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BQ24650EVM-639: BQ24650EVM-639

Mike Heath
Prodigy 20 points
Part Number: BQ24650EVM-639
Other Parts Discussed in Thread: BQ24650

Tool/software:

I have set up the EVM with a set of resistors so that I can select 1,2,3,4,5 or 6 cell charging.

The open circuit voltages at Vout are all correct, as calculate and measured, but the EVM will only charge 1,2,3 and 4 cells.

I have checked the voltages at the test terminals and all seem to be correct.

Is there something I am missing or could this be a faulty module?

//Mike

  • 0
    TI__Mastermind 31551 points

    Hello Mike,

    What is the input voltage?

    Can you provide the resistor divdors for each cell charging voltage?

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 20 points

    The input voltage was 25V.

    The values I calculated are as follows:
    Vout = 2.1 x (1+R13/R15)
    R13 =499k
    1 cell, Vout max = 4.2V so R15 =500k
    2 cells, Vout max = 8.4V so R15 = 499/3 = 165k
    3 cells, Voutmax = 12.6V so R15 = 499/5 = 99k
    4 cells, Vout max = 16.8 so R15 = 499/7 = 71k
    5 cells, Voutmax = 21V so R15 = 499/9 = 55K
    6 cells, Voutmax = 25.2 so R15 = 499/11 = 45k

    I Checked all the Vout voltages with a voltmeter and all were just under the max voltage for the number of cells.
    Further experimentation showed that all 6 cells can be charged, but there is an issue. I will explain.
    I had the current limit set to 500mA for all my tests. For 1 -4 cells this works fine, but for the 5 to 6 cells, the current varies between say 10 and 60mA , perhaps every second, or faster. When I set the current limit higher (it needs 1.4A) then charging seems to function. Even with the current limit set to 500mA, the charging LED is turned on to indicate charging, even though the current is so low.

    With cells 1-4, there is continuous operation if the input voltage is higher than the cell voltage, regardless of the current limit, but for cells 5 and 6, there is no charging unless the available current is more than 1.3A.

  • 0 in reply to Mike Heath
    TI__Mastermind 31551 points

    Hello Mke,

    For buck toplogy TI recommends an input voltage of at least 1.5 V to 2 V higher than the battery voltage, taking into consideration the DC losses in the high-side FET (Rdson), inductor (DCR), and input sense resistor (between ACP and ACN), the body diode drop of RBFET between VCC and input power supply, and battery sense resistor (between SRP and SRN). Power limit for the input supply must be greater than the maximum power required by either the system load or for battery charging (the greater of the two).

    I was able to replicate the issue with an EVM, it seems to be an issue with the power limitation of the power supply. I recommend increasing the input voltage or current limit. Another option is to reduce the charge current.

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 20 points

    I used a new resistor 0.08Ohms instead of the 0.02 ohms which came fitted. The only difference is that the current level has dropped from 1.3A to 0.4A. If I set the PSU current limit to less than 0.41Amps , then the EVM seems to go into a mode where it keeps trying to charge, every second or so, and indicates that it is charging, but it will only actually charge if the current is higher than set limit (R6 current shunt).

    This did not resolve my issue.

    Is there a better IC to do this job? This one seems to be 14 years old.

    //Mike

  • 0 in reply to Mike Heath
    TI__Mastermind 31551 points

    Hello Mike,

    Yes, it seems to be an issue with the power limitation of the power supply. The output power is requesting more than the input power can provide. 

    Input power=25V *0.4A=10W

    Output power= 25.2(6cell)*0.4A=10.08W

    The input power should be set higher than output power to support thermal and efficiency power loss.

    Mike Heath said:
    If I set the PSU current limit to less than 0.41Amps , t

    Why are you limiting the PSU current?

    Best Regards,

    Christian.

  • 0 in reply to Christian Moyer
    Prodigy 20 points

    I have a solar panel which can deliver 0.5A max. I am trying to simulate this on the bench. It works very well for a 4 cell battery where it allows current below the current limit, to charge the battery and power a device. I can do this on the bench easily, but as soon as I try 5 or 6 cells, the current limit is not a limit but set value, it never goes higher and as soon as the input current drops below that setting, the current goes to zero at the output.

  • 0 in reply to Mike Heath
    TI__Mastermind 31551 points

    Hello Mike,

    Did you change the value of the MPPSET resistors?

    Please refer to section 8.3.2 Input Voltage Regulation in the datasheet.

    In the default EVM the MPPSET voltage is set to around 17.8V, so when the current is limited on the power supply, input source cannot provide the total power of the system and BQ24650 charger, the input voltage drops (to 17.8V). When the voltage sensed on the MPPSET pin drops below 1.2 V, the charger maintains the input voltage (17.4V) by reducing the charge current.

    The input voltage is now 17.8V, and for the buck charging topology the input voltage has to be higher than the output voltage. I recommend changing MPPSET to 25V.

    Best Regards,

    Christian.

  • 0 in reply to Mike Heath
    Prodigy 20 points

    Thanks for your support. The system now works perfectly.

    I had read that section about MPPSET a few times but I didn't understand its function.