JPH04150775A - Switching power supply unit - Google Patents

Abstract

PURPOSE:To stop switching operation and also, prevent the occurrence of the maximum rush currents by comprising a smoothing capacitor, which is connected to the primary side of a switching power unit, and a resistor for discharge, and equipping it with a charge/discharge circuit and a switching control circuit. CONSTITUTION:When an AC switch 1 is turned on, it is rectified and smoothed to DC voltage with a rectifying circuit 3 and a smoothing capacitor 6 via a filter 2. Since the smoothing capacitor 6 and the resistor 5 for discharge constitutes a close circuit, the charge of the smoothing capacitor 6 is discharged gradually through the resistor 5 for discharge, so it will do to design the discharge time constant larger than the heat time constant of a power thermistor 4. Moreover, when the capacity of the smoothing capacitor 6 is let out by the discharge, the heat generation of the power thermistor 4 drops, and the resistance value recovers to the original value to become large enough, so even if the AC switch 1 is turned on again, it does not cause excessive rush currents.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a switching power supply device that prevents generation of excessive inrush current that occurs when the power source is 08.

[Prior Art] FIG. 3 is a block circuit diagram in which a power thermistor is provided in a conventional switching regulator to limit inrush current. In the figure, (1) is an AC switch that converts the AC power supply to 0N10FF, (2) is a filter that takes into account the noise of the AC power supply, (3) is a rectifier circuit that rectifies the AC input and converts it to a DC level, ( 4) is a power thermistor whose resistance value fluctuates due to temperature changes to prevent inrush current from occurring.

(5) is a discharge resistor that is connected in parallel with the smoothing capacitor (8) and protects the circuit when the capacitor is discharged, (7) is the primary coil of the high-frequency transformer, and (8) is a switching element, with the collector side being the primary side. connected to the coil (7),
The emitter side is connected to the power thermistor (4), and the base side is connected to the control circuit (9), and the control signal from the control circuit (9) controls the 0N10 F F of the negative side coil (7). Performs switching operation.

A device for limiting inrush current by providing a power thermistor in a conventional switching regulator is configured as described above, and FIG. 4 is an explanatory diagram showing the waveform of inrush current.
The operation will be explained below.

First, when you turn on the AC switch, the AC power is converted to DC voltage (-voltage) via the filter (2) and rectifier circuit (3), and the converted primary voltage is applied to the discharge resistor (5). and applied to both ends of the smoothing capacitor (6). At this time, the charging current input to the smoothing capacitor (6) is inversely proportional to the product of the input frequency (f) and the capacitor capacity (C) (1/2πfC), so the impedance of the smoothing capacitor (6) is high and the charging current should be smaller. However, in reality, the timing at which the AC switch is turned on is not constant, so it is impossible to predict at which phase of the AC input waveform the signal will be input. Therefore, as shown in FIG. 4, at time t1, the AC switch (1) is turned on.
When this occurs, a sudden voltage is applied across the smoothing capacitor (6), and since the applied voltage includes harmonics of integral multiples, the impedance of the smoothing capacitor (8) suddenly decreases, causing the smoothing capacitor to The maximum charging current (rush current) flows through (6). This inrush current ends in approximately half a cycle and becomes lower from the second wave, but when this high charging current (maximum inrush current) flows, the AC power supply decreases due to line impedance and enters a momentary sag state. This may adversely affect peripheral devices.

For this reason, a power thermistor (4) is installed on the AC line,
By utilizing the power thermistor's characteristic that its resistance value decreases due to self-heating, the inrush current is limited (as the resistance value of the power thermistor decreases, the voltage drop becomes negligibly small compared to the input voltage) to maximize the inrush current. Generation of current can be prevented. Incidentally, FIG. 4 shows the difference in rush current between the case where the power thermistor (4) is provided in the AC line and the case where the power thermistor (4) is not provided.

[Problems to be Solved by the Invention] In the conventional switching power supply device as described above, a power thermistor (4) is attached to the AC line, and when the temperature rises, this power thermistor (4) decreases its resistance value and reduces the inrush current. However, when the power is on, the AC switch (
1) is set to 0FFL and when the AC switch (1) is turned on again, the self-heating of the power thermistor (4) remains and the temperature does not drop immediately and the resistance value of the power thermistor does not increase, thus limiting excessive inrush current. The problem was that I couldn't do it.

The present invention has been made to solve this problem. Immediately after the AC switch is turned off, the switching operation of the switching power supply device is stopped, and the discharging time is extended due to the time constant of the smoothing capacitor and the discharging resistor. An object of the present invention is to obtain a switching power supply device that can prevent the occurrence of maximum inrush current.

[Means for Solving the Problems] A switching power supply device according to the present invention has an inrush current limiting circuit using a thermistor, in which a smoothing capacitor connected to the primary side of the switching power supply device and a discharging a charging/discharging circuit consisting of a thermal resistor whose time constant is larger than the thermal time constant of the thermistor;
The device includes a switch control circuit that forcibly stops the switching operation of the switching power supply device when the voltage outputted through the charging/discharging circuit becomes less than a certain value when the power is turned off.

[For production]

In the present invention, there is provided a charging/discharging circuit in which the time constant of the smoothing capacitor and the discharging resistor on the negative side is larger than the thermal time constant of the thermistor, and a voltage output through the charging/discharging circuit is a constant value. By providing a switch control circuit that forcibly stops the switching operation of the switching power supply when the following occurs, the switch control circuit stops the switching operation of the switching power supply and charges the smoothing capacitor immediately after the AC power is turned off. Because it remains, even if the AC power is turned on immediately, an excessive rush current will not flow to the smoothing capacitor, and when the AC power is turned off,
Even if you turn on the AC power after a while, the thermistor's thermal time constant is smaller than the time constant of the charge/discharge circuit, so the thermistor returns to its original resistance value and an excessive inrush current occurs. prevent the occurrence of

[Embodiment] FIG. 1 is a block circuit diagram of an embodiment of the present invention.
) to (9) are the same as above.

In the figure, (10) is after turning on the AC switch (1).
This is a switch control circuit provided to prevent excessive inrush current from occurring.
0c), N P N-type transistor (FOE) and P
It is composed of an NP type transistor (10r).

In addition, H is connected to the cathode side of the Zener diode (10a).
An AC line on the OT side is connected, and one end of a bias resistor (tab>) is connected to the anode side.

In addition, the other end of the bias resistor (10b) and the bias resistor (lod) are connected to the base side of the NPN transistor (10e).
is connected to one end of the The other end of the bias resistor (10d) is connected to the AC line on the GND side.

On the other hand, one end of a base resistor (10c) is connected to the collector side of the NPN transistor (TOE), and the emitter side is connected to an AC line on the GND side.

Furthermore, the other end of the base resistor (10c) is connected to the base side of the PNP type transistor (10f), the emitter side is connected to the control circuit (9), and the collector side is connected to the switching element (8). Connected to the base side.

The switching power supply device of the present invention is constructed as described above, and FIG.

First, when you turn on the AC switch (1), the filter (
2) Through the rectifier circuit (3) and smoothing capacitor (6)
When the DC voltage rises, the Zener diode (10a) becomes conductive, the NPN transistor (10e) turns on, and the base of the PNP transistor (IQf) turns on.

On the other hand, since a control signal is output from the control circuit (9) to the collector side via the emitter of the PNP transistor (10f), it controls the base of the switching element (9) and starts the normal operation of the switching regulator. .

Next, when the AC switch (1) is turned off, oscillation continues due to the charge of the smoothing capacitor (6), but the voltage (charge) across the smoothing capacitor (6) decreases and the Zener diode ( When the voltage is below the threshold level of 10a), the NPN transistor (10a)
e) and the PNP transistor (10f) are OFF, and the switching element (8) is turned off. In this case, the switching element (8) is turned off while a sufficient charge remains in the smoothing capacitor (6) to stop the oscillation of the switching regulator.

Therefore, even if the AC switch (1) is turned on again immediately, sufficient charge is stored in the smoothing capacitor (6), so generation of an excessive rush current can be prevented.

In addition, since the smoothing capacitor (6) and the discharging resistor (5) form a closed circuit, the electric charge of the smoothing capacitor (8) is transferred through the discharging resistor (5) as shown in Figure 2. It is discharged gradually, and the discharge time constant may be designed to be larger than the thermal time constant of the power thermistor (4). Also, when the smoothing capacitor (6) loses its capacity due to discharge, the heat generation of the power thermistor (4) decreases and the resistance value returns to its original value and becomes sufficiently large, so the AC switch (1) is turned on again. ) is turned on, no excessive inrush current will occur.

In this way, by preventing the charge of the smoothing capacitor from decreasing rapidly, it is possible to prevent the generation of excessive inrush current even when the AC switch is turned on immediately after the power is turned off, and to prevent the smoothing capacitor from discharging. The time constant of the power thermistor (4) is made larger than the thermal time constant of the power thermistor (4), so even if you turn on the AC switch (1) after a while, the power thermistor (4) will not work and generate an excessive inrush current. It can be prevented.

[Effects of the Invention] As described above, according to the present invention, the time constant of the negative side smoothing capacitor and the discharging resistor is made larger than the thermal time constant of the thermistor, and the switching operation of the switching power supply device is forced. Since it is designed to stop in a short time, even if the power is turned off and then turned on again, there will be no charge remaining in the smoothing capacitor and an excessive rush current will not flow through the smoothing capacitor. Even if the power is turned on again, the thermistor has returned to its original resistance value, making it possible to prevent excessive inrush current from occurring.

[Brief explanation of the drawing]

Fig. 1 is a block circuit diagram of an embodiment of the present invention, Fig. 2 is a charging/discharging characteristic diagram between both ends of a smoothing capacitor, and Fig. 3 is a block circuit 11 in which a power thermistor is provided in a conventional switching regulator to limit inrush current. 4 are waveform diagrams showing inrush current. In the figure, (1) is an AC switch, (2) is a filter, (3) is a rectifier circuit, (4) is a power thermistor, (
5) is a discharge resistor, (6) is a smoothing capacitor, (7) is a negative side coil, (8) is a switching element, (9) is a control circuit, and (1O) is an inrush current limiting circuit. Patent applicant Oki Electric Industry Co., Ltd. Agent Patent attorney Muneharu Sasaki

Claims (1)

[Claims] A switching power supply device having an inrush current limiting circuit using a thermistor, comprising a smoothing capacitor and a discharging resistor connected to the primary side of the switching power supply device, the time constant of which is set to the time constant of the thermistor. a charging/discharging circuit having a thermal time constant larger than a thermal time constant; and a switch control circuit that forcibly stops the switching operation of the switching power supply when the voltage outputted through the charging/discharging circuit becomes below a certain value when the power is turned off. A switching power supply device comprising: