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Registered Member #59110
Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74
I am making steady progress on a QCW project and have a beginner question about voltage waveform.
I got the DRSSTC part of the project running a while back and it looks good. The driver is loneoceans UD2.7 board and it's working very well. I used the inductor to tune the coil for ZVS operation and the waveforms show very little overshoot when tuned correctly.
My question has to do with the voltage waveform (Collector to Emitter) after the interrupter turns off. The voltage excursions after the interrupter turns off (and the tank current starts ringing down) are much higher than any during the on time. I have seen this on other DRSSTC waveforms on this forum.on these. So far I'm only running about 90 volts on the buss, but the voltage can go 10-20% higher than this during the ring down period.
This is not a problem at the low voltage I'm currently testing at but could be an issue at higher voltage. The only way to avoid the problem would be to use a fairly high voltage TVS. I notice that some designs use them, others not. And I can't figure out why.
Cheers.
Note: The voltage waveform was taken with a diff probe and has an effective scale of 100 V/div.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
Your waveform looks very normal (infact it looks extremely clean!).
Some increase in bus voltage is expected (the energy left in the system when the interrupter turns off is returning to the bus capacitors, charging them up), but I do not know either why the ringdown sometimes has large spikes.
I would not any TVS diodes until you look at behaviour at higher bus voltages - you may find that the spikes get no worse as the voltage is increased. TVS diodes aren't a magic solution either - it's hard to specify them to not conduct during normal operation but to clamp at a low enough voltage to reduce high-energy spikes to safe levels.
Registered Member #59110
Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74
Thanks.
It is good to get your assessment of TVS. I was looking at the specs and they are (obviously) far from ideal devices.
I am also kind of surprised at how nice the wave-forms are. I keep thinking that I am missing something. The most interesting thing that I noted was that the phase tuning got rid of the overshoot over the entire interrupter pulse. All of the scope shots I found on these forums had minimal overshoot at peak current but more at the beginning of the pulse.
I'll bet that things will start to uglify when I get higher in voltage.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
One thing I would check is whether the scope is capturing at a high enough sample rate to catch narrow spikes. I'm assuming that you've zoomed in a bit more than is shown in the screencap you attached and still not seen overshoot, but I do know that those old tek scopes are rather limited in memory depth (and thus sample rate at slower time/div settings).
I would be interested in more details about the coil, especially which IGBTs you're using.
Registered Member #59110
Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74
Yes I have zoomed in. Thanks for checking. It is an old Tektronics 60MHz/1Gs/sec. In the past I've looked at ~10nsec pulses from an LED/photodiode so it should be fast enough.
I wish I could say my design is original but I'm cloning almost exactly loneoceans QCW 1.0. I'm using his UD2.7 and the EasyBridge which uses paralleled IRGP50B60PD1 devices.
Since I hate programming I'm attempting to generate the ramp with an analog input using ideas from another coiler. So far it's moving along fine. I'm in no particular hurry.
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