SPV1001/SPV1002 performance evaluation in a typical photovoltaic

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Application Note
SPV1001/SPV1002 performance evaluation
in a typical photovoltaic application
Introduction
The SPV1001 and SPV1002 are system-in-package solutions for photovoltaic applications,
designed to increase system efficiency by implementing a bypass function through a power
MOSFET transistor instead of a conventional Schottky diode.
The SPV1002 differs from the SPV1001 in having a lower RDSon.
This application note provides an evaluation of the performance comparison between the
SPV100x and two standard Schottky diodes, in order to supply proper guidelines for the
correct use of both devices.
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Contents
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Contents
1
Application overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2
SPV100x functionalities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
Operating modes: forward and reverse . . . . . . . . . . . . . . . . . . . . . . . . . 6
4
Thermal runaway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6
SPV100x test description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7
6.1
Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.2
Instrumentation used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.3
Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Test results and device comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1
Free devices at ambient temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.2
Devices soldered on the PCB at ambient temperature . . . . . . . . . . . . . . 12
7.3
Devices soldered on the PCB at 85 °C chamber temperature . . . . . . . . . 13
7.4
Devices soldered on the PCB at 105 °C chamber temperature (Junction
Box) 14
8
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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Application overview
Application overview
The photovoltaic effect allows each PV cell to generate current once irradiated. Therefore,
the PV cell can be represented as a current generator, whose voltage and current generated
depend on the cell technology, cell size, and irradiation level.
Typically, the voltage provided by a single PV cell is too low for most of the applications; so
far, their connection in series is preferred. For this reason, PV panels are assembled by
connecting in series a proper number of PV cells.
In optimal conditions the PV cells of a PV panel are equally irradiated and generate the
same current, assuming negligible the spread among each PV cell.
Due to topological constraints, even if only one PV cell on the panel is partially shaded, the
whole series operates at the lowest current level forced by the shaded PV cell.
Therefore, shaded cells behave like a load and the current generated from the fully
irradiated cells can cause them to overheat (Hot spot) or in some cases, also lead to
permanent damage.
In order to prevent these events, the series of cells of a PV panel are arranged in strings and
a bypass device is connected in parallel to each string, as shown in Figure 1.
Figure 1.
Bypass diodes internal connection
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The following is a brief list of the main requirements of the bypass devices:
●
To prevent the hot spot issue, bypass devices are connected in parallel to the cell string
●
During normal operation (no shadows) the reverse leakage current must be very low
●
When the cells are shaded the voltage drop must be very low.
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Application overview
Figure 2.
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Bypass functionality in series panels
The SPV100x is a two-pin device like a standard diode which, being based on power
MOSFET technology, has very low reverse leakage current and very low forward voltage
drop. Details on the operating mode of the SPV100x can be found in the related datasheet.
Figure 3.
Bypass pin connection
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SPV100x functionalities
SPV100x functionalities
The SPV100x consists of a power MOSFET transistor properly controlled by a gate driver +
charge pump + tank capacitor system, such as that explained in Figure 4:
Figure 4.
SPV100x internal architecture
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This architecture allows the following functionalities to be performed:
●
To charge the integrated tank capacitor during the power MOSFET OFF time (Toff),
boosting, with a charge pump, the voltage drop on the body diode of the power
MOSFET itself.
●
To drive the gate of the power MOSFET with the charge previously stored in the tank
capacitor during the ON time (Ton).
So, the forward voltage drop between anode (source) and cathode (drain) terminals during
the MOSFET switching, is shown in Figure 5 below:
Figure 5.
SPV100x forward voltage drop
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Operating modes: forward and reverse
3
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Operating modes: forward and reverse
In forward mode the average voltage drop between anode (source) and cathode (drain)
(Vak) is:
Equation 1
V ak ⋅ T off + V ak ⋅ T on
off
on
V ak = ------------------------------------------------------------T
with T = Ton + Toff.
During the ON time the voltage drop is:
Equation 2
V ak
on
= R ds
ON
⋅ I ak
While in the OFF time the voltage drop is equal to the MOSFET body diode voltage drop.
The average power is calculated using the relation:
Equation 3
P ak = V ak ⋅ I ak
In reverse mode the leakage current results from the standard MOSFET value:
Ir < 1 µA @ Tj = 25 °C
Ir <10 µA @ Tj = 125 °C
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Thermal runaway
Thermal runaway
If the application is not properly designed in terms of heat dissipation, Schottky diodes can
go into thermal runaway. This phenomenon permanently damages the diode, which works
like short-circuit. As the SPV100x is based on MOSFET technology, it is free from the above
mentioned phenomenon.
Normally, Schottky diodes with lower forward losses, have higher leakage current and so
they are more sensitive to thermal runaway; for this reason the correct design of the
application comes also from a trade-off between forward voltage drop and leakage current.
When the diode is in forward mode the temperature increases due to the high power
dissipation, while, when it goes into reverse polarity it can have a relatively high leakage
current due to the high temperature coming from the previous condition.
If the power losses generated from the leakage current are higher than those in forward
mode, then the diode goes into thermal runaway until permanent damage occurs.
Figure 6.
Thermal runaway positive loop
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Therefore, in all the photovoltaic applications, the use of Schottky diodes, as the bypass
device, may be dangerous because of the risk of thermal runaway.
During forward mode, the forward current (Iak) and the forward voltage (Vak) define the
junction temperature (Tj):
Equation 4
T j = T a + R thJA ⋅ ( V ak ⋅ I ak )
where RthJA is the junction to ambient thermal resistance.
During the fast switching of the diode from forward to reverse mode, the junction
temperature, due to the preceding forward mode, stays continuos and determines the
leakage current (Irev) related to the reverse voltage Vrev. This leakage current determines
the new junction temperature trend.
This variation trend, between the initial junction temperature (due to forward mode) and the
new one (due to reverse mode), gives the Tj variation and the rotation sense that can be
seen in Figure 6.
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Thermal runaway
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Experimental results can confirm that:
The stability can be guaranteed only if P_forward > P_reverse @ t_change
In Figure 7 the details for the temperature increase that destroys the device is shown.
Figure 7.
Thermal runaway detail
°C
Canale 5
Canale 6
Canale 7
Canale 8
200
Diode Break event due
to the thermal runaway
Diode in forward
during the shadow
150
100
Diode in reverse
with cells fully sunned
50
17000
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Application information
Application information
Typically, standard panels are split into three different cell groups (strings) and each one
needs a bypass device.
In order to test the devices, such as they are already used in the field application, a
dedicated PCB has been realized. Its size is suitable for many junction box dimensions, with
three separated heat sinkers for each bypass device, and thermal vias through the two
layers. Layer thickness is 35 µm.
The PCB image is shown in Figure 4 and the sizes for the 3 heat sinkers are:
Left --> 10.0 cm2;
Central --> 12.5 cm2;
Right --> 10.5 cm2
Different characterizations have been carried out in order to evaluate the device
performance in terms of current capabilities, heat power dissipation, and average voltage
drop, in four different operative conditions.
1.
Device only, without any heat sinker @ oven temperature.
2.
Just one device soldered on the PCB @ ambient temperature.
3.
Three devices soldered on the PCB, at the temperature defined by IEC 61215
procedures (@ 85 °C).
4.
The same as point 3 but at a different temperature (105 °C, to emulate the temperature
inside a junction box when ambient temperature is 85 °C).
This analysis tries to evaluate the thermal performance of all devices in the conditions
mentioned above. But note that the performances are strictly related to the PCB design.
Also, performances can be affected by how the PCB is placed inside the junction box, and
by the junction box material itself. Each of these elements can create an important
bottleneck in the correct heat dissipation that must be guaranteed for every device used in
this application field.
Figure 8.
Typical junction box PCB to solder and connect the devices on PANEL
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SPV100x test description
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SPV100x test description
6.1
Purpose
To assess the device thermal performance, checking the adequacy of the PCB thermal
design and relative long-term reliability of the SPV100x diodes versus two standard
Schottky diodes with comparable current capability (20 A and 30 A) and reverse voltage
(40 V).
6.2
6.3
Instrumentation used
●
Thermal chamber MAZZALI SYSTEM model TESYS 1200h.
●
Data Logging PicoLog, high resolution until 1/100 °C
●
Thermocouples interconnected with PicoLog.
●
Power supply and current probe.
Procedure
Set up the environment in order to measure the following parameters:
●
For free devices: The Tj (junction devices temperatures), and power in forward mode
●
For the devices soldered in the PCB: The Tj and power in forward mode
●
For the devices soldered in the PCB in the heat chamber @ 85 °C and @105 °C:
The Tj and power in forward mode.
All of the current values are checked in order to keep the SPV100x Tj temperature below its
maximum operative value (150 °C).
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Test results and device comparison
Test results and device comparison
For every device and temperature condition an analysis has been done in terms of, power
dissipation and junction temperature.
7.1
Free devices at ambient temperature
For the average power the values are calculated using Equation 2.
Figure 9.
Power vs. Iak
Power vs Iak
Pak (W)
4.5
4.0
3.5
Schottky
30A
3.0
Schottky
20A
2.5
2.0
SPV1001
1.5
SPV1002
1.0
0.5
Iak (A)
0.0
0
1
2
3
4
5
6
7
8
9
10
11
12
In the same condition the measured junction temperatures are:
Figure 10. Junction temp. vs. Iak
Junction Temp vs Iak
TEMP (C)
120
Schottky
30A
100
Schottky
20A
80
SPV1001
60
SPV1002
40
20
Iak
(A)
0
0
1
2
3
4
5
6
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Test results and device comparison
7.2
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Devices soldered on the PCB at ambient temperature
Figure 11. Power dissipation vs. Iak
Pak (W)
Power Dissipation vs Iak
6
Schottky
30A
5
4
Schottky
20A
3
SPV1001
2
SPV1002
1
Iak
(A)
0
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Figure 12. Junction temp vs. Iak
Junction Temp vs Iak
TEMP (C)
160
Schottky
30A
140
Schottky
20A
120
100
SPV1001
80
60
SPV1002
40
20
Iak
(A)
0
0
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4
5
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7.3
Test results and device comparison
Devices soldered on the PCB at 85 °C chamber temperature
Figure 13. Power dissipation @ 85 °C vs. Iak
Power Dissipation @ 85 C vs Iak
Pak (W)
3.0
2.5
Schottky
20A
2.0
Schottky
30A
1.5
SPV1001
1.0
SPV1002
0.5
Iak
(A)
0.0
0
1
2
3
4
5
6
7
8
9
10
11
12
Figure 14. Junction temp @ 85 °C vs. Iak
TEMP
(C)
Junction Temp @ 85 C vs Iak
175
150
Schottky
30A
125
Schottky
20A
SPV1001
SPV1002
100
75
Iak
(A)
50
0
1
2
3
4
5
6
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Test results and device comparison
7.4
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Devices soldered on the PCB at 105 °C chamber temperature
(junction box)
Figure 15. Power dissipation @ 105 °C vs. Iak
Power Dissipation @ 105 C vs Iak
Pak (W)
2.5
Schottky
20A
2.0
Schottky
30A
1.5
SPV1001
1.0
SPV1002
0.5
0.0
0
1
2
3
4
5
Iak (A)
6
7
8
9
10
Figure 16. Junction temp. @ 105 °C vs. Iak
TEMP (C)
Junction Temp @ 105 C vs Iak
150
Schottky
20A
140
Schottky
30A
SPV1001
130
SPV1002
120
110
Iak
(A)
100
0
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Conclusion
Conclusion
According to the results shown in the plots, the thermal and power performances of the
SPV1001 and SPV1002 are better than the standard Shottky diodes.
The above results can be improved by changing the PCB heat-sinking characteristics
(increasing size, increasing thickness, increasing copper layers, changing number and size
of thermal vias).
Finally, from the application point of view it should be noted that the performance is strongly
influenced by the specific junction box where the devices are placed.
So, for every panel, device integration in the junction box, the material and the internal PCB
design, is an important key to reaching the target current capability.
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References
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References
1.
CEI EN 61215-2006/08
2.
SPV1001/SPV1002 datasheet
3.
AN1542 application note
4.
AN836 application note
5.
AN869 application note
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Revision history
Revision history
Table 1.
Document revision history
Date
Revision
05-Dec-2011
1
Changes
Initial release
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