Keep Your Device Cool with a Heat Sink
The hot device should be kept cool with heat sink to enhance its reliability. The total heat load, Q, on the hot side is the sum of the active heat loads. For DC-DC converter the heat load is the wattage difference between the input and output. Proper heat sink is needed for the hot side.
Keeping your device as low as practical is a good practice.
· D T = Q x Q Temperature rise is the heat load, Q, times the thermal resistance, Q .
Normally used for low-power applications, its thermal resistance, Q , 2.0 oC /watt for dimensions of 5 "x3 "x l"
A natural convection heat sink should be positioned so that:
(1). heat can be dissipate upward through the fins,
(2). no significant obstructions to impede air flow.
Any other heat-generating components near by the heat sink would increase the ambient air temperature.
A black anodized finish on aluminum heat sink can lower the Q by 4 %.
The Q is proportional to inverse of total surface area. It is desirable to use a heat sink as large as practical
without blocking the air flow. However, too reduce the Q by one-half, the extruded heat sink volume has to be
increased by a factor of four. The structure member of an enclosure or mounting frame is often used to remove heat from a device. This technique is effective for small amount of heat.
Substantial cooling power can be achieved by using forced convection. The Q can be reduced to below
0.2 oC/W.
Three general ways of applications:
(1). Mount the fan or blower to one side of the heat sink and force air through the heat sink parallel to the
extrusion direction,
(2). Mount the fan or blower near the center of heat sink on top of the fin and force the air downward to the
base and through two open sides,
(3). Same as (2). except force the air upward from the base and through two open sides.
These methods should be carefully assessed from the complete package point of view to reach a best solution for a particular application.
A black anodize finish does not improve much for the force convection heat sink.
Linear Feet per Minute (LFM) equals to the Cubic Feet per Minute (CFM) of the fan or blower divided by the orifice area in square feet.
The
fan, a moving device, has a much lower MTBF than the TEC. Easy maintenance
to the fan should be considered in designing the cooling assembly.
How to estimate the Q of a heat sink?
(1). Find the heat sink's Length (in inch) & surface area (S in inch2 per inch).
(2). Compute the figure of merit (F). F = 0.008 x square root of L.
(3). Q (oC/Watt) = 1 I ( S x F).
Example: L = 5 in, S = 20 in2 per in, then F = 0.008 x ~5 =0.0178, Q = 2.8.
If forced air cooling is used, find the fan's CFM Calculate the LFM and tweak the figure of merit,
F = 0.011 x square root of (L X LFM / 1O0)
Example: CFM =400, orifice=2 ft2, LFM= 400/2,
F = 0.011 x square root of (5 x 200 /100) = 0.0347, 1.4. Then, AT is only half of that without a fan.
The "Stacked fin" heat sink is better than extrusion in:
(1). Lower thermal resistance 30%
(2). Lower weight 30%
(3). Smaller size and light weight.
Ambient Temperature
Temperature of the air near the device under consideration, do not confuse with the room temperature. In most cases it is much hotter than room temperature, especially if the device is inside a enclosure without adequate air circulation.
Also called TE cooler or Peltier cooler or electronic heat pump, is a semiconductor device and functions like a heat pump.
By applying a DC current to the TEC, heat will be moved through the module from one side to the other. One module face will be cooled while the other side is heated.
Seebeck Effect:
An electric current would flow continuously in a closed circuit if heat is applied to the thermocouple junction. This principle was discovered by Thomas Seebeck, a German scientist.
Peltier Effect
If a voltage is applied to the semiconductor junction an electrical current will flow in the circuit. As a result of the current flow, a cooling effect will occur at one junction and a heating effect will occur at the other junction.
The cooling or heating is proportional to the magnitude of current. This effect is reversible. If the direction of current is changed, the original cooling side will become heating side and heating side becomes cooling side. Joule heating (1X R), R is the electrical resistance also occurs in the semiconductors as a result of current flow.
ThermoElectric Material
· Made of Bismuth Telluride Semiconductors
· Couple - pair of N & P bars connected in series
· Bar areas are from 0.2mm2 to 25mm2
· Pumping capacity is about 40 mW/mm2
· Roughly one amp/mm2 produces 30 oC temperature difference Technology up to 254 couples
Mean Time Before Failure (MTBF)
TEC, a solid state device, is highly reliable with MTBF 200K to 300K hours at room temperature.
· At 800 C, MTBF are estimated about 100K hours. Yet, actual field results are 2 to 3 times higher.
Failure return rate is less than 0.1% with over 90% of these returns are due to improper uses such as
too much mechanical force or overheating.
· Less than 0.01% is due to the production defect.
Mechanical Mounting: Uneven compression forces induced by improper torque, bolting patterns, and uneven surface of heat sink should be avoided.
· Recommended compression is 150 pounds/sq. inch. TEC is relatively weak in the shear direction.
· Avoid any shear force due to uneven torque. Shock and Vibration: TEC has been used for military
applications. However, it can handle sever shock and vibration in compression mode only.
Moisture: Prevent moisture from penetrating into the TEC. The presence of moisture will cause corrosion and degrade the TEC, conductors and solders. Moisture can also cause electrical and thermal shorts between the hot and cold sides.
· Sealing or dry atmosphere should be provided.
Overheating: Above 800 C, copper diffuses into the TEC. At 100-1 100C it could result in about 25% loss of device performance within 100 hours.
· Also above 850 C, a solid state reaction occurs between TEC and bismuth4in solder. In extended time
this may result in failures of the interface.
· For high temperature
applications be sure to use ACK's industrial & military grades.
Installation
There are three methods: adhesion, compression, or soldering.
For smaller area (<1 9mm) adhesion or soldering works fine since the thermal stress due to expansion mismatches of solder or epoxy and the TEC ceramic plate is not much.
· For area larger than 1 9mm, the compression with thermal grease is recommended. Thermal grease can
provide a more flexible interface to relieve the stress.
Preparing~ Surfaces The surface should be flat of less than 0.08mm over the TEC mounting area. The surface should be clean and free from oil, nicks and burrs. For multiple-stage application, the TEC thickness should vary no more than 0.05mm
Typical TEC Applications for cooling:
· Infrared Detectors e Charge Couple Devices
· Liquid Exchangers · Laser Diodes
·
Air to Air Exchanger. Black Body References
(Return to Top)
ThermoElectric Cooler (TEC) Selection
· Use the ACK rule of thumb on the inside of back cover for a quick selection and the following detail guides to verify the selection.
Active Heat Load --- Q = V x I. The heat load is the voltage, applied to the device, times the current through the device. Typical Intel P4, 3GHz CPU generates about 90W in active mode. A photo diode with bias of 50V and resistance of 0.5 mega-ohms generates only 0.005 watts.
Parasitic Heat Loads --- Heat loads due to:
(1) Convection:
When the air (Tair) flows over an object of different temperature (Tc), heat transfer takes place. Convective heat load on TEC can be result of natural convection or induced by a fan.
This loading, the most significant loss, is a function of
the exposed area and the difference in temperature (Tair -Tc). It is computed:
Q = h A (Tair - Tc) , where h is heat transfer coefficient
(w/m2 oC) 21.7 for a flat, horizontal plate in air at 1
ATM, A is the exposed area in m2
Example: A 0.01 m2plate (0.006m thick) is cooled from
25 oC to 5 oC. The heat load is about 5.4 watts.
· It is important to avoid condensation when cooling below the dew point. This can be achieved by enclosing the cooling system in a dry gas or a vacuum environment.
(2) Conduction
Conductive loads occur through lead wires, mounting screws, etc., which form a thermal path from the device being cooled to the heat sink or ambient environment. The loading can be expressed as:
Q=k A DT I L, where k is the thermal conductivity of the material (w/m oC), A is the cross section (m2), L is the heat path length (m), and AT is the temperature difference (oC). The thermal conductivity for a few commonly used materials. Unit: w/m oC AL(1100) 238, AL(6035), 205 CU 386, Epoxy 0.8, Grease 0.87, Air 0.026.
(3) Radiation
Heat load through the electromagnetic radiation between two objects of different temperatures. This load is not significant when the system is operated in a gaseous environment.
Coefficient of Performance (C.O.P. ) defined as the heat removed at the cold junction, divided by the input electrical power for the TEC.
· Operating at higher C.O.P. has the advantages of using less input power and therefore generating less heat on the hot side. In turn a smaller heat sink is required. To achieve higher C.O.P. it requires larger or additional TECs. This should be examined closely to reach a optimal design in terms of overall cost and reliability.
· Keep TEC hot side temperature as low as possible with proper heat sink.
Power Supply - Keep AC ripple <20%
TEC is a low impedance device. After a TEC has been selected, the required current and voltage can be used to determine the wattage of power supply needed. Power supply ripple filtering is not critical for TEC than for other typical electronic applications.
· TEC performance, AT, will degrade by 5% for a 20% AC ripple.
From the 115 VAC source, a simple full-wave bridge rectifier and a filter capacitor ( 4000 mf) can be used. A switching power supply, with small size and light weight advantage over a linear unit, should also be considered.
Temperature Controller --- Determine temperature accuracy needed, then use one of the followings:
Open Loop (f 10C) - Simple temperature control can be achieved by a variable DC power supply if the thermal load is relatively constant.
· By manually adjusting the DC power supply output a temperature of 11 oC can be maintained for over several hours.
Closed Loop ( +/- 0. 1oC) --- Feedback system is needed for 1 0. 1oC accuracy or it the heat load fluctuates. Special PID control loop is used to maintain 1 0.0 10C. TEC exhibits about 0.5% per oC increase in its electrical resistance.
Installation Instruction
Cold Station
1. Connect
the Red terminal to +Black terminal to -.
DC Voltage ONLY
for model CS 6 from 4.5
to 5.5 Volt ( not to exceed 5.5 V),
for model CS 10 and 12 from 8 to 12 Volt (
not to exceed 12V),
2.
Check that the two flat head screws on cold plate ( holding TE to heat sink)
is hand tight.
Never apply too much
force ! It may break the TEC.
3.
Make sure: a) fan rotating properly. b) no blockage of
air flow. The water condensation on Cold
Plate shall appear in a few
minutes.
*
The heat sink may get warm, but never too hot to touch.
4.
If a sample holder is placed on top of the cold plate, apply a thin layer of
the thermal grease (included) to
the interface. For better cooling, use
brackets, 4-40 screws (not included) and tapped holes to press
down the
sample holder for good contact.
SuperCooler
A. Turn off
the Computer and open the computer case.
B. Mounting
Method: Clamp or clip
-
Apply a very thin layer of thermal compound on the TEC surface.
-
Position the package on top of the CPU ( press, slide left & right for good TEC & CPU contact),
-
Use the clamp to tighten the SuperCooler to the CPU.
C. Connect the SuperCooler to DC power.
Cautions:
Max. Voltage for Fan is 12 VDC.
Max. Voltage for the TEC is ~ 10 VDC, nominal 5 V for CPU
applications.
( use voltage > 5 V, the hot side - heat sink may
overheat and damage the TEC. )
Super Cooler - Box
On your box it Requires:
-
A cut-out of ~ 65 mm x 65. For cooling module.
-
Two holes ( spaced x distance apart across the square cut out, make sure it matches the spacing of two holes on the large heat sink) to allow at least #4 screw to go thru). For fastening the large heat sink & fan package
-
One hole ( 0.25 dia.) ~ 1 away from the edge of cut out. For wires. ( a proper grommet is recommended.)
Installation
-
Put the small fan package thru the cut- out on the box .
-
Align the two holes on the box with the two hole on the large heat sink .
-
Use two # 4 screws and nuts to fasten the heat sink on to the box, put adequate insulating material
between the two heat sinks. -
Pull wires ( inside box) thru the wire-hole to the out side .
-
Screw back the larger fan on to the heat sink, make sure the fan wire is close to the heat sink (the air should blow down to heat sink).
Both fans require 12VDC.
Heat pump (thermoelectric module) can operate from ~5VDC to ~ 12VDC. You may adjust it to achieve optimal cooling for your applications.
Buena Park, CA 90621 USA
Tel: (714) 739
- 5797
Fax:(714) 739 - 5898
Sales@ACKTechnology.com