Except in the simplest situations, TEC selection is just one piece of a larger process required to design an efficient and reliable thermoelectric cooling system. Besides the basic steps of defining the heat load, required DT and physical footprint desired/available and selecting a TEC model accordingly, there are four other major parts of the process.
These four steps are heat sink selection, mounting method, sealing method and power supply design/selection.
Each of these four major steps entails making basic choices based upon many considerations, some of which are interdependent. For instance, there are five basic types of heat sinks, and in specifying a forced air or liquid-cooled heat sink, II-VI Marlow’s system experts consider approximately 15 factors.
The consequence is that system design is complex, and each aspect of the problem demands attention to avoid field failures.
A. Heat Sinks
Design or selection of the heat sink is crucial to the overall thermoelectric system operation and cooler selection. All thermoelectric coolers require a heat sink and will be destroyed if operated without one.
HSR = (T1 -T2)/Q
HSR = Thermal resistance (°C/W)
T1 = Heat sink temperature (°C)
T2 = Ambient or coolant input temperature (°C)
Q = Heat load into heat sink (W) (includes absorbed + TEC power)
The goal of the heat sink design is to minimize thermal resistance. This is achieved through exposed surface area and may require forced air or liquid circulation.
The following schematic shows how the heat sink resistance can be determined. Ambient temperature is 27°C, the desired rise across the heat sink is 10°C, or heat sink temperature at 37°C. The load that must be dissipated is 10 W.
This gives a resistance of 10°C / 10 W or 1°C/W.
The three most common types of heat sinks are natural convective, forced convective, and liquid cooled, with liquid cooled being the most effective. Typical values of HSR for natural convective range from 0.5°C/W to 5°C/W, forced convective from 0.02°C/W to 0.5°C/W, and liquid cooled from 0.005°C/W to 0.15°C/W.
In general, most applications involving thermoelectric cooling require forced convective or liquid cooled heat sinks.
B. Power Supplies
Thermoelectric coolers operate from a DC power input. These DC power supplies can range from simple batteries to sophisticated closed loop temperature control/power supply circuits. Both linear and switching power supplies can be used to operate TECs.
1. Pulse Width Modulation
Pulse width modulation (PWM) is a power conversion technique that converts the AC line voltage to a lower voltage DC signal. Pulse width modulation essentially controls the duty cycle as well as the frequency of the power applied to the TEC.
To prevent thermal cycling, most PWM circuits filter the DC output to provide a "smooth" DC component to the TEC. The frequency of the PWM can be as low as possible to maintain a continuous, smooth output voltage.
2. On/Off Control
On/Off, or thermostatic control, is the simplest and most crude technique for controlling the temperature of the TEC. This method of control is the least preferred. Because the power to the TEC is cycled from full ON to full OFF, thermal cycling of the TEC will occur and degrade the life of the unit.
3. Proportional Control
A proportional controller offers much better stability than an On/Off controller does. In proportional controllers, there is always a residual error, even after the controller has settled to the final state. This error is proportional to the difference between the set-point temperature and the ambient temperature.
The following diagram is a linear bipolar proportional control circuit.
- OP - AMP
- SET CURRENT LIMIT <2.0 AMPS.
- INCREASE GAIN UNTIL OUTPUT STARTS OSCILLATING, THEN CUT IN HALF.
- ¯ CHOOSE C1 TO OBTAIN STABILITY AT HIGH SETTINGS IN HEATING MODE.
- ° TEC IS A 2A @ 2V MAX, WHEN BASE TEMP = +27°C
- R1 IS CURRENT SENSE RESISTOR, .3 OHMS, 3 WATTS.
4. Proportional-Integral (PIC) Control
The residual error present in the proportional controller can be eliminated by the addition of an integrator. PI control is required for systems that have wide variation in either or both the thermal load and ambient temperature.
5. Proportional-Integral-Derlvative (PID) Control
Full PID control loops are the most complex and are less common. The PID controller adds a derivative circuit to the PI controller, which improves the transient response. This type of controller is mainly used in applications where large thermal loads must be quickly controlled.
C. TEC Mounting Methods
Thermoelectric coolers (TECs) are mounted using one of three methods: adhesive bonding, compression using thermal grease, or solder.
In general, for a TEC with a ceramic base of 19mm or less, you can solder or adhesive bond without fear of failure due to thermal stresses. If the TEC base is larger than 19mm, we recommend the compression method because thermal grease is not rigid and does not transfer thermal stresses.
A thin layer of copper metallization on the hot and/or cold ceramic allows soldering as a means of attachment. Keep in mind a TEC that has no metallization on either side cannot be mounted using solder. Adhesives and greases are prone to outgassing; therefore, they are not as appropriate for use in a vacuum package.
1. Surface Preparation
Surface preparation is important when using any of the assembly methods. No matter which method is used, the mounting surface should be flat to less than 0.08mm over the TEC mounting area. In addition, the surface should be clean and free from oil, nicks and burrs. When multiple TECs are placed in parallel thermally between common plates, the TEC thicknesses should vary no more than 0.05mm.
2. Mounting with Adhesive Bonding
When to Use:
- When you want to permanently attach the TEC to your heat sink
- When mounting with solder is not an option
- When the TECs need to be lapped to the same height after mounting
- When moderate thermal conductivity is required.
Because of the short amount of time needed for epoxy to set up, be certain to have your TECs cleaned and ready to mount before mixing epoxy. Clean and prepare mounting surfaces on both the TEC and heat sink using methanol, acetone or general-use solvent.
NOTE: It is recommended that acetone and cotton swabs be available so any excess or spilled epoxy (uncured) may be quickly removed.
Use Marlow Industries’ Thermally Conductive Epoxy. Follow the instructions on the package carefully. Be certain to mix the two pouches thoroughly, or the epoxy will not cure properly.
- Remove the epoxy pack from the protective pouch.
- Remove the divider.
- Knead well until thoroughly mixed.
- Cut a corner and dispense. The epoxy working time is approximately one hour.
CAUTION: Avoid prolonged or repeated breathing of vapor, and use with adequate ventilation. Avoid contact with eyes, skin or clothing. In case of contact with eyes or skin, flush immediately with plenty of water and get medical attention.
Coat the ceramic of the TEC with approximately a 0.05mm thick layer of epoxy.
Place the TEC on the heat sink and gently rotate the TEC back and forth, squeezing out the excess epoxy.
Using a clamp or weight, apply pressure (less than 689,465 N/m2), and cure for two hours at 65°C to maximize thermal and physical properties. Curing time at room temperature is 24 hours.
3. Mounting with the Compression Method
When to Use:
- When a permanent bond is not desired
- When multiple TECs are used
- When your TEC is larger than 19mm.
Prepare heat sink and cold sink surfaces by machining the module area to within +/-0.03mm.
Locate bolt holes in your assembly such that they are at opposite sides of the cooler between 3.2mm to 12.7mm from the sides of the thermoelectric. The bolt holes should be in the same plane line as the heat sink fins to minimize any bowing that might occur.
The recommended hardware that should be used is: #4-40 or #6-32 stainless steel screws, Belleville or split lock type washers as well as a fiber-insulated washer to insulate the screw head from the heat sink.
Remove all burrs. Then, clean and prepare mounting surface with methanol, acetone or general-use solvents.
Apply a thin 0.05mm layer of Marlow's Thermal grease to the hot side of the TEC. Place the TEC on the heat sink and rotate back and forth, squeezing out the excess thermal grease until resistance is felt.
Repeat Step 5 and rotate cold plate back and forth, squeezing out the excess thermal grease.
In a two-module system, torque the middle screw first. Be careful to apply torque in small increments, alternating between screws.
In general, apply less than 1,034,198 N/m2 (N/m2 = Pascal) per square meter of TEC area.
4. Mounting with Solder
When to Use:
- When you need minimal outgassing
- When the TEC is smaller than 19mm
- When you need a high-strength junction
- When high thermal conductivity is required
IMPORTANT: The device to which the TEC is being soldered should be placed on a thermal insulator. This will allow the device to become hot enough to reflow the solder. If necessary, the device may be placed on a hot plate set at 100°C to help heat it to the solder melting point.
Clean the surfaces to be soldered with methanol, acetone or a general-use solvent to remove oils and residues, which would inhibit soldering.
With a soldering iron and a new tip, pre-tin the bottom of the TEC (the side with lead wires) using II-VI Marlows' Solder 96°C or 117°C and General Purpose Acid Flux. Use small amounts. You can heat the soldering iron to a maximum of 150°C, but extreme care must be taken since most TECs are constructed with 138°C (min.) solder. CAUTION: Do not mix solders. Use a separate soldering iron (or a new tip) for each solder.
With soldering iron, pre-tin the header or heat sink with the same solder and flux as used in pre-tinning the TEC. Use small amounts.
To minimize flux residue, clean both the header and TEC. Rinse them first in hot water, then scrub with Marlow Industries' Cleaning Solution and rinse again with hot water, brushing away any excess flux residue. Finally, wash with methanol and use forced air to blow dry.
Prior to mounting the TEC to the header, add a small amount of II-VI Marlows' Blue Mounting Flux to the mounting site on the header.
Hold TEC with tweezers and align on header. While doing this, maintain a steady, downward pressure.
While holding the TEC in place, put the soldering iron to the header near the solder seam. When the solder junction flows, remove the soldering iron. The downward pressure on the TEC will expel excess solder.
REMEMBER: The solder that holds the TEC together flows at 138°C (min.), so if you are using the 117°C solder, do not leave the soldering iron on the header surface too long, or you will melt the TEC solder as well.
Continue holding the TEC in place until the solder solidifies.
Check along all four edges of the TEC, looking for voids, cracks or bubbles. A smooth seam ensures proper thermal conduction.
Sealing thermoelectrics is key to the success of designs in the field. Condensation can and will corrode the inner materials of thermoelectrics if not properly managed.
Sealing can be done at the module and/or system level. Permeability, durability, adhesion, dielectric properties, thermal conductivity and manufacturability affect the sealant choice. Sealant choices could include o-rings, RTV, epoxy, PIB, and combinations of these and other techniques.
Please contact the factory to discuss the proper sealing techniques for your specific application.