Choosing topswitch based on current limit

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Hi,
For choosing topswitch in application note 16 (https://ac-dc.power.com/sites/default/files/product-docs/an16.pdf) is written start with the smallest TOPSwitch based on minimum current limit spec such that 0.9 x ILIMIT(minimum) ≥ Ip. So we can choose any topswitch with ILIMIT more than Ip/0.9 for a specified Ip(peak primary current) .
But in PI expert when I change the topswitch with more ILIMIT, this error appears : Peak flux density limit exceeded.Despite its ILIMIT has increased. I don't understand how can I choose topswitch.
Best Regards

First things - the transformer in a flyback converter is an energy storage device as well as a transformer. If you feed a given flyback transformer design with too much current. it will cause the transformer to saturate and (hopefully) trigger the overcurrent fault protection in the TOPSwitch.

Application note AN-16 was written during the time of TOPSwitch I, when each device in the family had a fixed current limit. It was important then both to select a device with a current limit capable of delivering the specified output power, but not such a high current limit as to saturate the transformer. A common mistake at that time was to choose a larger TOPSwitch for a given design to correct a thermal issue without considering the effect the higher current limit would have on the transformer.

Newer devices TOPSwitch GX, HX, and JX have a variable current limit function introduced by the addition of the "X" pin, allowing adjustment of the device current limit. It is still good design practice to select for the smallest TOPSwitch for a given power for cost reasons, but you also have the option of selecting a larger device (lower RDSON) for thermal reasons and trimming down the current limit using the "X" pin function to avoid saturating your transformer.

In PI Expert and PIXLS, this current limit adjustment parameter is labeled KI. You can adjust the KI parameter to tailor the current limit of a given device (within limits). This can enable you to change to a larger TOPSwitch without triggering a peak flux limit warning.

Thank you kindly, your explanation was so helpful.

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