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Power dissipation of switching Mosfets in Bq24616

Rajesh Kaliyaperumal
Intellectual 930 points
Other Parts Discussed in Thread: BQ24616

Hi everyone,

We are using FDS8447 as a switching mosfet for the PMIC (BQ24616) . As per the theoretical calculation  mentioned in the datasheets ,the Power dissipation of the Mosfet  (Including Conduction and Switching Loss ) gives a temperature rise of about 5 deg cel from Ambient temperature . But the actual temperature read near the Switching Mosfets in PCB gives an rise of 22 deg cel which is contrary with the theoretical calculation.

Actual temperature is measured using an thermistor which gives an accuracy of +/-1 Deg cel and no inaccuracy of reading the temperature.

What could be reason for the change in theoretical and practical value of the switching mosfets power dissipation ??

Regards

Rajesh K

  • 0
    TI__Genius 11480 points
    Hi Rajesh,

    There could be a couple factors here:

    1) Using Rtheta- ja for temperature rise calculations is very often mis-used. You can refer to this document on how to more accurately calculate thermal rise: www.ti.com/.../spra953c.pdf

    When you boil it down, it is an inexact science, unfortunately. There are main factor that a system designer has control over is the PCB layout and copper area around the part. At the end of the day, using Rtheta will usually result in imprecise temperature rise calculations, but is a good metric for "relative" thermal performance against other packages and competitors. This is all because the thermal performance depends on so much more than just the package type.

    2) If operating in non-synchronous mode, the device will turn off the low side FET. This means that the switching current will then flow through the body diode only, resulting in worse efficiency