Author Topic: Maximum pulse width in freewheel mode.  (Read 476 times)

Offline fh89

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Maximum pulse width in freewheel mode.
« on: February 08, 2021, 07:07:01 AM »
I have a newly completed large DRSSTC utilizing the UD+ driver in freewheel (pulse skipping?) mode.

Specs:
Full bridge cm600ha-24h
Liquid cooled primary
2000A primary current limit
UD+ freewheeling driver

What is the limiting factor in maximum pulse width given adequate cooling, properly set peak current limit, and running in freewheel mode? Currently running at 360us. I have read that Philip Slawinski was able to run up to 7ms pulse width on his skinny coil in freewheeling mode utilizing a liquid cooled primary. Is there some calculation I can do to determine the maximum pulse width my system can handle? Or some other way to test how long the pulse width can go?

Offline acobaugh

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Re: Maximum pulse width in freewheel mode.
« Reply #1 on: February 08, 2021, 07:26:09 AM »
Mads has all the math laid out on his website for calculating Fmax2: https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/igbts/

I think you could pretty easily find max on time as a function of frequency and current instead.

Offline Hydron

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Re: Maximum pulse width in freewheel mode.
« Reply #2 on: February 08, 2021, 12:19:47 PM »
Send it until the breaker trips?

In all seriousness, maybe try and do some testing to see what sort of temperature rise you get from various components as you ramp up pulsewidths. Also bear in mind the size of the temperature swing of the IGBT/Diode dies between on and off mode - with longer pulse-widths this may start to increase (the 100s of us pulsewidths normally used in DRSSTCs may allow the thermal mass of the die to limit the excursion - this will not be the case in the millisecond region), and at some point the stress from thermal cycling will be the life-limiting factor even with everything well under Tj(max).

Finally if you have a cheap source of IGBTs then just running it till something goes pop (then back off a quarter turn :P) may be the most expedient way to find the limits.

Offline fh89

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Re: Maximum pulse width in freewheel mode.
« Reply #3 on: February 08, 2021, 06:00:17 PM »
Quote
Mads has all the math laid out on his website for calculating Fmax2: https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/igbts/

I think you could pretty easily find max on time as a function of frequency and current instead.
This is definitely over my head- I might be able to use the formulas there for their intended purpose with emphasis on might, but I definitely wouldn't be able to figure out how to modify them for calculating max on-time. I'm not sure it would correlate well or not either, as much of the current will be passing through the diodes in freewheel mode.

Quote
Send it until the breaker trips?
This was my first thought as well, as I am pretty limited right now on a 240v 50a breaker. I can install a a larger breaker and run the wire directly to the coil though. I think I could get away with a 100a breaker with the 6g power cord I am using for the coil. I am guessing that with the short run times of the coil, the wire heating would not be too significant. Even if it did get hot and melt the insulation, with it running directly to the main breaker box I wouldn't have to worry about burning my house down.

Quote
In all seriousness, maybe try and do some testing to see what sort of temperature rise you get from various components as you ramp up pulsewidths. Also bear in mind the size of the temperature swing of the IGBT/Diode dies between on and off mode - with longer pulse-widths this may start to increase (the 100s of us pulsewidths normally used in DRSSTCs may allow the thermal mass of the die to limit the excursion - this will not be the case in the millisecond region), and at some point the stress from thermal cycling will be the life-limiting factor even with everything well under Tj(max). Finally if you have a cheap source of IGBTs then just running it till something goes pop (then back off a quarter turn :P) may be the most expedient way to find the limits.
cm600ha-24h bricks are available on ebay here for as little as $18, so this might be what I end up doing.

I appreciate the input guys.

Offline davekni

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Re: Maximum pulse width in freewheel mode.
« Reply #4 on: February 08, 2021, 07:32:55 PM »
Does the controller alternate which diodes conduct to even out power dissipation?  If so, I'd guess 10ms to be fine.  Diode thermal resistance is only 1.5x that of the IGBTs.  The IGBTs will be conducting more of the time than the diodes.  Average power should be well distributed within each brick.
David Knierim

Offline fh89

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Re: Maximum pulse width in freewheel mode.
« Reply #5 on: February 08, 2021, 07:57:15 PM »
Quote
Does the controller alternate which diodes conduct to even out power dissipation?
Yes, I am using EVR's version of Philip Slawinski's UD+ driver http://www.classictesla.com/pslawinski/udplus.html that alternates between each diode sequentially.

Quote
If so, I'd guess 10ms to be fine.  Diode thermal resistance is only 1.5x that of the IGBTs.  The IGBTs will be conducting more of the time than the diodes.  Average power should be well distributed within each brick.
Wow, I wonder what the power draw would be at 10ms! Is this guess based on experience with a freewheeling driver?
« Last Edit: February 08, 2021, 08:01:59 PM by fh89 »

Offline davekni

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Re: Maximum pulse width in freewheel mode.
« Reply #6 on: February 08, 2021, 09:33:47 PM »
Unfortunately my DRSSTC can't run pulse-skipping.  My diodes are about 1/3rd the side of the IGBT die (3x thermal resistance).  I'm also limited to a 240V 40A circuit.

Are you running 240V into a bridge rectifier or into a voltage doubler?  That would make a big difference on line power at 2000A peak primary current.  How much bulk capacitance do you have?  As on-times approach line half-cycle times, performance may vary depending on when the pulse occurs relative to cap charging crests of line voltage.

I'm presuming you are running with appropriate phase lead and dead-time so that IGBT switching losses are low.  (BTW, what frequency are you running?)  2000A peak is 1000A RMS per package.  (1400 RMS current, with each package conducting for every-other half-cycle.)  Looking at the CM600HA-24A graphs, 1000A dissipates roughly 3400 watts.  That is about the continuous power rating (3670 watts) if you could keep the case at 25C.  If the case is 87C instead, that cuts allowed power in half.  The transient thermal resistance graph shows that the thermal impedance drops by just over 2:1 at 10ms, getting you back to the 3670 watt capability for 10ms.

Perhaps the above is a bit optimistic.  Even with ideal phase-lead timing, switching losses aren't negligible.  So perhaps 5ms would be more conservative.  Depends on overall power and cooling.

With pulse-skipping there is no way to calculate line power ahead of time.  It depends on what percentage of pulses are skipped.

Very nice coil with impressive results!
« Last Edit: February 08, 2021, 11:47:22 PM by davekni »
David Knierim

Offline fh89

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Re: Maximum pulse width in freewheel mode.
« Reply #7 on: February 09, 2021, 02:26:10 AM »
Quote
Are you running 240V into a bridge rectifier or into a voltage doubler?


Doubler

Quote
How much bulk capacitance do you have?

6800uF

Quote
I'm presuming you are running with appropriate phase lead and dead-time so that IGBT switching losses are low.

Yes, phase-lead is set for clean switching. I assume proper dead time is built into the controller, it is not changeable.


Quote
BTW, what frequency are you running?
About 50kHz

Quote
2000A peak is 1000A RMS per package.  (1400 RMS current, with each package conducting for every-other half-cycle.)  Looking at the CM600HA-24A graphs, 1000A dissipates roughly 3400 watts.  That is about the continuous power rating (3670 watts) if you could keep the case at 25C.  If the case is 87C instead, that cuts allowed power in half.  The transient thermal resistance graph shows that the thermal impedance drops by just over 2:1 at 10ms, getting you back to the 3670 watt capability for 10ms.

Perhaps the above is a bit optimistic.  Even with ideal phase-lead timing, switching losses aren't negligible.  So perhaps 5ms would be more conservative.  Depends on overall power and cooling.

I don't currently have a way to measure case temps, but I built the coil to have very robust cooling.

IGBTs are mounted to a 10"x10"x.375" heatsink with rectifier on a separate heatsink


with dual 240v fans (12vdc fan on rectifier heatsink)


MMC is built for high airflow and lead heatsinking with copper busbar


with 3 240v fans


and as I mentioned, the primary is liquid cooled with a dual fan radiator
 

so I believe the cooling is pretty good. I guess the only way to really know for sure is to crank it up and see what happens! 10ms would probably look incredible. Even 5ms is crazy long.

Quote
With pulse-skipping there is no way to calculate line power ahead of time.  It depends on what percentage of pulses are skipped.
Makes sense. I really appreciate your detailed feedback!
« Last Edit: February 09, 2021, 02:29:45 AM by fh89 »

Offline davekni

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Re: Maximum pulse width in freewheel mode.
« Reply #8 on: February 09, 2021, 06:52:33 AM »
Very nice build!  Your mechanical skills are well above mine.

Phase lead looks perfect.  Voltage switching is slightly before current zero-crossing, and the H-Bridge output voltage looks very clean.  (I presume the yellow trace is H-Bridge output.  I can't make out the volts/div display.)

Dead time is generally controlled by the resistor/diode networks between GDT outputs and IGBT gates.  I'm not spotting any in your build.  That may be fine.  There is some dead time resulting from the gate voltage slew rate combined with Vge threshold voltage.  Presuming all gate waveforms track, one pair of IGBTs starts turning off as the GDT waveform drops down to the ~7V threshold voltage.  The other pair starts turning on when the GDT waveform gets down to -7V (+7Vge for the other pair of IGBTs).  Have you scoped gate voltage to check for excessive overshoot and to measure slew rate?  Dead time must be reasonable or else it would show up in spikes in the H-Bridge output waveforms.

The one scope capture shows 29us period, 34.5kHz.  Is that capture of typical operating conditions?  Or is 50kHz more representative?  If 34.5kHz, that results in 8ohms reactance for primary MMC and coil, which is 16kV peak at 2000A peak.  That is slightly over 2kV/cap for your MMC.  Probably won't cause any problems given your aggressive cooling.  (At 50kHz, reactance is lower, so peak voltage will also be lower.)

2000A peak is 1273A average, so would draw 1273A from bus caps if not skipping any pulses.  Skipping 47% of the pulses gets to 680A, for 100V/ms drop in bus voltage between line half-cycles for your 6800uF capacitance.  By 5ms, bus voltage will be heavily modulated, unless many more pulses are skipped.  Wouldn't worry at all about this for now.  Just something to scope as you increase on-time.

Have fun with your impressive machine.  12' arcs is great!  Took me quite a bit of work and tweaking to achieve a reasonable number of 10' arcs.  My DRSSTC isn't quite a big, though.
David Knierim

Offline fh89

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Re: Maximum pulse width in freewheel mode.
« Reply #9 on: February 09, 2021, 07:55:58 AM »
Quote
Very nice build!  Your mechanical skills are well above mine.
Thank you, your electronics knowledge is way above mine! I've built numerous coils before, but this is the first one that I have built in almost 15 years, and I am finally in a financial position to try to build something really nice.

 
Quote
Phase lead looks perfect.  Voltage switching is slightly before current zero-crossing, and the H-Bridge output voltage looks very clean.  (I presume the yellow trace is H-Bridge output.  I can't make out the volts/div display.)
Correct, yellow trace is bridge output through HV diff probe.

Quote
Dead time is generally controlled by the resistor/diode networks between GDT outputs and IGBT gates.  I'm not spotting any in your build.

Ahh, got it. They are there, directly mounted to the IGBTs. I understand that the gate resistor affects switching characteristics, I guess it should've been obvious the effect on dead time as well. My superficial understanding of this is the reason I used EVR's convenient gate driver board kits.


Quote
Have you scoped gate voltage to check for excessive overshoot and to measure slew rate?  Dead time must be reasonable or else it would show up in spikes in the H-Bridge output waveforms.

I have not done this. I kind of just trusted the driver would work well because of track record of reliability. Hopefully this isn't too naïve

Quote
The one scope capture shows 29us period, 34.5kHz.  Is that capture of typical operating conditions?
No, this was captured while adjusting phase lead with no secondary in place and primary tap point not yet tuned. I have not measured Fres but calculated it to be around 50kHz

Quote
If 34.5kHz, that results in 8ohms reactance for primary MMC and coil, which is 16kV peak at 2000A peak.  That is slightly over 2kV/cap for your MMC.  Probably won't cause any problems given your aggressive cooling.  (At 50kHz, reactance is lower, so peak voltage will also be lower.)
They are 2kV rated caps so hopefully they hold up!

Quote
2000A peak is 1273A average, so would draw 1273A from bus caps if not skipping any pulses.  Skipping 47% of the pulses gets to 680A, for 100V/ms drop in bus voltage between line half-cycles for your 6800uF capacitance.  By 5ms, bus voltage will be heavily modulated, unless many more pulses are skipped.  Wouldn't worry at all about this for now.  Just something to scope as you increase on-time.
After speaking with Philip Slawinski, I am going to at least double the bus capacitance.

Quote
Have fun with your impressive machine.  12' arcs is great!  Took me quite a bit of work and tweaking to achieve a reasonable number of 10' arcs.  My DRSSTC isn't quite a big, though.
Really hoping for at least 16' as that would get me to 4x secondary winding length! I'll have to check out your coils as well. Thanks again for your very detailed insights!
« Last Edit: February 09, 2021, 05:20:14 PM by fh89 »

Offline davekni

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Re: Maximum pulse width in freewheel mode.
« Reply #10 on: February 09, 2021, 07:40:39 PM »
Since EVR picked 5.1 ohms for CM600 driving, that is likely a good value.  That presumes that your GDT leakage inductance is similar to what EVR tested with.  I'd still suggest scoping one of the IGBTs for gate-emitter voltage just to make sure.

At 50kHz, MMC voltage will be 11kV at 2000A, so under 2kV per cap.

BTW, here's one of my 10' arcs with the video frame sequence leading up to it:
https://highvoltageforum.net/index.php?topic=798.msg9200#msg9200
Your larger-diameter secondary is a big help.  I couldn't get coupling factor high enough with my 6.2" diameter secondary.  16' will be very impressive.  Might be a record for DRSSTC?
David Knierim

Offline Hydron

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Re: Maximum pulse width in freewheel mode.
« Reply #11 on: February 09, 2021, 10:44:20 PM »
First off: very impressive looking build! It will not be the biggest by a long way (see below), but very nice and tidy looking for sure, with good performance to boot.

Your larger-diameter secondary is a big help.  I couldn't get coupling factor high enough with my 6.2" diameter secondary.  16' will be very impressive.  Might be a record for DRSSTC?

Offline fh89

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Re: Maximum pulse width in freewheel mode.
« Reply #12 on: February 10, 2021, 08:07:23 AM »
Quote
Since EVR picked 5.1 ohms for CM600 driving, that is likely a good value.  That presumes that your GDT leakage inductance is similar to what EVR tested with. I'd still suggest scoping one of the IGBTs for gate-emitter voltage just to make sure.
I'm using his GDT's as well, so should be the same. I will scope it out when I get a chance.

Quote
At 50kHz, MMC voltage will be 11kV at 2000A, so under 2kV per cap.
I spoke some more with Philip Slawinski about running in long-pulse mode and I am going to have to build another MMC with higher Arms capacity.

I did some testing on my interrupter and found out that it can only go to 1.12ms, so that is my pulsewidth limit for now. At 48Hz (the minimum frequency my interrupter can run) that gives a 5.4% duty cycle. Even this low pulsewidth will be more A rms than my MMC can handle, but going to go ahead and see what happens at 1ms and if the MMC dies, I will build another more robust one.

Quote
BTW, here's one of my 10' arcs with the video frame sequence leading up to it:
https://highvoltageforum.net/index.php?topic=798.msg9200#msg9200
Your larger-diameter secondary is a big help.  I couldn't get coupling factor high enough with my 6.2" diameter secondary.
Impressive spark length from that secondary size!

Quote
16' will be very impressive.  Might be a record for DRSSTC?
As Hydron mentioned, not even close! There are some absolute monsters out there. Also https://4hv.org/e107_plugins/forum/forum_viewtopic.php?p=1&id=164773 and https://4hv.org/e107_plugins/forum/forum_viewtopic.php?p=1&id=165062

Quote
First off: very impressive looking build! It will not be the biggest by a long way (see below), but very nice and tidy looking for sure, with good performance to boot.
Thanks! Definitely going to be looking for even better performance by increasing the on-times. Will keep the forum posted.

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Re: Maximum pulse width in freewheel mode.
« Reply #12 on: February 10, 2021, 08:07:23 AM »

 


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