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# LM74701-Q1: Background of the calculations of MOSFET selection

Part Number: LM74701-Q1

Hello,

A customer wants to know a bit more details of the datasheet explanation.

In 9.2.2.2 MOSFET Selection of the datasheet, I think there are 2 estimations as listed below. Would you please explain a bit more details about them?

1. The resulting average current (IISO_AVG) can be approximated as one third of the peak current that is around 2.4 A.

Is the approximation of “one third” made as a graphical approximation? Namely, since the yellow area below is in appearance roughly 1/3 of the rectangle of 7A x 1ms, the average current for 1ms is 2.4A? Although the actual calculation is 2.33333A and even if 2.3A is closer? Or is there another calculation?

2. Selecting a MOSFET with SOA characteristics covering the load line of 43 V which can support drain current greater than (IISO_PEAK/2) for 1 msec is a good starting point.

Is the denominator of 2 in (IISO_PEAK/2) based on an assumption of the current never go below the blue line in the below figure? Namely, the actual current will be always above the blue line such  as the green line? Or is there another reason for the “2”?

Another question is, where did the -100V and 1ms come from? The customer seems to think 150V for 2ms should be more common. What is the reference?

Thank you.

Regards,

Oguri (TIJ atuomotive FAE)

• Hi Oguri,

Is the approximation of “one third” made as a graphical approximation? Namely, since the yellow area below is in appearance roughly 1/3 of the rectangle of 7A x 1ms, the average current for 1ms is 2.4A? Although the actual calculation is 2.33333A and even if 2.3A is closer?

Your understanding is correct. The current signal is an exponentially decreasing waveform from its peak and hence can be considered as  roughly 1/3 of the peak value of 7A.

Is the denominator of 2 in (IISO_PEAK/2) based on an assumption of the current never go below the blue line in the below figure? Namely, the actual current will be always above the blue line such  as the green line?

Your understanding is correct.  (IISO_PEAK/2) is considered so that there is some margin from the actual average of 1/3 x Vpeak.

-100V and 1ms is just an example, you can consider -150V for 2ms and follow the same calculations to find a FET with suitable SOA.

• Hi Praveen,

Thanks! But the customer wonders why you chose 1ms for the decay time where the ISO7637 Pulse 1 requirement is 2ms according to the customer. Why you bothered to select different value?

Regards,

Oguri

• Hi Oguri,

This is because device and external FET will be in VDS clamp operation close to 1ms.  After that ISO 7637pulse-1 voltage fall below -40V so FET gets out of VDS clamp operation.