How to balance efficiency and EMI in EPS switching frequency?

The selection of the switching frequency in an EPS (Electric Power Supply) is a crucial trade-off between efficiency and EMI performance. Increasing the switching frequency can reduce the size and weight of magnetic components (inductors, transformers) and smooth the output waveform, but it also has the following effects:

1. Negative for EMI: Increasing the switching frequency directly leads to higher fundamental and harmonic frequencies, shifting the noise spectrum to higher frequencies (>1MHz) that are more difficult to filter, and increased switching losses may worsen thermal management.

2. Negative for efficiency: Increased switching losses (proportional to frequency) reduce conversion efficiency.

Therefore, the trade-off strategy is to choose the lowest possible switching frequency to optimize efficiency and EMI, while meeting output harmonic requirements and minimizing the size and cost of passive components. For fixed-frequency designs, careful optimization of the drive and layout is necessary to reduce noise energy per switch. Alternatively, frequency conversion or spread spectrum techniques can be used to distribute noise energy across a wider frequency band, reducing peak values at specific frequencies and thus alleviating EMI pressure, but this may introduce low-frequency ripple.