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VIPER17 Startup problem

Question asked by nemets.lev on Feb 20, 2015

We are using VIPER17 in our products as a standby power supply for a couple of years in mass production. Recently we faced a problem that if the mains is off and on again this power supply can not start-up. Mains has to be off for a long time (more than hour) for this power supply to be able to start-up. The problem is not related to the heat, input voltage or load conditions. Replacing IC with another from old stock fixed this problem.  Increasing the value of CDD ( to pin 5)also fixed this problem.  Our tests show:

- after mains was OFF for a long time (>2Hr), discharge current IDDch was 2.4mA and unit started  successfully .

- if Mains was OFF  for short  time (but bulk capacitor is still  charged) , discharge current IDDch was 3.43mA and unit could not start.

- if Mains was OFF a bit  longer time (bulk capacitor was fully discharged) , discharge current IDDch was 2.95mA and unit could not start.

Any suggestions why the value of   IDDch is changing?