Characteristics of EMI in switching power supplies
As an energy conversion device operating in a switching state, the voltage and current change rate of a switching power supply is very high, resulting in significant interference. The interference sources are mainly concentrated during power switching and in connection with heat sinks and high-voltage transformers, and their locations are relatively clear compared to digital circuits. 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. Printed circuit board (PCB) traces are usually routed manually, which has greater arbitrariness, increasing the difficulty of extracting PCB distributed parameters and estimating near-field interference.
Within 1MHz—differential-mode interference is the main issue; increasing the X capacitor can resolve this.
1MHz-5MHz: Differential and common-mode interference is present. A series of X capacitors are connected in parallel at the input to filter out differential-mode interference and analyze which type of interference is exceeding the limit and resolve it. Above 5MHz: Common-mode interference is dominant. Common-mode suppression methods are used. For circuits with a grounded casing, wrapping a ferrite core around the ground wire twice will significantly attenuate interference above 10MHz (diudiu2006). For 25-30MHz: However, methods such as increasing the Y capacitor to ground, wrapping the transformer with copper foil, changing the PCB layout, connecting a small, two-wire, parallel-wound ferrite core before the output line (at least 10 turns), and connecting an RC filter across the output rectifier diodes can be used.
The frequency of 30-50MHz is generally caused by the high-speed turn-on and turn-off of the MOSFET. This can be solved by increasing the MOSFET drive resistor, using a 1N4007 slow transistor in the RCD buffer circuit, and using a 1N4007 slow transistor for the VCC power supply voltage.
The 100-200MHz frequency issue is generally caused by the reverse recovery current of the output rectifier diode. A ferrite bead can be connected in series with the rectifier diode.
In the 100MHz-200MHz range, most issues stem from PFC MOSFETs and PFC diodes. Currently, connecting MOSFETs and PFC diodes with ferrite beads is effective, largely resolving the problem horizontally, but it's less effective vertically.
The radiation from switching power supplies generally only affects frequencies below 100MHz. Appropriate absorption circuits can be added to the MOSFETs and diodes, but this will reduce efficiency.
Measures to prevent EMI when designing switching power supplies:
1. Minimize the PCB copper foil area of noisy circuit nodes; such as the drain and collector of switching transistors, and the nodes of primary and secondary windings, etc.
2. Keep the input and output terminals away from noisy components, such as transformer coils, transformer cores, heat sinks of switching transistors, etc.
3. Keep noisy components (such as unshielded transformer coils, unshielded transformer cores, and switching transistors, etc.) away from the edge of the casing, as the edge of the casing is likely to be close to the external grounding wire under normal operation.
4. If the transformer does not use electric field shielding, keep the shielding and heat sink away from the transformer.
5. Minimize the area of the following current loops: secondary (output) rectifier, primary switching power device, gate (base) drive circuit, and auxiliary rectifier.
6. Do not mix the gate (base) drive feedback loop with the primary switching circuit or auxiliary rectifier circuit.
7. Adjust and optimize the damping resistor value so that it does not produce ringing noise during the dead time of the switch.
8. Prevent EMI filter inductor saturation.
9. Keep the turning points and secondary circuit components away from the shielding of the primary circuit or the heat sink of the switching transistor.
10. Keep the oscillating nodes and component bodies of the primary circuit away from shielding or heat sinks.
11. Place the high-frequency input EMI filter close to the input cable or connector end.
12. Keep the EMI filter for high-frequency output close to the output wire terminals.
13. Maintain a certain distance between the copper foil on the PCB opposite the EMI filter and the component body.
14. Place some resistors on the rectifier circuit of the auxiliary coil.
15. Connect a damping resistor in parallel with the magnetic rod coil.
16. Connect a damping resistor in parallel across the output RF filter.
17. In PCB design, a 1nF/500V ceramic capacitor or a series of resistors can be placed between the stationary terminal of the primary winding and the auxiliary winding of the transformer.
18. Keep the EMI filter away from the power transformer; in particular, avoid positioning it at the end of the wrapping.
19. If the PCB area is sufficient, the PCB can be reserved for the pins for the shielding winding and the position for the RC damper. The RC damper can be connected across the two ends of the shielding winding.
20. If space permits, place a small radial lead capacitor (Miller capacitor, 10 picofarads/1 kV) between the drain and gate of the switching power MOSFET.
21. If space permits, place a small RC damper at the DC output.
22. Do not place the AC socket near the heatsink of the primary switching transistor.
