JPS58182920A - Overcurrent protection system of power switching transistor - Google Patents

Abstract

PURPOSE:To narrow the safe operation range of a power transistor (TR) in a turn-off stat, by stopping or reducing the supply of a base reverse bias current when an overcurrent flows. CONSTITUTION:When a current flowing through the power transistor TR1 is not an overcurrent, an overcurrent detecting circuit 10 outputs a signal with logic 1 to open AND gates A1 and A2. Therefore, a TR3 or TR4 turns on according to the logic 1 or 0 of a base driving signal to flow a forward bias current or base backward bias current to the TR1. If an overcurrent state occurs, the circuit 10 outputs 0 to close the gates A1 and A2, so neither the TR3 nor TR4 turns on regardless of the base driving signal. Therefore, the TR1 is supplied with neither the forward nor backward bias current. Consequently, the safe operation range of the power TR in the turn-off state is widened as much as posible to protect the TR1 against an overcurrent securely.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an overcurrent protection system for a power switching transistor, and furthermore, when turned on, a pace forward bias current is supplied by a pace drive circuit, and when turned off, a base reverse bias current is supplied. The present invention provides an overcurrent protection method for power switching transistors.

Generally, in a power switching transistor (hereinafter simply referred to as a power transistor), a pace drive circuit is used to improve switching characteristics, and when the transistor is turned on, a pace forward bias current is supplied from the drive circuit, and one transistor is turned off. Sometimes a base reverse bias current is supplied to shorten the turn-off time.

Although the turn-off time is shortened by increasing the base reverse bias current in this way, it is known that the extent to which the base reverse bias current is increased narrows the safe operation area of the power transistor during turn-off.

FIG. 1 is an explanatory diagram of a safe operating area of a power transistor. The horizontal axis shows the collector voltage VCI of the F transistor, and the vertical axis shows the collector current Ic. Now, when the value of the Haese reverse bias current II is l1ll, the safe operating area of the power transistor is In Fig. 1, it is assumed that the region surrounded by R-+ 8-+U→■→Q-+R is 0, that is, at turn-off, the relationship between collector current Ic and collector voltage Vc in the power transistor is as described above. If it is within the range, the transistor will not be destroyed. Now, change the value of the base reverse bias current IIB from l1ll to Ill! 1The above safe operating area is R-+8-*
It narrows down to the area surrounded by T-*P→Q→R.

Conventionally, when turning off the power transistor ★, a sufficiently large base reverse bias current has been supplied not only in normal times but also in the event of an overcurrent to shut off the power transistor at high speed. As a result, as explained with reference to FIG. 1, as the base reverse bias current IBH increases, the safe operating area is narrowed, and the operation locus at turn-off of the power transistor falls within this narrowed safe operating area. It was necessary to design the equipment accordingly. As a result, the main circuit configuration of the power transistor becomes more simplistic, or the loss generated by the snubber circuit (not shown) increases.
) A serious drawback arose, such as the inability to fully utilize the capabilities of the transistors.

For example, in the explanatory diagram of the safe operating area shown in FIG.
Let's consider a case where the power transistor operates along an operating trajectory of 0-4P - + Q at turn-off.

It is known that a snubber circuit (not shown) designed to obtain such an operating trajectory generates the least loss. When collector current Ic increases beyond 0 point, r
If we try to follow the operating trajectory of tIJ4Im, we will deviate from the safe operating area for the base reverse bias current Ill''IBI12. Therefore, the operating trajectory 0-+ P-+
It can be seen that the value of the collector current 1c at the 0 point, which is the starting point of Q, must not exceed this value even in the case of an overcurrent. As a result, it becomes necessary to set the collector current 0 during normal operation to a value even lower than the value indicated by point 0, and the range of application of the power transistor becomes extremely narrow. In this way, in the conventional overcurrent 11m1 method, a large base reverse bias current is supplied even in the event of an overcurrent, so there is a problem that the safe operating area at turn-off of the power transistor is always used. 0j12WJ is a circuit diagram showing a conventional current-carrying IL protection method for a power transistor. In the figure, l is a power transistor (main transistor), 3.4 is a control transistor, 5 and 6 are ft'L pulse transformers, 7.8 is a diode, 9 is a current limiting resistor, and 10 is a In the overcurrent detection circuit, CT is a current transformer, A is an AND gate, and ■ is an inverting circuit. In addition, overcurrent detection
The circuit 10 detects the current flowing through the power transistor 1 using the current threshold 11cT, and when it is overcurrent, it outputs a logic "0" and closes the AND gate, and when there is no overcurrent, it outputs a logic - 1" to open AND gate A.

Next, the operation will be explained. As mentioned above, when the current flowing through the power transistor 1 is not in an overcurrent state, the gate A is open, so it cannot be used as a pace drive signal.
When the @1' signal is applied to the other input side of the AND gate A, a logic 11 "l" output is generated from the AND gate A. This output makes transistor 3 conductive, so the pulse transformer is removed from the pace drive power supply. A drive current flows through the primary side of the pulse transformer 5, and as a result, a forward bias current flows through the power transistor 1 through the secondary side of the pulse transformer 5, the diode 71, and the resistor 9, and the power transistor 1 is in an on state.

Then join the gauge people and the base drive hurts but logic''O
'', the output of the AND gate A also changes to ``aug'', so the conduction of the transistor 3 stops and the pace forward bias current is no longer supplied to the power transistor 1. Since the transistor 4 becomes conductive due to the signal, a drive current flows from the pace drive power supply to the primary side of the pulse transformer 6. Therefore, the pace reverse flow is transmitted to the power transistor 1 via the secondary side of the pulse transformer 6 and the diode 8. Bias current flows%
When the overcurrent detection circuit 10, which turns off the transistor l, detects an overcurrent, as described above, the gate A is closed and outputs the logic im"o'".
As in the case where the base drive signal becomes 1I'''0'', a reverse base bias current flows through power transistor 1, causing transistor 1 to be shut off at high speed if it is still in the on state. Yes As mentioned above, in the conventional overcurrent protection method for power transistors, when an overcurrent occurs, a base reverse bias current flows through the power transistor, which narrows the safe operating area when the power transistor turns off. As a result, various disadvantages such as complication of the main circuit configuration of the power transistor, increased loss generated by the snubber circuit, and insufficient utilization of the power transistor's ability can occur. also explained.

The present invention has been made in order to eliminate the drawbacks of the prior art as described above, and therefore, an object of the present invention is to provide a power transistor in which the safe operating area is not compromised under any circumstances. 0 In devices using power transistors, it is common to stop the device when an overcurrent occurs, so there is no need to shut off the power transistor at high speed. In view of this, the present invention protects the power transistor from overcurrent without interfering with the safe operating area during turn-off of the buff transistor by stopping or reducing the supply of base reverse bias current at the time of overcurrent. The above objectives have been achieved.

No. 311EI is a circuit diagram showing an embodiment of the present invention.
The circuit configuration shown in ri4WA is different from the conventional circuit shown in Figure 2.
The difference from the circuit is that there are two antenna gates (A1 and The point is that A2 and the inverting circuit 1 are connected as shown.

Explain the operation. When the current flowing through power transistor 11 is not in an overcurrent condition, overcurrent detection path 10 is in logic 11.
Since the @l" signal is output and the AND gates Al and A2 are opened, the base drive signal is 111@l".
0'', either transistor 3 is conducting and a base forward bias current flows through power transistor 1, or transistor 4 is conducting and a base reverse bias current flows.Then an overcurrent condition occurs. When an overcurrent occurs, the overcurrent detection circuit 10 outputs logic m″'0” and the AND gauge A
Since transistors 1 and A are closed, neither transistors 3 nor 4 are conductive regardless of the base drive signal, so power transistor 1 is supplied with neither base forward bias current nor reverse bias current.

FIG. 4 shows the base drive signal, the collector current of power transistor 1, O, and power transistor 1 in the circuit of FIG.
2 is a wave'M diagram showing waveforms of a base forward bias current and a reverse bias current applied to the base. FIG. In the same figure, during normal times,
It is shown that base forward bias current and reverse bias current are supplied, and in the event of overcurrent, supply of pace forward bias current and reverse bias current is stopped.

FIG. 5 is a circuit diagram showing another embodiment of the present invention. In the figure, a PNP transistor 12 and a resistor 11 are connected between the pace drive power source and one end of the primary side of the pulse transformer 6.
The circuit configuration shown in FIG. This is different from the conventional circuit shown in FIG.

Explain the operation. When there is no overcurrent condition and the detection circuit 10 is outputting the logic "III" signal, the AND gate A opens and the PNP transistor 12 to which the logic "o" signal is applied by the inversion circuit 11 becomes conductive. Resistance 11
Since the circuit is short-circuited, the operation at this time is the same as that of the conventional enclosure shown in the second WJ. Next, when an overcurrent condition occurs and the detection circuit 10 outputs a logic m"o" signal, the AND gate A closes and the transistor 12 also becomes non-conductive.

Therefore, the transistor 4 becomes conductive, and the current flowing from the base drive current through the primary side of the pulse transformer 6 is reduced by the resistor 11. As explained above, according to the present invention, the value of the base reverse bias current supplied to the power transistor 1 is reduced without impairing the switching characteristics of the power transistor during normal operation.
By cutting off the base forward bias current to the power transistor and stopping the base reverse bias current supply or reducing the reverse bias current only when an overcurrent occurs, the power transistor can widen the safe operating area at turn-off as much as possible. This has the effect of reliably protecting the device from overcurrent. The present invention can be applied to all semiconductor conversion devices using power transistors as switching elements.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the safe operating area of a power switching transistor, Fig. 2 is a rigid diagram showing a conventional overcurrent protection method for a power transistor, 11311 is a circuit diagram showing an embodiment of the present invention, and 4th ffi is A waveform diagram of each part signal in the circuit of FIG. 3, and a fifth WJ is a circuit diagram showing another embodiment of the present invention. Code explanation 1...Power transistor, 3, 4...
・Control transistor, 5, 6...Pulse transformer, 7゜8...Diode, 9.11...
... Current limiting resistor, 10... Overcurrent detection circuit, 12... Transistor for curved bypass, A e
Al e A2...and gate, '*'
1 e '2... Four-turn circuit representative Santo Patent attorney Namiki
Akio Agent Patent Attorney Matsuzaki Seikan Figure 1 □ VCE (V Book 2 Figure 0 113 Figure Bay J ( ■ Shi 1411

Claims (1)

[Claims] 1) In a power switching transistor drive system consisting of a power switching transistor and a pace drive circuit that supplies a pace j[bias current to the transistor when the transistor is turned on and supplies a base reverse bias current when the transistor is turned off. , an overcurrent detection means for detecting when the current flowing through the transistor becomes an overcurrent, and an overcurrent detection signal from the detection means to control the pace drive Kamiroji to control the pace forward bias current. 1. An overcurrent protection system for a power switching transistor, comprising pace drive circuit control means that not only stops the supply of base reverse bias current but also stops or reduces the supply of base reverse bias current.