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TPS54260 internal FET on resistance / current limit is changed

Chethan KS
Prodigy 30 points
Other Parts Discussed in Thread: LMP8645HV, TPS54260

Dear TI team,

We are using TPS54260 with LMP8645HV as constant current source to charge a capacitor bank of 100mF with a peak current limit of 3A. Once the capacitor bank is charged, the current requirement of TPS54260 is less than 1A. 

Input Voltage : 48V

Output Voltage : 42V

Load : 100mF capacitor Bank (Alluminium Electrolytic 100V rated ) connected through 1 mtr wire. (47uF on PCB)

PCB : 4 Layer (120 cm2)

Heat Sink : >5 cm2 area each on top, ground and bottom layer

With 3A current limit, capacitor bank was charging to 42V in less than 3 seconds. It was working fine for last two months without any problem. But today when its powered ON, it is taking >90 seconds to charge 100mF capacitor and >30 seconds to charge 33mF capacitor. TPS54260 is limiting its current to less than 50mA. But it is maintaining 42V as set by feedback resistor divider.

Is it due to increased on resistance of internal FET or change in internal current limit and abruptly how it can happen? Please help.

  • 0
    TI__Guru**** 191445 points

    Can you post your schematic and some waveforms?  be sure to include PH and COMP.

  • 0 in reply to JohnTucker
    Prodigy 30 points

    Dear JohnTucker,

    I have attached schematic and waveforms for your reference. Thank you.

    Regards,

    Chethan K S

    TPS54260.zip
  • 0 in reply to Chethan KS
    TI__Mastermind 35155 points

    Hi Chethan,

    Could you get a new screenshot that zooms in on the PH waveform while it is charging up the capacitor? The time scale should be small enough so that individual cycles are clearly visible.

    Does replacing the IC on this board fix the problem? Let's make sure it is a change in the TPS54260 IC and not something else on the board.

    Best Regards,
    Anthony

  • 0 in reply to Anthony F
    Prodigy 30 points

    Dear Anthony Fagnani,

     

    Board is sent for replacement of IC and it may take few days. I have attached the screen shots taken previously, (with and without load) at small time scales.

    Images (2) & (3) are with 33mF load (After charging period)
    images (4) & (5) are without load (After charging period)

     

    No pulses were seen during charging period with single 33mF capacitive load. (Screen shot was not captured)

    Image (1) shows the screenshot taken during charging period with time scale of 5 seconds. In this waveform,

     

    a)      Capacitor voltage is not building linearly and it is similar to RC charging.

    b)      No drop or dip in the input 48V power supply near TPS54260 IC was found during the measurement.

    c)       During charging period no pulses were observed at PH and it shows that TPS54260 was working at near 100% duty cycle and bootstrap capacitor was charging to required voltage since output was less.

    d)      Once output reached the set voltage of 42 volts, TPS54260 was observed to be running in pulse skipping mode to compensate for the leakage current in 33mF capacitor.

    e)      From image (1), RC time constant was used to estimate the ‘ON Resistance’ of TPS54260 and it is found around 200 ohms.

     

    Once we receive the board, we will carry out the tests. If TPS54260 'ON Resistance' is changed, what might cause it to happen?

    Thanks & Regards,

    Chethan K S

    TPS54260_2.zip
  • 0 in reply to Chethan KS
    TI__Mastermind 35155 points

    Something does seem to be damaged on this board. I do not think the high-side FET in the TPS54260 is even switching on. Could you check the COMP voltage? I would expect this to be at the high clamp value. This is typically in the range of about 1.5V.

    If replacing the IC removes this issue it will show the IC is damaged. In your application, are there any conditions which are at risk to exposing the TPS54260 to voltages that exceed the devices ratings?