Take computer power supply as an example to introduce some knowledge about shielding technology

The shielding technology we talk about usually has two meanings

A. Block stray electromagnetic waves and other interference signals in the environment (including stray electromagnetic waves on the industrial frequency power grid) outside the shielded power system.

B. Block the electromagnetic waves radiated or propagated outward from the oscillating signal source and alternating power radiation source in the power system through various links and paths in the circuit within the power system.

Computer power supply is a switching power supply, so the high-frequency electromagnetic waves generated by the oscillator and power converter will pass through the internal transformer, inductor, capacitor, resistor, lead and PCB circuit board.

Etc. propagation and radiation.

Shielding Technology

1. Soft shielding

The so-called soft shielding is to use effective circuit technology (such as common mode filter technology, differential mode filter technology, bidirectional filter technology, low-pass filter technology, etc.)

The following figure is the schematic diagram

By comparing the schematic diagram, we can see how many parts the manufacturer has missed in the physical picture below.

The above is a single-stage one. If cost is not a consideration, you can use a multi-stage one.

The circuit diagram is as follows:

There is a calculation method for selecting capacitors here, but there is no need for me to calculate it, that is the designer's job.

C1, C2, C5, C6 generally use 2200pF/250V, C3, C4 use 0.1uF/400V.

We see that the capacitor connection leads on the circuit board are cut very short. This is to reduce the lead inductance.

For such a filter, when added to the input end of the switching regulated power supply, it only allows low-frequency signals below 400HZ to pass through (we use 220V/50HZ)

For high frequency signals between 1-20KHZ, the attenuation is 40-100dB.

A major feature of capacitors is that they block direct current but pass alternating current, while a major feature of inductors is that they block alternating current but pass direct current.

High-frequency capacitors are used here, and electrolytic capacitors cannot be used.

The previous section discussed input-side filtering, and this section discusses output-side filtering.

Reason: To prevent and reduce the high-frequency signal inside the switching power supply from being superimposed on the output DC voltage to form clutter noise, thereby affecting the normal operation of the load circuit system, and to prevent the high-frequency signal in the load circuit system from entering

The switching regulated power supply circuit affects its normal operation.

The first figure is a pure inductor filter (output end). The inductor is usually a single enameled wire wound on a ferrite rod with a diameter of 6 mm and a length of about 30 mm. The number of turns is generally 4-7.

Some well-made computer power supplies also use magnetic rings.

Features: Simple structure, good filtering and blocking effect on high-frequency spike interference signals.

The second figure is the storage capacitor filter (output end). In actual application, two capacitors are used here, one is a high-frequency capacitor and the other is an electrolytic capacitor.

A high-frequency spike pulse signal is obtained on the output voltage.

The third picture is a hybrid type of capacitors and inductors. Of course, the effect will be greatly improved.

After talking about input and output filtering, let’s talk about output wiring technology.

When the leads are long and the wiring is unreasonable, the parasitic capacitance generated between the wires will increase to a level that cannot be ignored. The common-mode noise signal will be transmitted and introduced into the load circuit system through these parasitic capacitances, affecting its normal operation.

The most prominent one is in the amplifier circuit. If we put the power line next to the signal input line, we can hear the AC sound from the amplifier. I believe that many netizens who have used poor speakers will find that there is AC sound as long as it is turned on.

Experiments and practice have proved that the transmission effect of twisted lines is much better than that of parallel lines.

In addition to the use of these wiring technologies to control, filter and attenuate clutter noise signals, the selection of the ground wire location, the connection method of the ground wire, the length of the ground wire, the thickness of the ground wire, etc. are all related to the size of the filtered noise signal.

Have a close relationship.

The other is hard shielding technology, which is simply a shielding cover. Its function is to shield the residual electric radiation and magnetic radiation of the switching power supply after soft shielding, and to prevent external stray electromagnetic noise.

Electromagnetic waves radiate into the switching power supply circuit, affecting the normal operation of the power supply.