JP2003111417A - Inrush current preventing circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent an inrush current from flowing in a filter capacitor when an AC input power source is instantaneously interrupted. SOLUTION: A parallel connection body constituted of a current detecting resistor 5 and an FET 6 is installed between a filter capacitor 1 and terminals t1, t2 via bridge diodes 4. This inrush current preventing circuit is equipped with a switching circuit responsive to cut-off constituted of a transistor 8 for controlling on/off of the FET 6 and diodes 11, 14 for turning the transistor 8 on and off. When an AC input power source is cut off in the state that a transistor of a photocoupler 13 is turned on, the diodes 11, 14 cut off a supply voltage to a base of the transistor 8, and the transistor 8 is turned off, thereby turning the FET 6 off.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a power supply circuit, for example,
So-called DC / DC such as switching regulator
The present invention relates to a rush current prevention circuit suitable for preventing an inrush current from flowing into a smoothing capacitor when an AC input power is supplied to the converter through a large capacity smoothing capacitor.

[0002]

2. Description of the Related Art Regarding this kind of inrush current prevention circuit,
So far, various types have been proposed and put to practical use.
For example, a large-capacity smoothing capacitor is connected in parallel to a DC / DC converter as a power supply circuit, and an AC input power supply is connected to a parallel connection body composed of these DC / DC converter and smoothing capacitor via a DC diode. There is something connected. In that case, a current is allowed to flow into the parallel connection body of the DC / DC converter and the smoothing capacitor via the parallel connection body of the current detection resistor and the inrush current prevention element connected in parallel therewith. ing.

The inrush current prevention element connected in parallel with the current detection resistor in the parallel connection body is, for example, a thyristor or a semiconductor element, and passes through the current detection resistor when the AC input power is turned on. Then, after the current flows into the smoothing capacitor, the current is switched from the current detection resistor to the inrush current prevention element to flow into the smoothing capacitor, thereby preventing the inrush current from flowing.

[0004]

By the way, in the conventional inrush current prevention circuit, as described above, a thyristor, a semiconductor element, or the like is provided as the inrush current prevention element connected in parallel with the current detection resistor. However, when the AC input power supply is interrupted for an extremely short time such as 10 msec, the rush current prevention element continuously causes the current to flow into the smoothing capacitor. There was a risk that an inrush current would flow in and damage the smoothing capacitor.

In particular, although a thyristor as an inrush current prevention element is often used because it is inexpensive,
Since the thermal temperature coefficient is high, the resistance value decreases when the temperature rises, and it is difficult to quickly cope with the intermittent input of the AC input power source for a short time.

The present invention has been made in view of the above problems of the prior art.
The purpose is to provide a quick-response inrush current prevention circuit that can reliably prevent inrush current from flowing into the smoothing capacitor even if the AC input power supply is momentarily interrupted. To provide.

[0007]

[Means for Solving the Problems] In order to solve the above problems, the present invention employs the following means. Claim 1
The present invention described in an inrush current prevention circuit for suppressing an inrush current to a parallel connection body composed of a power supply circuit and a smoothing capacitor when an AC input power supply is applied, flows from the AC input power supply to the parallel connection body. A current detection resistor that detects a current, an inrush current prevention element that is connected in parallel to the current detection resistor and that can short-circuit the current detection resistor, and a state in which the AC input power supply is turned on and off, and is cut off. And a cutoff-corresponding switching circuit that switches the inrush current prevention element from the on state to the off state at the time of detecting

According to a second aspect of the present invention, in the inrush current prevention circuit according to the first aspect, the cutoff response switching circuit includes:
It is characterized by detecting a DC input voltage and turning on / off a semiconductor element that controls the inrush current prevention element based on supply and interruption of the DC input voltage.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit configuration diagram showing an inrush current prevention circuit according to an embodiment of the present invention. In FIG. 1, reference numeral 1 is a large-capacity smoothing capacitor, and 2 is a DC / DC converter forming a power supply circuit. The smoothing capacitor 1 and the DC / DC capacitor 2 are connected in parallel to each other to form a parallel connection body 3. A parallel connection body including a current detection resistor 5 and a FET 6 as an inrush current prevention element connected in parallel with the current detection resistor 5 via a bridge diode 4 between the smoothing capacitor 1 and the terminals t1 and t2. Is provided. Current detection resistor 5 and FET 6
Are connected in parallel, and when AC input power is supplied from the terminals t1 and t2, the rush current is charged by the current detection resistor 5 to the capacitor 1 as a drop voltage at that time.

The collector of the transistor 8 is connected to the gate of the FET 6 via the Zener diode 7, and the emitter of the transistor 8 is connected between the bridge diode 4 on the terminal t2 and the parallel connection body 3.
A parallel body of a resistor 9 and a capacitor 10 connected in parallel with the resistor 9 is connected to the base of the transistor 8 and serves as a power supply function for the transistor 8.

Between the terminal t1 and the bridge diode 4, and between the resistor 9 and the base of the transistor 8, a series connection body composed of a diode 11, a resistor 12, and a collector of the transistor of the photocoupler 13 is connected. A diode 14 is connected between the diode 11 and the resistor 12 and between the terminal t2 and the bridge diode 4 in the series connection body, and the diodes 11 and 14 detect whether or not the AC input power is supplied. That is,
When the AC input power is supplied to the diodes 11 and 14 and the transistor of the photocoupler 13 is in the ON state, when the AC input power is cut off, the transistor 8 is turned on.
The transistor 8 is turned off by shutting off the supply voltage to the base of the FET, thereby turning off the FET 6.

On the other hand, the smoothing capacitor 1 of the parallel connection body 3
A monitoring series body including a resistor 15, a Zener diode 16, a diode of the photocoupler 13 and a resistor 17 is connected in parallel between the gate of the FET 6 and the series body including the Zener diode 7 and the transistor 8. Then, when the charging voltage in the smoothing capacitor 1 rises by turning on the AC input power, the FET is turned on via the transistor 8.
6 is turned on. The reference numeral 18 is
The bridge diode 4, the current detection resistor 5, and the FE
The resistor 18 is provided between the gate of T6 and the Zener diode 7.

Since the inrush current prevention circuit of this embodiment is constructed as described above, its operation will be described next. When AC input power is applied from terminals t1 and t2,
Immediately after that, the rush current is the current detection resistor 5 of the parallel connection body 3.
Is detected as a drop voltage by the capacitor, flows into the smoothing capacitor 1, and is charged.

When the smoothing capacitor 1 is charged and the voltage rises, the monitoring series body including the resistor 15, the Zener diode 16, and the photocoupler 13 monitors the state, so that the light emitting diode of the photocoupler 13 operates. Then, the transistor of the photo coupler 13 is turned on. When the transistor of the photocoupler 13 is turned on, the base of the transistor 8 is charged with a predetermined voltage and the transistor 8 is turned on.
Since the gate voltage of T6 becomes a predetermined voltage, the FET 6 is turned on and the current detection resistor 5 is short-circuited.

As a result, when the AC input power is constantly supplied to the smoothing capacitor 1 via the bridge diode 4 and the FET 6, after being interrupted for a very short time such as 10 msec, When it is supplied again, it is impossible to instantaneously follow the FET 6 alone in the extremely short time, and it is difficult to perform the off / on operation. This is because the electric charge from the large-capacity smoothing capacitor 1 is discharged through the monitoring series body such as the resistor 15 and the Zener diode 16, which requires a long time.

However, in this embodiment,
During the time when the supply of the AC input power is stopped, the terminal t
No current is supplied to the diodes 11 and 14 connected to 1, 1 and t2, and since the transistor of the photocoupler 13 is in the on state, power is not supplied to the base of the transistor 8, so the transistor 8 is turned off. At the same time, the transistor 8 is turned off and the gate and source of the FET 6 are short-circuited.
The FET 6 is turned off.

When the AC input power is supplied again after the momentary interruption, the diodes 11 and 14 and the resistor 1
2. The base of the transistor 8 is charged through the transistor of the photocoupler 13 and the transistor 8 is turned on to turn on the FET 6 to form a steady energized state.

Therefore, when the supply of the AC input power supply is instantaneously stopped, the FET 6 can be turned off so as to follow it. Therefore, compared with the conventional case where a thyristor or the like is used, an instantaneous short time is required. Even if the supply is interrupted in the meantime, it is possible to reliably prevent the inrush current from flowing into the smoothing capacitor 1, and thereby to improve the reliability. That is, this inrush current prevention circuit is a switching circuit (not shown) corresponding to a cutoff circuit, which includes a transistor 8 for on / off controlling the FET 6 as an inrush current prevention element, and diodes 11 and 14 for turning on / off the transistor 8. It is configured with.

When the supply of the AC input power is instantaneously stopped, not only the DC input voltage is detected by the diodes 11 and 14 as described above, but even if the supply of the AC input power is instantaneously stopped, Detect transistor 8
Since the FET 6 is turned off and the FET 6 is controlled to be turned off, even if the AC input power supply is instantaneously stopped and re-supplied, it can be surely dealt with. Note that, if the AC input power supply is stopped for a long time, the photocoupler 13 is turned off. Therefore, if the photocoupler 13 is supplied after that, it is the same as the case of the initial power-on.

[0020]

As described above, according to the present invention as set forth in claim 1, when the supply of the AC input power supply is instantaneously stopped, the FET can be turned off so as to follow it. Even if the power supply is interrupted in a momentary short time, it is possible to prevent the inrush current from flowing into the smoothing capacitor by surely responding promptly, and thereby to improve the reliability. The effect is obtained.

According to the second aspect of the present invention, even if the supply of the direct current is instantaneously stopped, it is possible to reliably turn off the inrush current preventing element so as to follow it.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram showing an inrush current prevention circuit according to an embodiment of the present invention.

[Explanation of symbols]

1 Smoothing capacitor 2 DC / DC converter as power supply circuit 3 parallel connection 4 bridge diode 5 Current detection resistor 6 FET as inrush current prevention element 7,16 Zener diode 8 transistors 9, 12, 15, 17 resistance 10 capacitors 11,14 Diode 13 Photo coupler

Claims (2)

[Claims] 1. In a rush current prevention circuit for suppressing an inrush current to a parallel connection body composed of a power supply circuit and a smoothing capacitor when an AC input power supply is applied, a current flowing from the AC input power supply to the parallel connection body. A current detection resistor for detecting, and connected in parallel to the current detection resistor,
An inrush current prevention element capable of short-circuiting the current detection resistor,
An inrush current prevention circuit comprising: an inrush current prevention element, which detects an on / off state of an AC input power source and switches an inrush current prevention element from an on state to an off state at the time of detecting an interruption. 2. The inrush current prevention circuit according to claim 1, wherein the cutoff response switching circuit detects a DC input voltage and controls the inrush current prevention element based on supply and cutoff of the DC input voltage. Inrush current prevention circuit characterized by turning on and off.