Datasheet

the Efficiency 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 airflow and/or
reduce the power output. Start with this equation:
Internal Heat Dissipation [Pd in Watts] = (Ts – Ta)/RQ [at airflow] [6]
where “Ta” is the enclosure ambient air temperature and,
where “Ts” is the heatsink temperature and,
where “RQ [at airflow]” is a specific heat transfer thermal resistance (in
degrees Celsius per Watt) for a particular heat sink at a set airflow rate. We
have already estimated RQ [at airflow] 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-pro-
ducing 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 RQ [at airflow]) + Ta [7]
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).
Heat Sink Example
Assume an efficiency 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. figure also allows some margin in case the ambient climbs
above +30°C or the input voltage varies, giving us less than 92% efficiency.
The heat sink and airflow combination must have the following characteristics:
8.7 W = (90-30) / RQ[airflow] or,
RQ[airflow] = 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 hypo-
thetical airflow constant of 0.005, we can rearrange equation [5] as follows:
(Required Airflow, LFM) x (Airflow Constant) = RQ[Nat.Convection] /
RQ[at airflow] –1
or, (Required Airflow, LFM) x (Airflow Constant) = 12.5/6.9 –1 = 0.81 and,
rearranging again,
(Required Airflow, LFM) = 0.81/0.005 = 162 LFM
162 LFM is the minumum airflow to keep the heatsink below +90°C.
Increase the airflow to several hundred LFM to reduce the heatsink tempera-
ture further and improve life and reliability.
MDC_UVQ Models.F04 Page 24 of 25
UVQ Series
Low Profile, Isolated Quarter Brick
2.5–40 Amp DC-DC Converters
0.10
(2.54)
*
* UVQ 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)
www.murata-ps.com/suppor t