JP2002354795A - Switching power circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power circuit consuming the excessive voltage fed back in a lightly-loaded condition. SOLUTION: The switching power circuit comprises a transformer 53 making a load voltage on a secondary winding by turning a power MOSFET30 connected to a primary winding into ON/OFF, a photo coupler 62 detecting the load voltage generated on the secondary winding, and a pulse-width modulation circuit 41 generating a signal turning the power MOSFET30 to OFF based on the feedback voltage from the photo coupler 62. The power MOSFET30 allows an excessive feedback current in OFF to be consumed through the diode 48.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply circuit for detecting a secondary voltage of a transformer with a photocoupler and controlling the primary voltage.

[0002]

2. Description of the Related Art Recently, a switching power supply circuit is used in an electronic device such as a television or an audio device.

FIG. 4 is a circuit diagram of a conventional switching power supply circuit. To the full-wave rectifier circuit 1, a commercial power source Vin at terminals X and Y is applied via a choke coil. The transformer 3 has a primary winding 4 and a secondary winding 5 for extracting an output voltage.
And a bias winding 6. One end of the primary winding 4 is connected to a terminal S of the full-wave rectifier circuit 1, and the other end is connected to a terminal T of the full-wave rectifier circuit 1 via a drain / source electrode of a power MOSFET 7.

[0004] The smoothing circuit 8 smoothes the DC voltage rectified by the full-wave rectifier circuit 1. The rectified and smoothed DC voltage is applied to a terminal via a starting resistor 9 to become a power supply voltage Vcc of the integrated circuit C. One end of the bias winding 6 is connected to the terminal via a diode 10.

[0005] A diode 11 and a capacitor 12 are connected to the secondary winding 5 of the transformer 3. The photocoupler 14 includes a light emitting diode 15 and a phototransistor 16.
And one end of the phototransistor 16 is connected to a terminal of the integrated circuit C.

The transistor 17 detects a change in the load voltage taken out from the terminals M and N. The base 17 is connected to the dividing resistors 18 and 19, and the emitter is connected to the zener diode 20.

Next, the operation will be described. The commercial power applied between the terminals X and Y is rectified by the full-wave rectifier circuit 1, smoothed by the smoothing circuit 8, and then integrated by the integrated circuit C via the starting resistor 9.
Is applied as a power supply voltage Vcc.

When the power supply voltage Vcc is applied to the terminal, a high-level signal is generated from the integrated circuit C, and the power M
Applied to the gate of OSFET7.

When a high level signal from the integrated circuit C is applied to the gates of the power MOSFETs 7, these power MOSFETs 7 are turned on. Then power MOS
The rectified and smoothed DC voltage is applied to the primary winding 4 of the transformer via the drain / source electrode of the FET 7.

A predetermined secondary voltage is obtained in the secondary winding 5 of the transformer according to the DC voltage applied to the primary winding 4 of the transformer 3. The obtained secondary voltage is rectified and smoothed by the diode 11 and the capacitor 12, taken out from the terminals M and N, and supplied to the load as a load voltage.

When the load voltage is equal to or lower than a predetermined voltage and the drain current value of the power MOSFET 7 is equal to or lower than a reference value, the high level signal from the integrated circuit C remains unchanged and the power M
OSFET7 keeps ON. The change in the load voltage taken out from the terminals M and N is detected by the dividing resistors 18 and 19, and changes the base voltage of the transistor 17.
That is, when the load voltage rises, the base voltage of the transistor 17 rises, and the light emitting diode 1
5 and the light emission also increases. As a result, the resistance value of the phototransistor 16 decreases, and the voltage applied to the terminal of the integrated circuit C increases.

When the voltage applied to the terminal of the integrated circuit C increases, the signal applied to the gate of the power MOSFET 7 goes low, and the power MOSFET 7
Since the FF is performed, the current from the commercial power supply does not flow through the primary winding of the transformer in the device.

When no current flows through the primary winding 4 of the transformer 3 and the load voltage drops, a high-level signal is again applied to the gate of the power MOSFET 7 and the power MOSFET is turned on.
Turn on T7.

Therefore, a rectified current flows from the commercial power supply to the primary winding of the transformer in the device via the drain electrode and the source electrode of the power MOSFET 7. According to the current flowing through the primary winding 4 of the transformer 3, the transformer 3
A current flows through the secondary winding 5 of the above, and a predetermined load voltage is obtained.

[0015]

As described above, the power MOSFET connected to the primary side of the transformer is turned ON / OFF.
It turns off and a predetermined secondary voltage is obtained. The secondary voltage is detected by a photocoupler and returned to the primary side, and the power M
The OFF start time of the OS transistor is controlled, and when the feedback voltage drops, the power MOS transistor is turned ON again.

As described above, the power MOSFET is
A predetermined secondary voltage is obtained by N / OFF, but in the case of a light load, the secondary voltage rises, and the current fed back from the photocoupler increases. When the current fed back from the photocoupler increases and the feedback voltage rises, the voltage on the secondary side decreases and the current from the photocoupler decreases, so that the feedback voltage cannot fully decrease, and the power MOSFET normally operates. O
FF control becomes impossible, and abnormal oscillation occurs, and a predetermined secondary voltage cannot be obtained.

[0017]

According to the present invention, there is provided a transformer for applying a voltage obtained by rectifying and smoothing a commercial power supply, turning on / off a power MOSFET connected to a primary winding, and extracting a load voltage to a secondary winding. A photocoupler for detecting a load voltage generated in the secondary winding, a signal for turning on the power MOSFET with a signal from an oscillation circuit, and a power MOSFET based on a voltage fed back by the photocoupler. A pulse width modulation circuit that generates a signal to turn off, a diode is connected to a signal path for extracting a voltage fed back from the photocoupler, and an excessive current fed back when the power MOSFET is turned off passes through the diode. A switching power supply circuit to be consumed is provided.

According to the present invention, there is provided a transformer having a primary winding to which a voltage obtained by rectifying and smoothing a commercial power supply is applied and to which a drain electrode and a source electrode of a power MOSFET are connected, and a secondary winding for extracting an output voltage. A light emitting diode that emits light in accordance with a load voltage generated in the secondary winding, a photocoupler including a phototransistor that receives light from the light emitting diode and changes impedance, and a power MOSFET based on an oscillation signal from an oscillation circuit. And a pulse width modulation circuit for generating a signal for turning off the power MOSFET based on the voltage fed back by the photocoupler, and a signal path for extracting the voltage fed back from the photocoupler. A diode based on an oscillation signal from the oscillation circuit. The power MOSFET is turned on by a signal generated from the modulation circuit, a current flows through the primary winding of the transformer, and when the load voltage generated on the secondary winding exceeds a predetermined voltage, feedback is performed via a phototransistor. A switching power supply circuit for turning off a power MOSFET with a signal generated from a pulse width modulation circuit based on the applied voltage and consuming excessive current fed back to a phototransistor through the diode when the power MOSFET is turned off. I do.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to FIGS.

FIG. 1 is a block diagram of an integrated circuit portion used in the switching power supply circuit of the present invention. The broken line frame indicates the integrated circuit C alone, and the Vcc terminal, TRG terminal, FB terminal, S
It has an ENSE terminal, a GATE terminal and a GND terminal.
The integrated circuit C is incorporated in a package P. The package P has terminals, terminals, terminals, terminals and terminals.

The Vcc terminal is connected to the terminal, the TRG terminal is connected to the terminal, and the FB terminal is connected to the terminal. A source electrode of the power MOSFET 30 is connected between the terminals.
The drain electrode is connected. Power MOSFE
The drain electrode of T30 is connected to the drain electrode of sensor MOSFET31.

The oscillating circuit 32 oscillates a sawtooth signal, and the oscillated sawtooth signal is converted into a pulse signal by an oscillation edge circuit 33 and applied to a latch circuit 34. The reference voltage generation circuit 35 generates a reference voltage Vref from the power supply voltage Vcc applied to the terminal. The low voltage detection circuit 36 compares the power supply voltage Vcc with the reference voltage Vref and a predetermined voltage, generates a stop signal when the power supply voltage Vcc is lower than the predetermined voltage, and generates a stop release signal when the power supply voltage Vcc becomes higher than the predetermined voltage. And the oscillation circuit 32 oscillates.

On the contrary, the high voltage detecting circuit 37
When cc is higher than the specified voltage, the stop signal is detected and applied to the latch circuit 34 for latching. The abnormal overheat detection circuit 38 detects an abnormal temperature rise of the chip. When the temperature of the chip rises to an abnormal temperature, a stop signal is applied to the latch circuit 34 and latched.

The oscillation level comparison circuit 39 includes a voltage supplied according to the reference voltage Vref from the reference voltage generation circuit 35 and a load voltage from the terminal, and a sensor MOSFET.
Comparing the sum voltage with the voltage applied via 31, the signal generated is low when the reference voltage is high. However, when the voltage supplied according to the load voltage or the like becomes higher than the reference voltage, the oscillation level comparison circuit 39 generates a high level signal.

The pulse width modulation circuit 41 comprises an RS flip-flop.
And the signal from the oscillation level comparison circuit 39 are applied via an inverter circuit 44. Also RS
A signal from the oscillation level comparison circuit 39 is applied to the ET terminal R. The signal from the Q bar terminal of the pulse width modulation circuit 41 is supplied to the power MOSFET through a driver 46.
T30 and the gate of the sensor MOSFET 31.

A diode 48 having a gate signal line as a cathode is connected between the terminal of the package P to which the FB terminal is connected and the gate signal line of the power MOSFET 30.

FIG. 2 is a circuit diagram of a switching power supply circuit using the integrated circuit C. The commercial power supply Vi of the terminal X and the terminal Y is connected to the full-wave rectifier circuit 51 via the choke coil 52.
n is added. The transformer 53 includes a primary winding 54, a secondary winding 55 for extracting an output voltage, and a bias winding 56. One end of the primary winding 54 is connected to a terminal S of the full-wave rectifier circuit 51, the other end is connected to a terminal of the package P, and a terminal of the full-wave rectifier circuit 51 is connected to a drain / source electrode of the MOSFET 30. Connected to T.

The smoothing circuit 59 smoothes the DC voltage rectified by the full-wave rectifier circuit 51. The rectified and smoothed DC voltage is applied to the terminal of the package P via the starting resistor 57 and becomes the power supply voltage Vcc of the integrated circuit. One end of the bias winding 56 is connected to the terminal via a diode 58.

A diode 60 and a capacitor 61 are connected to the secondary winding 55 of the transformer 53. The photocoupler 62 includes a light emitting diode 63 and a phototransistor 64, and one end of the phototransistor 64 is connected to a resistor 66 and a resistor 67. A diode 48 having a terminal side as a cathode is connected between a connection point 68 of the resistor 66 and the resistor 67 and a terminal of the package P.

The transistor 70 detects a change in the load voltage taken out from the terminals M and N. The base is connected to the split resistors 71 and 72, and the emitter is connected to the zener diode 73.

Next, the operation will be described. The commercial power applied between the terminals X and Y is now rectified by the full-wave rectifier circuit 51,
After being smoothed by the smoothing circuit 59, it is applied as a power supply voltage Vcc to the terminal of the package P via the starting resistor 57.

When a power supply voltage Vcc is applied to the terminal, a reference voltage Vref is generated by a reference voltage generating circuit 35. When the power supply voltage Vcc becomes equal to or higher than the set voltage, a stop release signal from the low voltage detection circuit 36 is applied to the oscillation circuit 32. At this time, the signal from the edge circuit 47 is also applied to the oscillation circuit 32 to start oscillating and generate a sawtooth signal.

The sawtooth signal is applied to an oscillation edge circuit 33, converted into a pulse signal, and applied to a latch circuit 34. At this time, since the stop signal is not detected from the high voltage detection circuit 37, the pulse signal is supplied to the SE of the pulse width modulation circuit 41 via the latch circuit 34 and the inverter circuit 44.
Applies to the T terminal S.

As shown in FIG. 3, the pulse width modulation circuit 41
When a pulse signal is applied to the SET terminal S, the signal at the Q bar terminal of the pulse width modulation circuit 41 becomes low level.
At this time, since the stop signal from the low voltage detection circuit 36 is applied to the driver 46, the driver circuit 4
6 is in operation. The pulse width modulation circuit 41
The low-level signal at the Q bar terminal is inverted to a high-level signal by the driver circuit 46, and is applied to the respective gates of the power MOSFET 30 and the sensor MOSFET 31.

When a high-level signal inverted by the driver circuit 46 is applied to the gates of the power MOSFET 30 and the sensor MOSFET 31, the power M
The OSFET 30 and the sensor MOSFET 31 are turned on. Then, the drain electrode of the power MOSFET 30
The rectified and smoothed DC voltage is applied to the primary winding of the transformer via the source electrode.

The secondary winding 55 of the transformer 53 according to the DC voltage applied to the primary winding 54 of the transformer 53
, A predetermined secondary voltage is obtained. The obtained secondary voltage is rectified and smoothed by the diode 60 and the capacitor 61, taken out from the terminals M and N, and supplied to the load as a load voltage.

When the load voltage is equal to or lower than the predetermined voltage and the drain current value of the power MOSFET 30 is equal to or lower than the reference value, the pulse signal oscillated by the oscillating circuit 32 and converted by the oscillating edge circuit 33 is used as described above. 4
1, the signal at the Q bar terminal of the pulse width modulation circuit 41 remains at the low level and the power M
OSFET 30 keeps on.

The change in the load voltage taken out from the terminals M and N is detected by the split resistors 71 and 72, and changes the base voltage of the transistor 70. That is, when the load voltage increases, the base voltage of the transistor 70 increases, the current flowing through the light emitting diode 63 via the transistor 70 increases, and the light emission also increases. As a result, the resistance value of the phototransistor 64 decreases, and the feedback voltage applied to the terminal increases.

A feedback voltage corresponding to the load voltage applied to the oscillation level comparison circuit 39 and a sensor MOS
When the sum voltage with the voltage applied via the FET 31 becomes slightly higher than the reference voltage Vref from the reference voltage generation circuit 35, a high-level signal is generated from the oscillation level comparison circuit 39, and the pulse width modulation circuit 41 RSE
A reset signal is applied to the T terminal R to make the signal at the Q bar terminal of the pulse width modulation circuit 41 high.

Therefore, since the signal applied to the gate of the power MOSFET 30 via the driver circuit 46 becomes low level, the power MOSFET 30 is turned off, so that the current from the commercial power supply flows to the primary winding 54 of the transformer 53 in the device. Not flowing.

When no current flows through the primary winding 54 of the transformer 53 and the load voltage decreases, the voltage applied to the oscillation level comparison circuit 39 becomes smaller than the set voltage Vref from the reference voltage generation circuit 35 again. Since the signal generated from the oscillation level comparison circuit 39 becomes low level, no reset signal is applied to the RSET terminal of the pulse width modulation circuit 41.

On the other hand, since the oscillation circuit 32 continues to oscillate, the pulse width modulation circuit 4
The pulse signal is applied to the SET terminal 1 and the signal at the Q bar terminal of the pulse width modulation circuit 41 goes low again. The low level signal at the Q bar terminal of the pulse width modulation circuit 41 is a power MOSFET 30 and a sensor MOSFET.
In addition to the gates of the ET31, these power M
The OSFET 30 and the sensor MOSFET 31 are turned on.

Therefore, a rectified current flows from the commercial power supply to the primary winding 54 of the transformer 53 in the device via the drain electrode and the source electrode of the power MOSFET 30. A current flows through the secondary winding 55 of the transformer 53 according to the current flowing through the primary winding 54 of the transformer 53, and a predetermined load voltage is obtained.

As described above, the load voltage rises with the rise of the secondary voltage, the base voltage of the transistor 70 rises, the current flowing through the light emitting diode 63 via the transistor 70 increases, and the light emission also increases. As a result, the resistance value of the phototransistor 64 decreases, and the feedback voltage applied to the terminal increases. By the way, when a light load is connected to the terminals M and N, the feedback voltage detected by the resistor 67 via the phototransistor 64 becomes too large.

Then, the voltage applied to the oscillation level comparison circuit 39 from the terminal becomes higher than the reference voltage Vref from the reference voltage generation circuit 35, and the oscillation level comparison circuit 39 generates a high-level signal. The voltage detected by the resistor 67 via the phototransistor 64 remains higher than the reference voltage Vref even after the modulation circuit 41 is reset, the signal at the Q-bar terminal is set to the high level, and the power MOSFET 30 is turned off.

Therefore, the current from the commercial power supply does not flow through the primary winding 54 of the transformer 53, and the oscillation level comparison circuit 39 continues to generate a high-level signal despite the decrease in the secondary voltage.

Therefore, the pulse width modulation circuit 41 is set by the pulse signal from the oscillation edge circuit 33, the signal at the Q bar terminal is set to low level, and the power MOSFET 30 is turned off.
Even if N, the pulse width modulation circuit 41 is reset by the high level signal from the oscillation level comparison circuit 39, the signal at the Q bar terminal becomes high level, and the power MOSFET 30
Is turned off, so that the secondary side voltage cannot be controlled and drops too much.

According to the present invention, a diode 48 having a gate signal line as a cathode is connected between the terminal of the package P and the gate signal line of the power MOSFET 30, and an excessive feedback generated in the resistor 67 is generated only when the power MOSFET 30 is OFF. The current is consumed through the diode 48 so that the feedback voltage decreases. Therefore, when the secondary voltage becomes lower than the reference voltage, the power MOSFET 30 is turned off again as described above.
N, and increase the secondary side voltage.

A capacitor 74 is connected to the terminal of the package P to which the anode of the diode 48 is connected, and the capacitance of the capacitor 74 is changed to reduce the detection voltage which is consumed through the diode 48. The speed at which it is done can be changed.

[0050]

According to the switching power supply circuit of the present invention, a diode is connected to a signal path for extracting a voltage fed back from a photocoupler, and the power MOSFET is turned off.
Since excessive current fed back at the time of F is consumed via the diode, if the secondary voltage becomes lower than the reference voltage even when a light load is connected, the power MOSFET is again turned on.
ET can be turned on to increase the secondary voltage.

[Brief description of the drawings]

FIG. 1 is a block diagram of an integrated circuit used in a switching power supply circuit of the present invention.

FIG. 2 shows a feedback voltage according to the present invention. Therefore, it is a circuit diagram in which the secondary voltage is equal to or lower than the reference voltage.

FIG. 3 is a waveform diagram of each part of a pulse width modulation circuit used in the switching power supply circuit of the present invention.

FIG. 4 is a circuit diagram of a conventional switching power supply circuit.

[Explanation of symbols]

 REFERENCE SIGNS LIST 30 power MOSFET 32 oscillation circuit 33 oscillation edge circuit 34 latch circuit 35 reference voltage generation circuit 39 oscillation level comparison circuit 41 pulse width modulation circuit 44 inverter circuit 48 diode 51 full-wave rectification circuit 53 transformer 54 primary winding 55 secondary winding Wire 56 bias winding 62 photocoupler 63 light emitting diode 64 phototransistor

Claims (3)

[Claims] 1. A voltage obtained by rectifying and smoothing a commercial power supply is applied, and a power MOSFET connected to a primary winding is turned on.
And a photocoupler for detecting a load voltage generated in the secondary winding, and a signal for turning on the power MOSFET by a signal from an oscillation circuit. A pulse width modulation circuit for generating a signal for turning off the power MOSFET based on a voltage fed back by the photocoupler, connecting a diode to a signal path for extracting a voltage fed back from the photocoupler,
A switching power supply circuit characterized in that excessive current fed back when ET is turned off is consumed via the diode. 2. A transformer having a primary winding to which a voltage obtained by rectifying and smoothing a commercial power supply is applied and to which a drain electrode and a source electrode of a power MOSFET are connected, and a secondary winding for extracting an output voltage; A photocoupler including a light emitting diode that emits light in accordance with a load voltage generated in the next winding, a phototransistor that receives light from the light emitting diode and changes impedance, and a power MOSFET based on an oscillation signal from an oscillation circuit.
Is turned on, and the power MOSFET is turned on based on the voltage fed back by the photocoupler.
A pulse width modulation circuit for generating a signal to be flipped, and a diode connected to a signal path for extracting a voltage fed back from the photocoupler, generated by the pulse modulation circuit based on an oscillation signal from the oscillation circuit Turn on the power MOSFET with the signal,
A signal generated by a pulse width modulation circuit based on the voltage fed back via a phototransistor when a current flows through the primary winding of a transformer and the load voltage generated on the secondary winding exceeds a predetermined voltage. Power MOSFE
A switching power supply circuit, wherein T is turned off, and an excessive current fed back to a phototransistor when the power MOSFET is turned off is consumed via the diode. 3. The switching power supply circuit according to claim 1, wherein the pulse width modulation circuit is an RS-FF.