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Registered Member #4054
Joined: Sun Aug 14 2011, 01:39PM
Location: The boonies, France
Posts: 26
Hello everyone, I am new to 4HV.ORG, even though I have been browsing the wealth of information for some time now. Pls allow me to introduce myself, I am an Electronic R&D engineer working for a company manufacturing high end Battery Charger. My field of expertise mainly is small signal/digital electronics as well as embedded programming. So, I am not a power electronics expert, but I have dreamed of building a DRSSTC for quite a while. I currently have all the bits and pieces (secondary, top load, IGBT, miscelaneous components) and I am currently routing the control and Hbridge boards. The controller, pretty much is Steve Ward's which appear to have proven itself, but with a twist: There is a fast motor control uC on the PCB with the option of being able to manage zero-crossing, OVC detection and IGT switching, thus, taking 70% of steve's current components off the board. The Bridge board is a full Hbridge using TO-247 IGBT, as I was not able to get bricks, either fron RS, Farnell, or Digikey. Also, the TO-247 IGBTs I use ( IRG4PC50UD) are paralleled (or at least the board can accomodate // IGBTs. There is little hard data on // IGBTs but please check this out: http://www.vincotech.com/fileadmin/user_upload/articles/PEE_0506-08.pdfhave a read through it, I think it is very promising. Also the Hbridge caps is made up of 10x450V 470uF caps. The interrest of using these instead of one big cap is that they come free of charge, and, by using several, ESR and ESL are paralleled and alot smaller that for a single cap. The board is 4 layers with 2 layers each supplying power to the IGBTs in order to minimize stray inductance. While Steve Ward's approach is interresting, it is in theory better to have a board's entire plan dedicated to supplying power instead of strips of copper as inductance should be minimal. There is a snubbing capacitor for each IGBT bank, as in Steve's design. Please tell me what you think of the ideas above. It's just that I'd like to know if I am going the wrong way before I get the boards printed...
Registered Member #4054
Joined: Sun Aug 14 2011, 01:39PM
Location: The boonies, France
Posts: 26
And why is that Gregory? what's your experience with it? it can obviously be done, refering to that app note I posted. Anyway, the plan was to test the single IGBT config before the // one...
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
serge petiot wrote ...
And why is that Gregory? what's your experience with it? it can obviously be done, refering to that app note I posted. Anyway, the plan was to test the single IGBT config before the // one...
Regards
Serge
It *can* be done, but requires careful thermal balancing and synchronization of gate drive signals, which becomes even more important when you drive the IGBTs beyond their ratings as in a DRSSTC. But it can be done...
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
I don't find it that hard really paralleling IGBTs. A lot of IGBTs out there have a positive tempco like MOSFETs making them very easy to parallel. even IGBT's that have a negative tempco (like 60N60s) can be run in parallel with little effort. Assuming you have the two IGBTs on the same heatsink and they are close together the temp difference won't be much more than 20C different at the max. Because it can be a self balancing system as one IGBT heats up it also heats up the other keeping the current draw in balance. Another very easy step to take is add a current sharing transformer.
You also don't really need any kinda special gate drive ether, I have paralleled 60N60s in my QCW and drive them with a simple GDT.
Overall I don't think paralleling IGBTs is as hard as every one thinks it is. Just take the necessary step and you should be fine.
Registered Member #4054
Joined: Sun Aug 14 2011, 01:39PM
Location: The boonies, France
Posts: 26
Thx Goodchild for your input, that is exactly what I was thinking. My company systematically uses MOS in parallel for the step-down converters used in 50 Amps battery chargers, without any problems. I hardly see why it can't be done with IGBTs. Considering the price of an IRG4PC50UD or a FGA40N60, compared to a brick, especially for a newbie like myself, I'd hate to blow a $200 brick, when I can blow $15 worth of TO-247s... In my design, everything is driven by a single GDT. Also, for a medium sized coil such as mine (4" by 24" size) it would allow to stress the IGBTs alot less. You'll find the schematics as attachment to this post. Regarding the caps any comment? These are the caps my company uses for 400V buses and I don't have to pay for them...
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
serge petiot wrote ...
Thx Goodchild for your input, that is exactly what I was thinking. My company systematically uses MOS in parallel for the step-down converters used in 50 Amps battery chargers, without any problems. I hardly see why it can't be done with IGBTs. Considering the price of an IRG4PC50UD or a FGA40N60, compared to a brick, especially for a newbie like myself, I'd hate to blow a $200 brick, when I can blow $15 worth of TO-247s... In my design, everything is driven by a single GDT. Also, for a medium sized coil such as mine (4" by 24" size) it would allow to stress the IGBTs alot less. You'll find the schematics as attachment to this post. Regarding the caps any comment? These are the caps my company uses for 400V buses and I don't have to pay for them...
Thx and regards
Serge
The reason MOSFET's parallel fine is that they act as a resistive element when turned on, and resistors are self balancing components. Not only do they fundamentally balance given circuit laws, but they also have a temperature profile which makes their resistance go up as they heat up, so if any one carries more current than another, it will get hot and stop carrying so much current.
IGBT's on the other hand act as diodes when turned on, which don't self balance as well since they have a (relative) fixed voltage drop across them, and all the current will just choose the path that has the lowest voltage, even if it's only different from the other transistors by a tiny bit. IGBT's also often have the opposite temperature profile, which makes them unbalance even worse if they heat up, leading to thermal runaways within one of the paralleled IGBT's.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I read an article recently (an application note from one of the manufacturers).
It said some IGBT's provided excellent results when paralleled, but they should be mounted as close together as possible on one heatsink.
The IGBT datasheet should say if they are suitable for paralleling.
I'll try and find a link.
EDIT: Here is one example:
]igbt_datasheet.pdf[/file]
EDIT: Here is the IXYS paralleling advice:
(1. Each IGBT device must have its own gate resistor. 2. Arrange lay-out of devices so that the current paths are symmetrical. 3. Mount the parallel parts next to each other on the same heatsink so that they are thermally coupled. 4. All the paralleled parts within "one switch group“ should have the following characteristics: a. Be made from silicon dice from the same wafer lot (see below). b. Threshold voltages should be matched to + 0.10V c. Saturation voltages at normal operating current level should be matched to + 0.05 V. 5. If the IGBT component contains an anti-parallel diode, match the forward voltage drop of the diode to within + 0.1 V. 6. If it proves impractical to perform the matching outlined in step 4, then insert a resistor in series with each emitter to force current sharing. Pick a standard value for the resistor that is closest to 0.2V divided by the nominal current per IGBT. Resistor tolerance should not be greater than + 1%.)
And here is the International Rectifier advice:
And an ST application note:
(Non-Punch Through and Field Stop IGBT's have positive Vce(sat) coefficients, while Punch Through IGBT's have negative Vce(sat) coefficients.)
I also found this:
It is a misconception however that PT IGBTs cannot be paralleled because of their negative temperature coefficient. PT IGBTs can be paralleled because of the following: •Their temperature coefficients tend to be almost zero and are sometimes positive at higher current. •Heat sharing through the heat sink tends to force devices to share current because a hot device will heat its neighbors, thus lowering their on voltage. •Parameters that affect the temperature coefficient tend to be well matched between devices.
Registered Member #4054
Joined: Sun Aug 14 2011, 01:39PM
Location: The boonies, France
Posts: 26
Good job I posted this before I finished routing the board... I was planning on 2 massive heatsinks (one top, one bottom), but, one dedicated to each bank of // IGBTs. Back to the drawing board, or, I might just give it a shot, as 4 IGBTs parameters might average with the other 4 as far as switching losses are concerned (wishful thinking)? As far as putting emitter (degeneration) resistors on the IGBT, like I've done routinely in audio amps with // transistors, I think it's a bad idea. even an 0.2 Ohms resistor as in audio amps would dissipate/waste massive amounts of power... When you run at 500 Amps... Any feedback on the multi-capacitor issue?
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
Your MMC buscap might have a higher inductance than an equivalent large capacitor; try to keep the traces short and such. I've seen a commercial bridge that used such a cap, so the idea is definitely viable.
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new here, building DRSSTC with // IGBT Hbridge.
