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

Posted: 27 Oct 2011 ?? ?Print Version ?Bookmark and Share

Keywords:electromagnetic interference? common-mode currents? non-isolated? power supply?

Do you consider common-mode currents in a non-isolated power supply as a potential electromagnetic interference (EMI) source?

In high-voltage supplies, such as one you might find in an LED light bulb, you may find that you can't. On inspection, it really is no different than an isolated supply. There will be stray capacitance to ground from switching nodes that will generate common-mode currents.

Figure 1 is a schematic of a LED power supply showing the parasitic capacitance that is the main cause of common-mode current in this buck regulator. It is the capacitance to earth from the switch node. It is surprising how small this capacitance can be and still create a problem.

Figure 1: Even just 100 fF of capacitance from the switch node can create an EMI issue. (Click on image to enlarge.)

The CISPR Class B (for residential equipment) conducted emissions limit allows a 46 dB?V (200?V) signal into a 50? source impedance at 1MHz. This translates into only 4?A of allowable current. If the converter switches with a 200 Vpk-pk square wave on the drain of Q2 at 100kHz, the fundamental will be around 120V peak. Since the harmonics decrease in proportion to frequency, there will be about 9 Vrms at 1MHz.

That can be used to calculate an allowable capacitance to ground of around 0.1pF, or 100 fF (or a 2-M? impedance at 1MHz) which is an entirely feasible amount of capacitance from this node. There is also capacitance from the remainder of the circuitry to earth that provides a path for the common-mode currents to return. (This is notated as C_Stray2 in figure 1.)

In an LED light application, there is no chassis connection: only hot and neutral are available, so common-mode EMI filtering is a problem. That is because the circuit is high impedance. It can be represented by a 9 Vrms voltage source in series with a 2 M? capacitive reactance as shown in figure 2, and there is no realistic way to add impedance to reduce the current.

Figure 2: Even 100 fF can cause you to exceed EMI limits.


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