Reducing the operating speed of a system cooling fan not only saves power, but also extends the operating life of the fan and lowers acoustic noise. Fan speed can be controlled by sensing the ambient temperature in the system enclosure and reducing the fan supply voltage unless a temperature rise is detected. A previous design by Don Alfano (“Fan Speed Control Adjusts to Temperature,” ELECTRONIC DESIGN, Analog Applications Issue, Nov. 18, 1996, p. 83) for a 12-V brushless dc fan used a linear-voltage-regulator approach to provide fan voltages ranging from 6 to 12 V.
An alternative to the linear regulator, which provides even greater power savings, is the temperature-controlled pulse-width-modulator (PWM) approach (Fig. 1). The circuit is a buck-converter based on the Unitrode UCC2805, a single-chip biCMOS PWM controller that contains all of the necessary circuitry (voltage reference, error amplifier, comparator, MOSFET gate drive, and oscillator) for closedloop PWM power-supply control.
The UCC2805, which can be configured for either voltage-mode or current-mode feedback, is used here in a voltage-mode configuration controlling the buck-converter formed by Q1, D1, L1, and C1. A P-channel MOSFET is used, rather than N-channel, because the required gate drive for the P-channel device is much simpler. The lower rDS(ON) of an N-channel device isn’t necessary, given the modest power requirement. As shown in the timing diagram in Figure 1, the duty cycle of the switch drive signal OUT at pin 6 is determined by on-chip comparison of the control voltage, COMP (pin 1), to a sawtooth waveform, CS (pin 3).
The frequency of the on-chip PWM oscillator is set to 120 kHz by R2 and C2. The R3/R4 network attenuates the charge/discharge waveform on the oscillator timing capacitor C2. As a result, a sawtooth waveform is developed at CS of the proper amplitude for comparison with the COMP.
Temperature information is incorporated into the control voltage by RT, a negative-temperature-coefficient (NTC) thermistor that’s included in the RT/R5/R6 network of the error amplifier. The effect of temperature on the fan speed can be seen by following the control signal around the loop from RT to the fan supply voltage. When the ambient temperature increases, the value of RT decreases, increasing the current in RT, and reducing the value of the control voltage, COMP, at the output of the error amplifier. This changes the duty cycle of the comparator output and MOSFET gate drive so that Q1’s “on” time increases. This in turn increases the output voltage of the buck-converter supplying the fan, thus boosting the fan operating speed.
The fan speed versus temperature profile is characterized by two temperature points: T1 (below which the fan runs at half speed, with a supply voltage of 6V) and T2 (above which the fan runs at full speed, with the full 12V supply). The desired temperatures for T1 and T2 determine the values of resistors R5 and R6, which scale and shift the control voltage at the output of the error amplifier. The resistances are determined from the values of the thermistor RT(T1) and RT(T2) at temperatures T1 and T2:
The values shown in the schematic correspond to T1 = 77°F and T2 = 86°F, at which the thermistor had values of 200k and 154k, respectively. An on-chip control block limits the minimum duty cycle to 50%, corresponding to a minimum fan voltage of 6 V. This prevents stalling, which occurs at a fan supply voltage of about 4 V. Measurements on the circuit show an efficiency greater than 90%. The power consumed by the fan when operating at half speed was 26% of the full speed power consumption.