UCC28780: how to adjust the frequency

Dear Jack,

Thanks for interested in TI's solutions.

You can find Fsw equation on the datasheet 8.2.2.2.3

The switching frequency is depend on some parameters, Lm, Vin, Io and Csw.

Best Regards

Kevin

Hello Jack,

I would like to expand on Kevin’s response to you, above. The equation for fsw that he cited (Eqn 29 in the UCC28780 datasheet) is a general equation to predict how fsw varies over widely changing input and output conditions. This equation depends on the value of Lm to have been already determined.

However, a previous equation (Eqn 23) uses a factor fsw(min) to calculate the value for Lm. The system designer is required to choose the minimum switching frequency at which the converter should operate. Fsw(min) occurs at maximum load and minimum valley voltage of the bulk capacitor. Eqn 29 will show the full range of variation of fsw based on this choice of fsw(min), and generally one can anticipate that fsw(max) can be over 2X, even up to 3X from fsw(min) at full load. In ACF, fsw(max) can range even higher at lighter loads.

The initial choice of fsw(min) depends to a large extent on the type of MOSFET to be used for switching. Silicon super-junction MOSFETs have high values of Coss and cannot switch efficiently at frequencies higher than a few hundred kilohertz. Given the expected fsw variation, Si-Fets are usually operated less than 300kHz, so fsw(min) can is typically selected around 150kHz. GaN MOSFETs have much lower Coss and switch much faster, so fsw(min) can start at 250kHz~300kHz.

There is a temptation to target >500kHz for GaN MOSFETs, but because of the fsw variation and because efficiency drops at higher fsw, it is still better practice to choose fsw(min) around 250kHz. Although the UCC28780 ACF controller can drive ZVS operation at higher frequencies, AC resistance, core losses and gate-drive bias losses increase as well, so efficiency suffers if you start fsw(min) too high.

Regards,
Ulrich