LM25149: Using same MOSFET for high-site and low-site

Hi Orlando,

The switching frequency is 2MHz

I agree the gate charge will give loss, but isn't it the same for the low site FET?
In the design I get from WEBENCH CSD18514Q5A is suggested as low-site FET
Why not use CSD18504Q5A  for both FETs

What puzzles me is that non of the design suggestions use the same FET for High and Low site

BR.
/Gert

Hey Gert,

The larger gate charge on the high-side FET will slow down the switch node rise time and fall time. 

Slower SW times increases switching transition losses. 

Try out the quickstart calculator where you can fill in the MOSFET specs and get a estimated loss for both the high-side and the low-side FET.

https://www.ti.com/tool/LM5149-LM25149DESIGN-CALC 

(btw use QG at 4.5V as the LM5148 is 5V VCC)

WEBENCH likely chooses FETs for lowest losses (of their calculations), and chooses best high-side FET which is usually lower QG, and best low-side FET which is usually lower RDSON.

Hope this helps,

-Orlando

Hi Orlando,

The simulations explains it - Thanks