Use a PWM Fan Controller in an EMI-Susceptible Circuit

Microchip Technology offers a family of cooling-fan speed controllers that operate in PWM mode for use with brushless dc fans (Reference 1). To control fan speed using the PWM waveform's duty cycle, you can use either an external NTC (negative-temperature-coefficient) thermistor or one of Microchip's PIC microcontrollers and its SMBus serial-data bus. Figure 1 illustrates a typical application that the data sheet describes for the TC664 and TC665 controllers (Reference 2). Using a frequency-control capacitor, CF, with a value of 1 µF, fan-controller IC1 generates a PWM pulse train with a nominal frequency of 30 Hz and a temperature- or command-dependent duty cycle that varies from 30 to 100%.

Although using the controller in PWM mode reduces power dissipation in transistor QA, which drives the fan, the 100-mA, square-wave motor-drive current can cause unwanted interference in a nearby high-sensitivity audio circuit. The circuit in Figure 2 solves the problem. An additional driver transistor, Q1, and an RC network comprising C3 and R3 form a simple PWM-to-linear converter. You can also use another PWM-to-linear-conversion circuit, such as an integrator based on an operational amplifier.

Figure 3 shows a graph of the dc voltage at Q2's collector versus IC1's PWM drive-output waveform's duty cycle. The voltage applied to the fan corresponds to the difference between Q2's collector voltage and the 12 V supply voltage. Even though a steady voltage appears across the fan, current pulses that the fan motor's commutation produces still develop a voltage across current-sense resistor RSENSE that connect to Q2's emitter, and all of IC1's protective and advisory features remain available.

The listed component values are valid for a 100-mA, 12 V, brushless fan. Use a general-purpose NPN transistor such as the 2N2222 for driver-transistor Q1 and an NPN transistor, such as Fairchild Semiconductor's PZT2222A, that can dissipate one-third of the fan's maximum power consumption for Q2. Note that you can vary the PWM's nominal frequency over a range of 15 to 35 Hz by altering the value of CF.