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Registered Member #2750
Joined: Sun Mar 21 2010, 08:47PM
Location: Poland
Posts: 46
Hi Finn Yes I noticed this issue and I use damping resistors too - as you say: not so elegant but it works wery well. Somehow I can't trust the semiconductor gate drivers - I don't know why but I prefer old GDT's they are simple and there is nothing what can fail. But ofcourse if something go wrong this time I build your gatedrivers if you shared it for me. I have three other coils with Predikter and all work perfect - two big what you can see at photo and third small. That small coil work with predikter and ~ 180kHz Frez. Only thing what I needed to change is burden network connected to CTs.
Registered Member #2750
Joined: Sun Mar 21 2010, 08:47PM
Location: Poland
Posts: 46
Finally, I ran all the electronics. Everything look good and work perfect. I probably get first light today or tomorow . But some things still are not leave me in peace. At first gate waveforms. They are look good for me but what is good for CM300 IGBTs ? I know I must turn off fast and turn on slow... But what is slow and not too slow ? Second thing is a crossconduction. How to measure it and what is good (It never be 0A ... ). Third thing is a current leading circuit. I testet a lot of inductors of diferent types and values but it still works best with a piece of wire coil wound on a finger. What exact inductor I should use for ~42kHz to get ~2uS current leading. Some new pictures of my setup:
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
Finn Hammer wrote ...
Dzejwor,
Great to see the Predikter on a pro. quality board. The predikter was designed to be used with discrete gate driver boards, using optical isolation on the timing signals. Due to a faulty gate driver design, I never dared to use the original gate drivers with any real power, so instead I used 1:4 gate transformer and intermediate bridge. This solution has drawbacks, the main one being that the predikter puts out a very short, IRRC 10nS, pulse, after the last cycle in a burst. It is due to a delay on the interrupter pin, which allows the oscillator input to turn the gate driver chip "on" just briefly before it is turned "off" by the interrupter pin. This short pulse was drowned in delay on the optical link, but with a gate transformer it transfers enough energy to the gate transformer to create a huge and long ringing oscillation which looks really bad. I fear that it could turn on the gates unsequenced. Both Daniel's Diablo Tonnere and Thumper use this drive scheme, and we damped the oscillation with a 5W (I think 50ohm) resistor across the outputs. Not elegant, but it works.. There are 2 possible solutions, the one I prefer is to use the new Gate driver boards that I have designed, and I will let you have the design if you want to use them. Discrete gate driver boards means that you don't have to fiddle with gate resistor/diode networks, and you can really dial in the perfect non-ringing switch times.
Cheers, Finn Hammer
EDIT, I just notice that you have experience with the Predikter already. Did we talk about these issues already? Did you notice the "trapped ripple " ringing in the gate transformers?
-Finn
I definitely prefer optical and powered gate drivers over GDT's. I was able to tweak out just about all ringing, even on the first cycles. At full bus voltage, it only rings about 50V above bus. I've just never found this kind of timing precision to be possible on a GDT.
I also notice that brief gate pulse at the end of each interrupt cycle. It does propogate to my gates, but I've never seen it cause any problems, and surprisingly it doesn't ring at all at the nanoseconds later turn off.
It took a bit of doing to get the drivers working, but after about 2 months of R&D (and literally about 25 etched board prototypes), I now I have a flawless gate driver on a single sided board that screws to my IGBT's, has dropout protection to kill the whole coil softly, almost no ground loops, and transient protections everywhere possible. I've driven that coil to 5000A peak (don't even ask what was going through my mind when I drove it that high severely out of tune) on CM600's (on a prediktor driver), which burned out due to crappy 60hz GDT power supplies that weren't rated for the standoff when bus reached 1000V. I now have a bridge on 1700V/1000A bricks with isolated switching supplies for all four gate drivers. Hoping to fire that bridge up soon.
Bottom line, I hate GDT's and absolutely love discreet isolated gate drivers. I even used gate drive chips on a mini TO247 coil that fits an entire driver on a small computer heatsink.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Looks great so far
You do have some overshoot on the gates. While it may not be problematic, i always try to get as close to a perfect square as i can. The leads on your GDT (especially primary) are somewhat long, so that could be causing the overshoot.
The rise time also looks a tiny bit slower than ideal, which also may not be a problem, but i really split hairs when it comes to the health of bricks. You seem to have some resistors in series with the primary, though i may be wrong. I would omit those, unless you get unbearable ringing without them. It takes a fair bit of peak power to drive a gate that is around 50-100nf. A slower rise time could just generate more heat, which while may not really cause problems, could require you to use an otherwise unnecessary amount of cooling.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Dzejwor wrote ...
What exact inductor I should use for ~42kHz to get ~2uS current leading.
I use 5.5uH of these @34kHz, and there should be ample delay in the predikter board for that, even at higher frequencies.
This type of coils come from many different manufacturers, but they all seem to work fine. I like them since the coil is shielded inside the ferrite loop.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
dude_500 wrote ...
I definitely prefer optical and powered gate drivers over GDT's. I was able to tweak out just about all ringing, even on the first cycles. At full bus voltage, it only rings about 50V above bus. I've just never found this kind of timing precision to be possible on a GDT.
I also notice that brief gate pulse at the end of each interrupt cycle. It does propogate to my gates, but I've never seen it cause any problems, and surprisingly it doesn't ring at all at the nanoseconds later turn off.
It took a bit of doing to get the drivers working, but after about 2 months of R&D (and literally about 25 etched board prototypes), I now I have a flawless gate driver on a single sided board that screws to my IGBT's, has dropout protection to kill the whole coil softly, almost no ground loops, and transient protections everywhere possible. I've driven that coil to 5000A peak (don't even ask what was going through my mind when I drove it that high severely out of tune) on CM600's (on a prediktor driver), which burned out due to crappy 60hz GDT power supplies that weren't rated for the standoff when bus reached 1000V. I now have a bridge on 1700V/1000A bricks with isolated switching supplies for all four gate drivers. Hoping to fire that bridge up soon.
Bottom line, I hate GDT's and absolutely love discreet isolated gate drivers. I even used gate drive chips on a mini TO247 coil that fits an entire driver on a small computer heatsink.
My latest setup also includes discrete gate drivers, isolated with optocouplers on the signal, which solves the problem with the rookie peak at the end of each cycle. Boards are powered with discrete boost converters to insure full supply even at brownout.
However, all this finery, I think it reveals my level of OCD instead of the "let good enough be good enough" attitude of those builders whose coils are making sparks all day
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Finn Hammer wrote ...
Dzejwor wrote ...
What exact inductor I should use for ~42kHz to get ~2uS current leading.
I use 5.5uH of these @34kHz, and there should be ample delay in the predikter board for that, even at higher frequencies.
This type of coils come from many different manufacturers, but they all seem to work fine. I like them since the coil is shielded inside the ferrite loop.
Cheers, Finn Hammer
What i did was take one of those can type variable inductors with the ferrite tuning slug (this one has static and magnetic shielding) and rewound it with 24 turns of 32 gauge. It seems to have a good tuning range for my IGBT and frequency, and the tuning slug about right in the middle is where the phase lead seems to be perfect. I used a 4.7 ohm 1-2w carbon film shunt resistor in series with said inductor.
Registered Member #2750
Joined: Sun Mar 21 2010, 08:47PM
Location: Poland
Posts: 46
So I can't make shorter connection betwen GDT and driver but i can (and I do) shorter connections betwen gdt and gates. Now I have 10ohm gate resistors bypased by 1N5819diodes. I dont have any resistance in series with primary but I have parallel - must have it to dump ringing after last cycle. I have three other coils with predikter here and all have this dumping resistors at gdt primary. I have 4:1:1:1:1 GDT and its impedance can be too high I have around 20 turns on huge TX52 core made of 3E25 ferrite - it is probably strongly oversized but as I alaways say - better too big than too small
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Huge DRSSTC Project
but I prefer old GDT's they are simple and there is nothing what can fail. But ofcourse if something go wrong this time I build your gatedrivers if you shared it for me. I have three other coils with Predikter and all work perfect - two big what you can see at photo and third small. That small coil work with predikter and ~ 180kHz Frez. Only thing what I needed to change is burden network connected to CTs.
. But some things still are not leave me in peace. At first gate waveforms. They are look good for me but what is good for CM300 IGBTs ? I know I must turn off fast and turn on slow... But what is slow and not too slow ? Second thing is a crossconduction. How to measure it and what is good (It never be 0A ... ). Third thing is a current leading circuit. I testet a lot of inductors of diferent types and values but it still works best with a piece of wire coil wound on a finger. What exact inductor I should use for ~42kHz to get ~2uS current leading. 
