As Moore's Law plunges us into the realm of multigigahertz processors and PCs with gigabytes of RAM, engineers face the task of removing the heat that these state-of-the-art components produce. Cooling such systems poses a dilemma. If you optimize the fan size and speed for nominal operating conditions, the system is susceptible to failure when conditions deteriorate. If, on the other hand, you select the fan to maintain acceptable operating temperatures under worst-case conditions, the fan may produce an annoying level of sound. Controlling fan speed is the obvious solution. If the system includes a system-management bus, you can add one of the many available sophisticated ICs for controlling fan speed. But if such a bus is unavailable, you need a stand-alone fan-speed controller (Figure 1).
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| Figure 1. | To control fan speed, thermistor RT1 adjusts the output voltage of this dc/dc converter. |
Power comes from the 12 V supply, and a dc/dc converter, IC1, steps down the input voltage to an intermediate voltage for powering the fan. The transfer function of this voltage is a function of resistors R1 and R2 and thermistor RT1. The thermistor is an NTC (negative-temperature-coefficient) type, so the output voltage increases with increasing temperature. The output voltage is approximately 5.5 V at room temperature and increases to 12 V at approximately 47 °C (Figure 2). You can easily select the ratio of resistors R1, R2, and RT1 by using a spreadsheet. Note that thermistor manufacturers' tables of resistance ratio versus temperature are easier to use than are the cumbersome equations for thermistor resistance.
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| Figure 2. | Output voltage for the circuit in Figure 1 varies with temperature. |
Because the circuit in Figure 1 does not monitor fan speed or current, it includes R3, C1, and D1 to ensure that the fan starts turning during start-up. The time constant of R3 and C1 serves that purpose by causing IC1’s output to overshoot during the first few seconds of operation. After the fan starts, it easily sustains rotation at the lower operating voltages. An important criterion in selecting a dc/dc converter is the ability to operate at 100% duty cycle. IC1 satisfies that requirement and offers the convenience of an internal power MOSFET. IC1 supplies as much as 1 A output current, which is enough to drive one to four standard fans. As an added benefit, its high efficiency helps to minimize the heat that the circuit removes.
