1. Home
  2. Electronics Design
  3. Suggestions for 1kW, unregulated SMPS

Suggestions for 1kW, unregulated SMPS

"Unregulated SMPS" probably sounds funny, but this is it: At my lab we frequently heat up metal samples in vacuum by placing them near a hot tungsten filament and applying a high voltage, essentially making the sample the anode of a vacuum diode. This typically happens at voltages of around 1kV and currents of .5A, but 1kW total power isn't unheard of.

Now we have some homebuilt units that take care of this. They consist of a custom-wound mains transformer with a bridge rectifier for the HV, and the power is regulated via filament temperature. These beasts work OK, but the transformer makes them heavy and expensive.

Now we need a few more, and the natural path to follow would of course be to make them switched-mode -- except that I've never done an SMPS. I may be wrong, but I think this should nevertheless be doable since I need no regulation at all; the first version would just have a pot to adjust the duty cycle. So this thing would consist of no more than a PWM controller, an IGBT H-bridge, a transformer and a diode bridge. The output needs to be short-circuit proof; I'd do that by adding some series inductance to make the primary dI/dt slow enough to be safely cut off by the overcurrent trip.

The only tricky part that I'm aware of is the design of the gate drive circuit -- it mustn't ring and it must be quick to reduce heat dissipation and avoid cross conduction. The transformer, of course, is a critical part in any SMPS design, but I figure that a poorly wound xformer can't have any ill side effects except having too little output power due to stray inductance.

Ah yes, and an 1kW SMPS running from a single-phase 230V line needs PFC and soft-start. Will have to read up about that.

Should I do it? Maybe it's a bit stupid, seeing that only 5 of these things are needed at the moment and there exists a design that works and can be assembled and tested by the techie who originally designed it (he wouldn't touch a switcher with a 10-foot pole). However I'd like to have a bit of fun of my own but I don't enjoy having to pick bits of black epoxy out of my face.

And I know that a 1000V supply capable of delivering 1A is no joking matter. But since the secondary won't consist of anything besides the rectifier bridge and the output jack I won't have to do much there.

BTW, I can't use the filament/sample constellation itself as a rectifying diode. For one thing this would require some kind of flyback design, and, more importantly, the anode gets often hotter than the cathode.

--Daniel

Reply to
Haude Daniel
Loading thread data ...

You'll probably get lots more detailed responses than this one, but here are a few questions and pieces of advice:

1) Do you need isolation? If not, you could consider using a boost converter with 1000 V ouput as a single stage PFC/PWM. There are also PFC topologies that provide isolation. Control loop BW is always rather poor, though. 2) Full-bridge output SMPS poses extra challenges keeping DC out of the transformer. Be careful, or use a series capacitor right from the start. Don't believe the app notes that say that current mode control prevents DC imbalance (flux walking). That promise is only realized under controlled conditions. 3) Push-pull topology requires a larger transformer, but circuit is less complex. 4) There is much literature available on gate drivers. For a project like yours, simply choose one of the many good integrated gate drivers. 5) The most important design element is layout. Study SMPS layout carefully. Where di/dt is high, no inductance is too small to be ignored. Where dv/dt is high, same is true for capacitance. 6) You will be needing some high voltage differential probes.

7) Get ready to make some smoke.

Paul Mathews

Paul Mathews

3)
Reply to
Paul Mathews

This may be of interest :

formatting link

Also, have you looked at microwave oven transformers as a cheaper alternative to a custom unit.

Reply to
Mike Harrison

On 4 Dec 2006 07:13:46 -0800, "Paul Mathews" Gave us:

Get an audio amp, class D at the expected wattage capacity plus a good margin. Pump it at 17 plus kHz. Get a Large "E-I" style ferrite transformer core and bobbin. Wind a transformer with a low turns count of SPC teflon coated wire at like 18 Gauge on top of your High strength, high temp, mag wire secondary.

You will be making a step up so you can feed a multiple stage voltage multiplier. This is where you will get you isolation (vacuum impregnate with varnish US HV Xfmr tapes). Then you need to decide how much energy you need to store to do the job, but not store so much that failures become more catastrophic, and costs inflate to make it do more than is needed.

Sometimes the energy in the multiplier stages is enough and extra storage at the end isn't needed. It all depends on the desired consumption rate.

Reply to
JoeBloe

Yeah, I can't imagine a current trip being fast enough to limit the inrush into what's practically a sudden dead short.

That's why I wanto to go PP.

Sure. I've been reading quite a lot on the subject, out of interest, and that's what actually got me started on this idea. This is one of the rare opportunities where it makes sense to design a custom SMPS, and a simple one to boot.

--Daniel

Reply to
Haude Daniel

Your requirements are similar to those of a microwave oven. Use whatever they use, and if any microwave oven does have an SMPS it is going to be the simplest SMPS available.

In fact a search on does seem to produce some interesting results.

--
Tony Williams.
Reply to
Tony Williams

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.