Software Design
5.2.1 Power Factor Correction Control
PFC control consists of
5.2.1.1 DC-bus Voltage Control
The actual value of the
5.2.1.2 Phase Shift Synchronization
Phase shift synchronization synchronizes the generated PFC reference sine wave to the frequency of the mains. It is realized by means of:
•Zero voltage crossing detection (zero voltage sensor)
The zero voltage crossing sensor generates a falling edge every time when the input voltage crosses zero from positive to negative polarity.
•External interrupt
An external interrupt is triggered by the zero voltage crossing sensor. The interrupt subroutine is used to get the content of timer TIM registers TCNTH:TCNTL for automatic microcontroller trimming and for PFC reference sine wave amplitude gain calculation in lamp run mode.
•MCU oscillator frequency trimming
For microcontroller automatic trimming, the timer TIM is used as an interval counter. The timer is incremented by the internal clock (divided by prescaler). Its content is cleared every IRQ interrupt produced by the voltage zero crossing sensor. A user defined value determines what number should be found in TIM registers TCNTH:TCNTL. On the basis of the comparison between required and actual counter values, the content of the oscillator trim register OSCTRIM is adjusted with ramp.
5.2.1.3 Reference Sine Wave Generation
Reference is performed by the PWM peripheral. The software contains a sine wave table with values for interval and maximum amplitude. The amplitude of the sine wave depends on the value of the
5.2.1.4 Generation of Output PFC Control SIgnal
The generated output PFC signal controls the PFC power MOSFET. An
Dimmable Light Ballast with Power Factor Correction, Rev. 1
36 | Freescale Semiconductor |