Download Modelling LED Lamps with Thermal Phenomena Taken into Account

Modelling LED Lamps with
Thermal Phenomena
Taken into Account
Krzysztof Górecki and Przemysław Ptak
Gdynia Maritime University
Department of Marine Electronics
Outline
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Introduction
Model form
Results of calculations and measurements
Conclusions
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Introduction (1)
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Every LED lamp contains several components.
LED Lamp Optical
Lens
LED Module
Power Supply
LED Lamp Case
LED Module Heatsink
LED Lamp
Case
E27 Thread

The switch-mode power supply stabilizes the output current or the
output voltage.
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Introduction (2)
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The light can be emitted by the single LED or by the module
containing several LEDs situated on the common basis.
The power supply contracted in the LED lamp converts energy from
the electroenergy-network into the constants voltage or the
constant direct current feeding lighting elements.
Because of the limited watt-hour efficiency of this circuit an
essential increase of their internal temperature is observed as a
result of a self-heating phenomenon.
Between the element emitting the light and the power supply,
mutual thermal coupling occurs, resulting from the mechanical
construction of the lamp, causing an additional increase of the
temperature of the mentioned components of the lamp.
To limit the value of this increase a heat-sink is used, to which both
the mentioned components of the lamp are mounted.
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Introduction (3)
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While designing electronic devices computer programmes that
perform an analysis of these circuits are used.
Very often SPICE software is used.
In order to take into account thermal phenomena in the
analysis, electrothermal models of the components of the
investigated circuit are indispensable.
In this paper the electrothermal model of the LED lamp
dedicated for SPICE is presented.
This model takes into account electrical and optical
phenomena occurring in the investigated lamp, self-heating
and mutual thermal coupling between the components of the
lamp.
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Model form
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T
T
The model of the LED lamp has the form of a
V
V
subcircuit for SPICE.
R
R
R
R
C
C
C
C
This model belongs to the group of compact
G
G
G
electrothermal models.
G
The electrical model of the LED module
V
E
E
R
describes the dc characteristics of the circuit
C
E
consisting of m diodes connected in series.
V
G
Thermal model
This model contains the controlled current
LED
module
electrical
model
P
power supply model
i
source G1, the linear resistor RS and the
G
R
E
R
v
lamp
controlled voltage source ERS.
optical E
E
G
G
model
The current source G1 describes the diffusive
component of the current of diodes contracted
in the module.
The resistor RS0 represents a series resistance of all the diodes contracted in the LED
module in the reference temperature T0, while the voltage source ERS describes the linear
increase of the voltage drop on these resistance on their internal temperature
The optical model of the LED module represents the controlled voltage source EL, whose
output voltage corresponds to power density of radiation emitted by this module.
In the description of the source EL the influence of temperature and of the current is taken
into account
j
j1
T1
thD

th2
thD
th3
th2
Pth2
Pth1

T3
th5
Pth3
Pth5
T2
T3
T1
th5
th3
th4
th4
T2
Ta
e
S
line

Pth4
G
1
RS
S0
L

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
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2
3
3
6
Tj
CthD
Tj1
RthD
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

Rth3
Cth5
Cth4
VTa
power supply model
line
RS
E3
Rth5
GPth5
VT2
ET2
G2
VT3
GPth3
ET3
ET1
lamp
optical EL
model

Cth3
GPth2
Pe

Rth2
Cth2
GPth1
Model form (2)
VT1
GPth4
Rth4
Thermal model
LED module electrical model
i
ERS RS0
v G G1
G3
In turn, the electrical model of the power supply contains the controlled
current source G2, the controlled voltage source E3 and the resistor RS (for
the power supply with the constant output voltage) or the controlled current
source G3 (for the power supply with the constant output current).
The source G2 describes the waveform of the current received from the
network with the properties of the power factor correction circuit taken into
account.
The current of this source is described by means of the sum of sinusoidal
functions of frequencies corresponding to the following harmonics of the
network frequency.
The description of the output voltage of the source E3 takes into account the
influence of the internal temperature of the power supply on the output
voltage of the not loaded circuit, and the resistor RS represents the output
resistance of this power supply.
The power supply producing the constant output current is modelled with
the use of the controlled current source G3
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Tj
CthD
Tj1
RthD
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
Rth3
Cth5
Cth4
VTa
power supply model
line
RS
E3
Rth5
GPth5
VT2
ET2
G2
VT3
GPth3
ET3
ET1
lamp
optical EL
model

Cth3
GPth2
Pe

Rth2
Cth2
GPth1
Model form (3)
VT1
GPth4
Rth4
Thermal model
LED module electrical model
i
ERS RS0
v G G1
G3
The thermal model of the LED lamp enables calculation of the value of the
internal temperature of every diode in the LED module Tj and the internal
temperature of the power supply Tj1.
The presented model takes into account both self-heating phenomena in
every LED, and in the power supply, and mutual thermal coupling between
diodes contracted in the LED module and between this module and the
power supply.
The controlled current sources contracted in the thermal model represent
thermal power dissipated in every diode contracted in the LED module Gpth1, power dissipated in the power supply - Gpth2, the sum of power
dissipated in the other diodes of the LED module - Gpth3.
The other controlled current source and controlled voltage sources are used
to describe the mutual thermal coupling between components of LED lamp.
RC elements describe self and mutual transient thermal impedances in the
considered model.
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Results of calculations
and measurements
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By means of the presented model a lot of characteristics of different types
of LED lamps were calculated and the results of calculations were
compared with the results of measurements.
In the next part of this section the results of investigations of two LED
lamps (CLA25 and CLA60) by OSRAM are presented.
The nominal values of electrical power and the emitted luminous flux of
these lamps are different values.
The LED module included in the lamp CLA25 consists of 4 power LEDs
connected in series, whereas the module included in the lamp CLA60
consists of 13 power LEDs .
The power supply included in the lamp CLA25 stabilizes the output voltage,
whereas the power supply included in the lamp CLA60 stabilizes the output
current.
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Results of calculations
and measurements (2)
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In the figures presented in the further part of this section points
represent the results of measurements, whereas lines – the results of
calculations.
From the point of view of quality of the energy in the electroenergynetwork, it is important, that the waveform of the current feeding the
lamp has a shape most nearing to the sinusoid and that the phase shift
between the current and the voltage is the least.
At first the investigations results of the lamp CLA25 will be shown.
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Results of calculations
and measurements (3)
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Figure presents the calculated and measured waveforms of the current feeding
the lamp CLA25 calculated at different RMS values of the supply voltage.
500
400
CLA25
80V
300
Iin [mA]
200
130V
100
0
-100 0
-200
0,005
0,01
0,015
0,02
230V
-300
-400
-500
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t [s]
The shape of the obtained waveforms strongly differ from the sinusoidal
waveform.
The amplitude of the input current decreases at an increase of the input voltage.
The obtained shape of the waveform of the current Iin shows that from the point
of view of the electroenergy-network the considered system is seen as the
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rectifier with non-linear capacitive load.
Results of calculations
and measurements (4)
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Figure illustrates the calculated spectrum of the input current obtained at different
values of the RMS value of the input voltage.
Up to 35 harmonics should be taken into account in order to properly describe the
measured waveforms of the input current.
140
80V
CLA25
120
Iin [mA]
100
80
60
130V
40
20
230V
0
1
3
5
7
9 11 13 15 17 19 21 23 25 27 29 31 33 35
Number of harmonics
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The parameter, which characterizes a discrepancy between the considered
waveforms and the sinusoidal wave is the total harmonic distortion (THD).
The calculated value of this parameter is very high and it is equal to
72.9% for Vin = 80 V, 83.96% for Vin = 130 V and 69.2% for Vin = 230 V.
Such a high value of THD could lead to worsening of quality of the electrical energy
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in electro-energy network.
Results of calculations
and measurements (5)
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The next two figures illustrate the influence of mutual thermal coupling between the
LED module and the power supply on the characteristics of the lamp CLA25.
Dashed lines correspond to the disassembled lamp, in which the LED module and the
power supply are not connected together, and solid lines - the mounted lamp.
Figure presents the calculated and measured dependences of power density of the
emitted radiation on the RMS value of the supply voltage.
1,2
CLA25
1
Pe [W/m2]
0,8
0,6
0,4
0,2
0
0
50
100
150
200
250
VinRMS [V]
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As it is visible, for the mounted lamp the value of power density is higher even by
25% than for components of the lamp operating separately.
It is worth noticing that for the input voltage lower than 70 V the dependence
Pe(VinRMS) is an increasing function. It is the result of the increasing output voltage of
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the power supply.
Results of calculations
and measurements (6)
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Figure shows the calculated and measured dependences of the internal temperature
of the LED module and of the power supply on the RMS value of the supply voltage.
140
CLA25
Ta = 21°C
120
100
T [°C]
TLED_MODULE
80
60
TPOWER_SUPPLY
40
20
0
0
50
100
150
200
250
VinRMS [V]
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As one can observe, for the disassembled lamp the temperature of its components is
even about 70oC higher than for the mounted lamp.
The element, which more intensively gets warmer, is the LED module.
The cooling system applied in the considered LED lamp allows reducing the
temperature of the LED module.
Thanks to this reduction of internal temperatures, we observe in Figure an increase
in the value of power density of the emitted light.
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Results of calculations
and measurements (7)
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In next Figures the selected results of measurements and calculation characteristics
of the LED lamp CLA60 are presented.
Figure illustrates the influence of the RMS value of the input voltage on the
waveforms of the input current.
150
CLA60
100
130V
230V
Iin [mA]
50
0
70V
-50
-100
-150
0
0,005
0,01
0,015
0,02
t [s]
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The shape of this waveform is nearer to sinusoidal waveforms than the waveforms
obtained for the lamp CLA25.
The values of THD are as follows: 52.3% for VinRMS = 70 V, 28.6% for VinRMS = 130 V
and 24.6% for VinRMS = 230 V.
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Results of calculations
and measurements (8)
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In Figure a) the dependence of power density of the emitted lighting on the RMS
value of the input voltage is shown, whereas in Figure b) the dependences of the
internal temperature of the LED module and of the power supply on the RMS value
of the input voltage are presented.
Dashed lines correspond to the disassembled lamp in which the LED module and the
power supply are not connected together, and solid lines – to the mounted lamp.
160
3,5
CLA60
120
2
1,5
Tpower_supply
40
0,5
20
0
0
50
100
150
VinRMS [V]
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80
60
1
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T LED_MODULE
100
T [°C]
Pe [W/m2]
2,5
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CLA60
Ta = 22°C
140
3
200
250
50
100
150
VinRMS [V]
200
250
After assembling components of the lamp the increased temperature of the power
supply and the lower temperature of the LED module occur.
As a result of a fall of temperature of the LED module, the power density of the
radiation emitted by the lamp increases.
Power density of a radiation is a growing function of the input voltage.
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Conclusions
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In the paper the manner of modelling LED lamps in SPICE was
presented.
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Electrothermal models of components of such a lamp, i.e. the power
supply and the LED module were proposed.
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These models have a simple form and they could be used both in dc
and transient analyses.
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In these models both self-heating phenomena and mutual thermal
couplings between the mentioned components of the lamp are
taken into account .
The correctness of the worked out model was verified
experimentally for two types of lamps and the good agreement
between the results of calculations and measurements was obtained.
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Conclusions (2)
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The presented results of investigations prove that the power
supplies used in the considered lamps are characterised by very
different values of the THD of the input current.
For both the considered lamps the value of THD is high and in order
to compensate their favourable influence on quality of the electrical
energy the power factor correction circuits (PFC) should be installed
at the input of these lamps.
The mutual thermal coupling between components of these lamps
makes it possible to reduce the value of the internal temperature of
the LED module and consequently to increase the value of the
emitted luminous flux.
The presented model can be useful for designers of lighting systems
and in teaching properties of solid state lighting sources.
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