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Designing a variable HT PSU

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Wolfram

Sat Jan 29 2011, 02:02AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Maybe you could try a half-bridge based driver. I've had success with them for these sort of applications.

Inducktion

Sat Jan 29 2011, 03:17AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

...Looking at ignition coils and flyback transformers, what's the difference, if any?

Adam Munich

Sat Jan 29 2011, 10:15PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Gah, core losses everywhere. I tried a ferrite GDT and even that got hot. I'm trying to minimize losses, so I'm going to use a gate drive IC; FAN7392. I should have one by monday.

The circuit is going to be run off of a 14.8V lipo, and it's going to be adjusted variably and linearly to a minimum of 11V. So, 3.8 volts lost isn't too bad as far as efficiency goes. Looks like one of the biggest losses is going to be the flyback itself. It seems like a lot of energy is being lost in the core via eddy currents or some crazy thing like that.

Just a question, should I use this IGBT or irfp250's? One of my fancy fets I wanted to use was dead. :-/

Adam Munich

Thu Feb 24 2011, 06:28AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Alright, some news. I decided to try the multipler using my really old flyback from that olympic tv. Good things happened.

I fed the 8 stage multiplier with it, and I got some pretty neat results. The transformer was powered by a zvs just because I had one next to me.

Unloaded: 20 or so kilovolts out of the tranny, started arcing to the primary, punched right through 2 overhead sheets in no time at all.

Loaded with the multiplier: 3.5kV out. A huge drop!

Multiplier put out 4 cm sparks and measured 40kV. So... 40kV. Since 3500V * radical2 * 8stages = 39.5kV things are checking out.

The multipler's sparks were uncomfortably loud, about as loud as 1mA sparks from a 60kz xrt. (yes, it's limited by 15 megs) Good news...

The input to the zvs was 18V @ 4.5A, or 81 watts. Assuming 100% efficiency that means there's 2mA coming out of the multiplier. Now realistically efficiency is probably more like 50%, but that's still 1mA.

Frequency of the zvs is most likely about 30khz. Fbt core got a little warm, but not much. Theoretically all this should not be happening at 30khz, but it is...

If things keep going this good then that means at 22V I should be seeing 75kV or so. That's good because 22v lipos do exist. I am thinking of another alternative though; D batteries. I could use a 14V lipo, then the rest of the voltage supply is a string of D batteries. This would allow me to completely eliminate the variable power supply problem and save a lot of wasted power in the process.

pix later.

Wolfram

Thu Feb 24 2011, 12:35PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

I don't think estimating the current from the sound of the sparks is a reliable method. A lot of factors can change how much sound the sparks make, without changing the delivered current. Maybe you could use a microphone and recording software to figure out how often it sparks, and multiply this by the total energy stored in the multiplier capacitors to get total output power. You can roughly estimate the output voltage from the size of the gap, and this should give you a good idea of the current.

Adam Munich

Thu Feb 24 2011, 06:56PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Hmm... not too many sparks at 4cm, most likely because the massive amount of corona spewing from the electrodes is robbing my power. At 32mm I'm getting what appears to be more than 30 sparks a second, looks a lot like my old xrt video. I still think that the corona is robbing some of my sparks. Either that or my voltage isn't as high as I thought it was.

I'm not sure how to go about calculating this, but there's 8 stages and each cap is 1700pF. Is there a formula I could use to do this? (I'm not a math person).

Hmm... It appears to be that as the electrodes move closer together i get more sparks (expected) but there's also a rather
large voltage drop at the flyback... I might need a bigger one. Or i'm just getting inaccurate readings, my kilovoltmeter is getting all staticy.

Nope, cleaned it up with static eliminator and it gives me 3.5kV. Still a bit of drop but not as severe as I thought.

3.5kV in, at 34mm I'm getting at least 30 sparks a second, 40mm about 10, likely get more if it wasn't for that dammed orona. Holy crap it's spewing oil into the air. (As you might have guessed I've been writing this post as I play with the thing)
'
Oh god, my digital caliper is reading 689.23mm. Funny how electrostatics do that sort of stuff. *presses zero button*

*proceeds to add another d battery and gets a longer spark*

Hmm... unloading could be a problem because with the multiplier unloaded the voltage on the flyback jumps way up...

Ahh... my d batteries were dead. I just used a lead acid in series with a 12V psu and I'm getting about 80 sparks a second at 55mm, 60 or so at 66mm, 20 or so at 70mm, only corona and spewing oil at 75mm. The sparks are also a bit louder.

A volt or so more and this thing is ready to be made permanent and I can order the lipo and finally start building the dang x-ray.

Ok, so the batteries are good. They just have a high esr and can't supply more than 5A. Hmm... there goes my d battery idea. Looks like I'll have to design a variable voltage supply thing now.

Lol, em field killed my dmm's voltmeter. Ammeter's still good though.

Christ almighty. I hooked the battery up to a charger since it was just about dead and I'm getting like 50 sparks a second at 72mm. My ears are still ringing.
24 volts into 70kV is perfect. The flyback doesn't seem to like it much though, so I may just put it under oil along with the multiplier.

For the record, really old ohmite carbon comp resistors don't seem to mind 22kv across them.

Adam Munich

Thu Feb 24 2011, 10:38PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Err... I just insulated the flyback under oil too and I'm getting 30 sparks a second at 120kV. Must have been a lot of corona losses!

*turns down the voltage just a tad*

Ok, so 18V (5v atx + lead acid) gives me 60kv at about 40sparks/sec and 25V gives me 120KV; so I'm proposing 20V will give me 70kV. Hmm... I need an adjustable PSU since D batteries aren't cutting it. :-/

I wonder if hot glue and popsicle sticks will hold up under oil, because that's what I'm using to hold the flyback upright.

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