Electronic Design

Low Noise Power For Thermoelectric Coolers

Thermoelectric coolers (TECs) are used to cool high-sensitivity transducers, such as CCDs and photodiodes, in an effort to raise their performance even further. Powering these TECs presents an interesting challenge. Electric power must be delivered to the TEC, while at the same time heat must be removed from the TEC as it, in turn, cools the sensor.

Because the power electronics are likely to be in close proximity to the cooler and sensor, they must not introduce any noise into the system. This suggests using linear control elements in the power path. However, these will generate additional heat, making the heat-dissipation task even more difficult. Ultimately, what would be desired is the efficiency of a switch-mode power regulator with the noise performance of a linear regulator.

The circuit presented here uses a new low-noise switching regulator that approaches this ideal. U1 (LT1534) is a monolithic switching regulator with programmable switching slew rates. Most noise problems arising from switch-mode power supplies result from high-speed switching in the power path. These fast edges couple noise to nearby circuitry. Since this coupling takes place through parasitic circuit elements, filtering or shielding this noise is nearly impossible. By slowing down these transitions, the noise can be eliminated at its source.

Here’s how the circuit works: U1 is wired as an input-referred negative buck regulator. U1’s internal oscillator and power switch apply a pulse-widthmodulated square wave to inductor L1. This square wave is filtered by L1 and C1, and its average appears across the TEC. The current through the TEC is monitored by sense resistor R1. Op amps U2A and U2B servo U1 to set this current proportional to a control voltage at node A.

Although U1 provides both current limit and thermal shutdown, there are two good reasons to actively control the current through the TEC. First, the system may already contain temperature-control circuitry; all that’s needed is a voltage-controlled current source. More importantly, the current through the TEC must be accurately limited. As pointed out in a previous Idea for Design (ELECTRONIC DESIGN, “Simple Design Equations for Thermoelectric Coolers,” Feb. 23, 1998, p. 132), if too much current is applied, the TEC will end up heating the sensor instead of cooling it. In a temperature-control loop, this can lead to “thermostatic latch-up.” R1, R2, and R3 set the current scale—in this case, 0 to 5 V at point A corresponds to 0 to 1.5 A through the TEC.

The slew rates of U1’s power switch are proportional to the current flowing out of the RCSL and RVSL pins. Resistor R4 programs the nominal slew rates, whereas R5 and Q5 raise the slew rates when power demand is high. When the system starts, the temperature-control loop will apply maximum power to the TEC to slew the temperature toward its set point. Here, the slew rates are high, so the circuit will deliver this power efficiently. Once the sensor is cool, the control loop will throttle back the TEC power, the slew rates will drop, and the circuit will deliver quiet power to regulate the sensor’s temperature. Some efficiency is sacrificed for optimum noise performance.

U3 and its associated passive components form a simple temperature controller. This op amp measures the error voltage across a resistor bridge formed by thermistor RT and potentiometer R6. RT monitors the sensor temperature and R6 sets the operating point. The output of this circuit is clamped with a Zener diode to limit the current through the TEC.

The controlled slew rates reduce the troublesome high-frequency noise. Optional components L3, L4, C3, and C4 reduce both the common-mode and the differential-mode ripple at the 60-kHz fundamental frequency and its lower harmonics.

See associated figure

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