UCC28780: frequency calculation formula and the RDM pin

Also I still didn't know the RDM function. From the datasheet it's the demagnetic time which means the on-time of QH.

But if the Rrdm is fixed, did it means the demagnetic time fixed? Then in different input/load, the on-time of QH is fixed?

Would you please help me?

Hello Fleming,

Thank you for your interest in the UCC28780 ACF controller.

The switching frequency equation (fSW) on page 40 is derived from some simplification of the ideal Transition-mode waveform timing plus modifications for the extra negative current portion and resonant ringing portion. The switching period tSW is the inverse of the fSW, and tSW is the sum of the on-time, the demagnetization time, the negative current time, and the resonant time.  The various parameters are arranged such that it is not necessary to know Ipk for any of the conditions.  Unfortunately, I cannot simplify it further for you.

The RDM resistor value is fixed, however the voltage on the RDM pin is variable and depends on the reflected output voltage sampled at the VS pin during the off-time.  The variable voltage on RDM drives a variable current which affects the internal timing circuit for QH.  So, the on-time of QH can be variable depending on both output voltage and input bulk voltage (also sampled at VS during the on-time).    

Regards,
Ulrich

Dear Ulrich,

Thanks for your quick response.

For RDM I know the answer.

But for frequency I still wonder the formula. I know that the frequency is the reverse of the period and also the period include the on/off time and resonant time.

But for the detail separate time I didn't know the concrete formula. I also list the deduced process of mine, which is much different with yours. Would you please take a look at the calculation and correct them?

Hello Fleming,

I believe that there is one error in the Tneg equation where you equate nVo with Vbulk(1-D).  I believe that this is not true.

I do concur that Ton, Toff, Ton + Toff, and Tres equations are correct.  I tried to work through this in detail and I think the system engineer made some simplifying assumptions to arrive at the fSW formula in the datasheet.  I need to confer with the system engineer to understand and validate these this assumptions.  Please give me a day or two to pursue this and find out the full derivation. 

Regards,
Ulrich

Dear Ulrich,

Thanks for your prompt reply.

I agree that Tneg is wrong by my clerical error.

Also I'm very appreciated your later derivation.

Thanks.