61 CHAPTER 4 PI CONTROLLER BASED LCL RESONANT CONVERTER This Chapter deals with the procedure of embedding PI controller in the ARM processor LPC2148. The error signal which is generated from the reference and load voltage is used to change the MI to maintain the constant output voltage. Further, PI controller can be used to control the PWM signal for closed loop control of converters. This changes the MI of PWM signal to have better control. Hence the modeling and analysis of LCL resonant converter with PI controller becomes more important. This chapter focuses the study of PI controller based LCL Resonant converter implemented using ARM processor LPC2148. The simulated results are compared with the hardware results to validate the same. 4.1 INTRODUCTION The block diagram of LCL RC with PI controller is shown in Figure 4.1. The resonant tank consisting of three reactive energy storage elements (LCL) against the conventional resonant converter that has only two elements. The first stage converts a DC voltage to a high frequency AC voltage. The second stage of the converter is to convert the AC power to DC power by suitable high frequency rectifier and filter circuit. Power from the resonant circuit is taken either through a transformer in series with the resonant circuit or series in the capacitor comprising the resonant circuit. In both cases the high frequency feature of the link allows the use of a high
62 frequency transformer to provide voltage transformation and ohmic isolation between the DC source and the load. In LCL RC the load voltage can be controlled by varying the switching frequency or by varying the phase difference between the inverters. The phase domain control scheme is suitable for wide variation of load condition because the output voltage is independent of load. The DC current is absent in the primary side of the transformer, so there is no possibility of current balancing. Another advantage of this circuit is that the device currents are proportional to load current. This increases the efficiency of the converter at light loads to some extent because the device losses also decrease with the load current. If the load gets short at this condition, very large current would flow through the circuit. This may damage the switching devices. Figure 4.1 Block diagram of PI controller based LCL resonant converter The resonant circuit consist of series inductance L 1, parallel capacitor C and series inductance L 2. S1-S4 are switching devices having gate turn-on and turn-off capability. D1 to D4 are anti-parallel diodes across these switching devices. The MOSFET (say S1) and its anti parallel diode (D1) act
63 as a bidirectional switch. The gate pulses for S1 and S2 are in phase but 180 degree out of phase with the gate pulses for S3 and S4. The positive portion of switch current flows through the MOSFET and negative portion flows through the anti-parallel diode. The RLE load is connected across bridge rectifier via L 0 and C 0. The voltage across the point AB is rectified and fed to RLE load through L 0 and C 0. In the analysis that follows, it is assumed that the converter operates in the continuous conduction mode and the semiconductors have ideal characteristics. 4.2 PI CONTROLLER BASED LCL RESONANT CONVERTER SIMULINK MODEL The closed loop simulation using PI controller for the LCL RC is carried out using MATLAB/Simulink software. Depending on error and the change in error, the value of change of switching frequency is calculated. Set parameter instruction and function blocks available in MATLAB are used to update the new switching frequency of the pulse generators. 4.2.1 PI Based Control Controllers based on the PI approach are commonly used for DC DC converter applications. Power converters have relatively of low order dynamics that can be well controlled by the PI method. PI based closed loop simulink diagram of LCL is shown in Figure 4.2. The system is simulated with a switching frequency of 50 KHz. The simulated converter output voltage V o and load current I o for applied at 10 milliseconds. It is observed that the PI for LCL regulates the output voltage with a settling time of 0.1 millisecond. The following parameter settings are considered for PI controller: Proportional gain constant (K p ) = 0.05 and integral time constant (K i ) = 25. Design of PI controller has been discussed in APPENDIX 5.
64 Figure 4.2 Closed loop simulink model of LCL using PI 4.2.2 RTOS Based Control ARM Processor LPC 2148 shown in Figure 4.3. In this work the applicability of the Philips ARM processor LPC 2148 is investigated as the controller for the LCL resonant converter. The time sharing feature of the LPC2148 offers ample possibility for its use in the designed LCL RC which has a resonance frequency of 50 KHz. The RTOS output waveform is shown in Figure 4.4. The LPC2148 standard features and program are provided in Appendix 3 and Appendix 4 respectively.
65 Figure 4.3 ARM processor LPC 2148 Figure 4.4 Software output using RTOS
66 4.3 RESULTS AND DISCUSSION The proposed model has been simulated using MATLAB/Simulink toolbox. The fuzzy controller and PI controller has been designed for LCLRC. The simulated wave forms of resonant voltage, resonant current, output voltage, and output current are shown in Figures 4.5 to 4.9. 4.3.1 Open Loop Response The response for a reference voltage of 50V and output voltage is 48V, in the open loop response, the overshoot and the settling time are very high, and the response is oscillatory. The proposed control strategy is able to eliminate the peak overshoot and reduce the settling time. The resonant inverter voltage, resonant current and output voltage are shown in Figure 4.5. Figure 4.5 Inverter voltage and current waveforms
67 Figure 4.6 LCL Resonant converter output voltage The output voltages of the open loop LCL RC are shown in Figure 4.6. Here the settling time 0.6 milliseconds for 50% of load and 0.9 milliseconds for 100% of load, the steady state error for 50% of load is 0.06 and 100% of load is 0.079. 4.3.2 PI Closed Loop Response In the closed loop response by using PI Controller, the overshoot and settling time is less compared to open loop, and the response is oscillatory. The plots of resonant voltage and resonant current are shown in Figure 4.7, the justified that settling time of output voltage in open loop controller is more than that of the settling time in PI controller. Figure 4.7 LCL converter output voltage and current (PI)
68 The slight drop in the resonant characteristics is due to the increase in conduction losses in the bridge inverter and resonant network. The output voltage of the LCL RC with PI controller are shown in Figure 4.7, here the settling time is 0.058 millisecond for 50% of load and 0.1 millisecond for 100% of load, the steady state error for 50% of load is 0.06 and 100% of load is 0.079. The Harmonic spectrum for open loop and PI control are shown in Figures 4.8 and 4.9 respectively. The THD value of LCL RC with open loop control is obtained 27.1% and using PI controller, it is obtained 8.9%. The result is justified that %THD in open loop controller is more than that of the PI controller. Figure 4.8 THD for 50% load (Open loop)
69 Figure 4.9 THD for 50% load (PI) 4.4 PERFORMANCE EVALUATION The open loop LCL and Closed loop RC have been estimated and provided in Tables 4.1 and 4.2. It is seen that the PI based closed loop controller provides better settling time. Figure 4.10 Prototype model for PI based LCL resonant converter
70 A50KHz, 133W, prototype, shown in Figure 4.10, is built to verify the PI based LCL resonant converters. The L 1 is chosen to be 185 H, L 2 is chosen to be 0.4 H, and the resonant capacitor (C) is chosen to be 0.052 F.The transformer turns ratio is unity. The primary and secondary-side switches are selected to be IRF 540. The secondary side inductor (L o ) is chosen to be 202 H. The transformer core is chosen to be EER4242. This ensures that the system can be controlled effectively. The percentage THD and Efficiency performance of both open loop and closed loop controller for various load conditions are given in Tables 4.1 and 4.2. Table 4.1 Summary of performance evaluation for open loop control Load Parameters Rise Time in ms Setlling Time in ms Steady State Error THD % Efficiency % Full Load Resistive 0.52 0.58 0.079 26.7 85.64 50% Load Resistive 0.4 0.66 0.06 27.1 80.61 11% Load Resistive 0.35 0.79 0.05 31.5 74.09 Full load Inductive 0.44 1.2 0.1 24.3 81.17 Full load Capacitive 0.52 1.4 0.12 27.6 81.68 Table 4.2 Summary of performance evaluation for PI closed loop control Load Parameters Rise Time in ms Settling Time in ms Steady State Error THD % Efficiency % Full Load Resistive 0.059 0.058 0.058 7.9 89.64 50% Load Resistive 0.04 0.1 0.03 8.9 85.61 11% Load Resistive 0.03 0.12 0.02 9.9 77.09 Full load Inductive 0.05 0.18 0.048 11.8 85.17 Full load Capacitive 0.51 0.2 0.052 13.7 86.68
71 From the Tables 4.1 and 4.2, it is obvious that the rise time and settling time of open loop controller has been compared and concluded that PI has got better performance. Figure 4.11 V AB and I rms at 50% resistive load Figure 4.11 shows voltage across the terminals A and B and current through the primary side of the high frequency transformer with the frequency is 50 KHz at 50% resistive load. Figure 4.12 Loads versus THD for open loop and closed loop controls The Figure 4.12 shows the graph for load versus THD for open loop and closed loop controls has been plotted which depicts that the THD
72 increases for lower load and gradually decreases with increase in load and remain constant at greater loads. Among the two curves PI is well defined. The Figure 4.13 shows the graph for load versus % efficiency for open loop and closed loop controls has been plotted. Among the two curves PI is well defined. The above discussion revels that the PI Controller parameters are easy to determine. The PI control strategy is used to reduce the load sensitivity. The results obtained indicate that the PI is an effective approach for DC-DC converter output voltage regulation. Figure 4.13 Load versus % Efficiency for open loop and closed loop controls 4.5 SUMMARY A PI based LCL RC circuit was simulated in MAT LAB/ Simulink and experimentally done. The effectiveness of PI with open loop controller was verified. ARM (Advanced RISC Machine) processor LPC 2148 was used for the controller for both PI and open loop based resonant converter.