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LM3481: Why output can not recover after OCP cleared?

Zoe Yang
Intellectual 2210 points
Part Number: LM3481


Tool/software:

Hi team,

My customer use LM3481 to power IGBT. Vin=24V from primary. Vout1=5V/GND---500mA Vout2=+15V/GND1 --200mA  Vout3=-15V/GND1---200mA. Their test requirement is: Under full load situation, secondary side could test under OCP. After cleared OCP, the output should auto recovery. 

 Below is 5V full load with 500mA current. Yellow is load current green is Vgs.

Below is Vout1 was shortened. Yellow is short current, green is Vgs

After short circuit is cleared, yellow is normal load current but Vgs could not recover from LM3481. Normal working frequency is 130kHz， OCP frequency is 12KHz。

Can you help advise how to recovery Vgs to normal working conditions after OCP cleared? 

  • 0
    TI__Mastermind 18475 points

    Hi Zoe,

    Thanks for reching out. Can you please provide the complete schematic as PDF?

    Also can you please measure Vout and current sense pin in your scopeshots above?

    Thanks and best regards

    Moritz

  • 0 in reply to Moritz
    TI__Mastermind 31690 points

    Hi Zoe,

    I received a mail request on this exact issue from your colleague Racheal.
    The response is summarized here:

    Please note that the LM3481 has two protections for overcurrent events:
    - overcurrent protection (160mV threshold)
    - short-circuit protection (220mV threshold)

    So far, customer talked about the short-circuit protection, but I would like to clarify the different effects of each.

     

    Overcurrent protection triggers first and turns off the gate if Vsense goes above 160mV each cycle. This will limit the duty cycle.

    If the ramp on Vsense is so steep, that it goes above 220mV before the gate is turned off from the regular OCP, the additional short-circuit protection should activate and frequency foldback should start.

     

    We always recommend to design the schematic for the 160mV OCP threshold, as this is the main overcurrent protection feature that activates first.

    It is possible that the OCP with duty cycle limitation is triggered during startup, as the output capacitors are charged as well, which adds to the load condition. However, the device is theoretically still capable of ramping up the output voltage in OCP state, but it is rather recommended to implement a softstart instead.
    Here is an application note that proposes a secondary side softstart implementation for flyback applications:
    https://www.ti.com/lit/an/snvaa44/snvaa44.pdf

     I will continue discussions via mail, so I will close this thread in the meantime.

    Best regards,
    Niklas