Safety in event of a secondary short

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I have a question about a safety related issue in an offline flyback converter, I plan to use the TOP267KG.

I am evaluating cases with a singular component failure that may expose a user to dangerous voltage levels at the output of a flyback converter with 24 V output voltage and wide range mains input. In case the secondary diode fails and shorts, the mains voltage divided by -Np/Ns of the transformer is applied on the output pins of the converter during the conduction phase of the primary FET (in my case: -70 V). I want to make this safe by putting a second diode antiparallel to the output capacitor (D58 in attached picture). This diode would get forward biased in above mentioned failure case and limit the output voltage of the circuit to safe levels.

Is this a valid approach, or does the TOP267KG already protect the output from such failures? Similar ICs from e.g. ON (FSQ0565) specifically have protections for these cases (in ON's case: Abnormal Over-Current Protection), but I couldn't read from the TOP267KG's datasheet if it features something similar.

Thanks and regards


TOPswitch.PNG39.59 KB

I forgot to mention, the converter has an output of 24 V, 1.5 A and an input of 100 V - 250 V AC.

Adding an antiparallel diode to the 1st output cap is an interesting belt-and-braces solution to the single-point output rectifier failure that concerns you.
With or without the added diode, the primary current should exceed the primary overcurrent trip point after an output rectifier short, when the switch is on. Primary overcurrent protection will engage after the blanking time, forcing the power supply into auto-restart.
Using a resistor from B+ to the TOPSwitch X pin will reduce the primary overcurrent trip point as a function of AC input voltage, as an extra measure of safety. This is covered in the data sheet.

TOPSwitch HX and JX have an additional safety feature that extends the off-time if the drain current reaches Ilimit within 500ns. This will certainly happen if your output rectifier shorts. This feature, and auto-restart will help reduce the stress on the primary switch during a fault situation.