dknguyen

A PWM Power supply is a called a switching DC converter. Boost (Step-up), Buck (step-down), Buck-boost (does both while being less efficient), or flyback (uses a transformer instead of an inductor for isolation of the output), are the 4 kinds.

It is a big topic. With proper sensors for things like current feedback and with a microcontroller, you can adjust the duty cycle (the PWM) and thus do current limiting. Google it. It is a big...big complicated topic that I cannot cover on here unless you know the basics first.

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bmachining

Thankyou! Yes is very involved but you have explained a few things already that i need to knoe to refine my search.

Needs to be Buck, (stepdown), using a transformer for step down from 240 to 40V.

blueroomelectronics

You want 40V at up to 30A, that's a pretty beefy 1200W, not likely a simple design.

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dknguyen

THe inductor that would be pretty big. The transistors aren't too bad though since you can find transistors that just barely pull that off, and you only need one for a switching converter so you can parallel them vairly easily.

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bmachining

Yep, its a serious power supply. I imagine the current control could be much like some of the new switchmode arc welders, but im not wanting switchmode. Just the old transformer for stepdown. Half the battle seems to the be terminology im not used to. Can inductors be paralled? it would save winding one big one and the difficuilty finding a suitable core?

Ubergeek63

The chip is meant for current sourcing power LEDs so it is a natural for driving current into a joint.

The problem with "Just the old transformer for stepdown" is that at 1200W it will be huge. You need the "high" frequencies to get the size of the inductor down. It is imperative that you use switchmode since even a 50VA transformer will run $30 and up in that power range $200 plus shipping on something 8" diameter by 4" high at 25lbs.

http://www.supertex.com/pdf/app_notes/AN-H50.pdf is a good appnote on it, though what I suggest is going a bit further than that. And http://search.digikey.com/scripts/Dk...?name=M8374-ND would be a suitable inductor if a bit weak on the current.

It would run buck mode and I believe will reliably deliver 27A into 20V from a rectified mains. The formulas are all in the sheet. The other irritating detail is that this will NOT be isolated, which means that the entire circuit is tied to the mains. On the other hand, it IS the least expensive.

I would add that at the high levels in this discussion I would add an FET driver.

bmachining

Hmm, well i have a couple of transformers suitable already. I would really like it to be isolated from the mains. Been bitten by 240 a few times.
Am i correct in saying i need something like u have mentioned to drive a mosfet or 2 or more,which is fed from the transformer, with either built in or add on current sensing circuit to control the output via the pwm controller?

Then the Inductor and maybe some caps to smooth the output.?

Ubergeek63

in that case use one for isolation. The 1.5-2KVA isolators go for over $200 for the cheapies and cost a lot to ship.

Welders do not typically worry about clean DC. That circuit already has current sensing, it is a current regulator not a voltage regulator. If the transformer you have puts out 40VAC so much the better, a relatively simple tweak will approximate PFC as well. It will run CCM and I can show you how to make it reasonably PFC and not use big caps.

The $10 inductor, a couple $5 caps, an ultrafast 50A diode, and a 50+A FET (they tend to be pickier)

bmachining

PFC as in power factor correction? just read up on it so i have a very vauge idea now hehe. Transformer can put out any AC voltage i desire, rewound it myself, and not hard to change.

Ubergeek63

Exactly. It is not the transformer or the rectifier that destroys the power factor but the filter cap.

With the current source chip you can use AC output of the transformer for your current source reference to the LD (Linear Dimming) pin and a pot to adjust the average current and be able to get almost the full VA from the transformer as power out to your load (I believe you are trying to do resistance welding)

bmachining

This will output DC or AC ? needs to be DC.
It will be used for electroplating. I have heard that fets amp rating will only be safe to about half value for continuos use ( constant), can these be paralled ?

Ubergeek63

pulsating DC. Electroplating makes sense, but I still do not think you need it fully filtered.

You generally want some headroom, but you do not need double the current capacity. It is more important that you have enough heat sinking. This one is reasonable and would handle it by itself: http://search.digikey.com/scripts/Dk...=497-5166-5-ND
Assuming you are using your step down.

bmachining

Unfortunatly it does need to be rather smooth, less than 5% ripple for some applications. does this complicate things?
My previous linear circuits are always well heatsinked, also with cooling fans.

Awesome work so far!! Thankyou for this help.

Ubergeek63

Not a problem...wont need nearly as much heat sinking.

I would not have figured it would need low ripple. There is a trade off in that case for what would cost less. a simple isolation transformer and high voltage parts or your 40VAC transformer with a big mother cap, this one might be a good trade off: http://search.digikey.com/scripts/Dk...name=P10035-ND ... you need to keep the ripple valley over the output voltage, the current regulator will do the rest.

The ripple current in the cap will determine it's life and you still want the small $5 caps around it to take the high frequency current.

kell

What are you electroplating? 40 volts it way higher than you need for most stuff, like zinc only takes up less than one volt and other metals 2 or 3 volts.
So you don't want linear current regulation, you would burn up just about every bit of your available power simply in the regualtion. You could do it with SMPS techniques, but a simple buck regulator probably isn't the ticket for such a big voltage ratio. Are you up to high power PWM current source regulation? Why not use a transformer to lower the voltage and increase the current first, then worry about current regulation?