How to solve the problem of the excessive radiation of the switching power supply unit
Switching power supply voltage, current change rate is very high, resulting in greater intensity of interference; interference sources are mainly concentrated in the power switching period and connected to the heat sink and high level transformer, relative to the location of the digital circuit interference source is clearer; switching frequency is not high (from tens of kHz and several megahertz), the main form of interference is the conduction interference and near-field interference.
Specific individual frequency points over the solution is as follows:
Within 1MHz:
Differential mode interference is the main 1. Increase the X capacitance; 2. Add differential mode inductance; 3. Small power supply can be used PI-type filter processing (it is recommended that the electrolytic capacitors near the transformer can be selected larger).
1M-5MHz:
Differential mode common-mode mixing, using the input side and a series of X capacitors to filter out the differential touch interference and analyze which kind of interference exceeds the standard and solve it;
5MHz:
Above to common touch interference is mainly, using the method of suppressing common touch. For the shell grounded, in the ground line with a magnetic ring around 2 circles will be more than 10MHZ interference has a greater attenuation (diudiu2006); for 25 - 30MHZ but can be used to increase the Y capacitance to the ground, in the transformer outside the bread copper, change the PCBLAYOUT, the output line in front of the connection of a two-wire and winding of a small magnetic ring, a minimum of 10 turns around the output rectifier tube ends and the RC filter.
1M-5MHZ:
Differential mode common-mode mixing, using the input side in parallel with a series of X capacitance to filter out the differential interference and analyze which interference exceeds the standard and to solve the problem, 1. For the differential mode interference exceeds the standard can be adjusted to X capacitance, add a differential mode inductor, differential mode inductance; 2. For the common-mode interference exceeds the standard can be added to the common-mode inductance, the choice of a reasonable amount of inductance to inhibit; 3. You can change the characteristics of the rectifier diode to deal with a pair of fast diodes, such as FR107 a pair of ordinary rectifier diodes 1N4007.
Above 5MHz:
The common touch interference is the main one, and the method of suppressing the common touch is used.
For the shell grounded, in the ground line with a magnetic ring string around 2-3 turns will be more than 10MHZ interference has a greater attenuation effect; can choose to stick copper foil immediately after the core of the transformer, copper foil closed loop. Deal with the back-end output rectifier absorption circuit and the size of the primary large circuit shunt capacitance.
For 20M-30MHz:
1. For a class of products can be used to adjust the Y2 capacitance to ground or change the Y2 capacitor position;
2. Adjust the Y1 capacitance position and parameter value between the primary and secondary sides;
3. Load copper foil on the outside of the transformer; add shielding layer to the innermost layer of the transformer; adjust the arrangement of the windings of the transformer.
4. Change the PCB Layout;
5. Output line in front of the connection of a two-wire parallel winding of a small common mode inductor;
6. Output rectifier in parallel with both ends of the RC filter and adjust the reasonable parameters;
7. Add BEADCORE between the transformer and the MOSFET;
8. Add a small capacitor to the input voltage pin of the transformer.
9. Can be used to increase the MOS drive resistor.
30M-50MHz:
1. Generally caused by high-speed turn-on/off of the MOS tube, can be solved by increasing the MOS drive resistor, RCD buffer circuit using 1N4007 slow tube, VCC supply voltage with 1N4007 slow tube.
2. RCD buffer circuit using 1N4007 slow tube;
3. VCC supply voltage with 1N4007 slow tube to solve;
4. Or the output line front end in series with a two-wire parallel winding of a small common mode inductor;
5. Connect a small absorption circuit in parallel with the D-S pin of the MOSFET;
6 . Add BEADCORE between the transformer and the MOSFET;
7. Add a small capacitor to the input voltage pin of the transformer;
8. PCB LAYOUT when the large electrolytic capacitor, transformer, MOS constitute the circuit loop as small as possible;
9. transformer, output diode, output flat wave electrolytic capacitor constitute the circuit ring as small as possible.
