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UC2906 - The transistor is getting very hot

Lefteris Skoutaris
Prodigy 30 points
Other Parts Discussed in Thread: UC2906, UC3906

Hello everyone. I have built a circuit to charge a 24V/5Ah lead acid battery using UC2906. Although the datasheet is not very helpful/detailed, I tried to follow the instructions and get this working. But when the UC2906 gets in bulk charge state, the transistor is getting almost burnt... Probably something is wrong with my circuit, and that's what I ask for help to resolve. Here is the schematic:

A few data:

Configuration of the UC2906: DUAL STEP CURRENT CHARGER

Vbuck charge=24,6V

Vfloat=27,4V

Vholding current=27,6V

Ibuck charge=500mA

Iholding current=15mA

Vsupply=35Vdc

  • 0
    TI__Genius 13700 points

    Hello,

    What sort of heat dissipation do you have for the Q4?

    At the peak of power dissipation it is going to be pushing out over 5W in heat. Looking at the device specs it is rated for a max of 2W of dissipation.

    You are going to have to find a different BJT. While the NZT660 is capable of handling your charge current that continuous current rating is for when it is run in satuation.

    Look for a BJT that can handle the following characteristics of your circuit:

    -Collector-Emitter Voltage > 35V

    -Continuous Current > 500mA

    -Power Dissipation > 5W

    All of those are the minimums and I would recommend selecting parts with a good deal of head room especially in the power rating if the thermals of the board are poor and there is limited airflow.

    You are going to need a heatsink/large copper pour of thick copper to handle the power being dissipated by the BJT.

  • 0 in reply to Ryan May
    Prodigy 30 points

    Hello Ryan. Thanks for your suggestions. I have tried also TIP32AG which is in specs you suggest, but this gets very hot too... And also (unfortunately I forgot to mention) the UC2906 is getting very very hot too! And while I can use a heatsink for TIP32AG, I can't do anything about the UC2906. Plus, the UC2906 getting hot is very bad (according to the datasheet) because the compensation of the reference voltage is not correct (not following the real temperature of the batteries).

  • 0 in reply to Lefteris Skoutaris
    TI__Genius 13700 points

    Hello,

    A few notes:

    -Regardless of what BJT you use it will get rather hot since it is having to dissipate 5W of power. Even with a sizable passive heatsink it will probably get too hot to touch.

    When it comes to the UC2906 heating up I am thinking it is a function of the power it is sinking from the power BJT and the large input voltage you are working with.


    I would recommend looking at this design guide for the sister part the UC3906 in particular the Composite Follower Topology which should reduce the power dissipation in the UC2906 by as much as 2/3rds.

    http://www.ti.com/lit/an/slua115/slua115.pdf