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RE: Choke-input power supplies, Part 1

Posted by Henry Pasternack on April 27, 2009 at 13:31:54

In Reply to: RE: Choke-input power supplies, Part 1 posted by SteveBrown on April 20, 2009 at 19:12:14:

I'm back from vacation and hope to find time to work on the second part of the article soon.

My definition of a choke-input filter is one where there's enough inductance relative to the DC load current that the full-wave rectifier never shuts off throughout the entire AC cycle. (This ignores the "dead" zone I mentioned earlier due to the non-zero forward voltage drop of the rectifiers.) This is is a pretty standard definition, I think.

If the inductor is smaller than the critical value, the rectifier current will try to swing below zero twice every AC cycle. Since the rectifier can't conduct backwards by definition, the current will cut off suddenly (and turn back on suddenly a little while later). To recap my earlier article, this makes more switching noise and degrades the choke-input filter's good DC regulation,

A cap-input filter has one set of characteristics, and a choke-input filter has another set. Up until I started studying Jeff's "Low DCR" ideas, I assumed for intermediate (sub-critical) values of L the resulting filter would have properties intermediate between the two types, and in proportion to the inductance. Instead, I found, with the right combination of L and C, there is another mode of operation distinct enough to deserve (in my view) a unique name, which I called the "flywheel" filter. The flywheel filter has some interesting and useful properties of its own and is not just a blending of cap- and choke-input characteristics.

So, really, the role of the inductor is a bit more complicated than it seems at first glance. I think you can blame the rectifier for that, because it's nonlinear, which blows away some of our intuitions about smoothly varying changes in behavior with small changes in component values.

Bearing in mind that no component is perfect, a small inductor in the real world does some things better than a large inductor (like block high frequencies). The fact that a big old choke-input filter has a particular advantage for a specific reason does not preclude a smaller choke having different advantages for different reasons. And of course, with the advantages come disadvantages, and these have to be factored into the overall design as well.

Again, the goal here is to look at the elements of circuit design and to talk about some of the tradeoffs that you might not read about in engineering textbooks or typical DIY articles. But if I say something is "superior", that doesn't mean it's best in any given application, or in any application at all for that matter. I'd like people to understand how circuits work, but after that it's up to them to decide how to design their gear according to their own ideas of what's best and most interesting.

-Henry

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