Implementation of a digital control for Cuk power factor correction (PFC)

Inhalt:
In this paper a new digital control concept of a Cuk-PFC with average current mode control will be presented. An input voltage measurement for current reference is not required, only input current and output voltage have to be measured. Two possibilities for the average current detection using a shunt or current transformer will be shown. Today's digital signal processors (DSPs) offer high performance with fast analog to digital converters. It is possible to implement any complex digital control strategy. Due to the cost factor a DSP is not used, the control algorithm is executed by a midrange microcontroller, therefore all the associated restrictions must be considered. The program code has to be optimized specifically for this purpose, short and effective algorithms are necessary. For the dynamic circuit modelling, the state space averaging technique is used. The limited performance of the microcontroller does not allow a quasi-continuous digital control design, dead times will produce non-negligible phase lag, which can lead to instabilities. The control loop analysis has to consider the computation time and the sample and hold from the PWM module. The open loop gain and phase plot will be shown for various operation points. Finally the results will be verified by some meas-urements on a hardware setup.