Measures to suppress radiated EMI

To reduce radiated interference, you can use a voltage buffer circuit, such as connecting a ROD buffer circuit in parallel at both ends of the switch tube, or a current buffer circuit, such as connecting a 20-80μH inductor in series on the collector of the switch tube.

The collector of the switch tube is a strong interference source. The heat sink of the switch tube should be connected to the collector to ensure that the current generated by the distributed capacitance between the collector and the heat sink flows into the main circuit. In order to reduce the distributed capacitance between the heat sink and the casing, the heat sink should be as far away from the casing as possible. If conditions permit, a heat sink with shielding measures can be used. The rectifier diode should use a diode with a small recovery charge and a short reverse recovery time, such as a Schottky diode. It is best to use a diode with a soft reverse recovery characteristic. In addition, putting magnetic beads and parallel RC absorption networks on both ends of the Schottky tube can reduce interference. The values ​​of resistance and capacitance can be a few ohms and thousands of microfarads. The leads of the capacitor should be as short as possible to reduce the lead inductance.

The greater the load current and the longer the reverse recovery time of the diode, the greater the impact of the peak current. Using multiple diodes in parallel to share the load can reduce the impact of short-circuit peak current.

The switching power supply must be shielded and adopt a modular fully sealed structure, generally using galvanized steel plates with a thickness of more than 1mm. The shielding layer must be well grounded to suppress electromagnetic coupling of interference. Adding shielding covers to magnetic components such as high-frequency pulse transformers and output filter inductors can limit the magnetic lines of force within the shielding body with low magnetic resistance.

For example, for a switching power supply whose radiated interference exceeded the standard limit by 20dB, the following measures that are easy to implement in the laboratory were adopted to improve it:

(1) Connect a 470pF capacitor in parallel across all rectifier diodes;

(2) A 50pF capacitor is connected in parallel to the G input terminal of the switch tube to form an RC low-pass filter with the original 39Ω resistor;

(3) Connect a 0.01 pF capacitor in parallel to each output filter capacitor (electrolytic capacitor);

(4) Put a small magnetic bead on the pin of the rectifier diode;

(5) Improve the grounding of the shield.

After the above improvements, the power supply can pass the limit requirements of the radiated interference test.