MOSFET Dissipation (for Power Scaling / VVR)

[nickb]

Homeboy, did you custom design the main board and power supply board to be made for this project?
That looks dynamite, man. Pretty inspiring work.
I haven't dug into this threat too much yet, but is this based on the concept of sampling the incoming AC to track the phase and set the ON/OFF duration of a switching FET to limit the peak voltage over each half cycle? Is there a name for that?
The first time I came across that was reading a later iteration of RG's Mosfet Follies. I remember immediately thinking, "damn... that is a great idea. This is how it should have been done all along", and if I was ever going to build one again, I would do it this way.

Yes I did the PCBs for both these. The micro has a counter that is reset when the AC goes thru zero. When it reaches a desired count the triac is triggered using a 10uS pulse. The power controller uses a triac runs cool, no heatsink required. Something to be aware when using phase control like this is the transformer current waveform gets very distorted and this leads to high rms current. The practical implication is you have to limit the controllable power range to 100% and then smoothly from 50% to almost 0.

[nickb]

Hey Nick,

I'd love to hear more about your PIC/Triac solution. Seems like overkill on the surface, but the parts are cheap and it avoids all the extra heat dissipation.

Not sure what you want to know but if you think about the linear circuitry needed to set the bias voltage as you vary the plate voltage, I expect you will realize that the PIC is by far the easiest and cheapest way to go. Just $0.72. Plus it adds great flexibility, for example 50/60hz detection and giving a nice "feel" to the control knob. For a linear FET dropper control the size of heatsink becomes substantial and thus expensive, thus the savings of this approach are substantial.

[soundmasterg]

Greg - are the 500W versions the same basic idea ( control plate voltage with a linear dropper) or something different?

I'll have to get back to you on that Nickb. I'm overseas at the moment and don't have access to my O'Connor books to look it up, and I can't recall from memory right now either.

Greg

[SoulFetish]

Something to be aware when using phase control like this is the transformer current waveform gets very distorted and this leads to high rms current. The practical implication is you have to limit the controllable power range to 100% and then smoothly from 50% to almost 0.

So, when adding to an existing amp, some care needs to be taken to make sure the added stress does cause insulation breakdown from exceeding max RMS... oh, right! I remember now the conversation about manufacturing specs on max RMS for a bassmam secondary supply. This was the project, yeah?
Not for nothing, but you did some nice work here I could see a commercial market potentially (small community, but still). Thinking about any commercial development as a aftermarket, add-on product?

[soundmasterg]

Greg - are the 500W versions the same basic idea ( control plate voltage with a linear dropper) or something different?

Hi Nick,

I'm back but have been getting over jet lag and bronchitis, so still haven't had a chance to look up the design of the power scale circuit in my TUT books, but KOC provides some details on his site in the description.

https://www.londonpower.com/power-sc...kit-fixed-bias

The older style I think he called SSK and that was what VVR was based on from my understanding. I'll see if I can get you the info you're looking for about it when I get a chance.

Greg

[soundmasterg]

Hi Nick,

Kevin O' Connor started up a new forum at http://theultimatetone.com/ and he put up a post today talking about power scaling vs VVR amoung other things in response to a question. He basically says Dana Hall made a copy of an earlier version of power scaling but it wasn't quite the same and had issues that were fixed with band aids by Dana Hall.

I'm hoping to get a chance to look at the power scaling info this weekend and get you an answer to your question. I'll let you know. You could also check out that forum and ask Kevin directly too. He could explain it way better than I could I'm sure.

Greg