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Any ideas for a compact high current power supply with variable voltage?

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LAN_403

Electorials

Fri Jul 06 2012, 07:56PM

Registered Member #5457 Joined: Mon Jun 25 2012, 06:42PM
Location:
Posts: 14

Hello,

I want to create a variable power supply which is capable of delivering a pretty high amount of current (10-20A)
It needs to be compact, so I was thinking of a switched mode power supply.
To make the heatsinks as small as possible, I was also thinking about zero voltage switching.

now the problem is that the mains do cross zero volts each 10ms (50Hz) but if I always want to switch there, and DO want to have a variable output voltage,
that would mean I need to skip some sines, and let trough some sines to get an average voltage which is lower.

At 50Hz (in my country), this seems pretty impossible. Example:
I want 10V at the output, and the input is 230V. That would mean I need to let trough approximately 4% of all incoming sines.
1 sine each 20ms and then only 4% of those sines would be like 20ms ON time and about half a second of OFF time.
That half a second is just too long, I'd need an epic output filtering capacitor for that.

Another possibility would be to use 4% of each sine, but then it's no longer zvs since I'm switching the sine voltage off somewhere where it's not 0V.

Maybe I can convert the AC input to DC first, and then use a chopper, but again, that's not ZVS so I'll need large heatsinks if I want to have a 15A output.

Any suggestions on how I could do this?

Greetings,
Electorials

Ash Small

Fri Jul 06 2012, 08:01PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Why not just use a 50 Hz transformer with a multiple tapped secondary?

Thomas W

Fri Jul 06 2012, 08:07PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Socket -> Variac -> 110v site transformer -> Bridge -> Smoothing caps,

the site transformers are centertapped to ground, so its acctualy 55v-0-55v
and the amperage avalable is ususaly rather high, i persoanly have a 3.3Kva site transformer this gives you quite a lot of amps at a broard range of voltages.

--Thomas---

P.S,
contary to popular belief i have no affiliation with variac companys,
they just seem to be a all in 1 solution, providing you can find one cheap :D

Electorials

Fri Jul 06 2012, 08:07PM

Registered Member #5457 Joined: Mon Jun 25 2012, 06:42PM
Location:
Posts: 14

Ash Small wrote ...

Why not just use a 50 Hz transformer with a multiple tapped secondary?

That's what I have right now
but I'm not really happy with it :/
it's a MOT with secondary windings rewound. For some reason it gets Really hot after running for 5 minutes, even if I'm not connecting any load.
And the 50Hz buzz when I'm drawing more current is also annoying :/
It's not a constant output voltage because the capacitor drains quickly with big loads.

I'm hoping this problem to be solved if the output works at a higher frequency.

Tom Williamson wrote ...

Socket -> Variac -> 110v site transformer -> Bridge -> Smoothing caps,

But then I still have kind of exactly the same output voltage as I've got right now. Variable 50Hz with smoothing capacitor.
I need a more steady output voltage which doesn't need an epic capacitor to achieve that.

And the variac + other transformer is also quite a big setup :/ and I also variacs aren't that cheap.

Ash Small

Fri Jul 06 2012, 08:19PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Well, you still want a stepdown xformer to drive an SMPS, unless you drive it directly from rectified mains, and you'll possibly want smoothing caps as well.

Harry

Fri Jul 06 2012, 08:29PM

Registered Member #4081 Joined: Wed Aug 31 2011, 06:40PM
Location: UK
Posts: 139

Depends really on what kind of voltage you want from it.
A common one that I'm sure plenty of people use is a computer PSU, they're reliable and powerful. Only 12V max though. These are good in general for powering all sorts.
Audio transformers are good, decent output voltage and centre tapped, not variable however.
To solve the MOT problem you can wind a few more primary turns to take it out of near saturation and reduce heating, also add a few more filter caps.
I think you need a variac though.

Electorials

Fri Jul 06 2012, 09:01PM

Registered Member #5457 Joined: Mon Jun 25 2012, 06:42PM
Location:
Posts: 14

Harry wrote ...

Depends really on what kind of voltage you want from it.
A common one that I'm sure plenty of people use is a computer PSU, they're reliable and powerful. Only 12V max though.

But I needed 10-20A.

And the output voltage should be from 0v to 50v.

I don't really want to use existing supplies like using a transformer with then an already existing smps connected to it.
What I'm talking about is creating one on my own.
I want to create the control circuit and power circuit myself, but I'm not sure yet what type of circuit it'll be since I want it to switch at zero volts across the FET's

m4ge123

Sat Jul 07 2012, 01:26AM

Registered Member #4118 Joined: Mon Oct 03 2011, 04:50PM
Location: MD
Posts: 140

Electorials wrote ...

Harry wrote ...

Depends really on what kind of voltage you want from it.
A common one that I'm sure plenty of people use is a computer PSU, they're reliable and powerful. Only 12V max though.

ZVS is not all that important at a 1kW power level. You can use a circuit similar to a computer PSU, except with variable output voltage. See

Dr. ISOTOP

Sat Jul 07 2012, 01:59AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

A hard-switched half-bridge driving a ferrite transformer will do for a few hundred watts. In fact, at that point you're still within range of a flyback converter.

Ben Solon

Sat Jul 07 2012, 04:48AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

I agree with bwang. A flyback is a good solution and is only limited by the core. But if you don't care about isolation, you could do a mains buck converter to eliminate the core. Keep in mind that you'll need a logic supply transformer(or another smps) to power the low voltage control circuit.

But I agree with the smps idea. I powered my zvs flyback driver off of a 12 smps and got a nice quiet hiss, but the I wanted more power. So I rewound a mot, got a 35vdc off of it, but the 15v 60hz ripple (see he exploding lytic thread :D) voltage put a humm on he arc.

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