Hot-spot heating occurs when the operating current of the module exceeds the reduced short-circuit
current of a faulty (or shadowed) cell(s). This will force the cell(s) into a reverse bias condition when it
becomes a load dissipating heat. Serious hot spot phenomena can be as dramatic as outright burns of all
the layers, cracking, or even breakage of the glass. It is important to note that even under less severe hot
spot conditions, with the intervention of the bypass diode, a part (also known as a string) of the module is
excluded thus causing a sensible drop in power output of the module.
The approach to simulate realistic hot-spot conditions of the relevant clause 10.9 in IEC 61215 is
constantly being debated.
It is well accepted by the main test laboratories that the current version of hot-spot method does not
represent, nor is able to represent a real hot-spot situation. An improved hot-spot method has been
drafted within TC82 of the IEC, and is expected to become normative with the 3rd edition of IEC 61215 in
2010. Some test laboratories have decided to already use the improved method.
Further insight and details will be provided in a future article.
Although failure rate statistics in different laboratories may differ, hot-spot still appears to be among the 5
most frequent failures for both c-Si and thin-film modules.
Bypass diode: is a thermal test.
Bypass diode is a very important aspect of module design. It is a critical component determining the
thermal behavior of the module under hot-spot conditions and therefore also directly affecting reliability in
the field.
The test method requires attaching a thermocouple to the diode(s) body, heating the module up to 75°C ±
5 °C and applying a current equal to the short circuit current Isc measured at STC for 1h.
The temperature of each bypass diode body is measured (Tcase) and the junction temperature Tj is
calculated using a formula using the specs provided by the diode’s manufacturer (RTHjc = constant
provided by diode manufacturer relating Tj to Tcase, typically a design parameter, and UD = diode
voltage, ID = diode current).
Then the current is increased to 1.25 times the short-circuit current of the module Isc as measured at STC
for another hour while maintaining the module temperature at the same temperature.
The diode shall still be operational.
Failures of bypass diode test still occur with a certain frequency, caused by either overrating by the diode
manufacturer, or incorrect electrical configuration with respect to the module’s Isc by the module
manufacturer.
In most cases, the bypass diodes are supplied as incorporated components in the junction box of the
whole sub-assembly (junction box + cable + connector). Therefore, it is of critical importance to make
sure that this small component is closely checked during the incoming goods controls by the module
manufacturer.
UV preconditioning: is an irradiance test.
The purpose is to identify materials that are susceptible to ultra-violet (UV) degradation before the thermal
cycle and humidity freeze tests are performed.
IEC 61215 requires to subject the module to a total UV irradiation of 15 kWh/m2 in the (UVA + UVB)
regions (280 nm – 400 nm), with at least 5 kWh/m2, i.e. 33% in the UVB region (280 nm – 320 nm), while
maintaining the module at 60 °C ±5 °C.
(IEC 61646 requires a UVB portion of 3% to 10% of the total UV irradiation). This requirement has been
harmonized also for IEC 61215 by a CTL Decision Sheet n. 733 within IECEE CB Scheme.