LM5146-Q1: MOSFET power loss calculation within calculator

John Gomez

Part Number: LM5146-Q1
Other Parts Discussed in Thread: LM5146

In the LM5146 calculator, where is the output charge (Qoss) parameter taken from in the NVMFS6B14NL datasheet?This quantity greatly affects the HS FET switching loss. I'd like to be able to identify key switching loss parameter data from device data sheets.

Thanks.

over 2 years ago

0 Timothy Hegarty over 2 years ago

TI__Mastermind 49827 points

Hi John,

The Qoss can be estimated from the area under the Coss curve. Some FET vendors provide this parameter, but Onsemi doesn't in this case.

Regards,

Tim

0 John Gomez over 2 years ago in reply to Timothy Hegarty

TI__Intellectual 2315 points

Thanks. When integrating the Coss curve, what is the voltage range that is used?

0 Timothy Hegarty over 2 years ago in reply to John Gomez

TI__Mastermind 49827 points

Hi John, the applicable voltage is the input voltage (or the effective voltage across the FET).

Tim

0 John Gomez over 2 years ago in reply to Timothy Hegarty

TI__Intellectual 2315 points

The nominal voltage across the low-side FET is 48 V in the calculator.

This is the Coss graph for the low-side FET used in the EVM and in the calculator.

Using Q = CV, the Qoss would be about 200 pF * 48 V = 9.6 nC. However, the value for Qoss in the calculator is 20 nC. Do you recall how this value was calculated?

Thanks

0 Timothy Hegarty over 2 years ago in reply to John Gomez

TI__Mastermind 49827 points

You need to estimate the total area under the curve, not just the rectangle of 48V*200pF.

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LM5146-Q1: MOSFET power loss calculation within calculator