Journal of Energy and Power Engineering 5 (2011) 356-360
Comparison of Different Solutions for Blocking Diode
Applications in a Photovoltaic Panel under Varying
Ambient Conditions
H.C. Neitzert1 and A. Astone1, 2
1. Department of Electrical Engineering (DIIIE), Salerno University, Via Ponte Don Melillo 1, Fisciano 84084, Italy
2. Telecom Italia Mobile (TIM), Via Pascoli 9, Trieste 34129, Italy
Received: June 22, 2010 / Accepted: October 30, 2010 / Published: April 30, 2011.
Abstract: We characterized a crystalline silicon based mini-module under varying ambient conditions, developed a PSPICE model for
this panel, including temperature and irradiation dependence and applied this model to the simulation of the impact of a blocking diode
under different shadowing conditions. Different blocking diodes were examined, like germanium and silicon homojunction diodes and
silicon Schottky diodes and compared to “intelligent” diodes, consisting of operational amplifiers with MOSFET switches. The
simulations indicate a strongly reduced power loss in a panel integrating the new “intelligent” blocking diodes even when compared to
silicon Schottky diodes, as the best performing traditional blocking diodes.
Key words: Blocking diode, bypass diode, photovoltaics, circuit simulation, shadowing, intelligent diode, MOSFET.
1. Introduction
A long lasting discussion regarding the sense of the
integration of blocking or bypass diodes in solar panels
[1, 2, 3] did so far not come to a conclusive answer. For
example it has been reported that bypass diodes have
been responsible for failures in solar battery charging
stations [4]. Under some ambient conditions, however,
in particular under partial shadowing conditions of
solar panels or solar fields or under non
homogeneously distributed temperature conditions,
without the use of this diodes, large power losses are
measured due to non matching currents or voltages [3].
On the other hand is the voltage drop of conventional
diodes not negligible. With the development of
low-cost analogue circuitry and MOSFET’s with
extremely low on-resistance values, the realisation of
“intelligent” diodes with almost no power losses
Corresponding author: H.C. Neitzert, professor, research
fields: solar cells, nanocomposite based sensors and reliability
of optoelectronic components. E-mail: neitzert@unisa.it.
becomes viable [5]. Regarding the use of traditional
single diode solutions for blocking diode applications,
different types of devices are employed. Mostly silicon
based pn-diodes or Schottky diodes are used. In the
case of high temperature environmental conditions,
also SiC blocking diodes have been proposed [6, 7].
Operation in a extremely wide temperature range
between -170 ℃ and 270 ℃ has been achieved in this
case [6] and good reliability during accelerated lifetime
testing has been reported [8]. This is for example of
interest for space based photovoltaic systems. In the
case of terrestrial systems, however, low-bandgap
semiconductors should be more interesting because of
the lower forward bias voltage drop. Historically
germanium and selenium have been preceding the
nowadays dominating silicon as material for rectifying
diodes. May be in the future, germanium can gain again
some importance for blocking diode applications in PV
panels.
Comparison of Different Solutions for Blocking Diode Applications in a Photovoltaic
Panel under Varying a Mbient Conditions
As a first step, a questionnaire has been sent to a
number of solar panel producers, asking if they use
blocking or bypass diodes within their solar panels and
what type of diodes they use. The responses,
summarized in Table 1, indicated that only few
producers integrated blocking diodes into their solar
panels, while some more integrated bypass diodes.
Most popular diodes were silicon Schottky diodes
because of the lower voltage drop as compared to
silicon pn-diodes, as mentioned before. Subsequently a
commercial
crystalline
silicon
photovoltaic
mini-module (see Fig. 1) has been characterized under
varying ambient conditions. In particular, we measured
at ambient temperature between 30 ℃ and 60 ℃ and
used three different irradiance values (illumination
with a halogen lamp). It should be noted that under
standard irradiation conditions (AM1.5G) a conversion
Table 1
efficiency of 12.13% has been measured for the
mini-module. In Fig. 2 for example the current-voltage
characteristics at different temperatures under
irradiation with a halogen lamp with an incident
irradiance of 2105 W/m2 is shown.
2.1 Characterization of a Silicon PV Mini-Module
For a single silicon PV cell of the mini-module, based
on the measurements that are partially shown in Fig. 2, a
PSPICE model (see Fig. 3) has been developed, that
accounts also for temperature and irradiance variations.
0.1
30 (°C)
0.05
Current (A)
2. Results and Discussion
0
-0.05
Use of blocking and bypass diodes–interviews.
Manufacturer
Solar-Fabrik
United Solar
Ovonics Corp.
First Solar LLC
Solar Integrated
Technologies
Blocking diode Bypass diode Diode type
no
yes
80SQ045
optional
yes
Schottky
no
no
-
no
no
-
yes
module
dipendent
Shell Solar B.V. no
357
Current measured
-0.1
Current simulated
0
0.1
0.2
0.3
Voltage(V)
0.4
0.5
0.6
Fig. 2 Measurement and simulation of the temperature
dependent (between 30 ℃ and 60 ℃ in 10 ℃ steps)
current-voltage characteristics under irradiation with 2,105
W/m2 of a single diode of the above shown mini-module.
3
girrad
V Irr
Rs
1
D
Rsh
I
2
1
3
2
Fig. 1 Characterized silicon mini-module.
Fig. 3 PSPICE model of single cell, taken into account the
solar irradiation condition and the temperature behavior.
358
Comparison of Different Solutions for Blocking Diode Applications in a Photovoltaic
Panel under Varying a Mbient Conditions
2.2 Characterization of Different Types of Blocking
Diodes
100
Selenium
1N5818
10-1
10-2
2.3 Comparison of Power Losses for Different Blocking
Diodes
OA81
-3
10
10-4
1N4007
10-5
10-6
Experiment
10-7
Simulation
10-8
0.4 0.5
0.6 0.7
0.3
Voltage (V)
Fig. 4 Comparison of the measured and simulated forward
bias current-voltage characteristics at room temperature of
4 different commercial diodes.
0
0.1
0.2
0.10
0.09
Current
0.08
0.07
Experiment
Simulation
0.06
0.05
60°C
0.04
0.03
0.02
0.01
30°C
0
0.05
0.1
0.15
0.2
Voltage(V)
0.25
0.3
12%
10%
10,73
9,96
60°C
30°C
OA81
1N4007
8%
Selenium
6%
1N5818
5,42
4%
3,62
2,71
2,18
1,81
2%
Subsequently a simulation of the power losses at
30 ℃ and 60 ℃, when using the 4 different blocking
diodes has been performed. The results as a function of
the number of series connected silicon solar cells are
shown in Fig. 6 for a given irradiance. The use of
0.5
Fig. 5 Measurement and simulation of the temperature
dependent current-voltage characteristics of the 1N5158
Schottky diode between 30 ℃ and 60 ℃ (in 10 ℃ steps).
Power Loss
Subsequently we characterized 4 different types of
possible blocking diodes: a conventional high power
silicon diode (1N4007), a silicon based Schottky diode
(1N5818) and additionally 2 different “historical”
low-bandgap semiconductor diodes (AEG selenium
rectifier) and a germanium diode (OA81). It should be
noted, that the OA81 diode is a low power device and
has been only included because we did not have access
to commercial germanium power diodes. Other types
of high power germanium diodes could, however, in
the future be an interesting alternative to conventional
silicon diodes.
In Fig. 4 the measured and simulated forward bias
diode current-voltage characteristics at room
temperature are displayed. The superior performance
of the Schottky diode as compared to the silicon
pn-diode is clearly seen. As expected we find that also
the selenium diode has a lower voltage drop for a given
current value when compared to the silicon pn-diode.
The germanium diode (OA81) suffers from the high
series resistance and has therefore the worst behavior
of the 4 investigated diodes under high forward bias
current conditions.
For every diode type also a complete
characterization at 4 different temperatures has been
done. As an example in Fig. 5 the current-voltage
characteristics between 30 ℃ and 60 ℃ (in steps of
10 ℃) of the silicon Schottky diode are shown and
compared to the PSPICE simulation. A good
agreement between measured and simulated
characteristics has been achieved.
1,61
0,40
0,80
0,32
0,26
0,53
0%
1
2
3
4
5
6
Number of modules in series connection
Fig. 6 Power loss as a function of the number of series
connected solar cells for different types of blocking diodes at
two different ambient temperatures (30 ℃ and 60 ℃).
Comparison of Different Solutions for Blocking Diode Applications in a Photovoltaic
Panel under Varying a Mbient Conditions
2.4 Intelligent Diode with Low Voltage Drops
A very strong further reduction of the power loss
(see Fig. 7), has been achieved using a combination of
a low power operational amplifier (type “OP290”) in
combination with a low on-resistance MOSFET (type
“IRF630”), working as “intelligent” diode with low
own power consumption and very low voltage drop in
forward bias direction. The circuit of the “intelligent”
2.0
1.8
1.6
1,61
30°C
1N5818
60°C
1.4
Power Loss (%)
silicon Schottky diodes (type “1N5818”) instead of
silicon pn-diodes (Type “1N40007”) resulted in a
factor of three lower power losses as well at 30 ℃ and
at 60 ℃ ambient temperatures.
359
MOSFET + OpAmp
1.2
1.0
0,80
0.8
0.6
0,53
0,40
0.4
0.2
0,10
0.0
1
0,09
0,06
0,05
0,32
0,03
2
3
4
5
Number of modules in series connection
0,26
0,03
6
Fig. 7 Power loss as a function of the number of series
connected solar cells for a Schottky diode and an
“intelligent” diode at two different temperatures.
Fig. 8 Configuration of an “intelligent” blocking diode inserted in a PV module.
blocking diode and the insertion into the solar panel are
shown in Fig. 8.
3. Conclusions
A comprehensive simulation of the impact of the
blocking diode choice on the power losses of a silicon
photovoltaic mini-module has been done. The
simulation parameters of the silicon mini-module and
of the investigated blocking diodes have been obtained
by extraction of the device parameters from electrical
measurements under different temperatures and
irradiation conditions. An appreciable reduction of the
power losses have been obtained using a silicon
Schottky diode instead of a silicon pn-diode. Also the
use of a “historical” low-bandgap selenium rectifier
resulted in reduced power losses as compared to the
Comparison of Different Solutions for Blocking Diode Applications in a Photovoltaic
Panel under Varying a Mbient Conditions
360
silicon pn-diode. Simulation showed, that a strong
further improvement can be achieved when, instead of
the silicon Schottky blocking diode, an “intelligent”
diode, consisting of an operational amplifier in
combination with a power MOSFET is used.
[4]
[5]
[6]
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