We have used the MP4559DN in a DC-DC converter producing +15V from +24V to +55V input voltage. The nominal current load is equal to 0.8A.
The switching waveform on the chip pin 1 looks as expected with the nominal 0.8A load current.
With decreasing load current (using an electronic load) the waveform becomes more and more "jittery" on the oscilloscope screen. The picture below shows the switching waveform with 0.3A load current.
The switching frequency is expected to decrease with reduced load, but it also appears to be less stable. Is this normal or a sign of the DC-DC converter malfunction?
Also, when increasing the input voltage from 24V to 55V with constant 0.8A load current the switching waveform can become unstable on the oscilloscope screen when the input voltage is around 50V or higher. On a few occasions the waveform as on the following picture has been captured.
What could be reason for this?
We are greatly concerned whether the MP4559DN can work with input voltages higher than 50V as we had one failure where the MP4559DN switch short circuited when powered from 55V damaging other two DC-DC converters on the same board powered from its 15V output. The exact cause of this failure remains unknown.
Any suggestions what to check would be appreciated.