168 CHAPTER 7 HARDWARE IMPLEMENTATION 7.1 OVERVIEW In the previous chapters discussed about the design and simulation of Discrete controller for ZVS Buck, Interleaved Boost, Buck-Boost, Double Frequency Buck, and SEPIC converters. In this chapter, implementation of Discrete PID controller using LabVIEW for all the above converters has been discussed. The advantages and working of this application software is also revealed and selection of semiconductor switches and various hardware circuits are explained. 7.2 OVERALL BLOCK DIAGRAM OF THE EXPERIMENTAL SETUP The overall block diagram of the Digitally controlled DC-DC converter with the entire setup is illustrated in Figure 7.1. The output voltage from the DC-DC converter is compared against the desired reference voltage using comparator 1. The operational amplifier IC 741 has been used as a comparator 1. The comparator output is nothing but an error voltage V e which in turn gets corrected by the Digital compensator. The Digital compensator is obtained from the LabVIEW and the error output is fed into block diagram section of the LabVIEW through data acquisition card (DAQ) NI 6009 or NI 9221. The block diagram consists of transfer function block H(Z) in which the designed controller values are entered. Hence the error value is corrected
169 by this compensator, the corrected error signal V C is acquired back by DAQ card and fed into comparator 2. Comparator 2 is also designed using operational amplifier IC 741. This corrected signal V C is compared against with the carrier signal (Ramp) whose frequency is as that of the switching frequency of the converter, which is obtained from the signal generator. The resulting Pulse Width Modulated switching pulses are fed to the MOSFET of the DC-DC converter through the gate drive circuit. Figure 7.1 Block diagram of Digitally controlled DC-DC converter The following sections clearly explain all blocks such as DC-DC converter, gate drive circuit, LabVIEW section, comparator1 and comparator 2 which are shown in the overall block diagram. 7.3 DETAILED EXPLANATION OF THE BLOCKS 7.3.1 DC-DC Converter In order to implement DC-DC converter in hardware, it requires inductors, capacitors, resistors and semiconductor switches. Suitable inductor,
170 capacitor and resistor have been chosen and the semiconductor switch plays a vital role in the DC-DC converter. In this portable model, IRF 840 (MOSFET) is used as a main semiconductor switch. This IRF 840 can withstand up to the drain source voltage of 500 V, gate source voltage of ± 20 V, and the drain current of 8 A with the operating temperature 25º C. Even though the MOSFET experiences higher loss than IGBT, it is preferred for the following reasons. High switching frequency Wide line and load variations dv/dt on the diode is limited High light load efficiency Suitable for Motor drives (less than 250 W), Universal input AC-DC fly back and forward converter power supplies, and Low to mid power factor corrections circuit. Diode 1N4001 has been used as supplementary switch in the hardware. It has the features of high current capability and low forward voltage drop, surge overload rating to 30 A peak and low reverse leakage current. 7.3.2 Comparator 1 Basic differential amplifier circuit is used as a comparator 1 circuit that can be made to act as a subtractor as illustrated in Figure 7.2. In this circuit all the resistor values are equal hence the output voltage is equal to the difference of input voltage. The desired reference voltage (V ref ) is given to the inverting input, and the output voltage (V 0 ) of the DC-DC converter is given to the non-inverting input of the operational amplifier. If the input voltage V 0
171 is zero, then the comparator circuit acts as an inverting amplifier and the output voltage V e1 of the comparator is -V ref. Figure 7.2 Schematic diagram of comparator 1 V e1 = V ref R f R i = V ref (7.1) Similarly, if the input voltage V ref is zero, the comparator circuit behaves as a non-inverting amplifier with the input voltage as V 0 2 output voltage V e2 of the comparator is V 0., then the V e2 = V 0 2 1 + R f R i = V 0 (7.2) The output voltage of the comparator V e due to both inputs V ref and V 0 can be obtained as, V e = V e2 V e1 = V 0 V ref (7.3) 7.3.3 LabVIEW Section The DC-DC converter with Discrete PID controller has been implemented using LabVIEW as a controller platform. LabVIEW (Laboratory Virtual Instrumentation Engineering Work Bench) is mainly used
172 as a platform for executing any closed loop system and it can be employed for the improvement of a control system. It is widely used software for evaluating the projects experimentally within a shorter duration due to its programming flexibility along with incorporated tools designed especially for testing, control and measurements. The key feature of LabVIEW is that it widely supports accessing the hardware instrumentation. The abstraction layers and drivers are offered for almost all types of instruments. The buses are also available for addition and the abstraction layers and drivers act as graphical nodes and make possible to communicate successfully with the hardware devices thereby offering standard software interfaces. This software is used to construct virtual instrumentation (VI) which consists of the front panel and a functional block diagram. Virtual instrumentation has an interactive user interface known as the front panel. 7.3.4 Front Panel Figure 7.3 Front panel for the Digitally controlled Buck converter The front panel is the interactive user interface of a VI s window through which the user acts together with the source code. The front panel
173 opens through which inputs pass to the executing program and receive outputs when run a VI. The front panel is essential for viewing the program outputs. The control circuit connected with the front panel is shown in Figure 7.3. Users act together with the front panel when the program is running. Users can manage the program, change inputs, and see data updated in real time. Every front panel control or indicator has a corresponding terminal on the block diagram. When a VI is run, values from the control runs through the block diagram, where they are used in the functions on the diagram, and the results are passed on to other functions or indicators through wires. 7.3.5 Graphical Block Diagram Figure 7.4 Graphical block diagram of Discrete PID controller for Buck converter The graphical block diagram consists of executable icons (called nodes) connected (or wired) together. The block diagram is the source code for the VI, and is mainly used for user communications. It is through the front panel the required transfer function of the Discrete PID controller is entered
174 and the equivalent parameters of the controlled process and hence the updated status of the system is obtained. The block diagram, data acquisition, transfer function and signal generation are built using the functional block diagram as illustrated in Figure 7.4. The Analog output voltage from the external circuit is obtained by Data Acquisition card and it gets converted into Discrete PID controller H(z) block. Block of H(z) is portrayed in the Figure 7.5. In order to get the Discrete transfer function of the converter, coefficients of numerator and denominator are posted in the corresponding row available in Discrete transfer function configuration block. Figure 7.5 Discrete transfer function block for Buck converter 7.3.6 Data Acquisition An electrical and physical signal such as current, voltage, pressure, temperature or sound are measured using Data Acquisition (DAQ) cable. DAQ includes signal, sensors, actuators, signal conditioning, data acquisition
175 devices and application software. It processes the signals from the real world, by digitizing the signals and the data has been analysed, presented and saved. Figure 7.6 Block diagram of Data Acquisition system DAQ system block diagram is shown in Figure 7.6. It involves input/output signal, data acquisition hardware and application and driver software. The physical input/output signals are electrical signal such as voltage and current signal. The voltage signal ranges from 0-10 V, while a current signal ranges from 4-20 ma. DAQ device is used to interface between the computer and the external world. It implies the function such as Analog input, Analog output, Digital I/O and counter/timers. Among the different DAQ devices namely desktop, portable and distributed systems, portable and desktop DAQ devices are employed in this work. The NI DAQ 9221 is a desktop device with screw terminal which has a 63-terminal, detachable connector. It has multifunction data acquisition (DAQ) devices that provide plug and play connectivity to a computer for acquiring, generating and data logging in a variety of portable applications. It comprises of 8 analog inputs, ±60 V input range and 800 ks/s aggregate sampling rate. It has single ended inputs, screw terminal or D-SUB connector type and 12-bit resolution. It incorporates hot-swappable operation, overvoltage protection, and isolation. The NI DAQ 6009 is a portable USB gadget that comprises of 8 analog inputs with referenced single ended signal coupling or 4 inputs with differential coupling, 2 analog outputs, 12 bits A/D and D/A converters and 32 bits counters. There are 12 channels of Digital input/output lines which
176 can be used either as input or output. It eventually provides an excellent platform for the proposed Discrete PID controller. The DAQ pad NI USB - 6009 is shown in Figure 7.7. Figure 7.7 NI-DAQ Pad - 6009 7.3.7 Driver Software Driver software is used for easy communication with the hardware. It acts as a mediator between the application software and the hardware. The driver software employed as the NI - DAQmx consists of DAQ assistant that guides for configuring, testing and acquiring data for the measurement. It makes complex operation easier and faster since it is menu driven and drastically reduces the time for observing the measurements. 7.3.8 Comparator 2 The ramp signal (V ramp ), with a desired frequency of 20 KHz is applied to the non inverting terminal of the operational amplifier (op-amp) and the output from LabVIEW section known as corrected signal (V C ) is given to the inverting terminal of the op-amp through 10 KΩ potentiometer.
177 The schematic diagram of comparator 2 is depicted in Figure 7.8. In the given comparator circuit, ramp signal V ramp is compared against the corrected signal from the LabVIEW section (V C ). The ramp signal is greater than the corrected signal (V ramp >V C ) then the positive pulse is generated otherwise negative pulse is generated. Figure 7.8 Schematic diagram of comparator 2 Figure 7.9 Generation of PWM pulses If V L is positive, and lesser than V ramp then the turn ON pulse is generated and, if it is greater than V ramp then the turn OFF pulse is generated as shown in Figure 7.9. Similarly if V L is negative and greater than V ramp then
178 turn OFF pulse is generated and, if it is lesser than the V ramp then turn ON pulse is generated. In practical, certain amount of time is being taken to switch from one voltage level to another voltage level. In operational amplifier (IC 741) slew rate imposed major limitations and is equal to 0.5 V/µS. ±V Sat is limited by slew rate in the value of ±13 V, which is sufficient enough to turn ON the device called MOSFET which requires a pulse with amplitude of about 10 V to 12 V. 7.3.9 Gate Drive Circuit In a DC-DC converter, MOSFET is a semiconductor switch that acts as a main switch. It is a voltage controlled device, when applying drain source voltage, whose current starts flowing in the drain only when a certain value of voltage is applied between the gate and source terminals. MOSFET has high gain and high input impedance hence a small amount of leakage current flows from the voltage source through the gate since the gate terminal is electrically isolated from the source by a silicon oxide layer. a) In order to turn ON the MOSFET device, sufficient current is needed and hence voltage pulse is applied to a gate source terminal, which charges the input capacitance at the desired time. The MOSFET input capacitance C ISS is the sum of the capacitors formed by the metal oxide gate structure, from gate to source (C GS ) and gate to drain (C GD ). The driving voltage source impedance should be very minimum, to achieve the high transistor speeds. The required driving current and driving impedance can be obtained from the following equations : R G = t r 2.2C ISS (7.4)
179 I G = C ISS dv dt (7.5) Where R G is the generator impedance in Ohm, C ISS is the MOSFET input capacitance in PF, dv/dt is the rate of change of generator voltage in V/ns and t r is the MOSFET rise time in ns. b) An excessive voltage and voltage transients may damage the device hence extreme care should be taken while designing the gate drive circuit. If an excessive voltage is applied to the gate terminal, it may result in the breakdown of the oxide terminal thereby causing permanent isolation and damage. c) The gate drive circuit can be designed in such a way that it is capable of sensing and controlling the fault current. MOSFET can be turned ON and OFF by transferring the charge from the gate terminal. The gate drive circuit can be designed using operational amplifier (IC 741) in which the op-amp is designed as a voltage follower buffer and this in turn improves the slew rate and bandwidth. Gate drive circuit for MOSFET using op-amp is illustrated in the Figure 7.10. Here the output from the operational amplifier drives the push-pull amplifier circuit (combination of Q 1 and Q 2 ) which in turn switches the MOSFET ON and OFF.
180 Figure 7.10 Gate drive circuit for MOSFET using Op-amp In the Figure 7.10, the op-amp acts as a voltage follower hence the output and input are equal (V O1 = V i ). If the input voltage (V i ) is positive, then the transistor Q1 turns ON, Q2 turns OFF, MOSFET gate receives the voltage V CC through Q1. Otherwise Q2 turns ON, Q1 turns OFF, MOSFET gate connected to the ground through Q2. R 1 and R 2 are the current limiting resistors. 7.3.10 Experimental Setup, Results and Discussion Figure 7.11 Experimental set up for Discrete controlled Buck converter
181 The experimental setup for Buck converter with Discrete PID controller has been implemented using LabVIEW as a controller platform is illustrated in Figure 7.11. The experimental prototype and the response of the Discrete PID controlled Buck converter have been illustrated in Figure 7.11 to Figure 7.14. Figure 7.12 Indication of the experimental set up Figure 7.13 Output voltage for 5V reference
182 Figure 7.14 Output voltage measured using CRO Experimental setup for Boost converter with Discrete controller is also depicted in Figure 7.15. LabVIEW (Laboratory Virtual Instrumentation Engineering Work Bench) is a system design platform and development environment for a visual programming language from National Instruments. The experimental setup for Buck converter is developed using the values tabulated in Table 7.1. Table 7.1 Experimental values for Buck converter Description Design Values Switching frequency f S 20 KHz Input voltage V s 12 V Inductor L 15 mh Capacitor C 1 µf Load resistor R 20 Ω MOSFET S IRF 840 Diode D 1N 4001 DAQ NI 6009
183 Figure 7.15 Experimental set up for Discrete controlled Boost converter To evaluate the performance, the input voltage and load resistance has been varied and the corresponding output voltage is measured for the reference of 5 V is illustrated in Figures 7.16, 7.17, and 7.18. In these Figures, Input voltage response is taken at one channel and the output voltage is taken at another channel. Figure 7.16 Output voltage obtained for 12 V input, R 0 = 5 Ω, and V ref = 5 V In the Figure 7.16 the input voltage and reference voltage are given as 12.045 V, 5 V respectively, and the load resistance as 5 Ω whose
184 corresponding output voltage is measured as 5.063 V. The rise time and settling time of the output voltage response is 2 ms and 5 ms respectively. It has very little overshoot and undershoot in the output voltage and the steady state error is less than 1%. Similarly in Figure 7.17, the input voltage, load resistance and reference voltage have been set at 14.01 V, 10 Ω, 5 V respectively and the generated output voltage is observed to be 5.03 V. The rise time and settling time of the output voltage is 1 ms and 1.5 ms respectively. In Figure 7.18, the rise time and settling time of the output voltage are shown as 2 ms and 2.5 ms respectively for the given input voltage 10 V and load resistance 12 Ω. All the output voltage response has neither undershoot nor overshoot but have some oscillation till the response settles down, but is well within the tolerable limit. Figure 7.17 Output Voltage obtained for 14 V input, R 0 = 10Ω, and V ref = 5V
185 Figure 7.18 Output Voltage obtained for 10 V input, R 0 = 12 Ω, and V ref = 5 V The output voltages for the references of 5 V and 7 V along with their switching pulses are shown in Figure 7.19 and 7.20 respectively. In the experiment, the output voltage response taken at channel 2 is observed to be 5.01 V and corresponding PWM pulses with the duty cycle of 41.7 % response is taken at channel 2. Similarly in Figure 7.20, channel 1 indicates output voltage value as 7.19 V and channel 2 shows their corresponding PWM pulses with the duty cycle of 58.7 %. Figure 7.19 Duty cycle obtained for 5V reference
186 Figure 7.20 Duty cycle obtained for 7 V reference As illustrated in Figures, the reference voltage has been changed from 5 V to 7 V which is proportionally instantiated in duty cycle to get the output voltage at same reference level. From the output waveforms, it can be inferred that the output thus observed shows better performance thereby providing that the controller tracks the references in spite of the variation in input voltage and load resistance. 7.4 CONCLUSION The Discrete PID controller for Buck converter is implemented using LabVIEW as a control platform with DAQ device of NI USB 6009. It acts as an outstanding platform for the execution of Discrete PID controller. The error signals from the comparator are acquired by the LabVIEW software quickly and the controlled output is produced as without any time delay. Hence the rise time and settling time of the converter is very low in the order of few milli seconds only. The steady state error thus observed is very minimum and is much lesser than 2 %. No overshoot and undershoot are observed and the Buck converter with Discrete controller is capable of tracking the two different references such as 5 V and 7 V.