CN1881769A - Switch power supply capable of filtering industrial frequency noise in floating system - Google Patents

Abstract

The present invention relates to a switching power supply that can filter out power frequency noise in a floating ground system. In order to solve the problem that the power frequency noise in the floating ground system of the switching power supply cannot be filtered out in the prior art, the present invention is based on the traditional switching power supply, The Y capacitor in it is deleted, so that the primary winding and secondary winding of the transformer are pure coil inductive coupling, so that the power frequency noise at the AC end has no way to pass to the secondary; at the same time, each rectifier diode in the rectifier bridge There is a capacitor connected in parallel to reduce the distortion of the AC signal generated when the diode is turned on and off. Through the above solution, the power frequency noise of the floating system of the switching power supply can be effectively filtered out, and it is ensured that the electromagnetic test of the switching power supply still meets the requirements of the EMC standard.

Description

A switching power supply that can filter power frequency noise in floating systems

technical field

The invention relates to a switching power supply in electronic products, and more specifically relates to a switching power supply capable of filtering power frequency noise in a floating system.

Background technique

At present, there are two types of power supply devices that can convert AC to DC, namely linear power supply and switching power supply. Both types of power supplies introduce common-mode power frequency noise in floating systems. Among them, the capacitance between the transformer windings of the linear power supply is small, and the isolation is good, so the noise can be greatly reduced. Compared with the switching power supply, its noise is small. On the other hand, switching power supplies are widely used in electronic systems because of their light and handy features, wide input voltage range, and high efficiency. For switching power supplies, due to their smaller transformers and more complex designs, it is more difficult to filter out common-mode power frequency noise than for linear power supplies. Therefore, in the design of switching power supply, how to filter out the power frequency noise introduced in the floating system becomes an important issue.

For most household electronic devices (such as televisions, audio equipment, telephones, modems, set-top boxes, etc.), the power supply generally adopts a floating design. That is to say: neither the electronic equipment nor its power supply circuit is really connected to the earth. Therefore, its power supply circuit cannot filter out the common mode noise introduced from the AC input voltage, and there will be a very high noise in the conversion from AC to DC signal. Common mode noise is introduced into the DC voltage. In this way, the power frequency common mode noise has completely entered the electronic equipment, which will eventually affect the normal use of the equipment.

For example, as shown in Figure 1, in the IP telephone voice system, the AC input signal needs to be sent to the ungrounded switching power supply 101 first. Since the switching power supply is not grounded, the output DC voltage at this time contains 50HZ power frequency noise , after the VoIP system 102, the power frequency noise is introduced into the interface of the telephone system 103, since most of the telephone 1031 does not add a high-pass filter circuit that can filter out the power frequency noise, the obvious noise will be heard in the microphone 1032 power frequency noise.

The working principle of a common switching power supply meter is shown in Figure 2. The capacitor C in the figure is the so-called Y capacitor, and the AC signal can be coupled from the primary to the secondary through this Y capacitor. Figure 3 shows the principle of common mode noise introduced by the switching power supply in Figure 2, where C11 and C12 are the junction capacitance in the rectifier diode; C13 is the Y capacitance between the primary and secondary of the transformer; C14 is the output pole pair The equivalent capacitance of the earth. Because the rectifier tubes use the same material, the values of C11 and C12 are equal; at the same time, because the N-phase line in the power grid is connected to the ground, the voltage of V1 relative to the ground is half of the input voltage, that is, V1=Vin/2; And because the capacitive reactance of C14 is much larger than that of C13, V2 is almost equal to V1. After the output terminal of this switching power supply is connected to the load, due to the impedance mismatch, the common mode noise will be converted into differential mode noise, so at this time it will appear as AC noise in the phone, that is, it will affect Call quality; in the modem, it is manifested as a decrease in the data transmission rate, affecting the quality of data transmission, etc.

In the prior art, there is no method to completely filter out the power frequency noise introduced by the floating system of the switching power supply, but the following methods can only be used to reduce and reduce the noise. One of the methods is to reduce the Y capacitance between the primary group and the secondary to reduce the energy transferred from the primary power frequency voltage to the secondary, but it still cannot achieve the desired effect. The second method is to change the primary full-wave rectification to half-wave rectification, as shown in Figure 4. Although this method can reduce noise, changing the full-wave rectification to half-wave rectification will cause a decrease in power supply efficiency and lead to a decrease in the input voltage. Narrowing the range, etc. It can be seen that these two methods can only play an improvement role, and cannot completely filter out the power frequency noise introduced by the switching power supply in the floating system.

Contents of the invention

In view of the above-mentioned defects of the prior art, the present invention solves the problem that the prior art can only reduce or reduce the power frequency noise of the floating system of the switching power supply, but cannot filter out the power frequency noise of the floating system.

In order to solve the above technical problems, the technical solution of the present invention is to provide a switching power supply capable of filtering power frequency noise in a floating system, including a rectifier bridge, a transformer connected to the DC output end of the rectifier bridge, and a series connection A switching switch in the primary winding of the transformer; wherein, the primary winding and secondary winding of the transformer are simply coil inductively coupled; each rectifying device in the rectifying bridge is connected in parallel with a capacitor.

In the present invention, preferably, a shielding layer for blocking noise transmission is provided between the primary winding and the secondary winding of the transformer.

In the present invention, the rectifier bridge may be a full-wave rectifier bridge or a half-wave rectifier bridge.

It can be seen from the above scheme that the present invention filters out the power frequency noise of the floating system of the switching power supply by deleting the Y capacitor in the traditional switching power supply and connecting the capacitor in parallel on the rectifier diode. This scheme can make the floating ground of the switching power supply The power frequency noise of the system is effectively filtered, thereby eliminating the adverse effects of such noise in electrical equipment, so as to improve the quality of calls and data transmission.

Description of drawings

The present invention will be further described below in conjunction with accompanying drawing and embodiment, in the accompanying drawing:

Fig. 1 is a schematic diagram of sources of power frequency noise in an existing VoIP telephone system;

Fig. 2 is a schematic diagram of the introduction of power frequency noise of the switching power supply in the existing full-wave rectification mode;

Fig. 3 is the transformation schematic diagram of Fig. 2;

Fig. 4 is a schematic diagram of reducing power frequency noise by using a half-wave rectification method in the prior art;

Fig. 5 is a schematic diagram of the switching power supply in the present invention.

Detailed ways

As shown in Figure 5, the switching power supply of the present invention that can filter out power frequency noise in the floating system includes a rectifier bridge, a transformer connected to the DC output end of the rectifier bridge, and a switch connected in series to the primary winding of the transformer , between the primary winding and the secondary winding of the transformer is pure coil inductive coupling, and each rectifier device in the rectifier bridge is connected in parallel with a capacitor. The rectifier bridge can be a full-wave rectifier bridge or a half-wave rectifier bridge, and the rectifier is preferably a rectifier diode.

It can be seen that on the basis of the existing switching power supply, the present invention deletes the Y capacitor therein to reduce the coupling degree between the primary and secondary transformers of the switching power supply and improve the quality of the AC signal; at the same time, in order to avoid signal distortion, the Each rectifier (diode in this embodiment) of the rectifier circuit is connected in parallel with a capacitor, that is, C1 to C4 in FIG. 5 .

When the Y capacitor between the primary and secondary of the switching power supply is removed, the primary winding and the secondary winding of the transformer are pure coil inductive coupling, and there is no coupling caused by the Y capacitor, so the power frequency noise at the AC end There is no way to pass to the secondary, so there will be no 50Hz power frequency noise between the output voltage and the ground, so that it will no longer affect the voice quality of the call in the IP telephone system, and will affect the data transmission in the modem. Negative effects such as speed.

On the other hand, since a capacitor is connected in parallel to each rectifier diode in the rectifier circuit, the distortion of the AC signal generated when the diode is turned on and off can be reduced. Without this capacitor, the noise between the output voltage and ground would be severely distorted, resulting in differential-mode noise between the two stages of the output voltage.

In order to better meet the requirements of the switching power supply to the EMC (electromagnetic compatibility) index, in the present invention, preferably also take the following measures:

1. Reduce the coupling effect between the primary and secondary of the transformer, for example, by adding a shielding layer between the primary and secondary windings of the transformer to prevent secondary noise from passing to the primary.

2. The switching frequency of the switching power supply should not be too high, and the switch on and off time should be slowed down as much as possible to reduce di/dt (the change in current per unit time), which can reduce the signal-to-noise ratio.

3. Try to design the loop area occupied by the high-frequency loop to the minimum, so that the adverse effects caused by the alternating magnetic field generated by the alternating current can be reduced by reducing the loop area.

Claims (4)

1, a kind of Switching Power Supply capable of filtering industrial frequency noise in floating system comprises rectifier bridge, the transformer that is connected with the dc output end of described rectifier bridge and is serially connected with diverter switch in the described primary winding; It is characterized in that,

It between the elementary winding of described transformer and the secondary winding simple coil-induced coupling;

Be parallel with electric capacity on each rectifying device in the described rectifier bridge.

2, Switching Power Supply capable of filtering industrial frequency noise in floating system according to claim 1 is characterized in that, is provided with the screen that is used to intercept noise transfer between the elementary winding of described transformer and secondary winding.

3, Switching Power Supply capable of filtering industrial frequency noise in floating system according to claim 1 and 2 is characterized in that, described rectifier bridge is a full-wave rectification bridge.

4, Switching Power Supply capable of filtering industrial frequency noise in floating system according to claim 1 and 2 is characterized in that, described rectifier bridge is the halfwave rectifier bridge.

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