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Gate Drive Pointers

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LAN_403

alan sailer

Mon Jan 15 2018, 01:17AM

Registered Member #59110 Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74

I am working towards the goal of building a medium power DRSSTC with the longer term goal of powering up a QCW.

I am wondering if anyone can point me towards a better understanding the gate drive issues with IGBJTs.
Specifically I read about people over-driving the gates (above the suppliers max Vge). This seems risky for long term reliability.

I do understand the risks of under-driving the gates (overheating). Is there some sort of sweet spot to aim for?

Also when observing the gate drive waveform on my GDTs I can see overshoot of about 2 volts with a 20ohm/3nF load.
When I go to 10 ohms I get closer to 7 volts which is obviously bad. I understand this gate resistor is another
compromise but again is there a sweet spot between overdamped/slow and under-damped/fast and risky?

Are TVS diodes good for gate protection?

Cheers.

Weston

Thu Jan 18 2018, 07:21PM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

First there is the whole concept of the IGBT gate being a capacitor, how to drive it, the impact of the miller effect and such. There are a large number of app notes online about this. Here is one that I randomly chose that seems useful:

The reason why some people over voltage the gates in DRSSTCs is that the IGBTs often experience peak currents in excess of the datasheet recommended values. At some point of excess current the IGBT can exit saturation where it stops exhibits a ~ constant voltage drop and the on state voltage begins to rapidly raise with increased current. At that point, the IGBT is dissipating a large amount of power and can easily fail.

If you look at the "output characteristic" charts in the datasheets you can see a graph of IGBT current / collector-emitter voltage and how it relates to gate voltage. Here is a random example I pulled. As long as you drive the gate to some voltage where the IGBT stays in saturation for your peak tank current you should be fine.

I have not been able to find good data online about the impact of excess gate voltage on device lifespan. However, looking at the output characteristic curves of most IGBTs the maximum rated gate voltage should keep the device in saturation for very high currents. Unless you are really, really, pushing the IGBTs there is little reason to exceed the maximum gate voltage.

The best way to reduce ringing is to reduce the leakage inductance of the GDT. Altering the drive resistance only really sets the damping on a LCR circuit. You can judge what gate rise time is acceptable based on percentage of total switching period it is / the power dissipation. DRSSTCs switch at close to / at the zero current crossings, so switching loss can be relatively low.

TVS diodes can protect the gate, but they also add some amount of extra capacitance which can slow down your gate drive.

alan sailer

Sat Jan 20 2018, 07:09PM

Registered Member #59110 Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74

Weston,

thank you very much. Your answer was just what I was after. I'll be studying
the paper you linked to as ell as the rest of the information to come up with a
better idea of the best gate drive components.

I'm trying my best to destroy my full bridge out of ignorance. It has eight parallel
devices so death would be expensive.

Cheers.

Hydron

Sun Jan 21 2018, 04:51PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

One of the reasons for the max gate voltage rating (other than long term reliability) is that IGBTs are often rated to survive a short circuited load for a specified period (e.g. 10us), long enough to detect the fault and disable the gate drive. With gate voltages over 20V the additional short circuit current due to increased drive is too high for the device to survive.

I recall seeing numbering in the 50-80V region for actual gate dielectric strength. If you are pushing the IGBTs past their rated Icm I would probably go for 24V or so with a TVS across the gate to catch any spikes, but if you're running at peak currents within the IGBT ratings then there's no need to run higher than ~15-20V or so (and running reduced voltage will also reduce gate charge requirements too!).

alan sailer

Sun Jan 21 2018, 10:34PM

Registered Member #59110 Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74

Hydron,

Thank you very much. It's always great to get a reply even if it's an issue everyone has long gotten over.

It is interesting learning the real world reasons behind the max gate drive spec. I'll read more about it.

I'm getting close to turning on the DRSSTC for initial tests. If I was less cautious I could do it very soon
but would rather wait until I understand more of the issues. Right now I want to study some of the ideas
behind tuning for upper/lower/middle or jumping (joke!) poles. And trying to measure some of these
things on the bare coil.

Should be fun.

Cheers.

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