UCC28780: UCC28780-TIDA-01622 TRANSFORMER & OUTPUT

robin gangopadhya18

Mastermind 6050 points

Part Number: UCC28780

Hello:

Can you restate the expected output for the transformer with ratios

pri: sec = 5.5: 1

Pri-AUX: 3.14:1

pri:aux2= 4.4:1

These numbers appear to be for 20V output. One can verify these from SLUC644c spreadsheet.

What is the purpose of AUX2?

Furthermore, in TIDA-01622 schematic, transformer is shown to be 22:4:5:13

22:5 would represent AUX2 coil not AUX1 as implied by the WE datasheet referenced in the BOM.

And 22:13 seems awfully high turns ratio : yet the schematic shows AUX2 is rectified, followed by emiter follower to drive UCC28780 . What is the rationale for this option?

If you want to get 5 v output, does it not mean that the converter has to operate in sub-optimal mode with 22:4 ratio? Unless dropped by LDO to 5V from 20V ?

Is it possible there is a typo: turns ratio should be 22:3.14 for AUX1 instead of 22:13?

It is quite confusing.

Any comment to review these ratios will be highly appreciated.

robin\

over 3 years ago

0 Mike O' over 3 years ago

TI__Guru 66175 points

Hello,

I have an applications engineer looking into this issue for you. They will be getting back to you shortly.

Regards,

0 Jaden Ning over 3 years ago in reply to Mike O'

TI__Expert 7080 points

Hi Robin,

This reference design is a standard PD output , from 5V to 20V adjustable.

For 20V output , AUX2 winding is used for VDD supply . so VDD voltage is around 20V * AUX2/SEC =20* 5/4 =25V

For 5V output, AUX2+AUX1 windings are used for VDD supply so rectified voltage is around 5* (5+13)/4 =22.5V , Please note that there is a LDO after AUX1 rectified path. and the reference voltage of LDO is 15V . that means the LDO will be disabled when output voltage at 20V and 15V

Two AUX windings circuit is very common used in PD design , it's a higher efficiency solution compared with only use one LDO.

Hope this can answer your questions.

Thanks.

0 robin gangopadhya18 over 3 years ago in reply to Jaden Ning

Mastermind 6050 points

Jaden

Oh, this is neat !

Thanks for clarifying.. & the transformer ratio

We will close this post now

robin

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UCC28780: UCC28780-TIDA-01622 TRANSFORMER & OUTPUT