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bq24450 - HEATING OF NPN TRANSTER

SANDEEP BHANAGE
Prodigy 20 points
Other Parts Discussed in Thread: BQ24450

WE USE BQ24450 IC FOR CHARGE THE BATTERY FROM SOLAR CELL IN EXTERNAL QUASI-DARLINGTON MODE INPUT VOLTAGE IS 40 OR 60 WATT SOLAR MODULE /OPEN CKT VOLTAGE 21VOLT VMP IS 18VOLT TRANSISTOR USED IS TIP35C NPN & 2N2905 AS PNP  FOR EQD, BATTERY IS 40AH FOR 40WATT MODULE & 60AH FOR 60WATT PANEL BUT MY NPN TRANSISTOR GET HEATED WHILE CHARGING EVEN FOR LOW CURRENT 0.5 AMP ALSO WHY. GUDE ME TO REDUCE TEMP.

  • 0
    TI__Guru**** 150730 points

    The power lost in the pass transistor of a linear supply is simply the voltage across the transistor times the current through it.  This is (Vin - Vout) * Ichg.  If you have a large Vin to Vout differential, this power will be large.

    How hot it gets is a function of how much power is being dissipated in it, how it is heat sunk, and the ambient temperature.  You can do the above power estimate to see how much power is lost in the transistor.  You can also improve the heat sinking of the transistor to spread the heat from it.  This might include adding more copper around it on the board, use of a fan on the transistor, or implementation of an external heat sink for the transistor.

    The goal should be to keep it cool enough to meet system requirements.  This may include keeping it below the manufacturer's temperature limits or keeping it below some end equipment safety threshold.