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LM25017 switch is triggered twice

Jake Liang
Intellectual 310 points
Other Parts Discussed in Thread: LM25017

I am using a LM25017 to make a fly-buck converter to get +/-15V rails for our application.

Configuration: input voltage range: 18V ~ 30V,  normal voltage is 24V

                                Output rails at full load conditions:

+15V rail  with 50Ohm load.

                                                                   -15V rail with 125 Ohm load.

I have found:  At full load conditions, LM25017 internal switch gets triggered twice  when input voltage is lower than 24V. Please see attached captured waveforms  and schematics.

My questions are: 

1.             Is there any way to solve this problem?  Can it be solved by redesigning  transformer or adjusting  R54, C86 values?  It looks like that it is caused by the reflect current from secondary winding.  If I remove the load on secondary side, the switch works normally ( triggered once for each cycle).

 

2.            Is the converter going to have potential stability problems with the double triggered switch?

Notes on captured waveforms:

Channel 1  ( yellow) is the waveforms on SW ( pin 8 of LM25017)

Channel 2 ( blue) is the waveforms on FB ( pin 5 of LM25017)                     

TEK00020.tif is captured when Vin= 18V

TEK00021.tif is captured when Vin= 19V

TEK00022.tif is captured when Vin= 20V

TEK00023.tif is captured when Vin= 24V

  • 0
    TI__Genius 11490 points
    The tiny pulse as shown is normal. This represents leakage energy going through the high side FET body diode to input or charging the SW node capacitances.
  • 0 in reply to Vijay86929
    Intellectual 310 points

    Thanks, Vijay.

    It is good to hear that. it means that both switches work properly.

    Does it only happens during the time period that both high-side and low-side switches are open? otherwise, low side switch should hold it at low level.

    As you said this narrow pulse represents leakage energy flow back to input power, it should not impact stability of this converter. please correct me if I am wrong.

  • 0 in reply to Jake Liang
    TI__Genius 11490 points
    It happens during the deadtime when the low side switch turns off. It is only leakage energy of the transformer. I don't think there is any stability issue because of that. Most of the time the energy is utilized trying to change the sw node capacitances.