LM5156H: Flyback Transformer for 12v@4.5A

Janis Sarikas

Part Number: LM5156H
Other Parts Discussed in Thread: TIDA-050023, LM51551-Q1

Hello,

I try to design a PoE++ converter with 12v@4.5A (54W) output. I want to use TPS2372-4 and LM5156H for that (similar to reference design TIDA-050023).

I followed the design guide of SNVA866 which is intended for CCM - if I understood that right.

Unfortunately all transformers that I can find have primary inductance of 15-20µH and I would have to increase the frequency over 550kHz to get a ripple current of 60%.

I'm not sure if frequencies above 550kHz are acceptable for flyback converters because in all reference designs that I found they typically use frequencies in range of 200-350kHz.

Is there a rule of thumb regarding the switching frequency of flyback converters? Or is the switching frequency only limited by the core losses of the transformers?

Even when I would use the transformer 750317065 of the reference design TIDA-050023, which has a primary inductance of 30µH, I would have to increase the frequency to round about 460kHz.

I recognized that the flyback converter of TIDA-050023 only uses a frequency of 313kHz (R_T = 71.5Ohm). Does the reference design work in DCM instead of CCM? If so do you have design guide for DCM?

Best regards

Janis

over 2 years ago

0 Stefan Schauer over 2 years ago

TI__Mastermind 42161 points

Hi Janis,

thank you for using the E2E forum.

There is not a limit for the frequency for a Flyback converter. The main decisions is on size and efficiency. But often it is avoided to use frequencies where the sub harmonics are in the range for the radio AM frequencies. Therefore so find most SMPS switching frequencies below the 350kHz or then above the 2MHz.

Best regards,

Stefan

0 Janis Sarikas over 2 years ago in reply to Stefan Schauer

Prodigy 155 points

Hi Stefan,

Thanks for your reply and the hints regarding the AM frequencies and the efficiency.

Could you tell me if the reference design TIDA-050023 is working with DCM or CCM? I just wondering about the 30uH of 750317065 and the adjusted 313kHz of LM51551-Q1 in this circuit. I would like to understand that and maybe DCM would be a solution for me.

I read that DCM has some pro and cons. For example it can use transformers with smaller inductance but may produce more EMI and are optimal for low output power.

I hope that EMI could be (easily) lowered with some output filters but I don't know what means "optimal for low output power"? Would 54W still be a low output and if not what would that mean for DCM? Lower efficiency or something like that?

Regards

Janis

0 Stefan Schauer over 2 years ago in reply to Janis Sarikas

TI__Mastermind 42161 points

Hi Janis,

I do not know this design by myself but i just put the values of the schematic into the Power Stage Designer (Power Stage Designer | Overview | Design Resources | TI.com). Based on that it should runs in CCM when running with full load (4.25A).

Best regards,

Stefan

0 Janis Sarikas over 2 years ago in reply to Stefan Schauer

Prodigy 155 points

Hi Steffan,

thanks. I also tried it with the designer tool and you are right but only if the ripple ratio is increased to 90%. I only tried it with max 70%, as recommended in SNVA866.

I didn't used this tool before. I guess it will help me to get a better overview what I can expected of my further design decisions.

Regards

Janis

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LM5156H: Flyback Transformer for 12v@4.5A