JPH0661815A - Overload protection circuit for semiconductor element - Google Patents

Abstract

PURPOSE:To protect a semiconductor element from overload by allowing a switch drive circuit to drive an equivalent switch to select the connecting state when the semiconductor element is overloaded due to a short-circuit fault of a transmission line. CONSTITUTION:A semiconductor element protection means 3 is inserted between an input terminal 10 and an output terminal 12 of a semiconductor element 1 and provided with an equivalent switch 31 cut off usually and selected to the connecting state only when the element 1 is overloaded with an equivalent switch drive circuit 32 driving the changeover operation of the switch 31. While the load of the element 1 is not faulty, the circuit 32 keeps the equivalent switch 31 to be cut off but drives the equivalent switch 31 to be in the connecting state when the element 1 is overloaded due to a short-circuit fault of a transmission line. When the equivalent switch 31 is set to the connecting state, since almost all load currents flow to the equivalent switch 31, the element 1 is protected from the overload.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for protecting a semiconductor element provided with a follower type connection from overload.

[0002] Here, a follower type connection means a connection of a semiconductor element defined in the following note. Conventionally, no special consideration has been given to the overload of a follower-type connected semiconductor element. However, if the load is installed in a remote place, the probability of a short-circuit accident increases, and in the follower type semiconductor device, the power supply terminal is directly connected to the voltage source and the load resistance, that is, the safety resistance is inserted. Therefore, there is a risk of burnout due to overcurrent, and the need for overload countermeasures is increasing.

[Note] Definition of follower type connection: At least a first electrode serving as a supply source of a carrier (electrically conductive carrier), a second electrode serving as a receiving / receiving destination of the carrier, and a transmission amount of the carrier are controlled. A semiconductor amplifying device including a third electrode, wherein a voltage source is directly connected to the second electrode, an input signal source is connected to the third electrode, and a load is connected to the first electrode. How to connect. Specifically, there are cathode follower connection of vacuum tube, emitter follower connection of bipolar transistor, source follower connection of FET, etc., voltage gain is almost 1, input and output signals are in phase, operation is stable, frequency characteristics and It has features such as good strain characteristics.

[0004]

2. Description of the Related Art The prior art will be described below. FIG. 8 shows an example of a conventional ECL circuit using a follower type semiconductor element as an output stage.

In the figure, V _I and V _O are input and output voltages of the ECL circuit, Q ₁ to Q ₄ are transistors, R ₁ to R ₃ are resistors, and V _CC and V _EE are collector and emitter, respectively. The power supply voltage, V _BB, is the base power supply voltage of the transistor Q ₂ , and V _CS is the control bias voltage of the constant current source Q ₃ .

As shown in the figure, Q ₄ is an emitter follower connection transistor which is an example of a follower type connection semiconductor device in the output stage, its collector terminal is directly connected to V _CC , and its base terminal is connected to the collector side of Q _2. , The emitter terminal is connected to a load installed at a remote place via a load resistor R installed in the vicinity shown by the dotted line or a transmission line L shown by the solid line, and the end is connected to V _EE . Has been done.

When the input voltage V _I changes from a low logic level to a high logic level, the collector output level of Q ₂ , that is, the base input level of Q ₄ also changes from a low level to a high level. Therefore, the output voltage V _O also changes from the low level to the high level and is applied to the remote load via the load resistor R or the transmission line L in the vicinity.

In the usual case where the load is installed in the vicinity like the resistor R, the leakage from the power source, the short circuit of the circuit, and the like are closely monitored, so there is not much problem due to overload. Therefore, conventionally, no special overload protection measures have been taken for the follower type semiconductor element.

[0009]

Recently, however, a lot of loads have been installed in remote places such as terminal devices, and these have been driven by so-called line driver circuits via transmission lines. It is difficult for the load including the transmission line to be sufficiently monitored, and there is a high probability that a semiconductor element in the output stage is overloaded due to a short-circuit accident or the like.

The damage due to this overload is particularly great in the case of a follower type connection semiconductor element. This is because in a follower type semiconductor device, the terminal of the device is directly connected to the voltage source and the load resistance, that is, the safety resistance is not inserted.

FIG. 9 shows the input voltage V in the circuit of FIG.
Output voltage V _O and output current I _O when _I is at a high logic level
It is a characteristic view which shows the relationship with. As shown in the figure, when the transistor Q ₄ is overloaded, V _O cannot maintain a constant value and suddenly drops, and I _O also increases sharply until finally burning at point X.

Therefore, an object of the present invention is to provide a semiconductor element overload protection circuit which protects a follower type semiconductor element from overload, except for the above-mentioned drawbacks in the prior art.

[0013]

FIG. 1 is a block diagram showing the principle of the present invention. In the figure, 1 is a semiconductor element having an input terminal 10, a power supply terminal 11 directly connected to a first voltage source V ₁ , and an output terminal 12, and 2 is a second voltage source V on the opposite side of the semiconductor element 1. ₂ is the connected load, 3 is the equivalent switch 3
1 is a semiconductor element protection means including 1 and an equivalent switch drive circuit 32.

In order to achieve the above-mentioned object, the present invention is based on FIG.
As shown in FIG. That is, at least a power supply terminal 11 directly connected to the first voltage source V ₁ , an input terminal 10 to which an input signal is applied, and a follower type connection that is connected to the second voltage source V ₂ via a load and outputs to the load. A follower type connection circuit of a semiconductor element 1 having an output terminal 12 for transmitting a signal, comprising a semiconductor element protection means 3, said semiconductor element protection means 3 comprising an input terminal 10 and an output terminal 12 of said semiconductor element 1. And an equivalent switch drive circuit 32 for driving the switching operation of the equivalent switch 31.

[0015]

In FIG. 1 showing the principle block of the present invention, the equivalent switch drive circuit 32 holds the equivalent switch 31 in a disconnected state while the load of the semiconductor element 1 is normal.

When the semiconductor element 1 is overloaded due to a short circuit of the transmission line or the like, the equivalent switch drive circuit 32 drives the equivalent switch 31 to switch the connection state. Equivalent switch 31
When is connected, almost all the load current flows through the equivalent switch 31, so that the semiconductor element 1 is protected from overload.

[0017]

2 is a circuit diagram of a first embodiment of the present invention. In the figure, Q _A is a transistor whose collector and emitter are directly connected to the base and emitter of the transistor Q ₄ , respectively, and V _A is its base input voltage.
Other symbols and signs are exactly the same as those in FIG. 8 showing an example of the conventional ECL circuit.

The relationship between FIG. 1 showing the principle block of the present invention and this figure is as follows. That is, the transistor Q _A of FIG. 2 corresponds to the equivalent switch 31 of FIG. 1, and V _A corresponds to the output drive voltage of the equivalent switch drive circuit 32.

In FIG. 2, the value of V _A is such that Q _A does not conduct when the load is normal, that is, the output voltage V A when the load is normal.
It is set to be slightly lower than the value obtained by adding the base-emitter voltage V _BE during transistor operation to _O , that is, V _A <V _O + V _BE .

FIG. 3 shows the input voltage V in the circuit of FIG.
Output voltage V _O and output current I _O when _I is at a high logic level
It is a characteristic view which shows the relationship with. As shown, when the input voltage V _I is at a high logic level, the voltage at the collector of Q ₂ , ie at the base input of Q ₄ , is approximately equal to V _CC , so V _O ≈V _CC −V _BE , When Q ₄ becomes overloaded, I _O increases sharply, V _O also cannot maintain a constant value, and suddenly drops, and V _O ≦ V before the point X at which burnout in FIG. Reach point A, which is _A- V _BE .

Then, since Q _A turns into a conductive state, not only is Q _A responsible for overcurrent due to overload, but thereafter R ₂ inserted between Q _A and V _CC is a safety resistor. , The output current I _O is rapidly reduced to a value corresponding to the point B, and Q ₄ is protected from overload. Since it is as described above, there is no risk of overload even if the power capacity of Q _A is small.

FIG. 4 is a circuit diagram showing a second embodiment of the present invention. In the figure, R _A and R _B are resistors for voltage division, and a voltage for driving Q _A by potentiometrically dividing the power supply voltage difference (V _CC −V _EE ) by R _A and R _B. Value V _A , that is, V _A = R _B (V _CC −V _EE ) /
(R _A + R _B ).

FIG. 5 is a partial circuit diagram showing a third embodiment of the present invention. That is, only the main part in FIG. 4 showing the circuit of the second embodiment is shown, D _A and D _B are diodes, and other parts and configurations are exactly the same as those in FIG.

The purpose of inserting D _A and D _B is to prevent the voltage V _A from going too far from V _CC , ie, too low, due to power supply voltage variations, especially V _EE variations. That is, for example, according to the second embodiment described above,
Since it is V _A ∝ (V _CC −V _EE ), V _{EE is}
If V is too low, V _A may become too low and Q ₄ may become overloaded and Q _A may not conduct.

In this respect, the diode has a small forward voltage drop and is almost constant, so that it is suitable for keeping V _A at an appropriate value that is not too far from V _CC . Of course, for this purpose, along with the value of R _A is necessary to select such that R _A «R _B, the number of diodes to be used may be three or more in only one, but properly It is necessary to select the number that can obtain various voltage drops.

FIG. 6 is a partial circuit diagram showing a fourth embodiment of the present invention. That is, only the main part in FIG. 4 showing the circuit of the second embodiment is shown, D _S is a diode, and the other parts and configuration are exactly the same as in FIG.

D_SThe purpose of inserting_AHigh reverse breakdown voltage characteristics
To give. That is, Q _AV when non-conducting_A
Is V_OCan be damaged if it gets too low
It This damage can be prevented by inserting a diode.
If a Schottky diode is used as the diode,
The operation of the protection circuit can be made more reliable.

FIG. 7 is a circuit diagram showing a fifth embodiment of the present invention. As shown in the figure, this embodiment is a differential input / output circuit, and the transistor corresponding to the equivalent switch is Q
_In addition to Q _A for overload protection of ₄ , Q _B for overload protection of Q ₅ is also used.

In this type of circuit, a suitable base input voltage for driving Q _A has a logic level of just Q.
_Since it matches the logic level of the base input voltage of ₁ ,
A suitable base input voltage for driving Q _B, which is obtained from the base input voltage of Q ₁ via the voltage fine tuning resistor R _T , is a logic level of the base input voltage whose logic level is just Q _2. Is obtained from the base input voltage of Q ₂ via the voltage fine tuning resistor R _U.

Although the above description of the embodiment has been made only for the case of the emitter follower connection of the bipolar transistor, the same can be applied to the case of the source follower connection of the FET.

[0031]

As described above, according to the present invention, it is possible to realize a semiconductor element overload protection circuit for protecting a follower type semiconductor element from overload.

[Brief description of drawings]

FIG. 1 is a principle block diagram of the present invention.

FIG. 2 is a circuit diagram of the first embodiment of the present invention.

FIG. 3 is a V _O vs. I _O characteristic diagram in FIG.

FIG. 4 is a circuit diagram of a second embodiment of the present invention.

FIG. 5 is a partial circuit diagram of a third embodiment of the present invention.

FIG. 6 is a partial circuit diagram of a fourth embodiment of the present invention.

FIG. 7 is a circuit diagram of a fifth embodiment of the present invention.

FIG. 8 is an example of a circuit according to the related art.

FIG. 9 is a V _O vs. I _O characteristic diagram in FIG.

[Explanation of symbols]

 3 semiconductor element protection means 31 equivalent switch 32 equivalent switch drive circuit

Claims (1)

[Claims] 1. A power supply terminal (11) directly connected to at least a first voltage source (V ₁ ), an input terminal (10) to which an input signal is applied, and a second voltage via a load in a follower type connection. A follower type connection circuit of a semiconductor device (1) having an output terminal (12) connected to a source (V ₂ ) and sending an output signal to a load, comprising a semiconductor device protection means (3), The means (3) is inserted between the input terminal (10) and the output terminal (12) of the semiconductor element (1) and is normally disconnected, and is switched to the connected state only when the semiconductor element (1) is overloaded. And an equivalent switch drive circuit (32) for driving the switching operation of the equivalent switch (31).