Electronic Design

Power Transformer Design Quells Noise On AC Mains

A power transformer technology developed by Plitron Manufacturing Inc. of Toronto, Ontario, attenuates differential and common-mode noises on the ac mains to the extent that external filtering may be eliminated. The company's Narrow Bandwidth Technology (NBT) relies on a combination of phase cancellation and a controlled increase in the transformer's leakage inductance (not to be confused with stray magnetics).

Phase cancellation is achieved by extending either the secondary or primary winding with an additional winding (fp). This winding contains the same number of turns as the secondary, but it's connected in reverse phase through a capacitor (Cfp). At low frequencies, the capacitor acts like an open switch. But above 1 kHz, it serves as a closed switch, causing the secondary and fp windings to generate magnetic fluxes that are 180° out of phase. As a result, the fluxes cancel, causing full cancellation of the high-frequency signals.

The corner frequency of the transformer's low-pass response is determined by the number of turns on all windings, the dc resistances of the windings, the secondary load impedance, and the value of Cfp. As a result, it can be adjusted through capacitor selection.

Plitron applies the NBT technique on a custom basis to its toroidal transformers to achieve low-pass filtering that exceeds the performance of most commercial filters (see the figure). Starting at 1 kHz (with no resonance effect), NBT provides attenuation of −20 dB per decade, achieving −70 dB at 500 kHz and −50 dB from 2 MHz to 1 GHz.

For more information, contact Brian Gladstone at (416) 667-9914 or see www.plitron.com/PDF/nbt.pdf.

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