JPH0757096B2 - Limited switching regulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a switching regulator for supplying DC power to electric equipment such as a VTR and a facsimile,
The present invention relates to a limiting switching regulator that limits a decrease in the oscillation frequency of a switching element.

[Prior Art] In a ringing choke converter type switching regulator that extracts a DC output from a secondary winding during the OFF period of the switching element, as shown in FIG. 10, the switching element increases as the load current increases. Oscillating frequency decreases. This is due to the increase in load current
This is because the ON period of the switching element becomes long in order to increase the input energy to the transformer, and the OFF period becomes long accordingly.

When the oscillation frequency of the switching element decreases, the power conversion efficiency of the transformer decreases. In addition, if the power conversion efficiency is to be kept high, the transformer becomes large.

The present invention has been made to solve the above problems, and an object of the present invention is to keep the conversion efficiency high without increasing the size of the transformer by limiting the decrease of the oscillation frequency due to the increase of the output current. I am trying.

[Means for Solving the Problem] A detection winding coupled to the transformer for detecting an off period of the switching element, and a detection winding for detecting the off period when the off period is longer than a predetermined time within the detected off period. A frequency limiting circuit that outputs an ON control signal for turning on the switching element after a delay of the predetermined time from the start point and shortens the OFF period is provided.

In a switching regulator of the type including a control winding coupled to a transformer and a control signal extraction circuit for extracting a control signal input to the control electrode of the switching element from the control winding, the detection winding is Can also be used as the control winding.

The frequency limiting circuit includes a diode that conducts during the OFF period of the switching element, a delay circuit that operates by conduction of the diode and outputs a delay signal at a timing delayed by the predetermined time from the start of conduction, and a delay signal. And an ON control circuit that receives and outputs the ON control signal.

[Operation] According to the present invention, during the off period detected by the detection winding, the frequency limiting circuit causes the on control signal to be delayed by the predetermined time from the start time of the off period when the off air is longer than the predetermined time. Is output to shorten the off period. Therefore, even if the load current increases, the off period is kept short at a predetermined time, and the on period is also kept constant accordingly, so that the decrease in frequency is limited.

Further, if the detection winding is also used as a control winding for controlling ON / OFF of the switching element, the number of parts is reduced.

Furthermore, a frequency limiting circuit, a diode, a delay circuit,
If it is configured with an ON control circuit, the circuit is simplified.

Embodiments Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit diagram showing a first embodiment, in which the primary winding 12 of a transformer 11 is connected to a DC power supply 15 via a switching element 13 and a current detecting resistor 14. The switching element 13 is a field effect transistor in this example.

An output circuit 21 including a diode 19 and a smoothing capacitor 20 is connected to a secondary winding 18 magnetically coupled to the primary winding 12 via a core 17, and by this output circuit 21, During the off period of the switching element 13, a DC output is taken out from the secondary winding 18 and supplied to the load resistor 2.

A control signal generation circuit 25 for generating an optical signal (control signal) 24 for keeping the voltage across the load resistor 22 constant is connected to the output circuit 21. This control signal generation circuit 25
Is a voltage dividing circuit 26, a constant voltage element 28 biased by the voltage at the voltage dividing point 27, a photodiode 29 and a current limiting resistor 30 connected in series to the constant voltage element 28, and a photodiode 29 It consists of an adjustment resistor 31 for adjusting the cathode voltage, and when a voltage higher than the set value is applied to the control signal generation circuit 25, an optical signal is output from the photodiode 29.
Generates 24.

An ON / OFF control transistor 35 for controlling ON / OFF of the switching element 13 and a starting resistor 36 for ON-starting the switching element 13 are connected to the gate (control electrode) of the switching element 13. . On the other hand, a control winding 37 is magnetically coupled to the transformer 11 via a core 17, and the switching element is connected to the control winding 37.
A control signal extraction circuit 38 for extracting a control signal input to the gate of 13 is connected. This control signal extraction circuit 38 is composed of a charging / discharging circuit 41 consisting of a capacitor 39 and a resistor 40, and a series circuit of a photodiode 42, a resistor 43 and a diode 44 which conducts upon receiving the optical signal 24 from the control signal generating circuit 25. And a constant voltage control circuit 45.

Further, the control winding 37 is connected to an ON control signal generation circuit 49 including a rectifying / smoothing circuit in which a diode 47 and a capacitor 48 are combined.

The transformer 11 is further magnetically coupled with a detection winding 51 for detecting the off period of the switching element 13, and a diode 52 that conducts while the switching element 13 is off is connected to the detection winding 51. A delay circuit that outputs a delay signal 53 with a delay of a predetermined time (however, within the off period) from a time point when a voltage is generated in the detection winding 51 in a direction of conducting the diode 52, that is, a time point when the switching element 13 is turned off.
54 and are connected. An ON control signal output circuit 58 is connected to the delay circuit 54, and the ON control signal output circuit 58 switches the ON control signal 57 composed of the DC voltage signal generated by the ON control signal generation circuit 49. It is applied to the gate of the device 13.

The ON control signal generation circuit 49 and the ON control signal output circuit 58 form the ON control circuit 59 of the present invention, and the diode 52, the delay circuit 54 and the ON control circuit 59 form the frequency limiting circuit 60 of the present invention. ing.

The ON control signal output circuit 58 includes an NPN transistor 61 and its bias resistor 62, a PNP transistor 63 and its bias resistor 64, a diode 65, and a resistor 66.
The collector of N-transistor 61 is connected to P
When the delay signal 53 from the delay circuit 54 is connected to the base of the NP transistor 63 and the collector of the PNP transistor 63 is connected to the base of the NPN transistor 61, the ON control signal 57 is output. Diode 65
It is adapted to be applied to the gate of the switching element 13 through.

Further, on the positive potential side of the current detection resistor 14 connected to the source of the switching element 13, the on / off control transistor 35 is provided by the voltage detected by the current detection resistor 14.
A current supply resistor 68 for supplying a current is connected to the base of the. Further, the primary winding 12 has a diode 71,
A spanner circuit 74 for discharging energy, which includes a capacitor 72 and a resistor 73, is connected.

In the above configuration, first, the on / off operation of the switching element 13 when the control signal generating circuit 25 and the frequency limiting circuit 60 of the present invention are not operated will be described.

When the DC power supply 15 is turned on, the gate-source capacitance of the switching element 13 and the charging / discharging circuit 41 are passed through the starting resistor 36.
The capacitor 39 and is charged. When the gate potential of the switching element 13 rises to a certain level due to this charging, the switching element 13 turns on. At this time, the switching element 13 is previously passed from the DC power source 15 through the starting resistor 36.
Since a small gate current is supplied to the switching element 13, the switching element 13 turns on rapidly. The timing at which the switching element 13 turns on is determined by the time constant determined by the resistance values of the starting resistor 36 and the resistor 40, the capacitance of the capacitor 39, and the gate-source capacitance of the switching element 13.

On the other hand, when the switching element 13 is turned on, the current 77 flowing in the current detection resistor 14 increases with time with a slope of E / L when the voltage of the DC power supply 15 is E and the inductance of the primary winding 12 is L. Then, when the voltage drop in the current detecting resistor 14 due to the current 77 reaches the threshold voltage VBE between the base and emitter of the transistor 35, the transistor 35 is turned on. As a result, the electric charge of the capacitor 39 is discharged through the transistor 35, the gate potential of the switching element 13 is lowered, and the switching element 13 is turned off.
The timing at which the switching element 13 turns off depends on the resistance value of the current detection resistor 14, the voltage of the DC power supply 15, and the primary winding.
Determined by 12 inductances. The voltage V13 between the source and drain of the switching element 13 at this time is the second
This is as shown in FIG.

When the switching element 13 is turned off, the output circuit 21 extracts the output from the secondary winding 18 of the transformer 11.
In this state, since the switching element 13 has a long ON / OFF cycle, the oscillation frequency is low.

Next, a case where the control signal generating circuit 25 operates will be described.

When the output voltage across the control signal generation circuit 25 rises above a certain value, the photodiode 29 of the control signal generation circuit 25 generates the optical signal 24. The phototransistor of the constant voltage control circuit 45 when the switching element 13 is in the ON state.
Since a forward voltage is applied to 42, the phototransistor 42 receives the optical signal 24 and becomes conductive in this state. When the phototransistor 42 becomes conductive, the base potential of the on / off control transistor 35 rises, so that the on / off control transistor 35 turns on. When the on / off control transistor 35 turns on, the gate potential of the switching element 13 drops and the switching element 13 turns off.

By thus advancing the timing of turning off the switching element 13, the energy input to the transformer 11 is suppressed and the output voltage is reduced to a constant value. When the phototransistor 42 is turned on, the on / off control transistor 35 is turned on, and when the switching element 13 is turned off, the positive voltage induced in the control winding 37 is inverted and becomes negative. The base potential of the off control transistor 35 drops, the on / off control transistor 35 turns off again, and the next time the switching element 13 is turned on by charging the capacitor 39.

The collector-emitter voltage V35 of the on / off control transistor 35 at this time is shown in FIG. 2 (B), and the source-drain voltage V13 of the switching element 13 is shown in FIG. 2 (C). In this state, the ON period Ton1 of the switching element 13 is shortened, and accordingly, the discharge time of the capacitor 39 is also shortened, so that the charging time of the capacitor 39 is shortened accordingly, and as a result, the OFF period of the switching element 13 is also shortened. It gets shorter. That is, the oscillation frequency becomes high.

Next, a case where the frequency limiting circuit 60 of the present invention operates will be described.

When the current flowing through the load resistance 22 increases, the output voltage decreases, so that the photodiode 29 does not generate the optical signal 24, the ON period of the switching element 13 is not shortened, and the ON / OFF timing of the switching element 13 is As shown in FIG. 2 (A), the oscillation frequency may decrease.

However, when the switching element 13 is turned off, the voltage V52 shown in FIG. 2D is generated at the cathode of the diode 52 of the frequency limiting circuit 60. In response to this voltage V52, the delay circuit 54 operates to generate the delay signal 53 consisting of the pulse signal shown in FIG. 2E with a delay of a predetermined time Td from the start time 81 of the off period within the off period.

The delay signal 53 is input to the base of the NPN transistor 61, the NPN transistor 61 is turned on, the PNP transistor 63 is turned on by the turning on of the NPN transistor 61, and the on control signal 57 obtained by the on control signal generation circuit 49. Is applied to the gate of the switching element 13 through the diode 65 to turn on the switching element 13. As a result, the off period Toff of the switching element 13 is limited to Toff = Td = constant. At this time, the switching element 13 is turned on.
Off-diming is shown in FIG.

When the off period Toff of the switching element 13 is limited, 2
As shown in FIG. 5, the current I18 flowing through the next winding 18 shows a waveform obtained by cutting the current waveform I180 when there is no limitation of the off period (Toff1 during the off period). The direct current component I0 is superimposed. As a result, the current I12 flowing through the primary winding 12 shows a waveform obtained by cutting the first half of the current waveform I120 when there is no limit of the off period, as shown in FIG. It is a form in which the component I1 is superimposed.

Therefore, the voltage drop in the current detection resistor 14 is accelerated,
The on-timing of the transistor 35 is advanced, and the switching element 13 is also turned off, which results in
On period Ton when Ton is limited and not limited, On period T
It is shorter than on1. As described above, both the off period Toff and the on period Ton have the value Tof when the off period is not limited.
Since they are shorter than f1 and Ton1, respectively, a decrease in oscillation frequency due to an increase in load current is prevented.

On the other hand, in the high oscillation frequency region in which the off period Toff in FIG. 2C is shorter than the predetermined time Td, the predetermined time Td is outside the off period of the switching element 13, that is, within the on period of the switching element 13, and this on Since the cathode voltage V52 of the diode 52 drops to zero volts within the period, the delay signal 53 is not output from the delay circuit 54, so the switching element 13 is not turned on. Therefore, in this high oscillation frequency region, the oscillation frequency decreases as the load current increases, as in the conventional case. That is, the characteristics shown in FIG. 9 are obtained.

FIG. 3 shows a second embodiment. In this second embodiment, the control for turning on / off the switching element by eliminating the detection winding 51 in the first embodiment of FIG. The use winding 37 also serves as the detection winding 51.

That is, in FIG. 3, the control winding 37 has a diode 82 which is conductive when the switching element 13 is off.
And a delay circuit 84 that outputs the delay signal 53 after a predetermined time (however, within the off period) from the time when the diode 82 becomes conductive, that is, the time when the switching element 13 is turned off.

Since other configurations are the same as those in FIG. 1, the same parts are denoted by the same reference numerals and detailed description thereof will be omitted.

In this second embodiment, as shown in FIG.
When 82 becomes conductive, a negative voltage V82 is generated at its anode, and the delay circuit 84 receives the voltage V82 and delay signal 53
Is output, and the ON control signal 57, which is a DC voltage signal, is applied to the gate of the switching element 13 through the diode 65 of the OFF control signal output circuit 58 of FIG.
Turn on 13.

According to the second embodiment, since the detection winding 51 is also used as the control winding 37, the number of parts is reduced.

FIG. 7 is an ON control signal generation circuit of FIG. 1 (first embodiment).
A modified example in which 49 is simplified, and FIG. 8 shows a modified example in which the ON control signal generating circuit 49 in FIG. 3 (second embodiment) is simplified. In both figures, the diode 47 of the ON control signal generation circuit 49 is also used as the diode 44 of the constant voltage control circuit 45. Therefore, according to the modified examples shown in FIGS. 7 and 8, there is an advantage that the circuit configuration is simplified.

It should be noted that, in each of the above embodiments, other switching means, such as an NPN transistor, can be used as the switching element 13. Further, the delay circuits 54 and 84 themselves may be provided with a function of detecting the starting point of the off period of the switching element, and the diodes 52 and 82 may be omitted.

Furthermore, the present invention can also be applied to a switching regulator of the type in which the output side and the input side are electrically connected and the output voltage is controlled by the frequency, without performing the constant voltage control as in the above embodiments.

[Advantages of the Invention] As described above, according to the present invention, the off period is maintained at a predetermined time even if the load current increases, and accordingly the on time is also kept constant. Limited.
Therefore, the conversion efficiency can be maintained high without increasing the size of the transformer 11.

Further, the detection winding 51 is connected to the switching element 13
If it is also used as the control winding 37 for controlling the off state, the number of parts is reduced.

Further, if the frequency limiting circuit is composed of the diodes 52 and 82, the delay circuits 54 and 84, and the ON control circuit 59, there is an advantage that the circuit can be simplified.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention, FIG. 2 is a signal waveform diagram of an essential part of FIG. 1, FIG. 3 is a circuit diagram showing a second embodiment, and FIG. FIG. 5 is a waveform diagram showing a signal waveform of a main part of the figure, FIG. 5 is a waveform diagram showing a current flowing through a secondary winding of the present invention, and FIG. 6 is a waveform diagram showing a current flowing through a primary winding of the present invention.
FIG. 8 is a circuit diagram showing a modified example of the first embodiment, FIG. 8 is a circuit diagram showing a modified example of the second embodiment, and FIG. 9 is a characteristic diagram showing the relationship between the load current and the oscillation frequency of the present invention, FIG. 10 is a characteristic diagram showing a relationship between a conventional load current and an oscillation frequency. 11 …… Transformer, 12 …… Primary winding, 13 …… Switching element, 15 …… DC power supply, 18 …… Secondary winding, 21 …… Output circuit, 37 …… Control winding, 38 …… Control signal extraction circuit, 51
...... Detection winding, 52,82 …… Diode, 54,84 …… Delay circuit, 57 …… On control signal, 59 …… On control circuit, 60 ……
Frequency limit circuit, Toff ... off period, Td ... predetermined time.

Claims (3)

[Claims] 1. A switching element, a transformer having a primary winding connected to a DC power source via the switching element, and an output circuit for extracting a DC output from the secondary winding during the OFF period of the switching element. In the limited switching regulator, a detection winding coupled to the transformer to detect the off period of the switching element, and a start time of the off period when the off period is longer than a predetermined time within the detected off period. And a frequency limiting circuit for shortening the off period by outputting an on control signal for turning on the switching element after a delay of the predetermined time. 2. A control winding coupled to the transformer according to claim 1, and a control signal extraction circuit for extracting a control signal input from the control winding to a control electrode of the switching element, The detection winding is a limited switching regulator that is also used as the control winding. 3. The frequency limiting circuit according to claim 1, wherein the frequency limiting circuit is activated by the conduction of the diode and a diode that conducts during the OFF period of the switching element, and the delay signal is delayed at the timing when the conduction starts. A limiting switching regulator including a delay circuit for outputting the ON signal and an ON control circuit for receiving the delayed signal and outputting the ON control signal.