Electronic Design

Speed Control Handles Isolated –48-V Fans

Chips are available to monitor and control ventilation fans. But less common are fan controllers for telecom applications. Such fans must operate in electrically isolated environments with a supply voltage in the 36- to 72-V range. For example, the MAX6650 has numerous control and monitor functions useful to system designers. But it best resides on the secondary (low-voltage) side of the isolation barrier. Also, it lacks the pulse-width-modulated (PWM) output often used to control high-voltage fans.

Designers can place a MAX6650 on the high-voltage side of the barrier and translate bidirectional I2C signals across to the low-voltage side. However, an easier alternative is to keep the MAX6650 on the low-voltage side and use opto-isolators to translate two unidirectional signals (control and tachometer pulses) across the barrier (see the figure).

U1 and Q1 produce a dc control voltage at U1, pin 10. Comparators U2a and U2b then convert the dc voltage to a PWM signal as required by the fan. U2a generates a 1.25-kHz voltage ramp that climbs from approximately 1.2 V to about 3.3 V. U2b compares this ramp to the control voltage, developing a PWM signal that's applied to the LED in opto-isolator U3. The phototransistor in U3 inverts the PWM signal and feeds it directly to the fan's control input, where the signal's duty cycle dictates the fan's rotational speed.

For feedback, the fan produces a tachometer pulse train consisting of two pulses per revolution. Those pulses drive the LED in optoisolator U4. U4's phototransistor delivers the signal to U1's TACH input. U1 then servos the OUT voltage as necessary to maintain the TACH frequency at a level corresponding to a value placed in U1's internal SPEED register. The system microprocessor precisely dictates the fan speed by writing to that register via the I2C bus.

The U1 control loop also compensates for factors such as variation in the fan's supply voltage. The microprocessor can also command U1 to do many other things, such as issuing an alert when the fan fails, turning the fan off, or turning the fan fully on, which can be used to check fan bearings by noting the resulting fan speed and comparing it to earlier tests.

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