Re: Smokin' gate drive chips on DRSSTC-2 board - Kick the habbit!

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Sue Gaeta <sgsparky@xxxxxxxxxxx>

I have ran that board open load and didn't have any problems.
BUT, as for your second question, I DID have a problem with the tantalum 
coupling capacitors. I actually had a feeling it would happen even before I 
applied power. Tantalum capacitors are not supposed to be RF coupling 
capacitors. They are made for DC and are polarised. I am sure that I put 
them in the right way, but even if you do put them in right, they will be 
in backwards 100,000 times a second (or whatever your resonance happens to 
be)! If you put the wrong polarity across them they usually explode like 
fire crackers, but because there is a load in series with them in this 
case, they could fail quietly. If it shorts out, and you are not monitoring 
the input current of the driver supply, you won't even know it, and won't 
even care because the circuit might appear to be working normally for a 
while, but the drivers will run hotter than they are supposed to. I know 
this because I tried shorting out the coupling caps just to see what would 
happen, and my driver supply c urrent doubled!

In my case, the tantalum opened up, so I became immediatly aware that 
something was wrong when my gate drive just simply went away. I didn't have 
room for any of my P.P. caps, so I used a ceramic multilayer cap that was 
designed to handle higher ripple than the average cap of that type. This is 
still not the best choice for RF coupling, but they work just fine, and 
they don't even get luke warm. Looks like I picked a winner :-)

By the way, if you find that the driver chips run hot when loaded, it could 
be any number of things, but one thing that you might want to do is place 
your finger on one of the zener pairs that is connected from gate to source 
(with the H bridge power disconnected!). If they are running hot, it means 
that your leakage spike is too big, and the zeners are clipping them to 
safe levels. This is fine for the IGBTs, but not good for the driver chips.
In my first SSTC, I was fine for the first half cycle because the gate 
resistor took care of the spike, but since I had shcottkeys across the 
resistors, the negative half cycle had a ringer of a spike on it! (pardon 
the pun). Coincidently, my plans for today was to put a small value 
resistor in series with the schottkeys just enough to get the spike below 
the zener threashold, while keeping a faster fall time. This should not be 
too much of a problem with the higher power devices since the higher gate 
capacitance will tend to smooth the leakage spike and slow down the rise 
and fall times on their own.
Also if you are into solid state, then you can afford a simple scope. 
Please take my advice and buy a scope! It really will help you kick the 
smoking habit! If you insist on playing with this stuff without one then I 
suggest you take up drinking!

I already have a scope, but Santa wants to buy me a cool new toy this year. 
I have one of these on order. 
<http://www.elexp.com/tst_ps40.htm>http://www.elexp.com/tst_ps40.htm
This is cool because it enables you to do forbidden things that you can't 
do with a scope that is not isolated from the wall outlet. The best part is 
the price. Don't get the HPS10, it's a toy! I even saw a website seriously 
advertising it as a great toy to get your kids for stocking stuffers!

Remember also that lower frequencies are less stressful on driver 
components too. I did a test which confirmed this.

Sue

Tesla list <tesla@xxxxxxxxxx> wrote:
Original poster: "Scott Hanson"

Terry -

I am building (3) of Dan McCauley's DRSSTC driver boards, and the first one
I powered up (with no gate drive transformer connected) instantly let out a
small wisp of smoke from one of the driver chips ......

Curiously, I was ramping up input power with a variac, and if I recall this
happened at about 80 or 90 VAC input. I think it was U36 that suddenly got
hot and emitted a very thin thread of smoke. I instantly cut power, checked
everything out, installed some small inductors across the gate driver
outputs to simulate the load of a transformer, and re-applied power. Despite
the initial wisp of smoke, U36 appears to operate normally now, and drives a
real gate drive transformer just fine. If I recall, the output waveforms
were somewhat irregular until I install a load across the driver outputs.

Here's a related question: Dan has used a pair of 10uF tantalum coupling
caps (10uF @ 35V, C45 & C46) in series with the gate drive transformer
primary windings. I'm not sure I understand how a polarized capacitor is
appropriate for this particular application. Any concerns? Would a
non-polarized, low-inductance polypropylene film cap be a better choice?

Regards,
Scott Hanson

PS - Any intention of buying some more of your power supply boards and
offering them to the List members who are building up Dan's driver boards?
I'm sure there would be some level of interest ......

---
[This E-mail scanned for viruses by Surfside Internet]