even if the AD converter 36 having a slow conversion operation is used.

(Advantages of the first embodiment)
As described above, in the switching power supply device 10 of the present embodiment, even if a low-speed AD converter 36 whose conversion time is slower than the ON period of the switching element 18 is used, the switching element passing current IM is normally converted into a digital value. can do.

This eliminates the need to use a high-speed and expensive AD converter, which has been a problem in the prior art, and switches when acquiring the current flowing through the switching element 18 using the AD converter 36 in a switching power supply device having a high switching frequency. It can solve the problem that the power supply becomes expensive.

Further, even in a switching power supply device having a low switching frequency, when the output voltage is set low by changing the output voltage or when the overcurrent protection operation for dropping the output voltage is performed by the overcurrent protection circuit, the switching element 18 Even if the on-time is shortened, since the voltage converted from the current flowing through the switching element 18 is held in the clamp capacitor 42, it is not necessary to use a high-speed and expensive AD converter, and the switching element is used by using the AD converter 36. It is possible to solve the problem that the switching power supply device becomes expensive when acquiring the current flowing through 18.

Further, since the voltage generated in the current-voltage conversion element 38 using the current detection resistance when converting the current flowing through the switching element 18 into a voltage is input to the AD converter without being averaged, the current detection resistance It is not necessary to increase the voltage generated by increasing the resistance value, and by using a current detection resistor with a small resistance value, it is possible to input a sufficient voltage to the AD converter even if the loss is low. The problem of reduced conversion efficiency can be solved, and the problem of high cost of the switching power supply device can be solved because it is not necessary to use an AD converter having high resolution.

(Direction of clamp element)
In the N-channel MOS-FET used for the clamp element 40 of FIG. 2, the source terminal S is connected to the current-voltage conversion element 38, and the drain terminal D is connected to the clamp capacitor 42. This is because the direction of the parasitic diode 44 of the N-channel MOS-FET used for the clamp element 40 is taken into consideration, and the forward direction of the parasitic diode 44 is the direction from the current-voltage conversion element 38 to the clamp capacitor 42. It is connected like. This prevents the electric charge stored in the clamp capacitor 42 from being discharged to the current-voltage conversion element 38 when the clamp element 40 is turned off.

Here, since a general parasitic diode has a voltage drop (about 0.3V to 1V) in the forward direction, no current flows in the forward direction even if a voltage smaller than this value is applied. Utilizing this characteristic, if the voltage value of the current conversion signals VRs generated in the current-voltage conversion element 38 is equal to or less than the forward voltage drop value of the parasitic diode 44, it is stored in the clamp capacitor 42 regardless of the direction of the parasitic diode 44. The charged charge is not discharged.

Therefore, when the voltage value of the current conversion signals VRs generated in the current-voltage conversion element 38 is set to be equal to or less than the forward voltage drop value of the parasitic diode 44, the drain terminal D of the clamp element 40 is converted to current-voltage. It can be connected to the element 38 and the source terminal S can be connected to the clamp capacitor 42.

When the source terminal S of the clamp element 40 is connected to the clamp capacitor 42, the effect that the clamp element 40 can be operated more stably than when the source terminal S is connected to the current-voltage conversion element 38 can be obtained. The reason will be described below.

When the clamp element 40 is an N-channel MOS-FET, a voltage is applied to the clamp element 40 by a predetermined threshold voltage higher than the voltage applied to the source terminal S by the gate terminal G with reference to the source terminal S. When it is applied, it turns on, and when a low voltage is applied, it turns off. In a general switching power supply device, a surge current often flows for a short time immediately after the switching element 18 is turned on. This surge current may be several times higher than that of the switching element passing current IM, and since it often has a high-frequency vibration component, the current-voltage conversion element 38 is accompanied by high-frequency voltage vibration. Generate a voltage.

When the source terminal S of the clamp element 40 is connected to the current-voltage conversion element 38, a high voltage generated by the surge current is applied to the source terminal S of the clamp element 40, and the surge current is flowing during the period. , The clamp element 40 cannot be turned on, or the clamp element 40 is repeatedly turned on and off at high speed.

On the other hand, since the clamp capacitor 42 maintains a stable voltage without being affected by the surge current immediately after the switching element 18 is turned on, the source terminal S of the clamp element 40 is connected to the clamp capacitor 42. In this case, a high voltage accompanied by high-frequency voltage vibration generated by a surge current is not applied to the source terminal S of the clamp element 40. Therefore, the clamp element 40 can be controlled with reference to the potential of the clamp capacitor 42, and the clamp element 40 can be stably turned on.

(Modification 1 of the first embodiment)
In the embodiment of FIGS. 1 and 2, a single-ended forward converter is used as the power conversion unit 12, but the input voltage Vin is converted into an intermittent voltage by turning on / off the switching element 18, and the intermittent voltage is rectified and smoothed. Any circuit method may be used as long as it is a circuit that generates an output voltage Vo, and for example, a bridge converter, a flyback converter, a chopper circuit, or the like may be used.

Further, in the embodiments of FIGS. 1 and 2, an N-channel MOS-FET is used as the switching element 18, but a P-channel MOS-FET or a bipolar can be used as long as an intermittent voltage can be generated by turning the switching element 18 on and off. It may be a transistor or the like.

Further, in the embodiment of FIG. 2, an N-channel MOS-FET is used as the clamp element 40, but a P-channel MOS-FET, a bipolar transistor, or the like may be used as long as the same operation can be realized.

(Modification 2 of the first embodiment)
FIG. 4 is a circuit block diagram showing a modified example of the first embodiment of the switching device that turns the clamp element on and off by the switching element drive signal.

In the embodiment of FIGS. 1 and 2, the control circuit unit 14 is provided with the switching element drive circuit 32 and the clamp element drive circuit 34, but in the embodiment of FIG. 4, the control circuit unit 14 drives the clamp element. The circuit 34 is not provided, and the switching element drive signal VTR from the switching element drive circuit 32 is output as it is to the clamp element 40 as the clamp element drive signal VQ.

Therefore, the clamp element 40 is turned on and off in synchronization with the switching element 18, and the same operation as that of the embodiment of FIGS. 1 and 2 can be realized.

Further, since it is not necessary to provide the clamp element drive circuit 34 in the control circuit unit 14, the circuit configuration can be simplified and the cost can be reduced accordingly.

[Second Embodiment of Switching Power Supply Device]
FIG. 5 is a circuit block diagram showing a second embodiment of the switching power supply device.

(Problem of the first embodiment)
In the switching power supply device 10 of the first embodiment shown in FIGS. 1 to 4, when an element having a large current rating is used for the switching element 18, there is a problem that the error of acquiring the current value by the AD converter 36 becomes large. There is. This can be seen when an N-channel MOS-FET or the like having a large input capacitance Ciss (gate-source capacitance of the switching element) is used for the switching element 18.

In the switching power supply device 10 of FIG. 2, the switching element 18 is controlled by the switching element drive circuit 32 in the control circuit unit 14. Since the switching element drive circuit 32 is located in the control circuit unit 14, it operates with the connection point between the negative side of the input power supply 11 and the current-voltage conversion element 38 as ground.

When the switching element drive circuit 32 turns on the switching element 18.
"(Switching element drive circuit 32 in the control circuit unit 14)-> (Input capacitance Ciss from the gate terminal G to the source terminal S of the switching element 18)-> (From one end to the other end of the current-voltage conversion element 38)-> (Control circuit unit 14 grounds) "
Current flows through the path of.

Therefore, when the switching element 18 is turned on, a current ITRON for turning on the switching element 18 flows in the current-voltage conversion element 38 in addition to the switching element passing current IM.

Here, in the case of an element having a large current rating for the switching element 18, the input capacitance Ciss also increases, so that the current ITRON for turning on the switching element 18 also increases. If the current ITRON for turning on the switching element 18 becomes large, an error occurs when the switching element passing current IM is converted into a digital value by the AD converter 36 and acquired. The switching power supply device of the second embodiment of FIG. 5 solves this problem.

(Configuration and operation of the second embodiment)
As shown in FIG. 5, the switching power supply device of the present embodiment has a configuration of the current detection unit 16 and a ground position of the control circuit unit 14 as compared with the switching power supply device of the first embodiment of FIGS. 1 and 2. different.

As shown in FIG. 5, in the present embodiment, the ground of the control circuit unit 14 is connected to the connection point at one end of the switching element 18 and the current-voltage conversion element 38. As a result, in the operation in which the switching element drive circuit 32 turns on the switching element 18.
"(Switching element drive circuit 32 in the control circuit unit 14) → (Input capacitance Ciss from the gate terminal G of the switching element 18 to the source terminal S) → (Ground of the control circuit unit 14)"
The current ITRON that turns on the switching element 18 flows in the path of, and the current ITRON that turns on the switching element 18 does not flow in the current-voltage conversion element 38.

Therefore, by using an N-channel MOS-FET or the like having a large input capacitance Ciss (gate-source capacitance of the switching element) for the switching element 18, switching is performed even if the current ITRon for turning on the switching element 18 becomes large. No error occurs when the element passing current IM is converted into a digital value by the AD converter 36.

Further, also in the present embodiment, the current conversion generated in the current-voltage conversion element 38 is caused by turning on the clamp element 40 when the switching element 18 is on and turning off the clamp element 40 when the switching element 18 is off. The operation in which the voltage of the signal VRs is stored in the clamp capacitor 42 as the clamp signal VCs is the same as that of the first embodiment of FIG.

(Differences in current detector)
The difference between the current detection unit 16 in the second embodiment shown in FIG. 5 and the first embodiment shown in FIG. 2 is that the current-voltage conversion element 38 is connected to the ground of the control circuit unit 14 and the negative side of the input power supply 11. It is connected to the point, and the other end of the clamp capacitor 42 is connected to the ground of the control circuit unit 14. In FIG. 2, the other end of the clamp capacitor 42 is connected to the ground of the control circuit unit 14 and the connection point on the minus side of the input power supply 11.

Therefore, the other end of the clamp capacitor 42 (the side connected to the ground of the control circuit unit 14) becomes a positive voltage, and one end of the clamp capacitor 42 (the side connected to the other end of the clamp element 40) becomes the control circuit. It will have a negative voltage with respect to the ground of the part 14.

It is not allowed to input a negative voltage because a general AD converter cannot convert a negative voltage into a digital value and causes a circuit malfunction.

Therefore, in the present embodiment, the bias circuit 46 is inserted between one end of the clamp capacitor 42 and the input of the AD converter 36. The bias circuit 46 is composed of a voltage source Vcc and a voltage dividing circuit of resistors 48 and 50, and uses the voltage generated in the resistors 48 as the bias voltage Vb.

The voltage of the current conversion signals VRs generated in the current-voltage conversion element 38 by the switching element passing current IM flowing by turning on the switching element 18 is charged and held in the clamp capacitor 42 by turning on the clamp element 40, and is input to the AD converter 36. Between and ground, the voltage of the circuit in which the resistor 48 of the bias circuit 46 and the clamp capacitor 42 are connected in series is applied, and this becomes the input of the AD converter 36.

That is, the voltage signal VCbs = Vb-VCs obtained by subtracting the clamp signal VCs from the bias voltage Vb is input to the AD converter 36.

Therefore, in the present embodiment, as the switching element passing current IM becomes larger, the clamp voltage VCs at one end of the clamp capacitor 42 becomes larger in the negative direction. Therefore, with respect to the maximum value of the switching element passing current IM to be acquired by the AD converter 36. It is necessary to set the bias voltage Vb higher than the voltage generated in the current-voltage conversion element 38.

For example, when the voltage generated in the current-voltage conversion element 38 and the bias voltage Vb are set to be equal to the maximum value of the switching element passing current IM to be acquired by the AD converter 36, the switching element passing current IM is zero. At the time, the voltage VCbs input to the AD converter 36 becomes the bias voltage Vb, and when the switching element passing current IM is maximum, the voltage VCbs input to the AD converter 36 becomes zero, and the voltage generated at the current-voltage conversion element 38. The digital value converted by the AD converter 36 changes in inverse proportion.

(Advantages of the second embodiment)
Due to the configuration and operation of the switching power supply device 10 of the second embodiment, in addition to the merits of the switching power supply device of the first embodiment, the current value of the AD converter 36 when an element having a large current rating is used for the switching element 18 It is possible to solve the problem that the acquisition error of is large.

(Direction of clamp element)
In the N-channel MOS-FET used for the clamp element 40 of FIG. 5, the source terminal S is connected to the clamp element 42, and the drain terminal D is connected to the current-voltage conversion element 38. This is because the direction of the parasitic diode 44 of the N-channel MOS-FET used for the clamp element 40 is taken into consideration, and the forward direction of the parasitic diode 44 is the direction from the clamp capacitor 42 to the current-voltage conversion element 38. (Since the polarity of the voltage generated in the clamp capacitor 42 is opposite to that of the first embodiment, the direction of the parasitic diode 44 is reversed).

This prevents the electric charge stored in the clamp capacitor 42 from being discharged to the current-voltage conversion element 38 when the clamp element 40 is turned off. Also in the second embodiment, if the voltage value of the current conversion signals VRs generated in the current-voltage conversion element 38 is set to be equal to or less than the forward voltage drop value of the parasitic diode 44, the direction of the parasitic diode 44 does not matter. Since the electric charge stored in the clamp capacitor 42 is not discharged, the clamp element 40 may be used so that the direction of the parasitic diode 44 is opposite to that in FIG.

[Third Embodiment of a switching power supply device]
FIG. 6 is a time chart showing signal waveforms of each part in the switching power supply device of the first embodiment using a clamp element that cannot respond quickly to the clamp element drive signal, and FIG. 6A shows a switching element drive signal VTR. 6 (B) shows the switching element passing current IM, FIG. 6 (C) shows the current conversion signals VRs which are the voltage signals of the current-voltage conversion element 38, and FIG. 6 (D) shows the clamp element drive signal VQ. FIG. 6 (E) shows the clamp signals VCs of the clamp capacitor 42.

Further, FIG. 7 is a circuit block diagram showing a third embodiment of the switching power supply device, FIG. 8 is a time chart showing signal waveforms of each part in the switching power supply device of FIG. 7, and FIG. 8 (A) is a switching element. The drive signal VTR is shown, FIG. 8 (B) shows the switching element drive signal VTR2 whose off is delayed, FIG. 8 (C) shows the switching element passing current IM, and FIG. 8 (D) shows the current-voltage conversion element 38. The current conversion signals VRs to be voltage signals are shown, FIG. 8 (E) shows the clamp element drive signal VQ, and FIG. 8 (F) shows the clamp signals VCs of the clamp capacitor 42.

(Problems of the first and second embodiments)
The switching power supply devices of the first and second embodiments shown in FIGS. 1 to 5 use elements that can respond to the switching element drive signal VTR and the clamp element drive signal VQ sufficiently quickly as the switching element 18 and the clamp element 40. If so, no problem will occur. However, when an element that cannot respond quickly to the clamp element drive signal VQ is used as the clamp element 40, the problem shown in FIG. 6 occurs.

In FIG. 3, the clamp signal VCs charged by the current conversion signal VRs is stored in the clamp capacitor 42 even when the switching element 18 is off, whereas in FIG. 6, the moment when the switching element 18 is turned off. A phenomenon occurs in which the clamp signal VCs drops, and the clamp signal VCs becomes a pulsating voltage.

This is because an element that cannot respond quickly to the clamp element drive signal VQ is used as the clamp element 40, so that there is a period during which the clamp element 40 cannot be turned off even if the switching element 18 is turned off and the current conversion signal VRs is lowered. This is because the charge of the clamp signal VCs charging the clamp capacitor 42 is discharged through the clamp element 40 in the conductive state until the element 40 is turned off.

When the phenomenon that the clamp signal VCs pulsates occurs in this way, the switching element passing current IM is not correctly reflected in the clamp signal VCs, so that an error occurs when the switching element passing current IM is acquired by the AD converter 36.

(Circuit configuration and operation)
In order to solve this problem, in the embodiment of FIG. 7, the timing at which the clamp element 40 is turned off is controlled to be earlier than the timing at which the switching element 18 is turned off.

Specifically, as shown in FIG. 7, an off-delay circuit 52 is provided in the signal line connecting the switching element drive circuit 32 and the switching element 18. As shown in FIG. 8, in the off-delay circuit 52, when the switching element drive signal VTR output from the switching element drive circuit 32 changes to the H level, the output switching element drive signal VTR2 becomes the H level as it is. When the switching element drive signal VTR changes to the L level, the output switching element drive signal VTR2 is set to the L level after a predetermined delay time Td.

Therefore, the timing of the time t1 when the clamp element 40 is turned off by the same clamp element drive signal VQ as the switching element drive signal VTR can be controlled earlier than the timing of the time t2 when the switching element drive signal VTR2 turns off the switching element 18. can.

Therefore, even if an element that cannot respond quickly to the clamp element drive signal VQ is used as the clamp element 40, when the switching element 18 is turned off and the current conversion signal VRs is lowered, the clamp element 40 is turned off, so that the clamp capacitor 42 The charge of the clamp signal VCs is not discharged, the clamp signal VCs charged by the current conversion signal VRs is held in the clamp capacitor 42, and an error occurs when the switching element passing current IM is acquired by the AD converter 36. There is no.

Further, in the embodiment of FIG. 7, since the switching element drive circuit 32 drives the switching element 18 and the clamp element 40, the off-delay circuit 52 is provided on the signal line that outputs the switching element drive signal VTR to the switching element 18. However, in the embodiment of FIGS. 1 and 2, since the switching element drive circuit 32 controls the switching element 18 on and off and the clamp element drive circuit 34 controls the clamp element 40 on and off, an off delay circuit is provided. Instead, the switching element drive circuit 32 may be set to control the clamp element 40 to be turned off earlier than the timing at which the switching element 18 is turned off.

(Advantages of Third Embodiment)
As described above, in the switching power supply device of the third embodiment shown in FIG. 7, in addition to the above-mentioned merits of the first embodiment and the second embodiment, the clamp element 40 includes an element that cannot respond quickly to the clamp element drive signal VQ. When used, it is possible to solve the problem that the clamp signals VCs pulsate and the error of acquiring the current value by the AD converter 36 becomes large.

[Modification of the present invention]
The present invention includes appropriate modifications that do not impair its purpose and advantages, and is not further limited by the numerical values shown in the above embodiments.

10: Switching power supply device 11: Input power supply 12: Power conversion unit 14: Control circuit unit 16: Current detection unit 18: Switching element 20: Transformer 20a: Primary winding 20b: Secondary winding 22, 24: Rectifying element 26 : Output inductor 28: Output capacitor 30: Load 32: Switching element drive circuit 34: Clamp element drive circuit 36: AD converter 38: Current / voltage conversion element 40: Clamp element 42: Clamp capacitor 44: Parasitic diode 46: Bias circuit 48, 50: Resistance 52: Off delay circuit

Claims (6)

In a switching power supply device equipped with a power conversion unit, a current detection unit, and a control circuit unit,
The power conversion unit converts the input voltage supplied by the input power supply into an intermittent voltage by turning the switching element on and off by the switching element drive signal, and then rectifies and smoothes the input voltage to generate a DC voltage.
The current detection unit is composed of a current-voltage conversion element, a clamp element, a clamp capacitor, and a bias circuit.
One end of the current-voltage conversion element is connected to the switching element, the other end is connected to the ground of the input power supply, and the current passing through the switching element is converted into a current conversion signal which is a voltage signal.
The clamp element is a switch element that is turned on and off by a clamp element drive signal, one end of which is connected to a connection point between the other end of the current-voltage conversion element and the ground of the input power supply, and the other end of the clamp capacitor. When the clamp element is on, the current-voltage conversion element and the clamp capacitor are connected to charge the current conversion signal converted by the current-voltage conversion element into the clamp capacitor, and the clamp element is charged. When it is off, the current-voltage conversion element and the clamp capacitor are separated to prevent the discharge of the charge of the current conversion signal charged in the clamp capacitor and clamp the clamp capacitor.
One end of the clamp capacitor is connected to the input of the AD converter in the control circuit unit via the bias circuit that generates a predetermined bias voltage, and the other end is the ground of the control circuit unit and one end of the current-voltage conversion element. Connected to the connection point of
Wherein the control circuit section, said comprising an AD converter, the current connection point between one end of the voltage conversion element and the other end of the clamp capacitor operates as a ground, the clamp capacitor the current conversion clamped from the bias voltage voltage signal obtained by subtracting the signal is input to the AD converter, and outputs the switching element driving signal and the clamp element driving signal so as to turn on and off in synchronization with said switching element and said clamping element,
A switching power supply that is characterized by this.
In the switching power supply device according to claim 1,
The clamping device is used the switching element having a parasitic diode, the switching power supply apparatus characterized by forward is connected from said clamp capacitor such that the orientation to the current voltage conversion element of the parasitic diode ..
In the switching power supply device according to claim 1,
The clamping element, the switching element is used with a parasitic diode, the forward direction of the parasitic diode is connected from the current-voltage converting element so that the orientation to said clamp capacitor, generated in the current-voltage converting element A switching power supply device characterized in that the voltage value of the current conversion signal is set to be equal to or lower than the forward voltage drop of the parasitic diode.
In the switching power supply device according to any one of claims 1 to 3.
The control circuit unit is a switching power supply device characterized in that the clamp element is driven by the switching element drive signal that drives the switching element instead of the clamp element drive signal.
In the switching power supply device according to any one of claims 1 to 3.
The control circuit unit is a switching power supply device characterized in that the timing at which the clamp element is turned off is controlled to be earlier than the timing at which the switching element is turned off.
In the switching power supply device according to claim 4,
Wherein said control circuit unit, the output switching element drive signal to the switching element and the clamping element, a signal line for outputting the switching element driving signal to the switching element, delaying the off timing of the switching element driving signal A switching power supply characterized by having an off-delay circuit.

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