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SAUSSUREA (ISSN: 0373-2525), 2016 Volume 6(3):PP. 219-240

RESEARCH ARTICLE

Design and Implementation of Single-Stage Grid-Connected Flyback

Microinverter Operates in DCM for Photovoltaic Applications

Turki K. Hassan1 and Mustafa A. Fadel

2

1PhD, Electrical Engineering Department, Faculty of Engineering, Al-Mustansiriya University, Iraq.

2 MSc, Electrical Engineering Department, Faculty of Engineering, Al-Mustansiriya

University, Iraq, Baghdad.

Abstract

In this paper, a single-stage grid-connected flyback microinverter is proposed. The proposed

flyback microinverter has some advantages such as high voltage gain, high efficiency, low cost, small

size, simple control and high power factor. The proposed system is used to connect the PV panel to the

grid with achieving maximum Power Point Tracking (MPPT) control. The converter operates in DCM to

inject a sinusoidal current into the grid with unity power factor. A complete system has been simulated

using PSIM program and the hardware is build using analog and digital devices. The simulation and

experimental results are obtained to validate the system.

Keywords: Single-Stage, Grid-Connected, Microinverter, Flyback, DCM, MPPT.

1. Introduction

In the last decades the fossil fuels have been widely used in order to cover the human needs of

energy. Moreover, extraction and transportation of fossil fuels cause environmental pollution along with

serious consequences such as air pollution, global warming, soil degradation and water deterioration.

Because of the risks of environmental contamination above, the searching for clean renewable sources of

energy are gaining more and more interest such as wind power, solar power, biomass, geothermal power,

wave and tidal power, and nuclear power. Now days the higher interest is concentrate in solar energy

exploitation, at present the power injection to the utility grid using photovoltaic panels (PV) is gaining more

attention [1, 2].

The photovoltaic panel is a device that converts luminous energy into electric energy through the

photoelectric effect. The electric energy is available at the terminals of the PV panel in the same instant that

the sunlight reaches it, most of the electric equipment of standard use cannot connected directly, this because

the generated current from the PV panel is continuous (dc) and at low voltage (generally between 12 and 68

volts, depending on the technology used in the panel construction) and the majority of the equipment

operates at alternating current (ac), at higher voltages [3]. This brings the need for power interface between

the PV panel and the grid through power electronic inverters. The latest technology for grid-connected PV

systems is the ac-PV module [4-6], where the inverter is integrated in the PV panel and both works as a

single unit that operates as an ac generator used to inject ac current to the grid.

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The ac-PV module can be easily installed at any place such as rooftop and open areas without needs

to any special technical knowledge or extreme safety caution. The power converter unit of an ac-PV module

is usually a single-phase inverter, ranging from 50 to 400W. There are many single or multistage topologies

about grid-connected inverters for PV modules [2]. As shown in Figure (1) single stage inverters boost the

input voltage and convert it into ac voltage in the same stage. Multistage inverters consist of two cascaded

stages, the first stage is a boost dc-dc converter and the second one is an inverter. Single stage inverters have

some advantages over multistage inverters; such as low cost, high power density, small size, high efficiency

and high reliability. [7].

Figure (1): Single stage and multistage configurations.

The proposed inverter in this paper consists of flyback inverter with PWM modulation. The inverter

operates in the DCM to inject a naturally sinusoidal current into the grid with unity power factor.

2. The Proposed Inverter System

The flyback inverter shown in Figure (2) performs energy flow from the dc side (PV panel) to the ac

side (utility grid), by using transformer with two identical secondary windings. Each of them is able to

transfer energy to the ac side during a utility grid half cycle. For this reason, a semiconductor switches (S2

and S3) are placed in each secondary winding and they appropriately controlled to turn on and off to ensure

an ac output waveform synchronized with the grid. The output LC filter is used to filter the output produced

by the rest of the converter so that the appropriate final output current is produced and fed to the grid.

Figure (2):Single-Stage Flyback inverter.

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While the main switch semiconductor S1 is modulated in high frequency (20–200 kHz), the switches

S2 and S3 are modulated in 50 Hz. The switching sequence of each semiconductor can be observed in Figure

(3).

Figure (3): Current waveforms and switching sequence diagram [8].

Figure (4) shows the operation of this converter during a line half cycle is as follows: When the

primary switch, S1, is on, the full PV panel input dc voltage is impressed across the transformer, thus putting

energy into its magnetizing inductance and making its magnetizing current rise. When S1 is off, the stored

energy in the transformer’s magnetizing inductance will transferred to the output, which is the grid. Since the

output is ac, which means it is either transferred through D1 and S2 when the voltage output is positive or

through D2 and S3 when the voltage output is negative.S1 duty cycle should be made to vary throughout the

ac voltage line cycle so that it is at its minimum when the ac voltage is at its minimum and it is at its

maximum when the ac voltage is at its peak.

S1 gate pulses

S2 gate pulses

S3 gate pulses

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(a)

(b)

(c)

Figure (4): Equivalent circuit of the current source flyback inverter during a line half cycle. (a) Each line

half cycle. (b) During S1 on time. (c) During S1 off time.

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In the following analysis, only discontinuous conduction mode (DCM) is considered due to its

simplicity of control. To ensure the inverter operates in DCM [9].

𝑡𝑜𝑓𝑓 _𝑝 ≤ 𝑇𝑠 − 𝑡𝑜𝑛 _𝑝 (1)

Where𝑡𝑜𝑓𝑓 _𝑝 is the off time of S1 in the switching cycle when the primary current reaches its peak value of

𝐼𝑝𝑟𝑖 _𝑝 and 𝑡𝑜𝑛 _𝑝 is the on time of S1 during the same switching cycle.

The on time can be expressed as:

𝑡𝑜𝑛 _𝑝 = 𝑑𝑝 𝑇𝑠 = 𝑑𝑝

𝑓𝑠(2)

Where 𝑓𝑠 is the switching frequency and 𝑑𝑝 is the duty cycle.

The current in Lm can drop to zero when S1 is turned off and the energy that is stored in it is transferred to the

output. When S1 is turned off, the voltage across the transformer is the output voltage that appears across the

secondary winding and is reflected to the primary. This voltage can be expressed as

𝑣𝑔𝑟𝑖𝑑 𝑡 = 𝑉𝑔𝑟𝑖𝑑 _𝑝 𝑠𝑖𝑛 𝜔𝑔𝑡 (3)

Where 𝑉𝑔𝑟𝑖𝑑 _𝑝 is the peak value of output voltage and 𝜔𝑔 is the grid frequency in rad/sec. Considering this

voltage reflected to the primary, the fall of current in Lm can be expressed as

𝑁 𝑣𝑔𝑟𝑖𝑑 𝑡 = 𝐿𝑚𝑑𝑖𝑝𝑟𝑖 (𝑡)

𝑑𝑡(4)

Where N is the turns ratio. So that 𝑡𝑜𝑓𝑓 can be expressed as:

𝑡𝑜𝑓𝑓 _𝑝 = 𝐿𝑚𝐼𝑝𝑟𝑖 _𝑝(𝑡)

𝑁 𝑉𝑔𝑟𝑖𝑑 _𝑝(5)

To determine the turn off time 𝑡𝑜𝑓𝑓 _𝑝 the peak primary current flowing through the flyback inverter 𝐼𝑝𝑟𝑖 _𝑝

needs to be determined. This can be done by considering the fact that the input dc voltage Vin is impressed on

the transformer primary when S1 is on. The rise in current when S1 is on can be expressed as:

𝑉𝑖𝑛 = 𝐿𝑚𝑑𝑖𝑝𝑟𝑖 (𝑡)

𝑑𝑡(6)

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Substituting (2) into this equation gives the following expression for 𝐼𝑝𝑟𝑖 _𝑝

𝐼𝑝𝑟𝑖 _𝑝 =𝑉𝑖𝑛𝑑𝑝

𝑓𝑠𝐿𝑚(7)

Substituting this equation into equation (5) gives,

𝑡𝑜𝑓𝑓 _𝑝 =𝑉𝑖𝑛𝑑𝑝

𝑉𝑔𝑟𝑖𝑑 _𝑝 𝑓𝑠 𝑁(8)

It can be seen from equation (8) that 𝑡𝑜𝑓𝑓 _𝑝 dependent on various parameters. Assuming a fixed switching

frequency 𝑓𝑠 , these parameters are fixed except for peak duty cycle 𝑑𝑝 and turns ratio N. Values for 𝑑𝑝and N

need to be chosen so that the converter remains in DCM. This can be done by considering the expression for

DCM operation given in equation (1) and substituting equation (8) into this equation to get

𝑉𝑖𝑛𝑑𝑝

𝑉𝑔𝑟𝑖𝑑 _𝑝 𝑓𝑠 𝑁≤ 𝑇𝑠 − 𝑡𝑜𝑛 _𝑝 (9)

Equation (9) can be rearranged to give the following expression for peak duty cycle

𝑑𝑝 ≤ 𝑉𝑖𝑛

𝑁 𝑉𝑔𝑟𝑖𝑑 _𝑝+ 1

−1

(10)

The next step is to determine an appropriate value of magnetizing inductance Lm that can store sufficient

energy to be fed to the grid, for a rated output power Po. Once this has been determined, the final step is to

confirm that the converter can operate with DCM with this value of Lm. The value of magnetizing

inductance Lm can be calculated using the following equation:

𝐿𝑚 =1

2 𝑑𝑝

2𝑉𝑔𝑟𝑖𝑑 _𝑟𝑚𝑠2

𝑓𝑠𝑃𝑜

𝑉𝑖𝑛𝑉𝑔𝑟𝑖𝑑 _𝑝

2

(11)

The flyback inverter transferred power Po is expressed by the following equation [8]:

𝑃𝑜 =1

4

𝑉𝑖𝑛2 𝑑𝑝

2

𝐿𝑚𝑓𝑠 (12)

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3. The Proposed Control

The proposed control is based on DCM operation. It requires the achievement of MPPT and unity

power factor for the output current of the inverter in addition to boosting the input voltage into its required

level. Figure (5) shows the schematic of the proposed control system, as shown in the figure, grid voltage

is sensed to provide control signal for the flyback inverter to operate in synchronization with grid voltage,

PV panel’s voltage and current (Vin and Iin) are provided to the MPPT controller to generate reference

signal (Eo). To provide sinusoidal modulation, grid voltage is rectified (S(t)) and then multiplied by the

output signal from the preoperational-integral controller (P-I controller), comparing its output with fixed

sawtooth signal to generate converter control signal (PWM signal for the main switch S1).

Figure (5): Proposed control circuit.

PV generation efficiency and power quality are the fundamental issues. PV power sources are

usually integrated with control algorithms that have the task of ensuring maximum power point (MPP)

operation. Many algorithms have been developed for tracking the maximum power point of a solar array

[10-12].

Most commonly used are perturb and observe (P&O) algorithm [13, 14] and the incremental

conductance algorithm [15]. The main advantages of these algorithms are easy to build and low cost

instruction. Consequently, researchers have been focused on the improvement of maximum power point

tracking (MPPT) control and the reduction of total harmonic distortion (THD). It is very important to

design the MPPT controller, so that the voltage ripple at the terminals of the PV panel is a minimum. The

Perturb and observe (P&O) method also called “Hill-Climbing” is the most widelytechnique used for

MPPT because of their simplicity and effectiveness. The perturbation in the operating voltage of the dc

link between the PV array and the power converter is repeatedly done. In this method, PV power P(k) is

measured and compared with the previous measured PV power P(k-1).

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If the power increases, the same perturbation is applied in the same direction to get the next PV

powerotherwise perturbationis made in opposite direction. By this process, the operating point of the

system gradually moves towards the MPP and oscillates around it. Based on these facts, the algorithm is

implemented. The process is repeated until the MPP is reached [16]. P&O maximum power point tracking

algorithm is used in this paper. Figure (6) shows the flowchart of the used MPPT control technique.

Figure (6): P&O algorithm for MPPT.

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4. Simulation Results

The proposed system as shown in Figure (7) is simulated using PSIM software. Table (1) shows the

simulated system specifications.

Figure (7): Proposed system simulation using PSIM program.

Table (1): System design specifications.

Parameters Symbol Value unit

Rated output power Po(max) 100 W

Switching frequency fs 20 kHz

Grid voltage Vgrid_rms 220 V

Input voltage Vin 22 ~ 33 V

Grid frequency fgrid 50 Hz

Total input capacitance Cdc 10000 μF

Turns ratio of transformer N 0.17 -

Output filter capacitance Cf 3 μF

Output filter inductance Lf 2 mH

Proportional constant gain kp 0.5 -

Integral constant gain ki 50 -

Figure (8) shows the input dc voltage from the PV panel, Vin=32.5 V, Which is not pure dc voltage

and it has a little value of ripple about Vripple=1V. However, any voltage ripple on the dc link will create

distortion on the output current waveform, and increase the total harmonic distortion (THD). In addition, the

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increased voltage ripple on the dc link introduces utilization losses on PV power.

Therefore, a pure or low-ripple dc-link voltage is necessary. The main factor that causes the voltage

ripple in single phase grid-connected inverters is the instantaneous power fluctuation with a magnitude

twice that of the average power and a frequency twice that of the grid frequency. Employing a power

decoupling device is essential to filter this power fluctuation. Using a large electrolytic capacitor (10000μF)

at the input is a simple method for power decoupling in flyback type microinverters [17, 18]. Figure (9)

shows the PV output current, whose value Iin=3.09 A, and it has a little value of current ripple of

Iripple=0.048A. Figure (10) shows the output power from the PV panel Pin=100 W, for S=530 W/m2 sunlight

radiation and T=25 ̊C temperature.

Figure (8): The output voltage from the PV panel.

Figure (9): The output current from the PV panel.

(V)

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Figure (10): PV output power.

Figure (11) shows the gate pulses sequence for the main switch S1(MOSFET) and switches pulses S2, S3

for the IGBTs.

Figure (11): (a) S1 gate pulses. (b) S2 gate pulses. (c) S3 gate pulses.

Figure (12) shows the drain-source voltage (Vds) for the MOSFET main switch S1, which has a

(a)

(b)

(c)

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spike voltage of Vspike=78.73 V. The spike voltage appears due to the leakage inductance of the transformer.

As shown in Figure (13), the switch S1 current (IS1) and D1 current (ID1). Due to the spike’s high voltage, it

may damage the MOSFET and it should be reduced, this is done by adding a snubber circuit to the switch

between the drain and the source.

Figure (12): Drain-Source voltage (Vds) of S1.

Figure (13): Switch S1 current and diode D1 current.

The primary current can be seen in Figure (14), as shown, the current has a shape like a rectified sine due to

the sinusoidal modulation. The peak current value of the primary currentis Ipri.=19.43A.

Vspike

Vin

Vin+NVo

IS1

ID1

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Figure (14): Primary current for flyback microinverter operates in DCM.

Figure (15) shows the voltage stress on the switches S1, S2 and S3. As shown in these figures, all

switches have lower stresses.

Figure (15):MOSFET Switch S1 voltage. (a) IGBT1 Switch S2 voltage. (b) IGBT2 Switch S3 voltage.

Figure (16-a) shows S2 output current Io1, Fig. (16-b) shows S3 output current Io2 and Fig. (16-c)

shows the output current Io before the output LC filter.

Ipri=19.43 A

(a)

(b)

(c)

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Figure (16):(a) Switch S1 current. (b) Switch S2 current. (c) Output current before the output filter.

For flyback microinverter which operates in DCM. Figure (17) shows the simulation result for the output

current fed to grid after filtering, whose value of Io(rms)=0.44 A, and it almost in-phase with the grid voltage

by power factor of P.F=0.9811 and the output current has a low value of (THD), which is 3.916% for 530

W/m2 sunlight radiation and 25̊ C.

Figure (17):Current fed to grid multiplied by 100 and grid voltage.

5. Experimental Results

The experimental results are obtained based on the prototype setup illustrated in Figure (18). A

flyback microinverter operates in DCM scheme experimentally examined on a 100 W PV panel with 33

V dc input voltage. The prototype is tested to feed 100 W maximum power transferred to the network

with 220 Vrms, 50 Hz. Measurements are obtained by using the oscilloscope spectrum analyzer (RIGOL

digital oscilloscope DS1102E 100 MHz, 1 Gsa/s sampling, USB storage) and probes (Hantek 2 X

(a)

(b)

(c)

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100MHz oscilloscope clip).

The sunlight radiation is measured using auto digitallux meter (victor 1010A), and the

temperature is measured by infrared thermometer (AR827D -50̊ C ~ 1050̊ C). The design parameters of

the implemented inverter are presented in Table (2).

v

Figure (18): Flyback microinverter prototype setup.

Table (2): Implemented inverter parameters.

MOSFET S1: APT5015BVR Core Material: 3F3

IGBT S2, S3: G40N150D N= 0.17

Diode D1,D2: RHRG75120 Cf =3 μF

fs=20 kHz Lf = 2 mH

Np=17 Turn, (Lm=50.5 μH) Standard aria wire gauge=19

lg= 0.3932 cm No. of primary strands=2

Core type: EE No. of primary strands=1

Figure (19) shows the output voltage from the PV panel (Vin) for S=530 W/m2 and T=25̊ C. As

MPPT Control

Circuit Current

Sensor

EMI filter

PIC

Microcontroller

Voltage

Sensor

Control Circuit Control Circuit

Power Circuit

HF Transformer

LC Filter S1

Cdc

S2

S3

G1

G2

G3

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shown, the voltage carries a little value of voltage ripple Vripple=3.2 V, which discussed before how to

reduce its value.Figure (20) shows the output current from the PV panel (Iin) for S=530 W/m2 and T=25̊ C.

As shown, the current carries a little value of current ripple Iripple=29 mA. Figure (21) shows the triangular

wave signal, which is internally generated by the PWM IC. The triangular wave peak voltage Vtri.=3.2 V

and the frequency is fS=20 kHz . Figure (22) shows the rectified sine wave signal S(t), which is generated by

the precision diode rectifier circuit.

Figure (19): output voltage from PV panel(1V/div.).

Figure (20): output current from the PV panel.

Figure (21): The triangular wave signal for (1V/div.)

and 20 kHz switching frequency.

Figure (22): The rectified sine wave signal S(t)

(1V/div.).

Figure (23) shows the gate pulses sequence for the main switch S1, and Figure (24) shows the gate

pulse for both S2 and S3 switches. Figure (25) shows the primary current, whose shape like a rectified sine

wave. The peak value of the primary current is Ipri.=19.2 A. Figure (26) shows the drain-source voltage (Vds)

of the main switch S1, the value Vds=60.8 V, for S=109.5 W/m2 and T=25̊ C.

Vin=30 V

Vripple=3.2 V

Iin=3.09 A

Iripple=29 mA

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Figure (23): Gate pulses sequence for the main

switch S1 (5V/div.).

Figure (24): Gate pulses sequence for switch S1 and

S2 (5V/div.).

Figure (25): The primary current Ipri (10A/div.).

Figure (26): drain-source voltage (Vds) of S1 switch.

The figures below shows experimental results for sinusoidal ac current fed to the grid by the single

stage flyback microinverter which operates in DCM with the grid voltage for different values of sunlight

radiation. Figure (27-a) shows the injected current to grid in phase with the grid voltage for S=515 W/m2,

T=25̊ C and the power generated from PV panel is Pin=93 W with respect to the given sunlight radiation.

Figure (27-b) shows the signals for S=357 W/m2, T=25̊ C and Pin=69 W. Figure (27-c) shows the signals for

S=257 W/m2, T=25̊ C and Pin=49 W. Figure (27-d) shows the signals for S=109 W/m

2, T=25̊ C and

Pin=16.5 W.

Ipri._p=19.2 A

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(a)

(b)

(c)

(d)

Figure (27): The ac current fed to grid (0.2 A/div.), 50 Hz in phase with voltage grid (10 V/div.) (a)

Io=0.381 A, P.F= 0.98 (b) Io=0.294 A, P.F= 0.961 (c) Io=0.21 A, P.F= 0.93 (d) Io=0.105 A, P.F= 0.579

The figures below shows comparison between the simulation results from the PSIM software and

the experimental results measured from the hardware device, the comparison is made for the power factor,

efficiency and total harmonic distortion, as shown in Figure (28) the power factor between the current

injected to the grid and the grid voltage in both simulation and experimental measurements almost unity

power factor at high power level, but at low power levels the power factor is poor due to the capacitor of the

output filter.

Figure (29) shows very good efficiency for both simulation and experimental results about 94.5%

efficiency at 100 W input power from the PV panel.

Figure (30) shows small values of THD (below 5%) at high power levels for both simulated and

experimental results, the THD increases above 5% at low power levels.

Io=0.381 AVgrid/10 Io=0.294 A Vgrid/10

Io=0.21 AVgrid/10 Io=0.105 AVgrid/10

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Figure (28):Calculated and measured P.F versus output power.

Figure (29): Calculated and measured efficiency versus output power.

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

15

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

Po

wer F

acto

r

Output Power Po (W)

Expreimental Power Factor

PSIM Power Factor

86

87

88

89

90

91

92

93

94

95

0 10 20 30 40 50 60 70 80 90 100

Eff

icie

ncy

(%

)

Output power Po (W)

PSIM Efficiency

Expreimental Efficiency

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Figure (30): Calculated and measured THD versus output power.

6. Conclusion

In this paper a single phase current-source flyback microinverter for grid-connected PV systems has

been investigated and implemented to inject alternating current to the national grid via a 100 W

photovoltaic panel. A design strategy for the operation scheme has been proposed in order to achieve high-

power density. Moreover, this paper has highlighted, both experimentallyandsimulation, the optimum

inverter behavior when theDCM operation mode is used, leading to a global and high-efficiency solution

for wide power range “ac–PV module” applications.

The complete DCM system was verified and a hardware prototype is build using analogue devices

and microcontroller for the flyback microinverter control. Based on the simulation and experimental results

the following aspects can be concluded:

1. Sinusoidal output current injected into the grid with approximately unity power factor is

obtained at high sunlight radiation (high output power from PV panel).

2. The output power of the microinverter depends on the solar energy absorbed by photovoltaic

panel, which is converted to electric energy. The maximum power point tracking algorithm

(Perturb and Observe method) is used to extract the maximum power from the PV panel.

3. Low THD of the output current is obtained due to the efficient control method and low-pass

output filter (LC filter).

The suggested flyback microinverter has high efficiency, high reliability, small size and low cost due to

single-stage of dc/ac conversion.

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

0 20 40 60 80 100 120

Tota

l H

arm

on

ic D

isto

rtio

n (

%)

Output power Po (W)

Experimental THD

PSIM THD

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RESEARCH ARTICLE

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