Download The New V/I Characteristic Expression

TC 40 Technical Meeting
Presenter
Danfeng Sun
Affiliation
Committee of Voltage Dependent Resistors,
Chinese Institute of Electronics
TITLE
A Proposal on the Revision Of IEC 61051-1 Edition 2
Why should the standard be Revised?
IEC 61051-1 – Varistors for use in electronic equipment – Part 1: Generic specification
The first edition was issued 22 years ago ( in 1991) , no significant change was made in the currently-used edition 2 of the
standard except that a few new clauses concerning surface mount varistors were added.
The present situation has become quite different from that 20 years ago when the standard was initially drawn up.
The difference mainly focuses on following four aspects:
New progress has been made
in the study of varistors
New parameters and requirements have come up
with the increasing applications of varistors
The need of coordination with new relevant
standards, e.g. IEC 62368-1: 2010, IEC 6164311: 2011, ITU/T-K77, etc.
Promote healthy market competition
Key Proposed Contents To Be Added
1. Add a new V/I Characteristic expression into the definition of varistors (Clause 2.2.3) .
2. Add following characteristics and parameters for application reference.
Pulse V/I characteristic formula/curve ( including its term and test method)
Maximum peak current derating characteristics ( including its term and test method)
TOV withstanding ampere-second characteristics ( including its term and test method)
TOV withstanding ampere-second value ( including its term and test method)
Thermal resistance (test method)
The New V/I Characteristic Expression
In Clause 2.2.3, the non-ohmic V/I characteristic of
varistors is expressed by the formula
U = CI b or I = AI g
in which β or γ are constants. But the empirical
formula is applicable only to a narrow current range as
shown in the right-hand figure.
Accurate V/I characteristic expression is crucial to
calculating peak currents and peak voltages for
varistors used in electronic circuits when overvoltages
occur.
A more reasonable and accurate V/I characteristic is
suggested be expressed as
U =10 A0 I B
in which B = (1+ A1 ) + A2 log I , A0, A1 and A2 are all
constants.
Actual V/I characteristic curve
V/ I characteristic curve expressed by U = CI b
Characteristics and Parameters for Application Reference
Characteristics and parameters for application reference are needed by users of varistors in the design and
maintenance of electronic equipment, and shall be provided by varistors’ manufacturers after testing according
to standardized methods. They shall reflect the true performance of the products, but are not subject to testing
in lot-by-lot or periodic inspection.
Although some of the above-listed characteristics and parameters for application reference have been
referred in the active Generic Specification or the Sectional Specification (IEC 61051-2) , none of the test
methods is specified. This make it difficult to judge the authenticity and validity of the information provided by
different manufacturers, and it’s also inconvenient for products’ interchangeability and quality assessment.
Characteristics and Parameters for Application Reference
Pulse V/I Characteristic Formula
Though “Clamping Voltage” is defined in Clause 2.2.11 in the active standard, which is necessary for quality
conformance inspection, it just represents A point on the V/I characteristic curve under 8/20 waveform and is
inadequate for users to design a overvoltage protecting circuit where varistors are used. Users have to
calculate the peak current flowing through a varistor and the peak voltage across it by utilizing its Pulse (8/20)
V/I Characteristic Curve or Formula in design which is often performed with the aid of computer circuit
simulation software nowadays. The Pulse V/I Characteristic is also the basis of matching technique for varistors
used in parallel and multi-level protection pattern.
Thus the Pulse V/I Characteristic Formula is a must-be in the application of varistors. Its definition and test
method shall be added into the standard.
Characteristics and Parameters for Application Reference
Maximum Peak Current Derating Characteristics
In IEC 61051-2 ( the lower-level standard of IEC 61051-1), “maximum
peak current derating curves” are required to be applied (Clause 2.2),
but its test method is not prescribed in the Generic Specification (IEC
61051-1 ).
We suggest a test method for Maximum Peak Current Derating
Characteristics on the basis of
C = LogI p + alog t + blogn
which is derived from the concept of “ Ampere-Second Resource”
C: the ampere-second resource of a varistor;
Ip: the peak value of a pulse current flowing through the varistor;
τ: the equivalent square-wave duration of the pulse curent;
a and b: constants.
Characteristics and Parameters for Application Reference
Pulse V/I Characteristic Formula
Though “Clamping Voltage” is defined in Clause 2.2.11 in the active standard, which is necessary for quality
conformance inspection, it just represents A point on the V/I characteristic curve under 8/20 waveform and is
inadequate for users to design a overvoltage protecting circuit where varistors are used. Users have to
calculate the peak current flowing through a varistor and the peak voltage across it by utilizing its Pulse (8/20)
V/I Characteristic Curve or Formula in design which is often performed with the aid of computer circuit
simulation software nowadays. The Pulse V/I Characteristic is also the basis of matching technique for varistors
used in parallel and multi-level protection pattern.
Thus the Pulse V/I Characteristic Formula is a must-be in the application of varistors. Its definition and test
method shall be added into the standard.
Characteristics and Parameters for Application Reference
TOV Withstanding Ampere-Second Characteristics / Value
Varistors used in supply circuits inevitably suffer TOV (temporary overvoltage) stresses, the thermal
breakdown of varistors by harsh TOV stresses may cause fires. Despite of its small probability, the
consequences are serious. We suggest that a parameter and a characteristics concerning TOV withstanding
ability be added into the standard.
The parameter is called “TOV Withstanding Ampere-Second Value” which is defined as
the product of a.c current (in rms) or d.c. current passing the varistor and the time duration before thermal
runaway of the varistor occurs when it is exposed to the specified a.c current (in rms) or d.c. current
The characteristics is called “TOV Withstanding Ampere-Second Characteristics” which is defined as
characteristic curve or mathematical formula expressing the relation between the power frequency current (in
rms) or d.c. current passing the varistor and the time duration before thermal runaway of the varistor occurs
over a specified current range.
Main Proposed Modifications of Existing Clauses
Clause 4.7 (voltage under pulse condition)
and Annex A
The test fixture ( as shown on the right ) referred
in clause 4.7 and annex A should be abolished
because it is not suitable for tests of high pulse
currents, and to our knowledge, none of varistor
manufacturers and testing organizations is
actually using this test fixture.
Main Proposed Modifications of Existing Clauses
Clause 4.9.3 foil method for voltage proof tests
A modification on the space of 1 to 1,5 mm between the edge of the foil and each non-axial
termination is suggested.
Reasons:
1.Frequent failures of voltage proof tests due to creepage or flashover when a power frequency
voltage of 2500 V is selected for the test.
2.According to table 17 of IEC 60335-1 (Household and similar electrical appliances - Safety - Part 1:
General requirements) , under the working voltage corresponding the test voltage of 2500 V, the
minimum creepage distance is specified to be 1,8 mm in the most favorable conditions (pollution
degree 1)
Main Proposed Modifications of Existing Clauses
Other suggestions
Clause 4.20 Fire Hazard
The problem of this clause lies in that no consideration is given to the variation of
flame application time with respect to different volumes of samples under the test in
order to ensure that test severities are basically the same.
It is suggested that a table as below be added.
Table X — The severities and requirements of the needle flame test
3
flame tender time (s) by volume V(mm )
category
maximum permissible
V>1750
duration of burning (s)
60
120
3
20
30
60
10
10
20
30
30
V≤250
250<V≤500
A
15
30
B
10
C
5
500<V≤1750
Other suggestions
Clause 4.21 Endurance at upper category temperature
In the sub-clause 4.21.3 under the clause, it is stipulated that the maximum continuous
d.c. or a.c. voltage shall be applied in cycles of 1,5 h on and 0,5 h off throughout the test
lasting for 1000 h. As far as we know, however, none of varistor manufacturers and testing
organizations is really using this method in practice, they simply applies the voltage
continuously on test samples.
The method currently prescribed in the clause is not only cumbersome, but its severity
has lagged behind the average level of the varistor industry. Thus a modification on this
method is suggested.
Now I’d like to end my presentation with my wishes that we are willing to have
a further discussion with all colleagues who are interested in the subject.
Thank you for your kind attention !
Committee of Voltage Dependent Resistors,
Chinese Institute of Electronics
Nan-Fa Zhang
Dan-Feng Sun