GB/T 44081-2024 Thermal runaway test for bypass diode applied in photovoltaic modules
1 Scope
This document provides a thermal runaway test method for bypass diode applied in photovoltaic modules. The method is used for evaluating whether a bypass diode as mounted in the module is susceptible to thermal runaway or if there is sufficient cooling for it to survive the transition from forward bias operation to reverse bias operation without overheating.
This document is applicable to the testing of Schottky barrier diodes, which have the characteristic of increasing leakage current as a function of reverse bias voltage at high temperature, making them more susceptible to thermal runaway.
The test specimens which employ P-N diodes as bypass diodes are exempted from the thermal runaway test required herein, because the capability of P-N diodes to withstand the reverse bias is sufficiently high.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
IEC TS 61836 Solar photovoltaic energy systems - Terms, definitions and symbols
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TS 61836 as well as the following apply.
3.1
reverse current
current flowing in the opposite direction to the polarity of the bypass diode
3.2
reverse bias voltage
voltage applied to the opposite direction to the polarity of the bypass diode
3.3
Tlead
temperature of the lead-wire of the bypass diode measured by thermocouple
4 Thermal runaway test
4.1 Diode thermal runaway
Some of the diodes utilized as bypass diodes in PV modules have characteristics where the reverse bias leakage current increases with the diode temperature. So if the diode is already at an elevated temperature when reverse biased, there may be a substantial reverse current and the diode junction temperature can increase considerably. The worst case occurs when this heating exceeds the cooling capability of the junction box in which the diode is installed. As a result of this increasing temperature and leakage current, the diode can break down. These phenomena are called "thermal runaway". The thermal design of the bypass diode in the junction box shall be verified to ensure that thermal runaway does not occur.
How the thermal runaway does or does not occur is illustrated simply in Figure 1.
The curve R indicates the relation of the power injected by the reverse bias voltage Vr versus the junction temperature Ti. As shown, the power injected will rapidly increase at the higher junction temperature. The cooling capability of the junction box is indicated by the curve "Heat dissipation” and the critical temperature Tc is the crossing point of the curve R and the curve "Heat dissipation".
If the reverse bias voltage is applied at a junction temperature higher than the critical temperature Tc, the injected power will be more than the cooling capability and the junction temperature will keep increasing until the diode undergoes thermal runaway.
On the other hand, if the reverse bias voltage is applied at a junction temperature lower than the critical temperature, the injected power will be less than the cooling capability and the junction temperature will gradually decrease toward the environmental temperature.
The curves F1 and F2 show the relationship of the power injected by the forward current IF1 and IF2 versus the junction temperature. The crossing points of these curves and the cooling capability “Heat dissipation” show the equilibrium temperature TF1 and TF2 when the forward current is applied.
The equilibrium temperature TF1 corresponding to the curve F1 is higher than Tc and the thermal runaway may occur when the diode is reverse biased. The equilibrium temperature TF2 corresponding to the curve F2 is lower than Tc and the thermal runaway will not occur when the diode is reverse biased.
4.2 Test conditions
The test conditions under which the thermal runaway test should be performed are as follows:
a) Initial module temperature: (90±2)℃.
Modules that carry a label that says “For use in open rack mount only” maybe tested at a reduced temperature of (75±2)℃ .
As the occurrence of thermal runaway is related to the temperature at the instance of the reverse bias voltage application, the thermal runaway test is to be performed under the highest environmental temperature the module could encounter during the normal operation.
The module temperature may be measured by Tlead
b) Specified forward current: 1.25× "Short circuit current (Isc) at STC" of the PV module for the bypass diode to be tested.
c) Specified reverse bias voltage: Open circuit voltage (Voc) at STC of the cell string of the module protected by the bypass diode to be tested.
Standard
GB/T 44081-2024 Thermal runaway test for bypass diode applied in photovoltaic modules (English Version)
Standard No.
GB/T 44081-2024
Status
valid
Language
English
File Format
PDF
Word Count
8500 words
Price(USD)
255.0
Implemented on
2024-12-1
Delivery
via email in 1~3 business day
Detail of GB/T 44081-2024
Standard No.
GB/T 44081-2024
English Name
Thermal runaway test for bypass diode applied in photovoltaic modules
GB/T 44081-2024 Thermal runaway test for bypass diode applied in photovoltaic modules
1 Scope
This document provides a thermal runaway test method for bypass diode applied in photovoltaic modules. The method is used for evaluating whether a bypass diode as mounted in the module is susceptible to thermal runaway or if there is sufficient cooling for it to survive the transition from forward bias operation to reverse bias operation without overheating.
This document is applicable to the testing of Schottky barrier diodes, which have the characteristic of increasing leakage current as a function of reverse bias voltage at high temperature, making them more susceptible to thermal runaway.
The test specimens which employ P-N diodes as bypass diodes are exempted from the thermal runaway test required herein, because the capability of P-N diodes to withstand the reverse bias is sufficiently high.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
IEC TS 61836 Solar photovoltaic energy systems - Terms, definitions and symbols
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TS 61836 as well as the following apply.
3.1
reverse current
current flowing in the opposite direction to the polarity of the bypass diode
3.2
reverse bias voltage
voltage applied to the opposite direction to the polarity of the bypass diode
3.3
Tlead
temperature of the lead-wire of the bypass diode measured by thermocouple
4 Thermal runaway test
4.1 Diode thermal runaway
Some of the diodes utilized as bypass diodes in PV modules have characteristics where the reverse bias leakage current increases with the diode temperature. So if the diode is already at an elevated temperature when reverse biased, there may be a substantial reverse current and the diode junction temperature can increase considerably. The worst case occurs when this heating exceeds the cooling capability of the junction box in which the diode is installed. As a result of this increasing temperature and leakage current, the diode can break down. These phenomena are called "thermal runaway". The thermal design of the bypass diode in the junction box shall be verified to ensure that thermal runaway does not occur.
How the thermal runaway does or does not occur is illustrated simply in Figure 1.
The curve R indicates the relation of the power injected by the reverse bias voltage Vr versus the junction temperature Ti. As shown, the power injected will rapidly increase at the higher junction temperature. The cooling capability of the junction box is indicated by the curve "Heat dissipation” and the critical temperature Tc is the crossing point of the curve R and the curve "Heat dissipation".
If the reverse bias voltage is applied at a junction temperature higher than the critical temperature Tc, the injected power will be more than the cooling capability and the junction temperature will keep increasing until the diode undergoes thermal runaway.
On the other hand, if the reverse bias voltage is applied at a junction temperature lower than the critical temperature, the injected power will be less than the cooling capability and the junction temperature will gradually decrease toward the environmental temperature.
The curves F1 and F2 show the relationship of the power injected by the forward current IF1 and IF2 versus the junction temperature. The crossing points of these curves and the cooling capability “Heat dissipation” show the equilibrium temperature TF1 and TF2 when the forward current is applied.
The equilibrium temperature TF1 corresponding to the curve F1 is higher than Tc and the thermal runaway may occur when the diode is reverse biased. The equilibrium temperature TF2 corresponding to the curve F2 is lower than Tc and the thermal runaway will not occur when the diode is reverse biased.
4.2 Test conditions
The test conditions under which the thermal runaway test should be performed are as follows:
a) Initial module temperature: (90±2)℃.
Modules that carry a label that says “For use in open rack mount only” maybe tested at a reduced temperature of (75±2)℃ .
As the occurrence of thermal runaway is related to the temperature at the instance of the reverse bias voltage application, the thermal runaway test is to be performed under the highest environmental temperature the module could encounter during the normal operation.
The module temperature may be measured by Tlead
b) Specified forward current: 1.25× "Short circuit current (Isc) at STC" of the PV module for the bypass diode to be tested.
c) Specified reverse bias voltage: Open circuit voltage (Voc) at STC of the cell string of the module protected by the bypass diode to be tested.
