TPS92411 RSNS and RSET resistor size/wattage [E]

LeonardEllis

Other Parts Discussed in Thread: TPS92411EVM-002

On the TPS92411EVM, RSNS and RSET are shown to be 1/4W and 1/8W 1206 size resistors. I'm not understanding why these are required to be so large, and in a new design, we are wanting to use 0402 size for reduced pcb space. There is only a few uA flowing in these resistors, and even with a 200V source, that would be <1mW. Please comment on why the seemingly large difference in wattage/size requirements.

~Leonard

over 9 years ago

0 David Zhang over 9 years ago

TI__Expert 3495 points

Leonard:
Standard 0402 size resistors are rated for 50V. Standard 1206 size resistors are rated for 200V. The EVM uses 1206 size resistors for high voltage rating.
David Zhang

0 LeonardEllis over 9 years ago in reply to David Zhang

TI__Guru 71516 points

Thanks David, and that would make sense if there was 200V in that part of the circuit, but I'm seeing 4V on RSNS, for example. Is it necessary to still use 1206 size?

~Leonard

0 David Zhang over 9 years ago in reply to LeonardEllis

TI__Expert 3495 points

Leonard:
If you use TPS92411EVM-002 with 230Vac input and measure the voltage across R8, you will see high voltage.
David Zhang

0 Alexander Mozgovenko1 over 9 years ago in reply to David Zhang

Prodigy 60 points

Hi David,

We're not talking about R8. We're talking about R4, R11, R17. Also, the application is for 110VAC. Is there a way to make those 3 resistors smaller? There doesn't seem to be a high voltage present at those resistors.

Please advise.

Alexander

0 David Zhang over 9 years ago in reply to Alexander Mozgovenko1

TI__Expert 3495 points

Alexander:

For 110Vac application, the voltages across those sense resistors are lower. Yes, you can use smaller size resistors. Please make sure the voltage rating of the resistor is higher than the peak voltage across the resistor.

David Zhang

0 Alexander Mozgovenko1 over 9 years ago in reply to David Zhang

Prodigy 60 points

David, mind giving some more specific information? There's no information on what's the actual voltage on those pins. Is it 4V? It'd be great to know, in order to make a decision on the minimum size possible.

0 David Zhang over 9 years ago in reply to Alexander Mozgovenko1

TI__Expert 3495 points

Alexander:
It is hard to tell you the exact voltages those resistors will see because of the voltage spike generated by triac dimmer firing. At 110Vac input without triac dimmer, R4 will see peak voltage about 75V, R11 and R17 will see lower voltages. So you can 0402 resistors for R11 and R17. But for R4, I suggest using at least 0603 resistors. If it is a triac dimmable design, I strongly recommend you measuring the actual peak voltages with different dimmers.
David Zhang

0 LeonardEllis over 9 years ago in reply to Alexander Mozgovenko1

TI__Guru 71516 points

Hi Alexander,

Just wanted to make sure you got all the information you needed, and if this helps reducing the pcb footprint for this design? If so, please click the VERIFY tab; I'm eager to know how this design is progressing and when it will go to production.

~Leonard

0 Alexander Mozgovenko1 over 9 years ago in reply to LeonardEllis

Prodigy 60 points

Hi Leonard,

Thank you for the follow up. I went with 0805's, which should cover all of the situations and was easier for layout in my case.

Regards,

Alexander

p.s. I don't see the Verify link anywhere.

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TPS92411 RSNS and RSET resistor size/wattage [E]