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MPPT DC/DC Converter Design for Photovoltaic

Powered Electric Vehicle Charging Systems

Progress Report – 19th December 2012

Energy Systems Engineering – Electrical Stream

Fearghal Kineavy

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Table of Contents 1. Project Outline ................................................................................................................... 3

2. Proposals for Tackling the Project ..................................................................................... 4

3. Progress to Date ................................................................................................................. 5

3.1. Electric Vehicles in Ireland .......................................................................................... 5

3.2. Nissan Leaf .................................................................................................................. 6

3.3. Solar PV Panels ............................................................................................................ 6

3.4. Maximum Power Point Tracking (MPPT) .................................................................. 10

3.5. DC-DC Converters ...................................................................................................... 13

3.5.1. Basic Switching Converter.................................................................................. 13

3.5.2. Pulse Width Modulation (PWM) ........................................................................ 14

3.5.3. Buck Converter ................................................................................................... 15

3.5.4. Boost Converter ................................................................................................. 17

3.5.5. Perturb and Observe MPPT ............................................................................... 19

3.6. Demonstration System .............................................................................................. 19

4. Task List and Project Plan ................................................................................................ 22

5. References ....................................................................................................................... 23

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1. Project Outline

The main aim of the project is to design a solar photovoltaic (PV) system for charging

electric vehicles using direct current (DC) exclusively without any DC/AC (alternating

current) or AC/DC conversion stages. This will be a system that would be suitable for a

building such as the Engineering building in NUIG where there are cars parked during the

day when the sun is shining. A small scale demonstration system will be designed, built,

tested and analysed to validate the circuit models.

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2. Proposals for Tackling the Project

The project will be largely research based, containing some practical elements for the

building and testing of the demonstrator model. Initial research into electric vehicles in

general will be required. This will include research into the number of EVs on the road now,

future uptake trends, current technology (specifically the kind of batteries used in EVs,

charging profiles, and limitations) etc. There will also be significant research done on solar

PV panels. Using this research, the author will be able to gauge what size PV systems would

be required in various EV charging schemes.

Once the author has an idea of the scale of a PV charging EV system, it can be scaled down

to a size that is testable in the lab. This will be the basis for the demonstration system. The

main part of the demonstration system will be a DC-DC converter with maximum power

point tracking (MPPT) to match the output voltage of the PV panel to the voltage required

to charge a battery. The demonstration system will be designed with the aid of a computer

circuit simulation program (most likely PSPICE or LTSPICE). Appropriate values for the DC-DC

converter components (capacitor and inductor) will need to be calculated and the circuit

simulation program will enable different values to be tested before any circuit is built.

When the demonstration system has been built, tested and analysed, the author intends to

scale up the computer circuit model to the size of a real system. This will be used as the

basis for a detailed simulation of a real PV charging EV system.

Finally this detailed simulation of a PV charging EV system using only DC current, will be

compared against a similar system which has DC-AC and AC-DC conversion stages between

the PV panel and the EV.

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3. Progress to Date

Firstly the author researched what electric vehicles are currently available in Ireland and

what kind of infrastructure is in place for charging them.

3.1. Electric Vehicles in Ireland

As of October 2012, 192 electric cars have been sold in the Republic of Ireland [1]. Several

car manufacturers, such as Renault, Peugeot, Citroen and Mitsubishi, have electric car

models, however, the Nissan Leaf is the most widely known commercial model. Official

government policy is to encourage the uptake of electric vehicles over the next decade.

Electric Ireland are rolling out charging infrastructure across the country to support uptake.

There are 3 different ways to charge EVs in Ireland [2]:

(i) Home charge points – these are installed in peoples’ homes and are the primary

source of electricity for EVs as the EV can be plugged in to charge overnight. An

EV will draw single phase 16A (3.6kW) when connected to a home charge point

meaning that a full charge takes 6 – 8 hours.

(ii) Public charge points - Public charge points are being installed on-street and in

locations such as shopping centres and car parks. They are connected to a local

3-phase electricity supply and depending on the car type and battery size,

charging takes between 1 and 6 hours. As of October 2012, there are 860 public

charge points [1].

(iii) Fast charge points – Fast charging is done with either a 3-phase, 63A AC (44kW)

supply or a 120A, 400V DC (50kW) supply. As of October 2012, there are 30 fast

chargers in ROI [1] and all of these are DC fast chargers. The Nissan Leaf,

Mitsubishi iMiEV, Peugeot iOn and Citroen C-ZERO support DC fast charging. A

50kW DC fast charge point can charge a compatible electric car up to 80% in 20-

30 minutes. The AC fast charging will cater for other models, the next available

being Renault Zoe.

The government has standardised charge points with 2 socket types. An IEC Type 2 socket

and plug (Mennekes) is used for home, public and AC fast charging while a CHAdeMO socket

and plug is used for DC fast charging.

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The author decided to take the Nissan Leaf as the typical electric car as it is the most

popular in Ireland.

3.2. Nissan Leaf

The Nissan Leaf is a 5 door family car and has roughly the same performance as a 1.6 litre

petrol engine. In place of an internal combustion engine, the Leaf is run by an 80kW AC

motor that gives the Leaf a maximum speed of more than 140km/h [3]. The motor gets its

energy from a 24kWh lithium-ion battery. The battery is composed of 48 modules in series.

Each module has 2 lines in parallel with 2 cells in series in each line. Each cell has a charge

capacity of 33.1Ah and an average voltage of 3.8V [4]. Therefore the energy capacity of each

module is

and the total energy capacity is

The Leaf has two connectors for charging the battery, a Mennekes connector for home and

public charging, and a CHAdeMO connector for DC fast charging. With home and public

charging, an onboard charger in the Leaf converts the AC electricity to DC electricity and

charges the battery. With DC fast charging, the charging station communicates with the

battery via the CHAdeMO connector and the DC current goes straight from the charging

station to the battery.

3.3. Solar PV Panels

Solar photovoltaic panels normally consist of photovoltaic silicon based cells connected

together either in series, parallel or a combination of the two. PV cells are capable of

converting light directly into electricity. When the PV cells are exposed to solar energy,

some of the energy from the light photons is absorbed by the cells and this energy knocks

some of the PV cell’s electrons loose, allowing them to move. PV cells also have an electric

field that forces the freed electrons to move in one direction and by connecting conductors

to the terminals of the cell, the energy can be harnessed as an electric current.

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Typical commercial PV panels have efficiencies of about 15%. That is, 15% of the solar

irradiation incident on the PV panel is converted into electricity. To calculate how many PV

panels would be required to charge a Nissan Leaf parked at the Engineering building, it was

first necessary to get data on the solar irradiation in Galway. Figure 1 shows a graph of daily

solar irradiation in Galway for each month of the year [5]. The circles represent the solar

irradiation calculated using a methodology that combines hourly observations of cloud

conditions with monthly sunshine hours data in order to determine the frequency of clear,

partly cloudy and overcast skies.

Figure 1 – Daily solar irradiation in MJ/m^2/day in Galway, Ireland [5]

Dividing the MJ/m^2 values by 3.6 gives solar irradiation in kWh/m^2. Once this was known

it was then possible to calculate what area of PV panels would be required to fully charge a

Nissan Leaf each day. Below is the equation used to calculate the Area required:

( ̈ )

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Where is the energy capacity of the EV battery in kWh, ̈ is the solar irradiation in

kWh/m^2 and is the efficiency of the PV panels. Finally to determine how many PV panels

are required, simply divide by the area of one PV panel.

For the calculations a 245W Ecologics PV panel was chosen. Table 1 contains the relevant

data about the panel.

Ecologics 245W PV Panel Details

Max. power 245W

Open circuit voltage 37.12V

Short circuit current 8.78A

Max power voltage 29.99V

Max power current 8.17A

Module efficiency 15.10%

Cell efficiency 17.20%

Height 1.637m

Width 0.992m

Area 1.6239m^2

Table 1 – Ecologics 245W PV Panel

Table 2 shows the results of the above calculations for the 12 months of the year. As would

be expected, there is a huge variation between how many panels are required in May and

June compared to December and January (21 panels in May, 235 panels in December). As

can be seen, the average number of panels needed is 39. If we only consider the 5 months

from April to August, the average number of panels required drops to 23, covering about

37m^2. Multiplying the maximum output of one panel by 23, this translates into

approximately a 6kW array to power a Nissan Leaf from April to August. Table 3 shows

different ways that the arrays could be wired.

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Table 2 – Number of PV panels required in each month to fully charge a Nissan Leaf everyday.

January February March April May June July August September October November December Average

Average

April-August

Solar Irradiation

(MJ/m^2) 1.7 4.5 7.5 13 17 17 16 14 9 5.5 2 1.5 9.06 15.40

Solar Irradiation

(kWh/m^2) 0.47 1.25 2.08 3.61 4.72 4.72 4.44 3.89 2.50 1.53 0.56 0.42 2.52 4.28

Area required to

power EV (m^2) 337 127 76 44 34 34 36 41 64 104 286 381 63 37

No. panels

required to power

EV 208 79 47 28 21 21 23 26 40 65 177 235 39 23

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Table 3 – Different solar array combinations

Possible Panel Combinations

No. Modules

No. modules

in series

No. rows in

parallel

Output

voltage (V)

Max power

output (kW)

Average daily output

April-August (kWh)

20 10 2 300 4.9 21.0

20 5 4 150 4.9 21.0

20 4 5 120 4.9 21.0

21 7 3 210 5.1 22.0

22 11 2 330 5.4 23.1

23 23 1 690 5.6 24.1

24 12 2 360 5.9 25.2

24 8 3 240 5.9 25.2

24 6 4 180 5.9 25.2

24 4 6 120 5.9 25.2

25 5 5 150 6.1 26.2

26 13 2 390 6.4 27.3

27 9 3 270 6.6 28.3

3.4. Maximum Power Point Tracking (MPPT)

Solar modules can operate over a range of different voltages, and as the voltage varies, the

current also varies. Figure 2 shows the current/voltage curve for the Ecologics 245W panel

[6].

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Figure 2 – Current/Voltage characteristic for Ecologics 245W PV module [6]

Every PV module has its optimum operating voltage under specific conditions. At this

voltage, the maximum power point (MPP), the module is delivering the maximum available

power. Moving the voltage away from the MPP will decrease the power being produced by

the module and therefore reduces the efficiency of the system. The MPP occurs in the

region where the current begins to decrease more rapidly, i.e. the “elbow” of the curve.

Figure 3 shows the power/voltage curve for a typical PV module [7].

Figure 3 – Power/voltage curve of a typical PV module [7]

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As can clearly be seen, there are significant power reductions when the voltage is not at the

MPP, especially as the voltage moves towards the open circuit voltage. If a module is

connected directly to a load, the module’s voltage will be forced to the load’s operating

voltage. Therefore, unless the load voltage voltage matches the MPP voltage of the module,

there will be efficiency losses. To complicate things further, the MPP of a module is not fixed

but fluctuates with module temperature and solar irradiation. Figure 2 shows the I-V curves

for the Ecologics 245W module at 25 , 50 and 75 . It can be seen that the maximum

available power and the MPP voltage decrease as the temperature rises. Solar irradiation

does not affect the MPP voltage as much as temperature, although the maximum available

power obviously decreases as there is less available light. This can be seen in the I-V curves

for the Ecologics 245W module at 25 at various solar irradiations in figure 4 [6].

Figure 4 – I-V curves for Ecologics 245W module at various solar irradiations

Clearly it is desirable to keep the PV module operating at its MPP at all times. This is

achieved by using maximum power point tracking (MPPT). MPPT keeps the module voltage

at the MPP voltage all the times. The module output voltage can then be input to an

inverter to convert it to AC electricity for the mains or it can be input to a DC-DC converter

to convert the module output voltage to the voltage required for whatever load it is

supplying. Oftentimes, a MPPT algorithm is incorporated into the DC-DC converter itself. As

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this project is all about bypassing the AC conversion stages of a PV system to power a DC

load, DC-DC converters will now be looked at in more detail.

3.5. DC-DC Converters [8], [9]

A DC-DC converter is a device that takes in one voltage at the input and converts it to a

different voltage at the output. Linear converters can sometimes be useful as they are

relatively simple and cheap to build. They generally consist of some kind of voltage divider

to obtain the desired output voltage. However, the output voltage can only be lower than

the input voltage meaning that they are unsuitable for boost applications. Because the

“extra” voltage is simply dropped across a resistor there can be large heat losses which

means lower efficiency. Thermal management can also be an issue because of this.

Switch mode converters on the other hand use power switching techniques to achieve the

DC-DC conversion. They are more costly and complex to build than linear converters but

have much greater efficiencies and can be used in boost applications. They use inductors

and capacitors to store energy for small periods of time. They also have switches to regulate

the output voltage. There are three main types of switch mode converters, boost

converters, buck converters, and buck-boost converters. Buck and Boost converters are

looked at in more detail below.

3.5.1. Basic Switching Converter

Consider the diagram in figure 5 of a basic switching converter.

Figure 5 – Basic Switching Converter

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When the switch is closed, . When the switch is open, . The duty cycle, D,

represents the percentage of the period, T, that the switch is closed. Figure 6 shows the

output voltage, , as a function of time.

Figure 6 – Output voltage as function of time

It is obvious that is a function of and D. In the case of this simple switching converter,

.

3.5.2. Pulse Width Modulation (PWM)

The switch in a switching converter is normally controlled by pulse width modulation. Figure

7 shows how PWM generates the duty cycle.

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Figure 7 – Pulse Width Modulation

A triangular sawtooth wave, Vsaw, is compared with a reference voltage, Vref. If Vref is

bigger than Vsaw then the switch is closed, while if Vref is smaller than Vsaw then the

switch is open. Therefore the duty cycle of a DC-DC converter can be varied by varying Vref.

3.5.3. Buck Converter

In a buck converter, or step-down converter, the input voltage is stepped down to a lower

output voltage, . It is similar to the basic switching converter discussed previously

but an LC low pass filter is inserted after the switch. This produces a more steady DC voltage

and current at the output. The converter operates in mode 1 when the switch is closed and

mode 2 when the switch is open. Figure 8 shows (a) the circuit diagram of a Buck Converter,

(b) the equivalent circuit for mode 1 (switch closed) and (c) the equivalent circuit for mode 2

(switch open).

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Figure 8 – Buck Converter

In mode 1 the switch is a short circuit and the diode is an open circuit. The inductor current

equals the input current. This current increases linearly but is prevented from changing very

quickly by the inductor. When the current through any inductor is changing, a voltage is

induced across the inductor that opposes this change in current. The inductor “wants” the

current through it to remain constant. This can be seen from the equation

Where L is the inductance of the inductor and

is the rate of change of the inductor

current. So in simple terms, the more the current tries to change, the more the inductor will

try to resist it. The output voltage is .

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In mode 2 the switch is an open circuit and the diode is a short circuit. The inductor now

supplies the current to the output. The inductor current decreases linearly but again the

inductor prevents it from changing very quickly as it “wants” a constant current. The polarity

of the inductor’s voltage reverses as it now attempts to stop the current decreasing. The

output voltage is .

Figure 9 shows (a) the inductor voltage and (b) the inductor current.

Figure 9 – (a) Inductor voltage and (b) inductor current

3.5.4. Boost Converter

In a boost converter the input voltage is boosted up to a higher output voltage, where

( )

For charging EVs from a PV array, this is probably the converter that would be needed as the

output voltage from a PV array is usually less than the approx. 360V required to charge an

EV. The same components are used in a boost converter as in a buck converter but they are

arranged differently. Figure 10 shows (a) the circuit for a boost converter, (b) the equivalent

circuit for mode 1 (switch closed) and (c) the equivalent circuit for mode 2 (switch open).

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Figure 10 – Boost Converter

In mode 1 the switch is a short circuit and the diode is an open circuit. The circuit is

essentially split into two separate circuits. On the left, and the inductor current

equals the input current. The current increases linearly. On the right, the load is sustained

by the capacitor which discharges keeping the output current constant.

In mode 2 the switch is an open circuit and the diode is a short circuit. The inductor current

decreases linearly. . However, , will be negative as it is acting against the

current decreasing and therefore will be greater than .

Figure 11 shows (a) the inductor voltage, (b) inductor current, (c) diode current, and (d)

capacitor current. In (d), R is the equivalent resistance of the load.

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Figure 11 – Boost converter waveforms

3.5.5. Perturb and Observe MPPT

DC-DC converters can be used with MPPT algorithms to track the MPP of a PV array. One

such algorithm is the Perturb and Observe algorithm. This is a simple and reliable method of

MPPT. Essentially the converter measures the power coming from the PV module. It then

slightly changes its duty cycle and measures the power again. If the power increased, the

converter changes its duty cycle again in the same direction. If the power decreased, the

converter changes its duty cycle in the opposite direction. So the converter doesn’t operate

at exactly the MPP but rather oscillates around it.

3.6. Demonstration System

Work on the small scale demonstration system is still in the preliminary stages. It has been

decided to use a Spectra 10W solar module from Radionics to charge one 3.7V 2.2Ah lithium

ion cell. The module outputs about 15V so this will have to be stepped down to charge the

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li-ion cell. A MPPT tracking buck converter will be used to step down the voltage. It is being

considered to build the buck converter and to use an arduino to provide the PWM to control

the duty cycle. The Perturb and Observe method will be used to track the MPP of the PV

module. The converter output voltage will then be fed into a li-ion charging unit.

Ideally Li-ion cells should be charged with a specific charging profile and this is why a li-ion

charger must be used. The li-ion cell is fully discharged when it reaches 3V. It is fully charged

when it reaches 4.2V. It is possible to discharge to approx. 2.5V however this is not

recommended as it can reduce lifetime and capacity. Figure 12 shows a Constant-Current

Constant-Voltage (CCCV) charge profile for a li-ion battery [10].

Figure 10 – Li-ion battery charge profile [10]

If the cell voltage is below a threshold voltage, 3V for the li-ion cell above, the charger

provides a trickle charge at a constant current, typically 0.1C. When the cell reaches the

threshold voltage, constant current charging commences. This is typically at a rate of 0.5C-

1C. Charging above 1C can be detrimental to the cell. A charge rate of 1C is the rate that

would charge a given battery in one hour. So for the 2.2Ah li-ion cell, a charge rate of 1C is

equal to 2.2A while a charge rate of 0.5C is equal to 1.1A.

Once the cell reaches its maximum voltage, 4.2V for the li-ion cell, it enters constant voltage

charging. The voltage is held constant while the current tapers off. Once the current drops

below a specified value, usually 0.02C-0.05C, the charge terminates. Li-ion cells are very

sensitive to overcharging and therefore it is imperative that the voltage is as close as

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possible to 4.2V, typically to 1%. Below 4.2V and the cell will not be fully charged, above

4.2V and the cell may be damaged.

A li-ion single cell charger from SparkFun has been identified, and it is being considered to

use this as the li-ion charger in the demonstration system. Figure 11 shows the circuit of the

charger.

Figure 11 – Li-ion charger

However this charger has its limitations if it is to be used with a PV module. Because the li-

ion cell will be charged in a very specific profile, if the PV module is not supplying sufficient

power it may terminate the charge. Likewise, any excess power produced by the PV module

will be wasted as heat. Further research is to be done into these problems.

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4. Task List and Project Plan

Below is projected task list project plan timescale. This may change as the project

progresses and unforeseen challenges arise however it serves as a rough baseline.

Task Sub-task Start date End date

Design, build and analyse demonstration system

Underway 1st Feb 2013

Research charging li-ion cells from a variable input. Choose appropriate li-ion charger.

Choose detailed specs for Buck converter: switching frequency, voltage ripple, current ripple, input voltage, output voltage

Test 10W panel to determine I-V curve and hence full range of operating voltages

Circuit model of system on SPICE. Choose values for inductor and capacitor

Predict demonstration system performance from SPICE model

Build demonstration system

Test and analyse demonstration system

Design and do detailed simulation of full size system

4th Feb 2013

22nd Feb 2013

Scale up demonstration SPICE model

Change SPICE buck converter to boost converter and alter inductor and capacitor values accordingly

Analyse the system (specifically efficiency) at various operating conditions, full-load, half-load etc.

Compare full size DC-DC system with alternative DC-AC-DC system (specifically efficiency)

25th Feb 2013

15th March 2013

Compile results and write final report

18th March 2013

4th April 2013

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5. References

[1] The Irish Times Wednesday October 17, 2012 – “Time to pull the plug on electric cars”

[2] www.esb.ie/electric-cars

[3] www.nissan.ie/vehicles/leaf

[4] http://www.eco-aesc-lb.com/en/product.html

[5] Hurley WG. A new methodology to optimise solar energy extraction under cloudy

conditions. Renewable Energy 2010; 35: 780-787

[6] www.ecologics.ie

[7] Koutroulis, E. Development of a Microcontroller-Based, Photovoltaic Maximum Power

Point Tracking Control System. Power Electronics 2001; 16 : 46-54

[8] Prof G. Hurley – EE411 Lecture notes

[9] “Power Electronics” by D.W. Hart

[10] Min Chen. Accurate, Compact, and Power-Efficient Li-Ion Battery Charger Circuit.

Circuits and Systems II 2006; 53 : 1180-1184