Modelling LED Lamps with Thermal Phenomena Taken into Account Krzysztof Górecki and Przemysław Ptak Gdynia Maritime University Department of Marine Electronics

Outline Introduction Model form Results of calculations and measurements Conclusions 2

Introduction (1) Every LED lamp contains several components. The switch-mode power supply stabilizes the output current or the output voltage. 3

Introduction (2) 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. 4

Introduction (3) 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. 5

Model form The model of the LED lamp has the form of a subcircuit for SPICE. This model belongs to the group of compact electrothermal models. The electrical model of the LED module describes the dc characteristics of the circuit consisting of m diodes connected in series. This model contains the controlled current source G 1, the linear resistor R S and the controlled voltage source E RS. The current source G 1 describes the diffusive component of the current of diodes contracted in the module. The resistor R S0 represents a series resistance of all the diodes contracted in the LED module in the reference temperature T 0, while the voltage source E RS 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 E L, whose output voltage corresponds to power density of radiation emitted by this module. In the description of the source E L the influence of temperature and of the current is taken into account 6

Model form (2) In turn, the electrical model of the power supply contains the controlled current source G 2, the controlled voltage source E 3 and the resistor R S (for the power supply with the constant output voltage) or the controlled current source G 3 (for the power supply with the constant output current). The source G 2 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 E 3 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 R S 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 G 3 7

Model form (3) The thermal model of the LED lamp enables calculation of the value of the internal temperature of every diode in the LED module T j and the internal temperature of the power supply T j1. 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 - G pth1, power dissipated in the power supply - G pth2, the sum of power dissipated in the other diodes of the LED module - G pth3. 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. 8

Results of calculations and measurements 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. 9

Results of calculations and measurements (2) 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 electroenergy- network, 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. 10

Results of calculations and measurements (3) Figure presents the calculated and measured waveforms of the current feeding the lamp CLA25 calculated at different RMS values of the supply voltage. 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 I in shows that from the point of view of the electroenergy-network the considered system is seen as the rectifier with non-linear capacitive load. 11

Results of calculations and measurements (4) 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. 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 V in = 80 V, 83.96% for V in = 130 V and 69.2% for V in = 230 V. Such a high value of THD could lead to worsening of quality of the electrical energy in electro-energy network. 12

Results of calculations and measurements (5) 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. 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 P e (V inRMS ) is an increasing function. It is the result of the increasing output voltage of the power supply. 13

Results of calculations and measurements (6) 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. As one can observe, for the disassembled lamp the temperature of its components is even about 70 o C 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. 14

Results of calculations and measurements (7) 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. 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 V inRMS = 70 V, 28.6% for V inRMS = 130 V and 24.6% for V inRMS = 230 V. 15

Results of calculations and measurements (8) 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. 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. 16

Conclusions In the paper the manner of modelling LED lamps in SPICE was presented. Electrothermal models of components of such a lamp, i.e. the power supply and the LED module were proposed. These models have a simple form and they could be used both in dc and transient analyses. 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. 17

Conclusions (2) 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. 18