Datasheet

www.murata-ps.com/support
UQQ Series
Wide Input Range Single Output DC-DC Converters
MDC_UQQ.D05 Page 17 of 18
Calculating Maximum Power Dissipation
To determine the maximum amount of internal power dissipation, fi nd the
ambient temperature inside the enclosure and the airfl ow (in Linear Feet per
Minute – LFM) at the converter. Determine the expected heat dissipation using
the Effi ciency curves and the converter Input Voltage. You should also compen-
sate for lower atmospheric pressure if your application altitude is considerably
above sea level.
The general proceedure is to compute the expected temperature rise of the
heatsink. If the heatsink exceeds +100°C. either increase the airfl ow and/or
reduce the power output. Start with this equation:
Internal Heat Dissipation [Pd in Watts] = (Ts – Ta)/R [at airfl ow] [6]
where “Ta” is the enclosure ambient air temperature and,
where “Ts” is the heatsink temperature and,
where “R [at airfl ow]” is a specifi c heat transfer thermal resistance (in
degrees Celsius per Watt) for a particular heat sink at a set airfl ow rate. We
have already estimated R [at airfl ow] in the equations above.
Note particularly that Ta is the air temperature inside the enclosure at the
heatsink, not the outside air temperature. Most enclosures have higher
internal temperatures, especially if the converter is “downwind” from other
heat-producing circuits. Note also that this “Pd” term is only the internal heat
dissipated inside the converter and not the total power output of the converter.
We can rearrange this equation to give an estimated temperature rise of the
heatsink as follows:
Ts = (Pd x R [at airfl ow]) + Ta [7]
These model numbers are correct for the UQQ series.
Heat Sink Example
Assume an effi ciency of 92% and power output of 100 Watts. Using equation
[4], Pd is about 8.7 Watts at an input voltage of 48 Volts. Using +30°C ambient
temperature inside the enclosure, we wish to limit the heat sink temperature to
+90°C maximum baseplate temperature to stay well away from thermal shut-
down. The +90°C. fi gure also allows some margin in case the ambient climbs
above +30°C or the input voltage varies, giving us less than 92% effi ciency.
The heat sink and airfl ow combination must have the following characteristics:
8.7 W = (90-30) / R[airfl ow] or,
R[airfl ow] = 60/8.7 = 6.9°C/W
Since the ambient thermal resistance of the heatsink and pad is 12.5°C/W, we
need additional forced cooling to get us down to 6.9°C/W. Using a hypothetical
airfl ow constant of 0.005, we can rearrange equation [5] as follows:
(Required Airfl ow, LFM) x (Airfl ow Constant) = R[Nat.Convection] /
R[at airfl ow] –1, or,
(Required Airfl ow, LFM) x (Airfl ow Constant) = 12.5/6.9 –1 = 0.81
and, rearranging again,
(Required Airfl ow, LFM) = 0.81/0.005 = 162 LFM
162 LFM is the minumum airfl ow to keep the heatsink below +90°C. Increase
the airfl ow to several hundred LFM to reduce the heatsink temperature further
and improve life and reliability.
Heatsink Kit *
Model Number
Still Air (Natural convection)
thermal resistance
Heatsink height
(see drawing)
HS-QB25-UVQ 12°C/Watt 0.25" (6.35mm)
HS-QB50-UVQ 10.6°C/Watt 0.50" (12.7mm)
HS-QB100-UVQ 8°C/Watt 1.00" (25.4mm)
* Kit includes heatsink, thermal pad and mounting hardware. These are
non-RoHS models. For RoHS-6 versions, add “-C” to the model number
(e.g., HS-QB25-UVQ-C).
0.10
(2.54)
*
* UQQ SERIES HEATSINKS ARE AVAILABLE IN 3 HEIGHTS:
0.25 (6.35), 0.50 (12.70) AND 1.00 (25.4)
1.45
(36.83)
2.28
(57.91)
MATERIAL: BLACK ANODIZED ALUMINUM
1.03
(26.16)
1.860
(47.24)
0.140 DIA. (3.56) (4 PLACES)
Dimensions in inches (mm)
Figure 8. Optional Heatsink