DRSSTC Primary Current not getting as high as expected - Page 1
Forum section: electronics / projects

author: Etesla
author_rank: Regular Contributor
timestamp: December 13, 2021, 02:14:24 am
content: So here's the story. I'm building my fourth DRSSTC and I'm having a problem I've never run into before. The primary current sort of levels out at 57A and does not get any higher no matter what. (Image attached, Vpp measurement = maximum current the way I had my CT and load resistor to CT set up).Some random specs:It's meant to be a small tabletop size DRSSTC < 2 feet in height overall.196 kHz Fres40n60 IBGT's for a half bridge with 20uF sister capacitorsPrimary coil is made of 1/4" refridgerator copper tube. 1.5" inside radius, ~5" outside radius, about 7.75 turns, .2" turn spacing, inductance estimated around 9uH.Primary capacitor is two series 940C30P15K-F (0.075uF 6000V equivalent)Powered from rectified 120V for about 170V across the busImplements phase lead, verified that the voltage across the primary tank circuit changes right at the zero crossing of the primary currentIn simulation, taking into account the expected ESR of my primary capacitors, and my primary coil, and from the bridge capacitors and everything else, I thought I could easily get primary currents of like 300A over maybe 10 or 12 cycles without the secondary. (sim also attached, you need the 74hc library to use)To get the simulated primary current waveform to match up with reality, I need to add about 2 ohms of extra series resistance in the primary tank circuit in simulation. I could not find another way to get reality to match simulation besides adding that extra series resistance.When I went back to my physical setup I was unable to find any loose connections or other strange things that would add 2 ohms into the tank circuit. I measured the ESR of my primary caps (two CDE 3000V 1.5uF caps in series to make a 6000V 0.075uF cap) and found they were within their 13 mOhm spec at 200 kHz. I did some math and figured that even with the skin effect in play, the effective resistance of the primary coil shouldn't be above maybe 10 mOhm... I of course ohmed everything out with a multimeter and found nothing suspicious...I then ran the coil without the secondary for 200uS on times at 400 Hz for about a minute. Based on simulation, anything with about 2 ohms of resistance should be dissipating > 100 watts in that situation, and so I should be able to notice a temperature rise. At the end of that minute, the mosfets were just a little warm, the primary capacitors were stone cold, the wires connecting the primary coil to everything else were just a little warm, but the primary coil itself was too hot to touch. It melted the ABS spacers I had make to hold it, so we're talking 200 - 300F at least. I also measured the resonant frequency of the primary coil without any driving circuitry attached and found it to be equal the the frequency it oscillates at with the driver attached at 196 kHz (No foreseeable driver issue...)So just based on the fact that the primary coil is where I saw the power dissipation, I am thinking that it's most likely for the primary coil itself to be the thing causing my issue and limiting my primary current, but I don't understand why. The 'resistance' of the primary coil that I should be able to use to predict power dissipation, even with the skin effect in play, is in the milliohm range, meaning I wouldn't expect more than a couple watts of power dissipation for the current waveform I was dealing with. I have used the same sort of primary coil setup (.25" hollow copper tube, 0.2" turn spacing) successfully without excessive primary heating on larger coils where primary currents showed no signs of slowing down even up to 400 ish amps. That coil's Fres was about 110 kHz so it's not even that far off from this one...So now the questions.1.) Do you know why my primary current is sort of limiting itself in this way?2.) Are there things other than excess primary DC resistance that could be limiting my primary current to 57 ish amps peak?3.) Why is my primary coil getting so hot? What part of the math am I missing?4.) How can I increase my primary current without changing my primary capacitor?

author: Wolfram
author_rank: Frequent Contributor
timestamp: December 13, 2021, 11:52:27 am
content: If the current is self-limiting to such an extent even without the secondary in place, your driver is either powering the primary circuit at a frequency where it has a high impedance (could it be operating with way too much phase lead?), or your unloaded primary circuit Q is much lower than one would expect.10 mohm seems a bit optimistic for the AC resistance of the primary at 200 kHz when you include losses from the proximity effect, but it still shouldn't get that warm under the conditions you describe. Is there any chance your primary is made from copper coated steel? This would explain the exaggerated losses, not only does it have significantly higher resistivity than copper, the high permeability also reduces the skin depth by a lot.You can verify with a magnet, and if that checks out, you can measure the DC resistance of the primary by passing a few amps through it and measuring the voltage drop with a multimeter that can read millivolts. You can also measure the AC resistance of the primary by parallel-resonating it with the tank capacitor, feeding it from a signal generator through a 10k resistor, and measuring the voltage drop at resonance. I would expect the copper coil to have a Q of around 100 at this frequency, which would mean that the parallel tank should look like 10 ohms (the tank impedance) * 100 = 1 kohm.If your phase lead looks correct and your primary measures both low DC and AC resistance, I would double-check the primary side current measurement to ensure that your scaling factors are correct.

author: Etesla
author_rank: Regular Contributor
timestamp: December 14, 2021, 04:31:01 am
content: WOW you're a smart cookie. The primary coil material I purchased is actually made out of steel and only coated in copper. It's actually really cool how well the math works out there, with an online 'Round wire AC resistance calculator' calculating exactly 2.08 ohms of equivalent resistance, just as was predicted in simulation. I thought it was a little difficult to bend when I was forming the primaryProblem solved, picked up some copper tubing from home depot today and it is working as expected. Wish I had checked with a magnet earlier lol. Thanks for the tip!tldr I thought I had copper tube but it was actually copper coated steel tubing.