JPH04133666A - Power supply circuit - Google Patents

Abstract

PURPOSE:To surely prevent rush currents and prevent the fusion of a fuse and the fusion of a power supply switch by providing a voltage detecting circuit, which compulsorily stops the operation of a switching control when the output voltage of a rectifying and smoothing circuit is below a certain value. CONSTITUTION:When it gets in power stoppage condition or a power supply switch SW1 is turned off, the charge voltage of a capacitor C1 falls gradually. When the charge voltage of the capacitor C1 gets below a certain value and the partial voltages of resistors R3 and R4 get below the Zener voltage of a Zener diode ZD1, a transistor Q3 is turned on. By this, the base potential of a transistor Q4 becomes the partial voltage determined by resistors R6, R8, R9 and R10, and the transistor Q4 get in on condition. As a result, the power supply voltage of a control IC2 becomes nearly earth potential, and the switching control is stopped forcibly.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Industrial Application Field The present invention relates to a power supply circuit that inputs a commercial power source and outputs a predetermined voltage and current.

(b) Conventional technology Power supply circuits that supply a predetermined voltage and current from a commercial power supply to a load are divided into switching regulators that have higher power conversion efficiency and can be configured in a smaller size than conventional ones, and switching regulators that rectify and smooth commercial Ha. and a rectifying and smoothing circuit that supplies DC voltage to the DC voltage.

By the way, a rectifying and smoothing circuit that rectifies and smoothes the commercial Q'll and supplies DC voltage to the switching regulator is composed of, for example, a diode bridge that rectifies the commercial power supply and a smoothing capacitor that smoothes the rectified output. When , is turned on, the charged NN current flowing through the smoothing capacitor flows as an inrush current, which may cause the fuse to melt or the power switch to melt. Therefore, conventionally, circuits have been provided to prevent such inrush current. FIG. 2 shows an example of a power supply circuit equipped with an inrush current prevention circuit.

In Figure 2, SWI is a power switch, Fl is a fuse, R1 is a power supply limiting resistor, and C2 is a resistor R1 when it is turned on.
TRIAC (switch element) that bypasses the TRIAC (switch element), Dl is a diode bridge that rectifies the commercial power supply, C1 is a capacitor that smoothes the rectified output, Ql is a switching transistor that cuts off the current flowing to the primary winding N1 of the transformer 1, and D2 and C2 is the secondary winding N2 of transformer 1
A diode and a capacitor that rectify and smooth the electromotive force.

In the circuit shown in FIG. 2, when the power switch SWl is turned on, current passes through the resistor R1, is rectified by the diode bridge D1, and charges the capacitor C1. When the charging IQ pressure of the capacitor C1 reaches a predetermined value and the switching transistor Q1 starts oscillating, the secondary output voltage output from the control winding Nc of the transformer 1 is applied to the gate of the triac Q2 via the resistor R2. Given. This causes triac Q2 to turn on and short-circuit both ends of resistor R1. Therefore, current flows through triac Q2 thereafter.

(Problem to be solved by the C1 invention) However, in conventional power supply circuits equipped with such inrush current prevention circuits, it is difficult to deal with situations where there is a momentary power outage with a relatively long period of time, or when the power switch is repeatedly turned off/on. Depending on the operation, it may not be possible to sufficiently limit the inrush current.In other words, in the example shown in Figure 2, the on/off operation of the switching transistor Q1 will cause the charging voltage of the capacitor c1 to reach the charging voltage in the steady state. In addition, due to a power outage of the commercial power supply or the turning off of the power switch SW1, the charge in the capacitor C1 is discharged and the switch notching operation is continued until the charging voltage reaches the threshold value.Switching During the period of operation, the triac Q2 is kept on by the voltage of the control winding Nc of the transformer.Therefore, when commercial power is supplied, the charging voltage of the capacitor fc1 rises, and the switching operation is started. If a power outage occurs or the t! switch is turned off, and then the power is restored or the power switch is turned on again with the triator Q2 in the on state, the charging voltage of the capacitor CI will drop below the charging voltage in the steady state. Therefore, when the power is restored or the power switch is turned on again, an inrush current will flow through the triac Q2.

The purpose of this invention is to reliably prevent inrush current even in the case of a relatively long instantaneous power outage of the commercial power supply or sudden off/on operation of the power switch, thereby solving problems such as blowing of fuses and welding of the power switch. The purpose of the present invention is to provide a power supply circuit that eliminates the problem.

(Means for Solving Problem d1) This invention comprises a rectifying and smoothing circuit that rectifies and smoothes a commercial power supply and applies a DC voltage to the primary winding of a transformer via a switching transistor, and a switching circuit that controls on/off the switching transistor. A power supply equipped with a control circuit and an inrush current prevention circuit in which a parallel connection circuit of a current limiting resistor and a switch element is inserted into the input line of the commercial power supply, and the secondary output voltage of the transformer is applied to the control terminal of the switch element. The circuit is characterized in that it includes a voltage detection circuit that forcibly stops the switching control operation when the output voltage of the rectifier and smoothing circuit is less than a certain value.

(e) Function In the power supply circuit of the present invention, the voltage detection circuit forcibly stops the operation of the switching control circuit when the output voltage of the rectifying and smoothing circuit is less than a certain value. By adding this voltage detection circuit to a power supply circuit equipped with a conventional inrush current prevention circuit, when the output voltage of the rectifying and smoothing circuit is less than a certain value, the secondary output voltage of the transformer is applied to the control terminal of the switching element. The switch/7ch element is turned off.

Therefore, when the output voltage of the rectified and smoothed voltage is less than a certain value, current is passed through the current limiting resistor.

As a result, even if the power outage time of a momentary power outage or the off time of the power switch is incomplete, inrush power is reliably prevented when the power is restored or the power switch is turned on again.

(f) Embodiment A power supply circuit according to an embodiment of the present invention is shown in FIG. 1. In FIG. 1, the differences from the conventional power supply circuit shown in FIG. 2 are as follows.
The control IC 2 that controls the switching transistor Q1 and the power supply circuit for its control XC2 are also shown, and a circuit 3 corresponding to the voltage detection circuit of the present invention is provided. This is an auxiliary winding provided, and its electromotive voltage is connected to the resistor R11 and the diode +"D.
3 and a capacitor C3, the voltage is made constant by a Zener diode ZD2, and power is supplied to the control IC2. This control TC2
detects the load supply voltage and switches the switching transistor Q
The on-duty ratio of 1 is controlled. Note that at startup, power is supplied via resistors R6R7.

In FIG. 1, a voltage detection circuit 3 detects the output voltage of a rectifying and smoothing circuit consisting of a diode bridge D1 and a capacitor C1, and when the output voltage is less than a predetermined constant value, the voltage detection circuit 3 lowers the power supply voltage of the control IC 2. This is a circuit that forcibly stops the operation. That is, resistors R3 and R4 constitute a resistive voltage divider circuit, and when the divided voltage exceeds the Zener voltage of Zener diode ZDI, Zener diode ZDI becomes conductive, and Zener current flows to resistor R5 and to the base of transistor Q3. . As a result, the base potential of the transistor Q4 becomes approximately the ground potential, and the transistor Q4 is turned off. That is, in this state, the power supply voltage is supplied to the control IC 2 and switching control is performed.

Here, when a power outage occurs or the power switch
When Vv'l is turned off, the charging voltage of capacitor C1 gradually decreases. When the charging voltage of the capacitor C1 becomes less than a certain value and the divided voltage of the resistors R3 and R4 becomes less than the Zener voltage of the Zener diode ZDI, the transistor Q3 is turned off. This results in transistor Q4
The base potential of the transistor Q4 becomes a divided voltage determined by the resistors R6, R8, R9 and RIO, and the transistor Q4 is turned on. As a result, the power supply voltage of the control IC 2 becomes approximately the ground potential, and the switching control is forcibly stopped. By stopping the on/off operation of the switching transistor Q1, the electromotive voltage of the control winding Nc of the transformer 1 also becomes zero.
As a result, triac Q2 turns off. This results in an inrush current prevention state in which current is supplied through the current limiting resistor R1. Power outage or power switch.

If the off state of switch SWI continues, the charging voltage of capacitor C1 will further decrease due to the current flowing through resistor R3R4, resistors R6 and R7, and transistor Q4. After that, if the power failure returns or the power switch SW1 is turned on again, current is passed through the current limiting resistor R1, and when the charging voltage of the capacitor C1 exceeds a certain value, the Zener diode ZDI and the transistor Q3 are turned on, and the transistor Q4 is turned on. The control IC 2 is turned off and the power supply voltage is supplied to the control IC 2. As a result, switching control is started, and the triac Q2 is turned on again by the electromotive force of the control winding Nc of the transformer.

Furthermore, if the power is restored or the power switch SWI is turned on again after a very short period of time after a power outage or after the TFI switch SWI is turned off, the voltage that prevents capacitor C1 from charging will hardly drop. Therefore, transistor Q4 is not turned on, so the triac Q4 is not turned on.
2 remains on, but in such a case, since the charging voltage of the capacitor CI has hardly decreased in the first place, no inrush current will flow when the power is restored or when the power switch SWI is turned on again.

In the embodiment shown above, a control IC is used as the switch control circuit, and the switching control is forcibly stopped by stopping the power supply voltage. The switching control may be forcibly stopped by controlling the potential of the error input terminal. Also,
The switching transistor Q1 may be used as an automatic oscillation circuit element, and the switching operation may be forcibly stopped by stopping its positive feedback operation.

(g) Effects of the Invention According to the present invention, when the output voltage of the rectifying and smoothing circuit becomes less than a certain value due to the interruption of the supply of commercial power, the switching control operation is forcibly stopped. Therefore, even in the event of a momentary power outage with a relatively long duration, or if the power switch is suddenly turned off and on, the output voltage of the rectifier and smoothing circuit will drop to the extent that an inrush current will occur when commercial power supply starts. Since the current is always passed through the current limiting resistor when the power source is on, rush current is prevented regardless of whether or not the commercial NB supply is interrupted, and blowing of the fuse and welding of the power switch are reliably prevented.

I Trance, Control I voltage detection circuit, current limiting resistor, Triac (Sui) (Sui switching control wholesale circuit) switch element)

Claims (1)

[Claims] (1) It includes a rectifying and smoothing circuit that rectifies and smoothes a commercial power source and applies a DC voltage to the primary winding of the transformer via a switching transistor, and a switching control circuit that controls on/off the switching transistor,
In a power supply circuit equipped with an inrush current prevention circuit in which a parallel connection circuit of a current limiting resistor and a switching element is inserted into the input line of a commercial power supply, and the secondary output voltage of the transformer is applied to the control terminal of the switching element, a rectifying and smoothing circuit is used. A power supply circuit comprising a voltage detection circuit that forcibly stops switching control operation when the output voltage of the power supply circuit is less than a certain value.