Author Topic: Slow rise & fall time  (Read 2505 times)

Online Laci

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Re: Slow rise & fall time
« Reply #20 on: August 13, 2018, 10:20:43 PM »
So I finished the building of my new coil,the resonant frequency is around 170kHz as the JavaTC calculated.Today I put everything together and started testing.Sadly it performs as badly as before-nowhere near in comparision with a halfbridge design.The input voltage was more than 50V,but it was drawing over 4A even at 20V in CW mode,with some mm weak arcs.At the 50V supply I had a 4700uF smoothing capacitor,which discharged really fast in interrupted mode,when the supply(transformer)was turned off.I suppose there are huge losses,probably at the points that I marked before.Tomorrow I will make some more test,maybe measuring the inverter output,gate drive waveforms vs input current(through a shunt resistor).

The ferrite bead you are talking about is something like this?


I have a similar thing salvaged from an angle grinder,sadly nothing else.

A picture of the current setup:
« Last Edit: August 13, 2018, 10:25:30 PM by Laci »

Offline T3sl4co1l

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Re: Slow rise & fall time
« Reply #21 on: August 16, 2018, 07:59:03 AM »
Just to clarify for this example we are talking about the MI1206L391R.  Is the 300mA saturation a derived value?  I did not see it listed but I could have missed it.

Yes. See where the impedance at 100MHz drops to 70% of the zero bias figure.  You'll have to roughly interpolate the curves they give.  The real value is somewhere around there.


Quote
When I look at its impedance graph, I see the impedance tends towards 0 below 1MHz.  To me this is saying there is little power dissipation below 1Mhz so temperature should not be an issue?  The ringing we want to get rid of is in the low to 10s of MHz but the amount of energy in the ringing should be far less than the signal.

Ah, but we are driving gates here, so the current waveform is very peaky, the derivative of a square wave (approximately)!

I have a project that runs FDP33N25s (Qg = 37nC typ.) at up to 2MHz, with a small ferrite bead (might've been Fair-Rite #2673000101?) on the gate, and they get warm!  Not up to Tc, though.  I've cooked (up to Tc) the same size beads, in #43 material, on the S or D leads of STP19NM50s without any load at all (just commutation current: also very peaky, but much sharper, more intense, than gate drive!)

More specifically: an H bridge, hard switching, no load current, 300kHz, 320VDC supply.  So, just charging Coss through stray inductance and ferrite beads.

Quote
I agree the current limit of 2 amps is the limit and this cannot be exceeded.  The package is only so big...

Which, again, is only a thermal rating, for some typical test condition.  You're welcome to run hotter, at risk of cracking or oxidation.  Or maybe you can't handle the losses, and you need to run less, anyway.

And core loss directly impacts the current rating, just as it does for regular inductors.  It might be rated 2A at DC, but 1A when, say, 300mA (RMS) at 1MHz is also applied, or whatever.  Or it might be 3A (DC) with generous copper pours, or less with minimal footprints.

Quote
I sent an email off to a contact at Laird to ask them for their opinion.  I will post the response if I get one.

Cool, will like to hear. :)

Tim

Offline Teravolt

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Re: Slow rise & fall time
« Reply #22 on: August 20, 2018, 02:29:41 AM »
hi Laci, I don't know if you got this problem solved or not but what I am seeing in these pictures is shoot through. if it were me I would add dead time that is a little grater than your 560ns rise and fall times like 1us. If this happens with your main bridge it will be undesirable. If you drive each mosfet directly with a driver with 1-3 ohms. I found this

http://omapalvelin.homedns.org/tesla/SSTC/half-bridge.htm
http://www.modularcircuits.com/blog/articles/h-bridge-secrets/h-bridge-control/
http://www.loneoceans.com/labs/ud27/

hope this helps

Online Laci

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Re: Slow rise & fall time
« Reply #23 on: August 20, 2018, 10:37:37 PM »
Hi Teravolt!Thanks for the useful informations!The coil is working well yet,I have not tested it at high power,but it is already too noisy to run indoors.I am planning to modify the current oneTesla interrupter to add a mains synced staccato controller to it for long and more quiet sparks.
The waveform is the same as before,a bit high rise and fall time.One thing I figured out is that there is no problem with the gate resistors,since the fall time is also high through the gate schottky diodes.I suppose my ferrite core is not the best in my situation.The deadtime is a great idea,not sure if I have enough empty space on the driver board for some more components.Hopefully the 440V TVS and MUR1560 duo will prevent the IGBTs from dangerous overshoots! :)

Offline TDAF

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Re: Slow rise & fall time
« Reply #24 on: August 21, 2018, 03:09:01 AM »
I don't think there's a need to add the MURs as the IGBTs already come with built-in diodes which are much more optimized for the task at hand

I would not suggest you to go for that

Online Laci

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Re: Slow rise & fall time
« Reply #25 on: August 21, 2018, 07:51:31 AM »
I have IRG4PC60U's which do not have built in diode.The 440V TVS diodes should be enough,I just keep them for safety against my high inductance bridge design.:P

Offline TDAF

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Re: Slow rise & fall time
« Reply #26 on: August 21, 2018, 06:46:11 PM »
Oh, oops
Should've read the intro post!

Anyways then I definitely recommend the MURs

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Re: Slow rise & fall time
« Reply #26 on: August 21, 2018, 06:46:11 PM »

 


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