JP3142755B2 - Switching power supply circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a so-called DC-D
The present invention relates to a switching power supply circuit suitably implemented in a C converter or the like.

[0002]

2. Description of the Related Art It is used in portable small electronic equipment and the like, and switches a DC current obtained by rectifying and smoothing a commercial AC or a DC current from a battery at a high frequency. A switching power supply circuit that transforms the voltage with high efficiency is widely used.

In such portable small electronic devices, there is a demand for reduction in power consumption during light loads such as during standby. For example, according to the guidelines for electronic devices in Switzerland, the power consumption during standby is reduced. , At the end of 1996, 6 W or less, and at the end of 1998, 3 W or less.

FIG. 7 is a block diagram showing an electrical configuration of a typical conventional switching power supply circuit 51 for realizing such low power consumption. The switching power supply circuit 51 is provided with two converters, a main converter 50 and a sub-converter 50a. Input AC currents from input terminals T1 and T2 connected to a commercial AC power supply or the like are rectified by a rectifier circuit 52 composed of a diode bridge or the like, then smoothed by a smoothing capacitor 53, and output between power supply lines 54 and 55. Is done.

In the main converter 50, a series circuit including a primary winding of a pulse transformer 56 and a switching element 57 is interposed between the power supply lines 54 and 55, and the switching element 57 is connected to a gate from a control circuit 58. Conduction / interruption is controlled by a pulse. As a result, when the switching element 57 is cut off, an induced electromotive voltage is generated in the secondary winding of the pulse transformer 56, and the current caused by the electromotive voltage is rectified and smoothed by the diode 59 and the smoothing capacitor 60, and the power line 61 , 62 to output terminals T3, T4, respectively.

The sub-converter 50a has the same configuration as the main converter 50. Similar and corresponding parts are denoted by the same reference numerals with the suffix a, and the description thereof is omitted. Control circuit 5 of the main converter 50
8 is controlled by the load state detection circuit 63,
The load state detection circuit 63 detects the load state from the voltage between the terminals between the output terminals T3 and T4, and determines the light load state when the voltage between the terminals becomes equal to or higher than a predetermined voltage. In this state, the switching operation of the switching element 57 by the control circuit 58 is stopped.

In this way, the power consumption is reduced by suspending the operation of the main converter 50 in a light load state such as in a standby state and reducing the loss in the main converter 50.

[0008]

In the prior art as described above, the main converter 50 and the sub-converter 50a
These two converters are required, which increases the cost and increases the mounting area.

An object of the present invention is to provide a switching power supply circuit capable of reducing cost and space.

[0010]

According to a first aspect of the present invention, there is provided a switching power supply circuit in which a primary current of a transformer is switched by a switching element to obtain a secondary current of a desired voltage. A load state detecting means for detecting a load state of an output terminal of the switching power supply circuit, an oscillation circuit, and an oscillation signal from the oscillation circuit.
Signal cycle, the load state from the load state detection means
Of the corresponding feedback voltage and the first reference voltage
A pulse with a pulse width corresponding to the lower voltage,
Pulse width modulation comparator for controlling the switching element
And detecting that the load state is a light load state.
2 and the feedback voltage.
When the feedback voltage is smaller than the second reference voltage
The oscillation frequency of the oscillation circuit
And a oscillation frequency lowering means for lowering, I light load
Therefore, the feedback voltage decreases, and the first
When the voltage is less than the reference voltage, the pulse width modulation comparator
Pulse width to short value corresponding to the feedback voltage
Thus, a constant voltage operation is performed.

According to the above configuration, in a switching power supply circuit used as a DC-DC converter or the like, the loss in the switching element or its control circuit, which is a major factor of the loss, and the switching frequency of the switching element at light load are reduced. It is reduced by lowering.

Therefore, there is no need for a complicated structure such as providing a plurality of converter circuits and selectively using them according to the load condition. A single converter circuit can handle from a steady load to a light load. Can reduce power consumption,
A low-cost circuit configuration and space saving can be realized .

Further, since the switching frequency is merely switched in accordance with the two load states of the steady load and the light load, the switching frequency can be realized by switching the time constant of the oscillation circuit to two types. It can be realized by switching the bias current to the circuit into two types, and can be realized with a simple circuit configuration.

[0014] In the switching power supply circuit according to the invention of claim 2, wherein the switching element, a power MOSF
The switching element , the oscillation circuit, and the oscillation circuit.
The wave number lowering means is characterized by being sealed in one package.

According to the above configuration, the mounting area can be further reduced.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described.
The following is a description based on FIGS. 1 to 5.

FIG. 1 is a block diagram showing an electrical configuration of a switching power supply circuit 1 according to an embodiment of the present invention. Input terminals P1 and P2 connected to a commercial AC power supply
Is rectified by a rectifier circuit 2 composed of a diode bridge or the like, is smoothed by a smoothing capacitor C1, and is output between the power supply lines 3 and 4. A primary winding 5 of a pulse transformer 5 is provided between the power supply lines 3 and 4.
A series circuit comprising a switching element Q1 composed of a and an N-channel type power MOSFET is interposed. The switching element Q1 is controlled to be turned on / off by a gate pulse from the control circuit 6. As a result, when the switching element Q1 is cut off, an induced electromotive voltage is generated in the secondary winding 5b of the pulse transformer 5, and the current caused by the electromotive voltage is rectified and smoothed by the diode D1 and the smoothing capacitor C2, and the power supply line Output terminal P from 8, 9
3 and P4.

A series circuit composed of a diode D2, a pull-up resistor R1, and a phototransistor Q2 is interposed between the power supply lines 3 and 4. The potential at the connection point 10 between the pull-up resistor R1 and the phototransistor Q2 is input to the oscillation frequency lowering circuit 11 and to the input terminal F2 of the PWM comparator 12. An oscillation signal from an oscillation circuit 13 whose oscillation frequency is controlled by the oscillation frequency lowering circuit 11 is input to an input terminal F1 of the PWM comparator 12, and an input terminal F3 of the PWM comparator 12 is Reference voltage source 14
Is input.
The PWM comparator 12 outputs a gate pulse having a pulse width corresponding to the cycle of the oscillation signal from the oscillation circuit 13 and the lower one of the input voltages to the input terminals F2 and F3, This gate pulse is supplied to the control circuit 6
, Is input to the gate of the switching element Q1.

The switching element Q1 and the control circuit of the switching element Q1, such as the control circuit 6, the oscillation frequency lowering circuit 11, the PWM comparator 12, and the oscillation circuit 13, are sealed in one package as shown in FIG. , And the converter 15.

FIG. 2 is a diagram showing the structure of the converter 15. The control circuit 6, the oscillation frequency lowering circuit 11,
The PWM comparator 12 and the oscillation circuit 13 are formed in an integrated circuit chip 16.
Are mounted on the ceramic substrate 17. The ceramic substrate 17 on which the integrated circuit chip 16 is mounted and the switching element Q1 are integrally sealed with a resin or the like.

In the switching element Q1 realized by a MOSFET (metal oxide semiconductor), the drain connected to the primary winding 5a of the pulse transformer 5 is connected from the lead frame 18 to the terminal K1. The terminal K2 is connected to the source of the switching element Q1 and the ground line of the converter 15, and the terminal K3 receives an input signal to the control circuit 6 from an overcurrent detection circuit (not shown in FIG. 1). Furthermore, the terminal K4
Is connected to the collector of the phototransistor Q2,
Terminal K5 is connected to power supply line 3.

On the other hand, a load state detection circuit 20 is provided between the power supply lines 8 and 9. The load state detection circuit 20 controls the cathode current of the diode D3 according to the line voltage between the power lines 8 and 9. The diode D3 and the phototransistor Q2 form a pair to form a photocoupler. Therefore, an output voltage between the output terminals P3 and P4, that is, a feedback voltage corresponding to a load state is applied to the connection point 10. Will be.

When the output voltage rises due to a light load, the current flowing through the diode D3 increases, so that the current flowing through the phototransistor Q2 increases and the feedback voltage decreases. When Vref1 is less than 2.8V, for example,
The pulse width of the gate pulse from the PWM comparator 12 to the switching element Q1 has a short value corresponding to the feedback voltage , and thus a constant voltage operation is realized.

FIG. 3 is an electric circuit diagram showing a specific configuration of the oscillation circuit 13 and the oscillation frequency lowering circuit 11.
The oscillation circuit 13 includes comparators 34 and 35 and constant current sources 36,
37; 38, 39 and voltage dividing resistors R11, R12, R13
And transistors Q11, Q12, Q13, Q14,
And a capacitor C11. A series circuit of voltage dividing resistors R11 to R13 is interposed between the power supply line 40 to which the high-level voltage Vs is applied and the ground line. The connection point between the voltage dividing resistors R11 and R12 is connected to the inverting input terminal of the comparator 34 and the non-inverting input terminal of the comparator 35, respectively. An oscillation signal output to the PWM comparator 12 is input to a non-inverting input terminal of the comparator 34 and an inverting input terminal of the comparator 35. A bypass transistor Q11 is provided in parallel with the voltage dividing resistor R13.
11 is turned on / off by a comparator 34. The output of comparator 35 is provided to the base of bypass transistor Q12.

A series circuit of a constant current source 36 and a transistor Q13 is interposed between the power supply line 40 and the ground line. The constant current source 36 is also provided with a constant current source 37 in parallel, and the constant current source 37 is selectively connected by a transistor Q16 in the oscillation frequency lowering circuit 11. The transistor Q13 forms a pair with the transistor Q14 to form a current mirror circuit. The bases of the transistors Q13 and Q14 are connected to the collector of the transistor Q13 and the constant current source 36 and to the collector of the bypass transistor Q12. Therefore, when the bypass transistor Q12 conducts, these transistors Q13, Q1
4 shuts off. The transistor Q14 is a capacitor C11
Is provided in parallel with the capacitor C11.
A charging current is supplied from the power supply line 40 via a constant current source 38. A constant current source 39 is also provided in parallel with the constant current source 38. The constant current source 39 is connected to the oscillation frequency lowering circuit 11 like the transistor Q16.
Are selectively connected by a transistor Q15 in the inside.

In the oscillation frequency lowering circuit 11, the transistors Q15 and Q16 are connected to the reference voltage source 3
When the reference voltage becomes equal to or higher than the reference voltage Vref11 set by 1, for example, 4 V, the comparator 32 conducts.

For example, the voltage Vs is selected to be 4 V, and the voltage dividing resistors R11, R12 and R13 each have a voltage of 9.
68 kΩ, 2.32 kΩ and 18 kΩ are selected. Therefore, the comparator 34 includes the bypass transistor Q11
In the state where is shut off, the output voltage of the oscillation circuit 13, that is, the terminal voltage of the capacitor C11 is: (R12 + R13) .Vs / (R11 + R12 + R13) = (2.32 + 18). .71V, a high-level output is derived, and the bypass transistor Q11 is turned on.

Thus, once the bypass transistor Q
When 11 is turned on, even if the output voltage drops from the 2.71V, the bypass transistor Q11 can be used until R12.Vs / (R11 + R12 + R13) = 2.32.4 / (9.68 + 2.32 + 18) = 0.8V. Is conducting,
When the voltage reaches 0.8 V, the bypass transistor Q11
Is shut off and remains shut off until the voltage again reaches 2.71V.

On the other hand, the output from the comparator 35 is opposite to that of the comparator 34, that is, the output voltage increases.
Up to 71 V, a high-level output is derived, so that the transistor Q12 is conducting. When the voltage reaches 2.71 V, a low-level output is derived until the voltage drops to 0.8 V, and the bypass transistor Q12 is cut off. I have.

Here, the current amount of the constant current source 38 is I
0, for example, 5.2 μA, whereas the current amount of the constant current source 39 is I1, for example, 10.4 μA.
A is set. The current amount of the constant current source 36 is 2
I0 = 10.4 μA is selected, and the current amount of the constant current source 37 is selected as 2I1 = 20.8 μA. Furthermore, the capacitance of the capacitor C11 is selected to be 31 pF.

Therefore, transistors Q15, Q16
When the bypass transistor Q12 is turned off while the transistor is conducting, the transistor Q14 is turned off, and the constant current sources 38 and 39 cause the capacitor C11 to output a current amount I0.
The battery is charged at + I1. Therefore, the terminal voltage, that is, the output voltage of the oscillation circuit 13 is represented by t = ΔV · C11 / (I1 + I0) = (2.71-0.8) × 31 × 10 ⁻¹² / (t) 10.4 + 5.2) × 10 ⁻⁶ = 5 (μsec), and increases from 0.8 V to 2.71 V in 5 μsec.

On the other hand, the bypass transistor Q1
2 is turned on, the transistor Q14 is turned on, and the capacitor C11 is supplied with the current I
While being charged by 0 + I1, it is discharged by the sum of the currents 2 (I0 + I1) of the constant current sources 36 and 37. Therefore, from the above equation, it is 5 μs from 2.71 V to 0.8 V.
ec.

As described above, the oscillation circuit 13 operates in a state where the transistors Q15 and Q16 are conducting, as shown in FIG.
As shown in (a), a period of 10 μsec, that is, 10
At 0 kHz, a triangular wave in the range of 0.8 V to 2.71 V is oscillated.

On the other hand, transistors Q15, Q1
When the oscillation frequency, which is cut off by the oscillator 6, decreases, I1 = 0 in the above equation, and as shown in FIG.
A triangular wave oscillating at 5 μsec, that is, at 33 kHz is output.

Therefore, the oscillation frequency of the oscillation circuit 13 becomes 100 kHz at the time of steady load, and becomes 33 kHz when the output voltage rises with a light load and the feedback voltage becomes lower than the reference voltage Vref11.

FIG. 5 is a graph showing the experimental results of the present inventor who measured the output current from the output terminals P3 and P4, that is, the change in power consumption with respect to the change in the load state. In FIG. 5, reference numeral α1 denotes an oscillation frequency lowering circuit 11
, The reference numeral α2 indicates the case where the oscillation frequency lowering circuit 11 is provided, and the oscillation frequency lowering circuit 11 is operated when the output current is 0.5 A.

As described above, the oscillation circuit 13 and the oscillation frequency lowering circuit 11 lower the oscillation frequency of the gate pulse to the switching element Q1 in a lighter load state than in a steady load state. , The loss of the converter 15 which increases in proportion to the power consumption can be reduced, and low power consumption can be achieved. In addition, such low power consumption can be realized by a single converter 15, and cost reduction and space saving can be achieved. Furthermore, as shown in FIG. 2, the switching element Q1 and a circuit for controlling the switching element Q1 such as the oscillation frequency lowering circuit 11 and the oscillation circuit 13 are sealed as a converter 15 in a single package. Further, space saving can be achieved.

FIG. 6 shows another embodiment of the present invention.
It is as follows if it explains based on.

FIG. 6 is an electric circuit diagram showing a specific configuration of the oscillation frequency lowering circuit 41 according to another embodiment of the present invention and the oscillation circuit 13 related thereto. In the oscillation frequency lowering circuit 41, the feedback voltage is input to the inverting input terminal of the differential amplifier 42. The output of the differential amplifier 42 is positively fed back through resistors R14 and R15, and is input to the bases of the transistors Q15 and Q16 via resistors R16 and R17, respectively. The emitter of the transistor Q15 supplies a charging current to the capacitor C11 via the resistor R18. Transistor Q1
The emitter of 6 supplies a current to the collector of the transistor Q13 via the resistor R19.

Therefore, as the output voltage increases and the feedback voltage decreases under a light load condition, the output voltage of the differential amplifier 42 decreases, and the transistor Q15,
The currents I1a and 2I1a flowing through Q16 decrease. Thus, the oscillation frequency can be continuously and linearly reduced according to the load state.

Therefore, the aforementioned oscillation frequency lowering circuit 1
In No. 1, the downsizing is possible, whereas in the oscillation frequency lowering circuit 41, the oscillation frequency can be finely changed according to the load state.

As described above, according to the above configuration, the steady negative
Switching cycle from load to light load according to load condition
Since the wave number changes continuously,
Fine switching frequency control can be performed,
Minimizes losses that increase with increasing switching frequency
Can be minimized.

[0043]

Switching power supply circuit according to the invention of claim 1 according to the present invention, as described above, in the switching power supply circuit used as such a DC-DC converter, the oscillation circuit
At the cycle of the oscillation signal from the load state detecting means.
The feedback voltage corresponding to the load state and the first reference voltage
And the pulse width corresponding to the lower of the two voltages
Pulse width for controlling the switching element with a pulse of
A modulation comparator, wherein the load state is a light load state;
A second reference voltage to be detected and the feedback voltage
The feedback voltage is higher than the second reference voltage.
Is smaller than when the oscillation circuit is large.
Oscillation frequency lowering means for lowering the oscillation frequency,
As the load becomes light, the feedback voltage decreases,
If less than the serial first base reference voltage, the pulse width modulation comparator
The pulse width corresponds to the feedback voltage.
The constant voltage operation is performed by setting the short value.

Therefore, the loss in the switching element and its control circuit, which is a major factor of the loss, is reduced by lowering the switching frequency of the switching element at light load, so that a single converter circuit can reduce the steady load. To light loads, and low power consumption can be realized with a low-cost circuit configuration and in a small space . Further, since the switching frequency is merely switched in accordance with the two load states of the steady load and the light load, it can be realized with a simple circuit configuration.

In the switching power supply circuit according to the second aspect of the present invention, as described above, the switching element comprises a power MOSFET, and the switching element and the oscillation circuit are connected.
The circuit and the oscillation frequency lowering means are sealed in one package.

Therefore, the mounting area can be further reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an electrical configuration of a switching power supply circuit according to an embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a converter in which a switching element in the switching power supply circuit and a control circuit of the switching element such as an oscillation frequency lowering circuit and an oscillation circuit are sealed in one package.

FIG. 3 is an electric circuit diagram showing a specific configuration of an oscillation circuit and an oscillation frequency lowering circuit used in the switching power supply circuit shown in FIG.

FIG. 4 is a waveform chart for explaining an oscillation frequency lowering operation of the oscillation circuit by the oscillation frequency lowering circuit.

FIG. 5 is a graph of experimental results of the present inventor showing a change in power consumption with respect to a change in output current (load state) for explaining the effect of the present invention.

FIG. 6 is an electric circuit diagram showing a specific configuration of an oscillation frequency lowering circuit according to another embodiment of the present invention and an oscillation circuit related thereto.

FIG. 7 is a block diagram showing an electrical configuration of a typical conventional switching power supply circuit.

[Explanation of symbols]

1 the switching power supply circuit 2 rectifying circuit 5 pulse transformer 6 control circuit 11 an oscillation frequency lowering circuit (oscillation frequency lowering means) 12 PWM comparator (PWM comparator) 13 oscillation circuit 20 load state detection circuit (load state detection means) 32 comparator 34 comparator 35 comparator 41 oscillation frequency lowering circuit C1 smoothing capacitor C2 smoothing capacitor C11 capacitor Q1 switching element

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M 3/28 H03K 17/78

Claims (2)

(57) [Claims] In a switching power supply circuit in which a primary current of a transformer is switched by a switching element to obtain a secondary current of a desired voltage, a load state for detecting a load state of an output terminal of the switching power supply circuit. Detection means, an oscillation circuit, and the load state detection based on a cycle of an oscillation signal from the oscillation circuit.
Feedback voltage corresponding to the load condition from the output means
Corresponds to the lower of the first reference voltage
The switching element is controlled by the pulse of the specified pulse width
A pulse width modulation comparator for detecting that the load condition is a light load condition.
The reference voltage is compared with the feedback voltage, and the
When the feedback voltage is smaller than the second reference voltage
Lowers the oscillation frequency of the oscillation circuit compared to when
Oscillation frequency lowering means for lowering the feedback voltage,
When the voltage becomes lower than the first reference voltage, the pulse width modulation comparison is performed.
The pulse width corresponds to the feedback voltage.
Switching value that operates at a constant voltage.
Source circuit. 2. The power supply device according to claim 1, wherein the switching element is a power MOSF.
The switching element, the oscillation circuit, and the oscillation circuit.
Wave number lowering means is sealed in one package
The switching power supply circuit according to claim 1.