JPS59216420A - Overcurrent protecting device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an overcurrent protection device that can be used in a DC-DC converter.

Conventional configuration and its problems FIG. 1 shows an example of a conventional overcurrent protection device.

This shows the case of a switching type power supply device that always keeps the output voltage constant despite fluctuations in input voltage. In the figure, 1 is a DC power supply, 2 is a converter transformer, and 3 is a switching operation. transistor to do,
Reference numeral 4 indicates a drive circuit that generates a drive pulse for controlling conduction and interruption of the transistor 3, and 5 and 6 indicate a diode and a capacitor that rectify and smooth the output generated from the secondary winding of the converter transformer 2, and this rectifier and smoother circuit A DC output voltage of 1 is supplied to the load 7. In addition,
Although not shown, the pulse width of the drive pulse is controlled by detecting the DC output voltage in order to stabilize the DC output voltage, comparing the detected output with a reference voltage, and feeding back the comparison output to the drive circuit 4. to stabilize the output voltage.

Furthermore, in order to protect the circuit from overcurrent, in the circuit shown in Figure 1, a resistor 8 for overcurrent detection is connected in series with the emitter of the transistor 3, and the voltage across this resistor 80 is directly detected, or Integrate and detect the integral output,
In the event of an overcurrent, the protection circuit 9 is operated and its output stops the operation of the drive circuit 4.

However, although this method works reliably, since the resistor 8 is inserted in series in the loop of the primary circuit of the converter transformer 2, power loss occurs due to this resistor 8, which inevitably lowers the conversion efficiency of the circuit. There is a drawback.

OBJECTS OF THE INVENTION In view of the conventional drawbacks as described above, an object of the present invention is to provide an overcurrent protection device that can protect a circuit from overcurrent without reducing conversion efficiency.

Composition of the Invention In the present invention, the voltage between the electrodes of a switching element such as a bipolar transistor or a power MOS field effect transistor connected to the primary winding of a converter transformer has a characteristic that changes depending on the current flowing through the switching element. Focused on.

The voltage across the switching element is detected during its conduction period, the detected voltage and the reference voltage are compared in a comparison circuit, and when the detected voltage exceeds the reference voltage, the protection circuit is activated by the comparison output. Therefore, the desired protection operation can be obtained without reducing the conversion efficiency.

DESCRIPTION OF EMBODIMENTS An overcurrent protection device according to an embodiment of the present invention will be described below with reference to FIGS. 2 and 3. In the drawings, elements that perform the same functions as those in FIG. 1 are designated by the same reference numerals.

In the circuit shown in FIG. 2, the drain of an MO3 field effect transistor 1o is connected to the collector of a switching transistor 3 consisting of a bipolar transistor or a power MOS field effect transistor, and the source of the field effect transistor 10 is connected to a resistor 11 and a capacitor 1.
It is connected to one input terminal of the comparator circuit 13 through an integrating circuit composed of two components.

The gate of the field effect transistor 10 is connected to the output terminal of the drive circuit 4 together with the base of the switching transistor 3, and both transistors 3.10 are applied with the same drive pulse.

The comparison circuit 13 applies a reference voltage set by a variable resistor 14 to the other terminal, and the output of the comparison circuit 13 is applied to the protection circuit 15.

In the above configuration, the field effect transistor 1o conducts and cuts off in synchronization with the switching transistor 3, and as shown in FIG. Take it out.

In the steady state of operation, the reference voltage of the comparator circuit 13 is set to be higher than the comparison input voltage supplied through the integrating circuit, so the protection circuit 15 does not operate and the switching power supply stops operating. Never.

On the other hand, abnormal current (overcurrent) such as when a load short-circuits, etc.
When the current flows through the transistor 3, the current flowing through the transistor 3 also increases, its collector-emitter voltage vCE_SAT becomes larger, and the comparison input voltage after integration becomes larger than the reference voltage. As a result, the output of the comparator circuit 13 is reversed from that in the normal operating state, the protection circuit 16 operates, and the operation of the drive circuit 4 is stopped. Now, let us assume that the average current flowing through the primary side of the converter transformer 2 is the resistance 8 in Figure 1.
When the value of is R, the configuration shown in FIG. 2 can reduce the conversion loss by 2R.

In the above configuration, the resistor 11 and capacitor 12 are as follows:
The protection circuit is installed to prevent the protection circuit from malfunctioning due to surge voltage generated when a power switch (not shown) is closed. If the length is made short and the influence of the rise and fall of the transistor 3 is eliminated, the integration circuit described above is unnecessary. Further, the field effect transistor 10 is not limited to this, and any other circuit or element can be used as long as it is a circuit or element that can supply a voltage proportional to the voltage generated across the switching transistor 3 to the comparator circuit 13. An element may also be used.

Effects of the Invention As described above, according to the present invention, the voltage between the electrodes of the switching element constituting the power supply device is detected during the conduction period of the switching element, and when this detected voltage becomes larger than the reference voltage for comparison, By activating the protection circuit, the circuit can be protected from overcurrent without reducing conversion efficiency.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a conventional overcurrent protection device; Figure 2 is a circuit diagram of a conventional overcurrent protection device;
The figure is a circuit diagram of an overcurrent protection device according to an embodiment of the present invention, and FIG. 3 is a waveform diagram for explaining the operation of the device. 2...Converter transformer, 3...Switching element, 4...Drive circuit, 5...
... Rectifier diode, 6 ... Smoothing capacitor, 10 ... Field effect transistor, 13
... Comparison circuit, 14 ... Variable resistor,
16... Protection circuit. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2 Figure 3

Claims (2)

[Claims] (1) Providing a detection means for detecting the inter-electrode voltage of the switching element during the conduction period in synchronization with the conduction interruption of the switching element constituting the switching power supply,
An overcurrent protection device configured to apply this detected voltage to an overcurrent protection circuit and operate the overcurrent protection circuit when the detected voltage is larger than a predetermined reference voltage. (2) The detection means includes a switching means that takes out a voltage proportional to the voltage between the electrodes of the switching element at the full output terminal, and the conduction period of the switching means is made shorter than the conduction period of the switching element. An overcurrent protection device according to claim 1.