JPH10135806A - Switch circuit with rush current limit function - Google Patents

Abstract

PROBLEM TO BE SOLVED: To limit stably a rush current to the power supply control switch in an ON state of a transistor(TR) independently of a load capacity without giving effect on an output current supply capability in the case of the operation. SOLUTION: In the circuit for a power supply, a transistor(TR) 17 whose output current is limited and a TR 14 that is conductive and supplies a sufficient collector current when a difference between an input voltage and an output voltage is a prescribed voltage or below are connected in parallel. Thus, when the difference between the input voltage and the output voltage is high and a rush current might be generated, only the TR whose collector current is limited is conductive to limit the current and when the difference between the input voltage and the output voltage gets smaller, the TR with a high drive capability is conductive and a high output current supply capability is taken.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for limiting an inrush current generated when a power supply is turned on.

[0002]

2. Description of the Related Art An ordinary power switch circuit is composed of FETs and bipolar transistors. In this case, at the moment when the switch is turned on, an inrush current occurs for charging the capacitor connected to the output side. In the case of a system having a high impedance in a power supply system such as a dry battery, the supply voltage is reduced due to the rush current, so that there is a problem that the operation of the system becomes unstable. In particular, in a low power consumption system using a battery or the like, control is often used to reduce power consumption by disconnecting a power supply and connecting a power supply circuit as necessary to obtain a required current according to a use state. Therefore, reducing the inrush current is an important issue.

As a simple method of suppressing the inrush current, there is a method of increasing the impedance of the power supply by inserting a resistor or the like into a power supply terminal where the inrush current occurs.
In this case, when a large current is extracted from the output, a voltage drop occurs due to the current limiting resistor. For this reason, the switch circuit becomes very sensitive to current load fluctuations. In addition, as shown in FIG.
There is a method of restricting the rush current by smoothly varying the gate voltage variation of the gate voltage by the resistor 31 and the capacitor 32.

[0004]

However, the gate voltage-ON resistance characteristic of the FET is steep, and the impedance rapidly drops when the voltage exceeds a certain voltage. It is necessary to greatly delay the transient, and the transient fluctuation that reaches the threshold of the FET is also delayed, so that an excessive delay occurs in order for the output voltage to be a sufficient voltage. Also, if the load capacitance changes, the impedance may drop before the output voltage rises, and the current limiting characteristics may not be obtained.
Adjustment is required for each circuit. For this reason,
This method is not suitable for circuits that can be added.

It is an object of the present invention to stably limit the inrush current at the time of ON without affecting the output current supply capability during operation regardless of the load capacity.

[0006]

According to the present invention, a power supply is connected to an input and a load is connected to an output to control connection between the power supply and the load and to limit inrush current flowing from the power supply to the load. In a switch circuit with an inrush current limiting function having a function, a current limiting circuit (13, 17 in FIG. 1 and FIG. 3) connected between an input and an output and for limiting a power supply output current when a current is supplied from a power supply to a load. 18, 20, 2
1, 22) and a detection circuit for detecting that a voltage difference between an input voltage from the power supply and an output voltage to the load becomes equal to or less than a predetermined voltage due to an increase in output to the load due to an output current of the current limiting circuit. (11, 12, 16 in FIGS. 1 and 3)
And a switch circuit with an inrush current limiting function including a low impedance switch element (14 in FIG. 1 and 19 in FIG. 3) connecting between the input and the output by the output of the detection circuit.

More specifically, the current limiting circuit is a constant current switch circuit that is turned on by a control signal, and controls so that a certain amount of current does not flow from the power supply to the load in the on state.

The detection circuit controls ON / OFF of the switch element by a logical sum of a comparison circuit for comparing an input voltage from a power supply with an output voltage to a load, and a control signal used for the current limiting circuit and the comparison circuit. ORing means for performing the above operation.

[0009]

Next, an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention using a bipolar transistor.

The input terminal 10A in the figure is connected to a power supply,
Output terminal 10B is connected to a load. CONT terminal 10
C is a control signal input terminal of the power switch, and the switch circuit of FIG. 1 is turned on when the signal is at a low level “L”. The PNP transistors 14 and 17 are connected between the input and output terminals and control the current. A diode 12 is connected to the input terminal 10A, whereby the input voltage is reduced by a constant voltage and supplied to the positive input of the comparator 11. Output terminal 1
The output of 0B is supplied directly to the negative input of comparator 11.
The output of the comparator 11 and the control signal from the CONT terminal 10C are supplied to the OR gate 16, and the output is supplied to the base of the PNP transistor 14. The base of the PNP transistor 17 is connected to the CONT terminal 10C via the resistor 13 for limiting the base current.

When the control signal is at the high level "H", the output of the OR gate 16 becomes "H".
The P transistor 17 is in the OFF state and
The transistor 14 is also turned off.

Next, when the control signal changes to "L", the current is not flowing through the output terminal 10B, the voltage is low, and the output of the comparator 11 is "H". PNP transistor 14 maintains the OFF state. On the other hand, since the base of the PNP transistor 17 is connected via the resistor 13, the PNP transistor 17 is turned on when the control signal becomes “L” level. However, resistance 1
By making the value of 3 sufficiently large, the collector-emitter current of the PNP transistor 17 is limited by the base current, and does not flow beyond a certain current. As a result, the rush current immediately after the switch circuit of FIG. 1 is turned on does not flow more than a certain current due to the current limitation by the PNP transistor 17.

On the other hand, when a current flows to the collector output of the PNP transistor 17, the output voltage gradually increases. When the difference between the output voltage and the input voltage becomes smaller than the voltage drop caused by the diode 12, the output of the comparator 11 becomes "L". As a result, the PNP transistor 14 is turned on. As described above, when the voltage difference between the input and the output becomes smaller after the current limitation by the PNP transistor 17,
Since the PNP transistor 14 between the input and the output is in the ON state and the current is not limited, even if the load current increases, the generation of a voltage drop by the present switch circuit is suppressed.

In FIG. 1, an embodiment using a bipolar transistor has been described. However, a similar circuit can be formed by using an FET. FIG. 2 is a circuit diagram showing a second embodiment of the present invention, in which an FET is used instead of the bipolar transistor of FIG.

In FIG. 1, the current limiting circuit comprises a PNP transistor 17 and a resistor 13. In FIG. 2, however, the current limiting circuit includes a resistor 20 for sensing an output current;
The difference lies in that an OP amplifier 21 whose output voltage is controlled by a voltage generated at both ends thereof and a P-channel FET 18 whose output is supplied to a gate via a pull-down resistor 22. Otherwise, the PNP transistor 14 of FIG. The OR gate is a NOR gate 23.
It is.

In this embodiment, when the control signal is "H", both the P-channel FETs 18 and 19 are turned off. That is, when the control signal is "H", the output of the OP amplifier 21 is brought into a high impedance state by the control input to the OP amplifier 21, and the P-channel FET 18
Is turned off by the pull-down resistor 22, so that the gate input is turned off. On the other hand, P channel F
ET19 is also in the OFF state because the gate input is at the low level.

When the control signal changes to "L", the OP amplifier 21 operates and the P-channel FET 18 is turned on. At this time, the output to the load is detected by the OP amplifier 21 by the resistor 20, and when the terminal voltage of the resistor 20 increases, the gate control of the P-channel FET 18 is controlled by feedback control so as not to increase. As a result, the rush current during power supply is limited. Thereafter, when the voltage difference between the input and the output decreases due to an increase in the output to the load, the output of the comparator 11 changes to “L” and the output of the NOR gate 23 changes to “H”.
9 turns on. Therefore, the power supply output can be supplied to the load without causing a voltage drop.

[0019]

As described in detail above, the inrush current limiting switch circuit of the present invention is connected between the input from the power supply and the output to the load, and when the current is supplied from the power supply to the load, the power supply output current And an output current to the load due to the output current of the current limiting circuit detects that the voltage difference between the input voltage from the power supply and the output voltage to the load becomes a certain voltage or less. It includes a detection circuit and a low-impedance switch element that connects between the input and the output by the output of the detection circuit. Current is prevented. After that, the detection circuit operates the low impedance switch element due to the increase in the load current, so that the input and output are almost directly connected, and there is almost no voltage drop.

The current limiting circuit is turned on by a control signal. When the current limiting circuit is in the on state, the current limiting circuit is a constant current switch circuit for controlling a current exceeding a predetermined value from flowing from a power supply to a load. And the circuit is not complicated.

In addition to the configuration of the current control circuit, the detection circuit includes a comparison circuit for comparing the input voltage from the power supply and the output voltage to the load, and a switch for turning on and off the switch element by the logical sum of the control signal and the comparison circuit. By including the OR means for controlling the OFF, there is an advantage that the current control circuit and the switch element can be controlled by the same control signal.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

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

FIG. 3 is a circuit diagram showing a conventional inrush current limiting circuit.

[Explanation of symbols]

 REFERENCE SIGNS LIST 11 comparator 12 diode 13 resistor 14 PNP transistor 15 resistor 16 comparator 17 PNP transistor

Claims (5)

[Claims] 1. A power supply is connected to an input, a load is connected to an output,
In a switch circuit with an inrush current limiting function having a function of controlling a connection between the power supply and the load and limiting an inrush current flowing from the power supply to the load, the switch circuit is connected between the input and the output, A current limiting circuit that limits a power supply output current when a current is supplied from the power supply to the load; and an output to the load is increased by an output current of the current limiting circuit, so that an input voltage from the power supply and the load are increased. An inrush current limit including a detection circuit for detecting that a voltage difference between the output voltage and the output voltage becomes equal to or less than a predetermined voltage, and a low-impedance switch element connecting between the input and the output by an output of the detection circuit. Switch circuit with function. 2. The current limiting circuit according to claim 1, further comprising:
2. The switch circuit with an inrush current limiting function according to claim 1, wherein the switch circuit is a constant current switch circuit that is controlled to be in an N state so that a current equal to or greater than a certain value does not flow from the power supply to the load when the ON state is established. . 3. The constant current switch circuit includes a resistor connected to a base input to which the control signal is supplied, an emitter,
3. The switch circuit with an inrush current limiting function according to claim 2, wherein a collector has a transistor connected between the input and output, and a current between the collector and the emitter of the transistor is limited by the base resistance. 4. A constant current switch circuit comprising: an FET that is turned on by the control signal; a feedback circuit that controls a gate of the FET by an output of the FET to control an output current of the FET to be constant. 3. The switch circuit with an inrush current limiting function according to claim 2, comprising: 5. A detection circuit, comprising: a comparison circuit for comparing an input voltage from the power supply with an output voltage to the load; and an on / off of the switch element based on a logical sum of the control signal and the comparison circuit. 3. A switch circuit with an inrush current limiting function according to claim 2, further comprising: a logical sum means for controlling the inrush current control function.