Power tip: Role of common-mode currents in non-isolated power supplies

To reduce the emissions at 1MHz, you need to reduce the voltage or reduce the stray capacitance. Two ways to reduce the voltage is with dithering or rise time control. Dithering varies the operating frequency of a power supply to spread out the spectrum.

Rise-time control slows the switching speed in the power supply, to limit the high-frequency spectrum and is better suited for EMI problems above 10MHz. Reducing the stray capacitance from the switching node can be as simple as minimizing the etch area or it may involve shielding. Capacitance from this node to one of the rectified supply lines does not create common-mode current, so you can bury the trace in a multilayer printed wiring board (PWB) and reduce much of the unwanted capacitance.

However, you can not completely eliminate it because there is still capacitance remaining from the drain of the FET and inductor. Figure 2 provides a graph and steps you through the calculation of the EMI spectrum.

The first step is to calculate the spectrum of the voltage waveform (red). This is accomplished by calculating the Fourier series of the drain-voltage waveform, or more simply by calculating the fundamental component and approximating the envelope as one divided by the harmonic number and the fundamental.

A further adjustment is made at high frequency [1/(Π × rise time)], as shown above 7MHz. The next step involves dividing this voltage by the reactance of the stray capacitance. Interestingly, the low-frequency emissions are flat with frequency until you cross the pole that is set by rise time.

Finally, the CISPR Class B limits are also plotted. With only 0.1 pF of stray capacitance and a high-voltage input, emissions are close to the limits.

EMI problems can also exist at higher frequencies due circuit resonances and radiated emissions caused by resonances of input cabling. Common-mode filtering can help these issues because there is a reasonable amount of capacitance in C_Stray2.

For instance, if it were 20 pF, its impedance would be less than 2 k? at 5MHz. Common-mode inductors of sufficient impedance can be added between the circuit and the 50 ? test resistor to reduce measured emissions. This is also true at higher frequencies.

To summarize, with high-voltage, non-isolated power supplies, common-mode currents can cause EMI emissions to exceed standard limits. In two-wire designs (no chassis connection), they are particularly difficult to handle because of the high impedances involved.

The best way to approach this kind of challenge is to minimize the stray capacitance and to dither the switching frequency. At higher frequencies, where the impedance of the distributed capacitance from the remainder of the circuit becomes small, common-mode inductors can reduce both radiated and conducted emissions.

About the author
Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.

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