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Registered Member #91
Joined: Thu Feb 09 2006, 03:03PM
Location: The Netherlands
Posts: 45
Good evening.
After a day full of exploding gate drivers and sloppy waveforms I have finally decided that I will not be using random cores from the junkbox in my GDTs again. (I am currently working on a second revision of my SSTC which I hope will be an improvement in many aspects).
However, it's not so easy finding a shop that carries suitable cores (or any cores at all...) at reasonable prices. I found one supplier who has a small selection of ferrite toroids of type Philips 3E25. Here's the material specification. The relative permeability seems to be ok and the AL value for a 36 mm diameter core is 7390 which I think should be good, right?
However, I'm not sure if these cores are good for GDT use (my TC resonates at about 300 kHz) as I'm not sure how to interpret fig. 1 in the material specification; unfortunately the concept of "complex permeability" is beyond what I've learned so far (I'm studying electrical engineering).
So I was wondering if any of you guys have any experience with this particular core material and/or can tell me if it's usable or not. Any responses will be greatly appreciated
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hey Kipmans,
Complex permeability isn't that hard to understand. The real part of the complex permeability, u', is what causes inductive reactance, and the imaginary part u'' causes loss resistance. So, to make a transformer you want a material that has high u' and low u'' at your operating frequency.
You can see that at 300kHz the u' is already starting to fall off and u'' is already quite high. This suggests that the core may run hot and not be very efficient. Since the material was designed for use in EMI filter chokes, that's how it's supposed to behave: they want a lossy inductor to absorb the EMI.
I mostly use Fair-Rite type 78 toroids for GDTs and CTs. If you look at the graph of complex permeability here: you'll see it's somewhat better.
But if you can get the Ferroxcube ones cheap, it may be worth trying anyway.
Registered Member #91
Joined: Thu Feb 09 2006, 03:03PM
Location: The Netherlands
Posts: 45
Raff: yeah, I could do that, however I want to be able to calculate (or at least get a good estimate of) the inductances and currents involved beforehand so I need a material with known properties.
Steve: thanks for the info. I searched around a little more and found another store which has Epcos toroids of material type N30, which seems to be better (not as good as the Fair-rite ones, but certainly better than 3E25). I might give them a go, however not very soon as the store requires a minimum order and I have to wait for my new gate drivers anyway.
In the meantime, I will do some calculations and maybe rig up a PSPICE simulation of some sorts. Let's see if I can find a library that contains the Epcos N30 cores...
Registered Member #2315
Joined: Tue Aug 25 2009, 02:35AM
Location: Leyte, PH
Posts: 161
Kipmans,
after MANY attempts on winding MANY type/s of "unknown" cores for CT and for GDT(more on GDT) I have NEVER exploded a chip driver(ucc37321/22)... so I never bothered.. BUT I do have an LCmeter to check the winding inductance... my rule of thumb: never go below 100uH on the primary winding(though my latest 1:2:2 gdt primary inductance goes to a low 72uH = unknown torroid core >1" OD)
though I agree with Steve, those unknown commonly found ferrite torroids are mostly for EMI suppression...
one thing FOR SURE is that monitor flybacks can run as high as 150KHz..good for low freq DR or SSTC...
to Steve: at low enough voltage supplied to the H-bridge(like 20Vac), I could "HEAR" my GDT emit sound with the interrupter set to 300Hz @ >400uS ON time (core used was from the monitor flyback UU-core) ... is this behavior normal with LARGE GDTs?
and WHICH do you think is better(if these two where the only ones available) an unknown ferrite torroid (>1" OD commonly found as EMI chokes) OR the PC monitor flyback core??
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
raff wrote ...
at low enough voltage supplied to the H-bridge(like 20Vac), I could "HEAR" my GDT emit sound with the interrupter set to 300Hz @ >400uS ON time (core used was from the monitor flyback UU-core) ... is this behavior normal with LARGE GDTs?
and WHICH do you think is better(if these two where the only ones available) an unknown ferrite torroid (>1" OD commonly found as EMI chokes) OR the PC monitor flyback core??
If you run a feedback-driven SSTC at a low voltage, there may not be enough feedback signal to operate the drivers. In this case, the coil won't operate and the GDT just gets driven with the interrupter waveform, which contains lower frequencies than the GDT was designed to deal with. So it may make an audible noise and/or saturate.
Since the ferrite toroid is "unknown" it's impossible to answer your second question. It could be great, or it could suck completely. You need to try both yourself and see which works best.
Registered Member #91
Joined: Thu Feb 09 2006, 03:03PM
Location: The Netherlands
Posts: 45
raff wrote ...
Kipmans,
after MANY attempts on winding MANY type/s of "unknown" cores for CT and for GDT(more on GDT) I have NEVER exploded a chip driver(ucc37321/22)... so I never bothered.. BUT I do have an LCmeter to check the winding inductance... my rule of thumb: never go below 100uH on the primary winding(though my latest 1:2:2 gdt primary inductance goes to a low 72uH = unknown torroid core >1" OD)
though I agree with Steve, those unknown commonly found ferrite torroids are mostly for EMI suppression...
one thing FOR SURE is that monitor flybacks can run as high as 150KHz..good for low freq DR or SSTC...
Yeah, I don't have an LC-meter... I know that sucks but it's something I'll have to cope with for at least a few more months to come. Anyway, I think I'm going to buy me some N30 cores as soon as I have time and money ( ) and see where that brings me.
(By the way, the core that I used last time was some random ferrite toroid and this time I used a small E-core taken from an old monitor.)
Then I have one more question regarding the series coupling capacitors in the GDT drive circuit. What is their function, except for blocking DC? As far as my simulations and calculations go, they don't seem to provide any benefit: on the contrary, they only seem to cause low frequency oscillations that die out after maybe a few ms. So why exactly do we use them and why should you choose a particular capacitance value?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
That is their only function, to block DC, which prevents the gate drive chips from exploding if the GDT saturates because of a drive waveform with a DC component.
The capacitance should be more than 10 times the sum of input capacitances of all your MOSFETs. If you use a 1:2 GDT to overdrive the gates of IGBTs, then make it 40 times. I usually use 4uF.
You can damp the oscillations by putting a 10 ohm resistor across the capacitor. If your gate drive hangs up the resistor will burn, but it's still cheaper than the gate drive ICs.
Registered Member #2315
Joined: Tue Aug 25 2009, 02:35AM
Location: Leyte, PH
Posts: 161
Steve:
I think its better to use the core from computer monitor since *at least* we know the freq of those device can reach 150KHz(the resonant mode of flyback)..Im also thinking those cores could possibly do even higher freq?
much better than using unknown EMI suppression ferrite torriods..
I think this is also much critical for GDT than in CT feedback..
Registered Member #91
Joined: Thu Feb 09 2006, 03:03PM
Location: The Netherlands
Posts: 45
Steve McConner wrote ...
You can damp the oscillations by putting a 10 ohm resistor across the capacitor. If your gate drive hangs up the resistor will burn, but it's still cheaper than the gate drive ICs.
Ah yes, that's what I should've figured.
Thanks for clearing this stuff up a bit. I think I now have enough information to go and design an decent gate driver. After that... a good H-bridge
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Gate drive transformer core selection
) and see where that brings me.