JP2000166218A - Current limiter circuit for power circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a current limiter circuit for a power circuit for suppressing a rush current without being accompanied with sharp cost up, without enlarging the power circuit. SOLUTION: A resistor R3 is connected between the base of a switching transistor Q1 and the pulse output terminal OP of a control circuit 3, and a control transistor Q2 is connected in parallel to the resistor R3. The base of the control transistor Q2 is connected to the junction between a choke coil L1 and an output capacitor C2 through the series circuit of a diode D2 and a resistor R5, and a diode D3 is connected between the base and the emitter of the control transistor Q2. A current limiter circuit 4 is composed of these resistors R3 and R5, and the control transistor Q2, the diodes D2 and D3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for suppressing an inrush current generated when starting a power supply circuit.

[0002]

2. Description of the Related Art In order to operate electronic equipment, a power supply is required. However, the voltage and current supplied from the power supply do not always match the circuits and devices of each part constituting the electronic equipment. Therefore, a power supply circuit is incorporated in the electronic device in order to supply power with a voltage and a current suitable for each circuit and device. At present, there are various power supply circuit systems. For example, when it is required that the output capacity is relatively small and the power supply conversion efficiency is high, the power supply circuit having the configuration shown in FIG. Is used. The power supply circuit shown in FIG. 2 has the following configuration. Reference numerals 1 and 2 denote an input terminal and an output terminal on the high potential side, and the input terminal and the output terminal on the low potential side are connected to the ground, and are not shown.

A switching transistor Q1 and a choke coil L1 are connected in series between an input terminal 1 and an output terminal 2, and an output capacitor C2 is connected between one end of the output terminal side 2 of the choke coil L1 and ground. A commutating diode D1 is connected between the other end of the choke coil L1 and the ground, and the base of the switching transistor Q1 is connected to a resistor R
3 to the pulse output terminal PO of the control circuit 3. A resistor R1 and a resistor R2 are connected in series between the output terminal 2 and the ground, and a connection point between the resistors R1 and R2 is connected to a voltage detection terminal FB of the control circuit 3. The capacitor C1 connected between the input terminal 1 and the ground is a filter capacitor.

In the circuit of FIG. 2 having the above-described configuration, the switching transistor Q1, the choke coil L1, the output capacitor C2, and the commutating diode D1 form a step-down chopper type converter. The circuit operation of the step-down chopper type converter is introduced in various documents,
In this case, a detailed description of the operation is omitted, but in short, the circuit shown in FIG. 2 changes the on-period of the switching transistor Q1 according to the output voltage, and maintains the output voltage at a predetermined value lower than the input voltage. It is a circuit.

[0005]

Here, consider the start-up of the power supply circuit shown in FIG. Since the output voltage is almost zero when the circuit is started, the ON period of the switching transistor Q1 extends to the maximum ON period that can be controlled by the control circuit 3. Then, the current flowing from the input terminal 1 into the circuit of FIG. 2 has an extremely large value as compared with the normal operation. The large current generated at the time of start-up is called an inrush current, and may induce noise in a power supply line or the like or cause a malfunction in other nearby circuits. Therefore, if it is necessary to suppress the inrush current of the power supply circuit, take measures to connect an impedance circuit that is short-circuited during normal operation to the input side of the power supply circuit, or to reduce the on-duty of the switching element at startup. Had been given.

However, when an impedance circuit is connected to the input side of the power supply circuit, a circuit for detecting current or voltage is required in addition to the impedance element and the short-circuit switch element, and the entire power supply circuit is increased in size. I do. To shorten the on-duty, it is necessary to use a control circuit 3 configured so that the on-duty can be changed according to an external signal, and such a control circuit 3 is expensive. Therefore, there is a problem that the cost of the entire power supply circuit increases. Therefore, an object of the present invention is to provide a current limiter circuit for a power supply circuit for suppressing an inrush current without increasing the size of the power supply circuit and without significantly increasing the cost.

[0007]

A current limiter circuit for a power supply circuit according to the present invention comprises a first resistor connected to a control terminal of a current control element constituting the power supply circuit, and a main current path having a first current path.
And a series circuit of a second resistor and a first diode connected between a control terminal of the control transistor and a smoothing circuit forming a power supply circuit, or And a second diode connected between the control terminal of the control transistor and the current output terminal such that the forward direction is opposite to that of the control transistor.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A switching transistor, a choke coil, an output capacitor, a commutation diode and a control circuit are interconnected to form a step-down converter. A base of the switching transistor is connected to a pulse output terminal of the control circuit via a first resistor, and a main current path between a collector and an emitter of the control transistor is connected in parallel to the first resistor. A base of the control transistor is connected to a connection point of a choke coil and an output capacitor forming a smoothing circuit via a series circuit of a first diode and a second resistor, and the first and second resistors, the control transistor, and the One diode constitutes a current limiter circuit. Alternatively, a second diode may be connected between the base and the emitter of the control transistor.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a current limiter circuit for a power supply circuit according to the present invention and a power supply circuit provided with the current limiter circuit without increasing the size of the power supply circuit and without significantly increasing the cost. The circuit shown in FIG. 1 has the following configuration. The switching transistor Q1, the choke coil L1, the output capacitor C2, and the commutation diode D1 are connected to each other so as to form a step-down chopper converter. Control circuit 3
And the pulse output terminal PO of the control circuit 3 is connected to the base of the switching transistor Q1 via the resistor R3, and the voltage detection terminal FB is connected to the resistor R1 and the resistor R2 connected in series between the output terminal 2 and the ground. Connect to the connection point.
A filter capacitor C1 is connected between the input terminal 1 and the ground.

A series circuit of a resistor R4 and a control transistor Q2 is connected in parallel with the resistor R3, and the base of the control transistor Q2 is connected to a connection point of a choke coil L1 and an output capacitor C2 via a series circuit of a diode D2 and a resistor R5. Connect to A diode D3 is connected between the emitter and the base of the control transistor Q2.
4, resistor R5, control transistor Q2, diode D2
And a diode D3 to form a current limiter circuit 4. The power supply circuit shown in FIG. 1 provided with such a current limiter circuit 4 operates at the time of startup as follows.

When the voltage between the terminals of the output capacitor C2 is almost zero immediately after startup, the control transistor Q2 is off, and the base current of the switching transistor Q1 is substantially determined by the resistor R3. Here, if an element having a high resistance value is used for the resistor R3, the base current of the switching transistor Q1 becomes a small value according to the resistance value of the resistor R3, and the peak value of the current passing between the emitter and the collector becomes small. Can be limited to a value. As a result, the inrush current flowing into the circuit from the input terminal 1 at the time of startup is suppressed by the collector saturation current of the switching transistor Q1.

The output capacitor C2 is charged by the current between the emitter and the collector of the switching transistor Q1 and the current passing through the choke coil L1, and when the voltage between its terminals increases, the control transistor Q2 is turned on, and the resistor R4 and the control transistor are connected. Current also flows through the series circuit of Q2. The current flowing through the series circuit of the resistor R4 and the control transistor Q2 is the output capacitor C
2 increases with an increase in the inter-terminal voltage, and gradually increases the base current of the switching transistor Q1.
Then, the peak value of the current passing between the emitter and the collector of the switching transistor Q1 also gradually increases, and more current is supplied to the external load connected to the output capacitor C2 and the output terminal 2.

When the voltage between the terminals of the output capacitor C2 exceeds a predetermined value, the control transistor Q2 is turned on, and substantially a parallel circuit of a resistor R3 and a resistor R4 is provided between the base of the switching transistor Q1 and the pulse output terminal PO. Becomes Thereby, the switching transistor Q
Since the base current of 1 and the current value passing between the emitter and the collector thereof become large, a sufficient current can be supplied to the external load connected to the output capacitor C2 and the output terminal 2. Thereafter, the switching transistor Q1 performs a normal switching operation according to a signal appearing at the pulse output terminal PO of the control circuit 3, and the power supply circuit shown in FIG. 1 enters a steady operation state.

That is, the circuit shown in FIG. 1 includes a resistor R3, a resistor R4, a resistor R5, a control transistor Q2, a diode D
The current limiter circuit 4 including the diode 2 and the diode D3 narrows down the base current of the switching transistor Q1 at startup and suppresses the current passing through the switching transistor Q1 by utilizing the constant current characteristic of the collector saturation current of the transistor element. And
The current passing through the switching transistor Q1 is gradually increased according to the voltage between the terminals of the output capacitor C2, that is, the output voltage of the power supply, and the circuit is started while preventing a large inrush current from flowing.

When verifying the voltages appearing in the respective parts of the circuit shown in FIG. 1, when the switching transistor Q1 is in the off state, the voltage at the position of the pulse output terminal PO of the control circuit 3 becomes substantially equal to the input voltage. On the other hand, since the circuit shown in FIG. 1 has a configuration of a step-down chopper type converter, the input voltage supplied to the input terminal 1 has a higher value than the output voltage appearing at the output terminal 2. If the diode D2 and the diode D3 are not provided in the circuit of FIG. 1, when the switching transistor Q1 is in the off state, the difference between the input voltage and the output voltage is applied between the base and the emitter of the control transistor Q2 constituting the current limiter circuit 4. Is applied. In particular, since the output voltage is close to zero when the circuit is started, the input voltage may be directly applied as a reverse bias between the base and the emitter of the control transistor Q2.

In the circuit of FIG. 1, the diode D2 is connected to the base of the control transistor Q2, and the diode D3
Is connected between the base and the emitter of the control transistor Q2 to protect the control transistor Q2 from being damaged even when the difference between the input voltage and the output voltage is higher than the breakdown voltage between the base and the emitter of the control transistor Q2. ing. That is, the diode D3 is turned on with respect to the reverse bias voltage between the base and the emitter of the control transistor Q2, and protects the control transistor Q2.
The diode D2 is connected to the pulse output terminal P of the control circuit 3.
It functions to prevent current from leaking from the position of O to the output terminal 2 side. When the output capacitance of the power supply circuit is small and the input voltage is not so high, the diode D3 can be omitted. In this case, the diode D2 protects the control transistor Q2 by sharing a part of the reverse bias voltage applied between the base and the emitter of the control transistor Q2.

[0017]

As described above, the current limiter circuit for the power supply circuit according to the present invention comprises a first resistor connected to the control terminal of the current control element, a control transistor connected in parallel to the first resistor, It is characterized in that it comprises a series circuit of a second resistor and a first diode connected to the control terminal of the control transistor, or a second diode connected between the control terminal of the control transistor and the current output terminal. I have. According to the power supply circuit current limiter circuit having such a configuration, the current passing through the current control element can be narrowed down at the time of starting the power supply circuit, and a large inrush current can be prevented from flowing. Further, the current limiter circuit for a power supply circuit of the present invention has few components and does not require expensive circuits and components. Therefore, the power supply circuit in which the inrush current is suppressed by applying the present invention can be configured small and inexpensively.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a current limiter circuit for a power supply circuit according to the present invention and a power supply circuit including the same.

FIG. 2 is a circuit diagram showing a configuration of a conventional power supply circuit.

[Explanation of symbols]

1: input terminal 2: output terminal 3: control circuit
4: current limiter circuit C2: output capacitor D2: diode (first diode)
D3: Diode (second diode) L1:
Choke coil Q1: Switching transistor (current control element)
Q2: control transistor R3: resistor (first resistor) R5: resistor (second resistor)

Claims (2)

[Claims] 1. A first resistor connected to a control terminal of a current control element constituting a power supply circuit, a control transistor having a main current path connected in parallel to the first resistor, and control of the control transistor. A current limiter circuit for a power supply circuit, comprising: a series circuit of a second resistor and a first diode connected between a terminal and a smoothing circuit constituting the power supply circuit. 2. A control device, comprising: a second diode connected between a control terminal of the control transistor and a current output terminal so that a forward direction of the control transistor is opposite to that of the control transistor. Item 2. A current limiter circuit for a power supply circuit according to item 2.