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DC to AC Inverter for Solar Panel
Stanislav Arendárik, (Aug 2010)
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Main Topics
Main topics:
•
•
•
•
Solar panel description and characteristics
Main parameters of the inverter
Structure and block schematic of the inverter
Blocks description
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Solar cell simple description
Photons
in sunlight hit the solar panel
and are absorbed by semiconductor
material – silicon.
Electrons
(negatively charged) are
knocked loose from their atoms, allowing
them to flow through the material to
produce electricity. Due to special
composition of solar cell, the electrons
are only allowed to move in a single
direction.
An
array of the solar cells converts
solar energy into a usable amount of
direct current (DC) electricity.
source: wikipedia
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Solar cell characteristics
 A solar cell may be modeled by a current source in parallel with
a diode, shunt and series resistances.
 IL represents the max current of the solar panel (short current)
 Diode forms the I-V characteristic
 Shunt resistor represents the leakage currents (very small)
 Series resistance represents the wiring losses
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Solar cell characteristics - variations
For most crystalline silicon solar cells the change
in VOC with temperature is about -0.50%/°C

 Impact of the product of the series resistor and
short-circuit current (ISCRS) is about 25mV/cell.
 Impact of the parallel resistance is small.
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Solar cell characteristics example
•
The maximum power point (MPP) on the I-V curve varies with real working conditions
– high dependency on the irradiance and lower dependency on the temperature.
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Maximum Power Point

In the technical parameters of the solar cell
panel are defined:
• VMP – voltage at MPP
• IMP – current at MPP

The Inverter for the solar cell panel must
achieve the operation on the MPP. This
method is called as MPPT – maximum power
point tracking.

The presented inverter has implemented the
P&O (perturb & observe) algorithm for MPPT.

To get maximum power from a PV panel
required operating at the optimum voltage.
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Selected range of the solar panels

The amount of the solar cells is used to build one solar panel. Usually 72, 36, or other.

For this design was selected panel with 72 cells, this implies MPP at 36V, which fairly
corresponds to 3 x 12V lead acid batteries in series as the back-up for the power source for
the inverter. This battery is used as the energy accumulator for the off-grid inverter usage.

The characteristic of the selected solar panel:
PMAX = 180W
VSC = 44.4V
VMP = 36V
ISC = 5.25A
IMP = 5A
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Inverter structure
Option
DEMO Inverter
Charger
Battery
MPPT
Boost
stage
PV String
Output
filter
Inverter
GRID
PV Panel
Isolation
Isolation
Main HW/SW
Selected design way
Possible design way
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Main Parameters of DC to AC Inverter

Input voltage source





Output voltage




Lower than 3%;
Fault Protection



400VA;
Harmonic distortion of the output voltage


This design aims 1-phase 230V 50Hz (115V 60Hz optional);
Single design, just different power components;
Tolerance +5% to -10%;
Output power


Voltage level from solar panel VMP = +36V
Solar energy can be stored in Lead-Acid batteries
Battery charger can be implemented (phase 2)
Selected 3 pcs Lead-Acid batteries in series
Output over-current, over-voltage, short-circuit;
Input under-voltage;
Signal output

Serial link RS-485 to main system
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Block Diagram of DC to AC Inverter

Block Diagram of Inverter

The whole DC to AC inverter consists of main power parts:
–
–
–
–
–


MPP Tracking for solar panel output – software implemented
DC low voltage to DC high voltage converter
DC high voltage to AC sine output voltage inverter
Output filter
Isolated RS-485 line
Associated control and fault detection circuits
Both DC-DC converter and DC-AC inverter is controlled by one DSC MC56F8023.
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DC to DC Converter

Push-Pull type with bridge rectifier on the isolated secondary side

Advantage:
• Simple transformer windings
• Simple control
• Good efficiency
• MPPT algorithm integrated in the software control loop
D1
+400V DCBus
T1
L1
PWM1
Q1
C1
+ C2
+
PWM2
Q2
0V DCBus
D2
0V DC In
+36V DC In
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DC to DC Converter

Push-Pull type control signals:
Dmin
Dmax
U
The duty-cycle for the PWM1 and PWM2 has the
same value

The PWM2 control signal is shifted of T/2 later
against the PWM1 signal
PWM 1

t
U
The actual duty-cycle value D depends on the
actual power transferred through the transformer

PWM 2
t
PWM period T
PWM Duty-cycle variation
PWM 2 = PWM 1 + T/2
Dmax < 0.5 T
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DC to AC Inverter

Full-bridge type inverter

Advantages:
• Simple +400V DCBus used to generate 230V AC
• Simple PWM control
• Simple reconstruction filter for AC line output
• Can be controlled by half-bridge drivers or isolated gate transformers
• Power MOSFETs or IGBTs can be used
+400V DCBus
AC Line_1
Q3
Q5
L2
C3
AC Line_2
PWM4
DRIVER
Q4
Q6
PWM3
DRIVER
0V DCBus
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DC to AC Inverter

+Umax
Full-bridge type inverter control signals:
-Umax
U

The PWM3 and PWM4 are complementary
PWM 3
The duty-cycle varies ideal from zero to full
period in sine amplitude and power line
frequency

t
U
Real duty-cycle varies from 5% to 95% of the
period

PWM 4
When the duty-cycle is <50%, the negative
part of the output sine voltage is generated

t
PWM period T
When the duty-cycle is >50%, the positive part
of the output sine voltage is generated

PWM Duty-cycle variation
PWM 4 = PWM 3
D = (5% - 95%) T
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Output filter

Integrated output EMI filter used

Advantages:
• Compact standard design
• Good EMI properties
• Sufficient load properties
AC Line_1
Line L
L3
C5
Line PE
C4
C6
AC Line_2
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Line N
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Auxiliary Power Supplies
Auxiliary power supplies power the on-board circuitry
The HW comparator enables the first
DC/DC converter if the input voltage is >
18V DC from PV panel

MOSFET
MOSFETs
DRIVERS
TR
PUSH-PULL
+5VA-ISO
DC/DC
0VA-ISO
The main +12V DC power supply acts as
the source for all other DC/DC converters
+12V-ISO
ISOL
IGBT
DC/DC
+36V
0V-ISO
DRIVERS
FULLBRIDGE
BAT Relay +
+5VA-ISOA
HW Comparator
ISOL
ANALOG
DC/DC
PV
Two isolated power supplies provide the
power for the isolated secondary side of the
inverter
PARTs
3 x 12V
+12V

ANALOG
+5VA
If the battery is connected, the inverter
runs from battery and battery is charged by
the solar panel

+5VA
0V-ISOA
PARTs

+36V
DC/DC
+12V
+5VA
DC/DC
+12V
0V
+5VA
0V
ANALOG
0VA
PARTs
+3.3V
DC/DC
The control DSC is placed on the primary
low voltage side

DSC
+3.3V
0V
Isolated circuitry
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Control circuits







DSC MC56F8023 used
Controls the DC/DC converter and DC/AC inverter
MPPT software algorithm for the solar panel implemented
Battery charger control – as option
RS-485 line interface for supervisor’s line communication
Output over-current, over-voltage, short-circuit and input under-voltage fuses implemented
ON-GRID or OFF-GRID mode
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Battery connector
Solar Panel Battery
Connectors present
relay
Low power
DC-DC converters
For control circuitry
Push-Pull
Primary side
DSC MC56F8023
Control Board
RS-485 Line
Battery Charger HW
Inverter view
Full-Bridge Inverter
+ L-C Filter
DCBus +400V
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Output Filter
Power
Output
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Conclusion

One standard DSC MC56F8023 used for whole inverter control

Possibility of use as the ON-GRID or OFF-GRID connected inverter

Possibility of use of wide range of the solar panels:
 One piece of the 36V type with the output power up to 450W
 Two pieces of the 18V types in series with the total output power up to 450W

Inverters can be connected in parallel at the output to boost the output power in the OFF-GRID usage

The inverter starts run when the sufficient amount of the power is available from the solar panel (if battery
not connected)

The 3 x 12V lead-acid batteries in series are used as the energy accumulator
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DEMO Connection
DEMO Start-up sequence:
Switch
OFF both switches (down position)
Connect
load bulb (60W to 100W) at output
Connect
the solar panel

Put
The green LED shines, red LED not
OFF-GRID switch “ON” (up position)
Switch
ON the main switch
The
“Fault” LED indicates the over-current – when
shines, switch OFF the main switch, wait about 2030 sec and switch ON the main switch.
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Literature
[1]: http://www.solarserver.com/knowledge/basic-knowledge/photovoltaics.html
[2]: http://www.nikhef.nl/~h73/kn1c/praktikum/phywe/LEP/Experim/4_1_09.pdf
[3]: http://en.wikipedia.org/wiki/Solar_cell
[4]: http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=56F802X&tid=mDHp
[5]: http://www.simosolar.com/uploadfile/learn/uploadfile/200904/20090417030623524.pdf
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