chapter 6 development of a control algorithm for buck and...

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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

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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

Perror = Reference - Feed back

Pout = Perror x Differential Gain (KD)

I error = Error – Previous error

Iout = Ierror x Integral Gain (KI)

Controller output = Pout + Iout

The KD and KI are designed in such a way that they reduce the

overshoot and settling time of the converter to a very low value.

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6.4 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, Vout is sensed and conditioned by the voltage sensing circuit and then

input to the DSP via the ADC channel. The digitalised sensed output voltage

Vo is compared to the reference Vref 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

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6.5 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.

6.5.1 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.

6.5.2 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.

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

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

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6.5.3 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.

6.6.1 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 Kp = 0.12 and

Ki = 0.03. 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.

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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.

Figure 6.6 Boost Converter Subjected to a Variation of input voltage

from 2 Volts to 4.2 Volts

X axis 2 V/dv. Y axis 500ms/dv


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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


Figure 6.7 Boost Converter Subjected to a Variation of input voltage from 3.8 Volts to 3 Volts



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


6.6.2 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.


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Figure 6.8 Boost Converter Subjected to a Variation of Load



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.




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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.


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


6.6.3 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 Kp=0.12 and

Ki = 0.03. 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.


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Figure 6.11 Buck Converter Subjected to a Variation of Input Voltage

from 15 Volts to 25 Volts



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.





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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.



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.

6.6.4 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.

X axis 5 V/dv.Y axis 500ms/dv

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Figure 6.14 Buck Converter Subjected to a Variation of Load

to



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.

Figure 6.15 Buck Converter Subjected to a Variation of Load from



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output voltage of 6 volts irrespective of a variation of load to


Figure 6.16 Buck Converter Subjected to a Variation of Load from



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.

X axis 5 V/dv.Y axis 500ms/dv

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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

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6.8 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.

chapter 6 development of a control algorithm for buck and...

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