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LM7481-Q1: MOSFET burn out when turning on with capacitors as load

Florian Krieger
Prodigy 40 points
Part Number: LM7481-Q1

Hi,

i want to use this as a ideal diode. Power in on the left ist 10V to 60V. When i got a load above 250uF my MOSFET Q1B burns out when I turn it on. I use Power MOSFETS from Infineon (IPG20N10S4L-22). I also tried vishay (78-SQJB80EP-t1_GE3), same result. I guess i made a mistake with the inrush current limiting?

1. Scematic

In green the Voltage on the Output. In Yellow the current on the Output.

2. Turning on with 60V on Input and 47uF on Output.

2. Turning on with 55V on Input and 380uF on Output.

Unfortunately the current got cut off by my Current clamp meter. After a spike like this the mosfet has a resistance of 5Ohm to 10Ohm between drain and source and can not be switched any more.

Have you any advise of how i can make sure that i can power devices from this output with even greater capacitors to load?

  • 0
    TI__Guru 57737 points

    Hi Florian,

    During startup, the FETs are operated in linear region where there will be power loss due to voltage across the FET (Vin-Vout) and current flowing through the FETs (Inrush Current to charge the capacitors + load current). We need to ensure that the FET selected can handle the power loss for startup duration.

    Few ways to ensure safe startup of FET is by

    • Turning ON the load only after the Vout has ramped up close to Vin. This way we can ensure that the only current flowing through the FETs during startup is only inrush current which charges the output capacitor which reduces the additional power loss in the FET due . 
    • We can increase the Cdvdt capacitor to reduce the inrush current and reduce the power dissipation in the FET. This will increase the startup time though.

    We have design calculator 'FET-INRUSH-SOA-CALC — FET SOA margin calculator for dvdt based startup' which helps you choose Cdvdt and check if the selected FET has sufficient SOA to withstand the startup. The calculator can be downloaded from https://www.ti.com/product/LM7480-Q1#design-tools-simulation . Please go through the training videos in the link provided in the calculator before using the calculator.