How to solve the problem of excessive radiation of switching power supply
The voltage and current change rate of the switching power supply is very high, and the interference intensity generated is relatively large; the interference source is mainly concentrated during the power switching period and the radiator and high-level transformer connected to it, and the position of the interference source relative to the digital circuit is relatively clear; the switching frequency Not high (from tens of kilohertz and several megahertz), the main forms of interference are conducted interference and near-field interference.
The specific solutions for each frequency point exceeding the standard are as follows:
Within 1MHz:
Mainly differential mode interference 1. Increase X capacitance; 2. Add differential mode inductance; 3. Small power supply can be processed by PI filter (it is recommended to choose a larger electrolytic capacitor close to the transformer).
1M-5MHz:
Differential mode and common mode mixing, using the input terminal and a series of X capacitors to filter out the differential interference and analyze which kind of interference exceeds the standard and solve it;
5MHz:
The above is mainly based on co-mouse interference, and the method of suppressing co-mouse is adopted. For the case grounded, using a magnetic ring on the ground wire for 2 turns will greatly attenuate the interference above 10MHZ (diudiu2006); for 25--30MHZ, you can increase the Y capacitor to the ground and wrap copper skin outside the transformer , Change PCBLAYOUT, connect a small magnetic ring with two wires in parallel in front of the output line, at least 10 turns, and connect an RC filter at both ends of the output rectifier tube.
1M-5MHZ:
Differential-mode common-mode mixing, using a series of X capacitors connected in parallel at the input to filter out differential-mode interference and analyze which kind of interference exceeds the standard and solve it. 1. For differential-mode interference exceeding the standard, you can adjust the X capacitance and add a differential-mode inductor , to adjust the differential mode inductance; 2. For common mode interference exceeding the standard, common mode inductance can be added, and a reasonable inductance can be selected to suppress it; 3. The characteristics of the rectifier diode can also be changed to deal with a pair of fast diodes such as FR107 and a pair of ordinary rectifier diodes 1N4007 .
Above 5MHz:
Focus on co-moting interference, and adopt the method of suppressing co-moting.
For the grounding of the shell, using a magnetic ring in series on the ground wire for 2-3 turns will have a greater attenuation effect on interference above 10MHZ; you can choose to stick copper foil to the iron core of the transformer, and the copper foil is closed-loop. Deal with the size of the snubber circuit of the back-end output rectifier and the parallel capacitance of the primary large circuit.
For 20M-30MHz:
1. For a class of products, you can adjust the capacitance of Y2 to ground or change the position of Y2 capacitance;
2. Adjust the Y1 capacitor position and parameter value between the primary and secondary sides;
3. Wrap copper foil on the outside of the transformer; add a shielding layer to the innermost layer of the transformer; adjust the arrangement of the windings of the transformer.
4. Change the PCB Layout;
5. In front of the output line, connect a small common-mode inductor with double-wire parallel winding;
6. Connect RC filters in parallel at both ends of the output rectifier and adjust reasonable parameters;
7. Add BEADCORE between the transformer and MOSFET;
8. Add a small capacitor to the input voltage pin of the transformer.
9. You can increase the MOS drive resistance.
30M-50MHz:
1. It is generally caused by the high-speed turn-on and turn-off of MOS tubes. It can be solved by increasing the MOS drive resistance, using 1N4007 slow tubes for the RCD buffer circuit, and using 1N4007 slow tubes for the VCC supply voltage.
2. RCD buffer circuit adopts 1N4007 slow tube;
3. The VCC power supply voltage is solved by 1N4007 slow tube;
4. Or the front end of the output line is connected in series with a small common mode inductor with two wires wound in parallel;
5. Connect a small snubber circuit in parallel with the D-S pin of the MOSFET;
6. Add BEADCORE between the transformer and MOSFET;
7. Add a small capacitor to the input voltage pin of the transformer;
8. When PCB LAYOUT, the circuit loop composed of large electrolytic capacitors, transformers and MOS should be as small as possible;
9. The circuit loop composed of transformer, output diode and output smoothing electrolytic capacitor should be as small as possible.
50M-100MHZ:
It is generally caused by the reverse recovery current of the output rectifier tube,
1. Magnetic beads can be strung on the rectifier tube;
2. Adjust the absorbing circuit parameters of the output rectifier;
3. The impedance of the primary and secondary side across the Y capacitor branch can be changed, such as adding BEADCORE to the PIN pin or connecting an appropriate resistor in series;
4. It is also possible to change the MOSFET to output the radiation from the body of the rectifier diode to the space (such as the iron clip MOSFET; the iron clip DIODE, change the grounding point of the radiator).
5. Add shielding copper foil to suppress radiation to space.
