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Original question:

Short Circuit Protection with SEPIC DC-DC Converter Topology

LM3481-Q1: Overcurrent protection

Dongbao Zhou
Expert 4170 points
Part Number: LM3481-Q1
Other Parts Discussed in Thread: LM3481,

Hi Team,

My customer use LM3481-Q1 to provide bias supply for IGBT module, they can't power it up. Here is the SW waveform of LM3481, it very similar to a hiccup though no such kind of description in d/s, and Vsense is 120mV. According to https://e2e.ti.com/support/power_management/isolated_controllers/f/188/p/630178/2325304?tisearch=e2e-sitesearch&keymatch=lm3481%20short#2325304, overcurrent protection may be triggered, but i don't find the description in the datasheet. 

This problem can be solved by reducing the switching frequency from 400KHz to 300KHz, just wanna to double confirm with you the possible reason of this issue.

Thanks.

Dongbao

  • 0
    TI__Guru* 78715 points
    If you lower the frequency and you can get it operates, you may run into the min ON time issue. At higher frequency the min ON time translates to larger duty cycle, which may be greater than your circuit need to maintain output regulation. Therefore, after switching a few cycles, the output voltage become higher than regulation setting point, then the COMP falls and stops to switching until the voltage drops down a little. In such case, you will have to reduce the switching frequency if you don't want the circuit operates in such mode.
  • 0 in reply to Youhao Xi
    TI__Expert 4170 points

    Hi Youhao,

    What about the OC trigger condition? What's the meaning to Vsense? A typical value is 160mA

    Dongbao

  • 0 in reply to Dongbao Zhou
    TI__Guru* 78715 points
    Hi Dongbao,

    Vsense is the peak current sense voltage avoiding hit the cycle by cycle peak current limit. The signal unit is in mV, which is the product of current and the current sense resistor. You should calculate your peak current at full load, and select a sense resistor such that the resulted peak voltage will not hit this threshold, in order to delivery the full load.

    Thanks,
    Youhao