top257 Y

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Hi we are using the top257y without OV and OC. X and V are connected to the source. It is an off line flyback 12V 3A supply.
Most of the supplies work but about 1 in 10 won't start up . They start , the output voltage reaches, 4V in about 8ms, stays at 4V for about 50ms then it shuts down for 2s and tries again.
I have tried it loaded/unloaded, Low frequency/high frequency no difference.
Changing the TOP doesn't fix so it has be the external circuitry but what ?

One thing you might check is the phasing of the transformer. If the transformer is constructed with either the secondary or the bias mis-phased, you can have startup problems, and the supply output will collapse with just a light load.

The voltage at the anode of both the output rectifier and the bias rectifier should swing negative when the primary switch is on - when the switch is off, the voltage will be clamped to the output voltage plus a diode drop. If you are not seeing this, then the transformer has been wired incorrectly.

.Also, if the transformer for some reason is not gapped, you can also have startup problems, as the transformer core will saturate almost immediately and trigger the TOPSwitch fault protection. To detect this, check the inductance of the primary winding.

It is also a good idea to check for things like solder shorts and reversed components.