• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 63 subscribers
  • Views 1215 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UC3906 pass transistor too hot

John Thomas71964
Prodigy 80 points
Other Parts Discussed in Thread: BQ24450, BQ2031

The transistor used is too hot. Is there an alternative where I can use a MOSFET? What is the bias resistor i shld use?

  • 0
    TI__Guru**** 150550 points

    Please look at the bq24450 EVM.  It has an option to use a MOSFET and an explanation on how to change the circuit for its use.

    But likely this will not help you.  The heat is from operating the transistor (FET or BJT) in the active region, not in saturation.  Thus, the power lost in the transistor is the charge current times the voltage across the transistor or roughly Vin-Vcharge.  You can use a buck converter to reduce your Vin or move to a switching charger, such as the bq2031.

  • 0 in reply to Chris Glaser
    Prodigy 80 points

    Can't i use a FET with smaller Rds?

    From the reference design, the input voltage is 18V. How much lower I can go?

     

  • 0 in reply to John Thomas71964
    TI__Guru**** 150550 points

    Like I said, this won't help you.  The FET will never be fully on so you won't see the Rds-on.  The FET is being operated as a resistor--not fully on or off.  If the FET were fully on, you would have no control over the charge current.

    To determine your minimum Vin, I need to know what is your maximum charge voltage and which schematic (configuration) are you using?

  • 0 in reply to Chris Glaser
    TI__Guru**** 150550 points

    Just to clarify everything and my assumptions:

    If you are using one of the higher dropout transistor topologies--NPN emitter follower or PNP in quasi-darlington--then a FET may reduce your power lost in the transistor IF you are able to lower your Vin.

    Moving to a MOSFET does reduce the power loss by 2 methods--eliminating any base current and reducing the required headroom across the transistor.  The base current in a BJT does create some heat as it drops across the P-N junction (diode drop) but this is usually negligible.

    The real savings would be reducing the minimum deltaV required across the transistor.  Assuming you use figure 9 in the bq24450 datasheet--with Dext and a PNP pass element in the common emitter configuration.  As explained on page 13 of the datasheet, the different transistor configurations give different amounts of power lost in the driver IC (bq24450) and have different amounts of headroom, deltaV.  This deltaV is the minimum drop across the transistor.  Other drops include the 250 mV drop across the sense resistor and the drop across Dext.  Also, you will have drops in the PCB traces and any cables/wires between the input to the bq24450 and the batteries.  The sum of all these drops added on top of your maximum charge voltage is the absolute minimum voltage for the input to the bq24450.  The MOSFET is an advantage as it reduces the deltaV across the transistor from anywhere from 0.5V to 2.7V, depending on topology, to just the fast charge current times the Rds-on of the FET. 

    A key point to remember is that this only matters if you are able to drop your Vin to the minimum required from the above analysis.  If Vin stays the same, then the FET will still have the same drop across it as the BJT did--the transistor absorbs any extra voltage that isn't applied to the pack. 

    Keep in mind that all values, Vin, FET rds-on, Dext drop, etc. have tolerance, so it is not realistic to plan to operate with a minimum Vin that is calculated from the above analysis.  You need to have some margin there if you want to get the full fast charge current.  For any design, you need to increase the amount of copper on the PCB around the transistor and pick a suitable package for it--DPAK2 or TO-220 are good ones.