IGBT Gate Driver Circuit Woes - Page 1
Forum section: electronics / projects

author: eerocketman
author_rank: Contributor
timestamp: September 05, 2017, 09:51:43 pm
content: Hi everyone,I am having some major issues with my mosfet totem pole driving an IGBT. I have attached images of the circuit below and the IGBT gate signal. Could some one please explain what I am doing wrong? I also believe that switching the mosfets to BJTs might fix my issues but it would be nice to have someone knowledgeable explain what exactly I am doing wrong. Bus voltage is around 400V. Q1 is a depletion mosfet.I worry that the power circuit may be what is causing the crazy oscillations.Thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 05, 2017, 09:56:06 pm
content: Ok, what's driving DISCHARGE_ENABLE?There's no -5V bias for turn-off drive?Tim

author: eerocketman
author_rank: Contributor
timestamp: September 05, 2017, 10:05:32 pm
content: Discharge enable is the output of an opto coupler ACPL-M49T-500E that goes from +0V to +18V.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 05, 2017, 10:16:31 pm
content: 1. Why use 2A+ transistors when IGBT gate drive is limited to 0.35A peak?2. Any time the input is between about 2 and 16V, the drive transistors are shorting out the supply. This is probably not good for the supply or the transistors.3. The ACPL-M49T-500E isn't a driver, it's a transistor. Or worse, just a phototransistor, if you're not taking advantage of its higher speed configuration, which seems to be the case. So, best case, it'll spend on the order of 20 microseconds transitioning between levels, but loaded with big gates like those (Ciss + Miller effect), probably more like 100us.At the very least, you need separate turn-on delays to the high and low side transistors, but preferably, you need much faster drive, too. A couple BJTs will be enough to push that around in under a microsecond.If this is intended to run at a very low duty cycle, it might also be desirable to use something a lot sloppier than a three-terminal regulator. For instance, an 18V zener diode and an NPN or NMOS follower. Then the output capacitor will experience some voltage change, without sinking huge currents through the regulator. (A 100us short circuit is more than enough time for the 7818 to ramp up to its current limit. That poor MOSFET...) A current limiting circuit can also be added to further protect the supply. Or, as long as you're on the subject of depletion MOS, you could use a single one as a current limiting diode (e.g. LND150), a zener to shunt regulate the supply, and that's that. (Or if Vbus doesn't change much, a series resistor is perfectly equivalent and much cheaper.)Tim

author: eerocketman
author_rank: Contributor
timestamp: September 05, 2017, 10:47:32 pm
content: Thank you so much for your advice.Using a BJT totem pole as attached should get rid of the shorting issue in the transition phase of 16V to 2V. Is this correct? And if use a BJT at the output of the optocoupler will this speed up my driver?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 05, 2017, 10:52:10 pm
content: It's a start.Show the opto connection as well.MMBT4401/3 is more than enough here for the resistor values shown, or 3904/6 aren't far off, for that matter.Tim

author: DBecker
author_rank: Frequent Contributor
timestamp: September 05, 2017, 10:57:17 pm
content: Uhggh, yes, that is an excellent example of "shoot through". Some integrated gate drivers use supplemental MOSFETs to provide full gate voltage, but the primary current is typically provided with a bipolar pair to eliminate shoot-through.You might reconsider using an 18V supply if your IGBT gate is limited to +-20V. Unless you are switching really slowly, the actual gate voltage might spike to over 20V.IGBTs are much slower to turn off than MOSFETs due to slow minority carrier recombination. Higher power IGBT driver modules typically address this by using a -5V or even -12V turn-off voltage. But you can avoid a lot of complexity if you accept a slower turn-off time, and the extra heat that creates.

author: eerocketman
author_rank: Contributor
timestamp: September 05, 2017, 11:07:36 pm
content: If I was to accept a slower transition time would the following circuit be satisfactory?Thanks again for everyone's advice

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 06, 2017, 02:47:59 am
content: Do it this way,Tim

author: eerocketman
author_rank: Contributor
timestamp: September 07, 2017, 03:52:34 am
content: Can someone explain why placing a resistor at the top of the totem pole does not fix the shoot through issue?Thanks!