UCC28950: Resonant Inductor current does not track transformer primary current

Prodigy 75 points

Part Number: UCC28950

Tool/software:

Hi,

I am working on a PFSB operating at 125 KHz frequency.

The resonant inductor is 30uH, Lprimary is 15mH.

The input voltage is 850V.

I can observe that the resonant inductor current does not track transformer primary current. I can't think of a reason why there is a difference.

The topology is phase shifted full bridge with clamping diodes on the primary and current doubler output on the secondary. The MOSFETs on the current doubler are with RC snubbers.

Resonant Inductor Current

Transformer Primary Current

Thanks.

over 1 year ago

0 Ning Tan over 1 year ago

TI__Genius 12795 points

what is the loading condition? Can you overlay the ABCDEF waveform with the current on the same image?

0 SS over 1 year ago in reply to Ning Tan

Prodigy 75 points

Hi. This is at zero load condition. I can take the primary and secondary voltage of the transformer with the same measurements and post it here.

0 Ning Tan over 1 year ago in reply to SS

TI__Genius 12795 points

please do

0 SS over 1 year ago in reply to Ning Tan

Prodigy 75 points

Hi. I have got the waveforms measuring VLresonant, Vprimary, Vsecondary, V(Hbridge), Iresonant_inductor, Iprimary.

Zoomed Version.

The resonant inductor current is almost 3A which is the peak current limit but the primary current is quite lower than the resonant inductor current.

The operating conditions are Vin=650V, Vout=28V, Iout=21A.

Please let me know your thoughts.

Thanks.

0 Ning Tan over 1 year ago in reply to SS

TI__Genius 12795 points

We will review it and get back to you later. Thanks for the patience.

0 SS over 1 year ago in reply to Ning Tan

Prodigy 75 points

Thanks. Please note that the inductor current is 50uH in this measurement. I have increased it to achieve ZVS at a lower load condition.

0 Ning Tan over 1 year ago in reply to SS

TI__Genius 12795 points

ok, noted.

0 Mike O' over 1 year ago in reply to Ning Tan

TI__Guru* 86266 points

Hello,

Your resonant inductor (Ls) should track your transformer current. If it does not it may not be connected correctly.

The following link will bring you to an application note that discusses how to design a phase shifted full bridge using the UCC28950. It goes through the step by step design process. It even shows how to properly connect and size the Ls inductor in your design.

https://www.ti.com/lit/pdf/slua560

Regards,

Power management

Power management forum

UCC28950: Resonant Inductor current does not track transformer primary current