DEVELOPMENT OF A STEP-UP CONVERTER FOR A PV

SYSTEM

(ELEC5510 – PRACTICAL LABORATORY SESSION 1)

WRITTEN BY

OLADAPO Opeoluwa Ayokunle

(200581534)

SUBMITTED TO

CHONG Ben

SCHOOL OF ELECTRONIC & ELECTRICAL ENGINEERING

THE UNIVERSITY OF LEEDS

LEEDS

LS2 9JT

11 April 2011

1. INTRODUCTION

The issue of cleaner energy production and sustainability has increased the demands for energy

production using renewable technologies including; wind energy, hydro energy, bio-energy and solar

energy.

The solar energy conversion system exploits the photovoltaic effect (PV) for the generation of power

and this has become a very popular source of energy due to the limitless source i.e. the sun.

Although, with the advantages of having a limitless source, no emission of greenhouse gases and

little maintenance costs, the PV energy system has the disadvantages of variation in the availability

of the sun, low energy conversion (due to the efficiency of the PV cells, typically 13%) and a high cost

of installation. The manufacturing technology of the PV cells is been improved to help increase the

energy conversion efficiency and government policies have also helped in subsidizing the cost of

installation. These factors make the solar energy generation very attractive.

The variation in the availability of the sun is a disadvantage which is being mitigated by the use of

power electronic devices for ensuring stability in the level of energy delivered to the load. Thus, this

laboratory session investigates the development of a step-up converter for a PV system aimed at

stabilizing the level of the PV energy system for the load requirements.

The aim of this laboratory exercise is enumerated as follows;

Implementation of pulse width modulated (PWM) signal generation technique using the

dsPIC microcontroller.

Analysis and implementation of the switching circuit consisting of an opto-coupler for driving

the power electronic device (i.e. the MOSFET switch).

Investigation of DC-DC voltage regulation via switching duty cycle variation of the converter

and application to the PV system.

Learn the various measuring techniques of the I-V characteristics of a PV system.

Learn the effect of weather conditions on the I-V characteristics if a PV system.

2. THEORY

2.1. The Photovoltaic (PV) System

The photovoltaic (PV) module converts solar radiation to electrical energy directly, without any

processes in between by exploiting the advantage of the electrical properties of doped

semiconductors p-n junction devices which can be triggered by light.

The basic steps in the operation of a PV module are:

The generation and collection of light-generated carriers to generate a current;

The generation of a very large voltage across the PV module; and

The dissipation of power in the load and all associated resistances in the PV module.

Since the conversion to electrical energy involves no moving parts, the PV module is advantageous

as very little maintenance is required. Also, the semiconductor materials are very abundant in nature

and renewable sources (especially silicon) and produce no known harmful waste products. In spite of

all the outlined advantages the most significant disadvantage is the typical module efficiency of up

to 15% for commercial crystalline silicon PV modules (although up to 25% efficiency has been

recorded in the laboratory) [1] which can be attributed to semiconductor materials and varying

environmental conditions.

A PV module usually consists of individual solar cells electrically connected (in parallel to increase

the current and/or series to increase voltage) to increase their effective power output, since a single

solar cell has a voltage output between 0.5V and 0.7V which is relatively low for power applications.

The electrical operation of a PV cell can be estimated by the equivalent circuit shown in figure 1

while the I-V characteristics is shown in figure 2.

ISCRLOAD

ID ISH

I

RP

RS

Dio

de

Figure 1: Equivalent circuit model of a single photovoltaic cell

ISC

I

IMPP

VOC V

Load Line

MPP

VMPP

Figure 2: I-V characteristics of a PV cell showing the maximum power point and load line

2.2. The Boost Converter implementation

The DC-DC boost converter is an example of a classical switched-mode power supply circuit which is

mostly used for power applications having the output voltages higher than the input voltages. The

DC-DC converter is also used to regulate the dc output voltage under varying load and input

voltages.

Equivalent Circuit model of a DC-DC Boost converter

The circuit of a boost converter is shown in figure 1 below;

Figure 3: DC-DC Boost converter circuit

The switching signal to the switch (typically a MOSFET switch) is usually a pulse-width modulated

(PWM) signal which consequently turns the switch ON and OFF. The duration of the ‘ON’ and ‘OFF’

(i.e. the switching sequence) is determined by the duty ratio of the PWM signal and their sum is

equal to the switching period. The circuit goes from its initial transient state when the inductor

current and capacitor voltage build up to a steady state.

When the switch is ON, the diode is reverse bias and the circuit is separated into two parts as shown

in figure 2.The left part of the circuit shows the dc supply voltage charging the inductor while the

right shows that the output capacitor maintains the output voltage using previously stored energy.

When the switch is OFF as seen in figure 3, both the dc supply voltage and energy stored in the

ISC – short-circuit current ID – diode diffusion current ISh – shunt current (ohmic losses) I – output current RP – shunt resistance RS – series resistance RL – load resistance MPP – Maximum Power Point IMPP – Current at MPP VMPP – Voltage at MPP

iC

Vi VO R C

Do

de

L

VSW

+

-

iL iO

+ VL -

VC

inductor will charge the output capacitor and also supply power to the load hence the output

voltage is boosted as shown by equation 1.

where is output voltage, is input voltage and is the duty ratio.

Once more, the output voltage can be maintained at the constant required level by changing the

duty ratio and consequently controlling the switching sequence.

Figure 4: DC-DC Boost converter circuit (when switch is ON)

Figure 5: DC-DC Boost converter circuit (when switch is OFF)

The boost converter specifications

The implemented boost converter for this exercise is of the specifications below and a block diagram

is also shown in figure 5 below.

Input Voltage, Vin (from the PV panel) = varying Load Resistance, RL = 50.4Ω Switching frequency, fs = 20 kHz Capacitors, C1 and C2 = 82µF Inductor, L = 1mH Duty ratio, k = varying

Microcontroller Opto-Coupler Driver Boost Converter

Figure 6: Block diagram of the implemented microcontroller-based boost converter

C

io

Vo R vc

ic

VI

L

+ vL -

iL

L1 L2

io

ic

VI Vo R C

D L

+ vL -

+v

c

-

iL P

L3 L4

The principles of operation of the implemented boost converter

The switching signal to the MOSFET switch (STB24NF1) of the boost converter is supplied by a

switching circuit comprising of a microcontroller which generates a digital PWM signal, an opto-

coupler which provides electrical isolation between the microcontroller and the boost converter,

and a gate driver which is used to provide the required voltage and current level for powering the

MOSFET.

2.2.1 The Microcontroller

A microcontroller is an integrated chip designed to execute only a single particular task by

running one specific program (stored in the read-only memory; ROM) to control a single

system. Similar to a standard computer, it includes; timers, input/output ports, memory (read-

only, random access) and a central processing unit (CPU). The microcontroller being used is the

18-pin dsPIC30F2011 microcontroller manufactured by MicroChip. The basic specifications and

the pin diagram of this microcontroller are shown below;

SPECIFICATIONS

Random Access Memory (RAM) 1024 Bytes

Read-Only Memory (ROM) 12K Bytes

Program length (approximate) 4000 instructions

Input / Output (I/O) Pins 18

Speed 15 million instructions/second

Table 1: Table showing the specifications of the dsPIC30F2011 microcontroller

Figure 7: Pin diagram for dsPIC30F2011 microcontroller

2.2.2 The OptoCoupler

An OptoCoupler is an electronic device designed to transmit electrical signals by using light

waves to provide connection with electrical isolation between its input and output. The major

function of the OptoCoupler is to provide isolation between the two circuits by preventing

overvoltage damage on the low-voltage side. An OptoCoupler is usually consists an optical

transmitter and an optical receiver, separated by a transparent barrier impedes any electrical

current flow but allows the passage of light. The OptoCoupler used for this exercise is the HCPL-

4503 and the pin diagram is shown in figure 8. When the LED is on, the output voltage is low

and when the LED is off, the output voltage is high as seen in figure 8.

Figure 8: Pin diagram for the HCPL-4503 OptoCoupler

2.2.3 The Gate Driver

A gate driver is a power amplifier that produces the required level of current gate drive for

power switches from low level input of controller IC chips. They are mostly used in PWM

applications where the PWM controller cannot provide the required level of output current for

driving the connected MOSFET. The gate driver utilized for this application is the advanced

TD351 gate drivers for IGBT and power MOSFET. It amplifies the low logic-level signal from the

OptoCoupler (5V – HIGH and 0V – LOW) to the required high logic-level signal (15V – HIGH and

0V – LOW) for switching the power MOSFET. The pin diagram is shown in figure 9 below.

Figure 9: Pin diagram for the TD351 gate driver

3. PROCEDURE

The datasheet for the components were checked and the following parameters which define the

limits for the designed circuit are stated below;

Capacitor:

Rated voltage - 160V DC to 450V DC. But for safety reasons, operating voltage is chosen

as70% of rated voltage. Hence, Operating voltage range: 112.0 V to 315.0 V

Inductor:

Rated current: 1 to 6A

Rated inductance: 0.7mH - 20mH

DC resistance: 0.1 - 3.0 Ω

Power switch:

Maximum drain source voltage: 100V

Supply voltage should be less than maximum drain source voltage

Maximum gates source voltage: -20V or +20V

Gate signal should be less than maximum gate-source voltage, Vgs

Switch current should be less than 26A

Gate threshold voltage: 3V

Static drain source on resistance: 0.055 Ω

Switching time: Turn on delay time: 60ns

Rise time: 15ns

Turn off delay time: 50ns

Fall time: 20ns

Power diode:

Reverse recovery time: 35ns

Reverse recovery time should be faster than switching period

Maximum forward current: 1 µA at T=25oC and V=300V

Threshold voltage: 0.74 V

Socket 2: Load bulbs

Diode

MOSFET

Socket 3: Driver

connection

Cout

CapacitorCin

Capacitor

Socket 1: Input voltage, Vin

Inductor, L

Figure 10: External connections to the PCB layout

3.1. The Switching Circuit

The dsPIC30F2011 microcontroller is programmed to generate the PWM signal using the C

programming language. The flowchart of the program code is shown in figure 10 while the

program listing is included as Appendix A. The requirement for operation of the microcontroller

involves the conversion of an analogue input signal (varied between 0V – 5V) supplied by a

potentiometer) into digital signals at the output. The variation of the potentiometer results in

the variation of the input signal between its maximum and minimum levels but within a

specified duty cycle range of 0 – 60%.

START

DEFINE THE CONSTANTS

FOR PWM GENERATION

DEFINE THE VARIABLES

FOR PWM GENERATION

SAMPLING OF ANALOG

SIGNAL.

IS SAMPLING DONE?

START CONVERSION OF

ANALOG TO DIGITAL

CONFIGURATION OF

PARAMETERS FOR PWM

OPERATION

STOP

YES

NO

Figure 11: Flowchart for the dsPIC microcontroller program for PWM generation

The external connections to the microcontroller as implemented are shown in figure 11. The

output is OC1 containing the PWM signal whose duty cycle is determined by the potentiometer

voltage.

Figure 12: Diagram showing the external connections to the microcontroller

The output from the microcontroller is connected to the optocoupler’s input pin 2 via an input

resistance Rin (functioning as the LED current limiter) of an approximate value of 220Ω which is

calculated using equation 2. The terms VF and IF are device specific with values of 1.5V and

16mA respectively.

Also, the output pin 6 also has a pull-up resistor of 4.7kΩ connected to VCC and a capacitor of

100nF is connected across VCC and ground. The device is supplied by two power supplies of 5V

and 15V connected to pins 3 and 8 respectively. The external connections to the OptoCoupler

circuit are shown in figure 12 below.

Figure 13: Diagram showing the external connections to the opto-coupler

The output of the opto-coupler is connected to the input pin 1 of the gate driver. For this

exercise, a one-level turn off was utilized hence a 4.7kΩ resistor is connected across pin 2 and 3

(reference voltage and delay pins respectively). Also, a 100nF capacitor is connected across the

power supply terminals and a 47Ω resistor (gating resistor) is connected to the output pin 7

which provides the required current capacity for sufficient switching of the MOSFET. The gating

voltage is connected to pin 5 which is a miller current clamp pin whose function is to reduce the

miller effect and hence improving the bandwidth of the gate driver. The external connections to

the gate driver circuit are shown in figure 13.

Figure 13: Diagram showing the external connection to the gate driver circuit

3.2. The Boost Converter

The switching circuit output (i.e. the gate driver output pin 7) was connected to the gate of the

MOSFET as shown in figure 13 above. The boost converter was a ready-built PCB model. The

external connections to the boost converter (the switching circuit, the load bulbs and the input

power supply) were made with the help of the PCB layout and the connections where made as

shown in figure 14 below.

Figure 14: The external connections to the boost converter with the load as two electric bulbs

Since the circuit setup is now a high power set up, the supplies where switched off initially and

the circuit connection was confirmed okay by the laboratory supervisor. Also, the safety

precautions taken are enumerated below;

The load resistance was measured and ensured it is not too high. This is to ensure the

load is not open-circuited thus preventing the capacitor to charge fully.

The appropriate power supply for the low-voltage end and the high-voltage end is used

and care is taken not to use a common ground between the two power supplies.

The connections to the terminal block are ensured to be small to help avoid

interference.

3.3. The identification of the current-voltage (I-V) characteristics of a practical PV

panel and the investigation of boost converter as a controller for the PV

terminal voltage

For this task, a practical PV panel with a sun simulator, a rheostat, an ammeter and voltmeters

were provided. The connection is made as shown in figure 15 below. To the identification of the

I-V characteristic of the PV panel, the rheostat was varied from minimum to maximum and the

values were observed and stored. For the investigation of the boost converter as a PV terminal

voltage controller, the rheostat was replaced with a constant load and the duty cycle of the

PWM signal to the converter was varied. The results were observed and stored also.

V

A

PV PANEL RHEOSTAT

Figure 15: PV Panel external connections

4. RESULTS AND DISCUSSION

The microcontroller circuit, the opto-coupler circuit, the gate driver circuit and the boost converter

circuit were singularly tested for potentiometer voltages of 1.5V, 2.5V, 3.5V and 4.5V. The input and

output waveforms, frequency and duty cycle of each circuit at each potentiometer voltage was

observed.

4.1. The Microcontroller, Opto-Coupler and Gate Driver Tests

The tests carried out on the individual circuits’ show that each circuit operates as expected

when the duty cycle is varied. The input to the microcontroller is in the range 0V – 5V while the

output from the gate driver is in the range 0V – 15V, which is enough to sufficiently switch the

power MOSFET. The frequency value in each circuit stage is observed to be an acceptable

variation of 1.25% from the 20 kHz given in the microcontroller code.

The output from the opto-coupler is observed to be inverted when compared with the output

from the microcontroller and the gate driver. This expectably follows the truth table operation

of the opto-coupler as earlier stated in figure 8 above.

The measured values at the output of each circuit stage and the observed output waveforms at

each circuit stage for the different potentiometer voltages is shown in table 2 and figure 14 – 17

respectively.

Input Potentiometer (V)

Duty Cycle (TON/TS)

Microcontroller Output (V)

Opto-Coupler Output (V)

Gate Driver Output (V)

1.5 0.18 4.40 4.42 15.21

2.5 0.30 4.48 4.55 15.05

3.5 0.42 4.80 4.81 15.07

4.5 0.54 4.80 4.92 15.20

Table 2: Showing the variation of output voltages of each circuit stage with input potentiometer voltage

VOLTAGE

TIME

TIME

TIME

Figure 16: Output waveform at each circuit stage with potentiometer voltage of 1.5V

Microcontroller

Opto-Coupler

Gate Driver

VOLTAGE

TIME

TIME

TIME

Figure 17: Output waveform at each circuit stage with potentiometer voltage of 2.5V

VOLTAGE

TIME

TIME

TIME

Figure 18: Output waveform at each circuit stage with potentiometer voltage of 3.5V

VOLTAGE

TIME

TIME

TIME

Figure 20: Output waveform at each circuit stage with potentiometer voltage of 4.5V

4.2. The Boost Converter Test

With an input supply of 5V, 2A the duty cycle of the PWM signal was varied between 0% and

60% and the output of the boost converter was observed.

As the duty cycle of the PWM signal is increased from 0% to 60%, it was observed that the load

bulb brightness also increases, indicating a proper working condition of the boost converter.

Microcontroller

Opto-Coupler

Gate Driver

Microcontroller

Opto-Coupler

Gate Driver

Microcontroller

Opto-Coupler

Gate Driver

Also, the voltage across the load bulb was observed not to satisfy the input-output relationship

of the boost converter as shown in equation 1. This variation from the theoretical values is

because the equations assume ideal components but the components are non-ideal. A detailed

analysis of each component shows the following;

Although the diode is a fast recovery diode which reduces turn-on loss in the

commutating switch, it still has an internal resistance of 17.7mΩ which causes a voltage

drop of 0.74V across the diode.

The MOSFET has a turn-on resistance of 0.06Ω which causes a voltage drop of

approximately 1.56V (at a drain current of 26A).

Therefore, the approximate voltage drop across the non-ideal components is 2.3V

The theoretical and measured values of the boost converter output for a given duty cycle is

shown in table 3 below.

Input potentiometer

voltage (V)

Duty Cycle

(TON/(TON + TOFF))

Measured converter

output voltage (V)

Calculated converter

output voltage (V)

1.5 0.15 4.59 5.88

2.5 0.30 5.51 7.14

3.5 0.42 6.38 8.62

4.5 0.54 7.81 10.87

Table 3: Converter output voltages at varying duty cycles and input potentiometer voltages (measured and calculated)

The difference between the measured and calculated output voltages of the converter is

noticed to vary. This is because of the different ‘ON’ times as it is observed that an increase in

the ‘ON’ time satisfies the approximate voltage drop stated earlier.

4.3. The identification of the current-voltage (I-V) characteristics of a practical

PV panel

The I-V characteristics of the PV panel was derived by the variation of the rheostat from

minimum to maximum and the measured values were recorded for two sets of temperature

(low and high temperature) under three irradiation levels. The recorded values are shown in

table 4 and 5. The recorded values are plotted and shown in figure 20 and 21.

The obtained I-V characteristics is related to the resistance value of the of the rheostat with the

linear relationship stated below;

A straight line from the origin through each data point (a set of I-V) is referred to as the load

line and if defines a single operating point of the PV panel.

T1 (24.00 ) T2 (25.00 ) T3 (25.75 )

G1 (140W/m2) 40% G2 (210W/m2) 60% G3 (370W/m2) 100%

VPV (V) IPV (A) VPV (V) IPV (A) VPV (V) IPV (A)

0.334 1.229 0.419 1.790 0.606 2.980

2.450 1.212 4.930 1.750 5.880 2.874

8.520 1.134 9.420 1.710 10.770 2.769

10.420 1.099 11.870 1.638 14.940 2.684

12.350 1.059 13.460 1.583 17.360 2.484

14.180 1.030 15.910 1.524 18.130 1.867

15.820 1.000 16.370 1.472 18.580 1.406

17.410 0.916 18.050 1.392 18.900 1.009

18.040 0.700 18.670 1.239 19.150 0.927

18.670 0.336 18.890 1.108 19.210 0.725

18.870 0.226 18.990 0.414 19.414 0.623

18.940 0.172 19.210 0.216 19.560 0.221

19.010 0.127 19.310 0.128 19.630 0.130

Table 4: PV Panel I-V measurement for low temperature (23 - 26 ) at different weather conditions (G1, G2, and G3)

Figure 9: I-V curve for low temperature (23 - 26 ) at different weather conditions (G1, G2, and G3)

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25

G1 (140W/m2)

G2 (210W/m2)

G3 (210W/m2)

Vpv

Ipv

T1 (33.00 ) T2 (34.00 ) T3 (33.00 )

G1 (140W/m2) 40% G2 (210W/m2) 60% G3 (370W/m2) 100%

VPV (V) IPV (A) VPV (V) IPV (A) VPV (V) IPV (A)

0.371 1.246 0.380 1.800 0.420 2.920

4.270 1.220 4.140 1.832 5.630 2.831

9.600 1.127 11.360 1.685 10.060 2.742

13.740 1.097 14.870 1.613 14.910 2.622

15.370 1.018 16.320 1.364 16.090 2.513

16.150 0.843 16.720 1.118 17.364 2.295

16.470 0.738 16.800 1.069 17.729 2.017

16.650 0.656 17.110 0.851 17.450 1.840

16.960 0.520 17.430 0.591 17.900 1.645

17.150 0.421 17.670 0.413 18.000 0.987

17.460 0.253 17.730 0.294 18.370 0.520

17.700 0.118 18.020 0.120 18.730 0.129

Table 5: PV Panel I-V measurement for high temperature (31 - 35 ) at different weather conditions (G1, G2, and G3)

Figure 10: I-V curve for high temperature (31 - 35 ) at different weather conditions (G1, G2, and G3)

It is observed that the variation of the rheostat resistance conforms to the relationship stated in

equation 3 above. However, the difference between the measured and the theoretical I-V

characteristics is due to the variation of the temperature of the PV panel under the various

weather conditions. The values of the open-circuit voltage and the short-circuit current also

varies with the changing weather conditions according to equations 4 and 5 below.

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20

G1 (140W/m2)

G2 (210W/m2)

G3 (370W/m2)

Vpv (V)

Ipv

(A)

The maximum power point for each weather condition and temperature is listed in table 6

below;

IRRADIATION LEVEL

LOW TEMPERATURE HIGH TEMPERATURE

Vpv (V) Ipv (V) MPP (W) Vpv (V) Ipv (V) MPP (W)

G1 (140W/m2) 17.410 0.916 15.948 15.370 1.018 15.647

G2 (210W/m2) 18.050 1.392 25.126 14.870 1.613 23.985

G3 (370W/m2) 17.360 2.484 43.122 16.090 2.513 40.434

Table 6: Showing the maximum power point variation with varying weather conditions at different temperatures

Therefore, it is worth noting the effect of increasing temperature as;

Significant decrease in the open-circuit voltage and a slight increase in the short-circuit

current but an overall decrease in the power output and the maximum power point.

Also, it is worth noting the effect of increased irradiation level as;

Significant increase in the short-circuit current and a slight increase in the open-circuit

voltage but an overall increase in the power output and the maximum power point.

4.4. The investigation of DC-DC converter as a controller for the control of the PV

terminal voltage

The rheostat was replaced with a constant load of 34.3Ω and the maximum power point at each

condition is measured and recorded. The maximum gate drive voltage ( ) across the

MOSFET is 15V and this is used to calculate the duty cycle at each test stage. The equivalent

resistance seen from the input of the converter determines the current the converter supplies

thus by changing the equivalent resistance; the current supplied to the constant load can be

varied accordingly to maintain a constant voltage across the load.

Analysing the converter as a lossless circuit, it can be assumed that the input power is equal to

the output power. And thus a relationship between the equivalent input resistance and the

constant load resistance can be derived as follows;

If , substituting equation 1 and resolving the equation gives the following;

The weather conditions were varied and the results were observed and recorded in table 7

below;

Weather condition

(measured)

(calculated)

40% 3.540 0.236 17.190 0.902 19.060 20.020

60% 5.570 0.371 17.330 1.440 12.030 13.570

100% 7.690 0.513 17.410 2.640 6.590 8.134

Table 7: Showing the variation of measured and calculated equivalent input resistance of converter with weather condition

It is observed that the DC-DC converter functions as a controller for holding the terminal voltage

of the PV panel at the requirement of the connected load. The difference in the measured and

calculated values is due to the assumption made that the converter is a lossless circuit which in

reality is not.

The regulation of the duty cycle varies the equivalent resistance seen from the input of the DC-

DC converter. This action of the duty cycle regulation ensures that the voltage across the PV

terminal and thus the load is kept constant by regulating the current supplied by the converter

to the load. It is observed from table 7 above that as the current supplied to the load increases

the equivalent input resistance reduces according to the basic ohms’ law (V=IR) thus keeping

the terminal voltage constant.

5. CONCLUSION

The microcontroller-based generation and control of a pulse width modulated (PWM) signal has

been learnt and the implementation of the switching circuit used to appropriately drive power

electronic devices has been learnt. The use of opto-coupler and its conversion efficiency has also

been learnt.

From earlier studies of the PV energy system, the measurement of the terminal voltage of a practical

PV panel has been learnt and the variation of the I-V characteristics with different weather

conditions (temperature, irradiation level e.t.c.) has been practically identified.

Although the theory of the control of the terminal voltage of a PV system has been earlier learnt,

this exercise has helped in the practical application of this knowledge by regulating the terminal

voltage of the PV panel using duty cycle regulation.

The efficiency of the DC-DC boost converter can be improved by carefully choosing the input

inductor and the output capacitance as they determine the ripple effect of the whole converter

circuit which consequently determines the efficiency of the converter. The efficiency can also be

improved by carefully choosing the sampling time of the PWM signal driving the power MOSFET

switch as this also has an effect on the ripple content of the converter.

REFERENCES

1. http://fayazkadir.com/blog/?page_id=460 : “DC-DC Converter”, accessed February 24, 2011

2. Ned Mohan, Tore M. Undeland and William P. Robbins, Power Electronics: Converters,

Application and Design, (John Wiley and Sons Inc., Second Edition), pp. 61-74, 172-177

3. Li Zhang (2010): Handout ELEC5564 – Power Generation By Renewable Sources, Section 2,

Power Converters and Applications to Renewable Generation

4. http://www.iea-pvps.org/pv/materials.htm : “Photovoltaic Cells”, accessed February 5, 2011

5. William Shepherd, Li Zhang, “Power Converter Circuits”, Marcel Dekker Inc. ISBN 0-8347-5054-

3, 2004

APPENDIX A

Program Code for Microcontroller-based PWM generation

#include <p30f2011.h>

#define FCY 29480000 //FRC w/ PLL 16x instruction cycle – clock rate 7.37MHz

//See Note 3 of Table 20-16 in Pg 161 of datasheet

#define MILLISEC 29480 // 1 mSec delay constant

#define SCALE 0.3597 //Scaling factor for PWM generation

int main(void); /* function declaraions */

void init(void);

void DelayNmSec(unsigned int N);

int ADCValue; /*Variable declarations*/

int DutyCycle;

int main() /*Main Program starts here*/

init(); /*Program initialization – Jump to this function*/

while (1)

ADCON1bits.SAMP = 1; // start sampling ...

DelayNmSec(100); // for 100 mS

ADCON1bits.SAMP = 0; // start Converting

while (!ADCON1bits.DONE); // conversion done?

ADCValue = ADCBUF0; // yes then get ADC value 12-bit

DutyCycle = ADCValue*SCALE;

OC1RS = DutyCycle;

asm ("clrwdt"); /* clear watchdog timer */

// repeat

return 0;

/* ***** INITIALISATION ***** */

void init(void)

//setup ADC – see Page 111 – 117 of data sheet for reference

ADPCFG = 0xFFFB; // all PORTB = Digital; RB2 = analog

ADCON1 = 0x0000; // SAMP bit = 0 ends sampling ...

// and starts converting

ADCHS = 0x0002; // Connect RB2/AN2 as CH0 input ..

// in this application RB2/AN2 is the input

ADCSSL = 0; //Channel scanning is disabled

ADCON3 = 0x0003; // Manual Sample, Tad = internal 2 Tcy

ADCON2 = 0; //

ADCON1bits.ADON = 1; // turn ADC ON

// config o/p compare module for pwm operation - refer to Page 88–90 of datasheet

OC1CONbits.OCM = 6;

// set output compare (PWM duty cycle) value

OC1RS = 1178; //(80%) refer to Eqn 12-1 and figure overleaf

//setup timer2 – refer to Page 77 – 81 of datasheet

T2CON = 0; //reset timer2 control bits – Pre-scale 1:1

/* set Timer 2 period register */

PR2 = 1473; /* 20kHz */

/* select internal timer clock */

T2CONbits.TCS = 0;

T2CONbits.TON = 1;

void DelayNmSec(unsigned int N)

unsigned int j;

while(N--)

for(j=0;j < MILLISEC;j++);