How to solve the problem of excessive radiation in switching power supplies
The switching power supply has a high rate of voltage and current variation, resulting in a high level of interference intensity; The interference sources are mainly concentrated during the power switch period, as well as the heat sink and high level transformer connected to it, and the position of the interference source in the digital circuit is relatively clear; The switching frequency is not high (from tens of kilohertz to several megahertz), and the main forms of interference are conducted interference and near-field interference.
Within 1MHz:
Mainly based on differential mode interference. 1. Increase the electrical capacity of X; 2. Add differential mode inductance; 3. Small power sources can be processed using PI type filters (it is recommended to choose larger electrolytic capacitors near transformers).
1M-5MHz:
Differential mode common mode mixing, using input and a series of X capacitors to filter out differential interference and analyze which interference exceeds the standard and solve it;
5MHz:
The above mainly focuses on co touch interference and adopts the method of suppressing co touch. For the case grounded, using a magnetic ring for 2 turns on the ground wire will have a significant attenuation of interference above 10MHz (didiu 2006); For 25-30MHz, it is possible to increase the Y capacitance to the ground, wrap a copper skin outside the transformer, change the PCBLAYOUT, and connect a small magnetic ring with a double wire winding in front of the output line, with a minimum of 10 turns, and install an RC filter at both ends of the output rectifier tube.
1M-5MHZ:
Differential mode common mode mixing uses a series of X capacitors connected in parallel at the input end to filter out differential mode interference and analyze which type of interference exceeds the standard and solve it. 1. For differential mode interference exceeding the standard, the X capacitance can be adjusted by adding a differential mode inductor and adjusting the differential mode inductance; 2. For common mode interference exceeding the standard, common mode inductors can be added and a reasonable amount of inductance can be selected to suppress it; 3. The characteristics of the rectifier diode can also be changed to handle a pair of fast diodes such as FR107 and a pair of ordinary rectifier diodes 1N4007.
Above 5MHz:
Mainly focusing on co touch interference, adopting the method of suppressing co touch.
For the case grounded, using a magnetic ring in series for 2-3 turns on the ground wire will have a significant attenuation effect on interference above 10MHz; You can choose to stick copper foil tightly to the iron core of the transformer, with copper foil closed loop. Handle the size of the absorption circuit of the back-end output rectifier tube and the parallel capacitance of the primary large circuit.
