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Re: Terry's DRSSTC - Bench Test

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Again,

I thought I would add some more test results ;-)

I checked the overall drive current to H-bridge output delay:


424nS... Sort of "so so"... I "think" I know how to wind a CT to go faster (that is where all the delay is). But it is good enough not to worry much with it right now.

Now this is cool!! I took "out" all the crossover dead time and just ran it full speed (each half of the H-bridge is a scope channel):


Looks just fine!!!

Check out this cross over!!


Big high power IGBT die going from turn off to other side conduction in darn near 50nS!!!! Symmetry is close to 5nS!! Surprising since the gate drive wiring is really sloppy and I am floating the gate drive power supplies that are "riding" on this too!! But the impedances are very low and the "electrical "mass"" is small too. Too small to make a good antenna at these frequencies ;-) Cheating a little since the load is nil, but it seems the controller alone is able to control cross conduction and all very well.

Cross conduction is a zero issue it "seems". I really like my controller circuit ;-))


U3a,b and the four gates there just make darn sure nothing is going to go cross conducting. The +- CT also insures full symmetry.

But the close crossover pushes losses up (but the "little batteries" I am using don't seem to mind at all.)... I wonder if I should just "go for it" and let the IGBT temp sensors worry if it dissipates too much heat and back off from there if needed... But I have to slow down the mess too or Miller shoot through will blow it all up...

I wonder if one could "feedback" the actual H-Bridge output drive signals to the controller. So if say shoot through did hang it up, the controller would "know" and just "not" turn on the other side, thus saving the day... With high speed current trip and temperature monitoring, that is just about the point to where the H-drive "can't" be blown up!! I suppose a microcontroller could "on-the-fly" optimize things like dead time too and even do a "black box" function to record the last uS of data if the bridge ever did fail...

Here it is at 500kHz:


Here at 500kHz CW, the drives and my low-Z amp just can't keep up so the "test equipment" is going away... But given that everything is going haywire, the signals are still perfectly symmetrical and cross over is a "don't care"...

The TLP250 will over heat if they are run at say 500kHz CW. Pulsed low duty cycle stuff is no problem and I had no problem with them tonight, but that is an issue with them to watch if you are playing with them at CW high frequency. There 25kHz rating is a "thermal thing"... The gate capacitance of my IGBTs looks mean at first but quickly diminishes... That's why I like International Rectifier stuff. They know all the cool tricks ;-))

I did note that my 1000uF HF electrolytic caps on the gate drive boards seems to be running about 4C over air temp... Maybe the lamp was too close or something, but I'll watch them incase there is an issue there.

So the problems are as expected but I seem to have full control over them. I don't know where to "set" anything to deal with them though... But I'll figure it out ;-)) I just got 32 more IGBTs buy UPS today "if" I mess up :o))) Tonight's stuff was just all low power battery stuff, so maybe it will get more interesting still :o))) But the test results so far are A++++



At 08:21 PM 1/21/2005, Terry Fritz wrote:
Hi All,

Tonight I ran everything on the bench just off four 9V batteries to check operation:


It all worked pretty much just as expected. Here is the output at 40kHz and 300kHz:



I have the dead times set really high and the load is only a 1k ohm resistor so things look a little odd, but this does verify remote control to bridge output operation. Yea!!

Interesting to look at the IGBT turn on and off times:


The turn on is super fast! You can see the blip at about 10V where the doping regions are tearing themselves on! The load is trivial here, but that will have to be slowed down or the IGBTs will tear themselves apart under a real load. Just a trivial bigger gate resistor... Turn on is rather dull since with such a low load you are just seeing the output capacitance and the output MOV discharge curve. A little cross conduction would fix that >:o)

Look at the gates!!


The "pumped" TLP250's and the opto-isolated floating gate drivers have no problem at all pushing the gates! Infact, they drive "too" fast... Again, more gate drive resistance will slow that to a tame speed. Not sure what the bonus down spike is on the turn off... Might be due to the drive power supplies floating or inductance. It only helps here ;-) Since I am just running off batteries, the gate drive signals here are far less than the actual 27V drives:


So I need to tighten the dead time and play with gate drive resistance. A few more measurements and then I will start bolting it all into the box!

Interesting that Antonio found that the freewheeling thing did not seem to help much. The added electronics to do that is pretty complex and I will not miss it ;-))

Steve Ward's data and experiments were really exciting!!! With 27V of gate drive I can run pretty high currents at high speed ;-)) I was planning on running very low currents and slow (the exact opposite), but it can run just about anything...

Steve Conner's PLL circuit is really cool too! I think one would have to preset the free running frequency pretty close to Fo so it could sync and lock in "really fast" (only have 10 cycles according to Steve Ward). I think the start up would also have to be synced with the free running oscillator too so the very first turn-ons start together so the PLL error signals get off to a good start.

I am still looking into getting those MMC cap PC boards. Not too many people want them so it is sort of on the edge. I think I can drop the order to 16 and still get them for like $37 which might be just right.