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yet another DRSSTC - reality check

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Dr. Shark

Sun May 28 2006, 05:19PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Yeah, this is about the 100th thread about DRSSTCs, but I know a cool toy when I see one for about 100 times, so I resumed work on mine...

working on a 24V supply, it seems to be OK, but before firing it up at full mains voltage (no, I don't have a variac here), I thought I'd better ask if everything looks OK so far:

The lower trace is what my halfbride of small, cute TO220 IGBTs sees, and the upper trace is picked up from the air, i.e. what is radiated from the secondary. For me this looks OK, I have tuned it so the secondary voltage rises as high as it goes before the interruptor kicks in. It does not break out at 24V, but when I put a screwdriver near it, it arcs about 1cm, making an awfull sound at about 4kHz from the interruptor

The circuit BTW is Conner Steves dirty weekend DRSSTC, simple enough for me (unlike his new design), but still fully featured with primary feedback and synchronised interruptor.

So, should I dare plug it in?

EDIT: completey forgot to talk about tuning: My secondary with its 5pF topload runs at a frequency of around 350kHz, so I went for a primary with 4.3uH and chopped up my way too big MMC to 40nF. Should that be allright, or would I be better of with more capacitance and less primary turns (5 at the moment)?

Steve Ward

Sun May 28 2006, 06:12PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

From the information given, it looks OK. Dont run it at 4khz PRF when you plug it into the wall! Start out low, like 60bps. You shouldnt need more than 180bps to achieve full spark length anyway, and the very high PRF just causes a lot of stress to your components. Good luck!

Dr. Shark

Mon May 29 2006, 06:17PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

WOOO-HA! I got it going! It only produces about 3" of streamers right now, but I think that I am on the right track. It just seems feedback is not working, neither with a current transformer nor with an antenna. I have no idea how to fix this, but after it took me a long time to figure out that the MMC and primary should be in series and not parallel (DOH!), I think I might never find the problem

Anyway, this is really funny: I am using a 3300uF smoothing cap on the 300V line, just like the Steves do. However, when I plug the thing out, it keeps firing away for about a minute, since the cap takes so long to drain at my low rep rate...

Dr. Shark

Mon May 29 2006, 06:30PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Actually I have massive arcing from the top turns of the secondary, I wonder why that could be?

Dr. Shark

Mon May 29 2006, 06:38PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Ah, I actually found the reason for using these huge electrolytics:

It gives a huge bang when you blow up the IGBTs!

Marko

Mon May 29 2006, 06:38PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

What are you using as divider caps?

With PLL it's nice that it oscillates always no matter if it's tuned or not.
I'm using steve ward's crappy

circuit and I can't get mine oscillating (doh).

I wonder why is so hard to get current feedback working reliably.

Dr. Shark

Mon May 29 2006, 07:10PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Wow, this time an even louder bang, and I did not even turn up the break rate!

but just as I was bending down to pull the plug (yeah, fuses are for whimps), this happened

and nearly deafend my left ear. Gotta be more careful around those TCs!

Firkragg, the halfbridge has two 3.3uF 250V caps. I am not sure if it is so nice that the PLL just forces oscillations, since I am pretty sure I am loosing my IGBTs due to non-zero switching, for the lack of feedback. Or maybe those 10A TO220 devices are just not up to the job :)

EDIT: Now I blew up the set of IGBTs for the third time, but I think I figured out why: I did not use Zeners on the gates, because I though they are only for static discharge protection, and not really needed. Wrong! Aparently the transients on the drain get coupled through. But I still need to fix that darn feedback issue. Maybe I blew the 4046 PLL, that has happened to me before...

Marko

Mon May 29 2006, 07:53PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

3,3uF seems way too low for a divider.

You have only 75 milijoules of energy for each half-cycle (before both caps can recharge from big one).

Just for you not to wonder why are your sparks small after (I use two 470uF caps for divider, for example. Sometimes even entire filtering pari is made of two caps and they are fed to halfbridge
(sometimes there is a doubler, etc).

I also have similar problems with feedback (but mine doesn't work at all), I guess we should wait for Steve

PS. antenna feedback should work, try getting it pretty close to coil and see if PLL is responding!

Did you check the circuit for bugs before, does it lock to weak external signal?

Dr. Shark

Mon May 29 2006, 08:03PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Should 25mJ not be plenty for each half cycle? My MMC is only 50nF now, so I think 3uF to feed it is plenty... besides I don't have any bigger capacitors, and I think you cannot really get much bigger. Electrolytics are probably just way too slow.

Regarding the feedback, with the antenna I get a nice clean squarewave into the PLLs input, it just doesn't seem to care about it. Maybe it is blown...

Well, waiting for one of the Steves sounds like a good plan. In the meantime, a macro of a dead IGBT:

Marko

Mon May 29 2006, 08:32PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

''IGBT's Die In Hot Arcs''

With DRSSTC, it's not just all about charging-discharging MMC. Far from that.

You store the energy in primary LC (voltage and current rise to terrible levels) and it is also transferred to secondary very quicky (depending on coupling)

You need few serious joules of bang energy during interrupter cycle, and you don't want these caps to limit it then.

You want the most of energy to be transferred in the shortest time so you can turn the burst off with full spark length.

Let's say it is fine for now, and later you can add some lytics parallel to these caps. (Im also using normal ones paaralel to my divider).

Check the PLL board for bugs, obivously problem lies in there

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