IEC 62979:2017 pdf download

IEC 62979:2017 pdf download.Photovoltaic modules – Bypass diode – Thermal runaway test
1 Scope
This document provides a method 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 test methodology is particularly suited for 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 61 836, 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 61 836 as well as the following apply. ISO and IEC maintain terminological databases for use in standardization at the following addresses: • IEC Electropedia: available at http://www.electropedia.org/ • ISO Online browsing platform: available at http://www.iso.org/obp 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 T lead 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 versus the junction temperature. 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 T C is the crossing point of the curve R and the curve “Heat dissipation”.On the other hand, if the reverse bias voltage is applied at a junction temperature lower than the critical temperature T C , 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 I F1 and I F2 versus the junction temperature. The crossing points of these curves and the cooling capability “Heat dissipation” show the equilibrium temperature when the forward current is applied. The equilibrium temperature T F1 corresponding to the curve F1 is higher than T C and the thermal runaway may occur when the diode is reverse biased. The equilibrium temperature T F2 corresponding to the curve F2 is lower than T C and the thermal runaway will not occur when the diode is reverse biased.