Help with placement of filter inductor

John

out

No, it's a double half wave:

: _______ : D1 | | : ---------||--+--L--+-----+-------+---- GND : D2

All three L's have to be independent.

Tim

Yes. But together it's effectively full-wave because they conduct on both half cycles of the AC input and the output ripple has twice the frequency of the source.

I think you drew D3 with the wrong polarity. D1 and D3 will just short the transformer when the upper terminal is negative. But why are D3 and D4 necessary (even if D3 is placed with the correct polarity)?

I understand that a second LC filter will further reduce ripple, but that isn't exactly the point of my question. It's whether placing an inductor in the common transformer terminal will adequately serve the same function as separate inductors in each half-wave section.

=A0out

Oops, good point. Yes, D3 has to point down. So D1-D4 could be a FWB, if they make them in high speed silicon.

D3 and D4 are required so the L's don't just flap in the wind. You'll see a waveform between -0.8V and +Vpk at the rectifier, if inductor current is continuous (which it should be).

BTW, you'll get 5V if you FWCT rectify both halves of the waveform together, which is how it's wired stock. Half wave gets half voltage, because the inductor's averaging it out. Half wave on the whole winding gets 5V again (at poor duty cycle), and FW doubler gets 10V (at poor duty cycle). So you won't actually get 20V out. To do that, you have to split the winding's CT, which I suppose isn't practical. (Then again, you said your 12V winding is being used for something else? Is it seperate? They usually use one monster winding with "+5", 0V and "-5" taps along it.)

At this point, the most economical (=3D lowest parts) solution involves stripping the 5V winding off, replacing it with a proper 20V winding and using a FWB, one choke and one filter cap (and maybe another LC filter for kicks). Or maybe you can use the 12V winding instead.

Oh, and the conventional cap-input rectifier won't work because it will explode your transistors. You can get away with it for small currents (under 1A??), but regulation is poor, depending on input voltage, not duty cycle.

Tim

Oops, I left in the middle-leg L, assuming he wanted to peak rectify. Your diagram is correct assuming he wants to average a pwm thing, except the D3 polarity.

In my defense, the morning caffeine has worn off and the chocolate supply is dangerously low.

I usually draw things with + up.

John

A oops from my side. I'm so much more used to capacitor-input filtering that I forgot about how an L-C filter would behave with half wave.

Yes, as mentioned in my opening post, I have one with separate 5V and 12V windings.

I've been considering that option even before I realised my oversight about inductive filters with half wave. Taking out the

5V winding is too much hassle with the two halves of the core glued together. But since the turns/volt and the load are both small, it may be possible to just add a 20V winding.

Nope. It's committed. I want to use it to get 24V with a FW bridge rectifier using Schottky or fast-recovery PN diodes. The load will be about a couple of amps max.

That much I know. Thanks for your interest.

Excuses! Excuses! ;-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
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Ouch, beaten by the rookie? ;-)

Tim