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Registered Member #1530
Joined: Tue Jun 10 2008, 03:34PM
Location:
Posts: 32
So, I intended on building a fullbridge design, but I figure I'll do a half bridge for my first coil due to it being a bit simpler... I hope. Also, I don't want to strain my gate driver, it is the pdip model and it gets quite warm running around 300khz.
So. I was thinking about running my sstc for a few cycles (about 10-100 microseconds) and then shutting it off for about half a second or so. Just a very low duty cycle to make sure everything is safe. I'd be hopefully running feedback off of an antenna fed to the gate drivers, and a 555 monostable will provide the enable pulses, perhaps controlled by me (I will be far away from the coil, believe me.
So now there are two questions: About how far can I extend the primary wires away from the primary itself, to distance the control and drive circuitry. Second one is: If I am running at such a low duty cycle, won't the diodes (recovery) have to conduct a whole lot of current every time the coil is disabled?
So... waiting on new gate drivers from TI and gotta order me some polypropylene caps. Also variac... Is isolation transformer very important as well? Thanks!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
1: About 6 feet if you use thick wire (say 14AWG) and twist the conductors tightly to minimize stray inductance.
2: Nah, it'll probably be fine. Low duty cycle reduces the stress on everything. I've used this low duty cycle trick a lot for troubleshooting, and I'm sure other solid-state coilers use it too.
BTW, isolation transformers are very useful, you ought to get one if you can.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Yeah, I'd second Steve's comments. Low duty cycle is the best way to reduce dissipation in the switches and the diodes when trouble shooting. In some cases it can even prevent destruction until you've had time to observe a problem. I once had my SSTC survive repeated pulsed shoot-through at full mains voltage with a low duty cycle when I inadvertedly wired up gate drive to top and bottom devices in phase! Because I didn't have a scope handy, and it was only one bridge leg that was wired up wrong, the problem went largely un-noticed until I tried to increase the duty cycle. Eventually the two afflicted MOSFETs objected violently. (A stupid mistake that should have been spotted and avoided though!)
If you've got an isolation transformer and a storage scope you can see a lot of what's happening from a few carefully performed one-shot tests with a much reduced chance of anything going bang. Still best to keep a safe distance and use eye-protection though.
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Interesting pulse question diodes conduct?
