Power tip: Minimise EMI from offline power supply
Common-mode currents are generated by the high-voltage switching waveforms applied across the stray capacitances. These capacitances can be quite easy to visualize, such as the primary-to-secondary transformer capacitance, C_Stray2.
Figure 1: Common-mode inductor (L1) may not be needed at low power.
Or they may not so easy to visualize, such as the stray capacitance from the transformer core to chassis represented by C_Stray1. These common currents flow through the chassis connection whether it is an intentional physical connection to chassis or just capacitive coupling. As the currents complete their path through the input source, they can cause a product to fail EMI testing.
The typical approach to reducing common-mode emissions is to return common-mode currents in the transformer of figure 1 through C1 and to add a common-mode inductor, L1, to limit current flow. The challenge is that the common-mode inductor adds cost and size to the product, which is particularly undesirable to low-power, high-volume products like cell phone chargers.
Figure 2: C1=4700 pF makes dramatic improvement in EMI.
The following describes a series of incremental changes to the EMI filtering of a real design with the goal of eliminating the common-mode inductor. Figure 2 is the baseline EMI measurement which displays the CISPR class B limits and the first two measurements.
We removed the common-mode inductor (L1) and common-mode capacitor (C1) and made the measurements. We measured emissions of over 30 dBuV out of spec due to common-mode current through the transformer capacitance C_Stray1. This current continued into the secondary circuits and through stray capacitance into the chassis.
With C1 equal to 4700 pF, we measured a significant reduction in emissions of 30 dBuV, as shown on the plot. This improvement is due to the return of the common-mode currents through the added capacitance (C1). Adding C1 also changed the frequency at which the emissions peaked. These emissions are generated by the transformer magnetizing inductance resonating with the total stray capacitance on the drain of the MOSFET.
With C1 open and no secondary chassis connection, there is significant impedance in series with CStray,2 so it did not add much to the total stray capacitance on the drain. With C1 in the circuit, CStray2 added to the total stray capacitance and reduced the resonant frequency.
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