Open output diode - TNY SW III gets Damaged

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Hello, We have designed a SMPS based on TNY switch III - TNY277. Universal Input, 14v DC o/p @ 0.5 Amps, TL431 based feed back. EE-16 core transformer. We have observed that, when no output diode is present and SMPS is powered up, then, TNY277 gets damaged. After replacing with new IC and putting diode circuit works OK. Why TNY gets damaged when o/p diode is open? Is this due to incorrect design / transformer? How to prevent ic, when open o/p diode is encountered? Regards Vinay
From your description is sounds to me that your clamp circuit is getting damaged first... Obviously since output is open the current flow is minimal and the power supply will be in autorestart mode. What circuit are you using for the primary clamp? Does the blocking diode also get damaged when the TNY-III damages? Please send me the schematic with component part numbers so I can understand and advise better.

I suspect the following:

When the output diode is removed that there is no-where for the stored energy in the core to go. This results in a high voltage on the primary which exceeds the BVDss rating of the internal MOSFET.

This is considered an abnormal test and therefore a safe failure of TinySwitch is acceptable to the safety agencies. Safe = no fire, damage to isolation barrier etc.

Cheers

PI-Chekov