TPS23753AEVM-003 overheating R15 and D7

Alex Nancekievill

We have an exact duplicate of the TPS23753AEVM-003 schematic on our custom board. Input voltage over ethernet is 48V and output voltage is 5V @ 1.1A.

Although the circuit is working and producing a clean 5V supply, we find that the 4.7 ohm resistor R15 is getting very hot as is D7. Further investigation shows that the output of the HA3802-AL pin 7 includes a 100ns transient voltage spike which reaches about -30V. D7 conducts to protect the MOSFET as intended so a significant voltage drops across R15. I estimate from the area of the voltage spike on the scope that R15 has to dissipate 6W during the spike and 80mW on average which is above the rated 1/16W in the BoM. This would seem to explain why it is so hot.

I have ordered a TPS23753AEVM-003 to see if it exhibits the same behaviour but it will probably take a week to arrive in the UK. While I'm waiting can anyone shed any light on what might be happening? Can anyone confirm if the EVM shows the same behaviour?

Many thanks,

Alex

over 13 years ago

0 Eric Wright over 13 years ago

TI__Mastermind 42795 points

Sorry for the trouble. We've addressed the issue with an assembly change to the flyback transformer (HA3802-AL). The mfgr now has the HA3802-BL which has been proven to alleviate issues of this sort. The EVM user guide is currently being updated to reflect this change.

0 Alex Nancekievill over 13 years ago in reply to Eric Wright

Intellectual 350 points

Eric,

Many thanks for picking this up. It's a shame we didn't know about this earlier, but a simple part swap should be relatively painless. Unfortunately Coilcraft seem to be denying the existence of a HA3802-BL. They have a BL version of the HA3803 but that's for 12V output. Can you help me track it down please? When was the new part introduced? Who was the contact in Coilcraft?

Best regards,

Alex

0 Leonard Crane over 13 years ago in reply to Alex Nancekievill

Prodigy 20 points

Alex,

Thanks for bringing this to light. Coilcraft has just finalized the data sheet for HA3802-BL and will have it posted on our website as soon as possible. In the meantime please contact us at cust_service@coilcraft.com so we can send you the datasheet directly.

Best Regards,

Len Crane

Coilcraft Technical Marketing Director

0 Alex Nancekievill over 13 years ago in reply to Leonard Crane

Intellectual 350 points

Len,

Thanks for the prompt follow-up. I've been in touch with customer services and now have the datasheet. I understand it's also up on the web now too.

Best regards,

Alex

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TPS23753AEVM-003 overheating R15 and D7