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TNY290PG burnt

Posted by: galaxyelect@gm… on 03/17/2022

I designed a power supply using PI Expert 9 about four years ago for TNY290PG. The design file is attached herewith. We observed a failure today when the supply was in use in our product connected to 240 V AC fairly stable supply with a load of 0.6 A. The load was steady and after an hour the TNY290 IC burst. The image of the IC is attached. The power supply was successfully tested at 100% of rated load for eight hours at 110V AC and 8 hours at 260 V AC before using it on our product.

The change to the original design is we have increased the clamp as below to compensate the range of the leakage inductance of the transformers and to keep the Vds below 500 V:

1. The original R1 is 27K, R1 in use is 11K

2. The original C3 is 2.7 nF, C3 in use is 4.7 nF

3. In addition, a TVS diode P6KE200A is used in parallel to the above two components.

The transformer from the failed power supply is recovered. The measured Nominal Primary Inductance is 1035 µH and the Primary Leakage is 59 µH.  

I checked the same design working power supply on oscilloscope with a load of 9 Ohm resistor at 110 V AC and 260 V AC. The images are attached herewith. The switching frequency seems half of the designed frequency. Don't understand why it is so. Is it correct?

Could you please help solving the failure issue?

With Best Regards,

John.

Files

Comments

Submitted by PI-Enderman on 03/19/2022

Hello John,

Thanks for using our product.

The design file you provided is quite old as you mentioned and does not seem to show the right schematic and component values when uploaded on PI-expert online. May I have a screenshot (.png, .jpg) of the schematic so we can help you better?

For clarification, are the captured waveforms on the recovered transformer tested with the newly design snubber or with the original design that resulted in failure?

Looking forward to hearing back from you.

Best Regards,

PI-enderman

Submitted by galaxyelect@gm… on 03/19/2022

Thank you. Attached is the PDF of the complete design file.

Later I tried reducing the additional clamp components which I had introduced but the input current didn't change. The purpose of reducing the components was to see if those parts have made more dissipation in the IC due to which the IC burnt.

With Best Regards,

John.

Attachment Size
150210 TNY290 12V 1.8A, -12V 0.1A.pdf 159.6 KB
Submitted by PI-Enderman on 03/21/2022

Hi John,

I was able to open the design file you provided on the PI Expert Suite offline software. According to your design the FS_AT_ILIMMIN is 137183 Hz. This frequency is only true at the VACMIN (85 VAC), ILIMITMIN (0.79 A) and full load IO (1.8 A). You may test again using these parameters to check but the switching frequency that you measured (which is lower) is correct with respect to your test parameters. You may still measure a lower switching frequency but this is due to the ILIMITMIN being unchangeable. I also looked over your design and I have found no issues.

Regarding the failure, the transformer leakage is the biggest culprit to the issue seeing as the recovered transformer did not burn out the IC with the better redesigned snubber. You may choose between the better snubber or reduce transformer leakage while retaining the original snubber design to reduce cost.

Best Regards,

PI-enderman

Submitted by galaxyelect@gm… on 03/21/2022

Thank you. It seems as though there is some confusion regarding the snubber.

At the first place the power supply was tested for full load at 110 V and 260 V AC for 8 hours each. It also worked on our final product for quite a few days. One day at almost negligible load the IC burnt.  

We use high value snubber in production than recommended by the PI Expert. I believe you have mentioned it as "the better redesigned snubber". We decreased the R, increased the C and added a TVS diode P6KE200A. The redesigned snubber was present when the IC burnt because we have standardised on the redesigned snubber.

Later I mentioned that I thought the additional parts in the snubber (clamp) may cause additional dissipation in the IC which might be the cause of buring. Hence I removed the additional parts and made the assembly as recommended by the PIExpert. However I mentioned that I observed there was no change in the input mains current even if I removed the additional parts in the snubber.

Do you recommend using stronger snubber (clamp) with decresed R, increased C and additional diode? Anyway we are using those as a standard.

The reason behind burning of IC still remains unknown. 

With Best Regards,

John.  

Submitted by PI-Enderman on 03/22/2022

Hello John,

The clamp does not cause more dissipation in the IC as it is meant to protect the Drain pin of the IC by absorbing the energy from the leakage inductance. The problem may be elsewhere, unless the original snubber design has lower Drain voltage measurements than the redesigned snubber.

Upon further checks on your schematic, I have noticed that there are only two resistors going into the EN/UV pin. Usually, three resistors are used to divide the voltage across the resistors to reduce the chance of arcing and damaging the EN/UV pin. This occurrence is uncommon but it is notable as you mentioned the IC burst at high voltage.

Best Regards,

PI-enderman

Submitted by galaxyelect@gm… on 03/22/2022

Thank you. Understood. The PIExpert always show two. Will regularly use three resistors in place of two.

With Best Regards,

John.