Datasheet
Figure 7. Model UVQ Heatsink Assembly Diagram
UVQ Series Aluminum Heatsink
The UVQ series converter baseplate can be attached either to an enclosure wall
or a heatsink to remove heat from internal power dissipation. The discus-
sion below concerns only the heatsink alternative. The UVQs are available
with a low-profile extruded aluminum heatsink kit, models HS-QB25-UVQ,
HS-QB50-UVQ, and HS-QB100-UVQ. This kit includes the heatsink, thermal
mounting pad, screws and mounting hardware. See the assembly diagram
below. Do not overtighten the screws in the tapped holes in the converter (3.5
n-m or 1.9 in-oz. max.). This kit adds excellent thermal performance without
sacrificing too much component height. See the Mechanical Outline Drawings
for assembled dimensions. If the thermal pad is firmly attached, no thermal
compound (“thermal grease”) is required.
Thermal Performance
The HS-QB25-UVQ heatsink has a thermal resistance of 12 °C/Watt of internal
heat dissipation with “natural convection” airflow (no fans or other mechanical
airflow) at sea level altitude. This thermal resistance assumes that the heatsink
is firmly attached using the supplied thermal pad and that there is no nearby
wall or enclosure surface to inhibit the airflow. The thermal pad adds a negli-
gible series resistance of approximately 0.5°C/Watt so that the total assembled
resistance is 12.5°C/Watt.
Be aware that we need to handle only the internal heat dissipation, not the
full power output of the converter. This internal heat dissipation is related to the
efficiency as follows:
Power Dissipation [Pd] = Power In – Power Out [1]
Power Out / Power In = Efficiency [in %] / 100 [2]
Power Dissipation [Pd] = Power In x (1 –Efficiency%/100) [3]
Power Dissipation [Pd] = Power Out x (1 / (Efficiency%/100) - 1) [4]
Efficiency of course varies with input voltage and the total output power.
Please refer to the Performance Curves.
Since many applications do include fans, here is an approximate equation to
calculate the net thermal resistance:
RQ [at airflow] = RQ [natural convection] / (1 + (Airflow in LFM) x
[Airflow Constant]) [5]
Where,
RQ [at airflow] is the net thermal resistance (in °C/W) with the amount of
airflow available and,
RQ [natural convection] is the still air total path thermal resistance or in this
case 12.5°C/Watt and,
“Airflow in LFM” is the net air movement flow rate immediately at the converter.
This equation simplifies an otherwise complex aerodynamic model but is a
useful starting point. The “Airflow Constant” is dependent on the fan and enclo-
sure geometry. For example, if 200 LFM of airflow reduces the effective natural
convection thermal resistance by one half, the airflow constant would be
0.005. There is no practical way to publish a “one size fits all” airflow constant
because of variations in airflow direction, heatsink orientation, adjacent walls,
enclosure geometry, etc. Each application must be determined empirically and
the equation is primarily a way to help understand the cooling arithmetic.
This equation basically says that small amounts of forced airflow are quite
effective removing the heat. But very high airflows give diminishing returns.
Conversely, no forced airflow causes considerable heat buildup. At zero airflow,
cooling occurs only because of natural convection over the heatsink. Natural
convection is often well below 50 LFM, not much of a breeze.
While these equations are useful as a conceptual aid, most users find it
very difficult to measure actual airflow rates at the converter. Even if you know
the velocity specifications of the fan, this does not usually relate directly to
the enclosure geometry. Be sure to use a considerable safety margin doing
thermal analysis. If in doubt, measure the actual heat sink temperature with
a calibrated thermocouple, RTD or thermistor. Safe operation should keep the
heat sink below 100°C.
Calculating Maximum Power Dissipation
To determine the maximum amount of internal power dissipation, find the
ambient temperature inside the enclosure and the airflow (in Linear Feet per
Minute – LFM) at the converter. Determine the expected heat dissipation using
When assembling these kits onto the converter, include ALL kit hardware to
assure adequate mechanical capture and proper clearances. Thread relief is
0.090" (2.3mm).
MDC_UVQ Models.F04 Page 23 of 25
UVQ Series
Low Profile, Isolated Quarter Brick
2.5–40 Amp DC-DC Converters
PAN HEAD SCREW
M3 X 7MM
4 PLCS
LOCK WASHER
M3
4 PLCS
FLAT WASHER
NO. 4
4 PLCS
HEATSINK
HEAT TRANSFER PAD
(Peel off white plastic
backing material before
attaching to heatsink)
www.murata-ps.com/suppor t