2.3 DC-DC Buck Converter with Resistive Load
2.3.4 Experimental Investigation
c2 = 15.
The efficacy of the ABSC method in regulating the DC-DC buck converter output voltage has been investigated by simulating the converter in Matlab-Simulink environment with a step size of 50µs.
The response of the controller during start-up is shown in Figure 2.4. The plot in Figure 2.4 (a) shows the estimation of unknown loadR during start-up. It is observed that the online estimation converges to the actual value of R= 20Ω in 0.25sand the output voltagevo tracks the desired reference of 10V in 0.45s as shown in Figure 2.4 (b). Subsequently the responses of the inductor current iL and the control inputu are provided in Figure 2.4 (c) and Figure 2.4 (d) respectively.
Next, the response of the ABSC method is evaluated by abruptly perturbing the load current iR with a step disturbance. The load resistance R is changed from nominal value of 20Ω to 6.66Ω and the response obtained is shown in Figure 2.5. It can be noticed in Figure 2.5 (a) that the ABSC method successfully estimates the perturbed load value of 6.66Ω in a time span of 0.2s. Figure 2.5 (b) exhibits an undershoot of 50% in vo prior to reaching back to its desired value of 10V in 1.5s. The relevant inductor current iL and control input u profiles are shown in Figure 2.5 (c) and Figure 2.5 (d) respectively. The inductor current iLis observed to be a smooth profile without any overshoot.
Further, the robustness of the ABSC method is tested by subjecting the DC-DC buck converter to a step change in input voltage E from nominal 25V to 17V at t = 3s as shown in Figure 2.6. The responses of vo and iL show a settling time of 0.5s to reach the desired operating point.
Lastly, the robustness of the DC-DC buck converter using the ABSC is verified by suddenly changing the desired output voltage vr in a step manner. The reference voltage vr is changed from its nominal magnitude of 10V to 15V and the response recorded is provided in Figure 2.7. The obtained results demonstrate the ability of the proposed ABSC control scheme in yielding a successful tracking of reference output voltage in 0.5s.
2.3 DC-DC Buck Converter with Resistive Load
Estimator Uncertainty Control Law
D/A Converter vo(t)
vr(t) +
Proposed Controller
DSpace1103
A/D Converter Voltage
Sensor Current
Sensor
DC Source LC Filter Resistive Load
DC-DC Buck Converter
vo(t) iL(t)
Drive Circuit Gate
Optocoupler
Pulse Width Modulator
-
(Backstepping)
Figure 2.8: Functional block diagram of the ABSC implementation in DC-DC buck converter.
Figure 2.9: Experimental set-up of DC-DC buck converter.
DC-DC buck converter
• Power MOSFET: An N-Channel Power MOSFET IRFP460 is used as a control switching device in the DC-DC buck converter. It is a product of Fairchild manufacturers having a current rating of 20A, voltage rating of 500V and on state Drain to Source resistancerDSof 0.270Ω. It operates with a Gate to Source voltage VGS = 20V.
• Diode: The diode used for freewheeling purpose is 6A4 MIC manufactured by MIC Group Rectifiers. Its maximum average forward rectified current is 6A and maximum repetitive peak reverse voltage is 400V. Further, its maximum DC blocking voltage limit is 400V.
• Input voltage source: Input voltage is taken from Aplab Regulated DC Power Supply. It can provide voltage in the range 0−32V with 0−2A current output. Additionally it can provide voltages ±15V and +5V, which can be used to power the voltage and current sensors.
Measuring Instruments
• Digital storage oscilloscope: A YOKOGAWA manufactured DLM2054 with 500MHz frequency mixed signal digital storage oscilloscope is used to record the real time response. It has 4 channels for simultaneous sensing of voltage/current signals.
• Voltage transducer: Voltage sensor LV 25-P by LEM Manufacturer is used for sensing the output voltage across the load. It permits nominal rms primary currentIP N of 10mAand nominal rms primary voltage upto 500V.
• Current sensor: For the purpose of inductor current sensing, ACS-712T from Allegro MicroSys- tems is utilized. This is a fully integrated, Hall effect based linear current sensor IC. It has an internal resistance of 1.2mΩ during conduction. It works with a DC supply of 5V. It offers a precise and low-offset current sensing mechanism. The Hall circuit is provided with a conduc- tion path made of copper and it is located near the surface of the die. This helps in creating a magnetic field when there is current sensing through this copper path. This magnetic field helps in converting the current to a proportional voltage.
• Voltage measuring probe: A YOKOGAWA manufactured 700924 differential voltage probe is used to measure the real-time output voltage. It has frequency bandwidth of 100 MHz and maximum permissible continuous input voltage range is 1400V with output offset voltage of
±7.5mV.
• Current measuring probe: Measurement of real-time current is done by using a YOKOGAWA manufactured current probe 701932 with 100 MHz as frequency bandwidth. It allows a maximum continuous allowable input current of 30A.
Control Equipments
The controller has been implemented using DS1103 PPC Controller board. It provides a floating point platform and facilitates an easy conversion of the control algorithm in C language for further
2.3 DC-DC Buck Converter with Resistive Load
• DS1103 PPC controller board: This generates an executable object code for processor. It is a single-board with comprehensive I/O and real-time processor. The controller has been simulated in Simulink and using MATLAB to DSP real-time interface the controller is executed. It uses a built-in unit of Texas Instruments DSP board TM320F240 for computation purpose. A brief mention of its peripherals is given below.
– TMS320F240 DSP: It is a 16-bit fixed point digital signal processor (DSP). The analog to digital converter (ADC) can perform conversions within 6.1µs. It includes the T320C2xLP Core CPU with an application memory of 32 MB.
– Analog to Digital Converters (ADC): ADCs are provided with 16 multiplexed channels equipped with 4 sample and hold circuitry. It has 8 ADC sampler channels (4 multiplexed and 4 parallel) for simultaneous sampling. Its resolution is 16-bit and requires input voltage of±10V.
– Digital to Analog Converters (DAC): DACs are present in 8 channels with resolution of 16-bit each and output voltage range of±10V.
– Serial interface is available with RS232/RS422 compatibility. In this work RS 232 has been made use of.
– Clock rate of the processor is 20M Hz.
• Optocoupler: It is a well known fact that the power circuitry and the control circuitry need to be isolated from each other for safety reasons. Isolation is required to prevent ground loops sharing a common return path. It is most likely that due to any fault, a high current flow from power circuit may damage the control circuitry. In this work IC HPCL-2611 is used for isolating purpose. This optocoupler works at a high speed of 10 Mbit/s. It has single-channel circuit which is optically connected to a high speed photodetector logic gate.
• Gate drive circuit (GDC): In order to finally operate the power switch with PWM, the gate pulse produced by the controller needs to be processed through a gate drive circuit. The GDC assists in amplifying the control pulses from a low-power to a high-power signal thereby providing an adequate signal strength to drive the power switch Sw. Gate drive IC IR 2110 is used in this work. It is a high voltage and high speed driver. It generates a gate voltage ranging from 10V −20V. Besides, it produces output gate current upto 2A. Further, it has fast turn-on and turn-off time periods typically around 120nsand 94nsrespectively.
2.3.4.2 Experimental Results and Discussion
The performance of the ABSC is evaluated on the experimental prototype by first injecting the disturbances at the load end. The load resistance is suddenly changed from 20Ω to 10Ω and again brought back from 10Ω to 20Ω, as a consequence of which the load current is perturbed from 0.5Ato 1.0A and again from 1.0A to 0.5A. The performance exhibited by the ABSC is shown in Figure 2.10 (a). It is observed that the ABSC method takes 7s to track the set 10V with an undershoot of 5V,
(a)
(b)
(c)
Figure 2.10: Experimental response curves of DC-DC buck converter under ABSC scheme during: (a) load resistance R change from 20Ω to 10 Ω and vice-versa (scale: x-axis; time (5s/div), y-axis: voltage (5V/div), current (500mA/div)); (b) input voltageEchange from 25V to 17V and vice-versa (scale: x-axis; time (1s/div),