How effective is OCD? / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

How effective is OCD?

Move Thread

LAN_403

Dosinski

Tue Jun 08 2010, 09:46PM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
Location:
Posts: 45

I'm wondering how effective ocd is in protecting igbts. I'm running my drsstc with 100 volts on the primary and my ocd circuit is tripping. I set the ocd to trip at 200 amps (for testing purposes). The maximum current rating for my full bridge is 240 amps. If i put 200 volts on the primary (the maximum output of my doubler), the current will surely exceed the rating of my igbts. Will the ocd kick in and save my igbts before it's too late?

Goodchild

Tue Jun 08 2010, 11:21PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

ya it should if you have it all set up right.
Just a note you can still kill IGBTs with a limiter
if you use to high of a BPS.

Good luck (and get some higher rated IGBTs 240A is just sad)

Dosinski

Tue Jun 08 2010, 11:58PM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
Location:
Posts: 45

Alright, i'll try it out next week. thx^^

Do you have any igbts in mind? (that come in a TO-247 package)

Goodchild

Wed Jun 09 2010, 06:03AM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

You will not do much better than 250A to 300A with the TO-247 package. I would highly recommend moving up to some
mini-blocks (SOT-227B)

The best one you can get for the price are here:

At $18 each it's a good deal (about the same you will pay for a good TO-247 IGBT)
And you can pump up to 500A+ though them. Ive personally got 4.5 feet from them

HV Enthusiast

Wed Jun 09 2010, 11:37AM

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

Dosinski wrote ...

I'm wondering how effective ocd is in protecting igbts. I'm running my drsstc with 100 volts on the primary and my ocd circuit is tripping. I set the ocd to trip at 200 amps (for testing purposes). The maximum current rating for my full bridge is 240 amps. If i put 200 volts on the primary (the maximum output of my doubler), the current will surely exceed the rating of my igbts. Will the ocd kick in and save my igbts before it's too late?

Well, assuming your OCD is under control through the proper physiological or proper medical treatments, your IGBTs should be okay. In fact, OCD could be beneficial in that it may cause you to check and recheck your IGBT and related circuits many times to ensure you have the right connections, etc...

In fact, Nikola Tesla himself was believed to have a very serious case of OCD which affected both his personal and professional life extremely negatively. Of course, he wasn't dealing with IGBTs though.

Dosinski

Wed Jun 09 2010, 07:17PM

Registered Member #2680 Joined: Wed Feb 10 2010, 09:23PM
Location:
Posts: 45

EastVoltResearch wrote ...

Well, assuming your OCD is under control through the proper physiological or proper medical treatments, your IGBTs should be okay.

xD that gave me a good laugh. I think i could push my igbts to 300 amps. Once i get money i'll upgrade to something more substantial. Would CM300's work for 80 kHz?

HV Enthusiast

Thu Jun 10 2010, 12:29PM

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

Seriously, OCD protection is very effective at protecting IGBTs from overcurrent protection.

However! If not implemented properly, the OCD could lead to over-voltage conditions across your IGBT which could destroy your IGBTs.

Basically what happens is that if you stop a pulse mid-stream thats pumping say 500A peak current through your IGBT, then depending on how much bridge inductance you have, you could develop a HUGE voltage spike across the IGBT. Remember that with an inductor, you cannot stop current instaneously. The inductor will do everything in its power to prevent this and what happens is that a huge voltage differential will form across the inductance.

To prevent this, you need to have synchronized OCD circuit. (And no, synchronized shutdown circuits are not something invented soley for Tesla coils - they've been used in similar circuits in power supplies for many decades - just for this reason) Basically, on your modulator input pulses, you want to have a logic network that once a fault is detected it will not disable the pulses until the present pulse has finished switching.

They are a zillion ways to do this. Just some logic gates and flip-flops will do the trick. Steve Ward has a straight forward circuit which does just this which you can find on his website. For my designs, I go one step further with my logic circuit and in addition to inhibiting a pulse during an OC condition, i also inhibit any additional pulses for the remainder of the burst.

So that said, if you have a good sync circuit, then your IGBTs will be very well protected from OC conditions.

Goodchild

Thu Jun 10 2010, 05:37PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Dosinski wrote ...

xD that gave me a good laugh. I think i could push my igbts to 300 amps. Once i get money i'll upgrade to something more substantial. Would CM300's work for 80 kHz?

ummm the jump from TO-247 IGBTs to CM300s is rather big, I would go upgrade to some SOT-227B IGBTs before going to the big bricks. And also no
you can't run CM300s at any more then 60KHz, unless you use phase lead.

Dr. H.

Thu Jun 10 2010, 07:29PM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Hey - don't underestimate the TO-247 devices - they pack a punch. It is "easy" to get more than 40 inches of sparks with those devices ( irg4pc50ud for example - wich costs about 5$ a piece). If you don't believe me - check Reachings site (CTC-labs). Together with a friend we've built a DRRSTC based on those IGBTS pumping 44 inches continuosly wothout a sweat.

Designing the tank circuit is the name of the game with small IGBTs.

Cheers

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob