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Original question:

BQ24030: USB and Solar Charger with Power Path (BQ24030 / BQ2407x vs. BQ24210 or MCP73871)

BQ24210: Use of Two Solar Panels and USB Like Input

Gary DeWitt
Prodigy 90 points
Part Number: BQ24210
Other Parts Discussed in Thread: LM74700-Q1

Hello,

I am using the BQ24210 as a charging circuit for a 5000 mAH battery in my application using 2 solar panels and/or a USB type input as the power source.  I utilized the BQ24210EVM to develop my application and it is working well.  The three inputs each feed into the VBUS through a Schottky diode (3 diodes).

The only jumpers I have set on the EVM board are JP4 (LPCM) closed and JP1 (iSET to 500 mA) closed. 

One primary objective is to have the current coming in from the solar panels be additive.  That is, due to sun angles, if one solar panel is exposed to produce 150 mA of charge current out of the BAT output and the other panel is exposed to produce 50 mA at the BAT output, the total charge current will add up to 200 mA for the battery charge current. 

Using this approach works fine with using the BQ24210EVM prototyping board in my large prototype of the system.  I can place one panel in full sun and the other covered and get 150 mA of charge.  Covering the first panel and exposing the second panel also produces the expected 150 mA of charge.  Exposing both at the same time produces the desired 300 mA of charge to the battery.

The EVM circuit was duplicated (without all of the extra option components) on a PCB with the connections the same as if the JP1 and JP4 connections are closed.  

During test, the input of each panel works fine but they are not additive.  Whichever panel has the larger current drives the output BAT current.  Using the example above, illuminating one panel at a time results in each producing 150 mA on its own but with both exposed the output is still only 150 mA.

Here is an extract from my PCB schematic.  The large blue lines were added during debug.  The one marked 1 goes to an ADC to determine input current value and the /EN input comes from the output of a GPIO pin on a processor.  The USB like input is from a 12V input stepped down through a 5 volt regulator - this is not connected during the above testing.  However, it was previously tested without the panels exposed to sunlight and the charge current varies based on charge level of the battery during the life cycle of the LPCM type charge.  LPCM was chosen since we are only interested in trickle type charging from the solar panels and 12v input.  

Do I need to set a bias on VDPM or the TS/VTSB inputs?

  • 0
    TI__Mastermind 19195 points

    Hi Gary,

    This is most likely due to issues with ORing of the diodes. See section 3 of this document explains what is happening. "Forward voltage drop of the schottky diode has a negative temperature co-efficient and it reduces with increasing temperature. This can lead to situation where a single supply carries the entire load current though second supply is still present and results in increased junction temperature TJ ."

    Section 8 of document explains how this issue can be resolved by using an ideal diode controller like LM74700-Q1 instead of normal shottky diode.

    Best,

  • 0 in reply to Nick Brylski
    Prodigy 90 points

    Hi Nick,

    Thank you very much for the rapid response - it was a pleasant surprise after some of the experiences I have had with other Forums.

    I followed up with your suggestion and reviewed the LM74700-Q1.  That was a fairly complex change to add for each input.  I further investigated the documentation on both the BQ24210 device and the associated EVM.  I found a note hidden in a cell of a row in one of the tables that VBUS expects both a 10 uF and 0.1 uF capacitor at the VBUS input (no explanation of why).  Even though I doubted that would work, I tacked those devices onto the PCB and the function is now working as desired (the inputs are additive in current flow).

    Again, I appreciate your support and will get back to you if there are any other questions.

    Best regards,

    Gary DeWitt

  • 0 in reply to Gary DeWitt
    TI__Mastermind 19195 points

    Hi Gary,

    The 0.1 uF cap is for high frequency noise filtering. It is optional and will depend on noise coming from source and noise requirement of load. 

    5/22/20 update: The 10uF is required (min 1uF).

    Best,

  • 0 in reply to Nick Brylski
    Prodigy 90 points

    Hello,

    I have an additional complication that showed up during testing.  With the solar inputs providing up to 5 volts in to their respective isolation diodes, everything is working fine.  If a 5.5V dc source is placed at the inputs in place a solar panel, the circuit works fine.

    However, when I connect the 12 v (with no solar panel inputs) source through the LM2940 regulator, the regulator reached over 70 degrees C (very hot).  With the charge current set to about 500 mA by the 750 ohm resistor, I thought that the power dissipation was from the regulator would raise the temperature too high for comfort.  I placed a 1.8K resistor in the current limiting circuit and the current went down to 214 mA max which allowed the regulator to cool down to about 50 degree C - still pretty hot.  But I also noticed in this configuration that the BQ24210 was reaching 70 degree C (way too hot).  

    The output of the regulator is 5V dc.  If I disconnect and place a 5.5V dc source on one of the solar inputs I would think the solar charger (BQ24210) would heat up since this was a higher input voltage than from the regulator (5V).  However, it was running very cool (30 degree C) and the circuit still produced the 214 mA max setting that the limiting resistor was set at.

    I am puzzled why the regulator and solar charger would react this way. The filter capacitors were already installed for the Vbus input as discussed above.

    Please let me know if I have to open a new question or this can serve as the vehicle for obtaining obtaining support.

    Attached is a copy of the updated.

    Thank you

    circuit used for testing.

    Thank you

  • 0 in reply to Gary DeWitt
    TI__Mastermind 19195 points

    Hi Gary,

    Yes, next time would be best to make a separate thread if the issue changes. For now we can discuss here.

    This seems like the BQ24210 is acting a heatsink for the regulator. Moving the IC's further apart on the board could help. Just to confirm, when 200mA charging through the solar panels you do not see BQ24210 with high temperature? Only when regulator is the source?

    Here is one of the layout guidelines we recommend: It is critical that the exposed thermal pad on the backside of the bq24210 package be soldered to the PCB ground. Ensure that sufficient thermal vias are right underneath the IC, connecting to the ground plane on the other layers.

    Can you show what the layout looks like? 

    Best,

  • 0 in reply to Nick Brylski
    Prodigy 90 points

    Hi Nick,

     

    Here is the layout.  I believe I missed the vias to the other side.   I will get that corrected.  Not shown here is the polygon ground fill on top and bottom of board.  Will send you an actual pic of that area with the mods done to add the filter caps to vbus.   I do not believe the devices are close enough together to have the '210 be a heat sink for the regulator.  I have plenty of real estate and placed components comfortably apart.

     

    Yes, the high temp is only when the regulator is used to feed the charger.  With the solar panels only connected, I could actually charge at 300 – 400 mA without high temperature on the BQ24210. 

     

    And the regulator is still running over 50 C at 214 mA charge current along with the ‘210 running hot.  I can put the same power supply used for 12v and set it to 5.5v and feed that DC through one of the solar panel inputs and get no temp rise out of the ‘210.  Very puzzling to me.  Could the regulator be oscillating?  I have not put a scope on the 5V output – just a meter to ensure it is at 5V.

     

     

     

     

     

  • 0 in reply to Gary DeWitt
    TI__Mastermind 19195 points

    Hi Gary,

    Try moving the 0.1uF and 10uF caps as close as possible to BQ24210 VBUS pin (not having these was definitely causing the initial ORing issue). See EVM layout for reference.

    There could be oscillations from the LM2940, take a look at section 8.2.2.1.2 of LM2940 d/s. ESR of the output cap needs to be within a certain range.

    Best,

  • 0 in reply to Nick Brylski
    TI__Mastermind 19195 points

    Hi Gary,

    Any updates on this?

    Best,

  • 0 in reply to Nick Brylski
    Prodigy 90 points

    Nick,

    In going back through the spec several times, I came across this part of the spec. It turns out that if you connect to a wall charger putting out a USB like charge source (which is what I am doing with the regulator), you have to have a series resistor to ground out of the VDPM pin. I have set up the “programming” or settings of the charger to follow the battery level where you leave VDPM open.

    I concluded that it might be a good idea to add a MOSFET switch to the VDPM output to switch in a resistor when charging from the fixed, low impedance output regulator and open when the +12V input is not available for charging the battery.

    I am in the process of procuring the parts to do this modification. Do you think that leaving that pin open during the connection to the USB like input could cause the significant rise in temperature and, if you do, why would it manifest itself in that way?

    I have attached the spec reference and the resulting corrective circuit I am planning on using to test the operation with the switched in/out of the resistor.  The input to the circuit below is the 5V output of the regulator and causes the resistor to switch in when the +5V is active out of the regulator.

    I will let you know the result of the breadboard test of this circuit.  

    Regards,

    Gary DeWitt

  • 0 in reply to Gary DeWitt
    Prodigy 90 points

    By the way, the ESR of the capacitor chosen is 500 mOhms - dead center of the range of ESR operation in the spec.

  • 0 in reply to Gary DeWitt
    TI__Mastermind 19195 points

    Gary,

    What has lowest voltage regulator has been outputting? If regulator is falling under 4.6V you will see reduction in current from adding the resistor. 

    Best,

  • 0 in reply to Nick Brylski
    TI__Mastermind 19195 points

    Gary,

    Also recommend adding copper to the left of the IC for better heat dissipation, there is a section of it missing. 

  • 0 in reply to Nick Brylski
    Prodigy 90 points

    Hi Nick,

    I saw that and have that on my list to fill those areas in when updating the board (should start in a week or so to update and go to second generation.

  • 0 in reply to Nick Brylski
    Prodigy 90 points

    Nick,

    It is a TI 5.0 V regulator - it should not drop below 4.9 V.

    Gary

  • 0 in reply to Gary DeWitt
    TI__Mastermind 19195 points

    OK, should be no difference with VIN DPM pin changes then.

    Best,