EMI


The sum of many smaller parts


Electro Magnetic Interference is often seen as a black art, but the truth of the matter is that with sufficient knowledge, due diligence and attention to detail you can design your power supply to have the optimum performance, as Shane Callanan explains


EMI is a subject of increasing importance to all manufacturers of electronic equipment, especially with the extensive use of microprocessors. As a result EMC has become vital in every aspect of life from air travel to hospitals. Ensuring compliance is crucial.


Switch mode power supplies, in particular off- line switching power supplies, have high voltage switching waveforms that generate EMI in the form of conducted and radiated emissions. Figure 1 shows the power section of a typical flyback converter connected to a LISN for conducted EMI measurements. The Line Impedance Stabilization Network (LISN) is also shown and is only for the purpose of isolating the equipment under test from unwanted signals and also defines the impedance at radio frequencies at the terminals of the EUT. Parasitic capacitances are shown at several locations on the power supply, and they represent some of the parasitic capacitances in the circuit. (Note that the LISN is set-up to measure the EMI conducted along the live input line.)


For the purpose of this discussion assume the following arbitrary values


CYN = CYL = 1.5nF


Each parasitic capacitance to earth is set an arbitrary value of 30pF Consider the current loop from point A,


through the primary winding, through the main parasitic capacitance CPB , and back tough the Y caps to point A again.


At the drain of a power fet we could easily see a voltage that could be up to several hundred volts (let’s assume 500 Volts for this discussion). For an AC/DC convertor and a switching frequency of 100 kHz, the associated harmonics will extend up to several megahertz, with the


tenth harmonic being located at 1MHz. Assume a square wave, and this will be 20dB down on 200V or 60V.


A parasitic capacitance CPB, exists between the switching transistor and the heatsink, which is connected to the ground plane at point C. From Ohms Law we get the current flowing into the ground plane via CP4 as


Ig = 2 * π * f * V * CPB Eq (1)


=> Ig = 9.42 mA This current loop is closed back to the


transistor by Y capacitors CYN and CYL. If we assume that the current splits equally between these two, then the voltage developed across CYN, points A to D, would be:


VA = (1/2 * Ig / (2*π * f * CYL) Eq (2) => VA = 300 mVolts


The EN55022 level A limit for conducted EMI at 1MHz is 60dBuV, that is 60dB up on 1uV, therefore :


dBuV V50Ω


20 log [V50Ω/ 1uV] 10dBuV/20 * 1uV


V50Ω 1mV


The input filter forms a voltage divider between point A and the 50Ωof the measuring receiver and would have to attenuate the voltage derived above from eqn (2) by :


Atten = 20 log [VA/ V50Ω] = 49dB


Figure 1: Flyback convertor 10 CIE Power Supplement April 2012


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