TN013 ESD Failure Analysis of PV Module Diodes and TLP Test Method

Technical Note
TN.013.D20150915 TN013 ESD Failure Analysis of PV Module Diodes and TLP Test Method
By Wei Huang and Jerry Tichenor
Page 1
ESDEMC Technology LLC, 4000 Enterprise Dr, Suite 103, Rolla, MO, 65409, USA
www.esdemc.com Tel: (+1)-573-202-6411 Fax: (+1) 877-641-9358 Email: info@esdemc.com
ESD Failure Analysis of PV Module Diodes
and TLP Test Method
`
1. Introduction
Bypass diodes inserted across the strings of the solar panel arrays are essential to ensure the
efficiency of the solar power system. However, those diodes are found to be susceptible to potential
Electrostatic Discharge (ESD) events in the process of solar Photovoltaic (PV) panel manufacture,
transportation and on-site installation. Please refer [1], where an International PV Module Quality
Assurance Forum has been setup to investigate PV Module reliability, and Task Force 4 has been setting
guidelines for testing the ESD robustness of diodes used to enhance PV panel performance. This
document explains the theory behind the ESD damage and the proper test and analysis methods for ESD
failure of diodes. To demonstrate the proposed testing methodology that follows, we will be evaluating
six different types of diode models as supplied by our customer, who manufactures solar panel arrays.
1.1 Bypass and Blocking Diodes in Solar Panel Arrays
To help maintain the efficiency and performance of solar panel arrays it is common for bypass diodes
to be inserted across individual PV panels, and blocking diodes to be inserted in series with a string of
panels that are used in a parallel array. Figure 1 demonstrates how these diodes are inserted into a panel
array [2].
How could ESD damage happened?
What is Cable Discharge Event?
What is Transmission Line Pulse test?

Technical Note
Page 2
Figure 1 Solar PV Module bypass and blocking diode connections
The bypass diodes provide a current path around a shaded or damaged panel, and if these are not
installed, the panel will act like a high impedance load when shaded. This effectively reduces the series
string output as the current produced by the remaining series connected panels will be forced to go
through the shaded panel, thereby reducing the voltage output of the string.
If the bypass diodes are installed, and one of them fails due to ESD, it typically fails to a short circuit.
When this happens, a scenario, as shown in Figure 2, can occur. The shorted diode does not allow any
power produced by its panel to enter the system, thereby lowering system efficiency.
Figure 2 ESD damaged bypass diode fails short, disable the PV module and lowering system efficiency

Technical Note
Page 3
Figure 3 ESD damaged blocking diode fails short, allowing battery discharge path at night
Blocking diodes keep current from the battery pack, or a parallel panel string from entering a
damaged string. This is important at night when the panel array cannot provide any power, thus
providing a path for the battery to discharge. When installed, the blocking diodes may have leakage
current on the order of nano- or micro-amps. However, if they fail due to ESD, they typically fail to a
short circuit providing another path for the battery to discharge. This discharge current can be milli-amps
or amps. An example of this failure scenario is shown in Figure 3.
Failure of even one of these diodes in the field is very expensive for companies to replace due to
the need for a qualified service technician, as most installations will require code requirements to be
met. Continued operation of the panel array with a damaged bypass or blocking diode will, at best,
hamper the array’s efficiency and, at worst, cause permanent damage as it consumes power rather than
produces power. It has been proposed that the damage to the diodes is caused by electrostatic discharge
(ESD) stress.
1.2 What is ESD and how it damage the solar PV module diodes
Electrostatic discharge is the sudden flow of electricity between two electrically charged objects
caused by contact, an electrical short, or dielectric breakdown [3]. Electrostatic discharge stress can
occur in many forms and, depending on the characteristics of the stress, can damage different parts of
solar PV module subjected to the stress. In particular, there are several ESD models with industrial
standards that describe the pulse shape, source impedance, and determines levels at which the device
should survive.
The commonly used ESD models are the Human-Metal Model (HMM) (IEC 61000-4-2 for system
level ESD testing or ANSI/ESD SP5.6-2009 for component level ESD testing), the Human-Body Model

Technical Note
Page 4
(HBM) for component level ESD testing (ANSI/ESDA/JEDEC JS-001-2014), and the Charged-Device Model
(CDM) for device level ESD testing (ANSI/ESDA/JEDEC JS-002-2014). There is also the Machine Model
(MM), but it has been discontinued due to poor repeatability. Further, a new ESD model that currently
has no established industrial standard, but has a different damage effect is the Cable Discharge Event
(CDE).
ESD Model Short Applied DUT Latest Industrial Standard Comments
Human Metal Model (HMM) System level IEC 61000-4-2 ED. 2.0 B:2008 Widely used standard
Component level ANSI/ESD SP5.6-2009 Pending to be a standard
Human Body Model (HBM) Component level ANSI/ESDA/JEDEC JS-001-2014 Widely used standard
Charged Device Model (CDM) Device level ANSI/ESDA/JEDEC JS-002-2014 Widely used standard
Machine Model (MM) Component level ANSI/ESD STM5.2-2012 Discontinued
Transmission Line Pulse (TLP) Component level ANSI/ESD STM5.5.1-2014 Widely used standard
Cable Discharge Event (CDE) System level NA Hard to be defined
Table 1 Wide used ESD models, data compiled 2015.Sep
Human Metal Model (HMM)
The human-metal ESD can take place when a charged person holding a pointed metal object, like a
screwdriver or a ballpoint pen, rapidly moves the hand against an electronic device. In regard to PV
module bypass and blocking diodes, this type of ESD events would most likely occur during junction box
assembly with metal tools like tweezers, pliers, or screw drivers, etc. Figure 4 demonstrates a HMM
event between a screwdriver and a screw that is part of an electrical installation in the junction box .
Figure 4 HMM event between a screwdriver and a screw that is part of an electrical installation in the junction box
Human-Body Model (HBM)
Human-body model simulates the transfer of charge from a human to a component, such as through
a fingertip as a device is picked up. This model is one of the most commonly used ESD tests for
component qualification. In regard to PV module bypass and blocking diodes, this type of ESD event

Technical Note
Page 5
would also most likely occur during junction box assembly, especially if the operator picks a diode and
mounts it by hand into the junction box. Figure 5 demonstrates a personnel picking up a PV module
diode with bare fingers.
Figure 5 Potential ESD from HBM, Picking up the diodes
Charged-Device Model (CDM)
Charged-device model simulates the transfer of charge from a device to ground. A device can collect
charge by sliding across a surface and then discharged by contact to a metal surface or ground [6]. In
regard to PV module bypass and blocking diodes, this type of ESD event would most likely occur during
junction box assembly.
Figure 6 Solar PV module diode drops onto grounded metal surface
Transmission Line Pulse (TLP)
The TLP technique is based on charging a transmission line to a pre-determined voltage, and
discharging it into a DUT. The cable discharge emulates an electro-static discharge event that has better
defined RF signal path, controllable rise-time, and pulse width. The test setup allows transient current
and voltage waveform to be monitored therefore the change of the DUT impedance can be monitored
as a function of time in ps details. The DUT performance degrade or failure check can be automated with
RF high voltage switch and help the system with faster ESD performance analysis. Regarding to the PV

Technical Note
Page 6
module diodes, this model is not a real-world event as the transmission line would not be well defined
as the TLP model, but the type of waveform is relatively similar to Cable Discharge Events (CDE) during
the PV module on-site installation process.
Cable Discharge Event (CDE)
Cable discharge event is a frequent real-world electrostatic discharge event that occurs when a cable
is connected onto a device and the cable has existing charge prior to making the connection. This can
also happen by connecting a charged cable (open on one end) to a device. It occurs because there is a
potential differential between the charge on the cable to be connected and the device. The resulting
waveform is highly dependent on the real-world current return path and specifications of the cable.
However, these events usually have a fast rise time of less than 500ps, potential for high current that
can reach over 100 Amperes, and a potential long pulse that can be several µs if the discharging cables
are long. The fast rise time, high current, and long pulse duration can result in a permanent performance
degrade or physical damage of device being subjected. Regarding to solar PV module, the cable
connections between the panels can be very long as figure 7 shows, resulting the ESD current waveform
could be very different from all previous cases. Because cable connection is an avoidable on-site
installation process, cable discharge event should be treated as a special ESD case with special test setup
for the PV module diodes quality assurance.

Technical Note
Page 7
Figure 7 Long cables at a PV system setup can be charged and discharged during installation
Although these ESD models describe how an ESD stress event may originate, the underlying
physics of these models point to two basic damage causes. Damage may occur as the device cannot
withstand the extremely fast voltage transient, or a device is not able to handle the current or the heating
caused by the current. Here the heating occurs within nanoseconds, such that there is no thermal
exchange with the surrounding. Further, the current distribution within the conduction area of thethe
device may not be homogeneous, such that local melting (“filament creation”) leads to damage at
current levels that the device could handle, if the current would flow with equal current density in the
device. The CDM model is used to qualify a device for the first of these damage types in that a very fast
rise time as 100 ps with a short duration pulse. This test can determine if the gate oxide layer of a
component is susceptible to a CDM type of event. The HBM model is used to qualify a device for the
latter of the damage types in which a long duration (100ns) pulse is applied. The HMM, CDE and TLP
model could possibly contain both types of damage. However, CDE or TLP type of model would result in
the worst possible damage in all of the casescases discussed above.
Some examples of damage to the semiconductor components are shown in Figures 8 and 9.
Figure 8 demonstrates a fused internal connection of a transistor as a result of being zapped by an ESD
pulse. Figure 9 demonstrates burn track damage on a Schottky chip caused by ESD. Schottky technology
is commonly found in solar PV module bypass and blocking diode manufacturing.

Technical Note
Page 8
Figure 8 Burn Trace on the Schottky Chip Caused by ESD
2. The Transmission Line Pulse Test Methodology
Given the nature of how the bypass and blocking diodes could be exposed to and damaged by ESD
events, the worst case that would be the Cable Discharge Event, in which both fast rise-time and high
energy pulses occur during the installation process. Therefore, based on the existing industrial ESD
testing methods, we propose to use the transmission line pulse test method that does not necessarily
replace the IEC 61000-4-2 standard, which may have been be used in the current qualification process.
Instead, the TLP test method subject a diode (a low resistance DUT) to a faster rise-time and higher
energy pulse up to 180A (pulse reflections being allowed to approach the real-world CDE case). This
provides a fully automated device ESD performance characterization system for transient IV signal and
degrade/failure inspection before and after each pulse. Compared to the other types of ESD models, the
advantages of using a TLP test are:
Well Defined Consistent Waveform Shape
Both circuit and waveform defined in ESD simulator standards are too flexible (no impedance control for
test path, 30% tolerance at only certain time …) This causes ESD simulators to provide very different ESD
test results between different test sites. A TLP pulse is very clean and consistent.
Highly Repeatable Test Setup
Fatigue from holding ESD simulator by hand can lead to inconsistent test setups. In TLP testing with Jigs
for mounting the DUT, a more controlled test is obtained.
Fast Automatic Measurement and Reporting
Typical TLP testing is done with full automatic control of Oscilloscope Scale Adjustment, Voltage Pulsing,
Failure Criteria Checking, and IV Curve update.
Important Device Behavior is recorded for ESD analysis and design
Many useful parameters can be extracted from TLP tests for device transient behavior analysis, modeling
and System-Efficient ESD Design (SEED). Traditional ESD tests only generate pulse for Pass/Fail results.

Technical Note
Page 9
2.1 Test Setup
The TLP test setup is shown in Figure 9. A Transmission Line Pulse (TLP) generator provides a
rectangular voltage pulse by charging a 50Ω transmission line to a test voltage, and discharging the pulse
to the DUT by a special relay which can withstand the voltage, and can switch to an on-‘on ‘status without
bouncing. The pulse then travels out of the TLP through a coaxial transmission line where it first reaches
a high voltage relay (A621-HVLKR). This relay is capable of withstanding up to 10kV, and is required for
the high current TLP testing used with these high power diodes. The relay provides a means of
transferring connection of the DUT between TLP Measurement System and the Failure Detection
System. In particular, during TLP pulsing, the relay connects the DUT to the TLP, the measurement
probes, and the oscilloscope. After each TLP pulse test waveform has been captured, the system
switches the DUT to the SMU to measure the diode reverse leakage current at Maximum Recurrent Peak
Reverse Voltage (VRRM). The A621-LTKSEM leakage test module also helps to facilitate these connection
changes on the low voltage side of the measurement probes.
The DUT current is measured indirectly using a resistive tee to voltage measurement. The current is
recovered by the overlapping reflection method. This method measures both the current through and
the voltage across the DUT, but for low-resistance devices, such as a diode in the ‘on-state’, this method
is not well suited for measuring the device voltage. Instead, the DUT voltage is measured directly at the
device, providing a highly accurate voltage probing measurement.
The current measurement is performed by first measuring the pulse as it passes by the first pick off
resistor that goes to Ch1 of the oscilloscope. A short delay later (as determined by the length of coaxial
cable between the pick off resistors) the pulse reflected from the DUT is measured at the same pick off
resistor yielding an overlapped waveform. Using transmission line theory and a pre-measurement
calibration pulse, the current into the DUT can be determined as:
IDUT = (V+ - V-)/Zo
Where V+, V-, and Zo are the incident pulse, reflected pulse, and characteristic impedance (50Ω) of the
transmission line system, respectively.

Technical Note
Page 10
Figure 9 ESDEMC High Current TLP Test Setup for Solar PV Module Diodes
2.2 Test Procedure
The test procedure is demonstrated in the flowchart shown in Figure 11. Upon entering the test loop,
the system measures the leakage current of the DUT to obtain the Initial Degrade Measurement. Next
the TLP charge voltage is set. For the testing reported herein, the charge voltage was set to sweep from
500V to 9600V, in 100V increments. For the first test point, the oscilloscope scale and trigger level are
set based on the initial charge voltage and a 50Ω DUT. As testing progresses, the scale and trigger level
are set based on if the waveforms clip, or is under scaled. If the waveforms do not clip or are not under
scaled the settings are kept.
After setting the oscilloscope parameters the DUT is pulsed and the captured data is compared to
the oscilloscope display range for each captured channel to check for clipping and whether the scale is
appropriate. If any of the waveforms are clipped, the scale is adjusted and the DUT is pulsed again. If the
waveforms are ok, or under scaled, the data is accepted and processed. Any under scaled waveform
corrections are made on the next pulse level. Processing is completed by scaling the data by the
measurement attenuator and probe values.
Another degrade/failure measurement is made to determine if the DUT has failed or not. If failure
occurs, the test is stopped. If. If not, the next pulse point is performed. This repeats until all pulse points
are done, or failure occurs.

Technical Note
Page 11
Figure 10 ESDEMC TLP Test Procedure Flow Chart
2.3 Dynamic IV Curve Measurement Principles
One of the goals of the measurement system described above is to obtain the Dynamic IV curve of
the DUT over the voltage range pulsed. Current and voltage waveforms resulting during pulse test are
demonstrated in Figure 11. The dashed lines near the end of the pulses represent the start and stop
points of the dynamic IV measurement window. The measurement window is typically 70 to 90% range
of the pulse but other ranges can be selected. Over this window, the average value of the time waveform
is taken as the current and voltage, respectively. This value is then plotted for each voltage pulse applied.
Figure 11 TLP test voltage and current waveforms during one pulse test
2.4 Degrade/Failure Measurement (Leakage Current Measurement)
The leakage current was measured using the Source Meter Unit (SMU) and, depending on the diode
tested, the bias voltage was varied between two and three different voltages with the maximum bias
voltage set to the Maximum Recurrent Peak Reverse Voltage (VRRM) for each diode. The VRRM voltage
is listed in each individual diodes datasheet.
Also, for the results reported below, for any diode that failed the leakage current limit was set to
2.5mA. This is the compliance limit of the SMU and is not an indicator of diode characteristic after failure,
other than they appear to fail to a short.

Technical Note
Page 12
3. ESDEMC Solar PV Module Diodes Test
Over the years, ESDEMC Technology has tested several diode models for solar PV module companies.
The Maximum Peak Repetitive Reverse Voltage (VRRM) of the diodes are listed in Table 3Table 1. The
VRRM values are important because they provide the maximum bias voltage applied to the diode for
leakage current measurement. This value is supplied by the device manufacturer, and can be found in
their respective datasheets.
Equipment Brand and Model Specification and Comments
ESD Test ES621-200 TLP System
(A specially developed
system, not a release
model. The ES620-200
model may be released
for a simplified turnkey
solution)
<=1 ns rise-time, 100 ns pulse width, Max 10 kV charge line with no reflection
rejection.
The 10 kV charge voltage is achievable in the real world under dry conditions.
The 100 ns pulse is represented approximately 10 meters of cable that is very
close to the grounding conduit. Please note that during installation, a charged
PV module connecting to a grounded cable will have the same effect as a
charged cable discharging to a PV module. A 10 meter cable may not be the
worst case since solar cables could be up to hundreds meters for certain
systems.
Failure Test Keithley Model 2400 or
Keithley Model 6487
This configuration depends on the diode VRRM
Oscilloscope Agilent DSO9204H 2 GHz, 10 Gs/s
(200 MHz min bandwidth per ANSI/ESD STM5.5.1-2014 standard)
Table 2 ESDEMC Solar PV Module Diodes TLP Test Equipment and Settings
Diode Model VRRM, [V]
HY25PV060 60
HY25PV050 50
Vishay VSB1545 45
Diotec SB1540 40
Tyco F1200DC 150
Diotec F1200D 200
Table 3 Diode models tested and their VRRM
In the following sections, the test results will be presented in terms of the best performer to the
worst performer in regard to diode failure during TLP testing.

Technical Note
Page 13
3.1 Tyco F1200DC, 150V VRRM
Out of the 80 Tyco F1200DC devices we tested, no failure occurred. The Dynamic IV and Leakage
Current curves for three samples are shown in Figure 12. The Dynamic IV curve is read from the Y-axis
to the bottom of the X-axis, and the Leakage Current from the Y-axis to the top of the X-axis. Note that
the top of the X-axis is logarithmic due to the dramatic change in leakage current once a device fails to
a short circuit.
Figure 12 Tyco F1200DC Dynamic IV and Leakage Current @ High Current TLP Test
Test Failure
80 Passed 0 Failed
0
1
2
10 30 50 70 90 110 130 150 170 190
Failure Number and Level Distribution
Figure 13 Tyco F1200DC Failure Rate @ High Current TLP Test

Technical Note
Page 14
3.2 Vishay VSB1545
The next best performer was the Vishay VSB1545, which had only one diode fail out of one hundred
units; failure occurring near the last few test pulse levels. The Dynamic IV and Leakage Current curves
are shown in Figure 14. Once the diode failed to a short, the resulting leakage current was at the
compliance limit of the SMU, and is not an indicator of the diode condition.
Figure 14 Vishay VSB1545 Dynamic IV and Leakage Current @ High Current TLP Test
Figure 15 Vishay VSB1545 Failure Rate @ High Current TLP Test
Test Failure
99 Passed 1 Failed
0
1
2
10 30 50 70 90 110 130 150 170 190

Technical Note
Page 15
3.3 Diotec F1200D
The Diotec F1200D had eleven failures out of eighty diodes tested. The minimum, maximum, and
average pulse current for each of them are listed in Table 2, and the Dynamic IV and Leakage Current
curves, for three of the failed diodes, are shown in Figure 16.
Figure 16 Diotec F1200D Dynamic IV and Leakage Current @ High Current TLP Test
Figure 17 Diotec F1200D Failure Rate @ High Current TLP Test
Test Failure
70 Passed 10 Failed
0
1
2
3
10 30 50 70 90 110 130 150 170 190

Technical Note
Page 16
3.4 Diotec SB1540
The Diotec SB1540 diode had eighteen devices fail out of 100 tested. The Dynamic IV and Leakage
Current curves, for three of the failed diodes, are shown in Figure 18.
Figure 18 Diotec SB1540 Dynamic IV and Leakage Current @ High Current TLP Test
Figure 19 Diotec SB1540 Failure Rate @ High Current TLP Test
Test Failure
82 Passed 18 Failed
0
1
2
3
4
5
10 30 50 70 90 110 130 150 170 190

Technical Note
Page 17
3.5 HY 25PV050
All of the 50 HY 25PV050 diodes failed. The minimum, maximum, and average pulse current for each
of them are listed in Table 4, and the Dynamic IV and Leakage Current curves, for three of the failed
diodes, are shown in Figure 20.
Figure 20 HY 25PV050 Dynamic IV and Leakage Current
Figure 21 HY 25PV050 Failure Rate @ High Current TLP Test
Test Failure
0 Passed 50 Failed
0
5
10
15
20
25
10 30 50 70 90 110 130 150 170 190

Technical Note
Page 18
3.6 HY 25PV060
All of the 50 HY25PV060 diodes failed. The minimum, maximum, and average pulse current for each
of them are listed in Table 5, and the Dynamic IV and Leakage Current curves, for three of the failed
diodes, are shown in Figure 22.
Figure 22 HY 25PV060 Dynamic IV and Leakage Current@ High Current TLP Test
Figure 23 HY 25PV060 Failure Rate @ High Current TLP Test
Test Failure
0 Passed 50 Failed
0
5
10
15
20
25
10 30 50 70 90 110 130 150 170 190

Technical Note
Page 19
4. Conclusion
Of the diodes tested, only the Tyco F1200DC model did not have failure up to 200 Amp or 10 kV of
the eighty diodes tested. The next best performer was the Vishay VSB1545, which only had only one
failure, and that particular diode failed near the last few test pulses (100 diodes tested). The Diotec
F1200D had ten diode fail out of eighty tested, and the Diotec SB1540 had eighteen diodes fail out of
one hundred tested. The worst performers were both the HY models, where all diodes failed (all diodes
tested failed for both models).
Figure 24 Diode Failure Rate Comparison
The chart shown in Figure 25 depicts the minimum, maximum, and average pulse current at failure
for the diodes that failed.
Figure 25 Minimum, Maximum, and Average Pulse Current during failure for all tested diodes

Technical Note
Page 20
Of the diodes tested the Vishay VSB2045 and Hornby 15SQ050 are the only models that are Schottky
Barrier type. However, as the data suggests, the Schottky Barrier type does not necessarily guarantee
better performance in regard to withstanding ESD stress.
It has been suggested that it is not necessarily the diode design type that determines if the diode is
more or less susceptible to ESD stress, but instead a result of quality control of the manufacturing. For
example, the process may be as follows: a diode as the 15SQ100 (not tested herein) is being checked in
quality control after manufacturing. Its reverse breakdown voltage is checked. If it does not pass 100V,
but passes 50V, it is re-labeled as a 15SQ050 model. This may not guarantee that the 15SQ050 model is
a high quality 050 design, and may instead be a poor quality 100 design relegated to the 050 model line.
Here, the problem is that the diode may not hold 100V reverse voltage due to a local defect. The local
defect will concentrate the current during ESD into a very small area and cause the diode locally to melt.
Thus, the robustness of such a diode is much worse than a diode that passes the 100V reverse voltage,
which may indicate that it does have few, and less severe local defects.
According to our customer, our findings on the diode failure rate, through TLP test methodology,
correlates to their field return failure rate. Therefore, we recommend that TLP testing be performed for
all solar PV module diodes. In addition, it may be in the best interest of both solar PV module and diode
manufacturers to investigate the quality control of the diodes selected, yielding a more reliable design
for field use.
References
[1] International PV Module Quality Assurance Forum, Task Force 4 (www.pvqat.org), Dec. 5-7,
2011.
[2] n.a., (2015, Sept. 8), Bypass Diodes, Retrieved from http://www.electronics-
tutorials.ws/diode/bypass-diodes.html.
[3] n.a., Electrostatic Discharge, Retrieved from
https://en.wikipedia.org/wiki/Electrostatic_discharge
[5] n.a., n.d., Part 5: Device Sensitivity and Testing, Retrieved from https://www.esda.org/about-
esd/esd-fundamentals/part-5-device-sensitivity-and-testing/
[6] n.a., n.d., CAB Products, Retrieved from http://www.cabproducts.com/cable-rings-solar/

TN013 ESD Failure Analysis of PV Module Diodes and TLP Test Method

More Related Content

What's hot (20)

Similar to TN013 ESD Failure Analysis of PV Module Diodes and TLP Test Method (20)

Recently uploaded (20)

TN013 ESD Failure Analysis of PV Module Diodes and TLP Test Method