MOSFETs (active rectification) in place of diodes in linear PSU

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Oops... I replyed too early! :D


poobah said:
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!


You are on your own to do the State-Space math - :D

yeah, that sounds like a though job... :rolleyes:

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


poobah said:
Oh,

By the way, I would use a buck derived smps before I would do all this work...
:D

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... :clown:

mmmh... would it be feasible to modify one such (resonant type) SMPS to operate at much lower frequencies from a normal mains PT? :scratch: probably not... :(
 
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?




:D
 
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.
 
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.
 
poobah said:
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... :rolleyes:

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? :confused:


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? :D

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. :-$
 
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.

:D
 
Hi Eva,

Eva said:
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. :confused:


[...]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!!! :cheerful:
 
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?
 
poobah said:
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... :D) one of the main goals.

As it should be clear by now, :D 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! :cool:


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.
:D

Absolutely!!! :yes:

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! :cannotbe:
 
poobah said:

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...

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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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.
 
poobah said:
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.
 
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.

:D
 
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