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Buck Circuit for QCW Coil

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alan sailer

Fri Apr 13 2018, 03:03PM

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.

Cheers.

|)4+80!|-|\\/

Fri Apr 13 2018, 03:30PM

Registered Member #61803 Joined: Thu Sept 21 2017, 09:18AM
Location: Bhopal,India
Posts: 34

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!!)

Alex Yuan

Tue May 15 2018, 11:34AM

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

alan sailer

Thu May 17 2018, 05:42PM

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.

Cheers.

Alex Yuan

Fri May 18 2018, 04:01AM

Registered Member #9614 Joined: Mon Jan 14 2013, 10:00PM
Location:
Posts: 44

Thanks for the corrections!

2. Found this pdf about adding hysteresis to comparators and it includes notes on selecting R_h:

alan sailer

Fri May 18 2018, 05:44PM

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.

Cheers.

|)4+80!|-|\\/

Sun May 20 2018, 12:52PM

Registered Member #61803 Joined: Thu Sept 21 2017, 09:18AM
Location: Bhopal,India
Posts: 34

How did you contact loneoceans (gao)??

alan sailer

Sun May 20 2018, 02:35PM

Registered Member #59110 Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74

I sent him a message on tiis forum.

Cheers.

|)4+80!|-|\\/

Mon May 21 2018, 06:52AM

Registered Member #61803 Joined: Thu Sept 21 2017, 09:18AM
Location: Bhopal,India
Posts: 34

What is tiis forum?

alan sailer

Tue May 22 2018, 06:44PM

Registered Member #59110 Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74

This, sorry. The forum allows you to sent private mail to another forum member.
I did this and he replied.

Cheers.

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