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

[UnixMan]

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.

OK... BTW, I would say that the return path was there already (via the ground connection).

There is no point in inserting the secondary windings of the main transformer in the trigger loop.

how would you sync the PWM driver to the mains frequency, then?

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

I'll have a look at it...

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

that could be done... but wouldn't a simple CR do the trick?

 

[UnixMan]

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

except the one I have.

Indeed, in my case (having only ~220Vac on the secondary) I've been a bit dumb not choosing a center-tapped PT, even if I was planning on using sand anyway.

But if you consider a PSU for tubes which requires several hundred or even over 1 KV dc, using a center-tap PT would be rather troublesome, or at least quite expensive.

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.

agreed.

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.

mmmh... intersting idea... it's definitely worth some search...

 

[poobah]

In any topology you will need a second low voltage winding to derive "house keeping power" for the start-up control. This winding can provide synch.

If you have no center tap, you are forced to use 2 diodes and 2 scr's.

 

[Lars Clausen]

We use it commercial products with good results.

Per Anders: I have always been wondering where you work in the daytime

All the best from

Lars

 

[Eva]

peranders:

Ouch!!

I have seen these BTSxxx ICs priced anywhere from 3 euro/piece the lowest current models to 12 euro/piece the 8 miliohm version, so I think that I will continue with my discrete solutions...