JPH0540710Y2 - - Google Patents

Description

[Detailed explanation of the idea] (a) Industrial application fields This invention relates to a switching power supply circuit.

(b) Prior Art Figure 2 shows a conventional separately excited oscillation type switching power supply circuit. In the figure, T is a transformer, and the primary winding L1 is provided with a switching transistor Q2 and a separately excited oscillation circuit 1 that turns on and off the switching transistor Q2. is outputting. AC is a commercial alternating current power supply, rectified by diode bridge DB, and smoothing capacitor C
1 and the obtained DC voltage is connected to the primary coil L1 of the transformer T and is also supplied to the starting power source 2 of the separately excited oscillation circuit 1.
This starting power supply 2 has a transistor Q1 connected in series, a Zener diode ZD, a resistor R5,
Along with R6, a series regulator type circuit configuration is adopted. When the separately excited oscillation circuit 1 starts oscillating with such a starting power supply, a voltage is induced in the coil L2 of the transformer T, this induced voltage is rectified by the diode D2, smoothed by the capacitor C3, and the separately excited oscillation circuit 1 is started. It is supplied as a power source for the circuit 1. Therefore, after that, the separately excited oscillation circuit 1 continues its oscillation operation mainly by the power supplied from L2 and D2.

(c) Problems to be solved by the invention In the conventional switching power supply circuit that supplies power to the separately excited oscillator circuit using the series regulator method at startup, the separately excited oscillator circuit performs stable oscillation operation. During the start-up period, the startup power supply consumes a lot of power, and the transistor Q1 generates heat. Therefore, conventionally, a transistor with a capacity corresponding to the collector loss of this transistor Q1 is used and a heat sink is provided according to its heat generation amount, but if the power switch SW of this switching power supply circuit is turned on and off frequently. or when the load is abnormal, the rise period of the separately excited oscillation circuit 1 becomes longer, and the transistor Q
There was a problem that 1 was generating abnormal heat.

The purpose of this invention is to reduce heat generation in the startup power supply that supplies power when starting up the separately excited oscillation circuit,
It is an object of the present invention to provide a switching power supply circuit which solves the problems of the conventional art.

(d) Means for solving the problem This invention consists of a switching element on the primary side of the transformer, a separately excited oscillator circuit that turns the switching element on and off, and a startup system that supplies power when starting up the separately excited oscillator circuit. In a switching power supply circuit that is provided with a power supply and supplies part of the power output from the secondary side to the transformer as a power supply for the separately excited oscillation circuit, the starting power supply is connected in series between an input section and an output section. a series transistor that conducts between the input section and the output section in an on state, a diode connected in series in the forward direction between the series transistor and the output section, and a capacitor that charges the input voltage from the input section. a CR time constant circuit consisting of a series circuit of resistors; a constant voltage generation circuit that generates a constant voltage equal to the power supply voltage at startup for the separately excited oscillation circuit; and an output voltage of the CR time constant circuit that exceeds the constant voltage. The present invention is characterized by comprising a series transistor control circuit that discharges the charge in the capacitor of the CR time constant circuit that is turned on via the series transistor, and turns on the series transistor.

(e) Effect In the switching power supply circuit of this invention,
As a starting power supply that supplies power when starting up the separately excited oscillation circuit, a series transistor is connected in series between the input section and the output section and conducts between the input section and the output section in the on state, and this series transistor and the output A diode connected in series in the forward direction between the parts,
a CR time constant circuit consisting of a series circuit of a capacitor and a resistor that charges the input voltage from the input section; a constant voltage generation circuit that generates a constant voltage equal to the power supply voltage at startup for the separately excited oscillation circuit;
The circuit comprises a series transistor control circuit that turns on when the output voltage of the time constant circuit exceeds the constant voltage to discharge the charge in the capacitor of the CR time constant circuit through the series transistor, and turns on the series transistor. As a result, the CR time constant circuit charges the input voltage from the input section to the capacitor via the resistor with a constant time constant, and the series transistor control circuit turns on when the output voltage of the CR time constant circuit exceeds the constant voltage. That CR
The charge in the capacitor of the time constant circuit is discharged to the load (separately excited oscillation circuit) via the series transistor, and the series transistor is turned on. Because of this configuration, when the output voltage of the CR time constant circuit exceeds the above-mentioned constant voltage after input power supply starts to be supplied to the startup power supply, the series transistor is turned on and the input power supply continues to the separately excited oscillation circuit. After that, the charge in the capacitor of the CR time constant circuit is discharged, and the series transistor is turned off by the action of the series transistor control circuit. The starting power supply is turned on and off as described above until the separately excited oscillation circuit oscillates and the switching power supply circuit starts up. According to this invention, unnecessary power is not supplied to the separately excited oscillation circuit, and the power loss is small and the amount of heat generated is small. In addition, since the peak voltage supplied to the separately excited oscillation circuit when the transistor is turned on is high, the power supply voltage of the separately excited oscillation circuit rises relatively quickly, and as a result, the switching power supply operates within a short time after the power is turned on. , the operating time of the starting power source is shortened, and the amount of heat generated is also reduced accordingly.

(f) Embodiment FIG. 1 shows a switching power supply circuit which is an embodiment of this invention. The difference from the conventional switching power supply circuit shown in FIG. 2 is the configuration of the starting power supply 2. In FIG. 1, a series circuit consisting of a Zener diode ZD1 and a resistor R1 constitutes a constant voltage circuit determined by the Zener voltage of the Zener diode ZD1, and that voltage is applied to the gate of PUT. The series circuit of resistor R2 and capacitor C2 constitutes a CR time constant circuit, and its output is
Connected to the anode of PUT. Zener diode ZD2 is provided to determine the upper limit of the charging voltage of C2, and its Zener voltage is equal to the above-mentioned value.
Slightly higher than ZD1's Zener voltage. Resistors R3 and R4 are connected to the cathode of PUT to determine the base bias of the transistor Q1. The collector of the transistor Q1 is connected to the power supply line, and the emitter is connected to the power supply input section of the separately excited oscillation circuit 1 via the diode D1.

In the circuit configured as described above, when the power switch SW is turned on, the anode potential of PUT rises exponentially from 0 to the Zener voltage of ZD2 due to the action of the time constant circuit composed of R2 and C2. As mentioned above, the Zener voltage of ZD1 is ZD
Since it is set lower than the Zener voltage of 2,
When the anode potential of PUT exceeds the gate potential, PUT turns on. As a result, the base potential of Q1 rises, turning it on and supplying power to the separately excited oscillation circuit 1. Due to the continuity of PUT,
Since the charge of C2 is discharged via R3, R4, D1 and separately excited oscillation circuit 1, the anode potential of PUT decreases. When the anode potential of PUT drops below the gate potential, PUT is turned off again, and as a result, the base potential of Q1 drops, turning Q1 off, and the power supply to separately excited oscillation circuit 1 is cut off.

By repeating the above operations, the startup power supply 2
performs chopping at a frequency determined by the Zener voltage difference between the Zener diodes ZD1 and ZD2 and the time constants of R2 and C2, and operates as a so-called chopper type regulator. At this time, even though transistor Q1 operates in the active region during the on period, the average power loss due to the transistor is small;
Therefore, the amount of heat generated can be kept low. Also, when the transistor Q1 is on, the smoothing capacitor C3 is connected to the power supply line of the separately excited oscillation circuit 1.
Since the charging voltage rises in a relatively short time, the separately excited oscillation circuit 1 is started up quickly. The oscillation operation of the separately excited oscillator circuit 1 causes switching of the transistor Q2, and thereafter, the voltage induced in the secondary winding L2 of the transformer causes the separately excited oscillator circuit 1 to switch.
operates, the operating time of the starting power supply 2 is shortened, and heat dissipation from the transistor Q1 is also efficiently performed.

(g) Effect of the invention As described above, according to this invention, the switching power supply is connected in series between the input section and the output section as a starting power supply for supplying power to the separately excited oscillation circuit when starting up the switching power supply, and when the switching power supply is turned on, the input CR consists of a series transistor that conducts between the input section and the output section, a diode connected in series in the forward direction between the series transistor and the output section, and a series circuit of a capacitor and a resistor that charges the input voltage from the input section. a time constant circuit, a constant voltage generating circuit that generates a constant voltage equal to the power supply voltage at startup for the separately excited oscillator circuit, and a constant voltage generating circuit that turns on when the output voltage of the specified number of circuits exceeds the constant voltage during CR, and turns on the CR. The configuration includes a series transistor control circuit that discharges the charge in the capacitor of the time constant circuit through the series transistor and turns on the series transistor, thereby reducing the amount of heat generated by the transistor of this starting power supply and reducing the heat capacity. A small heat sink and a transistor with small current capacity can be used,
It is possible to downsize the circuit and reduce costs.

[Brief description of the drawings]

FIG. 1 is a diagram showing a switching power supply circuit according to an embodiment of this invention, and FIG. 2 is a diagram showing an example of a conventional switching power supply circuit. 1... Separately excited oscillation circuit, 2... Starting power supply.

Claims (1)

[Claims for Utility Model Registration] The primary side of the transformer is provided with a switching element, a separately excited oscillation circuit that turns on and off the switching element, and a starting power source that supplies power when starting up the separately excited oscillator circuit. In the switching power supply circuit that supplies a part of the power output from the secondary side of the externally excited oscillation circuit as a power source for the separately excited oscillation circuit, the starting power source is connected in series between the input section and the output section, and the input section is connected in series between the input section and the output section in an on state. A CR consisting of a series transistor that conducts between the input section and the output section, a diode connected in series in the forward direction between the series transistor and the output section, and a series circuit of a capacitor and a resistor that charges the input voltage from the input section. a time constant circuit; a constant voltage generation circuit that generates a constant voltage equal to the power supply voltage at startup for the separately excited oscillation circuit; and when the output voltage of this CR time constant circuit exceeds the constant voltage, it is turned on to generate the CR time constant circuit. 1. A switching power supply circuit comprising a series transistor control circuit that discharges the charge of a capacitor of a constant circuit through the series transistor and turns on the series transistor.