7.7 Micromirror Temperature Calculations
The DMD Micromirror temperature cannot be measured directly, therefore it must be computed analytically from:
- the measurement point on the outside of the package
- the silicon-to-ceramic thermal resistance
- the mirror-to-silicon thermal resistance
- the internally generated electrical power
- and the illumination heat load
The relationship between mirror temperature and the reference ceramic temperature (thermal test TP1 in
Figure 20) is provided by the following equations:
TMIRROR = TCERAMIC + Delta_TSILICON-TO-CERAMIC + Delta_TMIRROR-TO-SILICON
Delta_TSILICON-TO-CERAMIC = QSILICON × RSILICON-TO-CERAMIC
Delta_TMIRROR-TO-SILICON = QMIRROR × RMIRROR-TO-SILICON
QSILICON = QELECTRICAL + (αDMD × QINCIDENT)
Equation 1. QMIRROR = QINCIDENT_MIRROR × [FFOFF-STATE_MIRROR × (1 - MR)]
Equation 2. αDMD = [FFOFF-STATE_MIRROR × (1-MR)] + [1-FFOFF-STATE-MIRROR] + [2 × αWINDOW]
where:
- TMIRROR = computed micromirror temperature (°C)
- TCERAMIC = measured ceramic temperature (°C) (TP1 location)
- Delta_TSILICON-TO-CERAMIC = temperature rise of silicon above ceramic test point TP1
- Delta_TMIRROR-TO-SILICON = temperature rise of an individual mirror above the silicon (°C)
- RSILICON-TO-CERAMIC = thermal resistance, silicon die to ceramic TP1 (°C/Watt) as specified in Thermal Information
- RMIRROR-TO-SILICON = thermal resistance, individual mirror to silicon die (°C/Watt) as specified in Thermal Information
- QSILICON = total DMD power (electrical + absorbed) on the silicon (Watts)
- QMIRROR = absorbed heat load on a single mirror (Watts)
- QELECTRICAL = nominal electrical power (Watts)
- QINCIDENT = total incident optical power to DMD (Watts)
- QINCIDENT_MIRROR = Incident optical power on an individual mirror (Watts)
- αDMD = absorptivity of DMD
- αWINDOW = absorptivity of DMD window (single pass)
- FFOFF-STATE-MIRROR = DMD off-state mirror fill factor
- MR = DMD mirror reflectivity
The electrical power dissipation of the DMD is variable and depends on the voltages, data rates and operating frequencies. The absorbed power from the illumination source is variable and depends on the operating state of the micromirrors and the intensity of the light source. The equations shown above are valid for a system operating at 1064 nm with 100% of the illumination contained within the 1280 × 800 active array mirrors. Silicon-to-ceramic thermal resistance assumes the entire active micromirror array is uniformly illuminated.
NOTE
Incident irradiation that concentrates on a subset of the micromirror array, results in an increase in effective package thermal resistance.