Digital Micromirror Device Thermal Considerations Including Pulsed Optical Sources
Abstract
Many applications for the DLP® Digital Micromirror Device (DMD) use Continuous Wave (CW) illumination. Under CW illumination conditions, the temperature rise of the DMD is straightforward to calculate, and equations needed are provided in the data sheet for each DMD. There are also applications where pulsed illumination is used. The temperature rise of the DMD is more complicated to calculate with pulsed illumination, but is important to understand to make sure the DMD micromirror temperature is kept within a reliable operating range. The time required for DMD mirror heating can be longer than the pulse duration of the optical source, creating the need for a transient thermal model instead of a steady-state thermal model to accurately calculate the thermal rise at the DMD mirror surface. This application note describes the equations necessary for calculating DMD mirror surface temperature rise and DMD mirror bulk temperature rise as a function of pulse duration and pulse rate of the optical source.
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1 Heating of a DMD Micromirror
When light from an optical illumination source is applied to the DMD, the majority of the light is incident on the mirror surface and is reflected. However, a small portion of that incident light is absorbed by the mirror material and must be dissipated through the DMD mirror structure into the DMD silicon, and then from the DMD silicon die to the backside of the DMD ceramic package. Another small portion of the incident light on the DMD falls within the gaps between mirrors directly on the DMD silicon along with any light that overfills the array and falls directly onto the silicon. This light is absorbed by the silicon as heat and combines with the heat generated by the electrical circuits on the silicon and the heat absorbed through the mirror surface to create an overall temperature rise at the DMD silicon. This heat is then dissipated through the DMD ceramic package. A model describing the path of DMD heat absorption and cooling is illustrated by the simple 1-D resistor network in Figure 1-1.
Figure 1-1 Simplified DMD Thermal
Resistor NetworkThis resistor network shows three temperature rises that must be calculated to determine the mirror surface temperature. TCERAMIC can be measured in a system using an attached thermocouple, but TSILICON, TBULK MIRROR, and TMIRROR SURFACE cannot be directly measured. Therefore, we need to calculate the following temperature delta's:
