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Registered Member #286
Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399
The only problem I see is that some people are not using gate resistors. They exist for a reason. If you drive a MOSFET without a series gate resistor, there will be huge di/dt happening during the inverting switch of the gate drive circuit. There will always be significant leakage inductance between the gate drive and the MOSFET's gate. This inductance will cause the gate capacitor and "inductance" to resonate during transistions. I don't know what VHF resonance can do to a MOSFET but it can't be good. Use a gate resistor, it help dampen it.
One more thing is that make shure that the drain and source current path do not share the gate to source current path. Some IGBTs have two source connections on the device with helps this problem. With a regular MOSFET, there could be some leakage inductance and resistance on the source pin and PCB traces and this could drive extra voltage on your gate and pop it. Make sure that your PCB designes have seperate current return traces for your gate and drain.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
LMAO at "Goldilocks" gate drive Steve - Not too hot, not too cold... I'll remember that one!
It's interesting to read what different people's experiences are, particularly the RF engineers as I feel that RF and power-electronics engineers sometimes independently re-discover the same techniques!
I've made a few notes on gate-drive optimisation from the perspective of power-electronics design for manufacture. It's a bit long compared to the other posts, and not the most enthralling read, so I've stuck it here:
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Just remember that unlike turning a gate off, turning a gate on is limited by physical characteristics of the device. This turn on delay is specified in the datasheet and no matter what you do, you aren't going to turn it on faster or harder than that.
The main reason for gate resistor is to critically damp your gate drive circuit so that you don't oscillate while driving which is especially important in unipolar drive schemes where there is no neg bias on the gate during turn-off.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Just letting you know I had another failure, this time with 22A MOSFETs with below 10A turn-off current (and much less average) in a hard switching half bridge. The gate drive was not very fast (~0.4Apk). So perhaps the reason for the deaths is even more mysterious than just "too fast gate drive".
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi Jan,
Have you read the kim ladha's last posts in IH thread?
kim ladha wrote ... Hi, as you know I had an unexpected 'cold' igbt failure under normal opperation. I fixed this and repeated the conditions under a more clinical environment with lots of probes.... I have found the cause!
When the high side transistor switches off, the current free wheels down through the low side igbt diode. The diode is fast but has a poor transfer characteristic. It allows the output to drop below the 0v line by a staggering 4v. This gets worse when the current is triangular. The current through my boot strap diode and the high side zenner is the only thing clamping it during this transistion. They get hot and fail- this causes everything else to break.
THE FIX- put a 47 ohm resistor in series with the bootstap diode. This lets the free wheel diode do its jub and stops all the problems... for now.
Those bootstrap driver IC's can be pain in the ass. With GDT's, you really don't have much to go wrong.
Inverter similar to yours driven by SG3525 and GDT and SCR overcurrent protection is lasting me a year now without having blown a single mosfet, ever, even though I done some terrible things to it.
I don't see any reason to use bootstrap chips unless it's absolutrely needed, like class D amps.
Registered Member #1217
Joined: Mon Jan 07 2008, 11:46AM
Location: Leicester, UK
Posts: 11
I have just finished reading this thread and thought I would make a contribution since somethings are missing.... I will point out now that my design goals prioritise reliability not reducing EMI. It gives me pleasure to know that people can hear my tesla coil many miles away on their old am radios!!
1) Gate drive resistance is a good thing! You use about 10 - 50% the minimum as a very general rule. The minimum is the required resistance to get a dv/dt on the gate enough to keep the transistor saturated on turn on. You take you max expected turn on di/dt (peak current * 2pi*f for a sinusoid or peak supply voltage/ inductance for inductive loads) and divide the transistor gain (siemens amps/volt) by this to get your minimum dv/dy on the gate. You then look at the drive chip and get the value of amps at the threshold voltage of the transistor (ensure resistor is limiting current- some drive chips are current limited). Take your driver supply voltage minus the threshold voltage- this is the resisor voltage at the switch on instant. multiply the dv/dt by the gate capacitance to get the gate current needed for you minimum dv/dt on the gate. Divide the resistor voltage by this current to give you your resistor value. It will be much bigger than you expected. Using this value will provide a device turn on where the transistor is only just controlling the current- less resistance than this means the device is saturated and not dissipating energy.
The dv/dt on the drain/collector at switch off just needs to be checked with a scope- it is made much worse by too low gate resistance- if this is higher than the data sheet- add a snubber or reduce stray inductance.
2) Nobody mentioned the problem with modern IGBT avalanches- basically if you try and switch them off while carrying a lot of current, if you apply a large dv/dt or over voltage them they avalanch. The smps ones are the worst and these are the no1 choice for drsstcs! The solution is to use a snubber for inductive loads and to make sure you zero current switch for drsstc's and similar circuits.
3) MOSFETs dont avalanche- they clamp over voltage like zenners and burn up very fast, they act like they have an awful reverse recovery anti-parrallel diode if you over dv/dt them at switch off (big current pulse) and most mosfet failures I have seen are due to over current. A mosfet heats up really fast if you over current it due to its apparent resistive property. Also, you MUST make sure the stray inductance and the max switched current give rise to no more than 10% the single pulse avalanch energy (use repetitive pulse avalanche energy if given and ignore the 10%).
Most of the 'unknown failures' are caused by unexpected transients not overheating. In the case of mosfets/igbts this can be dv/dt turn on, over current and gate overvoltage.
I have watched coleagues run long wires to gates and wonder why their transistors keep breaking- always have the minimum distance to the gate to avoid overshoot. Use transmission line style tracks and twisted pair wires. Check waveforms with a scope- obviously.
Transients caused by controller malfuction are difficult to spot. Keep things shielded and dont place high current tracks near the controller- star connect the grounds on a pcb for smps work. Driver chip failure can also be the cause of unexpected failures. Make sure you clamp all the maximum ratings from the datasheet with zeners- you can always remove them later if you think they are redundant.
I hope that at least the resistor selection bit is useful to some.
btw- them ir2153's are current limited so you dont need a resistor normally. If you have really long tracks you might get a bit of overshoot but i doubt it.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Marko- thanks, I've probably missed that post, it actually makes perfect sense! I'll try to run the high side driver from a separate isolated power supply and see how well it goes. This also explains other things such as: the high side supply voltage unexpectedly getting higher than the low side one, and often the failures happening at similar currents no matter of devices' current ratings.
As to the GDTs - I've had always problems with the transistors getting (sometimes much) hotter than with the level shifted driver, no matter how good my GDT was, I have no idea to this.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi kim, Jan,
2) Nobody mentioned the problem with modern IGBT avalanches- basically if you try and switch them off while carrying a lot of current, if you apply a large dv/dt or over voltage them they avalanch. The smps ones are the worst and these are the no1 choice for drsstcs! The solution is to use a snubber for inductive loads and to make sure you zero current switch for drsstc's and similar circuits.
3) MOSFETs dont avalanche- they clamp over voltage like zenners and burn up very fast, they act like they have an awful reverse recovery anti-parrallel diode if you over dv/dt them at switch off (big current pulse) and most mosfet failures I have seen are due to over current. A mosfet heats up really fast if you over current it due to its apparent resistive property. Also, you MUST make sure the stray inductance and the max switched current give rise to no more than 10% the single pulse avalanch energy (use repetitive pulse avalanche energy if given and ignore the 10%).
I don't know what you mean by this... I thought avalanching mechanism is pretty much same for mosfets, ''clamping like zeners'' as you say, turning the clamped energy into heat in the device. Snubber circuit would as I think simply move the heat away from the device.
As to the GDTs - I've had always problems with the transistors getting (sometimes much) hotter than with the level shifted driver, no matter how good my GDT was, I have no idea to this.
I think you should try again, laws of physics shouldn't be too different in mine and your country, as I think.
For frequencies you were running at GDT is really not a problem. I thought I'd have hard time constructing a GDT poorly enough to notice it at 100kHz! In MHz range they become a problem, though. GDT also gives advantage of bipolar drive. If anything, you could compare the GDT with bootstrapped driver.
You don't really need any insane speeds at these frequencies either, you should be quite fine just driving off the SG3525 or similar IC for like 4-5nF output load.
The failures you had with IR driver always looked suspicious to me. Hope you can get it working now.
Registered Member #1217
Joined: Mon Jan 07 2008, 11:46AM
Location: Leicester, UK
Posts: 11
Hi Marko,
To clarify. IGBTs have a pnpn internal structure like a thyristor. If the transistor avalanches then it remains 'latched' until the current is reduced to zero. IGBTs can also avalanche like a bjt this is leads to latch up. A bjt avalanches when you apply too much voltage and is characterises by the device suddenly switching on like a thyristor. You can help prevent this by tying the gate to the emmitter using a resistor or by using a lower impedance gate drive in the case of igbt's. The best way to stop transients latching the igbts is using a MOV or TVS diode.
When you overvoltage a MOSFET the electrons just squeeze through the channel- this looks like a zener diode characteristic although it is not really. If you apply a pulse of voltage to the drain, some charge gets to the gate through Cdg (drain gate capacitance or miller capacitance). This turns on the device until the charge is removed. If you have a low impedance driver, this charge is removed quicker and the dv/dt turn on is avoided. You can't latch a mosfet but you can get it to turn on by itself with enough dv/dt.
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Goldilocks Gate Drive?
