Tool/software:
Hi team
Customer get questions on LM7480x-Q1 MOSFET and VDS selection:
1. in DS, we have below description. Can you please explain the fundamental on why using 2 unidirectional TVS can de-rate the MOSFET from 60V(with a single bidirectional TVS) to 40V?
2. Detail guide on how to select MOSFET and TVS.
thanks!
Hi Terry,
1. During ISO pulse 1, Input voltage swing negative and needs to be clamped. This clamp has to be chosen based on 2 factors.
1. Device abs-max: Since the device is rated for -65V, we have to clamp it >-65V.
2. FET VDS rating: During ISO pulse 1, due to reverse current blocking FET is turned off and its VDS might get violated.
For a 60V rated FET, we can clamp till -44V. (60V-16V)
For a 40V rated FET, we have to clamp till -24V. (40V-16V)
Since clamp required is different, we need different TVS to clamp on negative side.
Now for positive side, we don't really do any clamping and need a positive side TVS just to block the power path otherwise if only negative TVS is present, we will short the input supply.
Now, why bi-directional for 60V FET and 2xunidirectional for 40V rated FET?
In 60V case, we have a diode that can clamp at -44V and also pass suppressed load dump on positive side, so using bidirectional is optimal.
In 40V case, we do not a diode that clamp at -24V and also pass suppressed load dump on positive side, because of which we need to use 2xunidirectional TVS.
Diode selection depends on the used MOSFET, it is not the other way around.
2. MOSFET selection guide is present in the datasheet. Please go through the section 10.2.4 and 10.2.5.
Regards,
Shiven Dhir
