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Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
Hi Everyone,
I've build a ramp generator for this will be my analog wave form input to my QCW Modulator using this schematic
the problem is, whenever i tried to run my modulator - buck converter with the DRSSTC attached to its output(real working system) it gets the signal distorted and as expected, the output wave form will distort too resulting in a gnarly-unpredictable shape of spark.. but when using a lamp (80 ohms) as a load of my modulator-buck converter, the output waveform is completely clean and without any distortion.. my question: is isolating the signal coming from this ramp analog input (approx 5v peak 10ms) using optocoupler and some low pass RC and opamp will correct the problems i'm encountering? because, currently, the output of this ramp gen is directly plugged in the modulator(comparator (+) input) with a 1k(R12) ohms resistor in between.. schematic attached too... thanks!
PS: The power supply for this ramp gen is isolated from the supply of the modulator control.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
In the past I have never isolated my interrupter from my buck modulator, I usually just have a run of RCA coax from the interrupter to the modulator board. Chances are if you are getting distortion when the TC load is connected it’s probably one or a couple things. The most likely culprit is switching noise from your TC inverter getting back into the feedback of your buck regulation loop. I would check to see if any noise is getting into your feedback.
A couple recommendations to help combat noise are to put a common-mode/differential-mode filter in between the buck and the TC inverter. Also be sure your filter on the buck regulation loop is up to snuff.
Another source of distortion would be a mismatch from your TC inverter input impedance and your buck output impedance. Your buck output impedance is only 2 ohms with your currently selected L and C values; this could also be part of the issue.
{EDIT} Chances are you are running 300V at 100A out of your buck or something close to that, making your input impedance about 2 or 3 ohms, so that may not be the issue.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Interference, as in 90% of solid state coil problems. The ICs are not designed to withstand electromagnetic interference levels 1000 times over the limit, and PCB layout can make this much worse.
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
Hi Eric,
surely I would check the feedback wiring, as currently, im using a non-shielded wire for my feedback(just a twisted pair). I will replace it and see what will happen, as for the interrupter output from the buck modulator, i used an optocoupler(4N35) with boosting transistor stage on its output. btw, the isolation i was questioning is on the ramp generator output and the input of the comparator, because i didn't implement optocoupler and RCs for DAC conversion(like in the schem) - this was my targeted question... do i have to isolate the ramp analog input using that DAC or i could just plug it in like i was currently doing, and might be causing me distortions? is this really the root of distortion?(non isolated input)
Distortion due to impedance mismatch is probably not much of that effect i think.. because i replaced those LC values to check that, i even set the Z of the buck output to 1.. still distorted.. and will check of that common/diff noise filter. thanks..
EDIT: is it true that it is okay if the halfbridge (DRSSTC power side) has higher impedance that of the buck output(say 1.5 ohms)? or it is required to equalize(same value) it?
@ Dark Current, is metal casing all the circuits is the solution for this?? thanks.
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QCW Modulator Ramp Analog Input
