Suppression of electromagnetic noise and electrostatic noise in switching power supply

Figure 1 shows a DC-DC converter with a built-in filter. Figure 1 (a) shows a DC-DC converter with an LC filter (L1 and C1 form a line filter). In order to give full play to the effect of LC filter in filtering noise, an external capacitor can be connected to form a π-shaped filter. PG is the frame ground, and the metal shell is connected to the frame ground. Figure 1 (b) shows a DC-DC converter with a common mode choke. Capacitors C1 and C2 and inductor L1 form a common mode noise filter, and C3 also has the effect of suppressing normal noise.

Figure 1 DC-DC converter with built-in filter

(a) DC-DC converter with built-in LC filter;

(b) DC-DC converter with common mode choke

Figure 2 shows the propagation of noise through the transformer. Since the primary and secondary windings of the transformer are tightly coupled, the high-frequency signal is coupled through the distributed capacitance Cps between the primary and secondary windings, and the common-mode noise current flows to the frame through C5. The normal noise current flows through Cps, and its magnitude decreases as the impedance of the series line with the load decays. However, the common-mode noise through C5 increases through the distributed capacitance between the wiring and the ground, and part of it becomes normal noise, so the noise on the load increases.

Figure 2: Noise propagation through a transformer

Figure 3 shows the situation where the noise generated inside the DC-DC converter is transmitted through the external circuit. This noise is transmitted in the circuit through static electricity or electromagnetic induction. When selecting an isolated DC-DC converter, it is necessary to confirm the coupling capacitance between the primary and secondary windings recorded in the product manual. The judgment standard is: a capacitance of 20 to 30 pF or less is a good product, a few hundred pF requires attention, and a few thousand pF cannot be used.

Figure 3: Noise propagation through external circuits

Even if an isolated DC-DC converter is used, the large leakage flux of the converter will have an adverse effect on the circuit and the purpose of isolation cannot be achieved. The so-called leakage flux here refers to the leakage flux generated by the isolation transformer, choke coil and printed circuit board wiring when the converter switch is working.

DC-DC converters may use plastic or metal casings. Of course, metal casings have better noise shielding effects.

(1) In order to suppress the electromagnetic noise and electrostatic noise of the switching power supply, the following points should be achieved:

1) The DC-DC converter should be placed in a metal casing.

2) The DC-DC converter must be well grounded.

3) The input end should be connected to an electrolytic capacitor with good frequency characteristics and small lead inductance.

4) The output end should be connected to a multilayer ceramic capacitor and an electrolytic capacitor with good frequency characteristics, and the lead inductance should be as small as possible.

5) Take measures to suppress common-mode noise.

(2) A DC-DC converter of good quality should meet the following conditions:

1) The isolation characteristics of the transformer are good, especially the AC isolation characteristics are more important.

2) There are measures to reduce electrostatic/electromagnetic coupling.

3) There is a noise filter inside and measures are taken to reduce switching noise.