M&M 3

Inductors are inductors, capacitors are capacitors

I remembered the days when I was a power electronics engineer. I was quite efficient of using capacitors and inductors and I was aware of their limitations.

For instance, I knew that an inductor core can be saturated when a high level of DC current exceeds the rated current and the inductor became useless. Well, simply replacing the inductor with a jump wire and I could see the conducted and radiated emission result didn’t budge. I was also aware of using capacitors at their lowest impedance frequency point.

But I was not aware that at high frequencies, the interwinding capacitance of the inductor could mean that the inductor effectively becomes a capacitor. Same with that at high frequencies, a capacitor could behave more like an inductor rather than capacitor.

An RF engineer might laugh at this, because for RF engineers, at a very early stage, they have leant how to tune an open ended transmission line to become either an inductor or a capacitor. They don’t like using discrete components when frequency goes beyond GHz range, simply because at high frequency, things become much more complex, for instance, nonlinearity of the dielectric materials, lossy behaviour of component, etc.

Most of the system engineers, electronics engineers, mechanical engineers I later met when I started my own consultancy business are not RF engineers by training. So they would not know the high frequency behaviour of discrete passive components like me when I was an electronics engineer. The challenge here is to pursue the ultimate goal of high efficiency, electronic circuit switches faster and faster, even with the power electronics industry.

10 years ago, in power electronics industry, people were fascinated by new technology brought by MOSFETs, but now, GaN and SiC is bringing the frequency of a circuit to a different level. Every time when the frequency increases by an order of magnitude, we need to review our component selection. It is worth knowing that inductors are not just inductors and Capacitors are not just capacitors.