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Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I'm doing some final tunings of my DRSSTC and am having some issues getting some transients out of the initial cycles. The coil runs CM400 bridges with isolated powered drivers, signaled through a GDT. The driver is of my own design but is quite similar to the prediktor, at least in the input stage. The current transformer has a 30uH inductor to set the phase lead.
The feedback signal at the board looks great:
Here is the current from a current transformer with no inductive phase leader:
And here it is from a third current transformer (again no inductive phase leader):
And now, here's the voltage on the output of the bridge with respect to ground (sorry, don't have a differential probe so it's a little ugly but it gets the point across of the transients at the beginning):
So, are these spikes at startup something to worry about, and if so is there some trick at getting rid of them? I'm guessing it's not on the phase sense feedback due to the reactance of the inductor to the HF transients, but not sure about that.
I'm mainly concerned about the bridge output voltage waveform since I have 600V bricks and want to get as much out of this as possible, and those transients would get me over 600V with about 400DC into the bridge (maybe the TVS diodes would take this, but I kind of like my TVS's to be protection, not actively in use)
I have my delay times adjusted for no controller delay on fall time, and about 1.4uS rise time. This was just experimentally determined, it gets rid of as many spikes in the current waveform, but I still can't seem to get rid of those ones at the beginning.
Maybe this is related to my input comparator? I'm using an LM393 biased at 30mV on the - leg to avoid off-oscillations.
Registered Member #2922
Joined: Sun Jun 13 2010, 12:08AM
Location:
Posts: 226
Did you adjust the RC delay step of your predikter circuit? The inductance at the feedback will not excatly anticipate the signal.. It will antecipate more that you need, than you make a delay with a ajustable RC circuit to get the perfect "time shift"..
And if your scope have two channel you can make diferencial measures: Put the second channel at the same voltage scale and in invert mode and than adjust the y offset to the same of channel one. Now you can make diferencial measures.. use the two probes with the grounds floating
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
Good point on the scope, I hadn't thought of that. I do have two channels that can invert and sum. I guess that basically is a differential probe in a way.
I do have RC delays, I have 4 pots, one for rise and one for fall controlling each UCC37322 in the output stage (just used four because I was curious if they might vary, turns out that optimal tuning seems to be at exactly the same rise time or fall time for each chip, so I could have gotten by with just single rise and fall time knobs, but I like to have too much control than too little).
Seems like optimal tuning is with 1.4uS controller delay on fall pots and 0 on rise pots (which I believe inverts in the system, else there would be awful shoot through). I don't really like that there are delay pots at zero, seems to imply it wants even less delay, maybe more inductance in the phase lead? I'm already running what seems to be pretty high compared to others with 30uH
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Have you compared your bridge output voltage with the current waveform to check where its actually switching? I find some of my home made CTs have a slight delay (like 50nS) compared to the pearson CTs, so you may need to over compensate some. Anyway, most that high frequency ringy crap should be able to go away if you get the timing dialed in right. I should mention that even with the ZCD prediction, i still see quite a bit of noise on the first cycle or 2 of switching where the primary current is its lowest, but its far less overshoot than im seeing on your pictures. I havent been able to figure out why it is that way.
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DRSSTC Final Tuning
