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Ben Solon

Mon Apr 02 2012, 02:48AM

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

Hey all-
A project high on my "priorities" is a 4kw boost converter for powering just about anything. I have done Many calculations and even built a mini proof-of-concept sort of thing on my breadboard. It put out 45vdc at a few tens of mA(I didn't measure it) powered with nothing more than 5v .5A USB power. Pretty good for a MOSFET with almost an ohm of on resistance.

For the big thing, I thought I should come here to get started. 4kw is an absolute maximum. I plan on using it for my drsstc(1kw). It needs to have 0-400v at 0-10A. Running a coil with a 330v bridge would only draw 3A. In reality I only need 2kw, but I want an overhead.

So on to the actual technical details. I am going for something similar to Steve's design, at a lower power. Should I use my 30n60 igbts? Active snubber or no? I don't want to use one, but I don't want to waste 800W either. The igbts dissipate 208W each, 4kw @ 90% efficiency means 400W. This is within the 400W I can dissipate. Steve simulated 93% efficiency without active snubbers. This seems good to me.

The specs:
170vin
10-20khz
400vout
10Aout
Current ripple?
Voltage ripple <4v?

Let me know what you think.

And almost forgot. When plugging Steve's converter's values into wolfram boost converter calculator, it says he draws 50A from the socket WHAT!?! Is this constant or just peak?

Edit: based on more calculations, Steve's ripple current with those inductors should be huge. I also read that ripple current is bad because if the current ever goes 0, funny things Happen to the voltage. Should I not care what the ripple current is so long as it never goes 0?

2Spoons

Mon Apr 02 2012, 03:48AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615

4kW seems a bit much for the boost topology. I would have thought a half bridge or full bridge would be better. Don't forget: boost has no input to output isolation, so if something bad happens on your load you could end up trying to short out the grid.

Patrick

Mon Apr 02 2012, 04:00AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

2Spoons wrote ...

4kW seems a bit much for the boost topology. I would have thought a half bridge or full bridge would be better. Don't forget: boost has no input to output isolation, so if something bad happens on your load you could end up trying to short out the grid.

I would second 2spoons comments, the isolation part is usually pretty important. Also the boost topology can be used at 4kw, but i wouldny want to waste a lot of developement time figuring it out. Besides transformers give you so much more flexibility with the math and physics than a single inductor (and inherent isolation, 2 birds dude).

you can also use smaller diodes and other power end components, and not have huge peak currents sloshing around. that reactance and peak current can be scary when you screw up (and i have) and glowing white liqiud copper drips onto the carpet.

Ben Solon

Mon Apr 02 2012, 04:23AM

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

My options are between winding the primary and secondays on a small mot core(I don't thing that setup would push 1.5kw) and a boost converter(which provides a lot more control). How would you go about doing 400v at up to 10 amps solid state? And if you still suggest a transformer, how big would it need to be? My maximum transformer size is MOT because I don't plan on buying a 500$ one. Plus Steve did it easy enough. I am only using 1/3 the max power he uses. 1/8 the normal operational power. Plus I want to build the boost converter not just for function, but because it sounds like a good fun challenge

Weston

Mon Apr 02 2012, 04:40AM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

I don't see any reference to power factor, so I assume you are not constructing this as a means to achieve PFC?

If PF is not a concern, a step up transformer and a variac would be a much simpler and most likely cheaper (when you factor in development costs), solution.

Ben Solon

Mon Apr 02 2012, 05:18AM

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

Yes I would like to achieve pfc. I don't have a variac, or power transformer(and don't want to go down that road anyways). I don't mind this being difficult or dangerous(enough danger no matter where you deal in the kw range). Not to sound like a stubborn jerk, but I am not really looking for alternate methods, just advice on this one.

Steve Conner

Mon Apr 02 2012, 06:44AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Steve's first boost PFC used gapped ferrite cores with discontinuous current. The ripple was indeed huge. He redesigned it with a massive iron powder toroid and continuous current. I don't know which version he published the schematics of.

A boost PFC won't go down to 0. It can't provide a voltage less than the rectified line: 160V in this case.

Dr. ISOTOP

Mon Apr 02 2012, 07:45AM

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

You want regulation and overcurrent detection to have anything remotely reliable.
Like so <--I've seen this thing evolve through many forms and get progressively more refined in real life, so I'd say it's a pretty damn reliable design right now.

Ben Solon

Mon Apr 02 2012, 01:02PM

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

I am good with the controller. I am going to use an atmega328p with optocouplers to the ucc chips and from external comparators. Should I be using continuous current or discontinuous? I would like to use my 30n60's but I don't know if they will hold up well...

Edit: I just dug up this

makes you wonder how many more buried links Steve has. He uses 1 1200v igbt with 160A Icm for 1-5kw. My plan is to use 2 600v at 220A Icm igbts, so regarding power, I should be fine. any more design tips before I start to build this with my own controller?

2Spoons

Mon Apr 02 2012, 10:01PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
Location:
Posts: 615

Whatever you do, don't use a MOT core at those frequencies : the core losses will be huge. MOTs are built for 50 / 60 Hz. You need ferrite or powdered iron at 20kHz. Consider going higher in f , up to 100kHz : all the reactive parts get smaller.

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