thermal resistance measurements of bypass diode /junction ... · tambient = 25 c, if = 10 a. for...
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Thermal Resistance Measurements of Bypass Diode / Junction Boxes for Predicting Field Stressors
Narendra Shiradkar1,2, Vivek Gade2
1University of Central Florida2Jabil Circuit Inc.
Email: [email protected]
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Bypass Diode Reliability● Bypass diodes are primary components in PV modules that
provide protection against shading by limiting the reversevoltage across a shaded cell.
● A bypass diode failed in short circuit results in immediate PVmodule failure, while a bypass diode failed in open circuit cancause significant safety hazards.
● The operating junction temperature (TJ) and difference betweenmaximum and minimum temperature during thermal cycling(ΔTJ) of bypass diodes in field deployed modules determinestheir long term reliability.
● The stressors TJ and ΔTJ need to be estimated for diodes inmodules deployed in various climatic zones during service life.
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Estimating Stressors in Field● In order to quantify the thermal stresses experienced by a
bypass diode in the field, it is necessary to estimate the junctiontemperature (TJ) as a function of environmental and shadingconditions.
● Worst-case shading for bypass diode is when the module isshort circuited and a cell is 100% shaded.
● Worst-case stressors experienced by bypass diodes in variousclimatic zones can be quantified if TJ of the diode can bepredicted from the Typical Meteorological Year (TMY) data.
● The goal is to perform thermal characterization of bypass diodes/ junction boxes in the lab and then use models to predict TJ infield based on the TMY data.
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Predicting Worst-Case Diode Forward Current from TMY Data
Mounting Tilt
Location
Time of the Day
Clouds
Irradiance
Ambient Temperature
Wind SpeedCell / Module Temperature
Module Short Circuit Current = Worst Case Diode Forward Current
Electrical Model for PV Module / PV Module Datasheet
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Predicting Diode Junction Temperature from Power Dissipation
Worst-Case Diode Forward
CurrentDiode Forward
Voltage
Power Dissipation in
Diode
Diode Device (I,V,T)
PropertiesInstanteneous Junction Temperature of Diode
(TJ)
Thermal Model f (Ambient Temperature, Thermal Resistance etc)
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Outdoor Measurements: Irradiance and Temperatures● Diode case temperature
follows the irradiance on a sunny day
● Even for low irradiance and ambient temperature values, diode temperature could be significantly higher.
● Wind has introduced noise in the ambient and cell temperature measurements
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Outdoor Measurements: Diode Current and Voltage● Diode Current reduces as
irradiance reduces over time.
● Reduction in current causes reduction in Tcase.
● However, Vf shows setp-wise behavior due to two balancing forces:
● Lower current → Lower Vf, and
● Lower current → (Lower temperature)→ Higher Vf
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Diode Temperature vs Power Dissipation● On a sunny day, steady reduction in
diode current results in gradual reduction in Tcase over time.
● A state of quasi-thermal equilibrium is maintained throughout the process.
● Correlation between electrical and thermal properties can be best described by an “apparent” ambient temperature and thermal resistance.
Tcase-Diode (C) = 22.50 + 22.42 x Power Dissipation in Diode (W)
“Apparent” Ambient Temperature (Tambient)
“Apparent” Thermal Resistance (Rth)
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Simplified One-Dimensional Thermal Model
퐴푡 푇ℎ푒푟푚푎푙 퐸푞푢푖푙푖푏푟푖푢푚:푅푡ℎ(퐽푢푛푐푡푖표푛 → 퐴푚푏푖푒푛푡)~(푇퐽 − 푇퐴푚푏푖푒푛푡)
(퐼푓 × 푉푓)
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Rth(JunctionAmbient) vs TJ● The value of Rth(JunctionCase) = 3.5
was used from diode datasheet to calculate TJ from Tcase.
● In an environmental chamber, Rth(JunctionAmbient) was calculated for Tambient = 25 C, 75 C and 90 C. and If = 6,8,10,12 A.
● However, Rth(JunctionAmbient) was found to vary significantly with Tambient and TJ itself.
● This may be because of increasing heat transfer by radiation at elevated temperatures which is not accounted for by simplified 1-D thermal model.
Tambient = 25 C
Tambient = 75 C
Tambient = 90 C
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Rth(JunctionAmbient) @ Standard Conditions for Comparing Performance of Junction Boxes
● Rth(JunctionAmbient) was compared for different types of junction boxes at Tambient = 25 C, If = 10 A.
● For large, air cooled junction boxes, adding more metal (and surface area) as a heat sink can significantly reduce the thermal resistance.
● For small junction boxes with metallic heat sink, addition of junction box pottantcan significantly reduce the thermal resistance
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Conclusions and Future Work● On a sunny day, under worst-case shading scenario for a diode, the
diode is in a state of quasi-thermal equilibrium as current changes gradually with the irradiance.
● The diode case temperature is directly proportional to the power dissipated in the diode with “apparent” ambient temperature and thermal resistance as the coefficients.
● However, Rth(JunctionAmbient) calculated from the simplified 1-D model is found to vary with Tambient and TJ itself. Therefore, this approach is of limited value for predicting field stressors.
● Rth(JunctionAmbient) when measured at standard conditions can be useful to compare thermal properties of bypass diodes / junction boxes.
● We are exploring other approaches for predicting field stressors from thermal characterization of bypass diodes / junction boxes.