LM3481: Is there any data about phase margin and gain margin for flyback converter using LM3481?

Kazuya Nakai59

Part Number: LM3481

Hello guys,

One of my customers is evaluating LM3481 with a flyback converter topology (Vin=9~18V, Vo=12V, Io=1A) on their own PCB for their next products.

They want to get any data of phase margin/gain margin for flyback converter like the customer conditions as the above. Do you have any data?

Also they want to know how to measure phase margin/gain margin on their flyback converter. Do you have any document how to measure the margins?

Your reply would be appreciated.

Best regards,

Kazuya Nakai.

over 6 years ago

0 Garrett Roecker over 6 years ago

TI__Mastermind 21720 points

Hi Kazuya,

Please take a look at this design guide outlining how to design the LM3481 as a flyback. Section 5 shows the control loop measurement.

www.ti.com/.../snva761.pdf

Thanks,

Garrett

0 Kazuya Nakai59 over 6 years ago in reply to Garrett Roecker

Guru 21515 points

Hi Garrett,

Thank you for the prompt reply. I think the document is very helpful for the customer. Could I ask you a few additional questions?

Q1. In the documents, R1/R2 and C1/C2 calculation ways are discribed at 4.4.3 and 4.4.4 section on page 14~15.
Which resistor and capacitor in figure10 (LM3481-Flyback Schematic) on page 11 corresponds to R1/R2/C1/C2?
I think it as the below. Is it correct?
R1 -> R12, R2 -> R17, C1 -> C15, C2 -> C19.

Q2. Section 4.4.3 on page 14 says "Meanwhile the gain-frequency must pass across 0 db with –20 dB/dec to ensure enough phase
margins.".
What phase margin do you recommend? Is it more than 45 degree? Or more than 60 degree?

Q3. What gain margin do you recommend?

Could you please give me your advice?

Thank you again and best regards,
Kazuya Nakai.

0 Kazuya Nakai59 over 6 years ago in reply to Kazuya Nakai59

Guru 21515 points

Hi Garrett,

Could you please give me your comments?

Best regards,
Kazuya Nakai.

0 Garrett Roecker over 6 years ago in reply to Kazuya Nakai59

TI__Mastermind 21720 points

Kazuya,

Please see my responses below.

Q1: Those are correct components

Q2: Typically it is recommend to shoot for a phase margin of 65deg to 70deg. This will provide a good load transient response.

Q3: Adequate Gain margin is typically around 15dB to 20dB

Thanks,

Garrett

0 Kazuya Nakai59 over 6 years ago in reply to Garrett Roecker

Guru 21515 points

Hello Garrett,

Thank you very much for your reply. Your reply is really helpful for the customer and me.

Best regards,
Kazuya Nakai.

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LM3481: Is there any data about phase margin and gain margin for flyback converter using LM3481?