LM51561: Application in datasheet

Chong Yu

Intellectual 1435 points

Part Number: LM51561

Hi,

My customer need assistance.

This schematic is according to Webench:

webench.ti.com/.../64

Performance: Input voltage is 24V by switching power supply. Output voltage is 0V while powered on. The switching power supply will have a overcurrent protection after powered on 3 seconds. The voltage is reducing. And the transformer is very hot, even has a smell. I tried two times but it happens the same. I tested the input and output. It is not short-circuit to GND.
Question 1: May I know how this design works?
Question 2: How to caculate the output voltage? Why is the feedback not connected to FB pin but COMP pin?
Question 3: How to caculate the spec of transformer? Input voltage is 24V (8V~36V). Output voltage is 12V. Output current is 5A. The total load is about 8A. It is assumed to have two flyback circuits to seperate GND.

Please help. Thanks.

over 4 years ago

0 Brigitte over 4 years ago

TI__Guru 65310 points

Hello Chong,

Can you share a schematic that is clear? I cannot read it.

Please share the Webench file as well.

Question 1: It should follow this app note: https://www.ti.com/lit/pdf/snva866

Question 2: You are already comparing the output voltage to a reference voltage on the secondary side and so the internal error amplifier is not necessary to repeat this comparison. The output of the optocoupler is used to directly set the duty cycle via influensing the COMP signal.

Question 3: Please check if the above app note answers your questions.

Best regards,
Brigitte

0 Chong Yu over 4 years ago in reply to Brigitte

TI__Intellectual 1435 points

I hope the attached schematic will be clear enough.

And the Webench is available: webench.ti.com/.../64

0 Brigitte over 4 years ago in reply to Chong Yu

TI__Guru 65310 points

Hello Chong,

This schematic is slightly better then the last one, but still hard to read. Could you please use this quickstart calculator to check the design https://www.ti.com/lit/zip/snvc240?

Best regards,
Brigitte

0 BAI LI over 4 years ago in reply to Brigitte

Prodigy 70 points

Hello Brigitte

Could you open WORD?The file format is .docx.If not can Orcad?LM51561反激隔离电路.docx

Best regards

BAI LI

0 Brigitte over 4 years ago in reply to BAI LI

TI__Guru 65310 points

Hello BAI LI,

I can open the word document and the schematic is well readable there. Thank you.

The quickstart calculator can be used to check if your external components fit your application.

Best regards,
Brigitte

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LM51561: Application in datasheet