How to ease EMI compliance in power converters

Most PWM converters are designed to switch at a fixed frequency, with several consequent benefits. One benefit is that the design of the conducted EMI-attenuating input filter is eased because the currents that the components of the filter have to process are at well-defined frequencies under all operating conditions.

Fig.1: Flyback converter with EMI filter

But a problem with this approach is that the converter's input current can cause it to violate EMI limits. To illustrate this problem for conducted noise, consider a typical DC-DC flyback converter, shown in Figure 1. Assuming continuous conduction without loss of generality the MOSFET current I_p is a trapezoid. This can be Fourier decomposed into components at the switching frequency and its harmonics. These Fourier components, if allowed to flow into the power source for the converter, can exceed the limits set by regulatory agencies.

These switching currents must therefore be low-pass filtered by the input capacitor and input EMI filter so that they are greatly attenuated by the time they reach the power source. However the filtering is not perfect, and often leaves enough residual switching currents to cause the system to fail conducted EMI tests as qualitatively illustrated by I_in in Figure 1.

Page 2: Spread spectrum switching
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