JPH0474929B2 - - Google Patents

Description

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

Conventional structure and problems thereof 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 the input voltage. In the figure, 1 is a DC power supply, 2 is a converter transformer, and 3 is a switching operation 4 is a drive circuit that generates a drive pulse to control the conduction/cutoff of this transistor 3; 5 and 6 are diodes and capacitors that rectify and smooth the output generated from the secondary winding of the converter transformer 2; The DC output voltage of the rectifier and smoothing circuit is supplied to the load 7. Although not shown, in order to stabilize the DC output voltage, the DC output voltage is detected, the detected output is compared with a reference voltage, and the comparison output is fed back to the drive circuit 4, thereby controlling the pulse width of the drive pulse. is controlled to stabilize the output voltage.

Furthermore, for circuit protection from overcurrent, the first
In the circuit shown in the figure, a resistor 8 for overcurrent detection is connected in series with the emitter of the transistor 3, and the voltage across this resistor 8 is directly detected, or it is further integrated and the integrated output is detected to detect the overcurrent. At times, the protection circuit 9 is operated, and the drive circuit 4 is activated by its output.
I am trying to stop the operation.

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 the conversion effect.

Structure of the Invention In the present invention, a bipolar transistor connected to the primary winding of a converter transformer and a power
Focusing on the fact that the voltage between the electrodes of a switching element such as a MOS field effect transistor changes depending on the current flowing through the switching element, the voltage across the switching element is detected during the conduction period of the switching element, and this detected voltage and a reference voltage in a comparator circuit, and when the detected voltage exceeds the reference voltage, the comparison output is used to operate the protection circuit, allowing the desired protection operation to be performed without reducing conversion efficiency. This is something that can be obtained.

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 a MOS field effect transistor 10 is connected to the collector of a switching transistor 3 made of a bipolar transistor or a power MOS field effect transistor, and the source of the field effect transistor 10 is made of a resistor 11 and a capacitor 12. It is connected to one input terminal of the comparator circuit 13 through an integrating circuit. 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 the same drive pulse is applied to both transistors 3 and 10. The comparison circuit 13 applies a reference voltage set by the variable resistor 14 to the other input terminal, and the output of the comparison circuit 13 is applied to the protection circuit 15.

In the above configuration, the field effect transistor 10 is turned on and off in synchronization with the switching transistor 3.
Collector-emitter voltage during the period when transistor 3 is turned off and conductive as shown in Figure 4
Takes out a voltage proportional to V _{CE SAT} to the source. In the steady operating state, the reference voltage of the comparator circuit 13 is
Since the comparison input voltage is set to be higher than the comparison input voltage supplied through the integrating circuit, the protection circuit 15 does not operate and the operation of the switching power supply device does not stop.

On the other hand, when an abnormal current (overcurrent) flows, such as during load shot, the current flowing through transistor 3 also increases, its collector-emitter voltage V _{CE SAT} increases, and the comparison input voltage after integration becomes becomes higher than the reference voltage. As a result, the output of the comparator circuit 13 is inverted from that in the normal operating state, the protection circuit 15 is activated, and the operation of the drive circuit 4 is stopped. Now converter transformer 2
If the average current flowing through the primary side of the converter is I, and the value of the resistor 8 in FIG. 1 is R, then the configuration shown in FIG. 2 can reduce the conversion loss by I ² R.

Note that in the above configuration, resistor 11 and capacitor 1
2 is set to prevent the protection circuit from malfunctioning due to a surge voltage generated when a power switch (not shown) is closed. For example, the drive period of the field effect transistor 10 is set to The drive period of
If the influence of the rise and fall of the transistor 3 is eliminated, the integration circuit described above is unnecessary.
Furthermore, the field effect transistor 10 is not limited to this, and may be any other circuit or element as long as it can supply a voltage proportional to the voltage generated across the switching transistor 3 to the comparator circuit 13. It may 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]

Fig. 1 is a circuit diagram of an overcurrent protection device in a conventional example, Fig. 2 is a circuit diagram of an overcurrent protection device in an embodiment of the present invention, and Fig. 3 is a waveform diagram for explaining the operation of the same 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, 15... protection circuit.

Claims (1)

[Scope of Claims] 1. A detection means is provided 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 device, and this detected voltage is transmitted to the overcurrent protection circuit. An overcurrent protection device configured to operate the overcurrent protection circuit when the detected voltage is larger than a predetermined reference voltage in addition to the detection voltage. 2. A patent claim in which the detection means includes a switching means for outputting a voltage proportional to the voltage between the electrodes of the switching element at the output terminal, and the conduction period of the switching means is made shorter than the conduction period of the switching element. The overcurrent protection device according to item 1.