Voltage Regulators Rev Up PWM-Based Fan Control

Nov 1, 2006 12:00 PM
By Tarlton Fleming, Editor, Steve Caldwell and Nancy Xiong, Customer Applications Engineers, Maxim I

The simplest of fan-control schemes — an on-off switch — is simple but inefficient, because it provides more cooling than necessary most of the time. When on, the fully powered switch-controlled fan also generates a maximum amount of noise. More recently, the use of pulse-width modulation (PWM) has improved the efficiency and regulation of fan controllers, but PWM is not a perfect solution, either. However, there are two alternatives to the direct use of PWM signals for controlling the speed of 3-wire fans. One is an inexpensive “bulletproof” technique based on a linear regulator, while the other is a more efficient approach that employs a switching regulator.

Typical fan-controller ICs provide a PWM output for regulating fan speed. Normally, this low-frequency signal (~30 Hz to 100 Hz) adjusts the fan rotations per minute by cycling the fan motor on and off via an adjustable duty cycle. Unfortunately, chopping power to a 3-wire fan (power supply, ground and tachometer output) inhibits the tachometer signal that provides feedback to the fan controller, because cutting off the signal during low portions of the duty cycle affects the control loop. Some fan controllers attempt to compensate for this effect, but with limited success. The on/off cycling also can produce an objectionable clicking noise.

One solution to these problems is to smooth the PWM signal with a low-pass filter and use that voltage to control a linear driver. For a 12-V fan, the resulting control voltage is typically 5 V to 12 V, and you can use an inexpensive linear voltage regulator to drive the fan. Otherwise, such circuits introduce an RC filter for smoothing the PWM output, which is then buffered by an op amp and current-amplified by an external pass transistor. That approach is functional, but without additional protection circuitry it is subject to damage. A shorted fan can blow out the circuit.

The popular linear voltage regulator is well suited to drive a fan. It combines the op amp, pass transistor, current limiter, short-circuit protection and over-temperature protection all in one package at a reasonable price. More importantly, a typical linear-regulator IC can supply 0.5 A to 1.5 A, which covers most of today's fan-control applications.

In a typical application, the controller applies a 100-Hz PWM signal to the base of a pass transistor, which controls fan speed according to the PWM duty cycle, by toggling the fan-motor current off and on. The circuit of Fig. 1 also controls the fan with a 100-Hz PWM signal, generated by the open-drain output of U1 (the MAX6639, a 2-channel temperature monitor with dual automatic PWM fan-speed controller).

Instead of the on/off control of a pass transistor, however, the PWM signal in Fig. 1 controls the output-voltage level of a linear voltage regulator (U2). U1's PWM output is smoothed by an RC network consisting of the Thevenin equivalent of R1, R2A and R2B, multiplied by the value of C2.

U2 regulates its output to maintain 1.25 V between the V_OUT and ADJ pins. Temporarily ignoring the effect of U1, the output voltage of U2 (V_OUT) is 1.25 V × (1 + R2/R1), where R2 = R2A + R2B. To account for the control influence of U1, note that R2A sets the minimum output voltage. With U1's PWM-polarity bit set for a positive duty cycle, a 0% duty cycle produces a low PWM signal that turns the open-drain output continuously on, removing R2B by placing a short across it. The R2A value shown (3.3 kΩ) then produces a minimum output of 4.7 V, which is low enough to minimize the fan's power consumption while ensuring a valid tachometer signal from the fan.

The sum of R2B and R2A sets the maximum value of V_OUT. At 100% duty cycle, the open-drain output is open (off), so the full value of R2B is present in the divider network. An R2B value of 7.5 kΩ produces a maximum V_OUT of 12.5 V. C1 and C4 are typical input-bypass capacitors, and C3 is U2's output capacitor, which smoothes the output voltage and supplies ac ripple current to the fan.

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