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ALTAIR04-900 spontaneously begins to oscilate and falls into burst mode

Question asked by shishkin.michael on Mar 4, 2014
I encountered some strange behavior of power supply based on ALTAIR04-900.

Most of the time it works as expected but sometimes the voltage on COMP pin begin to oscilate with negative spikes by 0.2-0.4V each 5-7 microseconds.
They can even drop bellow burst-mode threshold which lead to ALTAIR04-900 turnoff for 500 microseconds. It leads to the drop of output voltage.

Here is the oscillogram
  • Yellow - COMP pin voltage.
  • Green - secondary winding switching voltage.
  • Purple - ALTAIR04-900 Vcc voltage (about 23V)
  • Red - rectified bus voltage (about 330V).
At the left everething is ok. In the middle yellow line (COMP voltage) drops, oscilates and kicks IC into burst-mode.
After 500 mks IC starts switching again.

Here is the zoomed oscillogram of the crash.
There are negative spikes on Yellow (COMP pin) not connected to power switching and votage sampling.

I can't understand the reason of COMP pin oscilations since there is no power transistor switches and no secondary voltage sampling.
What could be the reason for such behavior?
Is there are any workaround (such as increasing COMP pin capacitor)?