JPS6111011B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a short-circuit protection circuit for a semiconductor switch, and in particular, in the event of an overcurrent such as a load short-circuit, the semiconductor switch is protected by being cut off using an increase in voltage drop of the semiconductor switch itself, without using an overcurrent detection resistor. It is something to do. In this type of semiconductor switch circuit, a short circuit in the load circuit may cause damage to the semiconductor switch or the power supply that drives the load circuit. A system that reliably prevents damage due to short-circuit accidents is desired. FIG. 1 shows a transistor switch circuit as an example of a general semiconductor switch circuit. In Figure 1, 1 is a switching signal generation circuit, 2 is a load circuit, T between them is a pulse transformer for insulation, D ₁ is a rectifier diode in series with the secondary side T ₂ of the pulse transformer T, and C ₁ is the smoothing capacitor connected in parallel, Tr ₁ is the switching transistor, R ₁ is the base of the transistor Tr ₁ , one end of the smoothing capacitor C _1, and the diode D _1.
The input current limiting resistor E connected between the cathode side and the input current limiter E is a DC power supply for driving the load circuit. Next, to explain the operation of the circuit shown in Fig. 1, when the input signal is input to the switching signal generation circuit 1 (switching signal generation section), the pulse signal is input to the primary side _T1 of the pulse transformer T, and the same Secondary side
An induced voltage is generated at T ₂ . This induced voltage is rectified by the diode _D1 , smoothed by the capacitor _C1 , and inputted to the base of the transistor _Tr1 through the resistor _R1 , and the transistor _Tr1 is turned on. When the transistor Tr ₁ is turned on, a driving DC voltage E is applied to the load circuit 2 . 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 ic of the transistor _Tr1 increases. Figure 2 shows the static characteristic curve of the collector current IC (vertical axis) and collector-emitter voltage V _CE (horizontal axis) of the transistor Tr ₁ in this case, and the operating point is indicated by the arrow on curve A from the normal operating point a. It moves toward point b in the direction of . As the collector current ic of the transistor Tr ₁ increases, the collector-emitter voltage V _CE of the transistor Tr 1 increases rapidly, and the operating point b, that is, the maximum collector loss curve B of the transistor Tr ₁ and the a-b transition curve A due to the parameter ib, Exceeding the crossing point will result in destruction of transistor _Tr1 . Also, along with this, the driving power supply E for the load circuit 2
There is also a risk of destruction 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 a short-circuit protection circuit is shown in FIG. In FIG. 3, for convenience, parts corresponding to FIG. 1 are given the same reference numerals.The following parts can be considered as additions in FIG. 3 to those in FIG. 1. In other words, the thyristor SCR connected between the base and emitter of transistor _Tr1 and the transistor
An overcurrent detection resistor Rs is connected in series between the emitter connections of Tr ₁ , and a diode D ₂ is connected to the emitter side of transistor Tr ₁ and the cathode to the thyristor gate side to supply the gate input to the thyristor SCR. is provided, and in the event of a short-circuit accident, the thyristor SCR lights up and works to short-circuit the base circuit of transistor _Tr1 . The circuit operation in Figure 3 is that transistor Tr ₁ is
The process is the same as in FIG. 1 until it is turned on and the load circuit 2 is driven. Next, if a short circuit occurs between both terminals A and B of the load circuit 2 in Figure 3, the collector current IC of the transistor Tr ₁ increases, and the overcurrent detection resistor Rs
When the voltage V _RS across the thyristor exceeds the voltage V _ON shown in equation (1) below, the _thyristor
A trigger voltage _VGF for ignition is applied to the gate of SCR, and thyristor SCR is ignited. As a result, the base of the transistor Tr ₁ becomes the ground level, the transistor Tr ₁ is turned off, and the transistor Tr ₁ is protected from being cut off before being destroyed. Also, once the thyristor SCR is fired, the thyristor SCR continues until the switching signal is no longer input to the input of the switching generation circuit 1.
It is not possible to extinguish the arc. In other words, even if the short-circuit fault is repaired during that time, the thyristor SCR will remain lit, so the transistor Tr ₁ will remain OFF, and only when the switching signal disappears will the current stop flowing through the thyristor SCR and the switch will turn off. That's what I do. V _ON =V _D +V _GF ≒0.6+0.8=1.4[V]...(1) However

[Table] However, in such a semiconductor switch circuit with a short-circuit protection circuit, the use of the resistor Rs for overcurrent detection poses a problem. That is, in the overcurrent detection method using the resistor Rs inserted in series in the load circuit 2, the following drawbacks become more noticeable as the load current capacity increases. First, a high power loss rated resistor must be used as the overcurrent detection resistor Rs, which increases the mounting space. Second, the power loss in the resistor Rs is large under normal operating conditions, which is disadvantageous in terms of heat dissipation and reliability. Taking these points into consideration, the present invention includes an overcurrent detection resistor to prevent semiconductor switches and load driving power supplies from being damaged due to short-circuit accidents, and to ensure that the load circuit operates efficiently in normal operating conditions. The method is to form a protection circuit by utilizing the increase in voltage drop of the semiconductor switch itself and applying it to a protection transistor, an overcurrent detection diode, etc. to cut off the semiconductor switch. FIG. 4 is a circuit configuration diagram of an embodiment of the present invention in a transistor switch. For convenience, the same reference numerals are used for the same parts in FIG. 1 as in FIG. 3. The portion to be newly inserted and connected according to the present invention is indicated by a portion C surrounded by a broken line. That is, this part C includes a protection transistor Tr ₂ , an overcurrent detection diode D ₂ , and a capacitor forming a first-order delay circuit.
_C2 , resistor R2 _, and resistors _R3 and _R4 for voltage division and base control of transistor _Tr2 . Furthermore, P ₁ is the point on the cathode side of diode D ₁ , P ₂
is a point on the anode side of diode D ₂ , P ₃ is a point on the collector side of transistor Tr ₁ , and P ₄ and P ₅ are ground points where one end of resistor R ₄ and the emitter of transistor Tr ₁ are connected. In addition, the collector of transistor Tr ₂ is connected to the connection between the resistor R ₁ and the base of transistor Tr ₁ , the emitter is connected to point P ₅ , and the resistor R ₄ is connected between the base of transistor Tr ₂ and point P ₄ , respectively, and the diode D ₁ is connected to the collector of transistor Tr 2.
A resistor R ₂ and a diode D ₂ are connected between the cathode side P ₁ point of the transistor Tr 1 and the collector point P 3 of the transistor Tr ₁ so that its cathode side becomes the point P ₃ , and the anode _side point of the resistor R ₂ and the diode D ₂ A capacitor C ₂ is connected between P ₂ and the point P ₄ , and a resistor R ₃ is connected between the same point P ₂ and the base of the transistor Tr ₂ . Also, Figure 5a shows the voltage waveform diagram at each point during normal operation, where is the input voltage waveform diagram, ,, is the input voltage waveform diagram,
It is a voltage waveform diagram of each point P ₁ , P ₂ , P ₃ , and b is a voltage waveform diagram of each point corresponding to a of the same at the time of a short circuit accident.
is the same input voltage as in a, and ,, is the input voltage P ₁ , P ₂ , P ₃ corresponding to , in a.
This is the voltage at each point. Next, the operation of the circuit shown in FIG. 4 will be explained in conjunction with the waveform diagram shown in FIG. First, during normal operation, when the input signal voltage shown _{in FIG} .
Smoothed by C ₁ . The voltage waveform at point _P1 is as shown in Figure 5a. Transistor Tr ₁ turns on when the voltage at point _P1 reaches the value at time Ti, and loads circuit 2.
is driven. Since the capacitance of capacitor _C2 is set larger than that of capacitor _C1 , the charging time is longer than that of capacitor _C1 . Therefore, the voltage at point P ₂ tries to become the waveform P ₂ b shown by the dotted line in Figure _5a due to the first-order lag circuit made up of resistor R _{2 and} capacitor C ₂ . The waveform becomes a solid line P ₂ a. Also, since the voltage at point P ₂ becomes 0V as shown in waveform P ₂ a, no current flows between points P ₂ and P ₄ , and protection transistor Tr ₂ remains in the OFF state, and the voltage at point P ₅ becomes 0 V. Transistor as shown in figure a
It becomes 0V while Tr ₁ is ON. Next, if a short connection fault occurs between both ends A and B of the load circuit 2 in Fig. 4, an input signal is input and the time t ₁
The process up to the point where transistor Tr ₁ is turned on by the voltage at point P ₁ at , is the same as during normal operation. However, in the static characteristic curve of transistor Tr _{1 shown} in FIG.
As ic increases, the collector-emitter voltage V _CE , that is, the voltage at point _P3 (shown in Figure 5b) increases rapidly. The potential at _three points P is V _P1 and resistance R ₂ and resistance
While it is smaller than the value divided by R ₃ and R ₄ (V _P2 max),
Since the potential at point _P2 is almost equal to the potential at point _P3 due to the diode _D2 , the potential at point _P2 also increases rapidly like the potential at point _P3 . When the potential at point P ₂ rises and reaches the operating voltage of transistor Tr ₂ determined by resistors R ₃ and R ₄ (the voltage at point P ₂ at time t ₂ in Figure 5b), the protection transistor Tr 2 turns on and the transistor Tr ₂ turns on. The base potential of Tr ₁ becomes 0V, and transistor Tr ₁ is turned off.
When the transistor Tr ₁ is in the OFF state, the potential at the P ₃ point is higher between the P ₂ point and the P ₃ point, but the diode D ₂
Since the high voltage on the load side does not affect the transistor Tr2, the high voltage on the load side does not affect the transistor _Tr2 . However, even if the short-circuit accident recovers, if the input signal entering the switching signal generation circuit 1 continues from the time of the short-circuit, the transistor Tr ₂ remains on and the transistor Tr ₁ does not turn on, so the load circuit 2 Although it is not driven, once the input signal is turned OFF, the transistor Tr ₂ is turned OFF, and the load circuit 2 is driven again by the next input signal. The short-circuit protection of a transistor switch according to an embodiment of the semiconductor switch of the present invention has been described above with reference to FIG. 4, and the block diagram of FIG. 6 shows the case where this is applied to a MOS.FET. The difference between FIG. 6 and FIG. 4 is that an N-channel transistor is used instead of the switching transistor _Tr1 .
Only the enhancement type MOS.FET, Tr' ₁ is used, and the gate of the FET, Tr' ₁ is connected to one end of the resistor R ₁ and the collector of the transistor Tr ₂ , and the drain is connected to one end of the load circuit 2 and the diode D _2. It is used with the source connected to the cathode and grounded. The circuit operation is exactly the same as that shown in Figure 4.However, if the static characteristics of the MOS.FET and T'r are shown in Figure 7, the relationship between the drain current iD and the drain-source voltage _VOS is shown, and this enhancement of
In MOS.FET, its static characteristics are similar to those of transistors as shown in the figure, and B' is the maximum collector loss curve A' for B in Fig. 2, and the parameter V corresponding to A is a' → b' transition due to _GS. It becomes a curve. In addition, the drain current iD is controlled by the gate-source voltage _VGS , and the enhancement type MOSFET,
Tr' ₁ can be easily used as a switching element because the drain current iD becomes 0 when the gate-source voltage _VGS becomes 0 [V]. In addition, FIG. 8 shows that the semiconductor switch circuit K including the drive power source E and the load side M including the load circuit 2 are connected via a long distance line.
The case shown is that they are connected by L ₁ and L ₂ , but in this case, using a load short-circuit protection circuit C as shown in Figure 4 for the semiconductor switch circuit K is effective because it prevents the cable from being short-circuited. It is. Thus, in the semiconductor switch circuit according to the present invention, the semiconductor switch is cut off by utilizing the fact that the voltage drop of the semiconductor switch itself increases in the event of an overcurrent, and the drive signal of the semiconductor switch is delayed in time to be used in the protection circuit. This prevents the protection circuit from operating when the semiconductor switch is turned on. In addition, the protection circuit can be operated by feedback using diode characteristics. Therefore, according to the present invention, when the load circuit is short-circuited, the protection circuit can be operated according to the static characteristics of transistor Tr ₁ , MOS.FET, and Tr' ₁ . Overcurrent is detected by using the increase in collector-emitter voltage V _CE and drain-source voltage V _DS , and since it does not use a conventional overcurrent detection resistor, there is no power loss in the operating state. Semiconductor switches can be protected from short-circuit accidents with a small and simple circuit.

[Brief explanation of the drawing]

Figure 1 is a general semiconductor switch circuit connection diagram, Figure 2 is the switching transistor in Figure 1.
The static characteristic diagram of Tr ₁ , FIG. 3, is a conventional well-known semiconductor switch circuit connection diagram when a short-circuit protection circuit is provided in the circuit configuration of FIG. 1. FIG. 4 is a circuit wiring diagram of an embodiment of the semiconductor switch circuit with short-circuit protection circuit according to the present invention, and FIGS. 5a and 5b are voltage waveform diagrams of the circuit wiring diagram of FIG. 4 during normal operation and during a short-circuit accident, respectively. , FIG. 6 is a circuit connection diagram of a modified embodiment of FIG. 4 according to the present invention, FIG. 7 is a static characteristic diagram of the N-channel enhancement type MOS.FET in FIG. 6, and FIG. 8 is a diagram of the semiconductor shown in FIG. 4. FIG. 3 is a circuit configuration diagram when a switch circuit and a load circuit are separated and connected using a long-distance line. In the diagram, 1: switching signal generation circuit, 2: load circuit, Tr ₁ : switching transistor, Tr ₂ :
Protection transistor for transistor Tr ₁ , D ₁ : Rectifier diode, C ₁ : Smoothing capacitor, R ₁ :
Base input current limiting resistor of transistor Tr ₁ ,
R ₂ , C ₂ : First-order delay circuit resistance, capacitor,
R ₃ : Base input current limiting resistance of transistor Tr ₂ , R ₄ : Base-emitter voltage limiting resistance of transistor Tr ₂ , D ₂ : Overcurrent detection diode.

Claims (1)

[Claims] 1. In a semiconductor switch circuit that generates a switching signal in response to an input signal and applies it to the control electrode of a semiconductor switch such as a switching transistor or an N-channel enhancement type MOSFET to drive a load circuit by switching, there is a connection between the control electrode of the semiconductor switch and the ground. A protection transistor whose collector and emitter are connected, a capacitor charged at a predetermined time constant by the switching signal, and a transistor connected in parallel to the capacitor and whose voltage dividing point is connected to the base of the protection transistor. It consists of a piezoresistor, and an overcurrent detection diode inserted between the collector or drain of the semiconductor switch and the voltage dividing resistor with a polarity that causes a charging current to the capacitor by the switching signal to flow to the semiconductor switch. A short-circuit protection circuit for a semiconductor switch characterized by the following.