JP2001095240A - Rush-current preventing circuit having input over- voltage limiting function - Google Patents

Abstract

PROBLEM TO BE SOLVED: To add an input over-voltage limiting function to a rush current preventing circuit. SOLUTION: This circuit is composed of a current-limited resistor 5 provided in the input side of a capacitor 7 connected in parallel with a primary input circuit of a DC-DC converter, a first switch element 1 forming a bypass circuit, a second switch element 2 for on/off control of the first switch element 1 by inputting an output signal from an input voltage monitoring circuit 4 which is connected in parallel with the capacitor 7 to a base terminal and a constant voltage setting circuit provided between the base terminal of the first switch element and the ground line.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inrush current prevention circuit having an overvoltage limiting function provided in an input circuit of a switching power supply.

[0002]

2. Description of the Related Art A circuit configuration of a conventional inrush current prevention circuit provided in an input circuit of a switching power supply is as shown in FIG. Main transformer 10 constituting DC / DC converter
The capacitor 107 is connected in parallel to a series circuit composed of a primary coil 111 and a main switch element 109 on the primary input side of the switch 8. On the input side of the capacitor 107, a current limiting resistor 103, a first switch element 101 provided in a bypass circuit thereof, a resistor 104 provided between a collector terminal and a base terminal of the first switch element 101, A resistor 105 inserted between the base terminal and the ground line of the primary input circuit, an input voltage monitoring circuit 106 connected in parallel to a capacitor 107, a collector terminal connected to the base terminal of the first switch element, and an emitter terminal To the ground line and the base terminal to the input voltage monitoring circuit 1.
The inrush current prevention circuit is constituted by the second switch element 102 connected to the output terminal 06.

[0003]

When the DC / DC converter is started, the second switch element 102 is turned on based on the voltage detected by the input monitoring circuit 106, so that the first switch element 101 is turned off. Remains. Therefore, the input voltage to charge a capacitor 107 through a current limiting resistor 103, charging the voltage across the capacitor 107 approaches the input voltage V _i is completed. Accordingly, the second switch element 102 is turned off, so that the first switch element 101 is turned on, and both ends of the current limiting resistor 103 are short-circuited. That is, the inrush current flowing into the input circuit is limited by the current limiting resistor 103, and flows into the primary coil 111 and the main switch element 109 via the capacitor 107.

However, since the inrush current prevention circuit does not have a function of suppressing an input overvoltage, when an overvoltage is applied to the input circuit, the primary coil is connected via the inrush current prevention circuit and the capacitor 107. 111 and main switch element 1
09 will be applied with an overvoltage. In order to prevent the destruction of the main switch element due to the input overvoltage, the breakdown voltage of the main switch element must be increased. However, in normal times, a switch element having a high withstand voltage is used at a low voltage, which causes problems such as a decrease in converter efficiency.

[0005]

SUMMARY OF THE INVENTION The present invention is intended to add an input overvoltage suppression function to the above-described conventional inrush current prevention circuit, and comprises a current limiting resistor having a switching element connected in parallel to a primary input circuit. At the start of the DC / DC converter, so that the current limited by the current limiting resistor flows into the primary input circuit. The switch element connected in parallel with the resistor is turned off, and in a steady state, the switch element is turned on to short-circuit the current limiting resistor and reduce the resistance loss. Further, when the input voltage rises above a predetermined voltage, the constant voltage setting circuit is activated,
A voltage higher than a predetermined voltage is not applied to the primary input side of the DC / DC converter.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIGS. 1, 2 and 3. FIG. FIG. 1 is a block diagram showing a first embodiment according to the present invention. DC / DC
A series circuit including a primary coil 11 of a main transformer 8 and a main switch element 9 constituting a converter includes a capacitor 7
Are connected in parallel. On the input side of the capacitor 7, a current limiting resistor 5, a first switch element 1 provided in a bypass circuit thereof, a resistor 6 and a base terminal provided between a collector terminal and a base terminal of the first switch element 1, and a ground. A constant voltage setting circuit (for example, a Zener diode) 3 provided between the lines, an input voltage monitoring circuit 4 connected in parallel between the current limiting resistor 5 and the capacitor 7, and a collector terminal connected to the first switch element 1 is connected to the base terminal, the emitter terminal is connected to the ground line, and the base terminal is connected to the output terminal of the input voltage monitoring circuit 4. Is composed.

Next, the operation characteristics of the inrush current prevention circuit having the input overvoltage limiting function according to the present invention will be described. DC
As described above, the inrush current flowing when the / DC converter is activated is suppressed by the current limiting resistor 5 provided in the inrush current prevention circuit.

The input voltage V of the DC / DC converter_iForgiveness
Capable voltage range is V₁≤V_i≤V_Two  And the gate terminal of the first switch element 1 and the ground
A constant voltage setting circuit provided between lines (for example, Zener
Set voltage V of 3)_zTo V_TwoSlightly larger voltage
V_ThreeIf the input voltage is V₁~ V_Three1 in the range
It is applied to the next input circuit as it is,_ThreeThe above voltage is
Blocked by the constant voltage setting circuit 3. FIG.
FIG. 3 shows an overvoltage waveform flowing into the next input circuit.
(B) is the input voltage suppressed by the constant voltage setting circuit.
5 shows a terminal voltage waveform in a parallel capacitor 7;
The input overvoltage is the set voltage V of the constant voltage setting circuit._zSuppressed by
Is controlled.

FIG. 2 is a block diagram showing a second embodiment according to the present invention. A first voltage dividing resistor circuit comprising voltage dividing resistors 16 and 17 is connected in parallel to the input side of a parallel capacitor 7. The detection signal from the connection point between the voltage dividing resistors 16 and 17 is input to the input voltage monitoring circuit 4. The detection signal from the connection point of the resistors 14 and 15 constituting the second voltage-dividing resistor circuit and the detection signal from the input voltage monitoring circuit 4 are connected to the constant voltage setting circuit 3 connected in parallel to the primary input circuit. To enter.

A current limiting resistor 5 is inserted in series to the ground line of the primary input circuit, and FETs 1 are connected in parallel at both ends. The source terminal of the FET 1 is connected to the gate terminal via the resistor 6, and the gate terminal is connected to the resistor 13.
And a detection signal from the constant voltage setting circuit 3 is inputted thereto. The functions of the input voltage monitoring circuit 4, the constant voltage setting circuit 3, the current limiting resistor 5, and the FET 1 are the same as those in FIG.

[0011]

As described above, the inrush current can be limited by controlling the on / off of the switch element connected in parallel to both ends of the current limiting resistor inserted in series with the line in the primary input circuit. Also, by connecting a simple constant voltage setting circuit in parallel to the inrush current prevention circuit, input overvoltage can be suppressed. Therefore, it is possible to prevent the converter from being damaged due to an instantaneous overvoltage, so that the switching element constituting the converter may be an inexpensive element having a low withstand voltage. .

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment according to the present invention.

FIG. 2 is a block diagram showing a second embodiment according to the present invention.

FIG. 3 is a waveform diagram.

FIG. 4 is a block diagram of a rush current prevention circuit according to the related art.

[Explanation of symbols]

 1, 2, 9 switch element 3 constant voltage setting circuit 4 input voltage monitoring circuit 5, 6, 13 to 17 resistor 7 capacitor 8 main transformer

Claims (2)

[Claims] 1. A capacitor connected in parallel to a series circuit comprising a primary coil of a transformer and a main switch element in a primary input circuit of a DC / DC converter, and an input voltage connected in parallel to a primary circuit on the input side of the capacitor. A monitoring circuit;
A current limiting resistor inserted in series with a primary input circuit on the input side of the input voltage monitoring circuit, and a first switch having a resistor connected between a collector terminal and a base terminal forming a bypass circuit of the current limiting resistor An inrush current prevention circuit comprising an element and a second switch element for inputting an output signal from the input voltage monitoring circuit to a base terminal and controlling on / off of a first switch element, wherein the first switch element A constant voltage setting circuit is provided between the base terminal of the transformer and the ground line of the primary input circuit to limit the input overvoltage to the primary input circuit comprising the primary coil of the transformer and the main switch element. Inrush current prevention circuit with input overvoltage limiting function. 2. An input voltage monitoring circuit for detecting an input voltage from a first voltage dividing resistor circuit connected in parallel to a primary input circuit on an input side of a capacitor; A constant voltage setting circuit connected in parallel to a primary input circuit for inputting a detection voltage from a second voltage dividing resistor circuit connected in parallel to a next input circuit and an output signal from the input voltage monitoring circuit; And a current limiting resistor inserted in series with a ground line and an FET which is supplied with a detection signal from the input voltage monitoring circuit and is turned on and off by being connected in parallel to both ends of the current limiting resistor. The input voltage monitoring circuit detects the input voltage accompanying the start of the / DC converter, and the FET functions to limit the inrush current by the current limiting resistor, and the input overvoltage is limited by the constant voltage setting circuit. Rush current prevention circuit having an input overvoltage limiting function according to claim 1, characterized in that the.