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GDT Arcing problem :'(

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syntroniks

Tue Jul 01 2008, 03:11AM

Registered Member #1530 Joined: Tue Jun 10 2008, 03:34PM
Location:
Posts: 32

So... I took a video a while back (Yesterday I do believe) of my GDT Arcing. Some background:

Tri filar green magnet wire, driven by ucc37321/ucc37322 through 2 1uF electrolytics back to back and one .1uf ceramic. I have access to a few large .68 and/or .33 polypropylene caps if needed.

It drives well. A few turns though, 15 this time, I think it was 10 before I re-wound.

I re-wound because I saw it arcing and thought insulation failure. But now... Not so much.

So what would cause arcing in there? It only does it in one frequency range, my scope says it is around 30khz, but I don't believe that, it should be more like 300khz (approx my coil fres)
Either way, purpleish arcing. Perhaps something is wrong?

It starts up around 30vac in, and I have pushed it to about 100vac in. I get sparks from the coil, about a cm every seconds or so. Please tell me what I did to deserve this :'( I want to finish this off so I can do other projects then build a real sstc with the correct parts :-/

Thanks for help/insight. GARRRRRRR

---
EDIT

Well, I got the coil to spark... As I thought, It was an order of magnitude higher frequency than what I was at.

I'ma record some data and be back. I don't think I could've been oscillating around the GDT resonant freq... <does calculations>

Wolfram

Tue Jul 01 2008, 05:19AM

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

Many of us have had problems with magnet wire arcing when used on GDTs, me included. The best solution is to use something else, like the wire from CAT5 cable if there is enough room for it, and if the space is tight, kynar wire.

Anders M.

GeordieBoy

Tue Jul 01 2008, 09:49AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

If you are getting corona between windings on the GDT it means that there is a huge voltage between the low-voltage "controller" circuit and the mains-connected switching circuit.

I would make sure the base of the secondary is securely earthed to either mains earth, a copper water pipe or a 6ft stake in the ground. Then I would make sure that the 0v rail of the controller (driver) circuitry is also connected to mains earth. Finally you might want to connect some Y-rated capacitors between each of the V+ and V- DCbus rails and mains ground.

I would also recommend against using magnet wire for GDTs, particularly the cheap single coated ("gold") type because any scratches or bends could crack the insulation. Others have had good success with PVC coated stranded conductors like the inside wires from CAT-5 cable. If it had to meet commercial voltage withstand requirements we'd probably all be using triple-insulated TEX-E for our GDTs!

-Richie,

Tom540

Tue Jul 01 2008, 03:00PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

Well technically cores are meant to be coated. If they aren't coated and they have sharp edges then magnet wire will cause problems. The cores I have which are surplus are super smooth and nicely rounded.

The problem with cat 5 is if you solder it and it gets too hot the insulation melts and can short to another wire coming off the core. I have had that problem once.

syntroniks

Tue Jul 01 2008, 04:58PM

Registered Member #1530 Joined: Tue Jun 10 2008, 03:34PM
Location:
Posts: 32

Checklist:

Magnet wire = X
Kynar = + <Wondered about insulation rating, have some
Cat5 = ++ <got plenty at work, thousands of feet :)

Grounding: Terrible

I tell you something. I didn't have the secondary grounded until just a few hours ago!! So, I connected the secondary base to the ground connection for power to my half bridge with 4 turns through an EMI filter. I noticed better output, and I do think that has solved the problem.

If I ever do make another coil (Probably fullbridge) I'll use Cat5 cable (outer insul stripped).

Other bungles of mine, I am powering control electronics off of a computer power supply, but it -is- grounded... We'll see if I end up getting a small transformer and doing it the right way when I box this all up.

Y rated? I'll see if I can scrounge some up. Hopefully it will be printed on there. I have access to many many many many PC power supplies, so I'll have a look around.

Thanks for the replies!

HV Enthusiast

Tue Jul 01 2008, 07:39PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

You shouldn't be using magnet wire for hv isolation.
When designing magnetics, rule of thumb is to always assume the magnet wire lamination is good for ZERO volts.

syntroniks

Wed Jul 02 2008, 06:43PM

Registered Member #1530 Joined: Tue Jun 10 2008, 03:34PM
Location:
Posts: 32

Then I guess that limits to something thin like teflon generally? What do you use?

I found out it was only arcing if I did not have a breakout point, or I was driving off-resonance

Dr. Dark Current

Wed Jul 02 2008, 07:17PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Dr. Shock wrote ...

You shouldn't be using magnet wire for hv isolation.
When designing magnetics, rule of thumb is to always assume the magnet wire lamination is good for ZERO volts.

Ok, then you cannot wind any coils because of the voltage between adjacent turns

syntroniks wrote ...

Then I guess that limits to something thin like teflon generally? What do you use?

I found out it was only arcing if I did not have a breakout point, or I was driving off-resonance

I found it is best to use screened stereo audio cable. You parallel the screening for primary and the inner conductors are your secondaries. This is also a nice way to get VERY low leakage, much better than any multifilar winding.

GeordieBoy

Wed Jul 02 2008, 09:26PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

I found it is best to use screened stereo audio cable. You parallel the screening for primary and the inner conductors are your secondaries. This is also a nice way to get VERY low leakage, much better than any multifilar winding.

It does indeed achieve the lowest possible leakage inductance. The only downside is that it results in a relatively high inter-winding capacitance between secondary connected to the bridge leg mid-point (source terminal of the top MOSFET) and the low-voltage primary side. This tends to capacitively couple the high dv/dt from the bridge leg mid-point back into the drive electronics. The symptom is narrow but intense spiking of the 0v rail on the low-voltage drive side when the power devices switch.

If you have tri-axial cable you can get close to the ideal gate-drive transformer for a bridge leg by winding it like this... The drive side primary consists of the outer screen and therefore its flux encompases the two inner conductors. This guarantees high coupling factor to both secondary windings and low leakage inductance. The secondary driving the low-side MOSFET consists of the inner screen, and the secondary driving the high-side MOSFET consists of the centre conductor of the cable. When constructed in this way the low-side secondary winding (which is connected to DC bus negative) acts as an inter-winding faraday screen between the noisey bridge output voltage on the centre conductor and the sensitive low voltage drive electronics on the outside shield. All of the high frequency hash from the bridge leg mid-point is capacitively coupled back to the DC bus negative. It is kept on the power side where it belongs, and out of the control and driver electronics.

I just thought i'd share this little bit of practical experience from engineering gate drives for commercial applications.

-Richie,

Steve Conner

Wed Jul 02 2008, 10:26PM

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

I did actually use GDTs with Faraday shields on Mjolnir. I wound them trifilar with one piece of miniature coax and two ordinary PVC insulated wires. The core of the coax was hooked up to the gate drivers, and the screen was connected at one end to the metal housing of the driver circuit, with the other end left open. The two remaining wires hooked up to two IGBTs. I used two GDTs like this to drive my H-bridge.

I don't know if this is the reason why, but Mjolnir has never blown an IGBT, or any other of his semiconductors for that matter. I gave away my stash of miniblocks to try and tempt providence, but nothing happened.

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