What are the effects of full load/light load on EMC of EPS?

The EMC performance of EPS varies under different load conditions (no load, light load, half load, full load) because load changes alter the operating conditions of switching devices and system parameters. The main effects include:

1. Switching waveform changes: Different loads result in different currents and junction temperatures of the switching transistors, potentially causing variations in switching speed (dv/dt, di/dt) and ringing amplitude, thus altering the noise spectrum. Full load leads to high switching losses and increased temperature, which may change noise characteristics.

2. Modulation depth and harmonics: The load affects the modulation ratio and current waveform, thereby changing the harmonic spectrum of the output voltage/current. These low-frequency harmonics may affect the low-frequency range of conducted emissions (close to 150kHz).

3. Filter operating point: Common-mode inductors may approach saturation under heavy loads and high currents, leading to a decrease in their high-frequency impedance and a deterioration in filtering performance.

Therefore, EMC testing standards typically require testing at multiple typical load points (such as minimum, rated, and maximum) within the product's nominal load range to cover the most unfavorable conditions.