MOSFETs (active rectification) in place of diodes in linear PSU - Page 3 - diyAudio
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Old 12th January 2006, 05:43 PM   #21
UnixMan is offline UnixMan  Europe
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Oops... I replyed too early!


Quote:
Originally posted by poobah
OK,

I have shown only one side of the circuit... it must be doubled for full wave.

Conditions: Steady state, Inductor is "dry" (no current), both switches are open,
[...]
OK, except that I guess you meant "secondary" where you've written "primary", I think I got it.

Really very interesting, indeed!


Quote:
You are on your own to do the State-Space math -
yeah, that sounds like a though job...

BTW: doesn't it exists any dedicated driver chip for this type of SMPS as they do exists e.g. for driving synchronous rectifiers? That could made things much simpler and avoid much of the work... :-?


Quote:
Originally posted by poobah
Oh,

By the way, I would use a buck derived smps before I would do all this work...
what I don't like about SMPSs is that (for several good reasons, when it comes to their goals...) they are operating at relatively high frequency and typically are terrible wide-spectrum noise generators... exactly the opposite of what I'm looking for...

mmmh... would it be feasible to modify one such (resonant type) SMPS to operate at much lower frequencies from a normal mains PT? probably not...
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Old 12th January 2006, 05:55 PM   #22
poobah is offline poobah  United States
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I don't think there any chips for THIS design. Power factor correction and efficiency is what drives the market.

Some intersting areas of study would be "ferroresonant transformers" and also "magnetic amplifers"... these utilise the effect of magnetic saturation in interesting ways. Low efficiency, but if you're building SE amps... who cares?




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Old 12th January 2006, 06:07 PM   #23
Eva is offline Eva  Spain
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The design that poobah has proposed operates at line frequency (100Hz or 120Hz), not at high frequencies.

L1 may as well be the leakage inductance of the mains transformer, maybe with some additional inductance in series.

The control scheme won't work because S1 can't be opened until L1 current has decreased to zero due to transformer leakage inductance (that may be even bigger than L1 and there is no way to clamp it), so S2 is not required.

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 and we would have something very similar to the PSU found in Carver's amplifiers. An alternative control scheme may be based in skipping mains cycles, this is quite common in microcontroller-driven electric heaters.
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Old 12th January 2006, 06:28 PM   #24
poobah is offline poobah  United States
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Eva,

Could a thyrister be put in the center tap leg. and then just use diodes (or a tube if you must) on the "ends" of the primary. Then use simple control of the conduction angle?
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Old 12th January 2006, 06:37 PM   #25
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Then you would need a thyristor in the center leg and one diode in each other leg, thus making the current flow through two diode drops. On the other hand, one thyristor in each leg produces just a single diode drop. Remember that a thyristor is almost like a conventional diode, but in order to turn on it doesn't only require to be forward biased but also a trigger gate pulse.
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Old 12th January 2006, 06:42 PM   #26
poobah is offline poobah  United States
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Yes, of course, I am only thinking of a way to simplify the gate/trigger drive for the thyrister. 2 diodes drops is small a for a tube amp. In this way, the gate drive circuitry could be ground referenced perhaps.

hmmmmmm.....
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Old 12th January 2006, 06:46 PM   #27
Eva is offline Eva  Spain
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In these high voltage circumstances, the most comfortable way to trigger a thyristor is through a small pulse transformer. A single one may be used to trigger both, since the reverse-biased one will ignore the pulses.
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Old 12th January 2006, 06:50 PM   #28
poobah is offline poobah  United States
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Paolo,

Are you following along here?
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Old 12th January 2006, 06:53 PM   #29
UnixMan is offline UnixMan  Europe
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Quote:
Originally posted by poobah
I don't think there any chips for THIS design. Power factor correction and efficiency is what drives the market.
ok, I should really go and read some docs about resonant SMPS...

but if I got it right from what you have said about them, then a "standard" resonant SMPS would perfectly meet our needs ( with the extra bonus of good efficiency! ) as long as the resonant frequency is kept low enough.

If the switches are only operated when currents are null, I guess there should be no switching noise. Then, the only possible source of noise should be from the charging current itself. If the resonant frequency is low enough, the spectrum of such currents should be limited to low frequencies which are easily filtered and will not easily "spread out" of the PSU... am I right?


Quote:
Some intersting areas of study would be "ferroresonant transformers" and also "magnetic amplifers"... these utilise the effect of magnetic saturation in interesting ways. Low efficiency, but if you're building SE amps... who cares?
that may be interesting, too... but I guess it would require specially made "irons" that I'm afraid would be rather difficult to get. Moreover, quite likely it would be heavy, bulky and... expensive. :-$
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Old 12th January 2006, 07:04 PM   #30
UnixMan is offline UnixMan  Europe
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Quote:
Originally posted by poobah
Paolo,

Are you following along here?
Yeah, I'm trying to keep up... this thread is becoming really very interesting!!!

...it's just that you're posting faster than I can get to read it!
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