Vietnamese German University

Faculty of Electrical Engineering and Information Technology

Frankfurt University of Applied Sciences

Faculty Computer Science and Engineering

Project Report

Design of a Photovoltaic Power System Using Boost

Converter for DC Applications

Student:

Nguyen Phan Trung Hieu 1067880

Supervisor:

Prof. Dr. Hartmut Hinz

Table of Contents

1. Introduction ................................................................................................................... 1

2. Principle of Operation ................................................................................................... 2

3. Hardware Design ........................................................................................................... 3

3.1. Data acquisition module ......................................................................................... 3

3.1.1. Current measurement ....................................................................................... 4

3.1.2. Voltage measurement ...................................................................................... 6

3.2. Boost Converter ...................................................................................................... 8

3.2.1. Theoretical analysis ......................................................................................... 8

3.2.2. Boost converter realization ............................................................................ 12

3.2.3. Electrical isolation ......................................................................................... 12

3.3. Controller .............................................................................................................. 13

3.3.1. PWM generator ............................................................................................. 13

3.3.2. Data conversion block ................................................................................... 14

4. Photovoltaic module .................................................................................................... 15

5. Proposal for a solar simulator ...................................................................................... 17

5.1. Requirements ........................................................................................................ 17

5.2. Mechanism of mounting angle adjustment ........................................................... 17

5.3. Light source .......................................................................................................... 18

6. References ................................................................................................................... 19

1

1. Introduction

Output power of a photovoltaic (PV) panel does not stay at a fix level, rather vary greatly as

the weather changes moment by moment [1]. Therefore, it is impossible to harness generated

power efficiently if a fix load is directly connected to a PV panel. In addition, output voltage

of PV panel or array of PV panels usually differs from specified operating voltage of DC

loads.

A DC-DC converter serves as a mediate impedance matching device is a well-known

solution for these two problems of photovoltaic power system. Impedance of loads are

relatively constant compared to output impedance of PV panels, which depend dynamically

on many parameters [2]. This suggests that the DC-DC converter of a suitable topology must

also be manipulated dynamically, usually by a PWM signal that controls the conversion

factor, in order to maintain a good matching so that the PV panels always operate at

maximum power-point (MPP).

The aims of this project are to design and to implement a photovoltaic power system that

has the capability of tracking maximum power-point (MPPT) in various testing conditions.

2

2. Principle of Operation

In contrast to the complex behavior of photovoltaic processes, the proposed controlling

method is relatively simple. The block diagram of figure 2.1 is a demonstration for operation

of the whole system.

Figure 2.1 Functional block diagram of the system

Source: drawn by the author

The DC-DC boost converter (Section 3.2) is chosen as impedance matching device for the

fact that output voltage of the PV panel is usually lower than operating voltage levels of DC

loads. The controller (Section 3.3) manipulates the conversion ratio of the boost converter

with the help of PWM generator module (Section 3.3.1). The data acquisition module

(Section 3.1) measures output current and output voltage of the PV panel. The controller

multiplies measured values to compute output power.

In each cycle, the controller adjusts duty cycle of PWM signal in one direction. Measured

values of output current and output voltage are then multiplied to get the value of output

power. The computed value is compared with that of previous cycle. If there is an increase

in output power, the duty cycle is then adjusted in the same direction. Otherwise the

controller make adjustment in the opposite direction.

3

3. Hardware Design

The PV power system designed to be functionality-oriented consists of three separate

modules: data acquisition, controller and boost converter. Some changes in design with

respect to the project proposal are made due to better understandings of the system. The

most important change is to implement electrical isolation in order to guarantee that high

power noises from boost converter does not corrupt the operation of sensitive components

of the controller.

3.1. Data acquisition module

This module performs both voltage measurement and current measurement with the help of

high-accuracy components. The transduced values in form of analog voltages are fed

directly to the controlling module where the digitization takes place.

The module has a separate power supply (9V battery) in order to prevent shutdown in case

the output voltage of the PV drops below 4.5V (which is the lowest high-side supply voltage

of the isolation amplifier [3]). A charging circuit could be easily added if there would be a

requirement of direct power supply from PV panel for better evaluation of efficiency.

Figure 3.1 Layout of the data acquisition module

Source: designed by the author

4

3.1.1. Current measurement

ACS722LLCTR-10AU from Allegro Microsystems is chosen for the implementation of

current measurement function [4]. The current IPV through the main conductor of the IC is

measured base on Hall effect that maintains a good isolation between high-power noisy

signal at the high-side and low-noise low-power signal of sensitive components at the low-

side. The 0.65m primary conductor resistance guarantees very low heating loss [4].

On the other hand, this current sensing method leads to lower accuracy with respect to

current shunt measurement. The specified maximum total output error of ACS722LLCTR-

10AU is 3% [4]. A error model is proposed and discussed later that suggests an empirical

method to eliminate some sources of error in order to achieve a higher accuracy.

Specifications:

Symbol Parameter Value Unit Note

Measuring range 0 10 A

S Sensitivity 264 mV/A

BW Bandwidth 20 kHz

UCC Supply voltage 3 V

Zero-current output voltage 0.3 V 0.1UCC

es Sensitivity error 2 %

uos Offset voltage 15 mV

uo Total output error 3 % without error model

Error model

Figure 4.2 describes three sources of error: Sensitivity error, supply voltage error and offset

voltage. The supply voltage is regulated at 3V by the precision shunt voltage reference IC

LM4040A30 with a tiny error of 0.1% [5] so that UCC is neglected in the error model. The

two remaining error sources are dominant contributors to total output error.

= + 0.1 + ( + )

5

Where:

uo is the output voltage

uos is the offset voltage

S is the sensitivity

es is the sensitivity error

UCC is the supply voltage (UCC = 3V0.1%)

Figure 3.3 Error model of the current measurement

From this model, higher accuracy of current measurement is achieved if the values of uos

and es could be measured using precision equipments. The following procedure should be

performed to determine uos and es :

Set IP = 0A then measure uo, uos could be determined using this formular:

= 0.3

Set IP = 1A then measure uo, es could be determined by:

= 0.3

1 264

Using this error model the resolving formula for the value of IP is derived:

= 0.3

264 +

6

3.1.2. Voltage measurement

Realization of voltage measurement function is based on the isolation fully-differential

amplifier AMC1200 which has a fix precision gain of 8 with maximum gain error of 0.5%

[3]. The output voltage of PV panel is scaled down by a factor of GV using a simple voltage

divider:

= 1 + 2

2=

120 + 1

1= 121

Due to tolerance of the resistors GV must have a small error of eGV. Additionally, a small

offset of output voltage also contributes to total output error at the low-side.

The bandwidth of high-side signal is limited down to about 4kHz by using a passive low-

pass filter:

= 1

2=

1

2(1//2)=

1

2 1 120

121 47= 3.414

The measuring range is designed to be from 0V to 30V. However, if a higher input voltage

is desired, the scale-down fator could be increased easily by replacements of RGV1 and RGV2.

Specifications:

Symbol Parameters Value Unit Note

Measuring range 0 30 V

GV Scale-down factor 121 adjustable

G Gain of isolated amplifier 8 fixed

eG Gain error 0.5 % maximum

eGV Scale-down error 1 %

BW Bandwidth 4 kHz

Error model

Another error model is proposed (Figure 4.3) in order to eliminate dominant error sources.

This model neglect the very small gain error of the amplifier. The following formula of

output voltage is derived from the model:

7

= 8 [

(121 + )+ ]

Where:

uo is the low-side output voltage

UPV is the output voltage of PV panel

GV is the scale-down factor

uos is the high-side input offset voltage

Figure 3.3 Error model of the voltage measurement

Procedure of dominant error sources determination:

Set UPV = 0V then measure the value of uo, uos is computed using:

= 8

Set UPV = 10V then measure uo, the value of eGV could be determined using:

= 10

8

121

With the help of this error model the voltage UPV could be resolve with higher accuracy

using:

= (121 + ) (8

)

8

3.2. Boost Converter

3.2.1. Theoretical analysis

The implementation of a non-isolated boost converter is shown in figure 3.2. The switch is

realized using a high-power MOSFET and a free-wheeling Schottky diode, which introduces

additional losses into the circuit beside heat loss of the inductor. These losses in most cases

deviate the operation of a practical boost converter from that of a ideal one. Therefore, non-

idealities are considered in this analysis in order to work out a more precise behavior of a

boost converter.

Figure 3.4 Schematic of the boost converter

Source: drawn by the author

The practical inductor, which is the energy-storage element of the converter, is modeled

with a small resistance rL in series with an ideal inductor L (figure 3.3). The MOSFET is

modeled with a resistance ron in the on-state and an open circuit in the off-state (figure 3.4).

The free-wheeling Schottky diode is modeled with a ideal voltage source in series with a

resistance in the off-state and an open circuit in the on-state (figure 3.3).

Applying the small-ripple approximation method [6], which treats approximately the

inductor voltage as well as the capacitor current as DC values, the operation of a boost

converter in steady-state with consideration of those losses can be analyzed as follows:

9

Figure 3.5 Boost converter in on-state

Source: drawn by the author

Figure 3.6 Boost converter in off-state

Source: drawn by the author

In the on-state, the MOSFET is turned on and behaves as an ideal resistance ron while the

free-wheeling diode becomes a open circuit. Applying Kirchhoffs voltage law (KVL) and

Kirchhoffs current law (KCL) we have:

: + + + = 0

=

: + = 0

=

In the off-state, the MOSFET is turned off and becomes an open circuit. As the voltage

builds up, the Schottky diode starts to conduct and behaves as a ideal voltage source UD in

series with rD. The following equations can be formulated:

10

: + + + + + = 0

=

: = 0

=

From fundamentals of electrical engineering we know the following relation between

current voltage of an inductor:

=

=

Integration over one switching cycle, says from t = 0 to Ts, yields

= =

0

0

0

In steady-state condition, there is no net change in inductor current. Therefore the right-

hand-side term reduces to zero, leads to

= 0

0

( ) + ( )(1 ) = 0

(1 ) (1 ) (1 ) = 0

The same method is applied for the output capacitor, which yields

( ) + ()(1 ) = 0

(1 ) =

=

(1 )

With this value of iL the previous equation becomes

[

(1 )] [

(1 )

] (1 ) [(1 )(1 )

] (1 ) = 0

(1 ) = [

(1 )+

(1 )

+ (1 )(1 )

+ (1 )]

= [(1 )

+ + (1 ) + (1 )2] [ (1 )]

11

Figure 3.7 Small-ripple approximation for current and voltage waveforms under steady-state operation

Source: drawn by the author

Figure 3.6 Simulation of output voltage and efficiency

Source: executed and visualized by the author

12

3.2.2. Boost converter realization

The main switch is realized using a FCH072N60 N-channel power MOSFET which has a

typical static drain-to-source on resistance of only 66m while maintains a drain-to-source

voltage of upto 600V and a continuous drain current of 33A at 100C [7].

The freewheeling diode is realized using a C3D06065I silicone carbide Schottky diode

which can block a peak reverse voltage of 650V and can maintain a continuous forward

current of 13A [8].

Figure 3.8 Layout of the boost converter

Source: designed by the author

3.2.3. Electrical isolation

As this boost converter is a high-power stage, isolation of power and signal is a crucial task.

The requirement is fulfilled using ISO5500 isolated MOSFET/IGBT driver which is capable

of driving up to 2.5A peak output current [9]. This IC blocks high voltage and prevents high-

power noise from interfering with the controller and from destroying sensitive components.

With this design, the controller can drive high-power MOSFET using low-power logic

signal without any bulky driver circuit.

The ISO5500 gate driver is supply by an external 20V power supply instead of being

supplied directly from the PV panel because ISO5500 has an undervoltage lockout function

13

which turns off its operation when supply voltage drops below 12V [9]. Moreover this

design has the flexibility to connect PV panels in series without violating the maximum

supply voltage of ISO5500 and the maximum gate-to-source voltage of the FCH072N60

power MOSFET.

3.3. Controller

The controller module consists of three blocks with three different functions: PWM

generator, data conversion block and controller block. It is reasonable to locate PWM

generator and analog-to-digital conversion blocks near the controller block in the same

circuit board as they have to communicate with the controller via high-speed serial

interfaces. This design reduces the wiring tasks which are prone to error and also prevent

high-speed digital signals from interfering with analog signal [10].

Figure 3.9 Layout of the controller

Source: designed by the author

3.3.1. PWM generator

The PWM generator is base on the Analog Pulse Width Modulation precision design from

Texas Instruments [11]. Some modifications are made in order to get a full-range of duty

cycle using only positive input voltage. The switching frequency is also limit down to

250kHz.

14

The controller controls the PWM generator by communicating with DAC8830 (a 16-bit

digital-to-analog converter) via a SPI interface. The full-range of duty cycle is mapped to

216 steps that results in high-resolution PWM signal:

= 100%

216= 0.0015%

Specifications:

Symbol Parameter Value Unit Note

D Duty cycle 0 100 %

Resolution 0.0015 %

Ui Input voltage 0 2.5 V

UOH Output high 5 V

UOL Output low 0 V

fs Switching frequency 250 kHz

3.3.2. Data conversion block

ADS131E04 24-bit analog-to-digital converter specified for industrial power applications is

the core of data conversion block [10]. It shares the same serial interface with DAC8830 to

communicate with the controller.

ADS131E04 has four input channels which work independently. Two channels are used for

voltage measurement, the other two are used for current measurement. As only input voltage

and input current from PV panel are monitored, two channels ared reserved for later use.

Specifications:

Symbol Parameter Value Unit Note

Resolution 16, 24 bit programmable

Sampling frequecy 1, 2, 4, 8,

16, 32, 64

kHz programmable

Reference voltage 3 V

Reference error 0.1 %

15

4. Photovoltaic module

Energy for the photovoltaic power system is generated by KC60 PV module from Kyocera

which has the following electrical specifications [12]:

Symbol Parameter Value Unit Note

Pmax Maximum output power 60 W

UOC Open-circuit voltage 21.5 V

ISC Short-circuit current 3.73 A

UMPP MPP voltage 16.9 V

IMPP MPP current 3.55 A

Because of highly ideal test condition of the provided specifications, the actual KC60 PV

panel to be used in this project is tested for real-life behaviors under two conditions as shown

in figure 4.1.

Figure 4.1 Experiment setups (left: testing under halogen lamp, right: testing under direct sunlight)

Source: photographed by the author

In the first experiment (right-hand side picture), a 400-W halogen lamp (available in the

laboratory) is used as light source of the PV panel. In this experiment setup, KC60 panel

only produces a tiny output power (figure 4.2). The result suggests that the proposed halogen

lamp is not suitable for testing of the system in further stages.

16

In the second experiment (left-hand side picture), the PV panel is tested under direct sunlight

(2 p.m, May 22th 2015, high sun, no cloud) and produces a much better output

characteristics (figure 4.3).

Figure 4.2 Output characteristic under halogen lamp

Source: photographed by the author

Figure 4.3 Output characteristics under direct sunlight

Source: photographed by the author

17

5. Proposal for a solar simulator

Experiences from previous projects show that the development of photovoltaic power

systems is slowed down greatly if the testing is dependent entirely on direct sunlight which

cannot be manipulated. Therefore a designed of a solar simulator is proposed, with which

small PV systems could be tested conviniently in laboratory.

5.1. Requirements

The proposed solar simulator should provide enough space to mount two KC60 PV panels

(650x750mm each one). The connection of this two panels, in series or in parallel, should

be managed conviniently with the help of a connection box. The angle of the frame, onto

which the two PV panels are mounted, must be adjustable in order to test the behaviours of

the system under various irradiance angle.

The light source should be able to produce an irradiance of up to 1000W/m2 with a good

spectral match. The light source should also be dismountable so that testing under direct

sunlight could be performed if needed.

5.2. Mechanism of mounting angle adjustment

Figure 5.1 describes how mounting angle of the two PV panels is adjusted.

Figure 5.1 Angle adjustment mechanism

18

AB is the frame on which PV panels are mounted. The frame is fixed at pivot B so that A

can rotate around B. The fix-length arm AS connects pivot A to slider S, which can move

in horizontal direction with the help of a linear actuator.

At the right-most point of the slider S, frame AB is in horizontal direction. Rotation of the

linear actuator brings slider S to the right. As the lenght of AS cannot be changed, point A

also rotates around fixed pivot B that results in a angle of the whole frame with respect to

the horizontal line.

5.3. Light source

The SUNLIKE products from FUTURELED appear to be a promising candidate for the

proposed solar simulator.

Ten modules of standard size SUNLIKE L (300x300mm) could be assembled together to

cover a total size of 600x1500mm, which fits perfectly to the total area of two KC60

panels.

A more detail design cannot be made at the moment due to lack of documentation of the

products.

19

6. References

[1] E. F. Fuchs and M. A. S. Masoum, Power Conversion of Renewable Energy

Systems, New York: Springer, 2012.

[2] T. Markvart and L. Castaer, Practical Handbook of Photovoltaics: Fundamentals

and Applications, Oxford: Elsevier, 2003.

[3] Texas Instruments, AMC1200 Fully Differential Isolation Amplifier, 2012.

[4] Allegro Microsystems, ACS722 High Accuracy, Galvanically Isolated Current

Sensor, 2014.

[5] Texas Instruments, LM4040Axx Precision Micropower Shunt Voltage Reference,

2015.

[6] R. W. Erickson and D. Maksimovi, Fundamentals of Power Electronics, New York:

Kluwer Academic Publishers, 2004.

[7] Fairchild Semiconductors, FCH072N60 N-Channel SuperFET MOSFET, 2014.

[8] Cree, C3D06065I Silicone Carbide Schottky Diode Rev. A, 2012.

[9] Texas Instruments, ISO5500 2.5A Isolated IGBT, MOSFET Gate Driver, 2015.

[10] Texas Instruments, ADS131E04 Analog Front-End for Power Mornitoring, Control

and Protection, 2013.

[11] Texas Instrument: Caldwell, John, TI Precision Designs: Analog Pulse Width

Modulation, 2013.

[12] Kyocera, KC60 High Efficiency Multicrystal Photovoltaic Module.