Circuit controls multiple thermoelectric coolers

By Frank Effenberger, Bellcore, Morristown, NJ -- EDN, 8/19/1999

Optoelectronic and other components sometimes use a thermoelectric cooler and a thermistor for temperature control. A typical thermoelectric cooler has 1A maximum current and 1µ impedance. These parameters make simple series-pass drive circuits inefficient, given the typical available supply voltages (5, 12, or 15V). Often, several thermoelectric coolers exist in a single circuit (for example, a multiple-wavelength optical transmitter). In this case, you can obtain improved efficiency by using the circuit in Figure 1. This circuit has three devices to cool, so it contains three coolers and three thermistors. On the sensor side, each thermistor connects to a standard proportional-integral-differential op-amp or µP-based-controller circuit. Each controller produces a command voltage that is proportional to the current its respective cooler requires.

The command voltages drive precision rectifier circuits (op amps IC₁ to IC₃) that generate the maximum control V_MAX. Op amp IC₄ limits V_MAX, thus allowing you to control the maximum current delivered to the coolers. This limited output, V_LIM, is the input to the main transistor-driver amplifier, IC₈. V_LIM commands the main transistor to pass a current corresponding to the largest demand, subject to the limit constraint. The thermoelectric coolers connect in series, and this chain receives its current via the main series-pass transistor Q₄. IC₈ drives this transistor such that the voltage drop across R_SENSE equals V_LIM. You choose R_SENSE to equal the impedance of one cooler. The coolers have shunt transistors (Q₁ through Q₃) that divert any excess current around the individual coolers. The shunt transistors receive their drive from op amps IC₅ through IC₇, which are connected as difference amplifiers. The op amps drive the shunt transistors such that the voltage across each thermoelectric cooler equals the command voltage for that cooler.

Because the sense resistance equals that of the coolers, the current passing through each cooler assumes the correct value. The actual values of the difference-amplifier resistors, R, and the transistor-base resistors depend on the type of op amps and transistors you select. In most cases, the desired cooler currents have an average value, I_MEAN, and a maximum value, I_MAX, that are almost equal. If you cool N coolers using separate circuits, the supply needs to deliver NXI_MEANA. Using the series-connected circuit, the supply current reduces to just I_MEAN. Even in the case in which you an optimize the supply voltage for either circuit, the power dissipation for the series-connected circuit is lower because it drops a smaller fraction of the supply voltage through the series-pass transistor. (DI #2395).