JP2826251B2 - Short circuit protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a short-circuit protection circuit for limiting current and voltage against overcurrent when a load is short-circuited.

[0002]

2. Description of the Related Art Conventionally, in a receiver or the like of a disaster prevention monitoring device, as shown in FIG. 5, in order to suppress an overcurrent when a short circuit occurs on a terminal side connected to a power / signal line drawn from the receiver. A short circuit protection circuit is provided. In FIG.
Is connected to the positive line on the signal line 12a side,
Subsequent to 1, a power transistor 1 in which two transistors are Darlington-connected is provided, and an output current of the power transistor 1 is connected to an S terminal via a current detection resistor R5 and a diode D1.

A constant base current is supplied to a power transistor 1 by a constant current source 2. Constant current source 2
Is connected to a control transistor 3 for shunting the base current to the power transistor 1, thereby controlling the output current of the power transistor 1. A resistor R2 is connected between the base and the emitter of the control transistor 3,
The primary side of the current detection resistor R5 is connected via the resistor R6, and the resistor R4 is connected between the ground and the ground.

The current-voltage characteristics of the protection circuit shown in FIG. 5 are as shown in FIG. FIG. 6 is a characteristic diagram in the case where a power supply voltage of 48 V is input and V = 39.5 V is applied to the load 4 as the steady voltage V. If there is no short circuit on the load 4 side, the current I according to the load side impedance is supplied while maintaining the steady output voltage V = 39.5 V by the characteristic 50. The maximum current I that can maintain this steady output voltage V = 39.5 V
_max is given at point P. Steady-state output voltage V = 39.5
When the output current I in the form of V exceeds I _max point P, the control transistor 3 is turned on, the protection operation to limit the voltage and current at the same time is performed according to characteristic 70. When the output voltage due to the short circuit becomes 0 V, the current is limited to the current at the point Q. This characteristic is generally known as a square character.

Here, the maximum current at the point P is a current value required to obtain a base-emitter voltage V _BE required to turn on the control transistor 3, and for example, R2 = 22 KΩ, R
If 4 = 120 KΩ, R5 = 5.6Ω, and R6 = 5.6 KΩ, then I _max = 317 mA. When the output voltage V = 0 V due to the short circuit, the current at the point _Q becomes I _Q = 136 mA.

The slope of the characteristic 70 depends on the value of the resistor R4. When the resistance of the resistor R4 increases, the slope of the straight line of the characteristic 70 passing through the point P becomes steeper, and when the resistance decreases, the slope decreases. Become gentle.

[0007]

However, in such a conventional short-circuit protection circuit, if the load side is not completely short-circuited and becomes stable in a state where the load impedance is reduced, the short-circuit protection circuit is not used. There is a problem that the power loss of the power transistor 1 provided in the circuit increases. In example characteristic diagram of FIG. 6, accompanying the output current to a decrease in the load impedance exceeds the maximum current I _max point P characteristics 70
In this case, if the output voltage at the point _R becomes stable at, for example, V _R = 22.5 V without completely short-circuiting, the output current continues to flow at I _R = 243 mA. .

The vicinity of the point R is a region where the power loss in the power transistor 1 increases at a peak.
Therefore, in order to compensate for the stable operation of the power transistor 1 near the point R, a transistor having a large power loss must be used, and the radiator needs to be enlarged in order to enhance the heat radiation effect. However, there has been a problem that the circuit size is increased and the cost is increased.

SUMMARY OF THE INVENTION The present invention has been made in view of such a conventional problem, and provides a short-circuit protection for appropriately limiting a current voltage against a decrease in load impedance without increasing a power loss of a current limiting transistor. It is intended to provide a circuit.

[0010]

In order to achieve this object, the present invention is configured as follows. The reference numerals in the drawings of the embodiments are also shown. First, a short-circuit protection circuit according to the present invention includes a main transistor 1 for controlling a current supplied to a load 4, a constant current source 2 for supplying a constant base current to the main transistor 1,
A control transistor 3 for shunting the current from the constant current source 2 and controlling a base current to be supplied to the main transistor 1;
A current detection resistor R5 for generating a current detection voltage ΔV proportional to the output current I; and a third bias resistor R6 for connecting the base of the control transistor 3 to the primary side of the current detection resistor R5.
And a base-emitter resistor R2 connected between the base and the emitter of the control transistor 3.

Further, a first bias resistor R4 is connected between the base of the control transistor 3 and the ground, and when a predetermined current value is exceeded in a steady voltage output state, the first current limiter simultaneously limits the voltage and current. Set the characteristics. A second bias resistor R3 is connected between the base of the control transistor 3 and the ground via a Zener diode ZD that conducts at a specified Zener voltage V _ZD or more lower than the steady output voltage. In the output state, when a predetermined maximum current value I _max (point P) is exceeded, the first bias resistor R
4 is set to simultaneously limit the voltage and current determined by the parallel resistance value, and when the output current exceeds the specified limit current at the steady output voltage, the characteristic of the first _square is determined by the Zener voltage V _ZD . The operation shifts to the operating point R of the limited current and voltage, and thereafter, the output voltage and the output current are limited in accordance with the first-fold characteristic.

Here, the resistance value of the second bias resistor R3 is made sufficiently larger than the resistance value of the first bias resistor R4. This means that the inclination of the first character characteristic with respect to the point P is made gentle, and the inclination of the second character characteristic with respect to the point R is made steep. Also, the Zener voltage V _ZD of the Zener diode ZD, which determines the operating point R of the first F-shaped characteristic, which jumps from the point P of the second F-shaped characteristic, is set so that the power loss of the main transistor 1 is sufficiently reduced. Select.

[0013]

According to the short-circuit protection circuit of the present invention having such a configuration, a characteristic in which two different square-shaped characteristics are switched by turning on and off the zener diode ZD can be obtained.
That is, when the load is short-circuited at a steady output voltage, the maximum current I
_If it exceeds _max , the current and voltage are limited according to the second-fold characteristic determined by the turning on of the Zener diode ZD.
However, the zigzag diode characteristic area that is effective until the zener diode is turned off is a negative resistance in which the load resistance must be reduced with an increase in the voltage. The operation jumps to the operating point R of the first character characteristic determined by V _ZD and stabilizes.

Therefore, by setting the area of the second-figure characteristic, which cannot exist as an operating point, in the area where the power loss of the main transistor 1 is large, the load side is not short-circuited completely and the impedance is lowered. Even if the power transistor becomes stable, the current and voltage can be limited by jumping to the operating point where the power loss of the power transistor is small, and a power transistor with small power loss can be used for the current limit. I'm done.

[0015]

FIG. 1 is an explanatory diagram of a disaster prevention monitoring device to which a short-circuit protection circuit according to the present invention is applied. In FIG. 1, reference numeral 10 denotes a receiver, which connects a sensor repeater 14, an analog sensor 16, and a control repeater 18 to a transmission line 12 drawn from the receiver 10. A sensor line 22 extends from the sensor repeater 14 and connects an on / off sensor 20.

When the on / off sensor 20 detects a fire, it short-circuits the signal lines of the sensor line 22 to a low impedance. Upon receiving the polling by the calling command from the receiver 10 specifying its own address, the sensor repeater 14 responds with the state of the on / off sensor 20 at that time.
The analog sensor 16 detects analog data such as heat and smoke density associated with a fire, and responds to polling from the receiver 10. The control repeater 18 is connected to a control load 24 such as a district bell or a smoke prevention device, and receives a control command specifying a terminal address from the receiver 10 to drive the control load 24.

The receiver 10 is provided with a control CPU 30, from which the transmission line 12 is drawn through a transmission input / output unit 32. This transmission input / output unit 32
Is provided with a short-circuit protection circuit 40 of the present invention. An operation unit 34 and a display unit 36 are provided for the control CPU 30, and a power supply unit 38 supplies a power supply voltage to each circuit unit. More power is being supplied.

FIG. 2 is a circuit diagram showing one embodiment of the short-circuit protection circuit 40 of the present invention provided in the transmission input / output unit 32 of FIG. In FIG. 2, reference numeral 1 denotes a power transistor as a main transistor, which has a Darlington connection structure of an NPN transistor and a PNP transistor. The primary side of the power transistor 1 is connected to the power supply side via a resistor R1, and a point A on the power supply side is, for example, D in this embodiment.
C48V is applied.

A current detection resistor R5 is connected to the secondary side of the power transistor 1, and is further connected to an S terminal to which a plus side signal line 12a of the load 4 is connected via a diode D1. A constant current source 2 is provided on a branch line from the primary side of the power transistor 1 so that a constant base current can be supplied to the power transistor 1. Subsequent to the constant current source 2, a control transistor 3 is provided, and a constant base current supplied from the constant current source 2 to the power transistor 1 is shunted to control the current of the power transistor 1.

The base of the control transistor 3 is connected to the primary side of the current detection resistor R5 via a resistor R6 as a third bias resistor. The emitter of the control transistor 3 is connected to the secondary side of the current detection resistor R5. The base and the emitter of the control transistor 3 are connected by a base-emitter resistor R2. Further, a resistor R4 as a first bias resistor is connected between the base of the control transistor 3 and the ground. The second bias resistor R4 is connected in parallel, that is, the second bias resistor R4 is connected between the base of the control transistor 3 and the ground.
The bias resistor R3 and the Zener diode ZD are connected in series. The load 4 is connected between the signal line 12a drawn from between the terminals S and SC and the common line 12b.

Here, the Zener voltages of the resistors R2 to R5 and the Zener diode ZD are determined, for example, as follows. R2 = 22KΩ R3 = 120KΩ R4 = 1MΩ R5 = 5.6Ω V _ZD = 24V And, in such a setting state of the resistance, DC48 is applied to the point A.
Output voltage V at point B in a steady state when V is applied
Becomes V = 39.5V.

FIG. 3 shows operating characteristics in the embodiment of FIG. In FIG. 3, a characteristic 50 is a steady voltage output V = 39.
5 shows a voltage-current characteristic with respect to a change in load impedance at 5 V. As is apparent from the characteristic 50, when the power is turned on, if the load is normal, the output voltage V
The output state is a current corresponding to the load impedance within the point P that gives the maximum current according to the characteristic 50 of = 39.5 V.

[0023] The steady output voltage V = 39.5V shorted state by the load 4 side of the characteristics 50 of happened, exceeds the maximum current I _max specified by point P, the operating point is in a totally different characteristics from the point P It jumps to the point R of the characteristic 60, and shifts to the point Q of the output voltage 0V due to the short circuit according to the characteristic 60 to make a determination. Characteristic 5
The region between the P point of 0 and the R point of the characteristic 60 is the output voltage V
It has no stable point because it is a negative resistance area in which the load impedance does not decrease but increases instead.
If the point P is exceeded, the program jumps to the point R of the characteristic 60.

FIG. 4 is an explanatory diagram showing the operating characteristics of FIG. 3 in detail. In the short-circuit protection circuit shown in FIG. 2, the Zener diode ZD differs depending on the on-state and off-state.
Create two current-voltage characteristics. First, in the on state of the Zener diode ZD in which the voltage applied to the point E becomes the Zener voltage V _ZD = 24 V or more, the voltage-current limiting characteristic indicated by the characteristic 70 determined by the parallel resistance value of the resistor R3 and the resistor R4 is reduced. Is achieved. Here, a characteristic formed by the characteristic 50 and the characteristic 70 is defined as a second-figure characteristic.

On the other hand, when the voltage at the point E becomes lower than the zener voltage 24 V and the zener diode ZD is in the off state, the resistance R3 is in the disconnected state, so that the characteristic 60 depends on the resistance value of the resistance R4. Voltage and current limiting characteristics can be obtained. Here, the characteristic formed by the characteristic 50 and the characteristic 60 is defined as a first character characteristic. When the zener diode ZD is turned on, the second fold-shaped characteristic 70 indicates that the output voltage V is 22.5 V when the minimum voltage at point E at which the zener diode ZD is turned on is 24 V. The R 'point up to 0.5V is the effective area. Meanwhile, the first
In the square-shaped characteristic 60, the output voltage V becomes effective when the output voltage V = 22.5V or less at which the zener diode ZD is turned off.
The area up to the point is the effective area.

Next, the maximum current I _{max at the} point P in FIG.
Short-circuit current I _{Q at} point Q when load is short-circuited, Zener diode Z
The current I _R when the voltage at the point E at which D turns on and off is 24 V, that is, the output voltage V is 22.5 V, and the characteristic 70 which becomes an unstable region due to negative resistance will be described in detail. explain. (1) Maximum current I _max maximum current I _max printable point P P point B point is obtained when the constant output voltage V = 39.5V. Here, when the forward voltage V _F of the diodes D1 and V _F = 0.9V, the voltage V _C at point C becomes _{V C = 39.5V + 0.9V = 40.4V} . Here, assuming that the base-emitter voltage V _BE when the control transistor 3 is turned on is 0.6 V, E
Voltage V _E of the point becomes _{V E = = V + V F} + V BE 39.5V + 0.9V + 0.6V = 41V. Therefore, current I _E flowing from point D, i.e. the resistance through the resistor R6 R4, R3, current I _E that flows in each R2 is The current _IE flowing from the point E flows from the resistor R6. Therefore, the voltage V _D at point D becomes _{V D = V E + I E} · R6 = 41V + 0.21mA × 5.6K = 42.176V. At this time, the voltage ΔV across the current detection resistor R5 is ΔV = V _D −V _C = 42.176V-40.4V = 1.776V. Therefore, the current flowing through the load 4, that is, the current I flowing through the current detection resistor R5 is Next, which is 3, the maximum current I _max of the point P in the characteristic 50 of Fig. (2) Output voltage at short circuit When the load 4 is completely short-circuited, the output voltage at the point B is V = 0V
Becomes Here, when the forward voltage V _F of the diodes D1 and 0.9V, the voltage V _C at point C is as 0.9V
Becomes Assuming that the base-emitter voltage V _BE when the control transistor 3 is turned on is 0.6 V, the voltage VE at the point _E is _VE = V + V _F + V _BE = 0 V + 0.9 V + 0.6 V = 1.5 V. The current _IE flowing out of the point _E is Becomes The current I _E since the flowing of resistors R6, the voltage V at point D is determined as _{V D = V E + I E} · R6 = 1.5V + 0.029mA × 5.6K = 1.6624V. At this time, the voltage Δ across the current detection resistor R5
V is as follows: ΔV = V _D −V _C = 1.6624V−0.9V = 0.7624V Therefore, at this time, the current flowing through the load 4, that is, the current I flowing through the current detection resistor R5 is Becomes This is the short-circuit current _IQ at the Q point of the first F-shaped characteristic 60 when the load is short-circuited. (3) Output current when point E is 24 V equal to zener voltage V _{ZD The} current-voltage characteristic with respect to the load changes at voltage V _E = 24 V at point E equal to the zener voltage. At this time, the current _IE flowing out of the point _E is Becomes Since the current _IE flows from the resistor R6, D
Voltage V _D of the point becomes _{V D = V E + I E} · R6 = 24V + 0.0513mA × 5.6K = 24.287V. Voltage across [Delta] V of this time, the current sensing resistor R5 _{ΔV = V D -V C = V} D - a _{(V E -V BE) = 24.287V-} (24V-0.6V) = 0.887V. Therefore, the current flowing through the load 4, that is, the current I flowing through the current detection resistor R5 is Becomes This is the output current when V _E = 24 V, that is, the current I _R at the point R of the first F-shaped characteristic 60 in FIGS. (4) Unstable region from point P to point R When point E is equal to zener voltage V _ZD and V _E = 24 V, B
The output voltage V of the point becomes _{V = V E -V BE -V F} = 24V-0.6V-0.9V = 22.5V. At this time, the output current I _R becomes 158 mA, so that the impedance Z of the load 4 becomes Becomes On the other hand, the maximum current I given at the point P of the characteristic 50
The load impedance Z _max of the _max value is Becomes Furthermore, the load impedance at the time of load-side short circuit is 0
Ω. Therefore, from the short-circuit state at the point Q at which the output voltage becomes 0 V to the point R at which the voltage at the point E becomes 24 V and the zener diode ZD turns on, the output voltage increases as the load impedance increases, Any point on the first F-shaped characteristic 60 depending on the impedance is given as a stable operating point.

However, when the voltage at the point E becomes 24 V, the output voltage 2 at which the Zener diode ZD turns on is reduced.
Between the point R at which 2.5 V is applied and the point P at which the maximum current _Pmax is applied, a negative resistance region in which the load impedance must be reduced from 161 Ω to 125 Ω when the output voltage is increased. The operating point cannot exist as a stable point in the region, and becomes an unstable region.

Therefore, the characteristic of the second fold is only the characteristic 50 up to the point P at which the maximum current is obtained at the steady output voltage of 39.5 V shown in FIG. The current jumps to the current-voltage limiting characteristic according to the first F-shaped characteristic 60. Further, the slope of the second square-shaped characteristic 70 in FIG. 4 is determined by the parallel resistance value of the resistors R3 and R4. The slope becomes steeper as the parallel resistance value increases, and the slope decreases as the parallel resistance value decreases. Therefore, the slope of the characteristic 70 can be determined by appropriately selecting the resistance values of the resistors R3 and R4.

This is the same for the resistor R4 that determines the first F-shaped characteristic 60. The greater the resistance value of the resistor R4, the steeper the slope as shown in the figure. In this embodiment, the resistance R that mainly determines the inclination of the
The resistance R4 for determining the inclination of the first F-shaped characteristic 60 is set to a sufficiently large value of 1 MΩ with respect to 120 KΩ of 3. Then, the region of the first character characteristic 60 indicated by the straight line QR is set to have a sufficiently steep slope so as to deviate from the region where the power loss of the power transistor 1 increases at a peak. The power loss of the power transistor 1 is suppressed to a small value even if the current voltage limited by the decrease of the load impedance at the point R is stabilized.

Further, the characteristic 50 and characteristic 60 shown in FIG.
There is hysteresis between the two operating characteristics. This is shown in FIG.
Is described as follows. If the resistance value of the variable resistor connected to the load side is reduced at an arbitrary operating point of the characteristic 50 where the steady output voltage becomes 39.5 V, the operating point moves to the point P, and the maximum current I _max = If it exceeds 317 mA, it reaches the R ′ point according to the characteristic 70. When the impedance is further lowered, the current reaches the point Q from the point R 'due to a complete short circuit of the impedance 0.

Subsequently, when the load impedance is gradually increased from the state at the point Q, the voltage / current increases according to the characteristic 60, and when the load current passes through the point R 'and reaches the point R, the P / P of the characteristic 50 is parallelized with the characteristic 70. Jump to point ´. That is, a jump of the operating point having hysteresis is performed between the characteristic 50 and the characteristic 60. In the above embodiments, specific values are shown for the voltage, current, resistance value and the like for convenience of explanation, but the present invention is of course not limited to these values. Further, although the embodiment of the present invention exemplifies a case where the present invention is used for a disaster prevention monitoring device, any device that supplies power to a load can be provided in a power supply unit of an appropriate device in exactly the same manner.

[0032]

As described above, according to the present invention, even if the load impedance lowers beyond the specified maximum current, the power loss of the power transistor used in the protection circuit is small. The power transistor used for voltage / current limiting can reduce the power loss because it jumps to the voltage / current limiting characteristics set in the area and stabilizes at the operating point of the voltage / current according to the load impedance. And a small radiator can be used.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a disaster prevention monitoring device to which the present invention is applied.

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

FIG. 3 is an explanatory diagram showing the operation characteristics of FIG. 2;

FIG. 4 is an explanatory diagram showing details of the operation characteristics of FIG. 3;

FIG. 5 is a circuit diagram of a conventional example.

FIG. 6 is a characteristic diagram of a conventional circuit.

[Explanation of symbols]

 1: Main transistor (power transistor) 2: Constant current source 3: Control transistor 4: Load R2: Base emitter resistance R3: Second bias resistance R4: First bias resistance R5: Current detection resistance R6: Third bias resistance 10 : Receiver 12: Transmission path 12 a: Signal line (S line) 12 b: Common line (SC line) 14: Detector repeater 16: Analog detector 18: Control repeater 20: On / off detector 22: Detector line 24: Control load 30: Control CPU 32: Transmission input / output unit 34: Operation unit 36: Display unit 38: Power supply unit 40: Short circuit protection circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) H02H 9/02 G05F 1/56 320 H02J 1/00 309

Claims (3)

(57) [Claims] An output voltage (V) and an output current (I) when an output current (I) supplied to a load (4) exceeds a predetermined value.
A main transistor (1) for controlling a current supplied to the load (4); and a constant current source (2) for supplying a constant base current to the main transistor (1). A control transistor (3) for controlling a base current supplied to the main transistor (1) by shunting the current from the constant current source (2); and a current detection voltage (%) proportional to the output current (I). ΔV) is connected between the base of the control transistor (3) and the ground, and when a predetermined current value is exceeded in a steady voltage output state,
A first bias resistor (R4) for setting a first F-shaped characteristic for simultaneously limiting voltage and current, and a specified Zener voltage (V _ZD ) lower than a steady output voltage between the base of the control transistor (3) and ground. The above-mentioned first current is connected through the Zener diode (ZD) which is turned on when the current exceeds a predetermined maximum current value (I _max ) in the conductive state of the Zener diode (ZD) and in the steady voltage output state. A second bias resistor (R3) for setting a second F-shaped characteristic for simultaneously limiting a voltage current determined by a parallel resistance value with the bias resistor (R4); and a base for the control transistor (3), the current detection resistor. A third bias resistor (R) connected to the primary side of (R5)
6), and a base-emitter resistor (R2) connected between the base and the emitter of the control transistor (3), and is determined by the zener voltage (V _ZD ) when a predetermined limit current is exceeded at a steady output voltage. A current limiting circuit, which jumps to a limited current and voltage operating point on the first curve characteristic, and thereafter limits an output voltage and an output current according to the first curve characteristic. 2. The short-circuit protection circuit according to claim 1, wherein a resistance value of said second bias resistor (R3) is sufficiently larger than a resistance value of said first bias resistor (R4). And short circuit protection circuit. 3. The short-circuit protection circuit according to claim 1, wherein a Zener voltage for determining an operating point of the first character characteristic jumping from the second character characteristic is changed to a power loss of the main transistor. A short-circuit protection circuit characterized in that the short-circuit protection circuit is selected so as to fall within a region where the value is small.