UCC3817: Can Rsense voltage be set to 0.1V not 1V?

Kazuya Nakai59

Guru 20600 points

Part Number: UCC3817

Hello guys,

One of my customers is evaluating UCC3817 using TI official EVM.

They calcurated Rsense(R14 in Figure 9 on page 16 of the device datasheet) value with the design procedure document in the following URL

and they for the calculated value resistor for R14.

　UCC3817: design of 1.5kw Boost pfc

https://e2e.ti.com/support/power-management/f/196/p/769506/2846576?tisearch=e2e-sitesearch&keymatch=ucc3817 design procedure Mathcad#2846576

.The EVM worked correctly but R14 became very hot. So they want to change R14 resistor value to smaller for reducing the the resistor temperature.

My question is that if R14 value is changed to one tenth of the caluculated value, does UCC3817A not work correctly because R14 voltage is chaged

from 1V to 0.1V? If the answer is yes, why UCC3817 doesn't work? Their peak current is 8A.

Could you please give me your advice or comments?

Your reply would be appreciated.

Best regards,

Kazuya Nakai.

over 5 years ago

0 John Griffin1 over 5 years ago

TI__Mastermind 34465 points

Hello Kazuya Nakai,
If you change the value of the current sense resistor then you will also need to change the multiplier output resistor.
This is R9 and R10 in the EVM.
Was this change made ?
Changing the Rsense value will also change the requirements for the current loop compensation.
The calculator tool needs to be re run in order to calculate the correct values.

Regards
John

0 Kazuya Nakai59 over 5 years ago in reply to John Griffin1

Guru 20600 points

Hello John,

Thank you for the prompt reply.

Yes. the customer replaced the original resistor (0.25 ohm) of R14 on UCC3817EVM with 0.03 ohm (30m ohm) resistor.
After the modification, they found the EVM didn't work correctly because of over current detecting.
They also changed both R9, R10 to 13k ohm.

Do you mean that the calculator is the mathCAD file stored under UCC3817 product folder?
Or another calculator?

Thank you and best regards,
Kazuya Nakai.

0 John Griffin1 over 5 years ago in reply to Kazuya Nakai59

TI__Mastermind 34465 points

Hello Kazuya Nakai,
The calculator tool is located here :
www.ti.com/.../getliterature.tsp
This is the only calculator available for the UCC3817.
Regards
John

0 Kazuya Nakai59 over 5 years ago in reply to John Griffin1

Guru 20600 points

Hello John,

The customer connects a motor to the PFC output as a load. The motor peak current is about 3 times of steady state current. In this condition, is it O.K. to use the steady state current as Pout (output power) for parts parameter calculation by the calculator?

Could you give me your comments or advice?

Thank you and best regards,
Kazuya Nakai.

0 John Griffin1 over 5 years ago in reply to Kazuya Nakai59

TI__Mastermind 34465 points

Hello Kazuya Nakai,
This question is different from your initial question.
Please close this post and open a new post for this new question.
Regards
John

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UCC3817: Can Rsense voltage be set to 0.1V not 1V?