JPH0345162A - Switching power source - Google Patents

Abstract

PURPOSE:To prevent the saturation of a transformer by charging the capacitor of a snubber circuit preliminarily at the time of start-up of the power source. CONSTITUTION:A switching power source is constituted of a DC power source 1, a main transformer 15, a switching element 16, a rectifying and smoothing circuit 17, etc., and this is driven by a switching element driving circuit 18, etc. so as to supply currents to load 26. Moreover, it is equipped with a snubber circuit consisting of a capacitor 11, etc., in parallel with the primary winding of the main transformer 15. And an auxiliary power source 10 is provided so as to constitute the preliminary charge circuit of said capacitor 11, whereby it charges the capacitor 11 in discharged conditions at the time of start up.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a switching power supply.

[Conventional technology]

Conventional switching power supplies have a snubber circuit on the primary side of the main transformer, and the snubber circuit resets the main transformer to prevent saturation of the main transformer.

[Problem to be solved by the invention]

In the conventional switching power supply described above, the snubber circuit resets the main transformer to prevent saturation of the main transformer, so when the switching element is turned off, the excitation energy of the main transformer is released to the snubber circuit. The capacitor of the snubber circuit is charged, but when the power supply is started, the capacitor of the snubber circuit is in a discharge state, and the time during which the excitation energy of the main transformer is released is compared to when the capacitor of the snubber circuit is charged. It takes a long time. For this reason, the switching element shifts to the on period before the excitation energy of the main transformer is completely released, so the main transformer is not reset, and the excitation energy increases, causing the main transformer to become saturated.

Furthermore, in order to prevent the above-mentioned drawbacks, the following measures have been taken in the prior art.

1. The core size of the main transformer is made large so that the main transformer does not become saturated even if the excitation energy increases to some extent.

2. When the power is turned on, turn on the switching element so that the reset is completed within 011 hours of the switching element.
A soft start circuit is used that shortens the period and gradually increases it.

3. In order to prevent the excitation energy of the main transformer from being released into the snubber circuit for a long time, the capacitance of the snubber circuit capacitor cannot be made too large, so select an appropriate value.

However, these measures have the following drawbacks.

1. Since the size of the main transformer increases, it becomes larger and more expensive.

2. The circuit becomes complicated.

3. The selectable range of the capacitance value of the snubber circuit capacitor becomes narrower, making circuit design difficult.

[Means to solve the problem]

The switching power supply of the present invention has a snubber circuit connected to the primary side of the main transformer to reset the main transformer, and includes a precharging circuit that precharges a capacitor of the snubber circuit at startup. have.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

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

In FIG. 1, this embodiment includes a series connection of an input power supply 1, an input smoothing capacitor 2, a capacitor 3, and a resistor 4, a series connection of a resistor 6, the primary side of a photocoupler 7, and a transistor 8, and a main transformer. The series connection of the primary winding 15 and the switching element 16 is connected in parallel to the input power supply 1, the connection point between the capacitor 3 and the resistor 4 is connected to the transistor 8 via the resistor 5, and the auxiliary power supply 10 is input power 1
The anode of the diode 13 connected in parallel with the capacitor 11 and the resistor 12 and in series with the diode 13 is connected to the primary winding of the main transformer 15 and the switching element 1.
6 and is connected in parallel to the primary winding of the main transformer 15, the first output a of the auxiliary power supply 10 is connected to the capacitor 11 via the diode 9, and the second output The resistor 14 and the secondary side of the photocoupler 7 are connected in series, and the secondary winding of the main transformer 15 is connected to the rectifying and smoothing circuit 17.
The series connection of resistors 24.25 and the load 26 are connected in parallel through the resistor 24.25, and the connection point of the resistor 24.25 and the reference voltage 23 are respectively connected to the error amplifier 220 input terminal, and the output of the error amplifier 22 and the oscillator 21 are connected in parallel. The output is voltage comparator 2
The output of the voltage comparator 20 and the connection point between the secondary side of the photocoupler 7 and the resistor 14 are respectively connected to the input terminal of an AND circuit 19, and the output of the AND circuit 19 is connected to a switching terminal. It is connected to an element drive circuit 18, and the output of the switching element drive circuit 18 is connected to the switching element 16.

Next, the operation of this embodiment will be explained.

When the input power supply 1 is first turned on, the auxiliary power supply 10 is first turned on.
starts working. Auxiliary type? , 10 charge a capacitor 11 via a diode 9. The capacitor 3 is initially in a discharged state, and a voltage is present across the resistor 4 while the capacitor 3 is being charged. Therefore, the transistor 81 becomes N, a current flows to the primary side of the photocoupler 7, and the potential at the connection point between the secondary side of the photocoupler 7 and the resistor 14 becomes low. Since one input potential of the AND circuit 19 becomes low, the output becomes low and the switching element drive circuit 1
8 is not operating, and the switching element 16 is not driven, so the power supply has stopped operating.

It is assumed that the constants of the capacitor 3 and the resistor 4 are such that the capacitance and resistance values are selected to provide a time constant that can keep the transistor 8 in an on state for a period of time until charging of the capacitor 11 is completed. When the preliminary charging of the capacitor 11 is completed, and the charging of the capacitor 3 is also completed, and the voltage is no longer present across the resistor 4, the transistor 8 is turned off, and the 1 of the photocoupler 7 is turned off.
The current on the next side stops flowing, and the potential at the connection point between the secondary side of the photocoupler 7 and the resistor 14 becomes high. AND circuit 1
9 becomes high, a signal from the voltage comparator 20 is output, and the switching element 16 is driven by the switching element drive circuit 18 to start a switching operation.

Since the capacitor 11 is precharged, the capacitor 11 has a capacitance of 1.
Any capacitance value may be used as long as the capacitance value is at least such that the terminal voltage of No. 1 does not fluctuate much.

When the switching element 16 starts a switching operation, an excitation current flows through the main transformer 15 during the ON period of the switching element 16. Next, when the switching element 16 is turned off, the excitation energy stored in the main transformer 15 by the excitation current is transferred to the capacitor 11 through the diode 13.
Then, it is released to the snubber circuit of the resistor 12. At this time, since the capacitor 11 is precharged by the auxiliary power supply 10 through the diode 9, the initial M value of the capacitor 11 is the output voltage value of the auxiliary power supply IO. Therefore, the time for excitation energy to be released from the main transformer 15 is short, and even when the switching element 16 is turned on in the next cycle, the reset of the main transformer 15 is already completed.
The excitation current does not increase and saturation of the main transformer 15 is prevented.

〔Effect of the invention〕

As explained above, the present invention secures the voltage stored in the capacitor by pre-charging the capacitor of the snubber circuit connected to the primary side of the main transformer in the pre-charging circuit when the power supply is started. Therefore, the reset operation can be completed within the off period of the switching element from the start of operation of the switching element, and therefore saturation of the main transformer is prevented and normal reset operation can be performed.Furthermore, the snubber circuit The range of capacitances of capacitors that can be selected is widened, which has the effect of facilitating the design of the snubber circuit.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention. 1...Input power supply, 2...Input smoothing capacitor, 3...Capacitor, 4.5.6...
...Resistor, 7...Photocoupler, 8...
...Transistor, 9...Diode, 10
...Auxiliary power supply, 11...Capacitor,
12...Resistor, 13...Diode, 14...Resistor, 15...Main transformer, 16...Switching element, 17・・・・・・
... Rectifier smoothing circuit, 18 ... Switching element drive circuit, 19 ... AND circuit, 20 ...
...Voltage comparator, 21... Oscillator, 22...
. . . error amplifier, 23 . . . reference voltage, 24.
25...Resistor, 26...Load.

Claims (1)

[Claims] 1 of the main transformer in order to reset the main transformer.
A switching power supply having a snubber circuit connected to the next side, characterized in that the switching power supply has a pre-charging circuit that pre-charges a capacitor of the snubber circuit at startup.