According to the design tool rev2.99
May I know the how the value of tgate_transfer and Ton max comes from?
“Tgate transfer” is half the time required for the MOSFET gate to ramp from its Vthreshold (when current starts flowing) to a gate voltage that supports current limit with the MOSFET source (or output) shorted to ground, i.e., the fault scenario is starting up into an output short circuit. The 1/gm of the MOSFET is assumed to be 0.5V / 25A. Therefore, this Vgs – Vth = Ilim * (0.5V / 25A) and Tgate transfer = 0.5 * Cgate * (Vgs – Vth) / Igateup. The 0.5 factor is because it’s a triangular power pulse during the ramp. Subtracting Tgate transfer from the total SOA time (1000us in your sheet) yields the timer’s “Ton max”.
thanks for the reply, however, May I know what does Ilimt stand for now? I startup current limit?
if I set I startup limit to 10A, and according to the SOA diagram below
even it's a SCP scenario , the MOSFET should be able to stand for at least 10ms ~100ms with 10A current and 12V Vds ( to consider derating, let's say 5ms) , but is still much longer than the time that the worksheet sussgest us (33nF, with Tonmax = 620 us)
sorry about that I wasn't able to upload the design tool worksheet to you, if you can provide me an Email address, I will send it to you.
I am still quite confused why the timer is set so low?
please correct me if my understanding is wrong
Ilim is taken to be “PSET I @ Vin” in Section 9, which is the typical power limit (“PWR” in Section 9) divided by nominal Vin (“Vnom” in Section 1). In previous question’s spreadsheet screenshot, you had 1000us (or 1ms) for “SOA line chosen” and “SOA const power” (these are derived from Section 8 and “FET SOA CP” worksheet Section 6). If you adjust those higher per your MOSFET SOA curve, then your Ton max will also be longer.
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