JPH0422571Y2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention relates to a power supply circuit that supplies power to a load in response to a control signal, and is particularly concerned with a power supply circuit that is inserted into a power supply path when the load is short-circuited or overloaded. The present invention relates to a power supply circuit that prevents an electronic switch from being destroyed due to excessive current flowing through it. (b) Prior Art Japanese Patent Publication No. 46-35106 describes an overload protection circuit using semiconductor switching elements. FIG. 2 is a circuit diagram showing the overload protection circuit, in which 1 is a current control transistor, 2 is a thyristor, and 3 is a load. When the load 3 is normal, the thyristor 2 is in a cutoff state, and the current control transistor 1 is forward biased by the resistors 4 and 5 and becomes conductive. Furthermore, when an abnormality occurs in the load 3 and the load current increases, the voltage drop across the resistor 6 increases. When the value reaches a predetermined value, the thyristor 2 becomes conductive, and the base current to the current control transistor 1 is no longer supplied. Therefore, the current control transistor 1 is cut off,
Destruction of the current control transistor 1 due to overcurrent can be prevented. (c) Problems to be Solved by the Invention However, the circuit shown in FIG. 2 has the drawback of causing power loss in the resistor 6 during normal operation. Furthermore, since the resistor 6 generates heat, there is a risk of damage to the package when integrated into an IC. In addition, the current flowing through the resistor 6 changes depending on the value of the load 3, and the voltage drop changes, resulting in a change in the supply voltage. (d) Means for Solving the Problems The present invention has been made in view of the above-mentioned points, and includes a first transistor that turns on an electronic switch in response to a control signal, and a first transistor that turns on an electronic switch in response to a control signal. A second transistor that turns on later than the transistor, a reference voltage generation source that generates a different reference voltage depending on the operation of the second transistor, and a terminal voltage of a load and a reference voltage of the reference voltage generation source are compared. It is characterized by comprising a comparator and a third transistor that turns off the first transistor according to the output signal of the comparator. (E) Effect According to the present invention, the terminal voltage of the load is detected, and when the voltage drops abnormally, the electronic switch of the power supply circuit is turned off to protect it, and when the power is turned on, the electronic switch of the power supply circuit is turned off. Since the electronic switch is forcibly turned on and the power supply voltage is applied to the load, it is possible to protect the electronic switch and provide a stable power supply. (F) Embodiment Figure 1 is a circuit diagram showing an embodiment of the present invention.
1 is a power supply terminal to which power supply voltage (+Vcc) is applied;
2 is an electronic switch consisting of transistors 3 and 4 connected in Darlington; 5 is a load; 6 is a smoothing capacitor; 7 is an integrating circuit that integrates a power on/off control signal applied to a control terminal 8; 9 is an integrating circuit; The first circuit to which the output signal of circuit 7 is applied to the base
10 is a second transistor in which the output signal is divided by resistors 11 and 12 and applied to the base; 13 is a bias source consisting of resistors 14 and 15 and generates a reference voltage; 16 is a second transistor whose base is applied to the second transistor; connected to the collector of the transistor 10;
transistors whose collectors are connected to point A of bias source 13 ; diodes 17 and 18 are connected in parallel with each other; one end is connected to the power supply line and the other end is connected to point B; A comparator 20 that performs comparison with the voltage is a third transistor whose base is applied with the output signal of the comparator 19 and whose collector is connected to the base of the first transistor 9.
It is a transistor. Next, the operation will be explained. First, before the control signal 8 is applied, the first and second transistors 9 and 10 are off.
When the first transistor 9 is off, the transistors 3 and 4 forming the electronic switch 2 are also turned off. Therefore, no voltage is supplied from the power supply terminal 1 to the load 5, and the voltage across the capacitor 6 is zero. On the other hand, when the second transistor 10 is off,
Transistor 16 is turned on and the voltage at point A is zero. Then, by the comparator 19 , point A
The voltage at point B and the voltage at point B are compared, but since the impedance seen from the base of transistor 21 is smaller than that of transistor 22, the base current of transistor 21 is larger than the base current of transistor 22, and transistor 21 On, the transistor 22 is turned off, and the third transistor 20 is also turned off. In this state, when an "H" level control signal is applied to the control terminal 8, the voltage at the output terminal of the integrating circuit 7 increases and the first transistor 9 is turned on.
Then, the base voltage of transistor 3 of electronic switch 2 decreases, so transistors 3 and 4 are turned on. Therefore, the voltage from power supply terminal 1 (+
Vcc) is applied to the load 5 via the collector-emitter path of the transistor 4. At this time, the voltage at point B is kept at a low value until the capacitor 6 is charged, but since the voltage at point A is kept at zero as described above, the third transistor 20 does not turn on. . Therefore, power is stably supplied to the load 5, and the voltage of the capacitor 6 is Vcc−V _CE sat−V _BE
(V _CE sat: collector-emitter saturation voltage of transistor 3; V _BE : base-emitter voltage of transistor 4), and a voltage equal to this is also applied to point B via resistor 23 and diode 17. Thereafter, when the voltage at the output terminal of the integrating circuit 7 increases, the second transistor 10 is turned on and the transistor 16 is turned off. Therefore, a voltage obtained by dividing the power supply voltage (+Vcc) by the resistors 14 and 15 is generated at point A.
If set lower than V _CE sat - V _BE ), the comparator 19 maintains the above state and the third transistor 20
remains off. Therefore, according to the circuit shown in FIG. 1, even in the initial state where power is applied to the load 5 in accordance with the control signal, a predetermined voltage can be stably applied without malfunction. Now, in this state, if the load 5 is short-circuited for some reason and the terminal voltage of the load 5 decreases to zero, an excessive current will be supplied to the load 5 from the emitter of the transistor 4 and the voltage of the transistor 22 will decrease.
A base current ib is supplied from the base of the circuit through a diode 18 and a resistor 23. Then, the voltage at point B becomes Vf+ib·R (Vf: forward voltage of diode 18, R: resistance value of resistor 23). Said point B
The voltage at point A is compared with the voltage at point A, but the voltage at point A is the voltage at the expected drop (Vf + ib・R)
Since it is set higher, transistor 22
is turned on, and the third transistor 20 is turned on. Then, the base of the first transistor 9 is forcibly grounded, so the first transistor 9 is turned off, and the electronic switch 2 is turned off. Therefore, according to the circuit shown in FIG. 1, even if the load 5 is short-circuited and an excessive current flows through the electronic switch 2 , the electronic switch 2 can be quickly turned off, and the electronic switch 2 can be turned off by the excessive current. It can be prevented from being destroyed. Incidentally, as in the embodiment, the diodes 17 and 18 connected in parallel with each other are connected to the resistor 23 and the transistor 2.
By inserting the resistor 23 between the base of the resistor 23 and the base of the resistor 23, the value of the resistor 23 can be reduced, which is advantageous for IC implementation. (g) Effects of the invention As described above, according to the invention, the terminal voltage of the load is compared with a comparator and if the voltage drops, the electronic switch is turned off, thereby preventing excessive current from flowing through the electronic switch. Therefore, the electronic switch can be protected.
Further, according to the present invention, since the electronic switch is forcibly turned on when the power is turned on, malfunctions during initial operation can be prevented and a stable power supply can be achieved.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG. 2 is a conventional overload protection circuit. 1...Power terminal, 2 ...Electronic switch, 5...
Load, 9...first transistor, 10...second transistor, 13 ...bias source, 19 ...comparator, 20...third transistor.

Claims (1)

[Scope of utility model registration request] A power supply circuit that detects the terminal voltage of a load and turns off an electronic switch inserted between the power supply and the load when the voltage decreases,
A first transistor that turns on the electronic switch in response to a control signal, a second transistor that turns on later than the first transistor in response to the control signal, and a different reference voltage depending on the operation of the second transistor. a reference voltage generation source, a comparator that compares a terminal voltage of the load with a reference voltage of the reference voltage generation source, and a third transistor that turns off the first transistor in response to an output signal of the comparator. The power supply circuit is characterized in that when the control signal is applied, the second transistor is turned off to make the comparator inoperable, and when the terminal voltage of the load decreases, the comparator is used to turn off the electronic switch. .