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Registered Member #59110
Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74
I have had some luck building the ramped buck controller for a QCW coil that I am making. The circuit is loneoceans asynchronous driver.
The circuit is working pretty well, driving up to about 250 volts into a 6 ohm load. It does tend to break up over that voltage and in addition drops out below ~50 volts. It will probably work fine for me because I am not after getting the longest sparks possible, just reliable operation to 2-4 feet. But I'd still like to understand and possibly tune the beast.
I have a few questions about the circuit that I am hoping someone can answer.
1) What is the function of the 22K/1nF in the buck feedback loop.
2) How is the 47kohm resistor selected? I think it has to do with the hysteresis of the comparator.
3) What is the function of the diode bypassed R/C circuit at the comparatator output (between the two inverters)?
I am trying to understand these parts so I can meaningfully tune the circuit for better regulation over a wider output voltage range. From studying lonesoceans buckconverter 1.5 he adds tuning and complexity to all three of these areas. The comparator feedback circuit has two resistors and a capacitor for more compensation.
Any illumination of these dark areas would be great. I'll be gone for three days so will not be able to reply to any helpful souls.
I suggest you to post this question on https://highvoltageforum.net/index.php There have been sightings of a certain ward of steve Who, then again was is the definitive authority on all things QCW(atleast I think so!!)
Registered Member #9614
Joined: Mon Jan 14 2013, 10:00PM
Location:
Posts: 44
Not sure if you've found any answers yet, but here's my understanding of the circuit (may or may not be correct):
1. High pass filter 2. At the beginning of a pulse it tells the buck to turn on and then the comparator takes over, value is not entirely important 3. Off time limiter
Registered Member #59110
Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74
Alex,
Thanks for your reply. I did finally get loneoceans and he described the function of these three parts of the circuit.
1) helps increase the comparator switching frequency by adding some phase lead to the feedback.
2) it does add some hysteresis to the comparator. Hysteresis helps make the circuit less likely to switch due to noise in the voltage feedback. To much hystersis slows down the buck switching frequency which leads to worse regulation. It's a trade-off.
3) this part of the circuit help also with noise immunity. To me it looks like part of a switch debounce circuit and may act the same way ie to only allow switching for the real on/off signal not the bounce noise.
Registered Member #59110
Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74
Loneoceans gave me a link to the TI app note tidu020a.pdf. This may be the .pdf you are referencing. It was very useful to me in getting some understanding of the hysteresis effect.
I also spent some time learning to use LTSpice and simulating a crude buck circuit. i was able to learn abou t the effect of several circuit element using this tool.
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Buck Circuit for QCW Coil
