Each a constant voltage but the input current

Each control method has its own qualities and drawbacks
based on the topology of the DCDC converter employed in PFC converter. Now a
day’s average current control method is taken as a standard strategy in the
industry for the boost AC-DC PFC converter, since it has merits of less THD
with improved noise and easy to shape input current waveform This control
method also allows a better improved input current waveform . A current error
amplifier is there as in the fig. 2.2, which filters the inductor current
sensed. The output of the current error amplifier drives a PWM modulator. The
inner current loop tends to reduce the error between the average input current,
ig and its reference.

Average
current mode control is based on a simple concept.
An amplifier is used in the around the boost power stage so that input
currentfeedback loop tracks the programming signal with very little error. This
is the advantage of average current mode control and it is what makes active
power factor correction possible. Average current mode control is relatively
easy to implement and is the method described here. The power circuit of a
boost power factor corrector is the same as that of a dc to dc boost converter.
There is a diode bridge ahead of the inductor to rectify the AC input voltage
but the large input capacitor which would normally be associated with the AC to
DC conversion function has been moved to the output of the boost converter. If
a capacitor follows the input diode bridge it is a small one used only for
noise control. The
output of the boost regulator is a constant voltage but the input current is
programmed by the input voltage to be a half sine wave. The power flow into the
output capacitor is not constant but is a sine wave at twice the line frequency
since power is the instantaneous product of voltage an current.

Out of which here only two methods are discussed below

4. Hysteresis Mode Control

3. Charge control method

2. Average current control method

1. Peak current control method

In general, the PFC control strategy is categorized  as 1) Active Control Method 2) Automatic
control of line current. For CCM, active control method is used, whereas, automatic
control method is related to the DCM operation of the converter . Several control
methods are there under active control methods. Some of them are;

There are different kind of
control methods, among which any one method can be used in PFC application. In
general, for any control strategy for PFC, two basic feedback compensating
loops are required shown in fig. 2.1. A voltage feedback compensating loop is
used as the outer loop to keep the bus voltage to a fixed DC (predefined
reference) value. An inner loop, known as current loop is to control the
inductor current to a specific level and to shape the inductor current with the
aim to be as like as possible to the rectified input DC voltage keeping almost
unity Powerfactor . The PFC power supplies with control loops implementation is
employed to achieve a stable system with a tolerable dynamic behavior
irrespective of the system loading conditions.

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