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Jrz126

Thu Jun 22 2006, 08:39PM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

So I think I'm going to go with the cardboard tube...(although, I think I might have enough 26# wire to do a 34" coil on that 18.5" diameter pipe that I have

). Will I have to coat the cardboard before I wind it?

My SISG parts wont arrive until Tuesday

Jrz126

Fri Jun 23 2006, 07:16PM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

Sweet! the fed ex package tracker dealy is way off. The parts are scheduled to be delivered today.

I have a 4.5" x 18" coil already wound, I think I'll set up a little test rig for now. I dont know if Ill be able to use the SiDac circuit, since it will only fire at 240-280V. I should have ordered some lower voltage ones.

I was reading up on how to drive these some more (see this ). on page 16,

An IGBT will be off when its gate voltage is zero. However, in order to ensure that the IGBT stays in its off state when dv/dt noise is present in the collector-emitter voltage an off bias must be used. Use of reverse bias also decreases turn off losses. The relationship between reverse bias voltage and switching losses is shown in Figure 4.45. For H-Series IGBTs an off bias of -5 to -15V is recommended.

So does this mean my Gate Drive should be capable of going negative?

Also, I was reading Steve's OLTC stuff and he mentioned something about secondary flashover...what is this exactly?

Marko

Fri Jun 23 2006, 08:43PM

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

So does this mean my Gate Drive should be capable of going negative?

Yes, It probably isn't that important here but it's main advantages are that it speeds up turn-off, and holds the gate strongly under zero.

Noise needs to be greater than this voltage (plus few volts) to accidently turn the IGBT on.

In high frequency bridge applications this is very important because miller (gate-drain capacitance) couples part of high supply voltage to the gate.
When IGBT closes shut voltage on it's drain quickly rises to supply voltage, and miller capacitance couples this dV/dt change to gate.
If gatedrive failed to hold off that 'spike', gate will turn on, IGBT will open when it was supposed to be closed, undergo shoot-trough with another IGBT in halfbridge, shorting the supply, and both die a horrid death.

In OLTC things are different, you have only one switch and switching is slow, so holding gate isn't necessary. As I figured out Steve Conner didn't use negative drive in his final design.

Also, I was reading Steve's OLTC stuff and he mentioned something about secondary flashover...what is this exactly?

As steve explained, single turn primary and MMC, switched by IGBT, aren't actually a very good resonant circuit.
Since we are operating on low voltage we try to get the tank capacitance as big as possible, and we must cut down the primary to single turn if we want a reasonably high Fres.

Due to losses on Primary circuit it must be closely coupled to secondary, so the energy is transferred as fast as possible before it's lost.

Fast energy transfer causes secondary voltage to rise rapidly, wich leaves little time for streamers to 'grow' and secondary tends to flash over and make racing sparks. It's hard to get multi-secondary-length streamer 'growing' off the breakout point and miraculously not flashing ovver, like with DRSSTC and less-coupled SGTC's (where energy transfer is much slower, especially with DRSSTC).

Jrz126

Sat Jun 24 2006, 03:32AM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

I made a quick test setup... secondary + rig
the brick

closeup of the brick

Now all I need is a gate drive

Marko

Sat Jun 24 2006, 10:41AM

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

Your MMC and secondary look pretty small for that brick. (Hihg Fres).

But for quick lash-up we can say it's OK.

What wire thickness did you use for this secondary?

Jrz126

Sat Jun 24 2006, 10:53AM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

I've got 24 of those caps to use.

I used 1/4" fridge tube.

I'll be working on it later tonight :)

Jrz126

Sat Jun 24 2006, 10:40PM

Registered Member #242 Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210

I'm designing my Gate drive right now, I'm going to use a IR21844 IC, a half bridge mosfet driver which has a programmable 'dead time' to prevent shoot through.
I'm going to use a 555 timer to trigger it. What duty cycle/on time do I want to use to drive my IGBT?

Marko

Sat Jun 24 2006, 11:13PM

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

Maybe we misunderstood?

I guess you meant to drive gatedrive-mosfets with this driver, and shoot-trough is not a danger there.
This is a high voltage driver for SMPS use, seems to be generally slow but you could use it if you really wish, for mosfet driving.

Look at Steve Conner's OLTC pages, you have lots of OLTC knowledge there.

You can use it but there are much simpler ways to do the same thing, and much faster.
I'd stick to Steve Conner's driver for the beggining, for a big OLTC improved version (with zero cross detection and crowbar) is very advisable.

NE555 and simple mosfet driver may be good for the beggining, at low power. Just take care not to blow up that nice IGBT.

Steve Conner

Sat Jun 24 2006, 11:21PM

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

Jrz and Aaron, 48 hours between double posts please

If you want to add something in a hurry, edit your original post.

Using the SISG as a power supply is pretty pointless. If you don't like voltage multipliers running off the 60Hz line, just go to an electrical salvage yard, get a 120 to 480v transformer, run it backwards, and rectify the 480V AC output. If you want to do something really 1337, make a boost converter power supply like Steve Ward's 7kW unit.

Terry Fritz

Sat Jun 24 2006, 11:30PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

I looked into the voltage doubler and trippler stuff. Pretty cool!! But the caps have to be pretty darn big and all!! In general, I would think one would be better off just using a MOT. Of course, maybe some folks like the voltage doublers and tripplers so more "power" to them

))) But a very simple MOT run at low voltage off a variac can kick some pretty big arcs too

)

Cheers,

Terry

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