Modelling Power LEDs with Thermal Phenomena Taken into Account

1. Modelling Power LEDs with Thermal Phenomena Taken into Account Krzysztof Górecki, Przemysław Ptak Gdynia Maritime University Department of Marine Electronics

2. Outline • Introduction • The model form • Results of calculations and measurements • Conclusions

3. Introduction (1) • 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.

4. Introduction (2) • 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.

5. The model form • 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

6. The model form (2) • In order to calculate the device internal temperature Tj, the compact thermal model is used. • The network representation of the thermal model includes: • 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.

7. The model form (3) • 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.

8. Results of calculations and measurements • 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.

9. Results of calculations and measurements (2) • 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).

10. Results of calculations and measurements (3) • tested diode excited by the jump of the current of the value equal to 1.4 A. • 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: • 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%.

11. Results of calculations and measurements (4) • 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 heat-sink) to 6000 s (for the diode situated on the big heat-sink).

12. Results of calculations and measurements (5) • 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.

13. Results of calculations and measurements (6) • 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.

14. Conclusions • 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.

15. Conclusions (2) • The results of calculations and measurements presented in the previous section prove that self-heating strongly influences the junction temperature of tested device and the power of the emitted light. • From the presentedresults 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: • the smaller value of illuminance can be obtained. • power consumption from the power source and the device internal temperature increase.