Required gate current for IGBT driver? - Page 1
Forum section: electronics / general

author: rubacava
author_rank: Newbie
timestamp: June 20, 2023, 12:15:51 am
content: Hello I'm designing a circuit that uses an isolated gate driver IC (1ED3890) to drive a 3 phase AC motor (600V). I was wondering what is the actual required current for bipolar operation to make sure IGBT switches reliably. I decided to use a bipolar DC-DC Converter which supplies 15V and -5V but at 80mA max at 15V and 40mA max for -5V output.The IGBT i'm going to use lists gate charge as Qg = 17,0 uC for Vge = -15...15V. Switching frequency is going to be 1KHZ. From what I researched, required gate current is frequency multiplied with gate charge but some sources also mention an extra multiplication with the delta voltage swing.Also there exists a peak source and sink currents which exceeds well beyond my converter does this current need to be supplied with bulk capacitors?Thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 20, 2023, 04:08:25 am
content: Voltage swing is only if you're calculating energy -- mind the units. Charge times voltage is energy; current times voltage, or charge times voltage times frequency, is power.You only need enough local bypass capacitance to ensure supply voltage remains nominal during/after the switching edge, and (DC) supply current to deliver the gate charge * maximum switching frequency.Since Ig = 0 (DC), it's often that a e.g. 20V supply is used (for +15/-5V drive), with a cute little zener shunt regulator (5.1V diode and 10k pull-up, say) to set emitter voltage with respect to the local supplies, and bypass caps handle the rest. It doesn't drift despite the low bias, because again, it's gate capacitance, hardly any leakage.And with this architecture, specialized drivers aren't necessary; a conventional (if higher voltage) driver plus digital isolator will do, for example (e.g. IXDD609 + ADUMxxxx).Is this for a motor drive application? 1kHz is pretty low for that; it might even be into DCM / forced CCM range, where core losses are particularly high.Tim

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 20, 2023, 10:28:28 pm
content: 17 uC, not nC, according to the original post. So that's more in line with 1 kHz switching frequency. Though even for very large IGBTs, 4 - 25 kHz would be more common. How much current does the motor draw?Dedicated drivers can often be cheaper than a digital isolator followed by a separate driver, at least if you consider the cost and complexity of adding in things like UVLO and desat. This particular example is pretty expensive, but it also includes a lot of (necessary? I'm not convinced) functionality. More basic chips with with just UVLO can be had for less than 50 cents per channel for similar current capability.

author: rubacava
author_rank: Newbie
timestamp: June 20, 2023, 10:35:51 pm
content: Hi Tim,Thanks for the quick response. Yes this is for a high power 3 phase motor which is controlled via SVPWM. It is for a test bench kind of setup for tram motors. The reason i chose to use this driver was it's protection, monitoring and scalability (if there occurs a need to combine multiple drivers into single PCB) features and I don't want to reinvent everything because testing is limited. Is the zener really necessary since I'm using an isolated DC converter (MGJ2D151505SC)? I have a buck converter feeding the DC Converter itself with 15V. My power stage looks like this 24V DC Battery -> 15.2V Buck converter -> Isolated DC-DC Converter.Thanks again.

author: Phoenix
author_rank: Frequent Contributor
timestamp: June 21, 2023, 06:34:41 am
content: Using MGJ2D151505SC means no additional regulation is required for +15V, -5V drive (i.e. no zener). at 2W it should be capable of running some pretty decent IGBTs, just put a big ceramic capacitor on each output (say 10uF 50V 1210 on each rail).With this kind of IGBT, protection features like soft turn off desaturation are valid to have. Some of the other internal measurement and configuration stuff - sure, it's for a test bench.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 21, 2023, 11:50:18 am
content: Oh right, that was micro. EditedThe savings [with zener] is a single (or single-output) converter; I guess you already have them, and since this isn't a production setup, that's perfectly fine.You'll need more bypass than that; I would suggest more like 100uF, which can be aluminum polymer.You may also want clamp diodes on the output, in case of total failure. This will be a couple of considerations together:- Blast shield to keep arc flash away from the gate driver board -- assuming it's a separate module, not an intimate part of the inverter assembly (e.g. pin-type IGBT that press-fit / solders into the board; at this power level, I suppose that's unlikely, but laying this out just in case)- Enough series gate resistance allowed, so that a TVS is able to sink fault current and blow the resistor (which will be fusible type)- A big enough TVS is allowed, to handle the surge without itself failing (or maybe TVS failure is acceptable, but it must fail short, not subsequently blow open)Probably at the low speed, this is acceptable; even a large TVS will have capacitance that pales in comparison to the IGBT gate, so hardly costs any gate drive power or speed. And at ~kHz, some ~us of switching speed would be "fast" I guess (it may even be a slow type IGBT where gate speed isn't particularly critical?).On the upside, the added resistance helps reduce peak current in the driver; on the downside, it restricts what you can do, as far as testing at higher frequencies, or with bigger transistors. You may want to target a larger driver (more supply power, maybe output current), and +/-15V (primarily, or as an option -- upside with the dual-output converter: just swap one in another voltage combo, assuming same family and pinout) to get more flexibility there. (Also I haven't surveyed IGBTs in a while, but I'm guessing the bigger ones are still largely +/-15V drive recommended, and the -5V here is either a somewhat special or newer type, or an attempt at economizing?)Tim

author: Phoenix
author_rank: Frequent Contributor
timestamp: June 23, 2023, 05:37:58 am
content: I looked up IGBTs around 17uC - huge things.Those modules likely still want +/-15V drive. And as Tim mentioned, plenty of bypass capacitance.You can also get off-the-shelf gate drives boards to suit standard module sizes.

author: rubacava
author_rank: Newbie
timestamp: June 23, 2023, 06:18:57 pm
content: I already have those gate limiting resistors to limit peak current to around 6A which is well inside the driver sink&source current specifications. An external TVS protection can be handy as a failsafe, I'll try to incorporate that into my design. Do I really need that much capacitance? I calculated my needs to be around 33uF minimum so around 50uF Low ESR with tolerances. Can higher than needed capacitance have any negative effects while switching?+15/-5V converted was selected mainly for availability purposes since this was the only part I could find last time I checked. The pin layout is similar though between murata converters so switching to ±15V should be trivial. I might also switch to center tapped isolated transformer as I progress.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 23, 2023, 07:12:11 pm
content: 33uF would be fairly minimal, yes. 47-100uF would be a good ballpark. Aluminum polymer, low ESR.Extra capacitance only loads your DC-DC during startup. Obey maximum capacitance spec if applicable.Tim

author: rubacava
author_rank: Newbie
timestamp: June 25, 2023, 03:16:46 am
content: Thanks for the replies.