# Understanding the safe operating area of a 1.7 kV rated MOSFET

I am looking at the datasheet of a 1.7 kV SiC MOSFET from CREE (C2M0045170D) which is attached here.

I am under the impression that if I control the MOSFET using PWM, during the turn-ON period (ton=10 us or lower) at a supply voltage of 1000 V, the voltage across the FET (VDS) will be pretty low (governed by the on-resistance and current through it.)

Should I consider the 1000 V point on the X-axis to calculate the current carrying capability of the FET at that supply voltage? In that case, the SOA graph indicates a current rating of 25 A. Is this correct or the current rating would be close to the maximum pulsed (<10 us) current limit of 160 A during the on-period?

My confusion stems from the fact that even at 1000 V supply voltage, the voltage across the FET should be pretty low during on period (Vgs=20 V.)

• The limits shown on the right-hand side, I believe, is called the action (action integral) of the device. This is what will "fuse" (or damage) the device. This will be proportional to the square of the current and time. So, if you ignore the voltage entirely and just focus on the current and the time, you should find a "constant value" that matches up with the current squared times the time. For example, at the 1000 V x-axis line, find (25 amp)^2 *10e-6 s= 0.00625 s A^2 and (2.5 amp)^2 *1e-3 s= 0.00625 s A^2. Note both are the same constant?
– jonk
Commented Sep 10, 2021 at 19:17
• The actual constant for the device will be in Joules^2 per second. In this case, I get $6250\:\frac{\text{Joule}^2}{\text{s}}$.
– jonk
Commented Sep 10, 2021 at 19:29
• @jonk, thanks for the comment. I think I get your point. However, to put my question in a single sentence, what would be the maximum current that the FET can allow during ON time at following operating condition: Vsupply= 1000V, ton< 10 us? Will it be 25 A or close to 160 A. Should I consider the voltage across the FET during ON period or the supply voltage while choosing a point on the X-axis? Commented Sep 10, 2021 at 19:29
• @Andyaka That's Joules-squared per second!! Not Joules. And their safe area chart stays completely within that figure on all boundaries shown. But I take your point that there are many more considerations than what is seen in a single chart. :) I was just addressing myself to interpretation of a single chart for the OP. That's all. Narrow view, admitted.
– jonk
Commented Sep 10, 2021 at 20:55
• @jonk I should probably have looked at what you wrote more carefully!!! Commented Sep 10, 2021 at 20:57

Is this correct or the current rating would be close to the maximum pulsed current limit of 160 A during the on-period (<10 us)

It is correct that the device can withstand 1,000 volts at about 23 amps for no more than 10 μs but, you'd be wise not to get within about 50% of the implied instantaneous power of 25 kW.

As for the 160 amps limit, that entirely depends on what your circuit produces.

So, when the MOSFET is activated (i.e. fully "on" as defined in the data sheet) it will withstand 160 amps but it certainly won't withstand 160 amps as the device is in the process of switching from off to fully on or vice versa.

My confusion stems from the fact that even at 1000 V supply voltage, the voltage across the FET should be pretty low during on period (Vgs=20 V).

A 1,000 volt supply voltage isn't the same as $$\V_{DS}\$$. The SOA graph has an x-axis of $$\V_{DS}\$$.

Here's a bit of advice; model your MOSFET and simulate the circuit and look very closely at the peak values of $$\V_{DS}\$$ and $$\I_D\$$. In fact, I would highly recommend that you use your simulator to calculate $$\V_{DS}\times I_D\$$ and look at that plot directly.

And finally, you should be aware that these devices will go "pop" during the transition from on to off and vice versa if you do not engineer a decent gate drive circuit. They are capable of switching in 20 ns or less and you should not entertain anything much slower than a 100 ns transition time if operating at 1,000 volts with tens of amps.

They are a good device BTW and good luck in getting hold of them. I placed an order in April this year with mouser and, at the time, they reported a delivery of late July this year but none arrived and the delivery date has fallen into mid 2022. I chose a different device!!

Don't forget to place a cut-out in the PCB around the drain so that 1,000 volts can be reliably switched.

• thanks for your helpful feedback. I will simulate the said operating condition to get a good idea of worst case scenario. I have faced the long delay issue with CREE MOSFETs as well this year. Hopefully thigs will get better by the end of this year. Can you elaborate a bit regarding the cut-out around the drain? I assume the reason is to increase the creepage. Commented Sep 10, 2021 at 20:45
• If you can live with 1,200 volt devices ON semi offer some very usable SiC MOSFETs Commented Sep 10, 2021 at 20:50