LM5022: About Demoboard PMP20551

WU YISYUAN

Expert 2165 points

Part Number: LM5022
Other Parts Discussed in Thread: PMP20551

Hi,

I saw the LM5022's demoboard PMP20551.

the transformer turns ratio is 3:1, it means duty about 80%

The general recognition is that the duty cycle of flyback is less than 50%.

What is the problem with this design?

thanks,

over 5 years ago

0 Garrett Roecker over 5 years ago

TI__Mastermind 21720 points

Hi Wu,

Thank you for reaching out with your question.

Typically flyback designs a configured to ensure that the duty cycle is not above 50%. However, the 50% duty cycle limit is no a requirement in all designs.

If the duty cycle is greater than 50% the designer must consider a lower frequency RHP Zero and the addition of slope compensation. Designs with a duty cycle lower than 50% are less complicated to design. There is nothing wrong with PMP20551, the designer decided it made sense to operate a t a higher duty cycle for the application.

Please let me know if you have any other questions.

Thanks,

Garrett

0 WU YISYUAN over 5 years ago in reply to Garrett Roecker

Expert 2165 points

Hi Garrett,

If the duty cycle is greater than 50% the designer must consider a lower frequency RHP Zero and the addition of slope compensation. <---Do you have any reference design for this condition compensation?

thanks.

0 Garrett Roecker over 5 years ago in reply to WU YISYUAN

TI__Mastermind 21720 points

Hi Wu,

The application note below goes through the design procedures for a flyback that operates above 50% duty cycle
www.ti.com/.../snva761.pdf

The basic theory is to set the cross over frequency about 1/5th the RHP zero frequency. The slope compensation should be at least 1/2 the falling slope of the sensed transformer current.

Thanks,

Garrett

0 WU YISYUAN over 5 years ago in reply to Garrett Roecker

Expert 2165 points

Hi Garrett,

got it.

Another question is on PMP20551's circuit,What is the function of D6, R12, C18?

Is it compensation ?

thanks.

0 Garrett Roecker over 5 years ago in reply to WU YISYUAN

TI__Mastermind 21720 points

Hi Wu,

These components just implement a secondary side soft-start of the output voltage. The RC slowly charges and the external reference will track the charging of the RC.

Thanks,

Garrett

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LM5022: About Demoboard PMP20551