JPH04325867A - Switching power source - Google Patents

Abstract

PURPOSE:To make it possible to operate a current limiting function when a short-circuiting state is continued at a load side by providing two systems for supplying power to a control circuit such as an FET, etc., at the times of rising of the power and a steady operation, and cutting OFF the system for supplying the power to the circuit at the time of rising of the power when shifting to the steady operation. CONSTITUTION:An electromotive force generated in a secondary winding 16 is rectified and smoothed by a second power source supply circuit (PS) B, and charged in a capacitor (CS) 43. When the charging voltage of the CS 43 becomes higher than a voltage to be supplied from a first PSB to a control circuit 15, a voltage is supplied from the second PSB to the circuit 15. A resistance-type voltage divider E divides a voltage VE to a voltage VE1, and inputs it to an operational amplifier 55 of an output control circuit F. The amplifier 55 turns OFF transistors(Tr) 38, 39 when the voltages become VE1>VC. A current limiter D detects an overcurrent state based on a reference voltage of a constant- voltage element 45, and so feeds back a control signal to the circuit 15 as to shorten an ON time of an FET 13.

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, and more particularly to an improvement in overcurrent prevention function.

0002

2. Description of the Related Art FIG. 3 is a circuit diagram showing an example of a conventional switching power supply, and shows an example of a forward converter system. In the figure, reference numeral 1 denotes an AC power supply, which is connected to input terminals a and b of a rectifier diode bridge 5 via a fuse 2, a switch 3, and an input line filter 4. One output terminal c of the rectifying diode bridge 5 is connected to a first common potential point on the primary side of the transformer 9. The other output terminal of the rectifier diode bridge 5 is connected to one end of a smoothing capacitor 6, one end of resistors 7 and 8, and one end of a primary winding 10 of a transformer 9.

The other end of the smoothing capacitor 6 is connected to the first common potential point, the other end of the resistor 7 is connected to the base of the transistor 11 and the cathode of the Zener diode 12, and the other end of the resistor 7 is connected to the base of the transistor 11 and the cathode of the Zener diode 12. The other end is connected to the collector of the transistor 11, and the other end of the primary winding 10 is connected to the switching FET 1.
3 to the first common potential point.

The anode of the Zener diode 12 is the first
connected to a common potential point. The emitter of the transistor 11 is connected to the anode of a diode 14, and the cathode of the diode 14 is connected to a control circuit 15 connected to the gate of the FET 13. These resistors 7 and 8, the transistor 11, and the diode 14 constitute a first power supply circuit A that supplies power to the control circuit 15.

The transformer 9 has two secondary windings 16 and 17.
is provided. One end of the secondary winding 16 is a diode 1
The other end is connected to the first common potential point. The cathode of the diode 18 is connected to the cathode of the diode 14 and also to the first common potential point via a capacitor 19. These secondary winding 16, diode 18, and capacitor 19 constitute a second power supply circuit B that supplies power to the control circuit 15.

One end of the secondary winding 17 is connected to the anode of the diode 20, and the other end is connected to a second common potential point on the secondary side of the transformer 9. Diode 20 catho
The terminal is connected to one end of the choke coil 21 and the cathode of the diode 22. The other end of the choke coil 21 is connected to one end of a capacitor 23. The anode of the diode 22 is connected to the second common potential point and also connected to one end of the current detection resistor 24.
The other end of 4 is connected to the other end of capacitor 23. These diodes 20, 22, choke coil 21, and capacitor 23 constitute a rectifying and smoothing circuit C.

Reference numeral 25 denotes an operational amplifier constituting the current limiting circuit D. The inverting input terminal of the operational amplifier 25 is connected via a resistor 26 to the connection point between the anode of the diode 22 and the current detection resistor 24, and is also connected via a resistor 27 to the positive electrode of a DC power supply 28. . A negative electrode of the DC power supply 28 is connected to a second common potential point. A non-inverting input terminal of the operational amplifier 25 is connected to a connection point between the capacitor 23 and the current detection resistor 24 via a resistor 29. The output terminal of the operational amplifier 25 is connected via a resistor 30 to the anode of a light emitting diode 32 constituting a photocoupler 31. The collector and emitter of the phototransistor 33 constituting the photocoupler 31 are connected to the control circuit 1.
5.

Reference numeral 34 denotes a voltage detection section that detects the voltage between both ends of the capacitor 23, the input terminal of which is connected to both ends of the capacitor 23, and the output terminal connected to the anode of a light emitting diode 36 constituting a photocoupler 35. It is connected to the. The collector and emitter of the phototransistor 37 constituting the photocoupler 35 are connected to the control circuit 15.

The operation of the power supply configured as described above will be explained. By turning on the switch 3, the capacitor 6 is charged, and the voltage across the capacitor 6 increases. As the voltage across the capacitor 6 increases, a voltage set by the Zener voltage of the Zener diode 12 is supplied from the first power supply circuit A to the control circuit 15 as a power source. The control circuit 15 starts oscillation when power is supplied, and turns the FET 13 on and off. FET
13 turns on, current flows through the primary winding 10 and is discharged to the secondary windings 16 and 17 as a secondary current.

The current discharged to the secondary winding 16 is rectified and smoothed by a rectifying and smoothing circuit configured as a second power supply circuit B, and is supplied to the control circuit 15 as a power source. and,
When the FET 13 turns on and off due to the operation of the control circuit 15 and enters a steady state, the output voltage of the second power supply circuit B becomes higher than the output voltage of the first power supply circuit A, and the control circuit 15 It is driven by the output voltage of the power supply circuit B.

On the other hand, the current discharged to the secondary winding 17 is rectified and smoothed by a rectification and smoothing circuit C, and output as a DC voltage. This DC voltage is fed back to the control circuit 15 via the voltage detection section 34 and the photocoupler 35. Control circuit 15
is based on the voltage fed back via the photocoupler 35, when the feedback voltage is higher than the set value, the on time of FET 13 is shortened, and when the feedback voltage is lower than the set value, the on time of FET 13 is shortened. FET13 so that
Controls the on/off time width. Further, the output current is detected by the current detection resistor 24 and fed back to the control circuit 15 via the current limiting circuit D and the photocoupler 31. Then, the control circuit 15 controls the FET1 when an overcurrent flows.
The on/off time width of FET 13 is controlled so that the on time of FET 13 is shortened.

0012

However, according to such a conventional configuration, the first power supply circuit A is always in a state where it can supply power to the control circuit 15 even in a steady state, so that the load The current limiting circuit D operates to shorten the ON time of the FET 13 when the side becomes short-circuited or overloaded, and the output voltage of the second power supply circuit B becomes the same as that of the first power supply. When the output voltage becomes lower than the output voltage of the circuit A, the control circuit 15 is driven by the output voltage of the first power supply circuit A again. As a result, the rectified and smoothed output voltage of the secondary winding 17 system does not drop to 0V, but continues to flow stably at several volts, and continues to put stress on each circuit element. may cause destruction of circuit elements. An object of the present invention is to solve such conventional problems, and to realize a highly reliable switching power supply that can completely reduce the output voltage to 0 V when an overcurrent flows.

[0013]

[Means for Solving the Problems] The present invention provides a primary winding to which switching elements are connected in series and to which an input voltage is applied;
A transformer has two secondary windings each connected to a rectifying and smoothing circuit, a control circuit that turns on and off the switching element, and detects and presets the current flowing through the first secondary winding. a first power supply circuit connected to the primary winding and having an output terminal connected to a power supply terminal of the control circuit; and a second secondary winding, the output terminal being connected to the power supply terminal of the control circuit together with the output terminal of the first power supply circuit, and an output voltage higher than the output voltage of the first power supply circuit. and an output control circuit that prohibits sending out the output voltage of the first power supply circuit after the control circuit is driven by the second power supply circuit. It is characterized by

[0014]

[Operation] When starting up the device, the control circuit is driven by the first power supply circuit. After startup, it is driven by the second power supply circuit, and transmission of the output voltage from the first power supply circuit is prohibited. On the other hand, when an overcurrent flows in a steady state of operation, the current limiting circuit controls the control circuit to limit the output current to a preset value, thereby reducing the output voltage of the second power supply circuit. Then, when the output voltage of the second power supply circuit becomes lower than the operable voltage of the control circuit, the on/off operation of the switching element is stopped, and the output voltage of the device becomes completely 0V.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG.
The same parts are given the same symbols. In Figure 1,
Transistors 38 and 3 forming the first power supply circuit A
9 is connected to Darlington. That is, the base of the transistor 38 is connected to the connection point between the resistor 7 and the cathode of the Zener diode 12, the collector of the transistor 38 is connected to the connection point between the collector of the transistor 39 and the resistor 8, and the transistor 38 The emitter of transistor 39 is connected to the base of transistor 39, and the emitter of transistor 39 is connected to the anode of diode 14.

The second power supply circuit B includes a diode 40
, 42, a choke coil 41, and a capacitor 43 constitute a rectifying and smoothing circuit. That is, the anode of the diode 40 is connected to one end of the secondary winding 16, and the cathode of the diode 40 is connected to one end of the choke coil 41 and the cathode of the diode 42. There is. The other end of the choke coil 41 is connected to one end of a capacitor 43, and also to the cathode of a diode 14 constituting the first power supply circuit A and the power terminal of the control circuit 15. The other end of capacitor 43 is diode 4
2 and the other end of the secondary winding 16.

The current limiting circuit D is composed of an operational amplifier 44, a constant voltage element 45, resistors 46 to 51, and a capacitor 52. The anode of the constant voltage element 45 is connected to the second common potential point, the connection point between the diode 22 and the current detection resistor 24, and the negative power terminal of the operational amplifier 44, and the cathode of the constant voltage element 45 is The connection point between the choke coil 21 and the capacitor 23 and the operational amplifier 44 are connected via a resistor 46.
It is connected to the positive power supply terminal of , and also connected to the inverting input terminal of operational amplifier 44 via resistor 47 . A non-inverting input terminal of the operational amplifier 44 is connected to a connection point between the capacitor 23 and the current detection resistor 24 via a resistor 48.
The output terminal of the operational amplifier 44 is connected via a resistor 51 to the anode of a light emitting diode 32 constituting the photocoupler 31. Further, a resistor 50 and a capacitor 52 are connected in parallel between the output terminal and the inverting input terminal of the operational amplifier 44.

E is a resistive voltage divider circuit composed of a series circuit of resistors 53 and 54. One end of the resistor 53 is connected to the output terminal d of the diode bridge 5, and the other end is connected to the first terminal through the resistor 54. connected to a common potential point.

F indicates that the control circuit 15 is connected to the second power supply circuit B.
This is an output control circuit that prohibits sending out the output voltage of the first power supply circuit A after being driven by the first power supply circuit A. This output control circuit F
is an operational amplifier 55, a DC power supply 56, a resistor 57,
It is composed of transistors 58 and 59. The inverting input terminal of the operational amplifier 55 is connected to the positive electrode of a DC power supply 56 that outputs the reference voltage VC, and the non-inverting input terminal is connected to the midpoint between the resistors 53 and 54. A negative electrode of the DC power supply 56 is connected to the first common potential point. The first output terminal of the operational amplifier 55 is connected to the transistor 5
The second output terminal is connected to the base of transistor 59. The collector of the transistor 58 is connected to the non-inverting input terminal of the operational amplifier 55 via the resistor 57, and the emitter of the transistor 58 is connected to the cathode of the diode 14 constituting the first power supply circuit A and the second power supply. It is connected to the connection point between the choke coil 41 and the capacitor 43 that constitute the supply circuit B and the connection point of the power supply terminal of the control circuit 15. The collector of the transistor 59 is connected to the connection point between the resistor 7 constituting the first power supply circuit A, the base of the transistor 38, and the Zener diode 12, and the emitter of the transistor 59 is connected to the first common potential point. It is connected.

The operation of FIG. 1 can be explained as follows: (A) rising state, (
B) Steady state and (C) Overcurrent state will be explained respectively.

(A) Rising state When the switch 3 is turned on, the capacitor 6 is charged by the rectified output of the diode bridge 5, and the terminal voltage VE of the capacitor 6 gradually rises. Then, current flows through the resistor 7, turning on the transistors 38 and 39. When transistors 38 and 39 are turned on, current flows through resistor 8, transistor 39, and diode 14, supplying power to control circuit 15. When power is supplied to the control circuit 15, oscillation starts internally, and the FET 13 starts to be turned on and off. By turning the FET 13 on and off, current flows through the primary winding 10 of the transformer 9, and the secondary winding 1
An electromotive force is generated at 6 and 17.

The electromotive force generated in the secondary winding 16 is rectified and smoothed by the second power supply circuit B, and charges the capacitor 43. When the charging voltage of the capacitor 43 becomes higher than the voltage supplied from the first power supply circuit A to the control circuit 15 via the diode 14, power is supplied to the control circuit 15 from the second power supply circuit B. will be done.

The resistive voltage dividing circuit E divides the voltage VE charged in the capacitor 6 into VE1, and inputs the divided voltage to the non-inverting input terminal of the operational amplifier 55 constituting the output control circuit F. The output voltage VC of the DC power supply 56 is applied to the inverting input terminal of the operational amplifier 55 constituting the output control circuit F, and the operational amplifier 55 constitutes the first power supply circuit A when VE1>VC. Turn off transistors 38 and 39. Note that the resistor 57 and the transistor 58 provide hysteresis characteristics to the rise of the output signal of the operational amplifier 55 when the switch 3 is turned on and the fall of the output signal of the operational amplifier 55 when the switch 3 is turned off. be. Here, the output voltage VC of the DC power supply 56 is set so that VE1>VC when the output voltage of the second power supply circuit B becomes higher than the output voltage of the first power supply circuit A. As a result, at the time when power is supplied from the second power supply circuit B to the control circuit 15, the first
The transistors 38 and 39 of the power supply circuit A are turned off.

On the other hand, the electromotive force generated in the secondary winding 17 is rectified and smoothed by a rectification and smoothing circuit C. The current detection resistor 24 is connected to the negative line of the rectifying and smoothing circuit C. The voltage across this current detection resistor 24 is detected by a current limiting circuit D. The current limiting circuit D detects an overcurrent state based on the reference voltage set by the constant voltage element 45, and when an overcurrent state occurs, the control circuit 15 controls the FET 13 via the photocoupler 31 to shorten the ON time. Feedback the signal.

(B) Steady state In steady state, input voltage is AC100V and VE is 1
The output voltage is 30V, the output voltage is DC24V, and the duty is 35V.
%. FIG. 2 shows the waveform of the electromotive force VS1 of the secondary winding 17. Generally, the output voltage VOUT is VOUT=VS
1 (ton/T). Therefore, when VOUT=24V, the electromotive force VS1 of the secondary winding 17 is VS1=VOU
From T (T/ton)=24/0.35, it becomes 68.6V. Similarly, from the second power supply circuit B to the control circuit 15
Assuming that DC12V is supplied to the secondary winding 1
The electromotive force VS2 of 6 is VS2=VOUT(T/ton)
=12/0.35 becomes 34.3V. That is,
In a steady state, the secondary windings 16 and 17 operate to generate the electromotive forces VS2 and VS1, respectively.

(C) Overcurrent state The operational amplifier 44 constituting the current limiting circuit D operates in response to the voltage generated by the overcurrent flowing through the current detection resistor 24, causing current to flow through the light emitting diode 32 of the photocoupler 31. By applying feedback to the control circuit 15, F
Shorten the on time of ET13.

Here, if there is no change in the charging voltage VE of the capacitor 6, the electromotive force generated in the secondary windings 16 and 17 is constant, but the ON time of the FET 13 is shortened and the duty cycle is reduced.
Since the output voltage VOUT becomes smaller, the output voltage VOUT decreases. Since the output voltage VOUT is supplied as a power source to the operational amplifier 44, as the output voltage VOUT decreases, the output voltage of the operational amplifier 44 also decreases, and eventually the current cannot flow through the light emitting diode 32 and the output voltage decreases. V.O.
As mentioned above, UT becomes stable at a few volts. For example, if the duty is 5%, the output voltage VOUT is VOUT
=VS1(ton/T)=68.6×0.05 to 3.
It becomes 43V. At this time, the power supply voltage supplied from the second power supply circuit B to the control circuit 15 is VS2×0.05
=34.3×0.05 becomes 1.72V. Normally, the lowest power supply voltage at which the control circuit 15 can operate is about 7V, and when the power supply voltage drops below that, oscillation stops.

Therefore, when the operational amplifier 44 operates in an overcurrent state and the duty becomes small, the second power supply circuit B
The power supply voltage supplied to the control circuit 15 from
The oscillation that drives T13 on and off naturally stops, and the output voltage VOUT becomes zero.

[0029] In the above embodiment, the forward converter
Although we have explained an example of the converter method, the reverse converter (
flyback) method, center tap (push-pull)
The present invention can be similarly applied to switching power supplies such as a half-bridge method, a full-bridge method, and the like.

[0030]

[Effects of the Invention] As explained above, according to the present invention,
There are two separate systems for supplying power to the control circuit that controls the on and off times of switching elements such as FETs, one for when the power is turned on and the other for normal operation, and the control circuit that supplies power to the control circuit that controls the on and off times of switching elements such as FETs is set up separately for when the power is turned on and when the power is turned on. Since the system that supplies power to the circuit is cut off, the current limiting function can be operated reliably in the event of a short circuit or overload condition on the secondary side (load side). Destruction of circuit elements such as switching elements and rectifier diodes can be prevented.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

FIG. 2 is an explanatory diagram of the operation of FIG. 1;

FIG. 3 is a circuit diagram showing a conventional example.

[Explanation of symbols]

A First power supply circuit B Second power supply circuit C Rectifier smoothing circuit D Current limiting circuit E Voltage dividing circuit F Output control circuit 9 Transformer 10 Primary winding 13 Switching element (FET) 15
Control circuit 16, 17 Secondary winding

Claims (1)

[Claims] 1. A transformer having a primary winding to which a switching element is connected in series and to which an input voltage is applied, and two secondary windings to which a rectifying and smoothing circuit is connected, and a transformer that turns the switching element on and off. a control circuit for driving the first secondary winding; a current limiting circuit for detecting the current flowing in the first secondary winding and outputting a control signal to the control circuit so as to limit the current to a preset value; a first power supply circuit connected to the control circuit and having an output terminal connected to a power supply terminal of the control circuit; and a second power supply circuit connected to a secondary winding and having an output terminal connected to the control circuit together with the output terminal of the first power supply circuit. a second power supply circuit that is connected to a power supply terminal of the circuit and generates an output voltage higher than the output voltage of the first power supply circuit; 1. A switching power supply comprising: an output control circuit for prohibiting output voltage from the power supply circuit of No. 1.