CHAPTER 4 PI CONTROLLER BASED LCL RESONANT...
Transcript of CHAPTER 4 PI CONTROLLER BASED LCL RESONANT...
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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
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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 L1, parallel
capacitor C and series inductance L2. 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
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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 L0 and C0. The voltage across the point AB is rectified and
fed to RLE load through L0 and C0. 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 Vo and load current Io 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 (Kp) = 0.05 and integral time
constant (Ki) = 25. Design of PI controller has been discussed in
APPENDIX 5.
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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.
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Figure 4.3 ARM processor LPC 2148
Figure 4.4 Software output using RTOS
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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
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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)
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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)
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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
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A50KHz, 133W, prototype, shown in Figure 4.10, is built to verify
the PI based LCL resonant converters. The L1 is chosen to be 185 H, L2 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 (Lo) 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
Parameters
Load
RiseTime in
ms
SetllingTime in
ms
SteadyStateError
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
Parameters
Load
Rise Timein ms
SettlingTime in
ms
SteadyStateError
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
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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 VAB and Irms 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
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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 loopcontrols
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.