Power supply fails to start

[itdontgo]

Hi,

I'm a little baffled by this issue.  Initially this prototype board we have worked fine.  It then appeared to struggle to start but touching the soft start capacitor made it run.  Eventually though it would not reliably start regardless unless you started it at 30V!  This is the same circuit we have built many times although in the past it has had a different PCB layout.  The main supply chip is the AOZ1284

http://www.aosmd.com/res/data_sheets/AOZ1284PI.pdf

When it runs it's a nice power supply with low noise.  All of the main parts have been changed.  When we changed the coil it worked perfectly but this had nothing to do with the coil.  We accidentally knocked the boot strap capacitor off.  It always starts with this removed, however, runs noisily at high voltages but does not take excessive current.    I'll be honest I don't understand the issue.  The general PCB layout is good when it runs.

[Apollyon25_]

Sounds like you took a perfectly good design and changed too much of it.
Schematic, PCB??

You have all the keys to answer this - you've mentioned many of them.

[rx8pilot]

post the schematic and all layers of the pcb. It's almost always the
 layout. Few people give enough credit to the impact of parasitics of the PCB. They will stop any converter.

Sent from my horrible mobile....