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CSD13383F4: How to minimize power dissipation in this application?

Lauren Mayfield
Expert 7915 points
Part Number: CSD13383F4

 Hi FET team,

Asking a question from a customer-

I have to super caps in parallel. One is a 3F and the other 0.5F. I charge the small cap first to 1V. The big cap has this FET in series as a switch to GND. When the first small cap reaches 1V, I turn on the FET essentially connecting them in parallel. There will be a large current initially an then decays when equilibrium is reached at 167mV roughly (1/6)V.

The total series resistances are 0.3 ohm for the 0.5F cap and 0.07 ohm for the 3F cap. The FET is 44mohm according to the DS.

I turning on the FET with 3.3V VGS.

This part is very attractive for its small size although the max. sustained power dissipation is only 0.5W.

I can afford to add an additional series resistance but probably no more than 1-2 ohms.

The other thing I could do which I much rather do instead of adding series R, is to slowly turn on the FET by putting a large RC time constant on the gate thus increasing its RDSon and limiting the initial current spike.

What would you recommend?

Thanks,
Lauren

  • 0
    TI__Mastermind 38200 points

    Hi Lauren,

    Thanks for the questions and promoting TI FETs at your customer. You're correct that the only thing limiting the current thru the FET is its rds(on). Adding a series resistor would help limit the current but slows down the charge transfer from the smaller super cap to the larger one. For example, 1 ohm x 3F = 3s time constant. Not sure if that is acceptable for the customer's application. Adding an R-C is a common way to slow down turn-on of a FET. It's also going to slow down turn-off of the device. From my experience, I'd probably opt for the R-C time constant. However, we'd also want to make sure that the FET operates within SOA limits specified in Figure 10 of the datasheet. Should be able to do some relatively simple simulations. Are there any timing requirements? That would help determine the appropriate R-C values.

  • 0 in reply to John Wallace1
    TI__Mastermind 38200 points

    Hi Lauren,

    I'm going to close out this thread. Please contact me via regular email if you have any additional questions.