This paper presents the result of high-temperature durability and thermal cycling testing and analysis for the selected diodes to study the detail of the thermal design and relative long-term reliability of the bypass diodes used to limit the detrimental effects of module hot-spot susceptibility

Kurtz, S.

Wohlgemuth, J.

Zhang, Z.

UNT Digital Library

The thermal reliability study of bypass diodes in photovoltaic modulesZhang, Z.1, 2,  Wohlgemuth J. 1, Kurtz, S.1National Renewable Energy Laboratory, Golden, Colorado, USAState Key Lab of Photovoltaic Science and Technology, Trinasolar Co. Ltd., Changzhou, ChinaBypass diodes are a standard addition to PV (photovoltaic) modules. The bypassdiodes’ function is to eliminate the reverse bias hot-spot phenomena which candamage PV cells and even cause fire if the light hitting the surface of the PVcells in a module is not uniform. The design and qualification of a reliablebypass diode device is of primary importance for the solar module. To study thedetail of the thermal design and relative long-term reliability of the bypassdiodes used to limit the detrimental effects of module hot-spot susceptibility;this paper presents the result of high temperature durability and thermal cyclingtesting and analysis for the selected diodes. During both the high temperaturedurability and the thermal cycle testing, there were some diodes with obviousperformance degradation or failure in J-box 1 with bad thermal design.Restricted heat dissipation causes the diode to operate at elevated temperatureswhich could lower its current handling capability and cause premature failure.Thermal cycle with forward biased current to the diode, is representative of hotspot conditions, can impose a strong thermal stress to diode, and may causefailure for bypass diodes in some PV module that may be able to pass thepresent criteria of IEC 61215.8090100110120130140150160170Temperature rise(°C)DateDiodes temperature rise of J‐box 1DT Max of Diode 1‐1DT Max of Diode 1‐2DT Max of Diode 1‐3Restart the testing (power supply)Diode case temperature reach 220°CRestart the testing (power supply)5055606570758011/Sep13/Sep15/Sep17/Sep19/Sep21/Sep23/Sep25/Sep27/Sep29/Sep1/Oct3/Oct5/Oct7/Oct9/OctTemperature rise(°C)DateDiodes temperature rise of J‐box 3DT Max of Diode 3‐1DT Max of Diode 3‐2DT Max of Diode 3‐35055606570758011/Sep13/Sep15/Sep17/Sep19/Sep21/Sep23/Sep25/Sep27/Sep29/Sep1/Oct3/Oct5/Oct7/Oct9/OctTemperature rise(°C)DateDiodes temperature rise of J‐box 2DT Max of Diode 2‐1DT Max of Diode 2‐2DT Max of Diode 2‐3Test samples(shown in fig.1 and fig.2) : 3 types of junction boxes for testing J-boxes were attached on mini laminate modules 3 diodes per j-box Diode rated current > 10A Thermocouples were bonded to diode casesData monitoring Measure forward and reverse characteristics of diodes before  each thermal durability test Monitor current and voltage data of diodes and/or power supply  Monitor case temperature of each diodeTest Procedure Test 1• Put the samples in chamber with controlled temperature of 50, 60, 75°C• Add forward current of 10A to bypass diodes• Monitor the bypass diode case temperature and forward voltage drop and current• 1000 hours Test 2• Chamber temperature cycled from -40°C to 85°C • 3 hours per cycle• Dwell time at both 85°C & -40°C are 10~30 minutes• Add forward bias current of 10A to diodes when the chamber temperature is higher than 25°C • One power supply is used for one J-box (3 power supplies). • 100 cycles Test3• Chamber temperature cycled from -40°C to 85°C • 3 hours per cycle• Dwell time at both 85°C & -40°C are 10~30 minutes• Add reverse bias voltage of 12V to diodes when the chamber temperature is higher than 25°C. • One power supply is used for one diode(9 power supplies). • 100 cycles Next step• Chamber temperature at 75°C• One hour of reversed bias (12 V ) plus one hour of forward bias(10A) per cycle• 20 cycles0.250.270.290.310.330.350.370.3927/Sep 30/Sep 3/Oct 6/Oct 9/OctForward voltage(V)DateV 1‐1V 1‐2V 1‐3V 2‐1V 2‐2V 2‐3V 3‐1V 3‐2V 3‐3‐5005010015020011 12 13 14 15 16 17Temperature of Box‐3(°C)Time(11:00~17:00)ChamberDiodecaseThermal cycle plus reverse bias endurance testing was applied to bypass diodesto assess diodes reliability under thermal cycling caused by ambienttemperature change without hot spot.Diodes case temperature are very close to chamber temperature during the testingDiodes performance after the testing: 12V reverse biased voltage was applied to diodes when the chambertemperature is higher than 25°C. Diode case temperature was close to chamber temperature. No failure or obvious degradation of diodes were observed during or after thetest.‐60‐40‐2002040608010011 12 13 14 15 16Reverse curremt(A)Time(11:00 ‐17:00)Diode B1ChamberDiode Z1Introduction ResultsExperimentsDiscussionReferenceAcknowledgmentsThe authors thank Peter Hacke and Kent Terwilliger of the National RenewableEnergy Laboratory for offering help on the experiments. The author appreciateVivek S. Gade of Jabil's Photovoltaic and Certification Test Laboratory and PaulRobusto of Intertek for insightful comment for the testing result analysis. Thiswork was supported by the U.S. Department of Energy under Contract No. DE-AC36-08-GO28308 with the National Renewable Energy Laboratory.1. Barreiro, C., et al. PV by-pass diode performance in landscape and portrait modalities. in Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE. 2011.2. Ben-Menahem, S. and S.C. Yang. Online photovoltaic array hot-spot Bayesian diagnostics from streaming string-level electric data. in Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE. 2012.3. Bower, W.I., M.A. Quintana, and J. Johnson, Electrical and thermal finite element modeling of arc faults in photovoltaic bypass diodes. 2012. p. Medium: ED; Size: 33 p.4. Al-Rawi, N.A., M.M. Al-Kaisi, and D.J. Asfer, Reliability of photovoltaic modules II. Interconnection and bypass diodes effects. Solar Energy Materials and Solar Cells, 1994. 31(4): p. 469-480.ThermocouplesThermal cycle plus forward bias endurance testing was applied to bypass diodes to assess diodes reliability under thermal cycling caused by ambient temperature change combined with hot spot current flow.Diodes case temperature during  the testing :Box - 1: - 40 ~ 214°C Box - 2: - 40 ~ 158°C Box - 3:  - 40 ~ 157°C Diodes performance after the testing:Diodes forwards bias voltage of Box-1 increase dramatically after 40 cycles.Diodes of Box-1 totally failed after this testing.Reverse current(at reverse voltage of 10 - 16V) of diodes 3-2 (middle diode ofbox-3) and 2-2 increased by 10~20%.Diodes forward bias voltage of Box-2 remained steadyDiodes forward bias voltage of Box-3 increased by 0.5VFig. 1. Junction box sample for testingFig. 2. Assembled testing samples in the chamber  High temperature endurance testing with forward biased current was applied to bypass diodes to assess diodes operating performance under long-term hot spot condition. Diodes temperature rise of 3 J-box during the testing(shown in fig.3 and fig.4) :• Box 1: Temperature  rises of diodes 1-1 and 1-2  increased by 20°C . The highest diode case temperature reached 220°C when the chamber temperature was 60°C• Box  2: Temperature  rises of diodes were very stable.• Box  3: Temperature rises of diodes 3-1, 3-2 and 3-3 increased slightly• Temperature rises of diodes decreased when ambient temperature increased.• Diode temperature rises of J-box 1 and 3 went up after restart testing. Diodes forward voltage of 3 J-box during the testing:• J-box 1: Voltages varied with testing time. Forward voltage of diodes 1-2 increased dramatically after restarted testing(Oct. 6), while voltage of diodes1-1, 1-3 decreased.• J-box 2: Voltages were stable• J-box 3: Voltages were stable No diode failed after the high temperature testing. Note: 1.Temperature rise is the temperature difference between diode case and chamber2.Diode 1-2, 2-2, 3-2 is the middle diodes of box 1, box 2 and box 3. 3.The temperature of middle one is highest in the box.Increase chamber temperature from 50°C to 60°CFig. 3. Diode case temperature rise for 3 J-box during high temperature testingTest 1Fig. 4 Diodes forward voltage of 3 J-box during the high temperature testingTest 2ConclusionsBased on the test result above, we can find if the heat dissipation is not good, thereis still some possibility of diodes degradation or failure in PV modules under hotspot condition. Thermal cycle condition with forward biased current to diode,really representative of hot spot conditions, can impose a strong thermal fatiguestress to diode, and may cause failure for bypass diodes of some PV module thatmay be able to pass present criteria of IEC 61215.05101520253035404510 11 12 13 14 15 16Reverse current(uA)Reverse voltage(V)Z2‐Before test Z2‐After test Q2‐Before test Q2‐After testTest 3Fig. 5. Chamber temperature and diode case temperature of box 3 during diodes thermal cycle plus forward bias testingFig. 6. Reverse characteristics of  diodes 2-2(Q2) and diode 3-2(Z2) before and after diodes thermal cycle plus reverse bias testingFig. 7. Chamber temperature and diode case temperature of box 3 during diodes thermal cycle plus reverse bias testingBias voltage /currentMonitor diode temperatureMonitor  Voltage /currentJ-boxTo assess diodes thermal reliability of PV modules, three indoor tests weredesigned to simulate 3 types of diodes operating condition. The related test resultswere shown in above section.High temperature endurance testing with forward biased current was applied tobypass diodes to assess diodes operating performance under hot spot condition. Minimodules with three types of junction boxes were put in chamber with controlledtemperature. Forward biased current of 10A was added to bypass diodes; and thebypass diode case temperature and forward voltage drop and current were monitoredduring the testing. After 1000 hours’ testing, though there is no abnormal appearanceof diode were found and no appreciable changes in terms of reverse diodecharacteristics were detected, the temperature rise of worst diodes in one J-boxincreased by 25°C. The temperature rises of diodes in J-box 1 and 3 went up by 2-15°C and their forward voltage increased dramatically after cool down the diodesand restart testing, while that of J-box 2 was stable. Based on the test result above,we can find if the heat dissipation is not good, there is still some possibility of diodesdegradation in PV modules in hot spot condition. When the diodes is forward biasedwith hot spot current flow, the forward current may make the diode hot enough forthe dopants that create the N- and P-type areas in the diode to diffuse across thejunction, wrecking the semi-conducting behavior that we rely on, and causeperformance degradation.Two types of thermal cycle testing were processed to assess the diodes’ durabilityof thermal cycling stress caused by ambient temperature change with or without hotspot in PV modules. Three types of J-boxes were tested in chamber with cyclingtemperature range from -40°C to 85°C. For the first 100 cycles, forward biasedcurrent of 10A was applied to diodes when the chamber temperature is higher than25°C. One of diodes totally failed with open circuit after the first 100 thermalcycles testing. The high temperature combined with thermal cycling will cause thediodes resistance increase and damage the PN junctions. For the second 100 cycles, -12V reverse biased voltage was added to diodes during the chamber temperature ishigher than 25°C. The diodes case and junction temperatures were close to ambienttemperature during the second 100 cycles test. And there was no failure or obviousdegradation of diodes were observed during or after the test. The diodes performanceof PV module is stable if there is no hot spot issue.The diode performance is stable if the diode is reverse-biased with low diodetemperature. However, the leakage currents doubles every 10°C as the temperatureincrease, and eventually the current may reach a level where the heat dissipationwithin the junction is high enough for the junction temperature to run away. For thefield operating condition, the PV modules may encounter momentary shadingcaused by cloud or bird, etc. The diodes in the modules will work under thecondition of high temperature with hot spot current flow firstly when the shading ison the modules. Then the diodes will be reverse-biased in high temperature conditionafter the shading is gone. For next step, the experiments need be designed to accessthe diode thermal reliability under simulated the field condition of momentaryshading .NREL is a national laboratory of the U.S. Department of Energy, Office of EnergyEfficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.2013 Photovoltaic Module Reliability Workshop  •  February 26‐27, 2013  •  Golden, ColoradoNREL/PO‐5200‐58225

Thermal Reliability Study of Bypass Diodes in Photovoltaic Modules (Poster)

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Thermal Reliability Study of Bypass Diodes in Photovoltaic Modules (Poster)

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