SuperCascode Designs

[kyledellaquila]

Hello guys!
Has anyone had any success with SiC Super-Cascode totem-poles?
My hopes for this is a modular method for economic High-voltage, high-current, High-frequency switching. In the scenario that one would need more current handling, the stack could be put in parallel. In the scenario that the one would need to drive at higher voltages - simply put more JFETs in the totem-pole.

Asking around, a problem with these Super-Cascodes is that their turn-on time/losses are significantly higher than the turn-off time/losses.

A nice thing about a distributed approach vs one big-ass transistor is that heat-dissipation my be much easier!

I envision future use & design requirements for my own applications:

• 1MHz and below
• 50 to 100 kW (single phase)

Thoughts?







-Kyle Dell'Aquila

[T3sl4co1l]

Sure, but what for?

I think you'll have problems doing it at high frequency, for multiple reasons:
- Note the balancing capacitances in the first example
- The avalanche diodes in the second example also have capacitance; worse, it's nonlinear
- The stack is long, i.e., stray inductance is unavoidable; at the very least the load current per stack will have to remain within some limit

Preferably, the assembly will also be very compact, perhaps a hybrid, and potted in some kind of goop to keep corona away and allow much smaller clearances.  This isn't friendly to prototyping.

You can apparently already get SiC devices up to 10kV or so, which are either academic samples, limited production, or special qualified customer access (probably because ITAR and such).  I haven't seen anything over 1800V in the usual places (well, some MOSFETs up to 4kV, but not very big or fast).

Tim

[johnf]

I have made cascades of IGBTs for a high voltage scan amplifier in an ion implanter. In the end the amplifier is just a super high voltage opamp, i used + and - 5kV rails and 2 amp igbts, higher curent ones have much higher gate capacitance which kills speed. Max freq for my one was around 10kHz. In my case the load on the amplifier was all capacitive ie the ion implanter x and y plates = 1n2 between plates all driven through RG8 coax which added another 1n5 to the amplifier load

[kyledellaquila]

I liked the idea that a method could be employed in which I could switch HV supplies by adding more in series or with more power handling capabilities with more in parallel. But I would agree that the voltage isolation may make prototyping much more complex than operating transmitting tubes.
And yes.. looks like the switching speed is not so hot.. Not a fan.

Most realistically, for future applications, it may be more savory to worry about putting mosfets in parallel for higher throughput. When it comes to load sharing Si Mosfets, there are all sorts of do's and dont's... but I heard that SiC Mosfets may have a better time load sharing. Is this true?

My aim is for modular design approaches for higher power or greater headroom. & Reducing heat dissipation complications with more headroom.
Anyone have success in working Mosfets in syncronysity?