JPH11149320A - Protection means for power circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To speedily protect the circuit element of a post-stage for the fault of a main switching element in a step-down chopper-type switching power source. SOLUTION: In the step-down chopper-type switching power source, a commercial power source 1 is rectified, it is converted into a DC power source, it is smoothed by input current regulation resistors 3 and 4 and a capacitor, and stabilized DC voltage output is obtained by a switching part constituted of the main switching element 5, a closure diode 6, a choke coil 7, a smoothing capacitor 8 and a voltage control means detecting output voltage and controlling on/off of a switching part. At least a part of input current regulation resistance is the overcurrent fusing-type resistance 3. A means 10 detecting the voltage rise of the output of stabilized power source, a current bypass route connected to the main switching element 5-side of the overcurrent fusing-type resistance 3, a second switching element 11 connecting the current bypass route to the reference potential side of the power source and constitution which drives the second switching element 11 from the detection result of the voltage rise and makes current sufficient for fusing overcurrent fusing-type resistance 3 flow are provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a protection means for a chopper type power supply circuit.

[0002]

2. Description of the Related Art A chopper type switching power supply is used as a small-capacity on-board power supply for home electric appliances and embedded devices. In particular, step-down power supplies are increasingly configured to obtain a low voltage by directly rectifying a commercial power supply. With this type of switching power supply, a desired stabilized DC power supply can be obtained with a relatively simple circuit configuration.

[0003]

However, since the high voltage of the commercial power supply is directly applied to the switching element for switching, a stress exceeding the rating of the switching element is applied due to noise such as an external surge, and the element is destroyed and its surroundings are destroyed. The circuit may be damaged.

Conventionally, for such a failure of the switching element, an increase in current is detected by using a positive temperature coefficient thermistor, and the resistance of the thermistor is increased, thereby preventing damage to a subsequent circuit element. The protection method against the failure of the switching element using the thermistor with a positive characteristic has a possibility of malfunction because the characteristic itself of the thermistor is affected by the ambient temperature, and is difficult to use when there is not enough room for the ambient temperature. Also, the time until protection is not instantaneous operation due to self-heating. Furthermore, if the change in current before and after the failure of the switching element is relatively small, it may take a long time to operate or may not operate, so that it is often unusable.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and rectifies a commercial power supply to convert it into a DC power supply, smoothes the DC power supply with an input current limiting resistor and a capacitor, and switches its output to a main switching circuit. A step-down chopper-type switching power supply that obtains a stabilized DC voltage output by a switching unit configured by an element, a return diode, a choke coil, a smoothing capacitor, and a voltage control unit that controls on / off of the switching element by detecting an output voltage; A part of the current limiting resistor is an overcurrent fusing resistor, means for detecting a voltage rise of the stabilized power supply output, a current bypass path connected to the main switching element side of the overcurrent fusing resistor, A second switching element that connects a bypass path to a reference potential side of a power supply, and a detection result of the voltage rise, Performs driving of the serial second switching element is obtained by a flow a sufficient current allowed to blow the overcurrent fusing type resistor.

[0006]

According to the present invention, a short-circuit failure of a main switching element can be detected by means for detecting an increase in output voltage, and the main switching element is blown by flowing an excessive current through an overcurrent fusing type resistor.
It is possible to protect the circuit element at the subsequent stage of the main switching element.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 is a main block diagram showing an embodiment of the present invention. In FIG. 1, a commercial power supply 1 is connected to a rectifier 2
And the main switching element 5 is connected through input current limiting resistors (overcurrent fusing resistors 3 and 4).
A freewheeling diode 6 is connected to the output terminal side of the main switching element 5.
Are connected in anti-parallel to the DC power supply, and the choke coil 7
And smoothing with a smoothing capacitor 8 to obtain a stabilized DC voltage output (hereinafter referred to as an output voltage).

The main switching element control circuit 9 controls the on / off of the main switching element 5 by detecting the output voltage, and operates so that the output voltage falls within a certain control voltage.

[0010] The overvoltage detection circuit 10 generates an overvoltage detection signal to the short circuit 11 when the output voltage rises above normal operation. The short circuit 11 is a switch element connected between a connection point of the overcurrent fusing type resistors 3 and 4 and a reference voltage (ground) terminal, and receives a detection signal of the overvoltage detection circuit 10 to conduct.

During normal operation, the main switching element control circuit 9 controls the on / off of the main switching element 5, so that the output voltage keeps a predetermined constant voltage. However, if the main switching element 5 is short-circuited for any reason,
Since the transistor is turned on regardless of the control of the main switching element control circuit 9, the output voltage increases.

FIG. 2 is an equivalent circuit of the power supply circuit when the main switching element 5 is short-circuited. Here, since the main switching element 5 is in a short-circuit state, the freewheeling diode 6 and the choke coil 7 can be neglected and are omitted. Assuming that the equivalent impedance of the power supply circuit is Z, the output voltage of the power supply circuit, that is, the input voltage of the overvoltage detection circuit 10, is obtained by converting the rectified voltage to the input current limiting resistance and the equivalent impedance Z.
And the partial pressure.

FIG. 3 shows a circuit example of the overvoltage detection circuit 10.

In FIG. 3A, a Zener diode 101 and a resistor 102 are connected in series, the cathode side of the Zener diode 101 is an output voltage terminal, the lower end of the resistor 102 is a reference voltage side, and the middle point is an overvoltage detection output. . FIG.
(B) is an example of a detection operation characteristic of the present circuit. When the output voltage exceeds the Zener voltage of the Zener diode 101, the overvoltage detection output increases. If the stabilizing power supply circuit is not operating normally and the Zener diode 101 that can detect the voltage rise when the main switching element 5 is short-circuited is selected, the protection operation is activated by the overvoltage detection output. It becomes possible.

FIG. 4 shows an example of a short circuit 11 for flowing a current sufficient to blow the overcurrent fusing type resistor 3. FIG.
In the case where the thyristor 111 is used as a short-circuit switch element. The anode terminal of the thyristor 111 is connected to the load side of the overcurrent fusing resistor 3, and the cathode terminal is connected to the reference potential. The gate terminal is connected to the overvoltage detection output via the resistor 112. Also, a resistor 113 is connected between the gate terminal and the reference potential to prevent erroneous firing. In the case of this circuit example, once the thyristor 111 fires, the ON state continues until the overcurrent fusing type resistor 3 is blown. As another method, a transistor can be used as a short-circuit switch element.

FIG. 5 shows a characteristic example of the overcurrent fusing type resistor 3. FIG. 5A shows an example of the fusing characteristic when the load factor is about ten and several times higher than the power rating of the resistor, and FIG. 5B shows an example of the fusing characteristic when the load factor is about 100 times. If the minimum voltage for the protection operation and the normal current value under normal conditions are determined and the resistance value is appropriately selected, the fusing characteristics shown in FIG. 5B can be applied, and the current can be interrupted within several tens of cycles of the commercial power supply. Can be.
Furthermore, if the current flowing through the thyristor 111 is the number of cycles within the specified value of the surge-on current rating of the thyristor, a thyristor having a relatively small current rating can be used, so that the cost can be reduced.

FIG. 6 shows an example of the timing from the short-circuit failure of the main switching element 5 to the fusing of the overcurrent fusing type resistor 3. Vout is an example of the output voltage of the power supply circuit, the output of the overvoltage detection circuit 10, and the current flowing through the overcurrent fusing type resistor. In the figure, at time T0, the main switching element 5
At the time T1, the overvoltage detection output starts to increase, and at the time T2, the thyristor 111 is ignited and the current starts flowing through the overcurrent fusing type resistor 3. At the time T3, the overcurrent fusing type resistor 3 is blown. ,. Io is a normal current at normal time, Is is a current value determined by the resistance value of the overcurrent fusing type resistor 3 and the voltage of the commercial power supply 1, and Vmax is a maximum voltage applied to a subsequent circuit when the power supply circuit fails.

After time T2, the current path is directly at the reference potential through the overcurrent fusing type resistor 3, so that the input voltage of the switching element becomes almost 0V, and the voltage applied to the subsequent circuits also becomes almost 0V. Therefore, the overvoltage state applied to the circuit element can be performed in a short time, and damage due to the overvoltage state can be minimized. Also, if components having a voltage rating with a margin are used, that is, by using components having a voltage rating equal to or higher than Vmax, after a short-circuit fault occurs in the main switching element 5 and before an overvoltage is applied to a subsequent circuit component. Therefore, the only components that need to be replaced are the main switching element 5 and its control circuit 9.

[0019]

According to the present invention, in a step-down chopper type switching power supply, a short-circuit failure of a main switching element can be detected, and a current path can be quickly and reliably interrupted. Damage can be minimized. This is an advantage in that protection with a positive temperature coefficient thermistor or the like as a protection element takes a relatively long time, and has an advantage that the influence of the ambient temperature is small.
Further, even if the change in current before and after the short-circuit failure of the main switching element has little change, the failure can be detected. In addition, the required elements can be composed of overcurrent fusing type resistors that also serve as input current limiting resistors, switch elements such as thyristors, and zener diodes, so that the number of parts can be reduced and the cost can be reduced. Can be.

[Brief description of the drawings]

FIG. 1 is a main part block circuit diagram showing one embodiment of the present invention.

FIG. 2 is an equivalent circuit at the time of the failure.

FIG. 3 is an example of the overvoltage detection circuit, and FIG. 3B is an example of a detection operation characteristic of the circuit.

FIG. 4 is an example of the short circuit.

FIG. 5 is an example of fusing characteristics of the overcurrent fusing type resistor.

FIG. 6 is an operation timing chart of the same.

[Explanation of symbols]

 3 Overcurrent fusing type resistor 5 Main switching element 10 Overvoltage detection circuit 11 Short circuit (switch element)

Claims (1)

[Claims] 1. A commercial power supply is rectified and converted into a DC power supply,
The output is stabilized by an input current limiting resistor and a capacitor, and its output is stabilized by a switching unit composed of a main switching element, a return diode, a choke coil, a smoothing capacitor, and a voltage control means for detecting an output voltage and controlling on / off of the switching element. (10) In a step-down chopper type switching power supply for obtaining a converted DC voltage output, at least a part of the input current limiting resistor is an overcurrent fusing type resistor (3), and means for detecting a voltage rise of the stabilized power supply output (10).
A current bypass path connected to the main switching element (5) of the overcurrent fusing resistor (3); and a switch element (11) connecting the current bypass path to a reference potential side of a power supply. A protection means for a power supply circuit, wherein the switch element (11) is driven based on the detection result of the voltage rise, and a current sufficient to blow the overcurrent fusing type resistor (3) flows.