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## Discharge Circuit MOSFET selection-Safe Operating Area

Level 1
Level 1

I'm using an Infineon MOSFET IPP60R120P7 in a discharge circuit. The MOSFET blows up during the discharge test. The total output Capacitance is 6.2 uF. The line to line voltage can be either 55V or 380V during discharge. And the total discharge resistors is 9.4 Ohm.

I was looking at the Safe Operating Area in the datasheet. I’m not sure how to calculate the Vds for the SOA.

For example, at 55V operation, Idsmax=55/9.4=5.85A and Vds=Rdson*Ids=0.58V (Rdson=0.1 Ohm)

And at 380V operation, Idsmax=380/9.4=40.4A and Vds=Rdson*Ids=4.04V (Rdson=0.1 Ohm)

Then checking the Vds and Ids combination in the datasheet SOA.

Is the above calculation correct to check the SOA? (Considering we get the right Rdson from datasheet based on temperature..)

Could you please advise me how to use the Safe Operating Area in the datasheet for a discharge circuit?

And how to select the right MOSFET for the application.

Thank you,

1 Solution
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## Re: Discharge Circuit MOSFET selection-Safe Operating Area

Moderator
Moderator

Hi @Siamak619 ，

Since your application is similar to load switch, so another application note attached for you, which has detailed derivation of modeling and drive control under capacitive load of MOSFET. Hope it helps.

Additionally, if stray inductances can be well limited in your system, the peak ringing will not be out of control when MOSFET is turned off, so you can choose a lower VDSS device.

Regards,

Neo

3 Replies
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## Re: Discharge Circuit MOSFET selection-Safe Operating Area

Moderator
Moderator

Hi @Siamak619 ，

Your calculation is correct, but only RDS limit has been verified.

In fact, the SOA of power MOSFET consists of 5 limit-lines, which illustrates below:

In your application, the load of the MOS is capacitive, a large inrush current is expected at MOS turn-on transition, huge Vd(t) * Id(t) overlap can easily damage the device.

You can get the V(t)/I(t) information by modeling or performing bench test, find out the most dangerous V(t)/I(t) point and plot it out in the SOA curve.

Note that the SOA curve is also temperature-dependent and must be derated when the ambient temperature rises.

Regarding to how to select the right MOSFET in your application, here are some suggestions:

1. According to the formula Vc= Vin * exp( -t/RC) derive the discharge time t_dis.

2. Slower but higher power MOSFETs maybe better suited for your application.

3. Carefully design the drive circuit to avoid excessive current spikes.

I attached some Infineon ANs for your reference.

Regards,

Neo

Attachments are accessible only for community members.

## Re: Discharge Circuit MOSFET selection-Safe Operating Area

Moderator
Moderator

Hi @Siamak619 ，

Since your application is similar to load switch, so another application note attached for you, which has detailed derivation of modeling and drive control under capacitive load of MOSFET. Hope it helps.

Additionally, if stray inductances can be well limited in your system, the peak ringing will not be out of control when MOSFET is turned off, so you can choose a lower VDSS device.

Regards,

Neo

Level 1
Level 1

Thank you, Neo.