UCC28950: Determining Maximum Duty Ratio to Select Transformer Turns Ratio

JSly

Part Number: UCC28950

Hello,

I am trying to determine the turns ratio for the transformer in my phase shifted full bridge converter. The relevant equation here is

Vout = D_max * V_inmin * N_s/N_p

My question is how do you determine D_max and where does that number come from? A first principles explanation would be great because I'm seeing conflicting sources of information.

At this link, it says maximum phase shift (duty cycle) should be 0.4.
At this link, its says maximum duty cycle is 70%.

Also, I'm looking to hone my understanding how duty cycle applies in the PSFB. The duty cycle for QA and QB is 50% and those are complementary signals. But, the duty cycle of the converter is the proportion of the switching cycle where voltage is applied to the transformer. Thus, the duty cycle of the converter is 100% if there is 100% "overlap" between QA and QD and between QB and QC. Is that correct?

Thank you for your help!

over 4 years ago

0 Benjamin Lough over 4 years ago

TI__Mastermind 27145 points

Hi JSly,

I couldn't open the first link unfortunately but it is fine to use 70% as a guideline. I would suggest taking a look at the power stage designer tool:https://www.ti.com/tool/POWERSTAGE-DESIGNER

It can be helpful for visualizing the waveforms for different SMPS topologies like PSFB as duty cycle, etc. changes.

Best Regards

Ben Lough

0 JSly over 4 years ago in reply to Benjamin Lough

Intellectual 410 points

Hi Ben,

Thanks for the quick response. Where does the 70% come from?

What's the advantage of going with a higher duty cycle? This increases the turns ratio, and therefore the size of the transformer, no?

0 Benjamin Lough over 4 years ago in reply to JSly

TI__Mastermind 27145 points

Hi JSly,

The 70% number is mostly rule of thumb. It comes from leaving some margin on the max duty cycle in ACDC applications where the PFC may experience a brownout for several ms and the PSFB must ride through the dip without issue. Smaller turns ratio will result in higher peak currents in the secondary rectifier. I would say it depends on what you care more about: size vs efficiency.

Best Regards,

Ben Lough

Power management

Power management forum

UCC28950: Determining Maximum Duty Ratio to Select Transformer Turns Ratio