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BQ2000T design help

Matko Orsag
Prodigy 40 points
Other Parts Discussed in Thread: BQ2000T, TPS40210

I would like to design a battery charger using BQ2000T, for a 24V NiMH battery pack with a 2A charge current. I looked at reference designs in bq2000T (fig 11) datasheet and DV2000S1 datasheet (last fig). I am wondering, is there some reference material on how to design switcher circuits (that connect and disconnect the power supply to battery) used in this examples.

  • 0
    TI__Guru**** 150900 points

    Yes, the bq200T can charge a 20 cell nickel pack.  What is your input voltage?  You will need an input at least 20 * 1.7V = 34V to fully charge this pack.  If your input voltage is not that high, you will need to boost it with the TPS40210.

    I'm not sure what kind of reference material you are asking for.  You can use the schematic in figure 11 and change out some of the components for higher voltage ones.  Mainly Q1, L1 (higher current), C5, C6, and maybe the gate drive/level shift circuity would need to be changed as well as the current sense resistor and BAT voltage divider.

     

  • 0 in reply to Chris Glaser
    Prodigy 40 points

    Thank you for your answer, I was asking for some material that could help me design resistors for transistors in that switcher, so that they can switch 2A >30V DC source for the battery. I think that is what you said when you mentioned hate drive/level shift circuits...

    Thanks

  • 0 in reply to Matko Orsag
    TI__Guru**** 150900 points

    I'm not sure I understand what you mean by switch the 2A 30V DC source.  The bq2000T drives a buck converter circuit that chops up the input voltage via a PWM signal to generate the proper charge voltage/current for the battery pack.  It does not simply apply and disconnect the 30V source from the pack, but provides a regulated current or voltage to the pack.

    You don't have to use a BJT for Q1.  A FET could be more efficient.  The main reason for the level translation is because MOD is only 5V or 0V.  Either of these voltages turn on Q1 since they are well below the input voltage.  So, a circuit must be implemented to allow the 5V to 0V MOD signal to switch between Vin and ground to turn on and off Q1.

    If using a FET, you need to pay attention to its gate source voltage rating.  Usually, 30 or 40V FETs cannot take more than 20V on their gate to source difference, so you will need to add a zener diode in series with your gate drive circuit.

  • 0 in reply to Chris Glaser
    Prodigy 40 points

    Thank you for your answer, but I am still wondering how to configure transistors Q2 & Q3 and their corresponding parts (i.e. C5, R5, D3 and R3). Bottom line of my question was how to choose this parts.

     

  • 0 in reply to Chris Glaser
    Prodigy 40 points

    One more question regarding your earlier post, do you suggest using a 48V source, or charging via additional 48-34V converter?

    matko

    Chris Glaser said:

    Yes, the bq200T can charge a 20 cell nickel pack.  What is your input voltage?  You will need an input at least 20 * 1.7V = 34V to fully charge this pack.  If your input voltage is not that high, you will need to boost it with the TPS40210.

    I'm not sure what kind of reference material you are asking for.  You can use the schematic in figure 11 and change out some of the components for higher voltage ones.  Mainly Q1, L1 (higher current), C5, C6, and maybe the gate drive/level shift circuity would need to be changed as well as the current sense resistor and BAT voltage divider.

     

     

  • 0 in reply to Matko Orsag
    TI__Guru**** 150900 points

    A 48V source is fine.  You don't need another converter in front of it.  Just make sure all your components are rated for the higher voltage.

    Those components just speed up the turn on and turn off of the BJT so as to lessen the power lost when switching.  You can simulate it in spice to see what values work best with your transistors.

    Before it starts switching, all those nodes are at Vin potential and Q1, Q3, and Q2 are off.  When MOD goes high, Q1 turns on.  Pin 1 of C5 immediately goes 5V lower, as the voltage across it (0V) must not change instantaneously.  (MOD is a 5V signal).  This turns on Q3 through D3 very quickly.  C5 charges up and R5 and R4 limit the base current.  MOD goes low, Q1 turns off and C5 now jumps up 5V.  This turns on Q2 through its base emitter diode which turns off Q3 quickly.  R3 keeps it off after C5 returns to steady state with 0V across it.

    So, Q2, D3, and R3 can likely stay the same value as long as their voltage ratings support your higher voltage.  R5 and C5 may need to be tweaked to optimize the turn on and off of the BJT.