LM7480: lm7480 Mosfet selection (SOA graph)

fedi makni

Prodigy 50 points

Part Number: LM7480

Tool/software:

Hello,

I am using the LM74800 as a protection from inrush current over voltage reverse polarity ...

My application will use 48V and it is estimated to draw around 8A

I am looking for a suitable mosfet to use in my case but something i still didn't quite get it.

I am looking into the SOA graph of some mosfets in order to know if it will operate safely or not. 2 things confuses me about that:

How long the LM74800 takes to turn off the mosfet in the worst case in order to make sure my mosfet can handle that correctly? Is the total gate or input capacitance of the mosfet matters in this case or not?
In normal operation when the mosfet is fully on so Vds will be low and 8A is flowing, in order to calculate the power dissipation and temperature rise of the mosfet do i need to find the Rds on value from the graph by the intersection Ids= 8A and find the corresponding Vds then calculate Rdson from that?

Thank you

5 months ago

0 fedi makni 5 months ago

Prodigy 50 points

Any update on this?

0 Shiven Dhir 5 months ago in reply to fedi makni

TI__Mastermind 47775 points

Hi Fedi,

Yes, total MOSFET input capacitance will play a significant role to decide the time it will take to charge.

When the FET is fully on, the power dissipation will be nothing but I^2 x RDS_ON

Generally, it is recommended to not turn-on the downstream load until FETs are turned-on since maximum PD is during startup where VDS is large.

Regards,

Shiven Dhir

0 fedi makni 5 months ago in reply to Shiven Dhir

Prodigy 50 points

Hello Shiven Dhir,

Thank you for your message, but what about the turn off time?

Best regards

0 Shiven Dhir 5 months ago in reply to fedi makni

TI__Mastermind 47775 points

Hi Fedi,

Turn-off time is pretty fast, and it also depends on CISS of FET. We have sink current spec in the datasheet.

Regards,

Shiven Dhir

0 fedi makni 5 months ago in reply to Shiven Dhir

Prodigy 50 points

Hello,

One last question if that is possible,

Regarding the inrush current value we need to select, is it the amount of current will pass through the mosfet during startup taking into consideration the soft start or is it something else.

As my load consume 8A, the datasheet doesn't specify exactly how to select that value and how long it takes to limit the current if the value chosen is Iinrush= 2A (for example)
Thank you.

0 Shiven Dhir 5 months ago in reply to fedi makni

TI__Mastermind 47775 points

Hi Fedi,

If Inrush is limited to 2A the total time will be as follows.

12 can be replaced with your input voltage.

Regards,

Shiven Dhir

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LM7480: lm7480 Mosfet selection (SOA graph)