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Old 11-10-2015, 10:30 PM   #2319 (permalink)
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thingstodo - I'd just like to add a big thank you for this amazing testing you're doing.

On my end, I've been working on the driver circuits for a TO-247 switch equipped controller.

This is awesome from an R&D perspective, because you can actually purchase all leading technologies ( IGBT, Mosfet, and SiC mosfets ) in the TO-247 package.

To get a feel for this, I got a sampling data sheets for 3 different versions from different manufacturers of the three different technologies. The common point with all of them is they needed to be able to carry about 100A. The standard mosfets could handle a maximum of 600V, while the IGBT's and SiC mosfets could handle 1200V.

The question I've been hoping to answer: Is it possible to make one driver topology that can drive all three types of switches. Answer - yes

Would the one in the Paul's most recent design (the one thingtodo is testing) work for all three? - yes, with very small mods. It looks like you only need to change the zener diode that determines the high/low voltage outputs. For lower voltage mosfets, it may be necessary to reduce the supply voltage. This can probably be done my changing the duty cycle for the power isolator circuit.

Anything new yet? no, honestly - Paul covered this a couple of posts back. However, there are a couple of interesting points I learned from some Powerex driver app notes:

The power required to drive a switch - whether a "bruiser" IGBT or a SiC switch is determined simply by multiplying the gate charge by the frequency. This is similar to the power required to charge/discharge a capacitor. A large TO-247 IGBT has a total gate charge of 410nC (nano Coulombs) and the biggest TO-247 SiC Mosfet I can find has a gate charge of 161nC. Because various tail effects and so on, you probably don't want to run the IGBT over 10kHz. However, the SiC can be run comfortably at 30kHz.
IGBT: 410nC*10kHz = 4.1mW
SiC: 161nC*30kHz = 4.8mW

This is NOT addressing all the issues, such as whether you'd want to drive the switch that fast - just the power required to drive one switch. Most importantly, it shows that the transformer used to supply isolated power is very safely spec'd as it can handle well over 3A - - - 72W.

The other issue is the current that must be instantaneously supplied to turn on a switch. This is simply E=IR, assuming the parasitic stuff like capacitor ESR, etc is small relative to the gate resistor value. This is where the SiC switches are more demanding, because they can handle a faster gate turn-on time. For example, a SiC switch with a 19V supply and 2Ohm gate resistor would require 9.5A PER SWITCH. A system that uses three switches, like the one I'm designing, will require a control that can handle 3*9.5A = 28.5A! Ouch! This would require a very capable control switch for the driver circuit. I've found ones that can handle peaks of 30A; they are actually physically smaller than the current ones - this may be a realistic limitation:

This then begs the question about all the connections for the switch gate pins and the emitter connections. At first I thought the Radsok connectors capable of 30A were silly overkill. The connectors I chose for the gate pins can handle 36A, but I chose them primarily for the contact reliability. At any rate, the standard "spring finger" type connector won't cut it. I'm looking at this technology for a reasonably small connector:
So far I've sourced the spring and I think the housing would be pretty easy to make.

Of course, if you're driving an IGBT @ 5kHz using a more normal gate resistance of 10Ohms or more, this isn't a problem.

- E*clipse
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MPaulHolmes (11-11-2015), thingstodo (11-11-2015)