Introduction :
In
recent years, there have been increasing demands for high power factor. Among
power factor correction techniques, the analogy control method is now
attractive in industry, but the digital control method is the trend in the
future. With the stringent requirements of power quality, power-factor
correction (PFC) has been an active research topic in power electronics, and
significant efforts have been made on the developments of the PFC converters.
In general, the continuous-conduction mode (CCM) boost topology has been widely
used as a PFC converter because of its simplicity and high power capability. It
can be used with the universal input voltage range. Recently, in an effort to improve the
efficiency of the PFC rectifiers, bridgeless PFC circuit topologies are used.
Generally, the bridgeless PFC topologies may reduce the conduction loss by
reducing the number of semiconductor components in the line current path. So
far, a number of bridgeless PFC boost rectifier implementations and their
variations have been proposed.
Conventional Boost Converter :
Diode rectifiers are the most commonly used circuits
for applications where the input is the ac supply. The power factor diode
rectifiers with a resistive load can be as high as 0.9 and it is lower with a
reactive load. With aid of a modern control technique, the input current of the
rectifiers can be made sinusoidal and in phase with the input voltage, thereby
having an input power factor of approximate unity. A unity power factor circuit
combines a full bridge rectifier and a boost converter.
Bridgeless Boost Converter :
The boost inductor is split and located at the AC
side to construct the boost structure. In this first half line cycle, MOSFET M1
and boost diode D1, together with the boost inductor construct a boost DC/DC
converter. Meanwhile, MOSFET M2 is operating as a simple diode. The input
current is controlled by the boost converter and following the input voltage.
During the other half line cycle, circuit operation as the same way. Thus, in
each half line cycle, one of the MOSFET operates as active switch and the other
one operates as a diode: both the MOSFET’s can be driven by the same signal.
The
difference between the bridgeless PFC and conventional PFC is summarized.
Comparing the conduction path of these two circuits at every moment, bridgeless
PFC inductor current only goes through two semiconductor devices, but inductor
current goes through three semiconductor devices for the conventional PFC
circuit.
Comparison:
PFC converter
|
Slow diode
|
Fast diode
|
MOSFET
|
Conduction path On/(Off)
|
Conventional
|
4
|
1
|
1
|
2 slow diode, 1 MOSFET/(2 slow diode, 1 fast diode)
|
Bridgless
|
0
|
2
|
2
|
1 body diode,
1 MOSFET/ (1 MOSFET
body diode,
1diode)
|
Table : Differences between conventional PFC and bridgeless PFC
As shown in Table 4.1, the bridgeless PFC uses one
MOSFET body diode to replace the two slow diodes of the conventional PFC. Since
both the circuits operating as a boost DC/DC converter, the switching loss
should be the same. Thus the efficiency improvement relies on the conduction
loss difference between the two slow diodes and the body diode of the MOSFET.
Besides, comparing with the conventional PFC, the bridgeless PFC not only
reduces conduction loss, but also reduces the total components count.
Automatic
Power Factor Controller :
This can be achieved by using microcontroller based
power factor controller .The main core of this work is to design power factor
controller. This system will be able to control the power factor of both linear
and nonlinear load system. The design aims to monitor phase angle continuously
and in the event of phase angle deviation, a correction action is initialized
to compensate for this difference by continuous changing variable capacitors
value via switching process. The overall system requires only one chip, a few
power electronic components and a bank of capacitors.
Figure : Block Diagram microcontroller based PFC
No comments:
Post a Comment