Unified fluorescent lamp dimming standard calculations

IEC/TR 62750:2012(E) applies to fluorescent lamp dimming systems. It deals with the interface of fluorescent lamps and dimming electronic controlgear. A unified framework for standardization of fluorescent lamp dimming systems and the associated parameter calculation method are described in this Technical Report.

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Publication Date
20-Feb-2012
Current Stage
PPUB - Publication issued
Start Date
21-Feb-2012
Completion Date
21-Feb-2012
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IEC/TR 62750
Edition 1.0 2012-02
TECHNICAL
REPORT
Unified fluorescent lamp dimming standard calculations
IEC/TR 62750:2012(E)
---------------------- Page: 1 ----------------------
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---------------------- Page: 2 ----------------------
IEC/TR 62750
Edition 1.0 2012-02
TECHNICAL
REPORT
Unified fluorescent lamp dimming standard calculations
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
ICS 29.140 ISBN 978-2-88912-939-3

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – TR 62750 © IEC:2012(E)
CONTENTS

FOREWORD ........................................................................................................................... 3

1 Scope ............................................................................................................................... 5

2 Explanation of the dimming requirements ......................................................................... 5

2.1 General ................................................................................................................... 5

2.2 Additional heating.................................................................................................... 5

2.3 Cathode heating limits ............................................................................................. 6

2.4 Substitution resistors for electronic controlgear qualification.................................... 7

3 Determination of limit values ............................................................................................ 8

3.1 General ................................................................................................................... 8

3.2 Minimum sum-of-squares – SoS (I ≤ I < I ) ............................................ 9

min D30 D Dtrans

3.2.1 Lamp and electronic controlgear systems – SoS' .................................... 9

min

3.2.2 Electronic controlgear qualification limits – SoS ................................... 11

min

3.3 Minimum cathode voltage – CV (I ≤ I < I ) ............................................. 12

min Dmin D D30

3.3.1 Lamp and electronic controlgear systems – CV' .................................... 12

min

3.3.2 Electronic controlgear qualification limits – CV ...................................... 12

min

3.4 Maximum cathode voltage – CV (I ≤ I < I ) ........................................ 13

max Dmin D Dtrans

3.4.1 Lamp and electronic controlgear systems – CV' ................................... 13

max

3.4.2 Electronic controlgear qualification limits – CV ..................................... 14

max

4 Example of calculation for 54W HO lamps ...................................................................... 14

4.1 General ................................................................................................................. 14

4.2 Calculation of lamp and ECG systems – SoS' ................................................... 15

min

4.3 Calculation of lamp and ECG systems – CV' ..................................................... 15

min

4.4 Calculation of ECG qualification limits – CV ...................................................... 16

min

4.5 Calculation of lamp and ECG systems – CV' .................................................... 16

max

4.6 Calculation of ECG qualification limits – CV ..................................................... 16

max

5 Glossary of symbols ....................................................................................................... 17

Bibliography .......................................................................................................................... 20

Figure 1 – Fundamental circuit for SoS test .......................................................................... 10

Table 1 – SoS parametric values ............................................................................................ 9

Table 2 – Fitted power law parameters ................................................................................. 11

Table 3 – Informative parameters for lamp and controlgear systems ..................................... 13

Table 4 – Datasheet parameters ........................................................................................... 14

---------------------- Page: 4 ----------------------
TR 62750 © IEC:2012(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
______________
UNIFIED FLUORESCENT LAMP
DIMMING STANDARD CALCULATIONS
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

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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

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6) All users should ensure that they have the latest edition of this publication.

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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.

The main task of IEC technical committees is to prepare International Standards. However, a

technical committee may propose the publication of a technical report when it has collected

data of a different kind from that which is normally published as an International Standard, for

example "state of the art".

IEC 62750, which is a technical report, has been prepared by subcommittee 34A: Lamps, of

IEC technical committee 34: Lamps and related equipment.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
34A/1511/DTR 34A/1546/RVC

Full information on the voting for the approval of this technical report can be found in the

report on voting indicated in the above table.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

---------------------- Page: 5 ----------------------
– 4 – TR 62750 © IEC:2012(E)

The committee has decided that the contents of this publication will remain unchanged until

the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data

related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.
---------------------- Page: 6 ----------------------
TR 62750 © IEC:2012(E) – 5 –
UNIFIED FLUORESCENT LAMP
DIMMING STANDARD CALCULATIONS
1 Scope

This Technical Report applies to fluorescent lamp dimming systems. It deals with the interface

of fluorescent lamps and dimming electronic controlgear. A unified framework for
standardization of fluorescent lamp dimming systems and the associated parameter
calculation method are described in this Technical Report.

Dimming of fluorescent lamps is becoming increasingly important as a strategy for conserving

global energy resources. This report is the result of many years of effort by global experts to

understand and test fluorescent dimming systems with the objective of standardizing these

systems to grow confidence and reliability in the marketplace. Two theoretical frameworks

have been merged to create this unified dimming standardization method: the SoS (sum of

squares of lead-in-wire currents) and CV (cathode voltage) models. The application of

dimming to actual fluorescent lamp and electronic controlgear (ECG) systems is the primary

concern for reliability in the application and end-user confidence. Characteristics of the

dimming parameter limits described in this report and observed in real system applications

such as in situ field diagnostics are offered as informative. The practical need to use

substitution resistors for ECG qualification is described in this report and also given as

normative parameters in the lamp and ECG standards. No attempt to treat the informative real

lamp-ECG system parameters as normative will be made in either the lamp or the controlgear

standards.
2 Explanation of the dimming requirements
2.1 General

This clause gives a general explanation of the dimming requirements found in the fluorescent

lamp and controlgear standards. Subclause 2.2 provides an overview of the theoretical

framework for the unified dimming standard. Subclause 2.3 provides an explanation of

informative limits for the cathode heating based on physical lamp and ECG systems.

Subclause 2.4 provides the basis for normative controlgear qualification using substitution

resistors. In this Technical Report, the use of primed quantities will signify values obtained

when measuring on actual fluorescent lamp and ECG systems. Unprimed quantities refer to

standardised quantities when testing ECG on substitution resistors. Although lead wire and

lamp discharge currents pertain to actual lamps, they will remain unprimed quantities in this

report.
2.2 Additional heating

It is a well-known fact that, when lowering the lamp current to decrease the luminous flux

(dimming) below a certain current value, the cathode is not heated sufficiently any more by

the lamp current. At these dimmed conditions without added ohmic heating, the cathode fall

will increase to sustain the lamp current and this results in an increased sputtering of the

cathode and thereby a decrease in lamp life. So additional cathode heating is necessary to

keep the cathode at a sufficiently high temperature for thermionic emission. The amount of

this additional heating current through the cathodes as a function of the lamp current is

however dependent on the controlgear circuit layout. There may be a phase shift between

these currents like in circuits with a capacitor parallel to the lamp. In other circuits, the

additional heating current is delivered by separate heating sources, in which case it is not

clear through which lead-in wire which part of the lamp current flows. For a generalized

description, these different circuits are included when describing the controlgear

requirements.
---------------------- Page: 7 ----------------------
– 6 – TR 62750 © IEC:2012(E)

It has been found that measuring the root mean square (r.m.s.) currents through the two lead-

in wires to the cathode and calculating the sum of the mean squares of these two currents as

a function of the discharge current can estimate the cathode heating. The sum of squares,

SoS’, needed to keep the cathode at a sufficient temperature, is found to have a linear

dependence on the root mean square (r.m.s.) discharge current:
2 2
SoS’ = I LH + I LL = X’ – Y’ × I
1 1 D
where
I is the discharge current;
I is I lead high, which is the highest current through either lead-in wire;
I is I lead low, which is the lowest current through either lead-in wire.

Alternatively, it has also been found that it is possible to describe cathode heating in terms of

root mean square (r.m.s.) voltage applied across the cathode, CV’, while dimming. To a

reasonable approximation in the deep dimming range, the voltage necessary to keep the

cathode at a sufficient temperature is also a linear function of the root mean square (r.m.s.)

discharge current,
CV’ = X’ – Y’ × I ,
3 3 D

where the coefficients X’ and Y’ are constants, different from X’ and Y’ in the SoS’

3 3 1 1
expression.

The dimming range of discharge currents for which additional heating is necessary is given by

a maximum value, I , which defines the transition between normal and dimming
Dtrans

operation, and a minimum value, I , specified in the lamp standard. These values are

Dmin

expressed relative to the cathode test current, I , designated in the relevant IEC

test

datasheets. This cathode test current is defined in the relevant IEC standard as the current to

be applied when measuring the cathode hot resistance. An indirect measure related to the

overall cathode temperature is the ratio, R /R , where R is the “hot” resistance during

h c h

operation and R is the “cold” resistance at 25 ºC. For a nominally performing filament, the

measured hot resistance will usually correspond to a hot-to-cold resistance ratio, R /R , of

h c

4,75. The range of discharge currents, I ≤ I ≤ I , specified for dimming is typically

Dmin D Dtrans

~10 % of I ≤ I ≤ ~80 % of I for many lamp designs . For discharge currents above the

test D test

dimming range, I ≥ I , additional cathode heating is not required, but not forbidden as

D Dtrans

long as the value for the maximum current in any lead, I , is observed. Explicit values for

LHmax

the dimming range of discharge current and cathode test current are specified in the lamp

datasheets.
2.3 Cathode heating limits

The SoS model is developed from assumptions about the cathode hot spot. This is the

location where the discharge arc attaches to the electron-emitting region of the cathode. As

the discharge current is lowered into the dimming region, a hot spot remains localized until

the lamp current is lowered below I , approximately 30 % of I . In this region of dimming

D30 test

where the lamp current is greater than or equal to I , values for the critical “minimum SoS”

D30
(SoS’ ) are set to prevent cathode sputtering and the resulting short lamp life.
min

At low dimming currents, the discharge attachment becomes diffusely attached to the cathode

and the hot spot no longer is localized or stationary. Also, the hot spot tends to lose a well-

defined location when high or excessive heating currents are applied to the cathode.

___________

It is important to note that I is defined as a cathode related current parameter and does not necessarily

test

relate to the lamp discharge current or the dimming range for lamp designs in some regions.

---------------------- Page: 8 ----------------------
TR 62750 © IEC:2012(E) – 7 –

Therefore, at low currents, or to describe the upper boundary of acceptable cathode heating,

the CV’ becomes a preferred approach to standardizing the cathode-heating requirement. In

the region of dimming where the lamp current is lowered below I , values for the minimum

D30

cathode voltage, CV’ , are set to prevent premature cathode destruction by sputtering.

min

Excessive additional heating will result in overheating of the cathode and thereby accelerated

end blackening of the lamp. To protect the cathode from overheating, resulting in excessive

barium evaporation (end-blackening) and possible mercury starvation in the lamp, a maximum

heating level should be set for the cathode voltage. This maximum cathode voltage, CV’ ,

max

is set to limit the cathode temperature below a temperature typically corresponding to a

cathode resistance ratio of R /R < 5,2.
h c

The uncoated part of the cathode can be overheated by the combination of high additional

heating and the discharge current itself (mainly in the higher dimming region). Setting a

, to the higher lead-in wire current, ILH, will protect these parts of the
maximum, I
LHmax
cathodes.

For controlgear design guidance, a target line SoS’ is also defined. It is a best setting for

tgt

the cathode heating to be sufficiently far away from the critical minimum and maximum.

To summarize, the cathode heating informative limits are given by the following set of criteria

uniquely defined over the dimming region of lamp currents. The voltage measured across the

leads of each cathode in the system should generally lie above the minimum heating line

CV’ and below the maximum heating limit CV’ . The measured SoS current values
min max

should lie above the minimum line SoS’ and the measured lead-in wire currents shall not

min
exceed the I limit.
LHmax
2 2
Lower limit: SoS’ [I + I ] = X’ – Y’ × I for I ≤ I < I
min LH LL 1 1 D D30 D Dtrans
Lower limit: CV’ = X’ – Y’ × I for I ≤ I < I
min 3 3 D Dmin D D30
Upper limit: CV’ , I for I ≤ I < I
max LHmax Dmin D Dtrans
2 2
Target: SoS’ [I + I ] = X’ – 0,3Y’ × I
tgt LH LL 1 1 D
2.4 Substitution resistors for electronic controlgear qualification

For normative ECG testing, the lamp discharge impedance is approximated using substitution

resistors, R , having values (R , R , R and R ) given at 10 %, 30 % and 60 %
L L10min L10max L30 L60

of the test current, I . Ambient temperature and lamp geometry are known to have a major

test
influence on the lamp impedance. At I = I a minimum, R , and maximum value,
D Dmin L10min

R , of the lamp substitution resistance is specified to allow for a rough approximation of

L10max

the thermal dependency of the lamp impedance. These resistor values are set at –30 % and

+ 30 % of the nominal lamp impedance at I = I . At I = 30 % (I ) and 60 % (I ) of

D Dmin D D30 D60

I , the nominal value of the lamp impedance is specified for the substitution resistance

test

value. These discharge current values have been chosen in the dimmed region where proper

setting of electrode heating is important for reliable lamp operation throughout the rated lamp

life. To summarize the lamp discharge substitution resistor set, the following abbreviations are

used:
• low impedance discharge at I : R ;
Dmin L10min
• high impedance discharge at I : R ;
Dmin L10max
• discharge impedance at I : R ;
D30 L30
• discharge impedance at I : R
D60 L60.
---------------------- Page: 9 ----------------------
– 8 – TR 62750 © IEC:2012(E)

For any ECG that does not operate with a continuous range of dimming current (e.g. step

dimming controlgear), the lamp substitution resistor is selected to approximate the lamp

discharge impedance. This impedance approximation uses linear interpolations for R from

Lmin
R to R , R from R to R , and R from R to R for the range of lamp
L10min L30 Lmax L10max L30 L L30 L60
discharge current from I ≤ I < I in this report,
Dmin D Dtrans
(R − R )
L60 L30
R = ⋅(I − I ) + R for I < I < I
L D D30 L30
D30 D Dtrans
(I − I )
D60 D30
(R − R )
L10min L30
R = ⋅(I − I ) + R for I ≤ I < I
Lmin D D30 L30 Dmin D D30
(I − I )
Dmin D30
(R − R )
L10max L30
R = ⋅(I − I ) + R for I ≤ I < I
Lmax D D30 L30 Dmin D D30
(I − I )
Dmin D30

The selected resistor shall have a resistance value within 20 % of the calculated R ,

Lmin

R , or R value for the lamp operating current of the ECG under test. For normative ECG,

Lmax L
qualification test conditions are specified in IEC 60929.

In addition, the cathode impedance is approximated with substitution resistors. Since the

2 2

heating characteristics of resistors P~V or P~I , differ significantly from actual cathodes,

1,4 3,2
P~V or P~I , three substitution resistance values, R , R , and R , are given for
test1 test2 test3

each cathode for controlgear qualification tests. The R value, chosen to account for

test1

typical cathode impedance variation and provide most cathodes with moderate auxiliary

heating R /R ≥ 4,3 is used when testing the lower cathode-heating limit, SoS . The R

h c min test1

value is approximately equal to 4,6 R for typical T5 cathodes. Nevertheless, as a rule of

thumb, R should be chosen on the order of 4,75 R . Note that, due to the selection of

test1 c

R values exceeding the typical cathode impedance, values for cathode heating limits will

test1

differ from the informative physical lamp-ECG system values when qualifying ECG on

substitution resistors. The R value, chosen to approximate the cathode impedance with a

test2

high level of auxiliary heating R /R ~5,2, is used when testing the upper cathode-heating

h c
limit, CV and I . The R value is chosen to approximate the typical cathode
max LHmax test3

impedance when heated only with auxiliary current to a temperature corresponding to

R /R ~4,3. The R substitution resistor is used when testing ECG for the lower cathode

h c test3

heating limit, CV , at the deepest dimming lamp currents that provide only negligible heating

min

from the discharge current. This selection of R is the most accurate representation of the

test3

typical cathode at the deep dimming current and therefore provides a robust test of ECG that

may use different cathode heating circuit topologies.

The normative qualification of ECG is specified only at the values of cathode substitution

resistances defined above, not continuously along the lamp current dimming curve. Related to

these values, a test procedure and a test circuit are given in the performance standard for

electronic controlgear, IEC 60929.
3 Determination of limit values
3.1 General

This clause provides the details by which the limit values for SoS and CV are determined. It is

important to keep in mind that the lamp discharge and cathode substitution resistor values as

well as the limit criteria for normative testing of ECG are negotiated quantities. The values are

set taking typical sources of cathode and lamp variation into account to provide reliable

dimming for compliant ECG operating in real systems. When developing normative limits for

the unified dimming standard, special care is taken to match the auxiliary heat delivered to a

typical cathode at I by the SoS’ and CV’ informative limit lines for the lamp and

D30 min min

controlgear system. A discontinuity in delivered auxiliary heat between these cathode heating

---------------------- Page: 10 ----------------------
TR 62750 © IEC:2012(E) – 9 –

limit lines should be avoided for practical system design. Due to the selection criteria of

substitution resistors for controlgear qualification (see 2.4) comparisons of the normative

quantities (SoS , CV and CV ) with informative quantities (SoS’ , CV’ and
min min max min min

CV’ ) will result in apparent discontinuities. This clause is divided into three subclauses.

max

Subclause 3.2 explains the derivation of the minimum SoS limits. Subclauses 3.3 and 3.4

describe the procedure for setting minimum and maximum CV limits respectively. Each

description builds from practical limits for the cathode heating based on physical lamp and

ECG systems to explain how the normative limits are determined for ECG qualification using

substitution resistors.
3.2 Minimum sum-of-squares – SoS (I ≤ I < I )
min D30 D Dtrans

Establishment of minimum auxiliary heating limits in the hot spot region begins with the

parametric values for the sum-of-squares (SoS) model. In this model, the values for the lamp

current range, I ≤ I < I , and for the SoS constants are coupled to the cathode test

D30 D Dtrans

current, Itest, at which a cathode reaches a specified resistance and temperature, given by

R /R ~4,75. These parametric values were determined through examination of reliability

h c

testing data and are considered applicable to many lamp designs. Table 1 gives approximate

mathematical expressions for these parameters.
Table 1 – SoS parametric values
Minimum dimming discharge current, I ≈ 0,1 Itest
Dmin
Maximum dimming discharge current, I ≈ 0,8 Itest
Dtrans
2 2
Lower limit of auxiliary heating X1 ≈ 1,8 I test, Y1 ≈ 1,85 Itest
SoSmin: I + I = X1 – Y1 × ID
LH LL
2 2 2
Target SoS : I + I = X1 – 0,3 Y1 × ID X1 ≈ 1,8 I test, Y1 ≈ 1,85 Itest
tgt LH LL
3.2.1 Lamp and electronic controlgear systems – SoS'
min

Since dimming operation is intended for actual physical lamps on ECG, system cathode

2 2
heating limits for SoS’ : I + I = X’ – Y’ × I over the dimming range above I are
min LH LL 1 1 D D30

desired for informative purposes. The selection of the cathode substitution resistor, R ,

test1

sets the cathode heating normative ECG requirement in the SoS model. As noted in 2.4, the

selection of R to account for cathode resistance variation results in differences between

test1

the normative SoS heating limit and the heating of actual cathodes in lamps operating on the

same ECG. Values for the coefficients X’ and Y’ are determined through a transformation

1 1

from the substitution resistor network to the lamp-ECG system. Since the auxiliary heat

delivered to the cathode is the fundamental parameter that maintains thermionic emission

under dimming conditions (see 2.2), it will be maintained invariant in the transformation. To

carry out this transformation, it is important to know the characteristics of typical cathodes. A

maximum lead wire resistance, R , is specified in the lamp standard datasheets to minimize

the biasing effect on the delivered auxiliary heat. Therefore, lead wire resistance is

considered negligible for this calculation. The following is a detailed description of the

transformation.

In the SoS model, the lead wire currents, I , I , I and I , consist of two components: the

11 12 21 22

auxiliary heating current and the lamp discharge current that also contributes to the heating of

the cathode. Similarly for the fundamental test circuit (see Figure 1) that simulates lamp

conditions using substitution resistors and i
...

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