MOSFETs (active rectification) in place of diodes in linear PSU - Page 4 - diyAudio
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Old 12th January 2006, 07:14 PM   #31
poobah is offline poobah  United States
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Well,

I am guessing, maybe incorrectly, that your biggest goal was to achieve regulation very early is the supply. Pre-regulate rather than post-regulate.

Eva's idea is great... my inductor, L1, is already hiding in the secondary of the transformer; and is not required. This elimanates the need for Switch 2 also.

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Old 12th January 2006, 08:17 PM   #32
UnixMan is offline UnixMan  Europe
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Hi Eva,

Quote:
Originally posted by Eva
The design that poobah has proposed operates at line frequency (100Hz or 120Hz), not at high frequencies.
well, sure... but if I understood it correctly, the switches can be operated at a higher frequency, depending on the resonant frequency of the L/C.


Quote:
[...]So S1 and S2 may be replaced by a single thyristor in place of S1, that would open automatically when inductor current reaches zero. Then the output voltage would be controlled through the firing angle of that thyristor
Wow, this sounds like a really GREAT idea!!!
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Old 12th January 2006, 08:29 PM   #33
poobah is offline poobah  United States
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This would give you your "pre"-regulation.

I would put the SCR in the center tap, you coould then used sand/snubbers or a tube for rectification on the "ends/legs" of the secondary.

I would use some heater supply to power a small circuit (sand) to: 1. control (regulate) your output voltage through conduction angle, 2. provide a B+ delay to prevent cathode stripping and output pops.

You may still need some small inductance where I have drawn L1... it depends on the leakage inductance of the secondary... with Eva's realization you do not need switch 2.

This could be the new rage?
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Old 12th January 2006, 08:31 PM   #34
UnixMan is offline UnixMan  Europe
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Quote:
Originally posted by poobah
I am guessing, maybe incorrectly, that your biggest goal was to achieve regulation very early is the supply. Pre-regulate rather than post-regulate.
well.. yes, this definitely was (has become? I've moved a bit along the line... ) one of the main goals.

As it should be clear by now, the other main goal was to keep the bandwidth of the whole "rectification noise" as narrow (low frequency) as possible... and Eva's idea seems promising also for that department!


Quote:
Eva's idea is great... my inductor, L1, is already hiding in the secondary of the transformer; and is not required. This elimanates the need for Switch 2 also.
Absolutely!!!

I was beginning to draw some rough sketch of Eva's idea as I have understood it. But now it's late and have to go... here is night and I still have to have dinner for tonight!
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Old 16th January 2006, 04:32 PM   #35
UnixMan is offline UnixMan  Europe
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Quote:
Originally posted by poobah

I would use some heater supply to power a small circuit (sand) to: 1. control (regulate) your output voltage through conduction angle, 2. provide a B+ delay to prevent cathode stripping and output pops.
Well, in some cases the idea is good, but in general I would like more a "self-booting" circuit (not requiring another external supply for the control circuit), if possible...

Quote:
This could be the new rage?
why not? indeed it offers some nice features...

BTW, let' sum up. Here is a rough sketch of the idea, as I have understood it; is that right?

The next step will be to design the "control unit" circuit.
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Old 16th January 2006, 05:33 PM   #36
poobah is offline poobah  United States
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I would use the SCR in the center tap and 2 diodes in the normal way for each leg of the secondary. Then use a little bit of rectifed heater power for control circuit.
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Old 16th January 2006, 06:08 PM   #37
Eva is offline Eva  Spain
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In order to trigger a thyristor, a small current should be made to flow from gate to cathode, so you should connect the secondaries of the pulse transformer between these legs. There is no point in inserting the secondary windings of the main transformer in the trigger loop.

As a control circuit you may consider the good old TL494 synchronized to the 50/60Hz waveform and with its internal clock programmed for 40Hz or so operation. The TL494 will be fine since it employs trailing-edge blanking (the pulses grow backwards from the end of the cycle as duty cycle is increased, instead of growing from the beggining as usual). Voltage mode control with enough frequency compensation will do the trick, and there is still a free op-amp in the IC that allows to implement some degree of output current limiting.
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Old 16th January 2006, 06:46 PM   #38
Eva is offline Eva  Spain
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I forgot to mention that you are going to need some kind of high frequency pulse generator with on/off in order to feed the trigger transformer. A LM393 comparator may do the job (Nat.Semi. datasheet contains some hints about that).
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Old 17th January 2006, 09:15 AM   #39
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Quote:
Originally posted by poobah
I would use the SCR in the center tap and 2 diodes in the normal way for each leg of the secondary. Then use a little bit of rectifed heater power for control circuit.

Yeah, that would made things a bit simpler, and would also avoid the need for a transformer to drive the thyristor, an CR coupling being probably enough.

But then it would require a center-tapped PT, while in the long run I would like to change this to a bridged setup in order to be able to use "normal" PT.
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Old 17th January 2006, 01:11 PM   #40
poobah is offline poobah  United States
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Well...

Either method would work... the method with 2 SCRS would be more efficient. The center tap method avoids the pulse tranformer and requires some careful thought about grounding.

I believe nearly all transformers with voltages for tube B+ will have center taps.

I would focus on the control strategy and topology... the fine points of design. Then, decide which SCR layout makes the most sense to you.

Just a thought, there may be silicon modules for light dimming that may be useful for AC synch and control of "firing angle"... might save you a lot of parts.

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