Closed Loop Control of PWM Based Soft Switched Non Isolated DC-DC Boost Converter

Closed Loop Control of PWM Based Soft Switched Non Isolated DC-DC Boost Converter

Aravind. G. S, P. Ebby Darney

PG Scholars, Associate Prof, EEE Department

SCAD College of Engineering and Technology Cheranmahadevi, Tirunelveli District, Tamilnadu,India

Abstract The conventional boost converter need high duty cycle to obtain high output voltage as well as high power. In order to obtain high voltage and power, the size and rating of the power semiconductor used should be increased. This semiconductor switch will increase conduction loss of the system and also reverse recovery problem is present. To reduce these problems nonisolated non isolated high step up DC-DC converter with soft switching technique is used. But PWM based this technique has high turn off switching losses. To reduce the turn off switching losses a new method called resonant PWM (RPWM) based soft switching nonisolated high step up DC-DC converter is used. The RPWM is performed by utilizing LrCr resonance in the auxiliary circuit; the capacitance is significantly reduced compared to the pulse width modulation method. Because of reduced switching losses, diode reverses recovery problems and increased step up ratio. A PWM based closed loop control is employed to this converter to enhance the load regulation and reliability of the control. The characteristics of Fuzzy Logic based closed loop control system is studied and the transient parameters are obtained. The Fuzzy Logic controller gives better performance on control the voltage under transient conduction.

1. INTRODUCTION

   The use of non-isolated high step-up dcdc converters has been increasing with the use of renewable energy sources like photo voltage system and wind energy systems and also used for the development of dc backup energy storage. Large input current and high output voltage are the two main factors related to the efficiency of the system The large input current results from low input voltage; therefore, low voltage rated devices with low on state resistance are necessary in order to reduce the conduction loss. The boost and buckboost converters are the simplest nonisolated topologies. The non-isolated converters can provide high step-up voltage gain without incurring extreme duty ratios. A coupled inductor high step-up dcdc converter utilizes a voltage doubler and adjusts the turn ratios of the coupled inductor. This converter has been highly efficient because it recycles the energy stored in the leakage inductor of the coupled inductor, but stress across the switches is high and large inductor leakage current may damage the switches.

   Most of the coupled-inductor and switched-capacitor converters are hard switched. The hard-switched CCM boost converter suffers from severe diode reverse-recovery problem in high-current high-power applications. That is, when the main switch is turned on, a shoot through of the output capacitor to ground due to the diode reverse recovery causes a large current spike through the diode and main switch. This not only incurs significant turn-off loss of the diode and turn-on loss of the main switch, but also causes severe electromagnetic interference (EMI) emission. The effect of the reverse-recovery-related problems becomes more significant for high switching frequency at high power level. Therefore, the hard-switched CCM boost converter is not capable to achieve high efficiency and high power density at high power level. Therefore, they are not suitable for high efficiency and high-power applications.

   Some soft-switched interleaved high step-up converter topologies have been proposed to achieve high efficiency at desired level of volume and power level. Among them, the soft-switched continuous conduction mode (CCM) boost converter demonstrated reduced voltage stresses of switches and diodes and zero-voltage switching (ZVS) turn-on of the switches in CCM and zero- current switching (ZCS) turn-off of the diodes. However, a drawback of this pulse width modulation (PWM) converter is high turn-off switch losses. In this project, an improved switching method, called resonant PWM (RPWM) is proposed for the soft-switched CCM boost converter in order to reduce the turn-off switching losses.

2. OPERATING PRINCIPLES OF THE PROPOSED CONVERTER

   Fig.1.Proposed ZVZCS RPWM DC-DC converter.

   PWM method and Resonant PWM method has same circuit topology. Main boost converter and an auxiliary circuit are the two main part of the proposed converter. The resonant capacitor Cr, resonant inductor Lr and two diodes DL and DU are the two main part of the auxiliary circuit. The two switches S1 and S2 regulated the output voltage of the system. This is a CCM mode converter. The turning of the converter is happen due to the high value of peak current. . But in the resonant PWM (RPWM) converter, the switches are turned off with resonance attained by the resonant capacitor Cr and inductor Lr of the system. The current through the resonant circuit decides which switch is turned on and off.

   control to regulate the output voltage is introduced which is based on PWM based control.

3. MODES OF OPERATION

   The operating modes and the key wave of the proposed converter are shown in figs.

   Mode 1(To-T1): This mode begins when upper switch Su which was carrying the current of difference between iLf is turned OFF.SL can be turned ON with ZVS if signal for SL is applied before the current direction reversed. Filter inductor current iLf and auxiliary current iLr starts to linearly increase and decrease, respectively, as follows.

   When closed loop system is introduced, response

   iLf = Vi

   (1)

   of the system is increased and also the steady state of the

   L1

   system is reduced. The resonant PWM (RPWM) converter

   iLr = Vc1-Vc2-Vc3

   (2)

   reduces the conduction losses of the system. It also reduced the reverse recovery problems of the switching diode. This will help to reduce the size of the system. The closed loop

   L2

   Fig 2 : Mode of operating of the proposed RPWM converter.

   Mode 2(T1-T2): In the beginning of this mode the Lr and Cr, auxiliary inductor and capacitor of the auxiliary circuit are resonant with each other. Current iLf is still linearly increasing. The voltage and current of resonant components are determined, respectively, as follows:

   The resonance mode ends when iLr reaches to zero. Note that DL is turned OFF under ZCS condition.

   Mode 3(T2-T3): There is no current path through the auxiliary circuit during this mode. Output capacitors supply

   the load. At the end of this mode the turn-off signal of SL is

   iL1 = Vi

   (3)

   applied. It is noted that the turn-off current of SL, ISL is

   L1

   limited to filter inductor current at T3, ILf which is much smaller than that of PWM method.

   =

   =

   iL2 Vc1-Vc3

   L2

   (4)

   iL1 = Vi

   L1

   Mode 4(T3T4): This mode begins when lower switch SL is turned OFF. SU can be turned ON with ZVS if gate signal for SU is applied before the current direction of SU is reversed. Filter inductor current iLf starts to linearly decrease since voltage vLf becomes negative.

   1. Specifications:

      Determinations of the operating requirements for the hardware design are

      Pout (max):2kW Vinput range: 70V

      Line frequency fL: 50 kHz

      iL1 = Vi-Vc3

      (6)

      Output voltage Vout: 380 VDC

      L1

   2. Selection of switching frequency (fs):

      iL2 = Vc1-Vc2

      (7)

      The switching frequency of the system must be high

      L2

      In this mode the other LrCr resonance of auxiliary circuit is started, and DU starts conducting as same as Mode 2. At the end of this mode the current iLr is equal to iLf .

      Mode 5(T4-T3): This mode begins when the currents iLr equals iLf , the direction of the upper switch iSU changes, then this mode begins. At the end of this mode, turn-off signal of SU is applied and this mode ends. The modes of operation of the system are shown in fig.2

4. CONTROL CIRCUIT OF PROPOSED CONVERTER.

   Fig.3 Closed loop control of the proposed converter

   The closed loop control of the proposed converter is as shown in fig.3. The closed loop control will help to maintain the system stability under transient condition. The closed loop control of the proposed converter is done with the help of a FUZZY Logic controller. The function of the FUZZY Logic controller is to maintain the system stability under transient conditions. This is done by comparing a part of the output voltage with a reference voltage Vref. The output of the error signal is given into a FUZZY Logic controller. The FUZZY Logic controller produce corresponding error signal and it is then given into a PWM converter. The PWM converter produces the corresponding PWM signals and is then given to gates of the switches. The switches will turned on and off with respect to the gate signal given to the system.

   DESIGN OF THE PROPOSED CONVERTER

   The load regulation of the converter is taken as 2kW. Although the design is developed for 2kW rating, the control circuit remains more or less the same for output ranging from 50 W to 5kW.

   enough to minimize the size of power circuit and reduce distortion. On the other hand it should be less for greater efficiency. Compromising between the two factors the value is selected as 50 KHz.

   1. Selection of resonant frequency (fr):

      The resonant frequency must be designed to reduce the total switching loss of the systems. In the below resonant condition both switch turn-off current and diode di/dt are smaller. Therefore, the resonant frequency can be determined by

      fr fs /2Deff

      fr = 50×103/(2×0.63) = 39.68 kHz = 40 kHz

      1. Input current.

         Pinput = Pout (max)

         Ipk = P/ Vinput= 2×103/70=28.571 A

      2. Ripple current.

         Ripple current is usually assumed to about 30% of the peak inductor current.

         I = 0.3 x Ipk= 0.3×28.57=8.57 A peak to peak

      3. Determination of the duty cycle of the system

         Vo = (2Vin / ( 1-D))-Vo

         Vo=5% of the output voltage D=1-(2Vinput/ (Vo+Vo))

         = 1-(2×70/ (380+19))=0.649

      4. To determine the effective duty ratio of the system

         Vo = (2Vin / (1-Deff))

         Deff = 1-(2 Vin / Vo)=1-(2×70/380)=0.63

      5. To determine the output current

   Io=Pout/Vo= 2000/380=5.263A

   1. To determine the input inductor

      Lf=DxVin/2Iin fs=(0.64×70)/(2×8.571x50x103)

      =52.27µH =50µH

      In the practical case input inductor is chosen as 50µH.

   2. To determine the value of auxiliary resonant inductor and auxiliary resonant capacitor. Auxiliary inductor is assumed to be Lr 6µH. Resonant frequency fr

      =1/2LrxCr

      Auxiliary capacitor Cr =1/4 2x(40×10 3)2 x6x10-6

      =2.64×10-6=2.7µF.

   3. Selection of output resistor with output current value, and output voltage. R =

      (Pout/ Io2) = (2000/5.2632) = 72.2

   4. PARAMETERS FOR SIMULATION

   Parameters	Design values
   Input voltage	70V
   Output voltage	380V
   Switching frequency	50KHz
   Input inductance	50µH
   Auxiliary capacitance	2.7µF
   Auxiliary resonant inductor	6µH
   Output current	5.45A
   Power	2 kW
   Output Resistance	72

   Table 1 Parameters for circuit design

   1. FUZZY LOGIC CONTROLLER DESIGN

      The block diagram of fuzzy logic controller (FLC) is shown in fig.4. It consists of three main blocks: fuzzification, inference engine and defuzzification. The two FLC input variables are the error e and change in error e*. Depending on membership functions and the rules FLC operates.

      Fig 4:Block Diagram of Fuzzy logic controller

      1. Fuzzification

         The membership function values are assigned to the linguistic variables using seventeen fuzzy subsets. Table-2 shows the rules of FLC. E and E* are input variables, where E is the error between the reference and actual voltage of the system, E* is the change in error in the sampling interval.

         E* E	NE	ZE	PE
         NE	NE	NE	NE
         ZE	NE	ZE	PE
         PE	PE	PE	PE

         TABLE 2: RULE TABLE FOR FLC

      2. Inference Engine

         Mamdani method is used with Max-Min operation fuzzy combination. Fuzzy inference is based on fuzzy rules. Rules are framed in inference engine block. The output membership function of each rule is given by MIN (Minimum) operator and MAX (Maximum) operator.

      3. Defuzzification

   The output of fuzzy controller is a fuzzy subset. As the actual system requires a non fuzzy value of Control, defuzzication is required. Defuzzifier is used to convert the linguistic fuzzy sets back into actual value

   Fig:5 Membership Functions of Error (e)

   Fig:6 Membership Functions of Change in Error(e*)

   Fig:7: Membership Function of Duty Cycle

5. SIMULATION RESULTS

   The simulation result of the system is as shown below.

   Fig:8 Output voltage wave form of the existing system

   Fig:8 Output current wave form of the existing system

6. CONCUSION

A DC-DC converter for high step up and high power applications is proposed. From the simulation results it is observed that ZVS turn on and ZCS turn off of all the switches is obtained. The voltage stress across the switches is much lesser. The Capacitor size and inductor size are reduced in the proposed topology by adapting proper switching scheme. The output voltage ripple can be reduced to 5%. The closed loop control adjusted the duty ratio of the system. The closed loop control limits the output voltage disturbances in particular limits.

.

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