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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 āSā 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 āSā 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= (š šššš š” ā š ššš ) 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 āV 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 š1 . 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 šššš 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:-