LTC4412 Voltage Drop after diode

Hello,

i'm building a UPS around LTC4412 to switch between wall adapter and battery. I have the following problem:

When i connect a load, almost the whole voltage drops across the diode, so the load never turns on.

Did i connect mosfet wrong?

Below are my schematic and PCB layout.

Thank you for your help.

Regards Jan

Parents
  • 0
    •  Analog Employees 
    on Aug 23, 2021 5:36 PM

    Hi Jan,
    Can you provide a larger picture of the schematic? The picture is too small.

    What is the nominal input voltage of the “18650” supply?
    What kind of load are you connecting when you report that the load doesn’t turn on?

    It is suspicious how you see the entire supply voltage drop across the diode. If the “19V” supply is present, D3 is forward biased, so you should get ~18.7V on the output. Maybe D3 was installed backwards. Let’s check this:

    Power off the circuit. Using a multimeter in diode mode, place the positive lead on the “19V” supply, and the negative lead on the output. You should see 0.3V to 0.7V. If the meter says "OL", reverse the leads. If you see 0.3V to 0.7V now, D3 is installed backwards.

    Now place the positive lead on the “18650” supply, and the negative lead on the output. You should see 0.3V to 0.7V (from the FETs body diode).

     

    -Aaron

  • Hi Aaron,

    the nominal voltage of the 18650 input is about 5 volts. And "19V" is 12 volts. I used these voltages for testing.

    The load that i connected is a DC motor and the LTC3780 board.

    As mentioned when there is no load connected i see the same voltage before and after the diode. As soon as i connect a load almost the whole voltage drops at the diode.

    In the meantime i tried the LTC4412 with two Mosfets as it's suggested in the datasheet. That seems to be working fine.

    Unfortunately my Multimeter doesn't support diode testing, so i attach a picture of the mounted PCB.

    Here is a larger picture of the schematic:

    Thank you for your help.

    Regards Jan

Reply
  • Hi Aaron,

    the nominal voltage of the 18650 input is about 5 volts. And "19V" is 12 volts. I used these voltages for testing.

    The load that i connected is a DC motor and the LTC3780 board.

    As mentioned when there is no load connected i see the same voltage before and after the diode. As soon as i connect a load almost the whole voltage drops at the diode.

    In the meantime i tried the LTC4412 with two Mosfets as it's suggested in the datasheet. That seems to be working fine.

    Unfortunately my Multimeter doesn't support diode testing, so i attach a picture of the mounted PCB.

    Here is a larger picture of the schematic:

    Thank you for your help.

    Regards Jan

Children