OLED模块的电气和热分析

This paper presents the electrical and  thermal analysis of an OLED module. The OLED  module consists of the OLED element and the  DC-DC converter incorporated in its backplane. The DC-DC converter is realized as an integrated  circuit and uses an external inductor. The inductor  is embedded in the substrate of the backplane and  has a spiral geometry. The DC-DC converter is  highly efficient, but a fraction of the electrical  power is dissipated in the integrated circuit and  the planar inductor. This thermal dissipation has  a negative consequence on the OLED  characteristics. Additionally the planar inductor  represents a loop antenna that radiates  electromagnetic energy. The radiated disturbance  of the planar inductor must conform to the EMC  standards. The COMSOL Multiphysics is used to  investigate both the electromagnetic and thermal  performance of the OLED module.

The electronics on flexible foils is an area of  intensive research and development. The  electronics on flexible foils include the areas of  wearable electronics, lightning devices and  displays. Such electronics applications may  require a power source embedded in the flexible  foil and DC-DC converters are regularly used for  this purpose because of their high efficiency.

The OLED module analysed in this paper is  powered by a DC-DC converter consisting of an  integrated circuit and a planar spiral inductor. The integrated circuit and the planar inductor are  embedded in the PET foil and form the backplane  of the OLED module. The backplane is connected  to the OLED tile via an adhesive layer. Figure 1 shows the general structure of an OLED module.

The DC-DC converter of the OLED module  places a number of requirements on the electrical  characteristics of the inductor. The series  inductance needs to be in a certain range for the  converter to operate properly. The series  resistance and the port capacitances need to be  low enough to reduce the power dissipation. The  resonant frequency needs to be significantly  higher than the operating frequency of the  converter f = 10 MHz.

The OLED is made of the organic material  that is extremely sensitive to thermal influences. The critical parameters that need to be  fulfilled for normal operation of the OLED  element is the operating temperature and the  maximum temperature difference across the  OLED element. Exceeding these requirements  contributes to accelerated aging of the OLED  element and its uneven illumination.

Fig1

The RF Module is used in simulations to  evaluate the electrical characteristics of the planar  inductor and then to evaluate the far-field  radiation and the heat dissipation due to the  resistive losses. The planar inductor is a 2 port  structure and it is defined by two Lumped Ports.  The electrical characteristics are evaluated based  on S-parameters. The RF Module is in this case  used to make the port sweep and the Lumped  Ports are set to Cable terminal type. When  evaluating the far-field radiation and the inductor  heating, the Lumped Ports are set to the Current  Terminal port.

The thermal modelling is based on the heating  produced by the planar inductor and the DC-DC  converter integrated circuit. The planar inductor  is defined as a Boundary Heat Source and it is  coupled with the RF Module physics via the  emw.Qsrh variable. The integrated circuit is as  well defined as a Boundary Heat Source over a  2x2 mm² boundary area. Its total boundary power  is set to the approximated thermal dissipation of  the integrated circuit. The heat spreader is defined  as a Highly Conductive Layer thus avoiding the  meshing of very thin films similarly as in the RF  Module. The heat spreader is defined in place of  the OLED cathode.

The model executes two separate studies. The  first study sweeps the ports over a range of  frequencies and thus produces the S-parameters  of the planar inductor. The second study consists  of two steps; the first step solves the  Electromagnetic Waves physics in the frequency  domain for a single frequency and the second step  solves the Heat transfer physics in the stationary  state. The second study sweeps the heat spreader  thickness in a logarithmic manner.