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rjon17469 30th December 2004 01:36 AM

Power Supply Design - Off My Rocker?
 
2 Attachment(s)
Hey everyone,

This is my first actual design, so I'm looking for input.

It's a power supply, but I am curious if it'll work. What I tried to do was convert the 120V 60Hz AC to DC, then convert it back to a 120VAC at a faster frequency.

The frequency is defined by the 555 timer running off a different power supply. The values I included set the 555 timer to oscillate at about 200Hz.

Am I off my rocker here, going after something that really won't work? My reasoning for doing this is so I won't need so many caps for the 15 amps my amplifier is requiring, ideally so the faster cycles will keep the caps charged.

For those who understand electronics better, I have a few questions:

1) Will this work?
2) If so, is it worth it?
3) Will this be too hard on the transformer/transistors/caps?
4) Will this provide better isolation from AC fluctuations than a typical unregulated power supply?

I found transistors rated at 150V @ 8 amps, which should be more than an enough.

All comments/suggestions welcome, including those related to how to draw schematics better! ;-)

Reece

Stocker 30th December 2004 03:01 AM

ah, you may want to check those specs again... 150V at 8A is 1200W, which may be above the SOA of your transistors... what transistor are you talking about?

Generally, If you are willing to use big fat transistors in your power supply anyway, and want to be sure it will work, just use big pass transistors on a regulator. Hardly even any figuring out to do.

rjon17469 30th December 2004 03:09 AM

Quote:

Originally posted by Stocker
150V at 8A is 1200W, which may be above the SOA of your transistors
MJE15030, rated at 150V 8A continuous, 16A peak.


Quote:

Originally posted by Stocker
If you are willing to use big fat transistors in your power supply anyway, and want to be sure it will work, just use big pass transistors on a regulator.
Sorry, I don't fully understand...

hummhoom 30th December 2004 03:32 AM

Well I'm not quite sure the H bridge you have driving the transformer will work. You'll have the pair of transistors on the positive rail that will need some form of level shifting to turn them on.

I think you'd be better off with just more caps if you're really worried. I read your schematic as 4x10,000uf on each rail. That's really not bad for an amp that draws 15 amps. You could even take those 2x10,000uf from the line voltage side, and put one extra on each rail. Save yourself the power transistors and heatsinks, and buy yourself another pair of caps, for 6x10,000 on each rail. That's really a fairly realistic value, and a good foundation for an amplifier.

I see you also have a housekeeping transformer that provides + and - 12V. Since your 555 circuit only needs the +12, I am assuming you're going to use that +12 and -12 V supply for your audio circuits. I highly advise against anything that could possibly connect line power to your low voltage ciruits, no matter how many improbable failures it would take.

If you still want to do this, check out this H bridge controller from IRF:
http://ec.irf.com/v6/en/US/adirect/i...uctID=IR3220S:
They even have modules that include IGBT's.

rjon17469 30th December 2004 03:41 AM

Quote:

Originally posted by hummhoom
I think you'd be better off with just more caps if you're really worried. I read your schematic as 4x10,000uf on each rail. That's really not bad for an amp that draws 15 amps. You could even take those 2x10,000uf from the line voltage side, and put one extra on each rail. Save yourself the power transistors and heatsinks, and buy yourself another pair of caps, for 6x10,000 on each rail. That's really a fairly realistic value, and a good foundation for an amplifier.

Yeah, I think you're right. It was a nice idea, but throwing the caps together would probably produce the same results with a lot less work.

Thanks for your help!

Reece

hummhoom 30th December 2004 03:58 AM

No problem.

There's really very little to be gained in quality in the power supply, except make it big enough.

Power switch, fuse, ransformer, rectifier bridge, and filter capacitors, and that's it. And make sure the power to any low level signal stages are regulated.

Enzo 30th December 2004 04:21 AM

Check the SOA chart again. The 15030 is rated at 150V and 8 A continuous, but not both at once. At 150v it is rated at less than an amp. The 8A rating is good only up to maybe 15v. It is only rated at 50 watts.

Are you running 240VAC into the thing? It looks like it with 120VAC on either side of the bridge with the cap common grounded. If you are running plain old 120VAC to the bridge recto, remember one side of that is already at ground, so that ground in the center of the filters will not cooperate.

PRR 30th December 2004 05:07 AM

> convert the 120V 60Hz AC to DC, then convert it back to a 120VAC at a faster frequency. ...I am curious if it'll work.

No. Aside from practical details like SOA, your "200Hz" is still heavily modulated with 120Hz ripple. You have more than doubled costs, for no advantage, and probable extra annoyance from having two sets of ripple spectrums. Plus you have a 160V square-wave, which is probably a very bad idea around audio.

rjon17469 30th December 2004 06:33 AM

Quote:

Originally posted by PRR
> convert the 120V 60Hz AC to DC, then convert it back to a 120VAC at a faster frequency. ...I am curious if it'll work.

No. Aside from practical details like SOA, your "200Hz" is still heavily modulated with 120Hz ripple. You have more than doubled costs, for no advantage, and probable extra annoyance from having two sets of ripple spectrums. Plus you have a 160V square-wave, which is probably a very bad idea around audio.


Well, I don't fully agree with that...

The safe operating area of the transistors was 2400 watts (16A @ 150V). I was looking at doing 3A @ 84V, so that wasn't really an issue (assuming heat was taken care of). With the capacitors on the AC line, I was getting about a 2.5 volt ripple at 84VDC, so that wasn't really bad. The signal coming out of the transformer would have been squarish, but that's all the better as rectified full wave that would have produced a near perfect DC on it's own, even without the 4 10,000uf caps I had smoothing each rail out!

The transistors cost $1.68 each, extra bridge rectifier a couple bucks, 555 timer maybe a buck or two, and everything else was negligable. I was already going to use the extra transformer, so that wasn't an issue. It would have cost maybe $10 more, which unfortunately is a lot less than the cost of 10 good quality 10,000uf caps.

But in the end, I probably don't want the extra trouble of it all. Fun thought though.

MikeB 30th December 2004 12:05 PM

Hi !

Hmm, i am not sure what exactly you try to do, but it looks like you
want to feed a squarewave into a transformator ?
If yes, DONT ! A transformator only works fine with sinus-signals,
a squarewave fed into a transformer results in VERY high voltagepeaks
at secondarywindings. Depending on the quality of the squarewave
these peaks will be several kilovolts. Also feeding the primary winding
with a squarewave results in very high voltagepeaks, also several
hundred volts. This will definetely blow your transistors at once, you
are dealing with severe inductive loads.
Also switching with transistors is not too easy, as transistors open
faster than they close, you always get a small short everytime you
switch, immediately blowing these transistors...
There are special transistors designed for switching, the mje15030
is designed to operate as audiodriver.

If you are generating a sinewave, the losses will be very high, this
means you are dealing with several 100 watts, needing the most
powerful bjts and heatsinks. So, using some more caps will be
smaller and less expensive.

If you are looking for a "ripplefree" psu, consider a switchmodesupply,
but this is also not too easy...

Mike


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