JPH0262053B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to semiconductor switch circuits. In this type of semiconductor switch circuit, a short circuit in the load circuit may cause damage to the power supply that drives the semiconductor switch and the load circuit. Therefore, there is a need for a method that easily and reliably prevents damage due to short-circuit accidents by adding a simple overcurrent protection circuit to the semiconductor switch circuit.

[Prior art and its problems]

Examples of general insulated semiconductor switch circuits are shown in FIGS. 1A and 1B. Note that in the description of each figure below, the same reference numerals indicate the same or corresponding parts.

In the circuit shown in Figure 1A, the switching signal generating circuit 1 is isolated from the load circuit 2 side by a pulse transformer T, where D1 is a rectifying diode, C1 is a smoothing capacitor, and _Tr1 is a smoothing capacitor. Switching transistor as a power transistor,
R ₁ is the base current of switching transistor Tr ₁
A resistor E that limits ib1 is a DC power source that drives the load circuit 2.

The operation of the circuit shown in FIG. 1A will be explained. When the input signal S is input to the switching signal generation circuit 1, the pulse signal is input to the pulse transformer T, and an induced voltage e2 is generated on the secondary side of the pulse transformer T.
This induced voltage e2 is rectified by the diode D1,
The signal is smoothed by the capacitor C1 and input to the base of the switching transistor Tr1, and the switching transistor Tr1 is turned on. When the switching transistor Tr1 is turned on, the voltage of the DC power supply E is applied to the load circuit 2. In other words, in addition to the switching signal, the switching signal generation circuit 1 also generates
Electrical energy for driving the base of the switching transistor Tr1 is also supplied via the pulse transformer T.

The circuit in Figure 1B connects the insulation to a photocoupler PC.
1, and in this case the photocoupler
By connecting the phototransistor Tr3 in the PC1 to the switching transistor Tr1 through a Darlington connection, a semiconductor switch circuit can be formed with a very simple configuration.

However, in such a semiconductor switch circuit,
When a short-circuit accident occurs between both ends A and B of the load circuit 2, the collector current IC1 of the switching transistor Tr1 increases. Figure 2 shows the static characteristic curve of the collector current IC1 (vertical axis) and collector-emitter voltage VCE1 (horizontal axis) of the transistor Tr1 in this case.
It moves on 2A towards point b in the direction of the arrow.
As the collector current ic1 of the transistor Tr1 increases, the collector-emitter voltage VCE1 of the transistor Tr1 increases rapidly, and the operating point b, that is, the transistor
Maximum collector loss curve 2B of Tr1 and parameters
If the intersection point with the a-b transition curve 2A due to ib1 (base current) is exceeded, the transistor Tr1 will be destroyed. Additionally, there is a risk that the DC power supply E that drives the load circuit 2 will also be destroyed due to overcurrent. Therefore, a circuit is required to protect the switching transistor Tr1 from such short-circuit accidents.

Next, a conventional method of a semiconductor switch circuit with an overcurrent protection circuit is shown in FIG.

This system was filed by the present applicant in Japanese Patent Application Laid-Open No. 57-131125, and in this circuit, the newly inserted and connected portion is shown as a portion C surrounded by a broken line. That is, this portion C includes a protection transistor Tr2, an overcurrent detection diode D2, and a capacitor C2 forming a first-order delay circuit.
, resistor R2 and voltage dividing and transistor
These are resistors R3 and R4 for controlling the base of Tr2.

Furthermore, P1 is a point on the cathode side of diode D1, P2 is a point on the anode side of diode D2, P
3 is a point on the collector side of transistor Tr1, P4
and P5 are ground points where one end of the resistor R4 and the emitter of the transistor Tr1 are connected. Also, resistor R1
The collector of the transistor Tr2 is connected to the connection point between the base of the transistor Tr1 and the base of the transistor Tr1, the resistor R4 is connected between the base of the transistor Tr2 and the point P4 to the emitter point P5, and the connection point between the cathode side P1 of the diode D1 and the collector of the transistor Tr1 is A resistor R2 and a diode D2 are connected between point P3 with their cathodes connected to point P3.
A capacitor C2 is connected between points P2 and P4 on the anode side of the diode D2 and a resistor R3 is connected between the same point P2 and the base of the transistor Tr2.

In addition, FIG. 4A shows a voltage waveform diagram at each point during normal operation, where is an input voltage waveform diagram as the input signal S, is a voltage waveform diagram at each point P1, P2, and P3, and FIG. is a voltage waveform diagram at each point corresponding to the same A at the time of a short circuit accident, is the same input voltage as the same A, and is the input voltage corresponding to the same A, that is, the voltage at each point P1, P2, P3.

Next, the circuit operation of FIG. 3 will be explained in conjunction with the waveform diagram of FIG. 4. This circuit detects an increase in the collector-emitter voltage VCE1 of the switching transistor Tr1 due to a load short circuit, and short-circuits the base current ib1 of the switching transistor Tr1 supplied from the switching signal generation circuit 1 through the protection transistor Tr2. By doing so, the switching transistor Tr1 is cut off,
It protects the switching transistor Tr1 and the DC power supply E.

That is, first, during normal operation, the voltage of the input signal S shown in FIG.
, the secondary side T2 of the pulse transformer T
The induced voltage e2 generated in the diode D1 is rectified by the diode D1 and smoothed by the capacitor C1. The voltage waveform at point P1 is as shown in FIG. 4A. The transistor Tr1 is turned on when the voltage at the P1 point reaches the value at time t1, and the load circuit 2 is driven. Since the capacitance of the capacitor C2 is set larger than that of the capacitor C1, the charging time is longer than that of the capacitor C1. Therefore, the voltage at point P2 tries to become the waveform P2b indicated by the dotted line in FIG.
Since transistor Tr1 is turned on, the solid line waveform P
It becomes 2a. Also, since the voltage at point P2 becomes 0V as shown in waveform P2a, no current flows between points P2 and P4, and the protection transistor Tr2 remains in the OFF state.
Further, the voltage at point P3 becomes 0V while transistor Tr1 is on, as shown in FIG. 4A.

Next, when a short-circuit accident occurs between both ends A and B of the load circuit 2 in Fig. 3, the input signal S is input and the voltage at the P1 point at time t1 turns on the transistor Tr1.
The steps up to this point are the same as during normal operation. but,
Assuming that a short circuit accident occurs at time t3 in FIG. 4B, the operating point in the static characteristic curve of transistor Tr1 in FIG.
2A in the direction of the arrow, and as the collector current ic1 of the transistor Tr1 increases, the collector-emitter voltage VCE1, that is, P3
The voltage at the point (FIG. 4B) increases rapidly. The potential at point P2 is the potential VP1 at point P1 by resistor R.
2 and the value divided by resistors R3 and R4 (VP2max)
However, during this time, while the potential at point P3 is smaller than the potential at point P2, the potential at point P2 is almost equal to the potential at point P3 due to diode D2, so the potential at point P2 also suddenly rises like the potential at point P3. To increase. When the potential at point P2 rises to the operating voltage of transistor Tr2 determined by resistors R3 and R4 (voltage at point P2 at time t2 in Figure 4B), protection transistor Tr2 turns on and the base potential of switching transistor Tr1 becomes The voltage becomes 0V, and the switching transistor Tr1 is abruptly turned off. In the OFF state of switching transistor Tr1, P
Between point 2 and point P3, the potential at point P3 is higher, but the high voltage on the load side does not affect the protection transistor Tr2 because it continues to be cut off by the diode D2.

However, even if the short-circuit accident recovers, if the input signal S entering the switching signal generation circuit 1 continues from the time of the short-circuit, the protection transistor Tr2
If it remains ON, the switching transistor Tr1 will be
Since it is not turned on, load circuit 2 is not driven, but
Once the input signal S turns OFF, the protection transistor Tr2 turns OFF, and the next input signal S drives the load circuit 2 again.

However, this circuit system requires (1) a relatively expensive pulse transformer T;

(2) A pulse transformer drive circuit (switching signal generation circuit 1) is required, making the circuit complicated.

(3) Since the overcurrent detection level is determined by the voltage change of the collector-emitter voltage VCE1 of the switching transistor Tr1,
There are large variations due to the characteristics of Tr1.

There are drawbacks such as.

[Purpose of the invention]

The purpose of this invention is to provide an inexpensive semiconductor switch circuit which eliminates the above-mentioned drawbacks and has an overcurrent protection function to prevent the semiconductor switch and the power supply for driving the load from being damaged due to a short circuit accident. .

[Key points of the invention]

The main point of the present invention is that in a semiconductor switch circuit in which a phototransistor of a photocoupler and a power transistor as a switching transistor are connected in a Darlington manner, at least an overcurrent detection means for detecting an overcurrent in a load circuit is connected to the power transistor. The collector of the protection transistor is connected in series with the collector-emitter to the emitter side, and in parallel with the series connection between the base-emitter of the power transistor and the overcurrent detection means, and has a polarity that short-circuits the base current of the power transistor. - By connecting the emitter, the overcurrent is detected and the base current of the power transistor is short-circuited by the protection transistor to cut off the power transistor, and at this time, the base of the power transistor and the phototransistor are connected. By inserting and connecting a resistor between the emitter of the transistor and connecting current limiting means for limiting the emitter current of the phototransistor between the base of the power transistor and the base of the phototransistor, Regardless of an increase in the voltage between the collector and emitter of the power transistor, an increase in loss of the phototransistor and an increase in short circuit current of the protection transistor are suppressed, and the emitter of the phototransistor and the emitter of the power transistor are By connecting a voltage dividing means between the voltage dividing means and connecting the voltage dividing point of the voltage dividing means to the base of the protection transistor, the base of the protection transistor is connected to the voltage across the overcurrent detection means (detection voltage). , driven by a positive feedback voltage of the collector potential of the power transistor by the voltage dividing means,
turning on the protection transistor abruptly;
The present invention is characterized in that a steep cut-off of the power transistor is realized.

[Embodiments of the invention]

FIG. 5 shows an embodiment of a semiconductor switch circuit with an overcurrent protection circuit using a photocoupler according to the present invention. In the figure, PC1 is a photocoupler for signal isolation similar to that in Figure 1B, Tr1 is a switching transistor as a power transistor, and Tr2 is a base current ib1 of the switching transistor Tr1.
R14 is an overcurrent detection resistor for detecting the current iL at the time of failure of the load circuit 2, and therefore the emitter current iE1 of the switching transistor Tr1. ZD, R11
are a Zener diode and a resistor for making the emitter current iE3 of the phototransistor Tr3 in the photocoupler PC1 a constant current after a failure is detected, and R12 is a protection transistor after an overcurrent is detected.
This is a resistor for applying positive feedback to the ON operation of Tr2. Also, R13 is a switching transistor Tr.
1 emitter point P14 and the phototransistor
The voltage between the emitter of Tr3 and the point P11 is divided together with the resistor R12 to connect the protection transistor Tr3.
2 and the protection transistor Tr2 is turned on.
When the operation (turn-on) starts, it is a low resistor that determines the value of the overcurrent iL at the trigger point, and therefore the value of the voltage (detection voltage) VD across the overcurrent detection resistor R14. When the potential at point P14 rises due to overcurrent iL, this circuit uses transistor Tr at the trigger point.
1 and Tr2 suddenly transition from the ON and OFF states to the OFF and ON states, respectively.

Next, the operation of this circuit will be explained in detail.

(1) When there is no input signal S, the phototransistor Tr of photocoupler PC1
3 is cut off, the base current ib1 is not supplied to the switching transistor Tr1, and the switching transistor Tr1 is also cut off, so that no load current iL flows.

(2) When there is an input signal S and a load current iL below the rating is flowing, the phototransistor Tr of the photocoupler PC1
3 becomes conductive and supplies the base current ib1 to the switching transistor Tr1 through the resistor R11, so the transistor Tr1 turns on and the load current decreases.
iL flows. That is, the phototransistor Tr3 and the switching transistor Tr1 of the photocoupler PC1 are connected in a Darlington manner. In this state, the detection voltage VD of the overcurrent detection resistor 14 based on the load current iL and therefore the emitter current iE1 of the transistor Tr1,
Therefore, the potential of point P14 is low, and therefore the base of protection transistor Tr2 (point P
13) Potential (base-emitter voltage VBE2)
is set so that the transistor Tr2 cannot be made conductive.

Note that the load current iL is divided into the collector current iC1 of the switching transistor Tr1 and the current flowing into the drive/protection circuit C1 of the switching transistor Tr1, that is, the collector current iC3 of the phototransistor Tr3. Sufficiently smaller than iC1,
Therefore, the load current iL is approximately equal to the collector current iC1, the emitter current iE1, or the current flowing into the overcurrent detection resistor R14.

(3) When an overcurrent iL flows When the load current iL becomes an overcurrent due to a short circuit in the load circuit 2, the detection voltage VD of the overcurrent detection resistor R14 increases, and the base of the protection transistor Tr2 increases.・The emitter voltage VBE2 also rises to a voltage sufficient to make the protection transistor Tr2 conductive, and the protection transistor Tr2
starts turning on. Therefore, a part of the base current ib1 of the switching transistor Tr1 is transferred to the collector of the protection transistor Tr2.
Since the current is shunted as iC2, the base current ib1 starts to decrease, and along with this, the collector current iC1 of the switching transistor Tr1 also decreases, that is, it operates in the blocking direction, so the transistor Tr
The collector-emitter voltage VCE1 of the phototransistor Tr3, and thus the potential at the collector point P10, also begins to rise, while the phototransistor Tr3 is in a sufficiently ON state, and the rise in the potential at the point P10 immediately increases the potential at the emitter point P11 of the phototransistor Tr3. The base current ib of the protection transistor Tr2, which leads to an increase in potential and therefore flows through the resistor R12,
2 increases, the collector current iC2 of the protection transistor Tr2 further increases, and the base current ib1 of the switching transistor Tr1 further decreases. OFF,
The protection transistor Tr2 shifts to a completely ON state. In the process of this transition, after the voltage drop across the resistor R11 reaches approximately the Zener voltage of the Zener diode ZD, the voltage drop remains almost unable to reach the Zener voltage, and the current i11 flowing through the resistor R11 becomes constant. . That is, at this time, the phototransistor Tr3 works as a constant current circuit, so even if the potential at the point P10 becomes high, the collector-emitter voltage VCE3 of the phototransistor Tr3 remains constant.
This increases the collector current iC3 of the phototransistor Tr3 from increasing.

The reason why the current i11 is constant here is that if the current i11 increases and the voltage drop across the resistor R11 exceeds the Zener voltage, the base and emitter of the phototransistor Tr3 will be reverse biased, and the phototransistor Tr3 will be reverse biased.
This is because Tr3 becomes unable to conduct.

Now, as described above, as a result of keeping the current i11 constant, the collector current iC2 of the protection transistor Tr2 is also limited, and its emitter-collector voltage VCE2 is also kept low, ensuring the OFF state of the switching transistor Tr1. Furthermore, since the voltage drop across the resistor R11 is constant and the protection transistor Tr2 is completely turned on, the potential at the emitter point P11 of the phototransistor Tr3 is also approximately constant, and therefore the current i12 flowing through the resistor R12 is also approximately constant. becomes. This means that the collector current iC of phototransistor Tr3
3 is also limited to a nearly constant value and the phototransistor
This means that the power consumption and heat loss of Tr3 are also limited. For this reason, the phototransistor
If Tr3 with a high withstand voltage is used, this circuit can be sufficiently applied to a relatively high voltage such as 100V as the voltage of the DC power supply E.

In this circuit, the protection transistor Tr2
When the base current ib2 of the switching transistor Tr1 and the protection transistor Tr2 is about to start conducting, that is, just before the base current ib2 starts flowing into the protection transistor Tr2,
The following relationship exists between the emitter voltages VBE1 and VBE2 and the detection voltage VD of the overcurrent detection resistor.

VD=VBE ₂ - (VBE1+VR) (R13/R12+R13) Here VR: Voltage drop of resistor R11 R12, R13: Resistors R12, R1 respectively
The resistance value is 3. Here, specific numerical examples using this formula are shown: VBE2=0.6V, VBE1=1.2V, VR=
When 0.2V, R12=8kΩ, R13=2kΩ,
VD=0.32V is obtained. In this way the detection voltage
The value of VD is the base of the protection transistor Tr2.
Since it can be set as low as about 1/2 of the emitter voltage VBE2, power loss due to the overcurrent detection resistor R14 can be kept relatively small when the rated load current iL is constantly flowing.

The above-mentioned reset of the overcurrent protection operation, that is, the cutoff of the protection transistor Tr2, is performed when the input signal S turns OFF, as in the conventional example described above.
This is done by also turning off the phototransistor Tr3.

In the above embodiment, overcurrent detection resistor R14
It can be easily inferred that it may be a nonlinear resistor or a semiconductor. Furthermore, the Zener diode ZD connected to the base of the phototransistor Tr3 may be something like a battery,
Also, using a resistor has the effect of suppressing the emitter current iE3.

〔Effect of the invention〕

As detailed above, according to the present invention, (1) the switching transistor drive/protection circuit is simple and can be constructed at low cost;

(2) Accurate overcurrent detection level.

(3) The loss of the overcurrent detection resistor is small.

(4) Can be used up to relatively high load voltages.

It is possible to realize a semiconductor switch circuit having an overcurrent protection function having the following features.

[Brief explanation of drawings]

Figure 1 is a diagram showing the configuration of a conventional semiconductor switch circuit, Figure 2 is a diagram showing the static characteristics of a switching transistor, Figure 3 is a diagram showing the configuration of a conventional semiconductor switch circuit with an overcurrent protection circuit, and Figure 4 3 is a waveform diagram showing the operation of FIG. 3, and FIG. 5 is a diagram showing a configuration example of a semiconductor switch circuit with an overcurrent protection circuit according to the present invention. Explanation of symbols, Tr1...Switching transistor, Tr2...Protection transistor, Tr3...Phototransistor, PC1...Photocoupler, R1
4... Overcurrent detection resistor, ZD... Zener diode, R11, R12, R13... Resistor.

Claims (1)

[Claims] 1. In a semiconductor switch circuit in which a phototransistor of a photocoupler and a power transistor are connected in Darlington, at least overcurrent detection means for detecting overcurrent in a load circuit is connected in series to the emitter side of the power transistor, and - Connect the collector emitter of the protection transistor to a polarity that short-circuits the base current of the power transistor in parallel with the series connection circuit of the emitter and the overcurrent detection means, and connect the collector emitter of the protection transistor to the polarity that short-circuits the base current of the power transistor, and Insert and connect a resistor between the emitter and
Current limiting means for limiting the emitter current of the phototransistor is connected between the base of the power transistor and the base of the phototransistor, and voltage dividing means is connected between the emitter of the phototransistor and the emitter of the power transistor. An overcurrent protection circuit for a semiconductor switch, characterized in that the voltage dividing point of the voltage dividing means is connected to the base of the protection transistor.