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BQ2000 reverse flow issue

Soumitra Bhattacharjee
Intellectual 810 points
Other Parts Discussed in Thread: BQ2000

Hi

I have an unusual issue with my setup. It uses a 5V/1A wall wart as the power source.
For 2 NiCD batteries in series this setup has been working fine but recently I needed to charge 4 NiMH batteries in series (1.2V AA type) ive had the LED indicator light up the moment the cells were connected (while the setup was not connected to the 5V 1A wall wart) suggesting a reverse current flow.

The switching mosfet was checked and its not shorted so I presume its probably the internal mosfet diode. Im using a TSM3457, which has similar specs to Si3455 and a Vds of -30V.

Would a series diode on the power rail between the BQ2000 VCC and the mosfet source pin be enough or is it a symptom of something else ?

Regards

Soumitra

  • 0
    Prodigy 80 points

    Soumitra,

    I don't think that this circuit will work to charge 4 NiMH cells. I think that this is a buck converter, where the input voltage has to be greater than the output voltage. So 4.8 volts going up to 6.0 volts is too high for a 5 volt input.

    I am sure that some one will correct me if I'm wrong, but I think that's right.

    Perhaps a TI person might suggest an alternative IC?

    Regards,

    Phil

  • 0 in reply to Phil Bromley
    Prodigy 80 points

    Soumitra,

    Thinking about this some more, You could raise the input voltage to say 9 volts, but then you have to ensure that the BQ2000 VCC does not go above 5 volts.

    I am using a similar circuit to charge a 3 cell NiMH battery. I have a resistor/zener diode combination to supply the IC with 5 volts. I also have two diodes from the 9v input, one supplies the IC, the other the pass transistor. This prevents any back current flow.

    That would be a better solution for you.

    Regards,

    Phil

  • 0 in reply to Phil Bromley
    Intellectual 810 points

    Hi Phil

    Thank you for your inputs. Yes I do have a provision for a 5v LDO for the IC ... just not used it yet. I can use a 12v 1A wallwart instead of  the 5V but I think a clamping zener (depending on the max supported charge voltage) at the gate of the mosfet would be required when using a higher voltage range. 

    So i guess its only the backflow of current which I can fix with diodes for the mosfet and the IC. That makes me breathe a little easier.

    Best regards

    Soumitra