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Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Dzejwor,
I applaud your input into this thread which is much too quiet, perhaps that will change as time passes. First thing, I have not lowered the switching frequency on purpose, I am trying to find a method where I can increase the switching frequency. From first light, this buck has been switching btwn. 5kHz and 12kHz, and increasing that figure into the thirties is my goal.
My schematic is lifted right out off the Loneocean site, from the QCW no. 1, where i omitted the 1uS r-c time delay between the two output schmitt trigger stages, because I don't understand the reason for them. I also use the LT1719 comparator, as recommended in the QCW 1.5 writeup. I feed the front end of the comparator with 9V, to gain benefit from the full 5 volts of signal from the AD8028 Buffer.
How did you get your buck to switch up to 30~70 kHz?
Registered Member #2750
Joined: Sun Mar 21 2010, 08:47PM
Location: Poland
Posts: 46
Now my buck is simple comparator like yours. I run at around 30kHz but I have those bad glithes at the ramp. When I found some free time I try to lower the switching ferq in order to check whether the glitch disappear. Previous version was on TL494 and run 65-70kHz witch no glitches on the ramp but very unstable and tend to fall in resonance. I tested synchronous buck configuration too but as expected no benefits. Now I use SKM100GB125 bricks in buck and in DRSSTC stage. They look much faster than CM400DY-12NF. I can Switch them at around 380kHz at 180A in my QCW using something what I call a hybrid gate driver - low voltage but powerfull bridge controled by semiconductor drivers supplies powerfull and well designed Gate Drive Transformer of main bridge. I stil get big delay on main bridge hard to compensate witch predikter and lot of switching losses. I'm not so happy with the results, my ramp is glitchy and unstable but IT WORK and sparks are beautiful suddenly quiet and unusual. This is the reason why I started to build QCW ;) I plan to reuild DRSSTC bridge witch FGH60N60 IGBTs, semiconductor drivers and probably the resonator will be changed. My actual results can be seen here
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
All,
The new board is an improvement, I am getting reasonably steady performance like the one below, 380 V on the bus, 350V ramp Peak into 2 ohms, translates into 175 amps, that's pretty cool. I have to keep the ramp short to peak out at 350V, otherwise the bus cap voltage sags.
But no matter what I do, I cannot seem to drive the switch frequency above 12kHz. (For a short while, it was 20kHz, but I forgot what parameters produced it) I would have thought that varying the feedback around the comparator was all I had to do to change the frequency, but it stays at 12kHz. I must be overlooking something very fundamental here, and I invite clever insights.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
All,
I wanted to make the Buck switch at 30kHz, but as this goal soon appeared to be up in god's domain, I had also made an unconscious mental agreement with myself, that if I managed to exceed 20kHz, then it was probably time to call it a day, with regards to tweaking the Buck. I finally reached that goal, by adding that single part of the circuit that I had omitted because I didn't understand it, that is the diode bypassed RC delay link between the 2 schmitt triggers in the gate drive chain.
Yellow is the ramp input from the Arduino Due Blue is 300 Vpeak into 2 Ohms for 150 Apeak. 30 Vripple
That made the difference btwn 10-12 kHz and 20+kHz, so I guess it is time to etch the final layout of the board, wrap it up in a case, and move on the actual coil. These intermediate prototype boards start out pristine, but very soon look horrribly messed up with afterthoughts:
Once more, I warm thank you to Gao Guangyan, of Loneoceans fame, for so generously publishing his work on this very special type of buck converter.
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The road to a QCW
