UCC28742: running at 100KHz

Hi Holger,

Thank you for the query on UCC28742.

I think it is safe to design for less than or equal to 80khz because of the part to part variation, designing for 100khz could potentially cause power limiting in the upper FM region. Even the calculator recommends the Fsw be less than 80khz.

Regards,

Harish

Hi,
they need the switching frequency of 100KHz because they are looking for a replacement of the NCP1217 which they use in their current design. I they don't want to change inductance and transformer.

Regards, Holger

Hi Holger,

Ok, they can use the same transformer but will have change the current sense resistor. P = 0.5Li^2Fsw

So in the best case if you get a part where Fsw is same as the nominal (typical) value, they should be fine.

Lm designed for 80khz vs 100khz is going to affect the magnetizing inductance. Let's if they use the same inductance and they were to get a part with lower Fsw, the smaller inductor will force the controller to hit frequency upper limit. In that case Rsense value would have to changed to pump the desired amount of power to permit operation of the controller at this reduced frequency and deliver full power.

I would also recommend UCC28740 which is more close to the NCP1217 where HV startup is also integrated.

Thank you

Regards,

Harish

Hi Harish,

he can't use the UCC28740 because of the brown-out protection of 68V. He can accept max 50V.

Regards, Holger

Hi Holger,

Thank you for the reply. Then with UCC28742 we can proceed as suggested above by adjusting the sense resistor.

Regards,

Harish