TEC controllers are used for thermoelectric cooling and heating in combination with Peltier elements or resistive heaters. Peltier elements are heat pumps which transfer heat from one side to the other, depending on the direction of the electrical current. TEC controllers are used to drive the Peltier elements.
This system design guide provides information about how to design a thermoelectric cooling application using TEC controllers and Peltier elements.

TEC Controller Product Overview

ContentsTEC Controller Products

Consulting

We are the electronics supplier for Elinter AG, a provider for complete solutions in thermal design. In cooperation with Elinter, we can assist you in designing your thermoelectric application. This includes simulation, design, mechanical construction as well as choosing the appropriate electronics, sinks and heat pipes. Contact us to profit from our know-how and experience.

Thermoelectric Cooling Video

This video explains the basics of thermoelectric cooling. We exemplify important design steps to sucessfully design a thermoelectric application using TEC controllers and Peltier elements.

Background Information

Thermoelectric cooling and heating can be used for various applications, even when active cooling below ambient temperature or high temperature precision (stability <0.01 °C) is required. A TEC controller – the current supply for the Peltier element – in combination with a Peltier element is actively regulating the temperature of a given object or case. This is done without acoustic and electrical noise, vibrations and mechanical moving parts. Changing from cooling to heating is possible by changing the direction of the current, without making any mechanical changes.

Temperature vs. Current Graph

There are temperature limits, when operating Peltier elements. Elements are available with a maximum operation temperature of 200 °C, where this limit is defined by the reflow temperature of solder and sealing. When using the Peltier element as a thermoelectric cooler, there is a limit where the temperature will rise again the more current is supplied. This is because of the power dissipation (I2R) within the Peltier element, when drawing more current than Imax.

When designing a thermoelectric application, cooling is the critical part. So we will take the case of cooling an object as example for the design guide.

System Overview

TEC Peltier Element Controller, Heatsink, Fan
The assembly of a thermoelectric cooling application. The different elements will be explained in the following chapters.

Please watch the following video for an overview about the TEC-family controllers and their features.

Design Process

The following steps are necessary when designing a thermoelectric cooling application:

  1. Estimate heat load of the object to be cooled
  2. Define temperature working range
  3. Choose a Peltier element that satisfies the requirements
  4. Choose a TEC controller with enough power
  5. Choose the object temperature sensor and the optional sink sensor
  6. Choose a heat sink for the Peltier element
  7. Choose a fan to air the heat sink (optional)
  8. Choose a power supply for the TEC controller
  9. Test your experimental setup, improve it

1. Estimate Heat Loads

An important parameter is the amount of heat to be absorbed from the object by the cold surface of the TEM or Peltier element. (QC [W]) Depending on the application, there are different types of heat load to be considered:

  • Power dissipation
  • Radiation
  • Convective
  • Conductive
  • Dynamic (dQ/dt)

The heat load QC will be transferred from the cold side to the hot side, where the heat sink is located.

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2. Define Temperatures

Usually an object has to be cooled to some given temperature. If the object to be cooled is in contact with the cold surface of the thermoelectric module, the desired temperature of the object can be considered the temperature of the cold side of the Peltier element after a certain time.

Two design parameters are important, when outlining a thermoelectric cooling application.

  • TO object temperature range (cold side temperature) [°C]
  • TS heat sink temperature range (hot side temperature) [°C]

The difference between TO and TS is ΔT (deltaT or dT) [K].

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3. Choosing a Peltier Element / TEM Module

The Peltier element is producing a temperature difference between its both sides due to the current flow.

  • Peltier Elements – functionality, technical basics, manufacturers and distributors

Besides mechanical properties Peltier elements are characterized by four important parameters: Qmax, ΔTmax, Umax, Imax

These are theoretical figures and they are used to describe the behavior of Peltier elements.

A Peltier element has a maximum heat pumping capacity Qmax if the temperature difference between both sides is 0 K. The current and voltage associated with Qmax are Imax and Vmax, respectively.

ΔTmax is the maximum temperature difference across the Peltier element, when absolutely no heat is pumped. Nevertheless, this maximum value is never reached in a thermoelectric application. It is given by the manufacturer to characterize the performance of the Peltier module.

It's important to understand, that the Peltier element will not produce these figures when just supplying enough current. The temperature difference ΔT is suppressed as heat is applied to the cold side of the Peltier element.

This normalized graph describes the relationship between heat pump capacity on the y-axis, current on the x axis for different ΔT.

Heat pumped vs Current

In a thermoelectric application, there is always a trade-off between heat pump capacity Qc and temperature difference ΔT.

At Imax either Qmax is zero and the temperature difference is at its maximum or vice versa.

It is important to understand that only for relatively small temperature differences ΔT a significant amount of heat can be transferred. Multi-stage Peltier elements are used when higher temperature differences are needed.

If a high temperature difference is produced or a large amount of heat is transferred, more Joule heating contributes to the total sum of heat to be dissipated. The power of Joule heating is exponentially proportional to the current driving the Peltier element.

P ∝ I2

The pumped heat load Qc and the temperature difference ΔT are inversely proportional to each other, as heat is applied to the cold side the temperature difference is suppressed.

Therefore, for an optimum performance of the cooling system, the temperature difference between the object and the heat sink must be kept as low as possible.

When outlining the system, the current for the Peltier element should be between 0.3 and 0.7 times Imax.

Maximum Efficiency

One important criterion is the Coefficient of Performance (COP) when choosing a Peltier element. The definition of the COP is the heat absorbed at the cold side divided by the input power of the Peltier element: COP = Q/ PP
The result of a maximum COP is minimum Peltier input power, thus minimal total heat has to be dissipated by the heat sink. (QC+PP) Therefore smaller heat sinks can be used, what allows a more space saving design. It is also important when the heat rejection of the heat sink has to be minimized. On the other hand, when optimizing costs, a design with a lower COP has to be chosen.

Peltier performance versus current

This is a plot showing the COP versus current relationship of a Peltier element and the optimum curve is plotted in bright green, showing the highest COP.

On the left side, we see that the COP is maximum at the lowest temperature difference. Hence we get a high amount of heat pumped per unit electrical power.

Therefore, for an optimum performance of the cooling system, the temperature difference between the object and the heat sink must be kept as low as possible.

As we can see, depending on deltaT the corresponding COP maximum is at different current levels – with higher deltaT it moves to the right.

If we follow the curve to the right, we find out that we must put a lot of electrical power into the system to get only a little heat pumped, what corresponds to a low COP value.

We can also observe that higher currents are needed to produce higher temperature differences.

Peltier performance versus current with marking

To understand the dynamics of the system, we can observe what happens if the ambient temperature changes—and therefore the deltaT—or when the heat load is increased.

If we operate the Peltier element with a current around 25% of Imax it is possible to compensate a rise of deltaT—point A to B—by increasing the current. The heat pump capacity stays unaffected. The heat pump capacity can be increased as well without changing deltaT, if we move from A to C.

If the working point is around 60% of Imax we need more current than in the previous example to compensate a rise of deltaT—point D to E—when the heat pump capacity should stay unaffected. The heat pump capacity can still be increased without losing temperate difference, if we move from D to F.

However, if the Peltier element is operated near its maximum current, a change in temperature can’t be compensated by rising the current. The transition from a lower to a higher temperature difference would result in a decrease of heat pump capacity

As a first conclusion, we see that we can add design margin by

  • choosing a Peltier element with greater than required heat pump capacity
  • by designing a system with an operating current well below Imax of the Peltier element
  • or as a third option by oversizing the heat sink or add a fan to it to keep the hot side temperature low

By applying this measures a change in ambient temperature or active heat load does not lead to thermal runaway

Example

There are two thermal parameters which are necessary to select a Peltier element.

  • Maximum cooling capacity Qmax
  • Temperature difference ΔT

As an example we assume an object to be cooled to zero degrees. (TO = 0 °C) Let's say the heat sink temperature TS is 36 °C. Thus the difference ΔT is 36 K. There is a heat load QC to be absorbed of 10 W.

As we have a temperature difference of ΔT = 36 K, a single stage Peltier element is sufficient. We will use one with a ΔTmax of 72 K.

First we calculate ΔT / ΔTmax = 36 / 72 = 0.5

Normalized performance graph for Peltier element
Normalized performance graph for Peltier elements with optimum line (red) in relation to COP. (QC=Q)

Obtain the optimum value of QC/Qmax at the intersection of the horizontal line of ΔT/ΔTmax = 0.5 and the optimum QC/Qmax line. Follow the blue lines and read out the value at the vertical axis QC/Qmax.
Optimum QC/Qmax = 0.25 (in the middle of the blue lines 0.2 and 0.3)
Optimum Qmax = Qc/0.25 = 10 W/0.25= 40 W

Obtain the maximum value of QC/Qmax at the right vertical axis that corresponds to ΔT/ΔTmax = 0.5.
Maximum QC/Qmax = 0.5
Maximum Qmax = QC/0.5 = 20 W

So a Peltier element with Qmax between 20 and 40 W has to be chosen. If a Peltier element with Qmax near to the optimum Qmax is chosen, a high COP and therefore maximum efficiency is achieved. An Element with Qmax near to the maximum Qmax will yield less cost but less cooling capacity. The Peltier element will be operated near its limits.

The ratio I/Imax is given by the diagram as well. At the intersection of ΔT/ΔTmax and the value of QC/Qmax, plot a vertical line. Note the value at the intersection of the vertical line and the bottom axis. In the optimum case I/Imax is 0.5. Using the value Imax of the Peltier element, we can calculate the resulting current I. I = Imax* (I/Imax)

Besides the thermal properties the mechanical properties are important as well. Questions to be answered are: What size fits my application? Do I need a special form of the Peltier element? What mounting options do I have? Also the choice of the heat sink for the Peltier element is crucial.

Please refer to the page Peltier Elements for a list of Distributors.

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4. Choosing a TEC Controller

The TEC controller is regulating the current supplied to the Peltier element, according to the desired temperature of the object and the actual measured object temperature.

Given the current Imax and voltage Umax of the Peltier element one has to select an appropriate TEC controller.

We have learned that a Peltier element should be powered with maximum 70% of its Imax value or even lower percentages. With this in mind, we look for a TEC controller that matches the operating current for the application and not Imax.

Single channel (stage) TEC controllers:

Dual channel TEC controllers in parallel mode:

Please refer to the TEC controller product page for an overview.

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5. Temperature Sensors

Temperature sensors are used by the TEC controller to measure the objects temperature and the temperature of the heat sink.

Object Temperature Measurement

To be able to control the temperature of the object, you have to place a temperature probe (sensor) on the object. Please note that it's important to place the sensor as near as possible to the critical point on the object where you need the desired temperature.

Because measuring the object temperature demands higher precision and a larger range, we suggest to use Pt100 sensors. To be able to measure temperatures far below 0 °C, Pt100/1000 probes are needed. This because, if the temperature gets too low, NTC probes can't be used as the resistance value gets too big. The resistance value of the sensor has to be smaller than the reference resistance in the TEC controller.

Temperature Ranges:

  • NTC (18K TEC configuration): 12 °C to 85 °C
  • NTC (39K TEC configuration): 0 °C to 60 °C
  • NTC (56K TEC configuration): -10 °C to 50 °C
  • NTC (1M TEC configuration): -55 °C to 60 °C
  • Pt100, Pt1000: -50 °C to 200 °C

When using Pt100/1000 sensors, the object temperature is measured using the four-terminal sensing technique (4-wire sensing) to achieve higher precision at low resistances. For NTC measurement 2-wire technique is used.

The term 4-wire doesn't mean that a sensor with four pins is needed. Separate pairs of current-carrying and voltage-sensing electrodes are used. (More information about four-terminal sensing)

Connecting the Temperature Sensor

Refer to the TEC Controller Notes page to learn how to connect your temperature sensor.

Heat Sink Temperature Measurement

Additionally, you can place a temperature probe on the heat sink as well – this is optional. In this case NTC thermistor is used as probe. Heat sink temperature measurement is used for a more efficient control of the system. If the heat sink temperature measurement is not used, it is possible to set a realistic fixed temperature value in the software.

The B-Value, β-Value or Beta-Value

An NTC temperature sensor has a negative resistance versus temperature relationship. The β-value is a material constant which describes the slope of the resistance vs. temperature relationship.

Thermistor Resistance vs. Temperature
Resistance of NTC as a function of temperature

A steeper slope means a higher β-value. Thus small temperature changes cause a significant change in resistance. More accurate measurements can be achieved.

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6. Heat Sink

The heat sink absorbs the heat load at the warm side of the Peltier element and dissipates it to the surrounding air.

The heat sink has to be large enough, so that its temperature doesn't get too high. This is a point that shouldn't be underdesigned. On the other hand, the heat sink has to fit into the application by its form and dimensions. The importance of an efficient TEC controller is clearly visible by the resulting size of the heat sink. Depending on your requirements a custom made heat sink or heat pipe might be a solution.

The thermal resistance is calculated by: RthSink = ΔT/P
RthSink = Thermal resistance of the heat sink [K/W]
ΔT = Temperature difference between the heat sink and the ambient air temperature [K]
P = Heat load to be absorbed [W]

The heat load P has to be assumed as heat load of the object plus heat load of the Peltier element due to losses.

As an example we assume P = 20 W. The temperature difference ΔT between the heat sink and the ambient temperature (25 °C) shall not be higher than 10 K. So ΔT = 10 K.

RthSink = ΔT/P = 10 K / 20 W = 0.5 K/W (minimum)

The smaller the thermal resistance RthSink the bigger the heat sink. So we choose a heat sink with RthSink smaller than 0.5 K/W.

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7. Fan

The fan is used to ventilate the heat sink and to avoid the temperature of the heat sink getting too high.

Up to 2 fans can be directly connected and controlled by the TEC controller.

The TEC controller is able to control fans which support the following features:

  • PWM control signal input to control the fan speed. The TEC generates a 1 kHz or 25 kHz PWM signal from 0 – 100%.
  • Frequency generator signal output which represents the rotation speed. The output should be a open collector output signal.

It is recommended to use a fan with the same supply voltage as the TEC needs. Please refer to the datasheet of the fan for the cable specification.

Fan Recommendations

For detailed information about the fan feature fan suggestions and optimal settings, please refer to the TEC Familiy User Manual chapter 6.3 (PDF).

Connecting the Fan to the TEC Controller

Refer to the TEC Controller Notes page to learn how to connect the fan.

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8. Power Supply Requirements

The power supply is the power source for the TEC controller.

Depending on the chosen TEC controller you can use a 24 V power supply. (Except TEC 1090 which uses up to 36 V) Make sure that the power supply is able to provide the power, necessary to drive the TEC controller with the Peltier element. (As a rule of thumb you can add 10% reserve. Multiply the necessary TEC output power times 1.1.)

Power Supply Recommendations

  • Mean Well, switched mode power supplies, vented housing, 15 .. 150 W,
    Distributors: PewatronDistrelec
  • Mean Well, switched mode power supplies, housing with fan, 320 .. 750 W,
    Distributors: PewatronDistrelec

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9. Test Your Setup

Now that you have selected the system components you set up the application and start testing and optimizing. To facilitate the assembly and initial set up using our service software please refer to our step by step TEC Controller Setup Guide.
The comprehensive service software can be downloaded and tested for free.

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Next Steps

Please refer to the TEC controller page for an overview of available products for your temperature controlling application.

Overview
Model
Detailed Data
Output Current
(no PWM, bipolar)
Output Voltage
Output Channels
 
TEC-1092 image TEC-1091 image TEC-1089 image TEC-1090 image TEC-1122 image TEC-1123 image
TEC-1092 TEC-1091 TEC-1089-SV TEC-1090-HV TEC-1122-SV TEC-1123-HV
more... more... more... more... more... more...
±0 – 1.2 A ±0 – 4 A ±0 – 10 A ±0 – 16 A 2 x ±0 – 10 A 2 x ±0 – 16 A
0 – 9.6 V 0 – 21 V 0 – 21 V 0 – 30 V 0 – 21 V 0 – 30 V
one two
 
 

If you still have questions about the design process or certain system components, don't hesitate to contact us.

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