JPH05236752A - Switching power supply circuit - Google Patents

Abstract

PURPOSE:To facilitate the suppression of common mode noise by connecting a parallel resonance circuit, comprising a series circuit of an inductor and a capacitor, for the stray capacity between a switching transistor and heat dissipating fins thereby blocking common mode noise components from returning into the circuit from line earth. CONSTITUTION:An AC voltage is applied from a line power supply 50 through a common mode choke coil 48 onto a full-wave rectifying circuit 14 and full wave rectified current is then smoothed through a smoothing circuit 22 and fed to a switching transistor 20. The switching transistor 20 is turned ON/OFF by switching pulses to induce an AC voltage in the secondary winding 18b of a transformer 18 and thus induced AC voltage is then converted through a rectifying circuit 44 into a DC voltage which is fed to a load 58. Noise to be produced from the switching transistor 20 is set such that the resonance frequency of a parallel resonance circuit 34 matches with the frequency of the basic wave of common mode noise, and the common mode noise is suppressed by blocking the basic wave component of common mode noise.

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 incorporating a common mode noise suppression circuit, for example.

[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, and in particular, a special element needs to be added in order to remove common mode noise flowing in 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 2 such as a toroidal coil 2, a line bypass capacitor 3 such as a 3 terminal capacitor, and an earth inductor 4 are incorporated to provide a noise source. Common mode noise transmitted from a certain switching transistor 5 as indicated by an arrow is removed.

[0004]

In the conventional switching power supply circuit 1, the common mode noise can be removed by the common mode choke coil 2, the line bypass capacitor 3 and the earth inductor 4, but another problem occurs in terms of heat generation and other points. It happened. 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]

The present invention is directed to a line power supply, a rectifying means for rectifying alternating current from the line power supply, a switching transistor connected to the output of the rectifying means and having heat radiation fins thermally coupled, and a switching transistor and heat radiation. A parallel resonant circuit that has a series circuit of a coil and a capacitor that are connected in parallel to the stray capacitance between the fins and whose resonant frequency is set to match the frequency of the fundamental wave of common mode noise. And a switching power supply circuit.

[0006]

The stray capacitance between the switching transistor and the heat radiation fin and the series circuit of the inductor and the capacitor form a parallel resonance circuit whose resonance frequency matches the frequency of the fundamental wave of the common mode noise. Therefore, the parallel resonant 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 to the stray capacitance between the switching transistor and the radiation fin. Common mode noise can be easily suppressed without causing another problem in heat generation or other points.

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

[0009]

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

As shown in FIG.
As can be seen clearly, the heat radiation fin 24 connected to the frame 12 is thermally coupled via the insulating sheet 26, and the series circuit of the coil 28 and the capacitor 30 is provided between the switching transistor 20 and the heat radiation fin 24. Three
2 are connected. Therefore, between the switching transistor 20 and the radiation fin 24, the stray capacitance C _S generated between them and the series circuit 32 constitute a parallel resonance circuit 34, which is equivalently shown in FIG. 3, for example. The resonance frequency of the parallel resonance circuit 34 (the frequency at which the impedance becomes infinite) is set so as to match the frequency of the fundamental wave of the common mode noise, as shown in FIG. 4, for example.

A known rectifying circuit 44 including diodes 36 and 38, a coil 40 and a smoothing capacitor 42 is connected to the secondary winding 18b of the transformer 18, and the output of the rectifying circuit 44 is provided on the frame 12. The output terminals 46a and 46b thus connected are connected. A line power supply 50 is connected to the aforementioned input terminals 16a and 16b via a common mode choke coil 48 such as a toroidal coil. Further, 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 applied 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 full-wave rectified and converted into direct current, and 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 AC voltage is generated.
Is converted into DC again by the output terminals 46a and 4
It is applied to a load 58 connected to 6b.

The noise generated in 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 fundamental frequency of the common mode noise. Since it is set (FIG. 4), the fundamental wave component of the common mode noise is blocked by the parallel resonant 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 significantly reduced, and the common mode noise can be substantially suppressed.

[Brief description of drawings]

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

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

FIG. 3 is an equivalent circuit diagram showing a parallel resonant 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]

 10 ... Switching power supply circuit 12 ... Frame 14 ... Full wave rectification circuit 18 ... Transformer 20 ... Switching transistor 22 ... Smoothing capacitor 24 ... Radiating fin 44 ... Rectification circuit 48 ... Common mode choke coil 50 ... Line power supply 52a, 52b ... Line bypass capacitor 54 ... Line earth 56 ... Earth inductor

Claims (1)

[Claims] 1. A line power supply, rectifying means for rectifying alternating current from the line power supply, a switching transistor connected to the output of the rectifying means and having heat radiation fins thermally coupled, and between the switching transistor and the heat radiation fin. Switching power supply having a parallel resonance circuit having a series circuit of a coil and a capacitor connected in parallel to the stray capacitance of the parallel resonance circuit, the resonance frequency of which is set to match the frequency of the fundamental wave of common mode noise. circuit.