IEC 62979:2017

Photovoltaic modules - Bypass diode - Thermal runaway test

IEC 62979:2017 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.

Modules photovoltaïques - Diode de derivation - Essai d'emballement thermique

l'IEC 62979:2017 donne une méthode permettant de déterminer si la diode de dérivation montée dans le module est susceptible de faire l'objet d'un emballement thermique ou si le refroidissement est suffisant pour lui permettre de résister au passage entre un fonctionnement en polarisation directe et un fonctionnement en polarisation inverse sans surchauffe. Cette méthodologie d'essai est particulièrement adaptée pour les diodes Schottky, qui ont la particularité d'augmenter le courant de fuite en fonction de la tension de polarisation inverse à haute température, ce qui les rend plus propices à l'emballement thermique.

General Information

• Status: Published
• Publication Date: 09-Aug-2017
• ICS: 01 - GENERALITIES. TERMINOLOGY. STANDARDIZATION. DOCUMENTATION
• ICS: 27.160 - Solar energy engineering
• Technical Committee: TC 82 - Solar photovoltaic energy systems
• Drafting Committee: WG 2 - TC 82/WG 2
• Current Stage: PPUB - Publication issued
• Start Date: 11-Aug-2017
• Completion Date: 10-Aug-2017

Standards Content (Sample)

IEC 62979 ®
Edition 1.0 2017-08
INTERNATIONAL
STANDARD
Photovoltaic modules – Bypass diode – Thermal runaway test

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from
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IEC 62979 ®
Edition 1.0 2017-08
INTERNATIONAL
STANDARD
Photovoltaic modules – Bypass diode – Thermal runaway test

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-4587-3

– 2 – IEC 62979:2017 © IEC 2017
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Thermal runaway test . 7
4.1 Diode thermal runaway . 7
4.2 Test conditions . 8
4.3 Preparation of test specimen. 8
4.4 Test equipment . 9
4.5 Test procedure . 10
5 Pass or fail criteria. 12
6 Test report . 12

Figure 1 – Illustration of how thermal runaway occurs . 7
Figure 2 – Circuit for measurement of T and forward voltage . 9
lead
Figure 3 – Circuit for flowing a forward current to the bypass diode . 10
Figure 4 – Circuit for applying a reverse bias voltage to the bypass diode. 10
Figure 5 – The typical pattern of thermal runaway . 11
Figure 6 – The pattern of non-thermal runaway . 11

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PHOTOVOLTAIC MODULES – BYPASS DIODE –
THERMAL RUNAWAY TEST
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 62979 has been prepared by IEC technical committee 82: Solar
photovoltaic energy systems.
The text of this International Standard is based on the following documents:
FDIS Report on voting
82/1269/FDIS 82/1311/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

– 4 – IEC 62979:2017 © IEC 2017
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

INTRODUCTION
During the normal operation of PV modules the bypass diodes are reverse biased. When the
PV module is partially shaded (for example by utility poles, buildings, or leaves), some of the
cells in the PV module may not be able to produce the current being produced by the other
cells in the series string. The shaded cells are then driven into reverse bias so the bypass
diode of the shaded cell-string becomes forward bias protecting the shaded cells.
Under these circumstances, the temperature of the bypass diode increases due to the forward
current flowing through the diode. It is in this condition that the diodes are tested in
accordance with IEC 61215-2:2016, 4.18.1: Bypass diode thermal test. When the shade is
removed, operating conditions return to normal and the bypass diode is again reversed biased.
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 will be a substantial leakage 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.

– 6 – IEC 62979:2017 © IEC 2017
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 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.
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 indica
...

IEC 62979 ®
Edition 1.0 2017-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Photovoltaic modules – Bypass diode – Thermal runaway test

Modules photovoltaïques – Diode de derivation – Essai d'emballement
thermique
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form
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About the IEC
The International Electrotechnical Commission (IEC) is the leading global organization that prepares and publishes
International Standards for all electrical, electronic and related technologies.

About IEC publications
The technical content of IEC publications is kept under constant review by the IEC. Please make sure that you have the
latest edition, a corrigendum or an amendment might have been published.

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IEC 62979 ®
Edition 1.0 2017-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Photovoltaic modules – Bypass diode – Thermal runaway test

Modules photovoltaïques – Diode de derivation – Essai d'emballement

thermique
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.160 ISBN 978-2-8322-7291-6

– 2 – IEC 62979:2017 © IEC 2017
CONTENTS
CONTENTS . 2
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Thermal runaway test . 7
4.1 Diode thermal runaway . 7
4.2 Test conditions . 8
4.3 Preparation of test specimen. 8
4.4 Test equipment . 9
4.5 Test procedure . 10
5 Pass or fail criteria. 12
6 Test report . 12

Figure 1 – Illustration of how thermal runaway occurs . 7
Figure 2 – Circuit for measurement of T and forward voltage . 9
lead
Figure 3 – Circuit for flowing a forward current to the bypass diode . 10
Figure 4 – Circuit for applying a reverse bias voltage to the bypass diode. 10
Figure 5 – The typical pattern of thermal runaway . 11
Figure 6 – The pattern of non-thermal runaway . 11

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
PHOTOVOLTAIC MODULES – BYPASS DIODE –
THERMAL RUNAWAY TEST
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 62979 has been prepared by IEC technical committee 82: Solar
photovoltaic energy systems.
This bilingual version (2019-09) corresponds to the monolingual English version, published in
2017-08.
The text of this International Standard is based on the following documents:
FDIS Report on voting
82/1269/FDIS 82/1311/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
The French version of this standard has not been voted upon.

– 4 – IEC 62979:2017 © IEC 2017
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
INTRODUCTION
During the normal operation of PV modules the bypass diodes are reverse biased. When the
PV module is partially shaded (for example by utility poles, buildings, or leaves), some of the
cells in the PV module may not be able to produce the current being produced by the other
cells in the series string. The shaded cells are then driven into reverse bias so the bypass
diode of the shaded cell-string becomes forward bias protecting the shaded cells.
Under these circumstances, the temperature of the bypass diode increases due to the forward
current flowing through the diode. It is in this condition that the diodes are tested in
accordance with IEC 61215-2:2016, 4.18.1: Bypass diode thermal test. When the shade is
removed, operating conditions return to normal and the bypass diode is again reversed biased.
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 will be a substantial leakage 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.

– 6 – IEC 62979:2017 © IEC 2017
PHOTOVOLTAIC MODULES – BYPASS DIODE –
THERMAL RUNAWAY TEST
1 Scope
This document provides a method for evaluating whether a bypass diode as mounted in the
...