Magnetics Business & Technology - Spring 2012 - (Page 12)

FEATURE ARTICLE By riCharD ozenBaugh • ozenBaugh engineering Four-Pole Technique for EMI Filter Design Tim Pullen, from Rockwell Collins, and I have written the 3rd edition of my book “EMI Filter Design.” Mr. Pullen has developed an interesting four-pole technique that splits the filter cutoff frequencies so that one filter section tends to remove the potential resonant rise of the other section. The equations are: The K value is the upper cutoff frequency multiplier. Therefore: For Example, if 150 kHz (FT ) needs 72 dB loss, then A is equal to 72, then B is equal to 63. If the multiplier is 3, then K equals 3. K is chosen by the engineer. Calculate F1 . The component for both sections follows. Z0 is 3 Ohms – the source resistance. Figure 1. Initial K 1, 3 and 5 Filters There are some very interesting properties that are quite useful. First, by adding the inductance together, the sum grows as the K value grows and so does the total capacitance values. However, the change between K values is not too great. I decided to use three different K values – 1, 3 and 5. I used the exact calculated component values, normally these values are increased to standard values. I used the original values of 3 Ohms, 72 dB loss and 150 kHz. The three filters are shown in Figure 1. To see the response, the following graph shows the three curves. The graph area is reduced to get a clearer view of the resonant rise of the three K values and to see that the 72 dB loss is met for the three K values. On a wider plot all three curves track the zero dB line and any one of these K values would be usable for 400 Hz. It is interesting to see what happen to the resonant rise frequency as the K values increase. The resonant rise frequency increase as K increases and the RC Shunt capacitor drops in value if the engineer thinks that the RC shunt is necessary. The increase of the resonant rise slope shows that the circuit Q decreases as the K value increases (see Figure 2). Figure 2. Curves of Figure 1 Using a higher the K value, the resonant rise frequency is increased and the smaller the RC Shunt capacitor value is. The total filter capacitor value does grow after adding the RC Shunt capacitor. The of each RC shunt capacitor is equal to the design impedance at the resonant rise frequency and the series resistor is also the design impedance, in this article this is 3 Ohms. There is more information on this in our book, EMI Filter Design, Third Edition. Please visit my web site at www.emi-filter-design.com, or email me at orichard_lee@hotmail.com for more information. 12 Magnetics Business & Technology • Spring 2012 www.MagneticsMagazine.com http://www.emi-filter-design.com http://www.emi-filter-design.com http://www.MagneticsMagazine.com

Magnetics Business & Technology - Spring 2012
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