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TPS40322: High Side Buck MOSFET too Hot

user5008999
Prodigy 10 points
Part Number: TPS40322

Hi,
I use TPS40322 to build a buck demo with two outputs.
Input: 5V
Output1: 4V/22A
Output2: 3V/22A

I design my circuit and layout according to TPS40322EVM-679
I changed the MOSFET to AP2606CMT for both high side buck MOSFET and low side SR MOSFET.
VBdss:25V
Rds_on:0.8mohm
Id:280A
Qg:75nC
Qgs:13nC
Qgd:38nC

When I power up the demo, I found the high side Buck MOSFET too hot.
ambient 25C, output 5A, the high side MOSFET is 60C,
ambient 25C, output 10A, the high side MOSFET will go up to 100C

I checked the gate waveform.
for low side MOSFET, rise time is 80ns, fall time is 20ns
for high side MOSFET, rise time is 100ns, fall time is 80ns.
and also high side gate amplitude is 4V, which is 1V lower than Vcc. Is it because of the bootstrap Diode's Vf?

my demo will be working in still air Natural Convection. No Fan will be used for cooling.
how can i cool down the high side buck MOSFET?
does it work if I add a Driver IC to enhance the Driving capability?

  • 0
    TI__Mastermind 24075 points

    Hello,

    With 5Vin, the BP6 voltage will be lower than 5V. Because of the internal boot diode Vf, the high-side gate voltage will be close to 4V as you observed.

    I suggest to use FET with low Vgs threhohold and low Vds rating.

    If high-side FET is still hot, consider to use two FET in paralle to reduce the conduction loss on high-side FET.

    Another method is to power TPS40322 VDD pin with a separate voltage higher than 5V, so  the high-side gate voltage will be higher.

    Thanks

    Qian