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Texas Instruments DLP High Power NIR Thermal Design Application notes
DLP® Products Thermal Design Guide:
Focus on High Power NIR Laser Illumination
Application Report
Literature Number: DLPA104
December 2018
Contents
1
2
3
Trademarks ......................................................................................................................... 3
Introduction ......................................................................................................................... 4
Digital Micromirror Device (DMD) Temperature Requirements ................................................... 4
3.1
4
5
Critical Locations for DMD Temperature Measurements ........................................................... 4
3.1.1
Sample Calculation 1 - Uniform illumination of entire DMD active array .............................. 7
3.1.2
Sample Calculation 2 – Partial DMD active array illumination with non-uniform iIlumination
peak: ........................................................................................................... 7
Determining DMD Heat Loads ................................................................................................ 8
4.1
Absorbed thermal load on active micromirror array and pond of mirrors (POM) ................................ 8
4.2
Absorbed load on DMD dark metal area and window aperture .................................................... 8
4.3
Absorbed load in the window glass and forced air window cooling ............................................... 9
4.4
Dump light heat load and diffraction losses
..........................................................................
9
DMD thermal resistances ...................................................................................................... 9
5.1
Silicon-to-Ceramic Thermal Resistance ............................................................................... 9
5.2
Mirror-to-Silicon Thermal Resistance .................................................................................. 9
7
8
............................................................................................. 9
5.4
Thermal Interface Resistance ......................................................................................... 10
5.5
DMD Heatsink Resistance............................................................................................. 10
Other design considerations and precautions........................................................................ 12
6.1
Overfill .................................................................................................................... 12
6.2
Optimal wavelengths ................................................................................................... 12
6.3
Illumination uniformity .................................................................................................. 12
6.4
Pulsed lasers ............................................................................................................ 13
Summary ........................................................................................................................... 13
Related Documents ............................................................................................................ 13
2
Table of Contents
5.3
6
System Mounting Interface
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List of Figures
1
1
DMD thermal test points .................................................................................................... 5
2
Active Array, POM, Dark Metal, Window Aperture ...................................................................... 8
3
DMD Thermal-Resistance Network ...................................................................................... 10
4
Simplified DMD Thermal-Resistance Network
5
Heat pipe/heatsink and fan thermal resistance vs. heatsink volume ................................................ 11
6
8 x 8 x 6 cm (384 cm3) heatpipe heatsink with three 6 mm diameter copper water heatpipes and
aluminum fins with 80 x 25 mm fan, 0.35 ºC/W thermal resistance (not including thermal grease interface
material) ..................................................................................................................... 12
.........................................................................
10
Trademarks
DLP is a registered trademark of Texas Instruments.
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List of Figures
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3
Application Report
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DLP® High-Power NIR Thermal Design Guide
Scott Overmann
2
Introduction
DLP® technology is often used with high power illumination sources for high brightness displays as well as
various industrial exposure equipment such as lithography systems, 3D printers, and laser marking
machines to name a few. Managing the thermal specifications requires certain DMD temperature
calculations. At higher power levels, important cooling techniques need to be incorporated to best control
the DMD temperature to maximize performance over time. This application report explains key DMD
temperature calculations and cooling best practices when integrating a DMD into a light engine design.
The examples provided focus on high power NIR applications in the 950 – 1150 nm range with high
incident power and high peak incident irradiance.
3
Digital Micromirror Device (DMD) Temperature Requirements
DLP® DMDs have been shipping commercially for over 20 years. Over the decades, TI has completed
extensive amounts of DMD characterization predominantly for display use cases that span 0 – 70 °C array
temperature operation. As a result, DMD performance has been found to be extremely robust over this
relatively large operating temperature range. However, there is a relationship with extreme temperature
conditions that leads to degraded performance. As a result, temperature limits are imposed on critical
locations of the DMD for operating and storage conditions, which are specified in each product’s
datasheet.
3.1
Critical Locations for DMD Temperature Measurements
The critical locations that define the device temperature limits (see Figure 1) are the:
• array micromirrors (cannot be measured directly, must be calculated)
• window glass edges (TP2 – TP5 reference points)
• back of the package (TP1 reference point).
The temperature of the array micromirrors is controlled by conduction through the ceramic header and the
heatsink that contacts the ceramic directly beneath the array.
The window glass is cooled primarily through conduction through a glass interposer to the silicon die. The
glass window is attached to the glass interposer (spacer) to the silicon die with very thin epoxy bonds and
therefore must be kept below specified temperatures to ensure package integrity is maintained.
The window glass temperature rise is primarily due to bulk absorption of the window glass and any
absorbed heat load on the window aperture. Therefore, it is recommended that the illumination design be
optimized so that direct light loading on the window aperture is eliminated since this will significantly
impact window temperature.
For this application report we will use the DLP650LNIR DMD as the example for all subsequent
calculations. Refer to the appropriate DMD datasheet for parameters used in the calculations. Figure 1
shows an image of the DLP650LNIR DMD along with the five reference test point (TP) locations for
monitoring critical temperatures on the device.
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Array
TP2
2X 12.0
TP5
TP4
2X 16.7
TP3
Window Aperture
Window Edge
(4 surfaces)
TP4
TP3 (TP2)
TP5
TP1
4.5
16.1
TP1
Figure 1. DMD thermal test points
External DMD temperature test points (TP1 – TP5) can be measured by attaching thermocouples to the
DMD at locations shown in Figure 1. Recommended thermocouple wire types are T or K with fine gauge
(36 gauge) wire. Thermocouple beads and exposed wire should be attached with glue such that the bead
and exposed wire is in close contact with the surface being measured. Ultraviolet (UV) cure glue is an
effective way to attach thermocouples to the DMD.
Window thermocouples (TP2 –TP5) can be read directly and compared to the appropriate datasheet
specification.
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Digital Micromirror Device (DMD) Temperature Requirements
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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 1) 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)
QMIRROR = QINCIDENT_MIRROR × [FFOFF-STATE_MIRROR × (1 - MR)]
αDMD = [FFOFF-STATE_MIRROR × (1-MR)] + [1-FFOFF-STATE-MIRROR] + [2 × αWINDOW]
(1)
(2)
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 the data
sheet
RMIRROR-TO-SILICON = thermal resistance, individual mirror to silicon die (°C/Watt) as specified in the data
sheet
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.
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3.1.1
Digital Micromirror Device (DMD) Temperature Requirements
Sample Calculation 1 - Uniform illumination of entire DMD active array
This calculation assumes that the entire DMD active array (1280 × 800) mirrors is uniformly illuminated
with zero overfill falling outside the Pond of Mirrors pixel border. The highest DMD temperatures typically
occur when the DMD mirrors are in the off-state (–12° landed) position. Therefore, the off-state fill factor
calculates the worst case mirror temperature. Calculate the mirror temperatures to assess the viability of
the illumination conditions.
• FFOFF-STATE-MIRROR = 75.3%
• MR @ 1064 nm = 94%
• αWINDOW@ 1064 nm = 0.7%
• RMIRROR-TO-SILICON = 3.39E5 °C/Watt
• RSILICON-TO-CERAMIC = 0.5 °C/Watt
• Array Resolution = 1280 × 800
• TCERAMIC = 30.0°C (measured)
• QINCIDENT = 160 W (measured)
• QELECTRICAL = 1.8 W
αDMD = [0.753 × (1-0.94)] + (1 - 0.753) + (2 × 0.007) = 0.31
QSILICON = 1.8 W + (0.31 × 160 W) = 51.4 W
QMIRROR = [(160W / (1280 × 800)] × 0.753 × (1 - 0.94) = 7.06E-6 W
Delta_TSILICON-TO-CERAMIC = 51.4 W × 0.5°C/W= 25.7°C
Delta_TMIRROR-TO-SILICON = 7.06E-6 W × 3.39E5 °C/W= 2.4°C
TMIRROR = 30.0°C + 25.7 + 2.4°C = 58.1°C
3.1.2
Sample Calculation 2 – Partial DMD active array illumination with non-uniform iIlumination peak:
This calculation assumes that only a subsection of the DMD active array 960 × 475 pixels in size is (nonuniformly) illuminated. This calculation assumes the illuminated area is in the center of the DMD. Noncentered area can affect the value of RSILICON-TO-CERAMIC. If the application requires offsetting the illumination
on the DMD, contact TI for more information on how to assess RSILICON-TO-CERAMIC. As in Sample Calculation
1, the off-state fill factor can be used to assess the highest temperatures that can occur. Calculate the
mirror temperatures which occur at the highest illumination intensities to assess the viability of the
illumination conditions.
• FFOFF-STATE-MIRROR = 75.3%
• MR @ 1064 nm = 94%
• αWINDOW@ 1064 nm = 0.7%
• RMIRROR-TO-SILICON = 3.39E5 °C/Watt
• RSILICON-TO-CERAMIC = 0.9 °C/Watt (higher than previous example due to reduced illumination area)
• Pixel Size = 10.8 µm = 0.00108 cm (square)
• TCERAMIC = 30.0°C (measured)
• QINCIDENT = 60 W (measured)
• QELECTRICAL = 1.8 W
• Peak Irradiance = 500 W/cm2 (measured)
αDMD = [0.753 × (1 - 0.94)] + (1 - 0.753) + (2 × 0.007) = 0.31
QSILICON = 1.8 W + (0.31 × 60 W) =20.4 W
QINCIDENT_MIRROR = Peak Irradiance (W/cm2) × Pixel Area (cm2) = [500 W/cm2 × (0.00108 cm)2 ] = 5.832E-4 W
QMIRROR = 5.832E-4 W × 0.753 × (1 - 0.94) = 2.64E-5 W
Delta_TSILICON-TO-CERAMIC = 20.4 W × 0.9°C/W = 18.4°C
Delta_TMIRROR-TO-SILICON = 2.64E-5 W × 3.39E5°C/W= 8.9°C
TMIRROR = 30.0°C + 18.4°C + 8.9°C = 57.3°C
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Determining DMD Heat Loads
4
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Determining DMD Heat Loads
Properly assessing the heat load on the DMD is critical to determining device temperatures and meeting
thermal requirements. Knowing how the heat load is distributed on the package is necessary to accurately
assess critical temperatures. Optical modeling can be used to predict the illumination light distribution on
the active array and aids in computing the spatial distribution of the thermal load. Thermal load due to the
electrical dissipation of the silicon die can be obtained from the appropriate device datasheet.
4.1
Absorbed thermal load on active micromirror array and pond of mirrors (POM)
Light incident to the active micromirror array falls on both the micromirror surface and in the gaps between
the micromirrors. Light incident on micromirrors is absorbed at a rate of (1 – mirror reflectivity). The DMD
micromirror surface has reflectivity similar to that of polished bulk aluminum. Light falling on the gaps
between the mirrors lands on the top of the silicon die which is intentionally designed to absorb most of
the light. Fill factor determines the percentage of light on the micromirror surface versus the mirror gaps.
The fill factor value is a function of several variables including pixel pitch, mirror state (on or off),
illumination angle, and f-number (f/#). When designing a system, it is recommended to consider the worst
case or lowest pixel fill factor condition, which is mirrors in the off-state when viewed from the illumination
pupil.
The heat loads on the DMD silicon, mirror, and mirror gap are determined from the following:
QSILICON = QELECTRICAL + QMIRROR +QMIRROR_GAPS
QMIRROR = QINCIDENT × (off-state mirror fill factor × (1 – mirror reflectivity))
QMIRROR_GAPS = QINCIDENT × (1 – off-state mirror fill factor)
The DMD has a region of mirrors outside of the active array approximately 10 pixels wide that are only
addressable to the off-state. This region is called the Pond of Mirrors (POM) (see Figure 2). From a
thermal standpoint, POM mirrors can be treated like active array mirrors in the off-state.
Dark Metal
POM
Array
Window Aperture Edge
Window Aperture
Window Edge
Figure 2. Active Array, POM, Dark Metal, Window Aperture
4.2
Absorbed load on DMD dark metal area and window aperture
A dark metal area exists outside of the POM on a DMD (see Figure 2). The dark metal area is specifically
designed to be dark in visible applications and therefore has highly absorptive properties. It is
recommended that there be no illumination load on the dark metal area since the absorption will be near
100% and will cause significant heating to the DMD with no benefit to system optical power output. For
this reason, it is recommended to slightly under fill the active array so that significant illumination loads are
not incident on the dark metal area.
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The DMD also has a dark aperture on the inside of the window glass. This aperture material is also highly
absorptive and has a high thermal resistance path to the back of the DMD where a heatsink would be
attached. Similar to the dark metal area, it is critical that significant incident light does not fall on the
window aperture. If it does, window heating will occur, due to the intense light load and relatively poor
thermal path, without providing any benefit to system optical power output.
4.3
Absorbed load in the window glass and forced air window cooling
The DMD window has both reflective and absorptive losses that are wavelength dependent. In the case of
the DLP650LNIR at 1064 nm, the absorbed load in the window is nominally 0.7% per pass. This results in
window heating that is significant at higher incident power levels.
For higher incident power levels, forced air cooling of the window is required to reduce window
temperature and ensure reliable operation of the DLP650LNIR DMD. Forced air window cooling can be
accomplished with an air stream from a simple blower-type fan ducted over the front of the window, or
clean dry air that may already be available within the system. Since the window glass has relatively low
thermal conductivity, but significant surface area, a relatively low flowrate of air provides substantial
window cooling.
Please refer to the DLP650LNIR datasheet for incident power and irradiance levels requiring forced air
window cooling as well as specific airflow details.
4.4
Dump light heat load and diffraction losses
When the DMD mirrors are in the off-state, incident light not absorbed by the DMD will be directed out of
the DMD but not through the projection optics. A dump light absorber is normally used to capture this light.
Since the dump light absorber and DMD are usually in close proximity, the dump light absorber should be
cooled properly such that parasitic heat conduction from the absorber to the DMD is minimized. The dump
light absorber may benefit from liquid cooling or forced airflow from a blower or fan since the heat load on
this component will likely be substantial.
The DMD also has diffractive light losses that will not necessarily be directed to the dump light absorber. It
is important to consider the location and impact of these optical diffraction orders as their heat loads can
be concentrated and potentially cause thermal damage to other parts of the system.
5
DMD thermal resistances
5.1
Silicon-to-Ceramic Thermal Resistance
The silicon-to-ceramic thermal resistance depends on DMD geometry, array size, die attach material, and
ceramic header. It is also a function of the heat input area which results in the silicon-to-ceramic thermal
resistance increasing when a subset of the array mirrors are illuminated. Thermal resistance provided in
the DMD datasheet assumes uniform illumination across the entire active micromirror array surface.
5.2
Mirror-to-Silicon Thermal Resistance
The mirror-to-silicon thermal resistance is the thermal resistance from the micromirror surface to the top of
the silicon die. Primary cooling paths include the air gap between the mirror and top of silicon and the
metal conductive paths of the micromirrors. This resistance is dependent upon pixel size. DMD datasheets
provide mirror-to-silicon thermal resistance.
5.3
System Mounting Interface
The DMD package mounting interface to the system can have a significant effect on the overall cooling of
the device. The typical interface consists of an optical housing on the window side and an electrical socket
on the back side that connects to an electronics board. Depending on the temperature of these parts
relative to the DMD, they act as either a heat sink or a source. If the optical housing and electronics board
are cooler than the DMD, then these parts act as heat sinks and will provide some additional cooling
benefit beyond the dedicated heatsink. However, sometimes these parts are hotter than the DMD and
thermally isolating them from the DMD is preferred.
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DMD thermal resistances
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A detailed thermal-resistance network of the DMD cooling paths is shown in Figure 3. A simplified thermalresistance that is defined from the device ceramic test point (TP1) to the external room ambient is shown
in Figure 4 below. This simplified resistance is termed the DMD system resistance (Rsys) and includes all
heat-transfer paths to and from the device. The value of the system resistance (Rsys) is that it can be
measured easily and used to gauge the effectiveness of the DMD cooling design.
T optics
R optics_housing-to-DMD
R silicon-to-ceramic
R th_interface
Q silicon
R heatsink
T silicon
T htsk_stud
R local-to-room_air
T local_air
T room_air
T ceramic (TP1)
R ceramic-to-PCB
T PCB_board
Figure 3. DMD Thermal-Resistance Network
R silicon-to-ceramic
R
Q silicon
sys
T silicon
T ceramic (TP1)
R sys =
T ceramic -
T room_air
Q silicon
Figure 4. Simplified DMD Thermal-Resistance Network
5.4
Thermal Interface Resistance
The DMD has a dedicated thermal interface area. Contact to this area with a thermal stud should be
maximized to reduce interface resistance. A low thermal resistance interface between the DMD ceramic
and attached heatsink stud is critical since it is part of the overall system thermal resistance. Assuming
18.9 x 11.4 mm interface area and thermal grease with a 3 W/m-K thermal conductivity and thickness of
50 µm results in an interface thermal resistance of 0.08 ºC/W.
5.5
DMD Heatsink Resistance
Most applications requiring high power illumination will require a high performance heat sink solution to
meet the temperature specification of the DMD. Typical high performance heatsink options include:
• liquid-cooled loops with a cooling stud
• heat pipe assisted heatsinks with an attached fan
In both cases, the liquid loop or the heatpipe only serves to transfer heat from one location to another.
Ultimately finned or radiator surface areas are required to reduce heatsink thermal resistance.
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DMD thermal resistances
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Returning to the DLP650LNIR example of 160W incident power, the calculated total DMD load was
51.4W. Temperature rise through the DMD package was 25.7ºC and additional temperature rise from
mirror-to-silicon was 2.4ºC. To maintain an array temperature of 70ºC, the back of the package must be
kept at 41.9ºC. Assuming a 20ºC room ambient, this requires a system heatsink resistance of:
R_sys = (41.9ºC – 20ºC) / 51.4 W = 0.43 ºC/W
This resistance includes the thermal interface to the DMD and also any preheating of air that may occur
from room ambient temperature to inside a product. However, if a high performance thermal interface
material such as a good thermal grease is used, and fresh air is brought in directly from outside the
product, then this resistance could be achieved either with a heat pipe heatsink assembly with a fan or
with a liquid cooling system.
Figure 5 provides an estimate of heatsink size versus thermal resistance. A 0.35 ºC/W heatsink with an
additional 0.08 ºC/W thermal grease interface resistance would achieve the required 0.43 ºC/W in the
equation above. It would have a finned volume of approximately 400 cm3. An example of a heatsink with
this resistance is shown in Figure 6.
Passive liquid cooling systems can achieve low heatsink thermal resistance. Active liquid cooling systems
have the potential to maintain the heatsink stud at room temperature resulting in a 0 ºC/W heatsink
thermal resistance. Care should be taken if heatsink temperatures below ambient air temperature are
used because there is risk of condensation if any part of the system is cooler than the dew point of the
surrounding air.
1.2
Thermal Resistance (ºC/W)
1
model results
curve fit
0.8
0.6
0.4
0.2
0
0
100
200
Heatsink Volume
300
400
500
(cm 3 )
Figure 5. Heat pipe/heatsink and fan thermal resistance vs. heatsink volume
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Other design considerations and precautions
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Figure 6. 8 x 8 x 6 cm (384 cm3) heatpipe heatsink with three 6 mm diameter copper water heatpipes and
aluminum fins with 80 x 25 mm fan, 0.35 ºC/W thermal resistance (not including thermal grease interface
material)
6
Other design considerations and precautions
6.1
Overfill
Controlling the illumination bundle close to the active array size is important to achieving acceptable DMD
temperatures. In the case of DLP650LNIR, if optical overfill extends past the 10 pixel POM significant
heating will occur since the region outside of the POM is highly absorptive. This will contribute to overall
DMD heat load as mentioned earlier. If the illumination bundle is significantly smaller than the 1280 x 800
active array, then the package thermal resistance, Rsilicon-to-ceramic will increase. It is therefore
recommended that the 1280 x 800 pixel array be slightly optically under filled to achieve the best thermal
performance.
As mentioned earlier, any significant illumination incident on the window aperture can cause substantial
window heating. Overfill that extends to the window aperture should be avoided. Stray light that is not part
of the primary illumination bundle should also be considered to ensure that very little light is incident on
the window aperture.
6.2
Optimal wavelengths
The window transmittance of each DMD depends on the window properties as well as the illumination
wavelength being used in a system design. The application note Wavelength Transmittance
Considerations for DLP® DMD Window is a good reference for determining DMD window efficiency values
for a given wavelength.
The DLP650LNIR DMD is optimized for operation between 950 and 1150 nm. The datasheet explains it
has operating capability beyond this range but at lower power levels. Designers must factor in that lower
window efficiency in these extended ranges leads to increased heat absorption levels. Care should be
taken to ensure all temperature specifications and performance goals are met depending on the desired
operating wavelength.
6.3
Illumination uniformity
From a thermal design perspective, the ideal illumination profile across the DMD would be spatially
uniform and resemble a “top hat” profile. For applications that have non-uniform illumination, the mirror-tosilicon temperature rise must be calculated based on the highest irradiance or peak intensity. This is
because DMD micromirrors are small and the temperature rise of each mirror above the silicon
temperature is dependent upon the absorbed heat load of that mirror.
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6.4
Pulsed lasers
The thermal time constant of a 10.8 µm DMD micromirror is very short, on the order of ~30 µs. Therefore,
pulsed lasers with low average power but very high peak power can possibly damage the micromirrors
since the thermal mass of an individual mirror is low and heating will occur rapidly.
7
Summary
Several thermal considerations must be managed when designing DLP systems using high power
illumination. These include determining heat loads, controlling illumination to minimize light outside the
active array, minimizing heatsink and thermal interface resistances, and measuring critical temperatures in
the system. Equations are provided for designers to perform thermal calculations to ensure the DMD is
kept within specified temperature ranges. An example is shown demonstrating that cooling of a 160W
incident power system is achievable while maintaining DMD temperature specifications.
8
Related Documents
•
•
•
•
DLPS136 DLP650LNIR Data Sheet
DLPA015 DLP® Series-450 DMD and System Mounting Concepts
DLPA067 Mounting Hardware and Quick Reference Guide for DLP® Advanced Light Control Digital
Micromirror Devices
DLPA031 Wavelength Transmittance Considerations for DLP® DMD Window
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