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Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Ash Small wrote ...
Sulaiman wrote ...
A small but maybe important thing I noticed, the SiC mosfet reverse gate voltages are much lower than the forward voltage, some designs may have problems with this.
It doesn't mention anything in the datasheet. (link in OP)
I presume you are referring to reverse voltage spikes when used in flyback topology.
This is exactly what I was planning on using them for. Are you not also considering using SIC's for flybacks?
It does mention something about not exceeding -4 volts between gate and source if using the integral freewheeling diode.
Mads Barnkob wrote ...
VGSmax Gate - Source Voltage -8/+18 V (Absolute maximum values)
Yes, most driver circuits will have to be changed, preferably to a modern mosfet driver ic that can do those exact voltages, atleast the lower turn off voltage. Like:
I presume you'd need to use the maximum voltages in order to achieve the published switching times. Does this mean blocking the integral freewheeling diode and adding an external Shottky, or something?
What Sulaiman and I pointed out, is that you can no longer just use a regular UCC37322 MOSFET driver with a GDT to drive these, like many SSTC schematics use(most MOSFETs can take +/- 20Vgate), you need a "special" MOSFET driver IC to handle the correct gate voltages.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Ash Small wrote ...
Sulaiman wrote ...
A small but maybe important thing I noticed, the SiC mosfet reverse gate voltages are much lower than the forward voltage, some designs may have problems with this.
It doesn't mention anything in the datasheet. (link in OP)
Page 1, Absolute maximum ratings, "VGSmax"
I presume you are referring to reverse voltage spikes when used in flyback topology.
The reverse voltage spikes across the transformer will appear as forward voltage spikes across the MOSFET, and this is what you need to watch out for, these should not exceed VDSmax.
The internal diode on these devices seems to perform worse with negative gate voltage, so if you rely on the diode you should not use negative gate voltages. If you know when the diode is supposed to conduct, you can turn on the MOSFET, this should lower the reverse voltage drop as MOSFETs conduct in both directions, it will effectively shunt the diode.
I presume you'd need to use the maximum voltages in order to achieve the published switching times. Does this mean blocking the integral freewheeling diode and adding an external Shottky, or something?
Switching times shouldn't change much with applied voltage I think. The internal diodes in SiC MOSFETs are very fast but soft, so they will have a high voltage drop (and therefore dissipation) with significant current flowing through them. If you want to improve it, you can add an external parallel diode with lower voltage drop, but this is only neccessary when you have high average reverse current flowing. No series schottky is neccessary.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Today I did a quick check on the 1700V device that I mentioned above, I only used the component test function of my 'scope at work (two wire, c50Hz 8Vrms via a resistor, X=voltage, Y=current) when Vgs = low; Vd > Vs ... very high resistance, Vd < Vs ... looks like a diode but with an unusually high forward voltage when Vgs = high; looks like a short circuit
8 Vrms gate-source does nothing ... no gate breakdown at c -11 V peak from the datasheet, Vgs abs. max. -10 /+25, Recommended -5 /+20 so I was probably stressing the gate but not too badly (I think that I read somewhere that mosfet gates accumulate damage ... unreliable memory/no references)
Registered Member #4465
Joined: Wed Apr 18 2012, 08:37AM
Location: Bucharest, Romania
Posts: 145
Done my home work. As Wolfram had suggested, I have tested some SiC MOSFET transistors for the body diode’s conduction. I have measured the MOSFET's body diode forward drop for different values of the gate voltage. Ids = 10 Amps The results.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
mister_rf wrote ...
Done my home work. As Wolfram had suggested, I have tested some SiC MOSFET transistors for the body diode’s conduction. I have measured the MOSFET's body diode forward drop for different values of the gate voltage. Ids = 10 Amps The results.
Excellent, that's exactly what I had hoped. Even if the diode is very soft, the FET conducts in reverse as well and shunts the diode when it's on. This means that these are excellent for pulse density modulation, where we rely on FETs to conduct significant current in both directions when on.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
On the subject of drivers, the driver linked to by Mads above has max rise time of 40nS and typical fall time of 8nS, max 20nS.
The IXDD614SI has typical rise time 25nS, max 35nS and typical fall time of 18nS, max 25nS.
Does anyone know of any suitable drivers with faster rise and fall times? How does the rise and fall times of the driver affect a switch with rise and fall times around 5nS?
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Rise time of the driver is usually quoted into some capacitance, usually a few nanofarads. These SiC devices have very low input capacitance, so the rise time will be somewhat better than the quoted figure. Due to the low gate charge, you don't need to deliver a lot of current to turn it on in a given time. This means that the most powerful driver won't neccessarily be the fastest one, a smaller driver is more likely to have lower internal delays, so a 2-5 A driver will probably be the fastest. The UCC27524 looks like a promising candidate.
If you want to switch actual currents at these time scales, then layout will be extremely critical. What sort of currents are you switching, and what sort of load are you driving?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Wolfram wrote ...
If you want to switch actual currents at these time scales, then layout will be extremely critical. What sort of currents are you switching, and what sort of load are you driving?
Without going into too much detail in this thread, I'm planning on putting a lot of current through them for periods of ~100nS, through a one turn primary, and into an inductive adder type pulse generator type circuit.I'm al;so planning on paralleleng the switches. I've not got into the maths too deeply yet, I was waiting for some suitable devices to be available at a reasonable price. The current plan is to use toroidal iron powder cores with distributed gap and flyback topology.
The second post on this page has a link to an imnductive adder pulse generator and a sketch of my proposed circuit:-
The circuit linked to is push-pull topology and uses a MetGlas core.
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New 900V SiC MOSFET launched by Cree, 11.5nS switching time
