JP3613731B2 - No-load power-saving power supply - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply device, and more particularly to a technique for reducing input power when there is no load.
[0002]
[Prior art]
Conventionally, as an example of a circuit for reducing the input power when there is no load, as shown in FIG. 3, the current on the secondary side of the power source is detected by a resistor 31, and the comparator when the current falls below a predetermined value. 9 outputs current, causes the light emitting element of the photocoupler 8 to emit light, turns on the light receiving element, turns on the transistor 10 connected to the gate of the switch element 2, and temporarily stops oscillation. It was used.
[0003]
In FIG. 3, once oscillation stops, it takes a predetermined time until oscillation starts again. Therefore, oscillation stop and oscillation start are repeated, but the average input power can be reduced by extending the oscillation stop period.
[0004]
[Problems to be solved by the invention]
In the conventional method shown above, in order to detect the current on the secondary side with a value close to no load, it is necessary to use a highly accurate comparison circuit, and the signal is changed from the secondary side to the primary side. The cost increase is large because a photocoupler is required for sending.
[0005]
SUMMARY OF THE INVENTION An object of the present invention is to provide a circuit that minimizes the cost increase and has the same effect as the conventional method.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, according to the present invention, in a power supply apparatus that sends a feedback signal for keeping a secondary side voltage constant to a primary side via a photocoupler, the load on the secondary side becomes no load. The current flowing through the photo-receiving element of the photocoupler is detected by the first resistor, and when the current is maximized, the voltage across the first resistor is A value is selected so as to reach the voltage between the emitters, the base and emitter of the transistor are connected to both ends of the first resistor, and the collector of the transistor is connected to the control signal input terminal of the oscillation control circuit of the switch element via the second resistor. Connected. A capacitor was connected between the terminal of the DC power supply that supplies current to the collector of the transistor and the light receiving element of the photocoupler and the terminal on the opposite side.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
When the load on the secondary side becomes no load, the current of the light receiving element of the photocoupler increases and the voltage across the first resistor rises. When the voltage between the base and emitter of the transistor rises and turns on, the capacitor The voltage is increased by charging, and eventually, a current is continuously supplied to the control signal input terminal of the oscillation control circuit, and the oscillation control circuit continues to lower the voltage of the control electrode of the switch element to stop the oscillation.
[0008]
When the oscillation stops, the voltage on the secondary side decreases and the current of the light receiving element of the photocoupler becomes zero, but the capacitor charge continues to flow to the control signal input terminal through the second resistor. When the discharge of the capacitor charge stops, the oscillation control circuit stops lowering the voltage of the control electrode of the switch element and resumes oscillation.
[0009]
In this way, the oscillation stop and start are repeated, but by appropriately selecting the time constant of the capacitor and the second resistor, the oscillation stop period can be set longer and the average input power is reduced. be able to.
[0010]
【Example】
FIG. 1 is a circuit diagram showing a self-excited ringing choke converter according to an embodiment of the present invention.
[0011]
In FIG. 1, the oscillation start is performed by the following mechanism. A current flows from the DC power source 23 through the resistor 18 to the gate of the switch element 2 and the capacitor 19, the gate voltage rises, and when the threshold value is reached, the switch element 2 is turned on and the primary winding 1a is turned on. The voltage of the DC power supply 23 is applied. At this time, since the polarity of the auxiliary winding 1c is selected so that the voltage induced in the auxiliary winding 1c pushes up the gate voltage, the gate voltage further increases and the switch element 2 is completely turned on. On the other hand, since the voltage of the auxiliary winding 1c charges the capacitor 22 via the resistor 21, when the voltage of the capacitor 22 reaches the base-emitter voltage of the transistor 3, the transistor 3 is turned on and the gate of the switch element 2 is turned on. The voltage is lowered and the switch element 2 is turned off. When the switch element 2 is turned off, an electromotive force in the reverse direction is generated in the winding of the transformer 1, and this power is supplied to the secondary side via the secondary winding 1b. The capacitor 19 is charged with a voltage that causes the gate side of the switch element 2 to be positive by the reverse voltage generated in the auxiliary winding 1c. When the electromotive force in the reverse direction is completely discharged through the secondary winding 1b, the voltage of the winding returns to zero. At this time, the electric charge charged in the capacitor 19 is discharged and the voltage of the gate of the switching element 2 is reduced. Then, the switch element 2 is turned on again.
[0012]
When the power supply is started, a current flows through the resistor 18 and the first turn-on is performed. However, when the oscillation continues, the charge of the capacitor 19 functions to turn on from the off state. When oscillating from the state where the electric charge of the capacitor 19 is completely discharged, it takes time until the oscillation starts because it starts from the position where it flows through the resistor 18.
[0013]
In FIG. 1, the voltage supplied to the load is divided by resistors 24 and 25 and applied to the reference terminal of the error amplifier 6 incorporating the reference power supply. The difference between this voltage and the voltage of the reference power supply is shown. Is larger, the current of the light emitting element of the photocoupler 7 is increased, and the current of the light receiving element is also increased. The diode 16 and the capacitor 17 rectify and smooth the voltage generated in the winding 1c to generate a DC voltage, and supply current to the light receiving element.
[0014]
The smaller the load current, the higher the voltage supplied to the load and the greater the current of the light receiving element. As the current of the light receiving element increases, the base voltage of the transistor 3 is always high. When the voltage of the winding 1c is applied to the base through the resistor 21 after the switch element 2 is turned on, the base voltage is reduced in a short period. Since the transistor 3 is turned on and the transistor 3 is turned on, the on period is shortened. However, when the switch element 2 is turned off, the voltage at the base of the transistor 3 immediately decreases. .
[0015]
When no load is applied, the voltage across the first resistor 11 turns on the transistor 12 and raises the voltage of the capacitor 13. When the voltage of the capacitor 13 increases, the current flowing through the transistor 12 flows through the second resistor 14 and the diode 15 to the base of the transistor 3 to turn on the transistor 3. When the transistor 3 is turned on, the gate voltage of the switch element 2 decreases and the oscillation stops.
[0016]
When the oscillation is stopped, the voltage applied to the load is reduced, the current of the light receiving element is reduced, and the transistor 12 is turned off. However, since the charge of the capacitor 13 continues to flow through the second resistor 14 to the base of the transistor 3, the oscillation is stopped. Continue for a while. Eventually, when the voltage of the capacitor 19 decreases and the current cannot continue to flow through the base of the transistor 3, the oscillation starts. However, since the restart of the oscillation starts from where the current flows through the resistor 18, the oscillation stop period accordingly. Becomes longer.
[0017]
In the embodiment of FIG. 1, the diode 15 is not an essential component constituting the circuit of the present invention, but the feedback signal of the photocoupler 7 causes the second resistor 14 when the oscillator 15 continuously oscillates at a load other than no load. When the capacitor 13 is charged, a delay in response due to the time constant of the capacitor 13 and the second resistor 14 occurs, so that a feedback signal is added so as not to charge the capacitor 13.
[0018]
In the embodiment of FIG. 1, the feedback signal that flows through the light receiving element of the photocoupler 7 and the current that flows through the transistor 3 are poured into the base of the transistor 12 that is the same control signal input terminal. It is also possible to use a method in which each of the input terminals is poured separately.
[0019]
FIG. 2 is a circuit diagram showing a PWM flyback converter according to an embodiment of the present invention.
[0020]
In FIG. 2, if the load current exceeds a predetermined value, the current flowing through the light receiving element of the photocoupler 7 is small, and the transistor 12 is in the OFF state. The transistor 12 whose load current falls below a predetermined value is turned on, and the voltage at the OFF terminal OFF of the PWM circuit 28 is increased to stop oscillation. When the voltage of the capacitor 13 decreases, the oscillation is resumed, so that the oscillation is repeatedly stopped and started. As a result, the average input power is reduced.
[0021]
The PWM circuit 28 has a built-in oscillation circuit, and has a feedback signal input terminal FB and an OFF terminal OFF as control signal input terminals. By changing the voltage applied to the feedback signal input terminal FB, pulse output is achieved. The pulse width output from the terminal OUT can be changed, and oscillation can be stopped by setting the voltage applied to the OFF terminal OFF to a predetermined value.
[0022]
【The invention's effect】
Compared with the conventional method shown in FIG. 3, the circuit is simple and the cost of parts can be reduced, so that the economic effect is great.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a self-excited ringing choke converter according to an embodiment of the present invention.
FIG. 2 is a circuit diagram showing a PWM flyback converter according to an embodiment of the present invention.
FIG. 3 is a circuit diagram showing an example of a conventional method.
[Explanation of symbols]
1 transformer 1a primary winding 1b secondary winding 1c auxiliary winding 2 MOSFET
3 Transistor 4 Diode 5 Capacitor 6 Reference amplifier built-in error amplifier 7 and 8 Photocoupler 9 Comparator 10 Transistor 11 First resistor 12 Transistor 13 Capacitor Second resistor 15 and 16 Diode 17 Capacitor 18 Resistor 19 Capacitor 20 and 21 Resistor 22 Capacitor 23 DC power supply 24, 25, 26 Resistance 27 Load 28 PWM circuit 31, 32, 33, 34 Resistance

Claims (1)

Oscillation control having a transformer having a primary winding and a secondary winding, a switch element connected in series to the primary winding, and one or more control signal input terminals connected to a control electrode of the switch element A circuit, a rectifying / smoothing circuit connected to the secondary winding, an error amplifier that compares a DC voltage generated by the rectifying / smoothing circuit with a voltage of a reference power source, and amplifies the difference, and the light emitting element includes the error A switching power supply comprising a photocoupler connected to an output terminal of an amplifier and having a light receiving element connected to one of the control signal input terminals, and a DC power supply for supplying a current to the light receiving element. a first resistor inserted in series with the DC power supply side, by adding a transistor to connect the base and emitter to both ends of the first resistor, the second to the control signal input terminal of the oscillation control circuit One end of the second resistor, the other end of the second resistor connected to the collector of the transistor, a connection point between the other end of the second resistor, the collector of the transistor, and the first of the DC power source. A no-load power-saving power supply device, wherein a capacitor is connected between a terminal to which a resistor is connected and a terminal on the opposite side.

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