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