Definition
where the real power is the average, over a cycle, of the instantaneous product of current and voltage, and the
apparent power is the product of the rms value of current times the rms value of voltage.
Critical Conduction Mode (CrM) Controllers
Critical Conduction Mode or Transitional Mode (also known as Borderline Conduction Mode BCM) controllers
are very popular for lighting and other lower power applications. These controllers are simple to use as well as
inexpensive.
The basic CrM PFC converter uses a control scheme (current mode control) similar to that shown above. An
error amplifier with a low frequency pole provides an error signal into the reference multiplier. The other input
to the multiplier is a scaled version of the input rectified ac line voltage. The multiplier output is the product of
the near dc signal from the error amplifier and the full-wave rectified sine waveform at the ac input. and is used
as the reference for the input voltage. The amplitude of this signal is adjusted to maintain the proper average
power to cause the output voltage to remain at its regulated value.
The current shaping network forces the current to follow the waveform out of the multiplier, although the line
frequency current signal (after filtering) will be half of the amplitude of this reference. The current shaping
network functions as follows: Vref is the signal out of the multiplier.
When the power switch turns on, the inductor current ramps up until the signal across the shunt reaches the level
of Vref. At this point the comparator changes states and turns off the power switch. With the switch off, the
current ramps down until it reaches zero. The zero current sense circuit measures the voltage across the inductor,
which will fall to zero when the current reaches zero. At this point the switch is turned on and the current again
ramps up. The frequency of this type of regulator varies with line and load. At high line and light load, the
frequency is at a maximum, but also varies throughout the line cycle (high frequency near zero crossing and low
frequency near the peak).
Continuous Conduction Mode (CCM) Control
First, the peak current stress is low and that leads to lower losses in the switches and other components. Also,
input ripple current is low and at constant frequency, making the filtering task much easier. The following
attributes of the CCM operation need further consideration.
Vrms2 Control
The boost converter is driven by an average current-mode pulse width modulator (PWM) that shapes the
inductor current (the converter’s input current) according to the current command signal, V i. This signal, Vi, is a
replica of the input voltage, Vin, scaled in magnitude by VDIV. VDIV results from dividing the voltage error
signal by the square of the input voltage (filtered by C f, so that it is simply a scaling factor proportional to the
input amplitude).
It may seem unusual that the error signal is divided by the square of the input voltage magnitude. The purpose is
to make the loop gain (and hence the transient response) independent of the input voltage. The voltage squared
function in the denominator cancels with the magnitude of V SIN and the transfer function of the PWM control
(current slope in the inductor is proportional to the input voltage). The disadvantage of this scheme lies in the
production variability of the multiplier. This makes it necessary to overdesign the power-handling components,
to account for the worst-case power dissipation.
PFC Inductor
The key consideration is in selecting the right core material and winding size for a given inductor. The higher
peak-peak current ripple means that the core losses are higher for the CrM operation. It is often more difficult to
use a cheap powder iron core for the CrM operation and achieve the required efficiency. The other key
consideration is the type of core. Toroids are the most popular because they offer low cost, but if the number
of winding turns is high, a bobbin based winding may facilitate easier winding. Some advanced designs use this
approach with Ferrite cores to get better flux control.
PFC Diode
At the instance of the boost switch turn-on, the diode is carrying significant current in the CCM operation. Since
this diode is a high voltage diode, it typically suffers from reverse recovery phenomenon (forced by slower
recombination of minority carriers) which adds to the losses and ringing. It is important to use ultrafast diodes
(preferably with soft recovery characteristics) to mitigate this problem.
UC3854
-Fixed-Frequency PWM Drive
-voltages that vary from 75 V to 275 V and line
-frequencies across the 50-Hz to 400-Hz range.
PFC de yüksek frekans akım harmonikler giriş LC de ki filtrelerden sönümlenir
Shunt direncinin L değerinin değeri düşük olmalı
VRMS
For best control, the VRMS voltage must stay between 1.5 V and 3.5 V.
The UC3854 uses average current-mode control to accomplish fixed-frequency current control with
stability and low distortion.
VCC MİN 16V
