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4. System design:
Our system comprised of 3 major parts.
1) Input low pass Filter for harmonic reduction
2) Boost converter for PF correction
3) Fly back converter for Voltage regulation
Block Diagram of the proposed System:
4.2 Power Factor Correction with boost converter:Boost converter is preferred here to serve as a PFC because component requirement is less.
Adaptive Switching Frequency Scheme is used to operate the single switch of boost converter. In
this scheme duty cycle of the wave given to single switch of boost converter is automatically
changes as input voltage varies. Fig.1 shows the block diagram of power factor corrector circuit.
The proposed power factor corrector circuit improves the power factor of overall circuit when it
is cascaded with electric control system for a laptop charger.
Figure 1: Basic Boost Converter
Figure 2: Mode 1
Figure -3: Mode 2
Working of Boost Converter Boost converter is shown in Figure1 .
Following condition should be satisfied for proper operation of boost converter when it is used in
power factor correction.
(a) Boost converter should operate in continuous conduction mode.
(b) The switching frequency is much higher than the line frequency.
Working of Boost converter is divided into two modes.
Mode1
When switch ‘S’ is closed, in this mode of operation the switch is in on state. The current flows
through switch and inductor, so the energy is stored in the inductor. At the same time, the
capacitor discharges and supplies current to the load. Mode1 is shown in Fig. 1.
Mode2
When switch ‘S’ is open, in this mode of operation the switch is in off state and current flows
through inductor, diode and the capacitor with the load and return to main. Mode 2 is shown in
Figure 2.
Design Consideration
If output voltage of the boost converter is represented as Vboost and input voltage is represented
as VRec, the duty ratio (D) of a typical boost converter is given by
D=
(𝑉 𝑏𝑜𝑜𝑠𝑡 − 𝑉 𝑟𝑒𝑐 )
V boost
The inductor shown in fig.1 can be designed using the following equation
Where f=switching frequency and R= Load Resistance The value of capacitance is given by.
Where ∆V is output voltage ripple.
Table 1: Simulation Parameter of Boost Converter
Input Voltage (VRec)
Output Voltage (V boost)
Duty Ratio(D)
Inductor (L)
Capacitance(C)
Load Resistance(R)
Switching Frequency(f)
Control Technique of the switch to control the boost converter:
Two Control loops are used to maintain the output voltage of the converter Vboost constant and
also make the power factor unity.
Voltage Control Loop
The error is estimated from the DC output voltage measurement. The DC output voltage control
loop maintains the rectifier voltage at a set reference value using feedback action. The error at
the DC output is regulated by a PI controller (voltage compensator or Integrator) and the PI
controller output is added to the current control loop to vary the duty ratio to maintain the DC
output voltage constant.
Current Control loop
The current control techniques have gained importance in ac to dc converters used for high
performance applications, where fast response and high accuracy are important. Various current
control methods have been proposed and classified as hysteresis control, predictive control,
linear control and timer controller with constant switching frequency. Here hysteresis control
method is used for current control loop.
Proposed Adaptive Switching Frequency Technique
The control technique (hysteresis control) is designed so that the inductor current follows the
shape of the rectified ac line voltage. To regulate the load, comparator senses the variation
between the output voltage and the fixed dc reference. This error voltage is multiplied with the
sensed rectifier line voltage to control the inductor current amplitude. The advantages of the
control are that one has no need of compensation ramp, converting a voltage source into a fastacting current source, the inductor is easy to design, operating switching frequency is high and
low distorted input current waveforms with fixed load. Fig. 4 shows hysteresis current control
technique to generate the switching pulse
Figure 4: Hysteresis Current Control Technique
4.3 DC Voltage Regulation with Flyback Converter
Figure 5 : Fly back Circuit
Flyback converters is the most widely used converters in applications where output is needed to
be isolated from input for low power ranges 5 W to 150W.
The basic circuit diagram of a Fly back converter is shown in Fig. 5. Its main parts are the
transformer, the primary switching MOSFET Q1, secondary rectifier D1, output capacitor C1 and
the PWM controller IC. Depending on the design of 𝑇1 . The Flyback can operate either in CCM
(Continuous Conduction Mode) or DCM (Discontinuous Conduction Mode). In DCM, all the
energy stored in the core is delivered to the secondary during the turn off phase (Flyback period),
and the primary current falls back to zero before the Q1 switch turns on again. For CCM, the
energy stored in the transformer is not completely transferred to the secondary; that is, the Flyback
current does not reach zero before the next switching cycle.
A flyback converter can either be used
I.
Continuous mode
II.
Discontinuous mode.
Figure 6
(a) CCM current waveform; (b) DCM Current Waveforms
In discontinuous mode the output winding goes to zero before the end of the 𝑇𝑜𝑓𝑓 period so that
all the stored energy is transferred to the load. Its best features are low system cost, simplicity, and
relative ease of implementation. For low current output and power levels below 50W, DCM fly
back is the usually the preferred operating mode, due to its simpler control loop implementation
and lower turn on loss.
In our project we have used discontinuous mode because the transformer is typically smaller and
because the power supply is more stable. In the discontinuous mode of operation the output current
drops to zero before the power switch turns on and the current ramp starts up. In this case the
secondary current ripple starts from a zero base on each cycle and the core only stores energy
during the switch on-time and during the fly back period. Every cycle will have a small dead time
in which nothing in the transformer is energized. It is this shut down time which is unique to the
fly back and which allows the fly back to regulate over a wide range of input voltages and over a
wide range of output currents.
Voltage Control Loop to control the switch of flyback controller:
The error is estimated from the output Vo voltage measurement. The DC output voltage control
loop maintains the output voltage at a set reference value using feedback action. The error at the
DC output is regulated by a PI controller (voltage compensator or Integrator) and the PI
controller output is added to the current control loop to vary the duty ratio to maintain the DC
output voltage constant.
5. Simulation Circuit design:-