JPH0246165A - Power supply circuit - Google Patents

Abstract

PURPOSE:To suppress a rush current by connecting a negative characteristic thermistor serious with a switching element which performs opening and closing of a power supply output. CONSTITUTION:In a switching power supply circuit, a switching transistor(Tr) Q1 is connected with the primary winding of a transformer T and said TrQ1 is ON-OFF controlled by a switching controller 1. Also, rectifier circuits D1-D2 and smoothing circuits C1-C2 are connected with the secondary side of said transformer T. Then, a switching element Q2 composed of a negative characteristic thermostor RT and a power MOSFET is series-connected between said rectifier and smoothing circuits D2, C2 and a power output end V0. Further, a thermostat TH is connected between said rectifier and smoothing circuits D1, C1 and a TrQ2 gate. Thus, the service current to a load is limited, when the TrQ2 is once turned OFF at the abnormal time and thereafter is again turned ON as the abnormal condition is removed.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention relates to a power supply circuit in which an output line is provided with a switch element that cuts off output in the event of an abnormality.

(b) Prior Art Conventionally, for example, switching power supply circuits are provided with various protection circuits. Among these, an overcurrent protection circuit and an overheat protection circuit are generally provided to particularly protect the power supply device itself.

FIG. 3 shows an example of a conventional general switching power supply circuit. In the figure, T is a transformer, the primary winding of which is connected to a switching transistor Q1, and a switching controller 1 controls on/off of the switching transistor Q1. A diode D is installed on the secondary side of the transformer T.
A rectifying and smoothing circuit including capacitors C1 and C2 is provided. D2. A switching element Q2 consisting of a power MO3FET is connected between the rectifying and smoothing circuit consisting of C2 and the power supply output terminal Vo. A thermostat T is connected between the rectifying and smoothing circuit consisting of DI and C1 and the gate of 02.
H is connected. Furthermore, a resistor R1 is connected between the gate and source of C2. Further, a resistor R2 for detecting the load current is inserted in the load current loop.

Normally, the thermostat TH is in the on state, and accordingly, the thermostat C2 is in the on state. Then, the switching controller 1 detects the voltage at point b and performs switching control of Ql so that the voltage becomes a constant value. When the power supply becomes overheated for some reason, the thermostat TH turns off. This turns off C2 and cuts off the power supply to the load.

As described above, conventional general power supply circuits are equipped with a protection circuit that cuts off output in the event of an abnormality such as overheating.

(C1 Problem to be Solved by the Invention However, conventional power supply circuits equipped with a circuit that cuts off output in the event of an abnormality have the following disadvantages.

In the example shown in Figure 3, the sensor is a thermostat) T
When thermostat TH is switched from off to on, if capacitive load 2 is connected as a load as shown in the figure, it turns off when it is overheated and turns on when it is in a steady state. , C2 is turned on, and charging current IO flows through load 2 as an inrush current. FIG. 4 shows the change in the load current Io immediately after the thermostat TH is turned on in that case. Since this current passes through the overcurrent detection resistor R2, the switching controller 1 detects the potential change at point a, and if the value (absolute value) exceeds a certain value, it operates in an overcurrent state and reduces the output. let If this overcurrent protection is an automatic recovery type, the output will be temporarily reduced, and if it is a shaft down type, the output will remain reduced unless the AC power is turned on again.

As described above, there were cases in which normal startup was not possible when the overheating protection operation was canceled. Furthermore, the inrush current is caused by the switching element Q2.
Since the 02 is likely to be destroyed by this current, a switch element with a high maximum rated current must be used to ensure reliability.

SUMMARY OF THE INVENTION An object of the present invention is to provide a power supply circuit that eliminates the above-mentioned problem by preventing inrush current even when a capacitive load is connected when a protective operation during an abnormality is released.

(Means for Solving Problem d1) In this invention, a switch element for opening and closing the power output is connected to the output line of a power supply, a sensor is connected to the control circuit of this switch element, and the switch element is turned off in the event of an abnormality. A power supply circuit equipped with an output cutoff circuit at the time of abnormality, characterized in that a negative characteristic thermistor is connected in series to the switch element.

(e) Function In the power supply circuit of the present invention, a negative characteristic thermistor is connected in series with a switch element that opens and closes the power output. Therefore, when the switch element is temporarily turned off in an abnormal state, and then the abnormal state is canceled and the switch element is turned on again, the negative characteristic thermistor will remain at the ambient temperature (room temperature).
Since the temperature is close to that of , the resistance value is high and the current supplied to the load is limited. Thereafter, the negative characteristic thermistor self-heats due to Joule heat, the resistance value quickly decreases as the temperature rises, and the load supply current becomes steady. In this way, inrush current is suppressed (f) Embodiment FIG. 1 is a circuit diagram of a switching TL source circuit which is an embodiment of the present invention. Here, T is a transformer, part 1
A switching transistor Q1 is connected to the next winding, and a switching controller 1 controls on/off of Q1.

Connected to the secondary side of the transformer T are a rectifier circuit made up of DI, C2 and a smoothing circuit made up of C1 and C2. Further, a switching element Q2 consisting of a negative characteristic thermistor RT and a power MO3FET is connected in series between the rectifying and smoothing circuit consisting of C2 and C2 and the power supply output terminal Vo. Dl, C1
A thermostat TH is connected between the rectifying and smoothing circuit consisting of the above circuit and the gate of C2, and a resistor R1 is connected between the gate and source of C2. Furthermore, it operates like a resistor R2 for overcurrent detection during the load current loop. First, in a steady state, the thermostat TI is in the on state, the gate-source voltage of C2 is high (becomes), and C2 is in the on state.

At this time, the switching controller 1 detects the voltage at point b and performs switching control of Ql so that the voltage becomes a predetermined value. As a result, D2. The voltage rectified and smoothed by C2 is supplied to the load from the output terminal 0 via RT and C2. If the power supply device becomes overheated for some reason and the thermostat Tll is turned off, the gate potential of C2 becomes the same as the source potential, and C2 is turned off. This cuts off the power supply to the load. After that, when the power supply device is naturally cooled and the thermostat TH is turned on again, C2 is turned on, but at this point, the negative characteristic thermistor RT is also naturally cooled and exhibits a high resistance value. Therefore, even if capacitive load 2 is connected,
The charging current 1o is limited by the thermistor RT.

After that, the RT self-heats due to Joule heat, its resistance value decreases rapidly, and the restriction on the load supply current is released.

Figure 2 shows the load current of 1o immediately after the thermostat TH is turned on.
It shows the change in In this way, even when a capacitive load is connected, its charging current is limited, so the first
The potential change at point a in the figure is small, and the switching controller 1 does not perform overcurrent protection. Furthermore, the maximum current value flowing through the switch element is suppressed.

(Effects of the Invention As described above, according to the present invention, the inrush current flowing to the load is suppressed when the protective operation at the time of an abnormality is canceled,
The maximum current value of the switch element can be suppressed, and its destruction can be prevented. Furthermore, in a power supply circuit equipped with an overcurrent protection circuit, malfunction of overcurrent protection due to rush current can be prevented.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a switching power supply circuit according to an embodiment of the present invention, and FIG. 2 is a diagram showing changes in load current of the switching power supply circuit. FIG. 3 is a circuit diagram of a conventional switching power supply circuit, and FIG. 4 is a diagram showing changes in load current of the conventional switching power supply circuit. RT - negative characteristic thermistor, C2 - switch element, TH - thermostat, 2 - capacitive load.

Claims (1)

[Claims] (1) A power supply circuit in which a switch element that opens and closes the power output is connected to the output line of the power supply, a sensor is connected to the control circuit of this switch element, and an abnormality output cutoff circuit that turns off the switch element in the event of an abnormality. A power supply circuit characterized in that a negative characteristic thermistor is connected in series to the switch element.