[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: DRSSTC eye candy (sparks)

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Ken,

At 01:07 PM 3/18/2005, you wrote:
...........

2. Why D19-D22, when 2 back-to-back zeners might do it?

The signal from the diodes is used for gate zero current detection in my case so they have to be super fast!! The gate capacitance and speed of "just" the zeners makes them too slow. But the added 1N5819 is super fast and takes care of things.

There is a trick too. With the diodes, the zeners charge up to their voltage and act like rail power supplies due the their high capacitance. Thus, they are always charged to say 5 volts and the 1N5819s are free to just do the very fast voltage limiting. The zeners charge to full voltage almost instantly so no extra capacitance is needed - and may in fact hurt the startup. The 1N5819s also take high current and have only 0.3V or drop across them. Very cool diodes!!!


>
> 2.  What IGBTs do you use--and would they work adequately @ 100-130
> KHz?  (My 2 secondarys are 100 & 130 or so KHz. & I wouldn't have the
> energy to make a new one.)

Im using some mini-block IGBTs made by fairchild, part number
HGT1N40N60A4D.  They are hefty little guys rated 110A.  They work just
fine at 100-130khz (i used them at 300khz in my mini coil).  In fact,
before i wound my new secondary i was using one that operated at about
130khz (this meant tuning the primary to 95khz).  I pushed them to
over 1000A pulsed at 95khz with no failure or signs of stress.  You
might want to check out the IGBTs terry is using (hopefully terry will
post the part number again, i dont remember it off hand) since you can
currently buy them.  Fairchild seems to have discontinued the
particular IGBT i use, though i still see people sell them on ebay
from time to time.
4. Terry?... 

Whaaa!? :o))  Busy reading wireless power posts >:o))))

I use IR IRG4PF50WDs.

http://hot-streamer.com/temp/irg4pf50wd.pdf

No worries about frequencies!! Normal IGBTs have a very low frequency specs since they have pretty high switching losses. If you ran an IGBT at say 1MHZ, the switching losses alone would be about 10X what it would take to overheat the IGBT!!

However!!! We run at say 5% duty cycle!! So the normal low frequency problem does not apply to us. The only "speed limit" is how fast and how accurately you can switch them. Probably 1MHz is the max. Newer IGBTs like IR's WARP and WARP-2 series are designed to have far lower switching losses thus they have higher frequency specs. But if you are not running CW, those specs alone mean nothing.


5. Perhaps one should always choose the lower freq. so as to minimuze efficiency-loss due to the IGBTs' transition-times.

It does not really matter enough to care about.... It would be impossible to keep the tuning so close for a normal coil being "played with" in many odd ways anyway. With primary current feedback, the controllers just ride along without problems.


7. I'd had somewhat the same problem with my feedback-t.c., in assuring that oscillations would start when I gated the drive on. I solved that by incorporating enough linear gain in the amplifying chain so that noise--picked up + internal--would provide the needed impulse to get things going. I had to use 3 linearized CMOS inverters in series followed by 1 digital, with the input-drive just the +/- 0.7V across a pair of back-to-back diodes in series with the secondary return. Couldn't employ all stages w/in one IC due to intra-substrate coupling; had to use 2, 74Cs--& not HCs.

Seems everyone has gone to a positive "start pulse" circuit to start their DRSSTCs. You sort of need a pulse long enough to get enough current into the primary for the CTs to take over. But you don't want the pulse too long or it might interfere with the running frequency. I just used a discrete pulse chip (74HC221) and was done with it. I would worry trying to do it with "noise"...

http://drsstc.com/~terrell/schematics/MainController-1.0.gif


BTW - Very nice sparks Steve!!! Glade to see you can work outside again!! Interesting to see that primary feedback is working better than secondary feedback for you!! Your controller is looking very simple and easy too!! You might have a look at optical gate drives. Works perfectly for me, but my IGBTs are smaller too. But it gets rid of GDTs and the need for external isolated power:

http://drsstc.com/~terrell/schematics/OpticalGateDrive-1.0.gif

http://drsstc.com/~terrell/pictures/IGBT-DriveCards.JPG

Never have had a single problem with it and no damage after 3 IGBT pops...



Cheers,

        Terry