JP2705303B2 - Power circuit - Google Patents

Description

The present invention relates to a power supply circuit that inputs a commercial power supply and outputs a predetermined voltage and current.

(B) Conventional technology A power supply circuit that supplies a predetermined voltage and current from a commercial power supply to a load is a switching regulator that has a high power conversion efficiency and can be configured in a small size, and a switching regulator that rectifies and smoothes a commercial power supply. And a rectifying / smoothing circuit for supplying a DC voltage to the power supply.

By the way, a rectifying and smoothing circuit that rectifies and smoothes a commercial power supply and supplies a DC voltage to a switching regulator includes, for example, a diode bridge that rectifies a commercial power supply and a smoothing capacitor that smoothes the rectified output thereof. The charging current flowing through the smoothing capacitor flows as an inrush current, which may cause a fuse to be blown or a power switch to be welded. Therefore, a circuit for preventing such an inrush current has conventionally been provided. FIG. 2 shows an example of a power supply circuit having an inrush current prevention circuit.

In FIG. 2, SW1 is a power switch, F1 is a fuse, R
1 is a power limiting resistor, Q2 is a triac (switch element) that bypasses resistor R1 when it is on, D1 is a diode bridge that rectifies commercial power, C1 is a capacitor that smoothes rectified output, and Q1 is the primary winding of transformer 1. A switching transistor that interrupts the current flowing through N1, and D2 and
C2 is a diode and a capacitor for rectifying and smoothing the electromotive voltage of one secondary winding N2 of the transformer.

In the circuit shown in FIG. 2, when the power switch SW1 is turned on, the current passes through the resistor R1 and the diode bridge D1
And the capacitor C1 is charged. Capacitor C1
Charging voltage reaches a predetermined value and the switching transistor
When the oscillation of Q1 starts, the secondary output voltage output from the control winding Nc of the transformer 1 becomes triac via the resistor R2.
Given to the gate of Q2. As a result, the triac Q2 turns on and short-circuits both ends of the resistor R1. Therefore, thereafter, a current flows through the triac Q2.

(C) Problems to be Solved by the Invention However, in the conventional power supply circuit having such an inrush current prevention circuit, when an instantaneous power failure with a relatively long interruption time occurs, or when the power switch is suddenly turned on / off, In some cases, a sufficient inrush current cannot be limited depending on the off operation. That is, in the example shown in FIG. 2, the on / off operation of the switching transistor Q1 is started until the charging voltage of the capacitor C1 reaches the charging voltage in the steady state,
Further, due to the power failure of the commercial power supply or the turning off of the power switch SW1, the charge of the capacitor C1 is discharged, and the switching operation is continued until the charged voltage reaches a certain threshold value. During the period when the switching operation is being performed, the triac Q2 keeps the ON state by the voltage of the control winding Nc of the transformer. Therefore, when the commercial power is supplied, the charging voltage of the capacitor C1 rises, and once the switching operation is started, the power is turned off, or the power switch is turned off, and then the power is restored or the power switch is restarted while the triac Q2 is turned on. If the power is turned on, the charging voltage of the capacitor C1 is lower than the charging voltage in the steady state. Therefore, when the above-mentioned power failure is restored or the power switch is turned on again, an inrush current flows through the triac Q2.

SUMMARY OF THE INVENTION An object of the present invention is to make it possible to reliably prevent an inrush current even by an instantaneous power failure of a commercial power supply having a relatively long disconnection time or a sudden ON / OFF operation of a power switch, thereby causing a problem of fuse blowing and power switch welding. It is an object of the present invention to provide a power supply circuit which solves the problem.

(D) Means for Solving the Problems The present invention rectifies and smoothes a commercial power supply and applies a DC voltage to a primary winding of a transformer via a switching transistor, and controls on / off of the switching transistor. A power supply circuit including a switching control circuit, and a gate control circuit that inserts a parallel circuit of a current limiting resistor and a triac into an input line of a commercial power supply and applies an output voltage of a control winding of a transformer to a gate terminal of the triac. A series circuit comprising a diode D4 and resistors R3 and R4 is provided between the input line of the commercial power supply and the ground on the primary side of the transformer, and a resistor R5 is provided between the connection point of the resistors R3 and R4 and the ground.
A series circuit consisting of a Zener diode ZD and a light-emitting part PCl of a photocoupler is connected in parallel with the capacitor C3, and the gate control circuit is connected to the control winding Nc of the transformer and a triac. The light receiving section PCa of the photocoupler and the diode D3 are connected in series between the gate of Q2 and the photocoupler.

(E) Operation In the power supply circuit of the present invention, the diode D4 rectifies the input of the commercial power supply voltage, and the resistors R3 and R4 divide the input, and
The resistor R5 and the capacitor C3 are charged with a time constant determined by the circuit constant. The charging voltage of the capacitor C3 is changed from the Zener voltage of the Zener diode ZD to the light emitting portion PC of the photocoupler.
When the voltage exceeds the voltage obtained by subtracting the voltage drop of 1, the light receiving section PCa of the photocoupler is turned on by the output voltage of the control winding, and the triac Q2 is turned on. With such a circuit, even if the commercial power is turned on again while the switching operation is continued, the triac Q2 is turned on with a delay of a certain time due to the delay operation due to the charging of the capacitor C3. As a result, even if the power outage time of the instantaneous power outage or the off time of the power switch is halfway, the inrush current at the time of the power recovery or the power switch on again is reliably prevented.

Further, in the power supply circuit of the present invention, since the ON condition of the photocoupler PC, that is, the ON condition of the triac Q2 is determined by the setting of the voltage dividing ratio of the resistors R3 and R4 and the Zener diode ZD zener voltage, the commercial power is turned on again. Can be set strictly until the triac turns on, and stable control characteristics can be obtained.

Further, in the power supply circuit according to the present invention, since the ground for the circuit for detecting the input of the commercial power supply is common to the ground on the secondary (DC) side of the diode bridge, the circuit configuration on the primary side of the transformer is simplified, and Using a photo coupler,
Since the detection result of the commercial power supply input is given to the gate control circuit provided in the control winding of the transformer, the circuit configuration on the output side of the control winding of the transformer is also simplified.

(F) Embodiment FIG. 1 shows a power supply circuit according to an embodiment of the present invention.

The circuit shown in FIG. 1 is different from the conventional power supply circuit shown in FIG. 2 in that a diode D4, resistors R3, R4, R5, a capacitor C3, a Zener diode ZD, A circuit consisting of a light emitting part PCl of a photocoupler is connected, and a control circuit consisting of a light receiving part PCa of a photo wrapper and a diode D3 is inserted into the output of the control winding Nc of the transformer and the gate signal supply line of the triac Q2.

In FIG. 1, a diode D4 rectifies an input voltage of a commercial power supply, and resistors R3 and R4 divide the rectified output voltage.
Further, a CR charging circuit is constituted by the resistors R3 and R5 and the capacitor C3, and a CR discharging circuit is constituted by the resistors R4 and R5 and the capacitor C3. The portion indicated by reference numeral 2 in the figure is a delay circuit according to the present invention, and when a commercial power supply voltage is supplied,
The charging voltage of capacitor C3 rises with the time constant determined by resistors R3, R5 and capacitor C3, and when the input of commercial power is cut off, the charging voltage of capacitor C3 decreases with the time constant determined by resistors R4, R5 and capacitor C3. I do. The Zener diode ZD conducts when the charging voltage of the capacitor C3 exceeds the Zener voltage (more precisely, the voltage obtained by adding the voltage drop of the photocoupler PCl to the Zener voltage of ZD), and the light emitting part PCl of the photocoupler emits light.

On the other hand, the light receiving portion PCa of the photocoupler connected to the control winding Nc of the transformer 1 supplies the output voltage of the control winding Nc to the gate of the triac Q2 only when the light emitting portion PCl emits light. Note that the diode D3 is the light receiving section PCa of the photocoupler.
To prevent a reverse voltage from being applied.

With the above configuration, the entire circuit operates as follows. First, when the power switch SW1 is turned on, a current flows through the resistor R1, rectification and smoothing are performed by the diode bridge D1 and the capacitor C, and the charging voltage of the capacitor C1 is changed by a switching control circuit (a separately excited oscillation circuit (not shown) or a switching transistor Q1). When the self-excited oscillation circuit (including a self-excited oscillation circuit) exceeds a threshold value at which the operation starts, the switching transistor Q1 repeats the on / off operation to interrupt the current flowing through the primary winding N1 of the transformer. Thereby, the electromotive voltage generated in the secondary winding N2 is rectified and smoothed by the diode D2 and the capacitor C2 and output to the load. Further, an electromotive voltage is also generated in the control winding Nc of the transformer, but at this time, the photocoupler PCa is not yet driven, and no gate trigger signal is supplied to the triac Q2. Power switch SW1 above
Immediately after turning on, the charging voltage of the capacitor C3 rises,
When the voltage exceeds a certain value, the Zener diode ZD conducts, and the photocoupler PCl is driven, whereby the light receiving portion PCa of the photocoupler is driven. As a result, the output voltage of the control winding Nc of the transformer is supplied to the gate of the triac Q2, and the triac Q2 turns on to short-circuit both ends of the resistor R1. Therefore, thereafter, power is supplied through the triac Q2.

Here, if a power failure occurs, or if the power switch SW1
Is turned off, the charging voltage of the capacitor C1 decreases, but the decrease is relatively gentle, the operation of the switching transistor Q1 is continued, and the triac Q2 remains on. On the other hand, the charge of the capacitor C3 is low resistance R4 and
Discharged quickly through R5, and when the charged voltage falls below substantially the Zener voltage of Zener diode ZD, Zener diode ZD is cut off and photocoupler PCl is turned off.
Therefore, the light receiving section PCa of the photocoupler is turned off, and the gate voltage of the triac Q2 is cut off. As a result, the triac Q2 is turned off, and the commercial power returns to a state where current is supplied through the resistor R1 (inrush current prevention state). Therefore, even if the power is restored or the power switch SW1 is turned on again thereafter, the charging current flows to the capacitor C1 through the resistor R1 and the diode bridge D1, so that no rush current flows.

Note that if a power failure is restored or the power switch SW1 is turned on again after a very short time after the power failure or the power switch SW1 is turned off, the Zener diode ZD will not be turned off, and therefore the triac Q2 keeps the ON state, but in such a case, since the charging voltage of the capacitor C1 hardly decreases, no rush current flows when the power is restored or when the power switch SW1 is turned on again.
Conversely, if the power failure state or the OFF state of the power switch SW1 continues for a long time, the charging voltage of the capacitor C1 drops sufficiently and the on / off operation of the switching transistor Q1 also stops. Or, even if the power switch SW1 is turned on again, the rush current is prevented in the same manner as when the normal power switch is turned on.

In the embodiment described above, the diode D4 is used as the rectifier circuit for rectifying the input voltage of the commercial power supply. However, a part of the diode bridge D1 may be used.

(G) Effects of the Invention According to the present invention, the triac connected in parallel to the current limiting resistor is turned on with a delay of a certain time after the commercial power is supplied, so that an instantaneous power failure with a relatively long interruption time occurs. Even when the power switch is suddenly turned on / off, the power is always supplied through the current limiting resistor at the start of the supply of the commercial power, so that the rush current is reliably prevented. Further, the time from when the commercial power is turned on until the triac turns on (the time until the inrush current prevention state is released) can be strictly set, and stable control characteristics can be obtained. Further, since the circuit configuration of the primary side of the transformer is configured above a single ground potential, the circuit configuration is simplified, and the configuration of the output side of the control winding of the transformer is also simplified.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a power supply circuit according to an embodiment of the present invention. FIG. 2 is a circuit diagram of a conventional power supply circuit. 1 Transformer 2 Delay circuit R1 Current limiting resistor Q2 Triac (switch element) (ZD, PCl, P
Ca): Switch circuit, D4: Diode (rectifier circuit).

Claims (1)

(57) [Claims] A rectifying / smoothing circuit for rectifying and smoothing a commercial power supply and applying a DC voltage to a primary winding of a transformer via a switching transistor; a switching control circuit for turning on / off the switching transistor; A power supply circuit having a gate control circuit that inserts a parallel circuit of a current limiting resistor and a triac into an input line of the triac and applies an output voltage of a control winding of the transformer to a gate terminal of the triac. A series circuit consisting of a diode D4 and resistors R3 and R4 is provided between the ground of the primary side and a resistor R5 between the connection point of the resistors R3 and R4 and the ground.
A series circuit consisting of a Zener diode ZD and a light-emitting part PCl of a photocoupler is connected in parallel with the capacitor C3, and the gate control circuit is connected to the control winding Nc of the transformer and a triac. A power supply circuit comprising a light receiving portion PCa of the photocoupler and a diode D3 connected in series between the gate of Q2 and the photocoupler.