Transcript Nieizotermiczne charakterystyki impulsowych układów zasilających

Modelling Power LEDs with
Thermal Phenomena Taken into
Account
Krzysztof Górecki, Przemysław Ptak
Gdynia Maritime University
Department of Marine Electronics
Outline
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Introduction
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The model form
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Results of calculations and measurements
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Conclusions
Introduction (1)
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Power LEDs are the most important part of
semiconductor lighting sources
Temperature strongly influences properties of these
diodes, particularly their lifetime.
The value of the internal temperature of the LED
depends on the ambient temperature and self-heating
phenomena in the diode.
In order to analyse properties of the system containing
LEDs before constructing such a system or optimising
its construction, the computer-aided analysis of
electronic circuits is used.
Very often such an analysis is realised with the use of
SPICE software.
Introduction (2)
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In such an analysis the electrothermal model of
the power LED is indispensable.
The electrothermal models of power LEDs do
not take into account the inertia of considered
devices.
In this paper, the new form of the power LED
electrothermal model dedicated for SPICE
software is proposed.
This model has the form of a subcircuit for
SPICE software and it takes into account
thermal inertia in the power LED.
The model form
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v Electrical model
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i
A
G1
vG
ERS
RS0
vRS0
K
Tj
Rthj-a1
E
Rthj-c
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TC
Cthj-a1
Cthj-c
GT
VTa
Thermal
model
EL
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Optical
model
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A group of compact electrothermal models.
Thermal, electrical and optical models.
The optical model makes it possible to
calculate the illuminance value at different
operating conditions of the considered LED
and at different cooling conditions of this
device.
The optical model contains the controlled
voltage source EL, representing illuminance
E of the lighted area.
The electrical model contains the controlled
current source G1, the resistor RS0
representing series resistance of the diode
and the controlled voltage source ERS
describing an additional voltage drop on
this resistance, resulting from the
temperature rise.
The source G1 models the diffusive
component of the diode current
The model form (2)
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In order to calculate the device internal temperature Tj, the compact
thermal model is used.
The network representation of the thermal model includes:
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the controlled current source GT representing the thermal power
pth dissipated in the considered LED,
 the RC elements represent transient thermal impedance between
the junction and the case.
 The voltage source VTa represents the ambient temperature.
Voltages on the terminals Tj and TC of the model correspond to the
junction temperature and the case temperature of the diode,
respectively.
The external thermal network consisting of resistors Rthj-ai and
capacitors Cthj-ai representing the heat flow between the case of the
investigated device and the surroundings is connected in-line to the
terminal TC.
The model form (3)
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The thermal power, represented by the controlled
current source GT, is equal to the difference between
the electrical power supplying the considered diode and
the power of the emitted light popt
In the model it is accepted that for every LED the
optical power popt is equal to the settled part of the
product of the current and the voltage, and the
proportion coefficient is marked with the symbol hp.
The power popt depends on the diode forward current.
This power is equal to zero for negative values of the
diode current, whereas for the positive diode current
the power popt depends on the power dissipated in the
diode and on its luminous efficiency.
Results of calculations and
measurements
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The usefulness of the presented model in the analysis of
electronic networks is verified for the power LED OF-HPW-5SL
by Optoflash.
The allowable value of the forward current of this diode is equal
to 1.2 A, the admissible power Ptot = 5 W, the maximum
junction temperature Tjmax = 135oC, and the emitted luminous
flux V = 175 lm.
The tested LED is situated in the case STAR.
The examined power LED is installed, in turn, on two different
heat-sinks of the dimensions:
 175x118x8 mm (the large heat-sink)
 100x75x2 mm (the small heat-sink)
 it operates without any heat-sink.
Results of calculations and
measurements (2)
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Using the authors’ methods which estimate parameters values of
the power LED model, values of electric and optical parameters of
the considered element are calculated.
Values of thermal parameters of the model were measured by
means of the authors’ method.
the dimensions of the heat-sink causes even a quadruple decrease
in the value of thermal resistance of the examined device (from
8.91 K/W to 37.4 K/W) and about a quadruple extension of the
longest thermal time constant (from over 480 s to 2060 s).
Results of calculations and
measurements (3)
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tested diode excited by the jump of the current of the value equal to 1.4 A.
6000
big heat-sink
5000
small heat-sink
E [lx]
4000
3000
LED without
any heat-sink
2000
1000
0
0
500
1000
1500
2000
2500
t [s]
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the result of a self-heating phenomenon is a raise in the value of illuminance.
The observed decrease in the value of this parameter is greater, when
cooling conditions of the examined device are worse:
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for the diode situated on the large heat-sink - 6%
for the diode situated on the small heat-sink - 16%,
for the diode operating without any heat-sink – up to 72%.
Results of calculations and
measurements (4)
200
180
LED without
any heat-sink
160
DTj [°C]
140
120
100
small heat-sink
80
60
40
big heat-sink
20
0
0,0001
0,001
0,01
0,1
1
10
100
1000
10000
t [s]
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The internal device temperature increases, when the cooling
conditions worsen.
The time indispensible to obtain the steady state in the diode
increases from 1500 s (for the diode operating without any heatsink) to 6000 s (for the diode situated on the big heat-sink).
Results of calculations and
measurements (5)
6000
big heat-sink
5000
E [lx]
4000
small heat-sink
3000
LED without
any heat-sink
2000
1000
0
0
200
400
600
800
1000
1200
1400
I [mA]
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only during the operation of the considered diode on the large
heat-sink (small value of thermal resistance), the dependence
E(ID) is a function monotonically increasing.
when the diode operates without any heat-sink, the considered
dependence possesses the maximum.
after exceeding a certain value of the forward current, in this case
0.8 A, an increase of the current causes a decrease of illuminance.
Results of calculations and
measurements (6)
200
180
160
TC, Tj [oC]
140
LED without
any heat-sink
120
100
80
small heat-sink
60
big heat-sink
40
20
0
0
200
400
600
800
1000
1200
1400
I [mA]
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The internal device temperature and the case temperature is
increasing function of the forward current.
An increase of the case temperature caused by self-heating
depends on the cooling conditions.
Conclusions
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The electrothermal model of the power LED taking into account its
electrical, optical and thermal proprieties is proposed.
This model takes into account thermal inertia in the considered
device and it can be used both in transient and dc analysis.
The presented model has a simple form, adequate for typical uses
of the modelled device.
The characteristics obtained by means of the proposed model
match well the results of measurements, both for waveforms of
illuminance and the device internal temperature, as well as for dc
optical, electrical and thermal characteristics of the power LED.
The differences between the obtained results of calculations and
measurement do not exceed several percent, which confirms the
correctness of the presented model.
Conclusions (2)
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The results of calculations and measurements
presented in the previous section prove that selfheating strongly influences the junction temperature of
tested device and the power of the emitted light.
From the presented results of calculations and
measurements it is visible that it is not a justifiable
operation of this device for the high current, because in
this range:
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the smaller value of illuminance can be obtained.
power consumption from the power source and the device
internal temperature increase.