JPS6292615A - Overcurrent protection circuit of power transistor - Google Patents

Abstract

PURPOSE:To improve the protection effect by detecting a voltage of collector- emitter of a power transistor (TR) by a voltage detector so as to detect an overcurrent thereby reducing the interruption time of the overcurrent. CONSTITUTION:In addition to a similar protection circuit as a conventional circuit, a collector-emitter voltage VCE of the power TR 1 is detected by a voltage detector 8, the detected voltage activates a base drive circuit 2 to interrupt the current of the TR 1. The voltage VCE shown a characteristic shown in figure with respect to a collector current IC when the base current is constant. In setting the detection point of the voltage detector 8 to a value within both voltages V1 and V2 where the voltage VCE changes linearly, e.g., a current I1, when the collector current IC of the TR 1 increases from the current I1 and the voltage VCE is increased, the voltage detector 8 detects it and the collector current IC is interrupted via the base drive circuit 2. Thus, not only the interruption time is reduced but also the protection circuit is duplicated.

Description

[Detailed description of the invention] [Technical field to which the invention pertains] The present invention relates to an overcurrent protection circuit for power transistors in devices using power transistors such as transistor inverters. [Prior art and problems thereof] In a device using a power transistor, when the load on the device increases, an overcurrent may flow through the power transistor and cause damage to the power transistor. To prevent this, an overcurrent protection circuit is provided. An example of a conventional power transistor overcurrent protection circuit provided in such a transistor inverter is shown in FIG. In FIG. 3, a power transistor 1 is the main part of a transistor inverter (not shown), and is controlled by a base disturbance circuit 2 to convert a DC voltage into an AC voltage. The train is running. The overcurrent protection circuit of the power transistor 1 includes a current transformer 3 that measures the AC load current, an amplifier 4 connected to the secondary coil of the current transformer 3, and an amplifier 4 that compares the output voltage of the amplifier 4 with a reference voltage. A comparator 5 that generates a signal when the input on the 4 side exceeds a reference voltage, an M circuit 6 that uses the signal of this comparator 5 as one input and the control signal of the power transistor 1 as the other input, and an AC side and It consists of an isolator 7 that insulates the DC side and transmits only signals, and a base drive circuit 2. When the AC load is normal, the output voltage of the transformer 3 is low, so the voltage that is amplified by the amplifier 4 and input to the comparator 5 is 0 does not exceed the reference voltage. Therefore, the comparator 5 does not generate a signal and one input of the AND circuit 6 is at a low level. Therefore, even if the other input is at a high level, the signal from the AND circuit 6 is at a low level, and the base drive circuit 2 controls the power transistor 1 in normal operation and continues to operate as an inverter.

When the load increases and the voltage of the secondary coil of the current transformer 3 rises, this voltage is amplified by the amplifier 4 and applied to the comparator 5, and when this voltage exceeds the reference voltage, the comparator 5 outputs a signal. At this time, if the control signal of the power transistor 1 is input to the AND circuit 6, the AND circuit 6 outputs 7.
・The base drive circuit 2 is operated via the isolator 7 by sending out an irregular signal, and the current of the power transistor 1 is cut off to protect it.

By the way, the overcurrent protection circuit of this power transistor is
The overcurrent of the power transistor is detected by the current transformer on the load side, so even if an overcurrent flows to the power transistor, unless the overcurrent flows to the load, the overcurrent of the power transistor cannot be detected and the power transistor cannot be protected. There is a drawback.

Another drawback is that there is a long delay from the occurrence of overcurrent to the interruption of the base current of the power transistor.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide an overcurrent protection circuit for a power transistor that can quickly cut off an overcurrent.

[Key points of the invention]

In the present invention, in the quasi-saturation region of the power transistor, the collector-emitter voltage with respect to the collector current is approximately IJ =
an overcurrent detector configured to detect an increase in voltage between the collector and emitter within a range where the voltage changes rapidly; and a base drive circuit in which the base current of the power transistor is cut off by a signal from the overcurrent detector. By utilizing the quasi-saturation region in the relationship between the output side current and output side voltage of the power transistor, changes in the collector current can be directly detected by the magnitude of the voltage between the collector and emitter. When the voltage between the collector and emitter exceeds a predetermined value, the base drive circuit operates to cut off the collector current, quickly interrupting the overcurrent.
And we want to make sure.

[Embodiments of the invention]

Embodiments of the present invention will be described in detail below with reference to FIGS. 1 and 2. In FIG. 1, the output side of the power transistor inverter generates an alternating current voltage of a required frequency, and supplies power to a load (not shown) via a current transformer 3. This transistor inverter also has a current transformer 3 and an amplifier 4 as in the conventional case.
, comparator5. A protection circuit for the power transistor is formed by an AND circuit 6, an isolator 7, and a base drive circuit 2. In addition, a voltage detector 8 detects the voltage VCE between the collector and emitter of the power transistor 1.
)-/7'l It differs greatly from the conventional one in that the current of the power transistor 1 is cut off by turning off the current of the power transistor 1 by turning off the current of the power transistor 1.

When the base current is constant, the collector-emitter voltage VCE of the power transistor l exhibits the characteristics shown in Fig. 2 with respect to the collector current. The current Il changes to a holinya, and the voltage ■1. The current I2 is the voltage V2. Therefore, the detection point of voltage detection 'a8 is set to this voltage VCE.
If the voltage is set within the range of 1°V2, for example, the current Il, then when the collector current IC of the power transistor 1 increases by 11, the voltage V(
Since l also increases, the voltage detector 8 detects this and cuts off the collector current Ic of the power transistor 1 via the base drive circuit 2. Of course, current transformer 3, amplifier 4
, the comparator 5, the MO circuit 6, the isolator 7, and the circuits maintained by the base drive circuit 2 are also delayed, but the power transistor 1
It functions as a protection circuit, and is equipped with a double protection circuit.

r Effects of the invention] According to the invention, since the voltage between the collector and emitter of the power transistor is detected by a voltage detector to detect an overcurrent, an increase in the collector current of the power transistor is directly detected, and as a result, The operation is fast because the collector current is cut off.

What's more, current detection is a collector! Since it is set to operate in the quasi-saturation region of the current IC-collector voltage VCI characteristic and in a range where voltage changes are large for small current changes, its protective operation is reliable and highly effective.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of an overcurrent protection circuit for a power transistor according to the present invention, FIG. 2 is a curve diagram showing collector current-collector-emitter voltage characteristics, and FIG.
The figure is a block diagram showing an example of a conventional power transistor overcurrent guarantee circuit. 1...Power transistor, 2...Pace pivot circuit, 8...Voltage detector. l'Engineering・T・・Bi・−几＋、、：ll'/Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1) a voltage detector configured to detect a rise in the voltage between the collector and emitter within a range where the voltage between the collector and emitter changes almost linearly with respect to the collector current in the quasi-saturation region of the power transistor; An overcurrent protection circuit for a power transistor, comprising a base drive circuit that cuts off a base current of the power transistor by a signal from the voltage detector.