Author Topic: increasing primary current without increasing IGBT losses  (Read 330 times)

Offline TMaxElectronics

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increasing primary current without increasing IGBT losses
« on: June 14, 2022, 11:50:29 PM »
Has anybody else ever tried to use a transformer between the bridge and the primary resonant circuit to step up the current? I've been thinking about that for a while and actually given it a go and it really seems to work very well in deed. The two main benefits I can see are a slower rise in primary current (only good if you want it to slow down of course :P) and a lower current in the power semiconductors. Second of which of course means you can either use smaller IGBTs or drive more current through the coil with your existing ones.

I tested two setups by modifying the primary circuit of my big coil (2m coil with 3m sparks). Design specs are 50kHz F_res, 800A primary current and 20kW peak power draw. Using skm400 IGBTs

First test setup was the transformer for the welding machine I'm building. 18 Primary turns and 2x3 secondary in series giving me a 6:1 ratio. That however didn't work, and ran into very significant current limitations as well as non linearities because of high stray inductance. Presumabley on the primary coil, meaning the core saw all of the current without any load applied and saturated, resulting in big losses. Scoping the secondary voltage of the transformer showed a hughe sinusoidal swing as opposed to the nice square wave I was expecting.

Second setup was the two secondaries arranges as an autotransformer, with a 2:1 ratio. Here the stray inductance is a lot lower, which results in the output voltage being pretty much exactly 0.5xV_bridge even with a high primary current.

Using the second setup I did some static load testes with a sledge hammer as the load ,250V on the bus and 400us pulses at 200Hz. For reference my coil would reach around 35°C heatsink temperature at 800A current limit without the Transformer in place. After installing the trafo I went up to 1000A limit and it only got to around 27°C. Testing with 1200A got me to the point where everything in the coil started overheating... except for the IGBTs. I'd need to re-design my coil to be able to keep up with the new capabilities :P

Something that I'd now want to do is to acutally do some engineering calculations on that transformer (calculate flux density swing etc) so I can optimize the transformer a little bit more. Even though I suspect that won't be easy beacuse the primary has a voltage imposed on it while the secondary a current. Netzpfuscher also suggested using the primary coil as the autotransformer, feeding the outer most winding from the bridge, which might be interesing to try out. Though I suspect the stray inductance of that setup could be a little too much. And of course finally I need to do a test run with a real secondary as the load... who knows maybe I'll crack 4m :P

I really do think that for low resonant circuit impedance coils or those that operate with a high bus voltage a transformer might be a very viable way of pushing beyond what your IGBTs could do on their own. Maybe somebody else would like to give it a try :)

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Re: increasing primary current without increasing IGBT losses
« Reply #1 on: June 15, 2022, 05:26:39 AM »
A transformer could be useful for altering characteristics of an existing coil that is difficult to modify.  For anyone designing a new coil, it is probably easier and cheaper and smaller to build a higher-impedance primary.  A perfect 2:1 transformer (no parasitics of any sort) is exactly the same as 4x primary inductance (2x turn count occupying the same space) and 1/4x primary capacitance (at 2x peak primary capacitor voltage).  Requires the same number/size of MMC capacitors arranged with twice the number in series and half the number in parallel.  Of course, if this higher-impedance primary allows increasing coil peak power, then MMC voltage/current capability needs to be increased proportionately.  Same holds true for a transformer-driven version.

Your transformer (ie. higher-impedance primary) experiment should provide valuable information on arc length.  I can imagine two competing effects.  Higher primary current should allow arcs to start with more detuning.  Lower primary H-bridge drive voltage might reduce how low a secondary frequency that primary drive can feed.  Not certain these are valid ways to view arc length effects, nor which one is more significant if they are valid.  Anyone else have thoughts about what to expect for arc length change?

BTW, my first coil (SSTC) is transformer-coupled.  Two H-bridges (for two separate 120V line circuits) drive transformers (series-connected secondaries) feeding the bottom of the SSTC "secondary".  There is no SSTC primary coil.
« Last Edit: June 15, 2022, 06:22:19 AM by davekni »
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Offline klugesmith

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Re: increasing primary current without increasing IGBT losses
« Reply #2 on: June 15, 2022, 06:37:08 PM »
Applause for Dave's point about, where possible,
rewinding the TC primary to better match the driver.

The principle works for pretty much anything that has coils: Motors, electromagnets, various "launchers", wireless device chargers, and of course transformers.
Same coil volume and copper mass is partitioned into more or fewer turns, for more or less inductance and resistance.
With no change in operating frequency, current density, efficiency, etc.

Offline Uspring

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Re: increasing primary current without increasing IGBT losses
« Reply #3 on: June 15, 2022, 08:08:10 PM »
davekni wrote:
Quote
Lower primary H-bridge drive voltage might reduce how low a secondary frequency that primary drive can feed.
I don't understand this. Can you please explain?

I've written here a bit about high impedance primaries. https://highvoltageforum.net/index.php?topic=1928.msg14441#msg14441
As has been pointed out, a higher impedance primary tank is basically what you get if you drive it with a step down transformer.

Offline TMaxElectronics

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Re: increasing primary current without increasing IGBT losses
« Reply #4 on: June 16, 2022, 01:25:12 AM »
Well the impedance of the primary increasing is only a side point really. The main benefit is that you can simply drive twice as much primary current with the same IGBTs.

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Re: increasing primary current without increasing IGBT losses
« Reply #5 on: June 16, 2022, 03:25:02 AM »
Quote
Quote

    Lower primary H-bridge drive voltage might reduce how low a secondary frequency that primary drive can feed.

I don't understand this. Can you please explain?
That wasn't a complete thought.  I agree, it doesn't make sense.  I think the down-side of too-high primary impedance is as you mention in your other referenced post:  As secondary frequency drops to primary frequency, primary Q may drop too low to draw enough power from H-bridge.  So, the issue is at matched frequency, not after secondary frequency drops below primary frequency.
Your other referenced post also brings up a possible bright side.  If enough primary energy can be stored up before secondary frequency drops to match, that energy might be enough to extend the arc to the point where primary frequency is below secondary, skipping past the problematic low-Q primary point.

Quote
Well the impedance of the primary increasing is only a side point really. The main benefit is that you can simply drive twice as much primary current with the same IGBTs.
Don't understand this comment.  2x primary current behaves exactly the same as 1x primary current through 2x primary turns (4x primary impedance).  Same advantages without the need for a transformer.  Whatever you hope to gain by 2:1 transformer can be gained with higher-impedance primary instead.  Choice depends on which is easier to build given your existing coil.
David Knierim

Offline Uspring

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Re: increasing primary current without increasing IGBT losses
« Reply #6 on: June 16, 2022, 12:39:00 PM »
@davekni: Thank you for the explanation. I see now, what you were getting at.

@TMaxElectronics: The IGBT current limit is not the only issue limiting primary current. Once the arc breaks out, the energy dissipated there will manifest itself as a lossy primary tank. This implies, that you will need a certain input voltage to the primary in order to establish a certain input current. The higher the input voltage is, the larger is also the achievable current.
A step down transformer will lower the input voltage to the tank, so in the end input current and with it input power might suffer too much.
This is not to say, that there might be no advantage to a transformer. That depends on your present tank impedance and whether it matches the bridges voltage and current capabilities.

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Re: increasing primary current without increasing IGBT losses
« Reply #6 on: June 16, 2022, 12:39:00 PM »

 


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