inductor sizing

You'd probably be happier in one of those newsgroups devoted to amateur psychoanalysis and human potential and stuff like that. This group is about electronic design.

John

The cheap yellow paint that Micrometals uses smells terrible when it burns.

John

Its not paint. It's a long distance, over-temperature indicator.

--
Regards,

John Popelish

--
LOL, really????

What, have you run out of recipes and anecdotes about beans and
pictures of the environs of SF and your building and restaurant
recommendations and on and on and on?

Not to my knowledge, the OP started with 250uH which would have resulted into a large filter so I was just trying to point out that a smaller total size is optimum. I suppose you could make the same argument for minimum cost.

Interesting, because of the PWM and duty>90%, there are a ton of harmonics. To do a good job attenuating the first harmonic the smaller LC would have to start early so the fundamental trap may not be too effective. Do not quote me on this! Cheers, Harry

Why would I run out of things? I keep doing things.

Last night we went to a restaurant decorated with - almost counstructed from - electrical conduit. I discovered Little Gem lettuce, truly superb with their dressing, with roasted hazelnuts and radish slices.

In the Mission District, the dining philosophy is to grab any parking spot that may magically appear, and then walk into the closest restaurant.

John

Good point. PWM does complicate the harmonics situation.

Incidentally, if one does a 2-stage filter, the cheap iron powder cores are usually fine for the second stage.

John

Excellent point, the second inductor has very little 200KHz causing core loss, so 60Hz material, with higher flux density would be suitable. Litz wire is also not needed but parasitic capacity should be reduced as much as possible. Cheers, Harry

--
Geez, I guess that since, according to you, 

"This group is about electronic design."

What\'s good for the goose isn\'t good for the gander?"

Gosh, you sure have a way with cliches.

John

--
Gosh, you sure have a way of missing the point.

On purpose, usually.

John

Ok Jamie, You sound like a nice person so here is a filter design that will meet your needs with a bottle of tequila to be sent upon the design's approval. This is the first LC section for one phase and will survive the 200KHz/440VDC stress and present a nice 60Hz sine wave to the second LC section input which you can design using high flux 60Hz material to meet you unspecified output requirements. As stated earlier the THD depends on the control loop and not the HF output filter. For the 25uH/30Apk inductor use a Ferroxcube or similar EC-70 (3F3 material) core set and bobbin with 21 turns of #10 AWG LITZ wire wound in 3 layers of 7turns/layer. Insulate all for 3500VDC. The LITZ wire should be made up with 256/#34 AWG wires. This will require a 0.21" gap across all three legs of the core. Do not grind the center leg for a 0.42" gap because this will cause increased Rac losses. The total dissipation will be

--
Ergo, intentionally devious...

On Mar 7, 8:44 am, "Harry Dellamano" wrote: ...

One reason to stay with smaller inductor values is to minimize the parasitics, so they perform well at high frequencies. Given the

200kHz fundamental, it doesn't take all that much inductance/ capacitance to kill the harmonics. There are "a ton of" harmonics at 50 percent duty cycle, too; it's more a matter of there being MORE fundamental at 50 percent duty cycle. For killing harmonics, a multiple-stage filter is far more efficient than a single L section. You can add zeros to the filter if you want, to kill lower frequencies (i.e. the fundamental) more effectively, and the tradeoff is lower attenuation at higher frequencies (which are likely lower amplitude to begin with, so the higher attenuation isn't really necessary there).

Although I probably wouldn't seriously do a power supply like this with an air core coil, I thought it worthwhile to give as an example because it sets an upper limit on the required coil size. I don't really agree that an air coil would be "impossible" to shield; I'm quite used to getting in excess of 120dB shielding on filters using air core coils, and since it's a linear system, I'd expect to be able to do that without too much trouble for such a power supply filter, if I needed to. I made a crude estimate of the Q of an air core coil like the one I suggested, and decided it would be fairly easy, using Litz wire, to get it above 100 at 200kHz.

Cheers, Tom

This filter is interesting because the first inductor is about protecting the first cap's RMS ripple current at 200KHz. The design iterations are; select a size and cost of first cap with as much Irms current as possible. Calculate input inductor necessary to maintain the cap's ripple current. If inductor size is too big go back and increase the cap's size and recalculate the inductor. When you are happy with the size, cost and temp rise of the first stage, then put on your 60Hz hat and design the second stage for attenuation requirement. The first stage air core sounds outrageous to me. The air core is a primary winding with 220VAC, 25A at 200KHz. Every loop of wire within 6" of it would be a secondary. A u-metal shield would be necessary. We have problems with radiated magnetic fields when we gap a power transformer by >0.050", your air core has a >2" gap. Regards, Harry

So, I agree with the first stage design. If I'm not mistaken, the Wima MKP10 caps I mentioned in an earlier posting can handle the sort of ripple current that even a very small inductance would yield, given a small stack of parallel ones.

As for designing the following stage for 60Hz, I gotta disagree. That's what control systems are for. But because of parasitics that are bound to be in the layout and the parts, a second stage is very useful for further reducing the 200kHz+harmonics, as I see it. I specifically do NOT want a low-frequency filter there, or it will play havoc with the control system design.

I'll repeat that it's unlikely I'd actually use an air core coil in that application, but to me, it's far from outrageous. People have been building resonant tank circuits using air-core coils that handle a lot more power than we're talking about here (and at a lot higher loaded Q, so there's much more energy stored in the coil's field), and appropriate shielding hasn't been all that difficult to achieve. Please correct me if I'm wrong, but as far as I know, linearity holds at these still fairly modest power levels, so the shielding that gives me 120dB attenuation for moderately large air core coils at low power (I use them for high Q; air core for extremely low distortion) should give me similar attenuation for considerably higher power.

Cheers, Tom

Well, it's more fun than being intentionally obtuse. Pays better, too.

John

--
So, ATY, what matters is bullshit and bucks?