May 8, 1997

Controller provides closed-loop temperature regulation

David Salerno, Unitrode Corp, Merrimack, NH

Sometimes, you need to test a circuit element over temperature, but it's impossible to put the entire application circuit in the temperature chamber. Freeze sprays and hair dryers are convenient for gross benchtop troubleshooting, but the temperature and slew rate are highly uncontrolled and may actually damage the part. Forced-air systems are good, but they're cumbersome and expensive. The circuit in Figure 1 uses a thermoelectric cooler (TEC) to provide closed-loop control of the element's temperature.

TECs use the Peltier effect and act as small, solid-state heat pumps when a current passes through them. You can reverse the magnitude and direction of heat transfer by simply reversing the magnitude and direction of the current. You can achieve a temperature difference as great as 50°C across a TEC if you provide proper heat sinking on one side of the device. You can produce larger temperature gradients by stacking multiple elements. You can use TECs effectively as part of a closed-loop temperature-regulation system.

The circuit in Figure 1 uses an H-bridge controller, four MOSFETs, a differential LC filter, and a TEC to form a closed-loop regulator. The bridge topology allows heating or cooling using a single supply; PWM minimizes losses. This topology requires a sophisticated PWM controller to prevent such problems as bridge cross-conduction. IC₁ contains the building blocks for closed-loop PWM control: a voltage reference, an error amplifier, a pulse-width modulator, an oscillator, a current-sense amplifier, and FET drivers. The IC also contains the biasing circuitry for single-supply operation.

R₁₅ and C₁₃ set the 100-kHz PWM frequency. The voltage on the DB pin, provided by the R₁₂/R₁₄ divider, sets the dead time between commutation of the bridge switches. The voltage on PVSET determines the amplitude of the triangle-wave oscillator in the PWM modulator. Schottky diodes D₁ and D₂ clamp any ringing below ground on the outputs of IC₁'s low-side FET drivers. Sensing resistor R₆ limits the peak current to 5A. IC₁ amplifies the current-sense voltage. C₁₂ and a 100 ohms resistor in the IC filter any noise spikes.

The LC output filter comprising L₁, L₂, and multilayer-ceramic capacitors C₂ to C₆ converts the PWM output from the bridge to a dc voltage. This conversion is necessary because ac ripple is detrimental to the TEC and causes its efficiency to drop off rapidly. Ripple lower than 10% is advisable. The resulting architecture is a low-bandwidth, class-D amplifier that can deliver a variable dc voltage--to ±12V--at several amperes to the TEC. The heat sink for the TEC uses a Pentium heat sink with a 12V dc, brushless fan, mounted in close contact with one side of the TEC. An aluminum "cold plate" is mounted on the other side, forming a sandwich with the TEC in the middle. The aluminum plate, acting as the control surface, adds thermal mass to stabilize the loop, and the plate protects the TEC's brittle ceramic surface. You mount this plate in intimate thermal contact with the device under test. Thermistor R_T1, mounted in a hole in the aluminum, provides good thermal feedback to close the loop. R₇ helps linearize the thermistor's response.

The circuit uses proportional gain because it's difficult to compensate an integral loop because of the mechanical system's long thermal time constant. The error amplifier's dc gain, set by R₁₀ and R₁₁, is high enough to produce a full output with a temperature error lower than 1°C. C₁₀ provides a pole to filter any noise that could reach the modulator. You calibrate the temperature-control potentiometer, R₉, by using a thermocouple temporarily mounted to the aluminum plate. The calibration is accurate and repeatable to within approximately 1°C. LEDs provide a visual indication of whether the TEC is heating or cooling. You should mount all the passive components and the diodes associated with IC₁ close to the IC. Also, keep the wires from the current-sense resistor short; if that's impossible, use twisted-pair cable.

The entire circuit operates from a 12V dc, 2.5A power supply and provides closed-loop temperature regulation for a surface approximately 1 in. sq. You can vary the temperature of the control surface from 0 to 80°C in a room-temperature ambient environment. Higher temperatures are possible, depending on the rating of the TEC. You can also obtain colder temperatures if you provide proper heat sinking or if you stack TECs. Remember that cooling can occur only if the opposite surface can dissipate the heat, including the heat stemming from efficiency losses in the TEC. ( DI #2021)

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