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Output-side LC filter (2023 Update)

I have a bunch of these eBay-special flybacks I'd like to slap one to a piece of FR4 and use for prototyping a thing at +/- 100, but the output is kinda ugly and has about 3-500mV of ~70kHz noise I want to reduce before it goes to the circuit.

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Looking for a ballpark for a starting point for the design of the LC, I have a number of ferrite toroid cores in stock so if it's feasible I'll just wind it myself like a common-mode choke. Is there a preferable material for this application? Is it better to have two Cs to ground, a C across the rail (like a differential filter), or combination?

Reply to
bitrex
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If you output +100 and -100, two Ls and 2 Cs would be better. Just L-C twice, caps grounded.

Reply to
jlarkin

The inductance values I'm coming up for a Butterworth that gives -40dB of rejection at 70kHz, without coupling them end up requiring ~200 turns on a FT50-43 toroid which seems physically impossible with magnet wire of a gauge I can easily work with, even if I wanted to take the time to do it. Maybe need different core material to go that way.

If I couple them that comes down by a factor of 4 which is doable on a core that size, kinda.

Reply to
bitrex

The inductance comes down by a factor of 4, rather, so the windings gets cut by half.

Reply to
bitrex

Coupled inductors, namely a common-mode choke, only filters common-mode noise.

An LC Butterworth (or other classic form) filter assumes driving and load impedances. One doesn't usually try to match those in a power supply filter. A small L and a big electrolytic cap (and maybe a ceramic too) will filter well and have a low Q.

maybe 10 uH and 100 uF, something like that. Do that in each leg.

Reply to
jlarkin

The noise seems to be predominantly common-mode induced from the MOSFET switching/parasitic capacitance into the ground common to both sides. These bricks aren't isolated flybacks unfortunately they share a ground between input and output.

But if 10u is all that's required don't really need to try to make the inductance smaller by using the same core.

Reply to
bitrex

Those values were wild guesses, but not bad ones. They resonate at about 5 KHz. With a typical electrolytic cap, Q will be low, and a parallel ceramic cap won't change that much.

Reply to
John Larkin

So, can you connect switch-side drive and ground through a CM choke? There's a big gain if you WANT drive and return currents to sum to zero current; passing power current through an output filter inductor, it's got DC bias...

Reply to
whit3rd

Is it noise from the FET or from the transformer?

That size transformers look like the capacitance from the transformer would dominate

A solution could be to shield the core. Not a complete turn...

Reply to
Klaus Kragelund

Beats me. How do I check?

Reply to
bitrex

Also to consider, the inner winding's outer wrap should be ground (or unswitched power), and the outer winding's inner wrap as well. That doesn't take hardware, just attention to which lead is which.

For radiated emission, a complete turn wrap IS appropriate, around not just the center leg of the E's, but around the whole thing.

Reply to
whit3rd

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