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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.