UCC2897A: The UCC2897A parameter is abnormal.

Tracy Song

Part Number: UCC2897A
Other Parts Discussed in Thread: TL431,

Hi，Peter.
I am using UCC2897A for ACF power supply. VDD power supply is 12.7v and VIN power supply is 75V.According to the internal block diagram of datasheet, the internal reference source should be 2.5v. Why are measuring pins 3, 4 and 5 all 3.2v?I set the frequency at 250KHz, but I'm actually measuring 312.5KHz.

over 6 years ago

0 Tracy Song over 6 years ago

Intellectual 350 points

Another problem is that FB is 5V when there is no load. When I configure FB to be in the range of 2.4-4.5v, the output voltage will exceed the set value.I would like to ask what the reason is.The reference voltage of TL431 is 2.49V.

0 John Griffin1 over 6 years ago in reply to Tracy Song

TI__Mastermind 34465 points

Hello Tracy,

You have a number of questions here and very little information as to how you observed these issues.

So I will try to address the issues as best I can.

(1) What do you mean by ACF ?
Do you have a schematic for this ?

(2) The block diagram shows pins 3,4 and 5 as a current source.
The voltage provided on these pin is not specified on the data sheet.

(3) Use the Excel calculator tool to set the switching frequency.
Many of the reference designs use a 250kHz switching frequency so you will get help there.

(4) I dont understand your last question

I recommend that you obtain the UCC2897AEVM and do all of your testing on this system before you are ready to proceed.
http://www.ti.com/tool/UCC2897AEVM

Regards

John

0 Tracy Song over 6 years ago in reply to John Griffin1

Intellectual 350 points

Hello,John
ACF means active clamp forward. And my frequency is 250KHz calculated by UCC2897A Excel, but the actual measured frequency is 312.5KHz.I don't know what caused it.

0 Tracy Song over 6 years ago in reply to Tracy Song

Intellectual 350 points

One problem is that when the optocoupler is not connected, Vref is 5V, FB pin is 1.36v and series resistance is 1K.FB should also be 5V, but this is not the case

0 John Griffin1 over 6 years ago in reply to Tracy Song

TI__Mastermind 34465 points

Your schematic is not wired up properly. If you need further help, please post an accurate schematic.

Regards

John

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UCC2897A: The UCC2897A parameter is abnormal.