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Safety agency testing of TNY276GN & optocoupler current limiting

Posted by: KevinF13 on

If safety agency applies a short-circuit between pins 1 & 4 of TNY276GN, EOS occurs at the detector of the optocoupler. Subsequently, the optocoupler suffers catastrophic failure caused by the rapid temperature rise and explosion which, in turn, compromises the DTI and the unit no longer passes Hi-POT. The fuse preceding the diode bridge opens but it is not fast enough to clear the fault and protect the optocoupler. What practical minor design change is recommended for the PI reference design (see attached, page 8) to limit the optocoupler collector current <50mA (<0.2W) under this short-circuit condition? Thank you.

 

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TNY276GN Datasheet 1.74 MB

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Submitted by PI-Kazuya on 01/26/2022

Hi KevinF13,

Typically, with regards to short-circuit test, the safety agency applies it in adjacent pins of the component, instead of in between pins, like this pins 1 & 4 of TNY276GN. 

That would be the practical way if the adjacent pins got shorted and when there is no required safety spacing to maintain between them. 

Though for some reason, the agency may also conduct the short-circuit test in between pins. I'm not sure if it is really included in the safety standard intended to qualify the product. 

 

Anyway, in terms of shorting the pins 1 & 4, I could not provide you a direct solution to limit the collector current during the short circuit condition. But, I would suggest to add zener diodes on those pins or suitable protective device into a circuit to prevent any blow up issue and severely damaging the IC device. It would help to protect the power supply design from that potential safety failure. 



I hope it helps. Thank you.



Regards,

Kazuya

Submitted by KevinF13 on 02/01/2022

Hi Kazuya,

UL 62368 B.4.1 specifically states that the circuit is to be examined to determine any single fault conditions that might bypass a safeguard or otherwise affecting the safety of the equipment. For single fault conditions, a component failure is to be simulated “by short-circuiting any two leads and open-circuiting any one lead of the component one at a time” and the insulation serving as a safeguard is required to withstand Hi-POT after the single fault test. There is no PI reference design w/ optocoupler feedback that can pass all single-fault tests because the optocoupler suffers catastrophic failure w/ breakdown of its double insulation when pins 1 (Enable) & 4 (Drain) of TNY276GN are shorted. Will PI publish new reference designs to specifically address the new challenges of UL 62368-1 single-fault testing?

Submitted by PI-Kazuya on 02/02/2022

Hi KevinF13,



I understand your point. However, our power supply design is for reference only and not intended to satisfy all applicable safety requirements.

We could not publish a new reference design to compliantly resolve the potential issue that is meant to certify the equipment according to UL62368-1 standard.

Safety should not only relies in IC, and full safety evaluation should be done in the end system. You may also try to consult with your own experts in circuit design and verify the safety compliance.

Thank you.

Regards,

Kazuya