JP2000354368A - Switching power unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the number of overvoltage protection circuits by preventing the occurrence of overvoltages. SOLUTION: A switching power unit is provided with an FET 40, which converts a DC voltage into an AC voltage supplied to the primary winding of a transformer 32 by switching the DC voltage. A diode 42 and a filter capacitor 44 convert the AC voltage induced in one secondary winding 36 of the transformer 32 into a DC voltage. Another diode 48 and a filter capacitor 50 convert the AC voltage induced in the other secondary winding 38 of the transformer 32 into a DC voltage. Both terminal voltage of a resistor 54, indicating the voltage across the filter capacitor 50, is supplied between the gate and anode of a shunt regulator 56. The cathode of the regulator 56 is connected to a photocoupler 58, and the output of the photocoupler 58 is supplied to a control circuit 60 which controls the FET 40. To the serial circuit of the regulator 56 and photocoupler 58, a DC voltage is supplied from the filter capacitor 33 via a resistor 62, and in addition, another voltage is supplied from the filter capacitor 50 via a resistor 64 also.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device, and more particularly to a switching power supply device for suppressing an overvoltage generated in an abnormal state such as a short circuit or an open state of components constituting the switching power supply.

[0002]

2. Description of the Related Art Conventionally, the above-mentioned switching power supplies include, for example, those shown in FIG. This switching power supply comprises a DC power supply 2 having both ends connected to a primary winding N1 of a transformer 4.
And the switching element 6 are connected to a series circuit.
The transformer 4 has two secondary windings N2 and N3, and a DC conversion circuit including a diode 7 and a smoothing capacitor 8 is provided in the secondary winding N2. The diode 10 and the smoothing capacitor 12 are also connected to the secondary winding N3.
The voltage across the smoothing capacitor 12 is the voltage dividing resistor 1
Shunt regulator 18
Is supplied to This shunt regulator 18
The light emitting diode 20 a of the photocoupler 20 is connected in series, and this series circuit is connected between both ends of the smoothing capacitor 8. A voltage proportional to the voltage across the smoothing capacitor 12 is supplied between both ends of the voltage dividing resistor 16,
When this becomes higher than the reference voltage of the shunt regulator 18, the light emitting diode 20a emits light, and an electric signal is generated in the light receiving transistor 20b of the photocoupler 20. This is supplied to the control circuit 22, and the switching element 6 is controlled. Overvoltage protection circuits 24 and 26 are attached between both ends of the smoothing capacitor 8 and between both ends of the smoothing capacitor 12, respectively.

[0003]

In this switching power supply, two overvoltage protection circuits 24 and 26 are provided. This is for the following reason. For example, the secondary winding N2
Alternatively, when the diode 6 is opened, no voltage is generated between both ends of the smoothing capacitor 8. As a result, the photocoupler 20 and the shunt regulator 18 do not operate, the control circuit 22 cannot control the switching element 6, and an excessive voltage is generated in the secondary windings N2 and N3 of the transformer 4.

However, providing the overvoltage protection circuits 24 and 26 in each output system in this way increases the number of parts and increases the cost.

It is an object of the present invention to provide a switching power supply device in which the number of overvoltage protection circuits is reduced by preventing occurrence of overvoltage.

[0006]

SUMMARY OF THE INVENTION A switching power supply according to the present invention has a switching element for switching a DC voltage to convert it to an AC voltage, and has a primary winding and at least two secondary windings. , The AC voltage is supplied to the primary winding. The first DC converting means converts the AC voltage induced in one secondary winding of the transformer into
Convert to DC. The second DC converting means converts the AC voltage induced in the other secondary winding of the transformer into DC. The comparing means compares a voltage representing the output voltage of the first DC converting means with a reference voltage. The control means controls the switching means based on the comparison result of the comparison means. An operation voltage supply unit supplies an operation voltage from each of the first and second DC conversion units to the comparison unit.

The comparing means has a conducting element, and the conducting element has first to third terminals, and represents an output voltage of the first DC converting means between second and third terminals. When a voltage is supplied and exceeds a predetermined reference voltage, conduction between the first and third terminals is established. The comparing means further has a voltage generating element, which is connected in series with the conducting element. The operating voltage supply means includes a first current limiting element connected between a series circuit of the conducting element and the voltage generation element and an output of the first DC conversion means, and an operating voltage supply means for the series circuit and the second DC conversion means. A second current limiting element connected between the output and the output.

According to the switching power supply device, the comparing means is supplied with the operating voltage from both the first and second DC converting means via the operating voltage supplying means. Therefore,
Even if a failure occurs in any of the first and second DC converting means and the secondary winding of the transformer connected to the first and second DC converting means, one of the DC converting means may output the comparing means. Is supplied with an operating voltage, and the comparing means operates normally. Therefore, since the control means also controls the switching means normally, the AC voltage induced in any of the secondary windings does not become abnormally high. Therefore, it is not necessary to provide an overvoltage protection circuit on the output side of each of the first and second DC conversion means, and the number of components of the switching power supply can be reduced.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The switching power supply of this embodiment has a DC power supply 30.
For example, it can be obtained by rectifying and smoothing a commercial AC power supply. The switching power supply 30 further includes a transformer 32. The transformer 32 includes a primary winding 34,
It has a plurality, for example, two secondary windings 36 and 38.
The primary winding 34 is a switching element, for example, a power FET.
40 is connected in series with the drain / source conductive path, and this series circuit is connected to the DC power supply 30. FET4
By performing on / off control of 0, a switched DC voltage, that is, a rectangular wave voltage is applied to the primary winding 34. Note that a bipolar transistor, an IGBT, or the like can be used as the switching element in addition to the FET. Further, instead of a single switching element, an inverter using a diode bridge including a plurality of switching elements may be configured.

[0010] The secondary winding 36 is provided with a DC conversion circuit including a diode 42 and a smoothing capacitor 44. The output of this DC conversion circuit is supplied to a load (not shown) as a non-control system output. Note that an overvoltage protection circuit 46 is provided between output terminals of the DC conversion circuit, that is, between both ends of the smoothing capacitor 44.

The other secondary winding 38 is also provided with a DC conversion circuit including a diode 48 and a smoothing capacitor 50. The output of this DC conversion circuit is the control system output. One end of the output terminal of each of the DC conversion circuits is grounded. In each of the DC circuits, a single diode is used as a rectifier circuit. But,
Instead of these, a diode bridge circuit composed of a plurality of diodes may be used. Further, the secondary windings 36 and 38 may have an intermediate tap.

Resistors 52 and 54 are connected in series as voltage dividers between both ends of the smoothing capacitor 50. The voltage between both ends of the resistor 54 represents the voltage of the smoothing capacitor 50, that is, the voltage proportional to the output voltage of the control system. This voltage is supplied between the second and third terminals of the conducting element constituting a part of the comparison means, for example, the shunt regulator 56, that is, between the anode and the gate. Between the first and second terminals of the shunt regulator 56, that is, between the cathode and the anode, voltage generating means, for example, the light emitting diode 58a of the photocoupler 58 is connected in series. The photocoupler 58 has a light-receiving transistor 58b, and a signal received by the light-receiving transistor 58b is supplied to a control circuit 60. The control circuit 60 controls each ON / OFF period of the FET 40, The voltage between both ends, that is, the output voltage of the control system is maintained at a predetermined value. Accordingly, the voltage between the smoothing capacitors 44 also becomes a predetermined voltage.

A series circuit of the light emitting diode 58a and the shunt regulator 56 is connected to the smoothing capacitor 44 via operating voltage supply means, for example, a current limiting resistor 62.
Connected to both ends. Also, the light emitting diode 58a
A series circuit of the shunt regulator 56 and the shunt regulator 56 is similarly connected to both ends of the smoothing capacitor 50 via operating voltage supply means, for example, a current limiting resistor 64.

In the conventional switching power supply shown in FIG. 2, an overvoltage protection circuit is provided between both ends of the smoothing capacitor 12. In this switching power supply device, no overvoltage protection circuit is provided between both ends of the smoothing capacitor 50 corresponding to the smoothing capacitor 12. Instead, the voltage of the smoothing capacitor 50 is supplied to the series circuit of the light emitting diode 58a and the shunt regulator 56 via the resistor 64.

In the switching power supply having such a configuration, in the normal state, the light emitting diode 58a and the shunt regulator 56 are connected to each other by the higher one of the voltage across the capacitor 44 and the voltage across the capacitor 50. An operation voltage is supplied, and the control circuit 60 controls the voltage of the smoothing capacitor 50 to a predetermined voltage.
Controls the FET 40.

For some reason, for example, when the secondary winding 36 or the diode 42 is opened, the smoothing capacitor 4
4 to light emitting diode 58a and shunt regulator 5
6, no operating voltage is supplied. However, the light-emitting diode 58a
Since the operating voltage is supplied to the shunt regulator 56 and the shunt regulator 56 via the resistor 64, they operate normally. Therefore, even if the secondary winding 36 or the diode 42 is open, the control circuit 60 normally controls the FET 40 so that the voltage between both ends of the smoothing capacitor 50 becomes a predetermined value. It is not necessary to provide an overvoltage protection circuit between both ends of the switching power supply, and the number of circuit components of the switching power supply can be reduced.

In the above-described embodiment, the shunt regulator 56 and the photocoupler 58 are used as comparison means. However, the present invention is not limited to this. For example, the voltage of the voltage dividing resistor 54 and the predetermined reference An error amplifier that outputs an error from the voltage can also be used. Also in this case, the operating voltage to the error amplifier is
50 from each other. Further, a transformer can be used in place of the photocoupler 58.

[0018]

As described above, according to the present invention, the number of overvoltage protection circuits can be reduced, and the number of parts can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram of a switching power supply according to an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional switching power supply device.

[Explanation of symbols]

 Reference Signs List 30 DC power supply 32 Transformer 34 Primary winding 36 38 Secondary winding 40 FET (switching element) 42 48 Diode (DC conversion circuit) 44 50 Smoothing capacitor (DC conversion circuit) 56 Shunt regulator (comparison means) 58 Photocoupler ( Comparison means) 62 64 resistor (operating voltage supply means)

Claims (2)

[Claims] A switching element for switching a DC voltage into an AC voltage; a transformer having a primary winding and at least two secondary windings, wherein the AC voltage is supplied to the primary winding First DC converting means for converting an AC voltage induced in one secondary winding of the transformer into DC, and converting DC voltage induced in the other secondary winding of the transformer into DC A second DC conversion unit, a comparison unit that compares a voltage representing an output voltage of the first DC conversion unit with a reference voltage, and a control unit that controls the switching unit based on a comparison result of the comparison unit. A switching power supply unit comprising: an operating voltage supply unit configured to supply an operating voltage from each of the first and second DC conversion units to the comparison unit. 2. The switching power supply according to claim 1, wherein said comparison means has first to third terminals and represents an output voltage of said first DC conversion means between second and third terminals. When a voltage is supplied, and when the voltage exceeds a predetermined reference voltage, a conductive element that conducts between the first and third terminals and a voltage generating element connected in series with the conductive element are provided. The first means is connected between a series circuit of the conducting element and the voltage generating element and an output of the first DC converting means.
And a second current limiting element connected between the series circuit and the output of the second DC converter.