As Moore's Law plunges us into the realm of multi-gigahertz processors and PCs with gigabytes of RAM, engineers are stuck with the task of removing the excess heat produced by high-speed components. Cooling such systems poses a dilemma. If you optimize the fans for nominal operating conditions, the system is susceptible to failure when conditions deteriorate. On the other hand, if you select the fan to maintain acceptable operating temperatures under worst-case conditions, it may produce an annoying level of sound.
The obvious solution is to control fan speed. If the system includes a system-management bus, you can add one of the many available sophisticated ICs for controlling fan speed. But some systems preclude the use of such a bus, requiring a stand-alone fan-speed controller (Figure 1).
Figure 1. To control fan speed, the thermistor (RT1) adjusts the output voltage of this dc-dc converter.
Power is delivered via the 12V supply, and a dc-dc converter (U1) steps the input voltage down to an intermediate voltage for powering the fan. The transfer function of this voltage is determined by resistors R1 and R2 and thermistor RT1. The thermistor is an NTC type, which causes the output voltage to increase with increasing temperature. The output voltage is about 5.5V at room temperature and increases to 12V at about 47°C (Figure 2).
Figure 2. Output voltage for the Figure 1 circuit varies with temperature as shown.
The ratio of resistors R1, R2, and RT1 is easily selected using a spreadsheet. Note that the tables of resistance ratio versus temperature provided by thermistor manufacturers are much easier to use than the cumbersome equation for thermistor resistance.
Because the circuit does not monitor fan speed or current, it includes R3, C10, and D2 to ensure that the fan starts turning during startup. The time constant of R3 and C10 serves that purpose by causing the U1 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 criteria in selecting a dc-dc converter is whether it is capable of 100% duty cycles. U1 satisfies that requirement while offering the simplicity of an internal power MOSFET. It supplies up to 1A of output current, which is suitable for one to four standard fans. As an added benefit, its high efficiency helps to minimize the heat that this circuit is designed to remove.
