JP3094625B2 - Switching power supply circuit - Google Patents

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 circuit, and more particularly to a novel switching power supply circuit having a built-in common mode noise suppression circuit.

[0002]

2. Description of the Related Art In this type of switching power supply circuit, switching noise is generated due to the on / off operation of a switching transistor. Switching noise includes common mode noise and normal mode noise. In particular, it has been necessary to add a special element in order to remove common mode noise flowing through both the signal line and the ground line.

For example, in a conventional switching power supply circuit 1 shown in FIG. 5, a common mode choke coil such as a toroidal coil 2, a line bypass capacitor 3 such as a three-terminal capacitor, and a ground inductor 4 are incorporated and used as a noise source. Common mode noise transmitted as shown by an arrow from a certain switching transistor 5 has been removed.

[0004]

In the conventional switching power supply circuit 1, common mode noise can be removed by the common mode choke coil 2, the line bypass capacitor 3 and the ground inductor 4, but there is another problem in heat generation and other points. May have occurred. Therefore, a main object of the present invention is to provide a switching power supply circuit that can easily suppress common mode noise without causing another problem.

[0005]

SUMMARY OF THE INVENTION The present invention relates to a line power supply, a rectifying means for rectifying an alternating current from the line power supply, a switching transistor connected to an output of the rectifying means and having a heat radiation fin thermally coupled thereto, and a switching transistor and a heat radiation parallel resonance times that have a series circuit of a parallel-connected coil and capacitor with respect to the stray capacitance between the fins
A switching power supply circuit including a parallel resonance circuit , wherein the parallel resonance frequency is set to be equal to a frequency of a fundamental wave of common mode noise.

[0006]

According to the present invention, a stray capacitance between a switching transistor and a radiation fin and a series circuit of an inductor and a capacitor constitute a parallel resonance circuit whose resonance frequency matches the frequency of a fundamental wave of common mode noise. Therefore, this parallel resonance circuit prevents the common mode noise component from returning from the line ground into the circuit.

[0007]

According to the present invention, the return of the common mode noise component can be reduced only by connecting the series circuit of the inductor and the capacitor in parallel with the stray capacitance between the switching transistor and the radiation fin. Common mode noise can be easily suppressed without generating another problem in terms of heat generation and other points.

The above objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments with reference to the drawings.

[0009]

Referring to FIG. 1, a switching power supply circuit 10 according to this embodiment includes a full-wave rectifier circuit 14 housed inside a chassis or a frame 12. Full-wave rectifier circuit 1
4 is connected to an input terminal 16 provided on the frame 12.
The primary winding 18a of the transformer 18 and the switching transistor 20 are connected in series, and the smoothing capacitor 22 is connected in parallel to the output.

As shown in FIG.
As can be clearly understood from the figure, the radiating fins 24 connected to the frame 12 are thermally coupled via the insulating sheet 26, and a series circuit of a coil 28 and a capacitor 30 is provided between the switching transistor 20 and the radiating fins 24. 3
2 are connected. Therefore, between the switching transistor 20 and the radiation fin 24, a parallel resonance circuit 34 equivalently shown in FIG. 3, for example, is formed by the stray capacitance C _S and the series circuit 32 generated therebetween. The resonance frequency (frequency at which the impedance becomes infinite) of the parallel resonance circuit 34 is set to match the frequency of the fundamental wave of the common mode noise, for example, as shown in FIG.

The secondary winding 18b of the transformer 18 is connected to a known rectifier circuit 44 composed of diodes 36 and 38, a coil 40 and a smoothing capacitor 42. The output of the rectifier circuit 44 is provided on the frame 12. Output terminals 46a and 46b are connected. A line power supply 50 is connected to the input terminals 16a and 16b via a common mode choke coil 48 such as a toroidal coil. Line bypass capacitors 52a and 52b such as three-terminal capacitors are connected in parallel to the common mode choke coil 48, and a line ground 54 is connected to the frame 12 via a ground inductor 56.

When the line power supply 50 is turned on, an AC voltage is supplied to the full-wave rectifier circuit 14 through the common mode choke coil 48 and the input terminals 16a and 16b. In the full-wave rectifier circuit 14, the applied AC voltage is subjected to full-wave rectification and converted into direct current. This direct current is smoothed by the smoothing capacitor 22 and then the switching transistor 20.
Given to. The switching transistor 20 is turned on / off by a switching pulse (not shown) applied to its base. Therefore, an AC voltage is generated in the secondary winding 18b of the transformer 18, and this voltage is generated by the rectifier circuit 44.
Is again converted to direct current, and the output terminals 46a and 4
6b.

Although the noise generated by the switching transistor 20 flows as shown by the arrow in FIG. 1, according to this embodiment, the resonance frequency of the parallel resonance circuit 34 matches the frequency of the fundamental wave of the common mode noise. Since it is set (FIG. 4), the fundamental wave component of the common mode noise is cut off by the parallel resonance circuit 34, and does not return to the switching transistor 20, that is, the power supply circuit. Therefore, the common mode noise component returning from the line ground 54 is greatly reduced, and the common mode noise can be substantially suppressed.

[Brief description of the drawings]

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

FIG. 2 is a perspective view schematically showing a state in which a radiation fin is attached to the switching transistor.

FIG. 3 is an equivalent circuit diagram showing a parallel resonance circuit.

FIG. 4 is a graph showing a relationship between a resonance frequency of a parallel resonance circuit and a frequency spectrum of common mode noise.

FIG. 5 is a circuit diagram showing a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Switching power supply circuit 12 ... Frame 14 ... Full-wave rectifier circuit 18 ... Transformer 20 ... Switching transistor 22 ... Smoothing capacitor 24 ... Radiation fin 44 ... Rectifier circuit 48 ... Common mode choke coil 50 ... Line power supply 52a, 52b ... Line bypass capacitor 54 ... line earth 56 ... earth inductor

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H02M ^7/ 00-7/40 H02M 1/12 H02M 3/28

Claims (1)

(57) [Claims] 1. A line power supply, rectifying means for rectifying an alternating current from the line power supply, a switching transistor connected to an output of the rectifying means and having a heat radiation fin thermally coupled thereto, and between the switching transistor and the heat radiation fin. a parallel resonant circuit have a series circuit of a parallel-connected coil and capacitor with respect to the stray capacitance of a parallel to the parallel resonance frequency is set to match the frequency of the fundamental wave of common mode noise A switching power supply circuit including a resonance circuit.