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BQ24650: Delay charging turn on with cap on MPPSET line

Part Number: BQ24650
Other Parts Discussed in Thread: MSP430F5419A

Hello,

   We have a design that uses the BQ24650 to charge a 13.8V, 10Ah LiPo battery. The problem is that when the system is powered from a solar panel, the battery charging starts immediately and causes motorboating. This is because the solar panel is not able to supply enough current AND the microcontroller (which sets MPP voltage, enable/disable charging etc.) doesn't have time to turn on to control the system.

So obviously we need a way to disable charging until the MCU can turn on. The optimal way would be to pull MPPSET to zero for a short time to allow the MCU to start. On the MPPSET pin we have a voltage divider of 523k connected to Vin, 47k to GND; and this has a 100nF cap to GND plus a 18pF cap to Vin. In this case Vin is the system input voltage, e.g. from the solar panel.

Question: What are the ramifications if I change the 100nF cap to 1uF? That would increase the MPPSET turn on time from 20mSec to 100mSec (according to simulation) which should allow the MCU to startup and disable charging until we can determine optimal MPP setting. The only downside that I could foresee is that this makes the system slower to change MPP tracking during a change in Vin. However, the timeframe we're talking about (100mSec) is still very fast compared to the speed of clouds across a solar panel. Are there any other downsides?

Thanks,

Derek

  • Dear Derek,

    I have an alternate solution. What if you had an NMOS at the MPPSET node? The gate of this NMOS would be pulled up to some conditioned voltage level from the input voltage with a large resistor value, say 100 kOhms and also connected to the open drain of your MCU. When the open drain is disconnected, the gate of the NMOS is pulled high and charge would be disabled (the drain of the NMOS is tied to MPPSET and the source is to GND). When the open drain pulls the gate low, the NMOS turns off and the MPPSET node is allowed to charge. This will only work if the open drain pin is OFF both when the MCU is off and when it powers up. Please verify that your MCU has this failsafe operation before making this change.

    Thanks,

    Mike Emanuel

    Please click "Resolved" if this answered your question.

  • Hello, I do have an NFET at the MPPSET node, and it is controlled by the microcontroller (MSP430F5419A), and has a pull-down resistor (100k) at the gate. Unfortunately I didn't add a population option to put a pull-up there, so adding a pull-up on that line would require a board spin. We already have about 100 boards out in the field so my goal is to do as minimal change as possible.  

  • Hello,

    What design do you have in place for different solar panel operating points? For example, when starting up, the panel transitions from supporting lower voltages do higher voltages. In addition, the temperature will affect the open circuit voltage of the panel, not to mention the irradiance will affect the short circuit current rating. What do you have in place to make sure every operating point will cause your MCU to turn on first?

    Thanks,

    Mike Emanuel

    Please click "Resolved" if this answered your question.