Compensating for Parasitic Effects in Low-Pass Filters

Parasitic inductances are effectively transferred; parasitic capacitances are effectively neutralized.

February 1, 2007

Naval Research Laboratory, Washington, DC

Techniques to compensate for the effects of parasitic inductances and capacitances have been developed as part of an effort to improve the performances of low-pass filters in electronic power circuits. As used here, "parasitic" refers to departure from an ideal inductive or capacitive characteristic. No inductor, capacitor, or other electronic component is ideal: wherever a current loop exists, there is inductance, and wherever two conductors are near each other, there is a capacitance between them. Parasitic capacitances in inductors and parasitic inductances in capacitors degrade the performances of low-pass filters, especially at high frequencies.

The Compensating Winding and Capacitor are added to the toroidal inductor to reduce the effect of parasitic capacitance in the main winding.

Heretofore, the usual approach to improving the high-frequency performance of a low-pass power filter has involved adding filter stages and components, thereby adding significantly to cost and weight. The present compensation techniques make it possible to achieve order-of-magnitude reductions in effects of parasitic inductances and capacitances with smaller increases in cost and weight.

The compensation techniques, denoted inductance cancellation and capacitance cancellation, are useful for improving the high-frequency performance of a filter capacitor or a filter inductor, respectively. The details of implementation of these techniques are complex and highly susceptible to application- specific variation. Notwithstanding this complexity, the underlying principles can be stated simply:

Inductance Cancellation — The effect of parasitic inductance in a capacitor can be shifted from a circuit branch where it is undesirable to other circuit branches where it is desirable or at least acceptable. In inductance cancellation, this shift is accomplished by use of magnetically coupled windings. From the perspective of traditional lumped-element circuit analysis, the net effect is to place a negative inductance in series with the parasitic inductance while placing larger positive inductances in the other circuit branches. Although the total inductance is increased, the effective decrease in the parasitic inductance improves the high-frequency performance of the affected capacitor.
Capacitance Cancellation — The undesirable effect of parasitic capacitance in a filter inductor is to couple highfrequency current from a noisy input port to an output port that is desired to be quiet. In capacitance cancellation, one introduces additional passive components to inject a high-frequency current approximately equal in magnitude and opposite in sign to the current flowing through the parasitic capacitance so that the net high-frequency current arriving at the output port is greatly reduced. In the example illustrated in the figure, capacitance cancellation of a toroidal filter inductor is effected by adding a compensating winding in series with a compensating capacitor. To a first approximation, cancellation could be achieved by choosing m(1-m)C_comp = C_p, where m is the ratio between the number of turns in the compensating winding and the main winding, Ccomp is the compensating capacitance, and Cp is the parasitic capacitance of the main winding.

This work was done by Timothy C. Neugebauer, Brandon J. Pierquet, and David J. Perreault of Massachusetts Institute of Technology for the Naval Research Laboratory. For further information, download the free white paper at www.defensetechbriefs.com under the Electronics/Computers category. NRL-0003