CHAPTER 6 DEVELOPMENT OF A CONTROL ALGORITHM FOR BUCK AND BOOST DC-DC CONVERTERS USING DSP

Transcription

1 115 CHAPTER 6 DEVELOPMENT OF A CONTROL ALGORITHM FOR BUCK AND BOOST DC-DC CONVERTERS USING DSP 6.1 INTRODUCTION Digital control of a power converter is becoming more and more common in industry today because of the availability of low cost, high performance DSP controllers with enhanced and integrated power electronic peripherals such as analog-to-digital (A/D) converters and pulse width modulators (PWM). Digital controllers are less susceptible to aging and environmental variations, and have better noise immunity. Modern 32-bit DSP controllers with processor speed up to 150 MHz and enhanced peripherals such as, a 12-bit A/D converter with conversion speed up to 80n Sec, a 32 x 32-bit multiplier, 32-bit timers and real-time code debugging capability, give the power supply designers all the benefits of digital control and allow the implementation of high bandwidth, high frequency power supplies without sacrificing performance (Bibian 2001, Jinghai Zhou 2001 and Zumal 2002). DSP-based digital control allows the implementation of more functional control schemes, standard control hardware design for multiple platforms and flexibility of quick design modifications to meet specific customer needs. The extra computing power of such processors also allows the implementation of sophisticated nonlinear control algorithms, the integration of multiple converter control into the same processor, and optimize the total system cost (Wanfeng Zhang 2004, Rabiner and Gold 1975). Due to these useful features of the DSP system, it is used as the

2 116 implementation platform of the proposed PI algorithm. This chapter aims to design a controller for buck and boost converters using DSP and to study the performance under input voltage and load variations. 6.2 SPECIFICATIONS OF THE TMS320LF2407A DSP The important features of the DSP used for the implementation of prototyping buck and boost converters are: it is a powerful TMS320LF2407A DSP. It is a cost effective, algorithm development based motion control application tool. Its basic configuration is similar to (both hardware and software) the Texas Instrument EVM kit. The Micro 2407A has many additional features like on board external memory, 16x2 LCD back light, 16 bit DSP processor working at 40MIPS, 16 PWM outputs,2x3 channels, 10 bit ADC, 48K x 16- bit EPROM for monitor, 16K X 16 bit RAM for program memory, 32K X 16 bit RAM for data memory, RS232 compatible serial port, based inductance provided for RF EMI rejection it also contains windows based powerful program development software, used to develop and compile the program. 6.3 PI ALGORITHM The control algorithm used for the design of the digital controller is as follows P error = Reference - Feed back P out = P error x Differential Gain (K D ) I error = Error Previous error I out = I error x Integral Gain (K I ) Controller output = P out + I out The K D and K I are designed in such a way that they reduce the overshoot and settling time of the converter to a very low value.

3 BLOCK DIAGRAM The proposed DSP controller is implemented as a discrete-time digital system using a digital signal processor (DSP) TMS320LF2407A and is shown in Figure 6.1. The DSP is mounted on an evaluation module (EVM) that allows full-speed verification of the TMS320LF2407A code. In addition, an interface board is built to sample and convert the analog switching converter output voltage into digital data and then convert the inferred results into control signals, which from the duty cycle. The instantaneous output voltage, V out is sensed and conditioned by the voltage sensing circuit and then input to the DSP via the ADC channel. The digitalised sensed output voltage V o is compared to the reference V ref depending on the error signal. The PI controller generates the control signal which is given to the PWM and it generates the control pulse. The control pulse generated by the PWM is given to the buck/boost converter and it is switched according to the duty cycle so that the output voltage is maintained constant. Figure 6.1 Block Diagram of the DSP Based Controller for a Buck/ Boost Converter

4 CIRCUIT DIAGRAM A digital signal processor is implemented as the controller for a buck and boost converters for which the circuit diagrams are explained in following the sections Buck Converter The circuit shown in Figure 6.2 is used to drive the actual buck converter circuit. The optoisolator used after the DSP is used to isolate the driver circuit from the DSP. The output control pulses from the DSP are given to the optoisolator (6N 137). It gives the same signal in the output, but it is in the inverted form. The inverted signal from the optoisolator is given to the inverter to get the actual signal which was given to the driver (IR 2110). The driver gives the 15V signal to the MOSFET of the buck converter and it is turned ON and OFF with respect to the control signal given from the DSP to maintain the output voltage constant, irrespective of the input voltage and load variation Boost Converter The circuit shown in Figure 6.3 is used to drive the actual boost circuit. The optoisolator used after the DSP is used to isolate the driver circuit from the DSP. The output control pulses from the DSP are given to the optoisolator (6N137) and it gives the same signal in the output, but it is in the inverted form. The inverted signal from the optoisolator is given to the inverter to get the actual control signal which is given to the driver (IR 2110). The driver gives the 15V output to turn ON and OFF the MOSFET of the buck converter with respect to the control signal from the DSP to maintain the output voltage constant, irrespective of the input voltage and load variations.

5 Figure 6.2 Complete Circuit Diagram of the DSP Based Controller for the Buck Converter

6 Figure 6.3 Complete Circuit Diagram of the DSP Based Controller for the Boost Converter

7 Power Supply The power supply to the buck/boost circuit is shown in Figure 6.4. The transformer used is a step-down transformer, it will give an output voltage of 15 V. The voltage is given to the diode rectifier and it is rectified to DC voltage which is filtered using the filter. The filtered output voltage is given to the series regulator which regulates the output voltage and it gives a constant output voltage of 15V. Figure 6.4 Circuit Diagram of Power supply 6.6 RESULTS AND DISCUSSION Experimental investigations have been performed for the various input voltage and load conditions to the buck and boost converter with a controller implemented using the DSP; these are given below Boost Converter Subjected to an Input Voltage Variation The PI control algorithm is implemented in a TMS320LF2407A DSP to drive the actual circuit of the boost converter with K p = 0.12 and K i = The parameter of the circuit is L = 0.16 mh, C= 47 and the load resistor R = 8 an increasing and decreasing manner. The set value of the output voltage is 6V. The effectiveness of the controller with respect to overshoot and settling time is studied.

8 122 X axis 2 V/dv. Figure 6.5 Boost Converter Subjected to a Variation of input voltage from 3.8 Volts to 2 Volts Figure 6.5 shows the output voltage plotted against time. It is found that the controller acts very effectively and it maintains the constant output voltage of 6 volts irrespective of the input voltage variation. The peak overshoot voltage at the time of input voltage variation is 50% and the settling time is 175 milli seconds. X axis 2 V/dv. Figure 6.6 Boost Converter Subjected to a Variation of input voltage from 2 Volts to 4.2 Volts

9 123 Figure 6.6 shows the output voltage plotted with respect to time. It is found that the controller acts very effectively and maintains the constant output voltage of 6 volts irrespective of the input voltage variation. The peak overshoot voltage at the time of input voltage variation is 60% and the settling time is 90 milli seconds. X axis 2 V/dv. Figure 6.7 Boost Converter Subjected to a Variation of input voltage from 3.8 Volts to 3 Volts Figure 6.7 shows the output voltage plotted against time. It is found that the controller acts very effectively and it maintains the constant output voltage of 6 volts irrespective of the input voltage variation. The peak over shoot voltage at the time of input voltage variation is 20% and the settling time is 100 milli seconds Boost Converter Subjected to Load Variations The boost converter is subjected to a variation of load an increasing and decreasing manner. The effectiveness of the controller with respect to overshoot and settling time at the time of load variations is studied.

10 124 X axis 2 V/dv. Figure 6.8 Boost Converter Subjected to a Variation of Load Figure 6.8 shows the output voltage plotted against time. It is found that the controller acts very effectively and it maintains the constant output voltage of 6 volts instead of a variation of load overshoot voltage at the time of load variation is 30% and the settling time is 150 milli seconds. X axis 2 V/dv. Figure 6.9 Boost Converter Subjected to a Variation of Load

11 125 Figure 6.9 shows the output voltage vs time. It is found that the controller acts very effectively and it maintains the constant output voltage of 6 volts irrespective of the variation of load shoot at the time of load variation is 40% and the settling time is 100 milli seconds. X axis 2 V/dv. Figure 6.10 Boost Converter Subjected to a Variation of Load Figure 6.10 shows the output voltage plotted with respect to time. It is found that the controller acts very effectively and it maintains the constant output voltage of 6 volts irrespective of the load variation from The peak overshoot at the time of variation of load is 30% and the settling time is 150 milli seconds Buck Converter Subjected to an Input Voltage Variation The PI control algorithm was implemented in a TMS320LF2407A DSP to drive the actual circuit of the buck converter with K p =0.12 and K i = The parameter of the circuit is L = 1 mh, C= 1000 and the load an increasing and decreasing manner. The set value of the output voltage is 10 V.

12 126 X axis 5 V/dv. Figure 6.11 Buck Converter Subjected to a Variation of Input Voltage from 15 Volts to 25 Volts Figure 6.11 shows the output voltage vs time. It is found that the controller acts very effectively and it maintains the constant output voltage of 10volts irrespective of the variation of input voltage from 15 volts to 25volts. The peak overshoot voltage at the time of input voltage variation is 30% and the settling time is 300 milli seconds. X axis 5 V/dv. Figure 6.12 Buck Converter Subjected to a Variation of Input Voltage from 15 Volts to 22 Volts

13 127 Figure 6.12 shows the output voltage vs time. It is found that the controller acts very effectively and it maintains the constant output voltage of 10volts irrespective of the variation of input voltage from 15 volts to 22volts. The peak overshoot voltage at the time of input voltage variation is 40% and the settling time is 150 milli seconds. X axis 5 V/dv. Figure 6.13 Buck Converter Subjected to a Variation of Input Voltage from 25 Volts to 16 Volts Figure 6.13 shows the output voltage plotted against time. It is found that the controller acts very effectively and it maintains the constant output voltage of 10 volts irrespective of the variation of input voltage from 25 volts to 16 volts. The peak overshoot voltage at the time of input voltage variation is 10% and the settling time is 150 milli seconds Buck Converter Subjected to Load Variations The buck converter is subjected to load variation from 100 controller with respect to overshoot and settling time at the time of load variations are studied.

14 128 X axis 5 V/dv. Figure 6.14 Buck Converter Subjected to a Variation of Load to Figure 6.14 shows the output voltage plotted against time. It is found that the controller acts very effectively and it maintains the constant output voltage of 6 volts irrespective of a variation of load to t at the time of load variation is 25% and the settling time is 125milli seconds. X axis 5 V/dv. Figure 6.15 Buck Converter Subjected to a Variation of Load from

15 129 Figure 6.15 shows the output voltage plotted against time. It is found that the controller acts very effectively and it maintains the constant output voltage of 6 volts irrespective of a variation of load to time is 125 milli seconds. X axis 5 V/dv. Figure 6.16 Buck Converter Subjected to a Variation of Load from Figure 6.16 shows the output voltage plotted against time. It is found that the controller acts very effectively and it maintains the constant output voltage of 6 volts irrespective of a variation of Load from to time is 100ms. 6.7 HARDWARE IMPLEMENTATION Figures 6.17 and 6.18 show the hardware implementation of the DSP-based controller for a buck and boost converter.

16 130 Figure 6.17 Photograph of the DSP Based Controller for a Buck Converter Figure 6.18 Photograph of the DSP Based Controller for a Boost Converter

17 CONCLUSION In this research study the controller for the buck and boost converters is successably implemented with a DSP using the PI algorithm. The experimental result shows that, for the buck and boost converter, the design method is able to provide fast transient recovery for load and input voltage disturbances. It also provides a steady state output voltage and low overshoot at the time of parameter variations when compared to the existing methods. Further work will be pursued incorporating more intelligent schemes in the controller, so that it can perform the operation very efficiently.