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Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
I know for a fact that the 24Vregs work fine even when driving large bricks, I have a single To-220 24Vreg driving 4 CM600HA-25H bricks in my larger DRSSTC. The reason it is able to do this is because gate drive doesn't really need that much average power, it dose however need very short burst during turn on and turn off, but that is usually taken care of by good decoupling practices.
there are a couple common things that I find to heat up the regulator in a gate drive circuit these are:
saturation of the GDT core no DC blocking or incorrectly sized on GDT primary shorts between windings in secondary and primary shorts from windings to the core shorted or bad IGBT gate incorrect core type (even if it's ferrite doesn't mean it's necessarily the right type!)
On top of these things a couple additional thing you may want to do:
Move the GDT as close to the gates as possible, you want the secondaries as short as they can be. The primary is not as critical.
Also not related to gate drive, but I really hope those blue caps are not your bus capacitors.... They should be mounted right on the IGBTs for a low inductance path. If this is not done the larger bursts of current flowing through from the caps to the IGBTs (which can be 100's or even 1000's of amps) will create massive voltage spices across your IGBTs that will exceed there voltage rating and destroy them, V=L(di/dt) you do that math....
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
Are you running your interrupter when you test your gate drive? A 24V regulator is not going to handle four CM300 gates running CW at 100 KHz. How much power is your driver board pulling? Can you test it off a regulated bench supply?
Registered Member #2405
Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140
Goodchild wrote ...
I know for a fact that the 24Vregs work fine even when driving large bricks, I have a single To-220 24Vreg driving 4 CM600HA-25H bricks in my larger DRSSTC. The reason it is able to do this is because gate drive doesn't really need that much average power, it dose however need very short burst during turn on and turn off, but that is usually taken care of by good decoupling practices.
there are a couple common things that I find to heat up the regulator in a gate drive circuit these are:
saturation of the GDT core no DC blocking or incorrectly sized on GDT primary shorts between windings in secondary and primary shorts from windings to the core shorted or bad IGBT gate incorrect core type (even if it's ferrite doesn't mean it's necessarily the right type!)
On top of these things a couple additional thing you may want to do:
Move the GDT as close to the gates as possible, you want the secondaries as short as they can be. The primary is not as critical.
Also not related to gate drive, but I really hope those blue caps are not your bus capacitors.... They should be mounted right on the IGBTs for a low inductance path. If this is not done the larger bursts of current flowing through from the caps to the IGBTs (which can be 100's or even 1000's of amps) will create massive voltage spices across your IGBTs that will exceed there voltage rating and destroy them, V=L(di/dt) you do that math....
Thanks for the input, I do agree that the bus caps should be closer without a doubt, -- The IGBTs are all good brand new actually and have been indidually tested -- GDT transformer turns are all ok no shorts / 12 turns on each winding Actually I tried different turns on several different tests and 12 turns seems to be the sweet spot inductance of each winding was posted earlier -- GDT core is the same as I use in my other DRSSTC that one is using a full bridge of TO-247 IGBTs that one runs fantastic also the same wire gauge 24 gauge --Actual DC blocking cap value 20uF two 10uF in parallel with a 10 ohm resistor in parallel (I removed the resistor to test without it and no change)
What I am concerned with is the gate charge on these bricks at 24 volt gate voltage its like 2600uC. The CM300DU-24H is half that, and that is what I originally ordered but somehow they switched them for these DU-12FNH bricks.
Now maybe I am concerned over nothing and there is something else going on here. So I just need to find a way make these work.
Like I said before though I did try different turns on the GDT and different cores and the results are the same. The Gate resistors are 5 watts, with 2600uC at 112Khz I come up wth 7 watts or so unless my math is wrong that would explain the 5 watt resistors getting hot.
You say that you run CM600 bricks with the same regulator??
Registered Member #2405
Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140
bwang wrote ...
Are you running your interrupter when you test your gate drive? A 24V regulator is not going to handle four CM300 gates running CW at 100 KHz. How much power is your driver board pulling? Can you test it off a regulated bench supply?
Yes, I am running the interrupter during these tests, the interrupter has to be on and so does the feed back, so I use a signal generator for the feedback.
I also lowered the signal from the signal generator which is feeding the feed back circuit to 60Khz and there was no difference other than the frequency, that was my frst thought is that I was switching to fast so I lowered it and well the same results. I added turns to the GDT as a matter of fact I went as high as 20 turns just to see of my core was saturating and again no difference.
I just wonder if the gate charge on these bricks is just through the roof and that is what is causing this heat issue. However I may have debunked that by lowering the gate voltage to 15 Volts, which put the charge about the same as the "H" series at 24 volts and only a slight improvement was shown so I dont think thats it entirely.
Registered Member #2405
Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140
bwang wrote ...
Are you running your interrupter when you test your gate drive? A 24V regulator is not going to handle four CM300 gates running CW at 100 KHz. How much power is your driver board pulling? Can you test it off a regulated bench supply?
Driver board is drawing about 250mA or so if a remember correctly
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Even if that core worked with your smaller IGBTs it could still be saturating under the heavy load of the big ones. Eceeding the volt/turn ratio.
Something you can try doing is to drive each CM300 half bridge with one GDT core with primaries in parallel and see if that improves the heating issue.
Saturation of the main core even only partly saturated, can create significant heating because the inductance is basically dropping down to 0 or close to it, this can lead to basically a short circuit situation.
You can also try adding more turns to the transformer from the looks of your images you have maybe 8 or 9 turns. I usually use 14 or 15 on my GDTs that drive larger bricks. This can also help with the saturation issue if that dose turn out to be a problem. You can do some math to also see if your in the ball park for your transformer design, using the universal transformer equation.
Attached is a spreadsheet that I created that I use to calculate turns in GDTs and other transformers. Be sure to use between 0.3 and 0.4 tesla for flux density for ferrite cores. You can even use 0.2 T to be on the supper safe side.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Sorry for the double post the first post gave me an error on the attachment, attached it the universal transform equation spread sheet that I mentioned in the other post. ]transformer_universal_emf_equation.xlsx.zip[/
file]
Registered Member #2405
Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140
Goodchild wrote ...
Sorry for the double post the first post gave me an error on the attachment, attached it the universal transform equation spread sheet that I mentioned in the other post.
Thanks for the file, you must be using Excel 2007, I am still using 2003 and cant open it up, I do use MathCad and have several work sheets made up for this purpose, I am really thinking the issue is caused by the high gate capcitance of these bricks. Everything else seems to check.
On your CM600 bricks what is the wattage size of your gate resistor? I am using 5 watt and no matter how I crunch these numbers I still come with 7 watts going across these 5 watters.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
I use half watt resistors on my CM600HA-24H bricks, once again pulsed power is high, but with DR's the duty is very low usually less than <10% So even at the worse case of a 10% duty cycles you would be seeing 500mW, if your 5W figure is for CW operation.
You may be able to see the gate resistors in this photo, full sized photos of my setup are here:
I also don't think your gate charge is the issue ether, because once again I drive bricks that have a whopping 4600nC each at 24V drive with the same 24V 1A reg. You must have some other external problem relating to your gate drive that is making it draw excessive power. It's up to you as an EE to find it
I reattached the spreadsheet that should be compatible with excel 2003.
Registered Member #2405
Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140
Goodchild wrote ...
I use half watt resistors on my CM600HA-24H bricks, once again pulsed power is high, but with DR's the duty is very low usually less than <10% So even at the worse case of a 10% duty cycles you would be seeing 500mW, if your 5W figure is for CW operation.
You may be able to see the gate resistors in this photo, full sized photos of my setup are here:
I also don't think your gate charge is the issue ether, because once again I drive bricks that have a whopping 4600nC each at 24V drive with the same 24V 1A reg. You must have some other external problem relating to your gate drive that is making it draw excessive power. It's up to you as an EE to find it
I reattached the spreadsheet that should be compatible with excel 2003.
Yep I was able to open that one, Thanks for the spread sheet, I think there is something else causing the heat build up too, I am going to figure it out one way or the other. I ran another test on the bricks and they are fine so I know its not them. I see you Gate resistors and they actually look smaller than mine (wattage size) so I should be able to run these, anyway I am will look around and see what I can find.
Nice work on the coil it looks very good. This is my fourth DRSSTC however the first one I did was using 40n60 in a full bridge then of course those became difficult to find so then I went to a leaded TO-247 style I actually had that coil at the cheesehead teslathon a couple years ago and that coil is still running good, its about 4 or 5 years old now but never had a problem with it, its 120Vac with a doubler. I was going to change the silicon over to the CM300 bricks and up the power on it, but then I got caught up in other projects and have not changed it over yet
I will continue to search for the issue here and let you know what I came up with.
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CM300DU-12NFH Full Bridge
