Custom-wound flybacks

[GCA]

Hey all, I've been trawling for quite a while and I had a question.

I'm trying to design a flyback supply for tube preamps with a fair amount of wiggle room. I've pretty much decided on a TopSwitch-HX as the switcher and FET as they're inexpensive and take a lot out of the design complexity, and also have a few other cool features besides having a chip in the exact range for my application.

I'm a little curious before I jump in and ask for quotes on the transformer. I'm hoping a few people can shed a little light and give somewhat of a consensus on what I should expect to pay for a transformer like this:

~128kHz
300V .15 A main winding
12.6V CT 2A winding
12.6V 2A winding
properly gapped, etc.

It'll be a roughly 125W supply, though I'll be running it on light load most of the time. I just don't want to design another and another and another every time I build something new because I'm a little sick of the "can't build that, your supply won't do it" dance.

I realize there is a LOT of room in the specs but I was just hoping to tap into some of the experience of those on the board who've ordered things like this. I'm only looking for a ballpark.

I tried to do it linear with a toroid but the quotes for the transformers were in the area of $250 a pop since no one makes any with a secondary winding that can be elevated for CF heaters. Honestly... hell no, that's a car payment for a single transformer that won't fit in a 1U case so that idea got scratched and I began learning a bit about switchmodes!

Anyone got an off-the-top-of-head clue?

Thx in advance
GC

[GCA]

Wow.. I guess no one orders these, or everyone winds them themselves.

[Sch3mat1c]

That's a lot for a flyback. Around 100W, forward converters start looking better.

I built this, which makes 250V 0.2A and 6.3V 10A. It could be reduced in size a bit by removing the special bits.

http://webpages.charter.net/dawill/I...TubeScope3.jpg

Tim

[GCA]

The 12.6V secondaries don't technically need 2A, but i wanted the capability there. 1A each would most likely do the trick, and the 150mA on the 300V is also a little much though not by as large a margin. It would run mostly between 25% and 50% duty, I've just got a little headroom. It's for a vacuum tube preamp that I wanted to make with a few optional bits, some require extra tubes, some don't, so I wanted to design in the maximum I could see it ever taking.

I didn't wanna do a forward convertor because I wanted an easy way of isolating it - if you know of a way ofo doing that with a forward then I'm all ears!

Or in this case, just plain interested in that there doohicky in the pic..

[Sch3mat1c]

A non-isolated forward converter is called a buck converter. Forwards are isolated. The controllers usually aren't, which usually ends up requiring two or three more transformers in the circuit.

The circuit pictured uses a TL494 driving MOSFETs driving a transformer up and down (half bridge), which is rectified and filtered (choke input filter, since the FETs are switching into the load, not like a flyback). The controller is powered by a startup supply on the left, which also produces a few extra voltages. It's very similar to the average computer PSU.

If you're only building a preamp, I don't see you needing even 20mA at 200V or so. A couple 12AU7s is a very easy load to drive, and an excellent use of a flyback supply.

Tim

[GCA]

Preamp's for bass guitar and will use an average of about 40-60mA at around 300V. There are 12ax7's and au7's in common cathode (not that much current, I know) and in direct-coupled CF arrangements, and I have a 4-band semi-parametric tone control using 2 more whole au7's, buffered FX loop, etc. Total whole tube count in this config is 5, making for 10 cathodes sucking current. Generally ~2-4mA for each gain stage, plus ~10mA for each CF / buffer. It's a lot of current for a pre, I know, but it ain't meant for hi-fi I've built all of the parts and tested them individually, but I prefer to have an elevated heater supply for everything in CF, and that requires 3 outputs. @12V DC I can cut the current requirement for each whole tube's heater to .15mA, but there will be times when I'd like to use the same supply for a different pre, hence the upped capacity. If it were for reproduction use, I'd just use my existing linear supply and go with a pair au7's like you mentioned since that would be far easier and I wouldn't need the elevated heaters like I do with a 300V B+.

*ah* on the forward vs. buck. I've been concentrating on the flybacks for multiple outputs and isolation. I'll check out forwards and see if it'll be worth switching gears. Even a radio shack 12V xformer could supply my heater current.

[Sch3mat1c]

Hmm, lots of outputs. Might do flyback anyway -- all the filters you put in a forward converter make them kind of annoying for that (though you can at least put all the filter windings on a common core, as they do in computer PSUs). The flyback will use a pretty big transformer, but just one, and the rest will be smaller. You might see this in computer monitors: one moderately sized ferrite transformer, with lots of windings coming off it; at least one I've seen is bank wound on a bobbin with lots of seperators, so a few slots are the primary, then a few slots are a couple secondaries, then a few slots are a couple of others. Lots of overlap, individual windings spanning multiple slots, to minimize leakage inductance and parasitic capacitance. Oh, and all the line-level insulation necessary also takes up a lot of space, requiring a larger core than you might expect.

Tim

[GCA]

I'm all for small transformers. Still reading Billings on the forward converter, and am kinda hoping for a revelation that all is well for under 1lb. and less than $40 into the power supply.


I wish I had "extra" tubes for that obscene drag race contest ;P