UCC28951: PSFB shim inductance vs duty cycle

marco ciambottini

Part Number: UCC28951

Hello,

I have run some simulations of a PSFB with Pspice and I have seen that there is some relashionship between the shim inductance and the maximum duty cycle.

Theoretically I could avoid the shim inductance and only use the leakage but:

1) If I use a large inductance, the DC/DC is not able to regulate at max load, the output voltage decreases some volts.

2) If I use a smaller inductance, some unwanted turn-offs appear on the primary mosfets when the load decreases.

Could someone explain me this behaviour?

Regards,

over 1 year ago

0 Mike O' over 1 year ago

TI__Guru* 85616 points

Hello,

Your inquiry has been received and is under review.

Regards,

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

TI__Guru* 85616 points

Hello,

Your shim inductor will form a voltage divider with your transformers primary magnetizing and if you don't account for it could have duty cycle issues.

The way to avoid this make shore the shim inductor is 10% or less than the magnetizing inductance. This way you can achieve the desire output voltage you require with 90% duty cycle.

Generally the shim inductor is not large enough to cause an issue. The following link will bring you to an application note that discusses how to design a PSFB using the UCC28951 controller. Gives recommendation on how to select the transformer turns ratio and magnetizing inductance to avoid these issues.

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

Regards,

0 marco ciambottini over 1 year ago in reply to Mike O'

Prodigy 90 points

Hello,

it was because of the turn ratio. Decreasing it a little, it works.

Regards,

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UCC28951: PSFB shim inductance vs duty cycle