----- Original Message ----- From: "Frosty" <frosty90@xxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx> Sent: Tuesday, July 29, 2008 8:51 AM Subject: Re: [TCML] sstc
Hi, Thanks for the info, I'm in the process of finding somewhere to order moremosfets, so I will let you know how I go. But there is one question which I have'nt heard much about in regard to tesla coiling; that is can mosfets beparralled easily for TC use? Has anyone done something like this? I am guessing that doing so will require proportionally as much gate drivecurrent which brings me to another question; since I like to use driver IC'srather than discreet components, can multiple driver IC's be parralled as well? Cheers, Jesse Frost
Why would you need to parallel them ? You can get CM600 brick IGBT's off eBay used pretty cheap, 1,200volts 600amps. I would be surprised if you needed more than 100amp mosfets ?
In paralling them, you will double the gate capacitance and it will make harder work for your gate drivers just for starters.. then there maybe small mismatches in switch times and other things... it would be easier just to obtain a larger current single mosfet..
Also, you could work out your current rating, if you know the size of your capacitor, voltage, resistance of mosfets, you can work out the max current pulse.. however I think realworld figures will be less as capacitors can be pretty "sluggish"... The best caps around seem to be CDE/CKE caps ? CDE I think is the proper name, CKE are a eBay copy which seem to work just as well ( but a fraction of the cost) the guys on the list will be able to help you out in that respect.
Some mosfets can be 10amps RMS, but can take 1KA pulses or more... Some capacitors can only pulse 10 to 50amps anyway, some of the better ones you maybe able to get 250amps or more... depending on frequency also, I would favour fast mosfets, switching losses are a real problem ( I killed lots of fets trying to run them at 2mhz!) Also switching times get longer as current ratings go up....the IRFP460 mosfets are not to bad really.. you maybe be able to find better mosfets.. I think Richie used them ? I ust STW series, 20NB50 or something I used. I always tried to run them at 500khz or more and it never worked very well. Though if you coil is under 100khz ( I forget what Richie's was) you shouldn't really have a problem.
I suspect its poor gate drive which is the problem your having killing fets... unless you used capacitors much larger in value than Richie used.....
OH.. also be careful about the number of primary turns you use.. if you circuit cannot pulse more than 50amps, then there is no use winding a single turn primary which will try to pull 500amps! It is still a good idea to use a tappable primary on these things, maybe 2 to 6 turns ? Also a good idea to monitor the current pulled from the DC supply rails.. if you have a single turn primary and your fets are only 10amp and you run on 500V then chances are you could pull 50amps or more from the DC supply and kill your fets that way.. so a good idea to monitor the current pulled on these things...
HTH chis _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla