JPS61266069A - Switching power source - Google Patents

Abstract

PURPOSE:To reduce a common mode noise by inserting a capacitor between a reset winding and a ground or frame ground. CONSTITUTION:The primary winding 21 coupled in series with a power transistor 4 of a switching element, the secondary winding 22 connected with the secondary circuit and a reset winding 223 for returning magnetic energy stored in a transformer 2 when the transistor 4 is ON at OFF time are provided in a transformer 2. A capacitor 40 is connected with the winding 23, and the other terminal of the capacitor 40 is connected to a ground. The ground capacity of the collector of the transistor 4 is cancelled by inserting the capacitor 40.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a power supply circuit, and particularly to a circuit configuration effective in reducing common mode noise in a switching circuit.

[Conventional technology]

FIG. 2 schematically shows a conventional switching power supply circuit. In FIG. 2, the main switching element, here the power transistor 4, is for example
By repeating the on-off operation 00x10 times, the primary side DC voltage E is converted into a pulsed high-frequency AC voltage having a peak value according to the turns ratio of the transformer. A rectifying and smoothing circuit consisting of a capacitor 7 and a capacitor 8 converts the voltage into a DC voltage with very little pulsation, and outputs it from the output terminals 12 and 15.The DC output voltage changes depending on the load, that is, the magnitude of the output current. The error voltage is detected from resistors 9 and 10, and 1
PWM (PtLtJF# Width Mod
*blation (abbreviated as PWM) control.

Now, the collector potential of the power transistor 4 takes two values, zero potential and DC voltage E, when it operates on and off.
The transition time is very short, several μm or less. However, as shown in FIG. 5, the power transistor 4 is attached to the heat sink 14 via an insulator 15 in order to efficiently dissipate the internal loss of the transistor to the outside.
Common. Therefore, a grounded capacitance is formed between the collector of the power transistor 4 and the heat tank 14. For this reason,
When the power transistor turns on and off, a small amount of high-frequency current flows through this grounded capacitance. This minute current was the cause of an increase in so-called common mode noise. In particular, for safety reasons, the heat sink 14 should be grounded, or the frame ground
), this causes an increase in common mode noise for the reasons mentioned above.

[Problem that the invention seeks to solve]

For safety reasons, the switching power supply circuit that connects the heat sink to the ground or frame ground should be connected to the common ground.
There was a problem that mode noise was large. An object of the present invention is to provide a switching power supply that can reduce common mode noise.

[Means for solving problems]

The present invention is a switching power supply characterized in that a capacitor is inserted between the reset winding and ground or frame ground.

〔Example〕

FIG. 1 shows an embodiment according to the present invention. Although this embodiment shows the case where the heat sink is connected to the ground, the same effect can be achieved even if the heat sink is connected to the frame ground, so the gist of the present invention will be described using FIG. 1. Now, in Fig. 1, the transformer 2 has a power transistor 4 which is a switching element.
A primary winding 21 connected in series with the secondary winding 22 connected to the secondary circuit, and a reset winding for returning the magnetic energy stored in the transformer when the power transistor 40 is turned on to the DC power supply when it is turned off. A line 25 is provided. Further, a capacitor 40 is connected to the reset winding 25 at a point in the figure, and the other end is connected to ground. Furthermore, the collector ground capacitance formed by the power transistor 4 and the heat sink is represented by a capacitor 50 in the figure, and the hole-spun capacitor 50 is virtually connected to the junction point @ of the power transistor 4 and the primary winding 21. Adding the capacitor 40 as shown in the figure has the following effects. When the power transistor 40 turns on and off, voltages of opposite polarity are generated between the 0 point and the point in the figure, so a high frequency current flows through the capacitor 50, but by inserting the capacitor 40, the grounding capacitance of the collector is canceled out. ,
Prevents propagation of common mode noise.

〔Effect of the invention〕

According to the present invention, common mode noise can be significantly reduced. Furthermore, normal mode noise generated on the secondary side of the transformer 20 is no longer induced on the primary side due to the configuration of the transformer bridge consisting of the capacitors 50 and 40 centered around the 0 point and the 0 point. From the above explanation, it is possible to obtain a switching power supply with an extremely low level of noise by applying the present invention.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram according to the present invention. FIG. 2 is a circuit diagram showing a conventional example. FIG. 5 is a schematic diagram of the heat sink mounting structure of the buff transistor. It is. 1: Commercial frequency AC power supply 2 Nidorance 5.5, 6: Diode 4: Power transistor 7: Chiraku coil 8.40: Capacitor 50: Equivalent capacitor 12 representing collector grounding capacitance
, 15: Output terminal 9.10: Resistor 21: Primary winding 22: Secondary winding 25: Reset winding Fig. 2 Fig. 3

Claims (1)

[Scope of Claims] 1. A switching power converter circuit having a transformer with a reset winding, characterized in that a capacitor is connected between the reset winding and ground or frame ground. 2. The switching power supply according to claim 1, wherein the capacitor capacity is approximately equal to the ground capacity formed between the switching element and the heat sink.