AN2459 Datasheet, PDF (6/35 Pages) STMicroelectronics – Digital Power Factor Correction for Tube Lamp Ballasts

Power Factor Correction (PFC)
AN2459 - Application note
Figure 2. PFC Transition Mode principle (frequency is not to scale)
Inductor current
Peak current
enveloppe
Average current
On
MOSFET
On
AI12647
2.2
Digital implementation - Enhanced One Pulse Mode
To provide good switch control, as described in Chapter 2.1 above, a simple 8-bit
microcontroller may be used and a special PWM timer mode has been introduced. The
timer mode, called "Enhanced One Pulse Mode" of the PWM generator (12-bit autoreload
timer) is found inside the ST7FLITE19B microcontroller. It is explained in Figure 3 and in
datasheet ST7Lite1xB (4). In principle, when a zero current event occurs the microcontroller
will reset the timer and turn-on the PFC MOSFET. If there is no signal from ZCD, the timer
will overflow and turn-on the MOSFET anyway (it means a minimum switching frequency is
secured). The on-time of the MOSFET is set by a software control routine and is constant
during the mains half-cycle (this is detailed below in Chapter 4). The control routine
executed by the MCU alters the on-time depending on the input voltage level and the load
current.
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