Electronic Design

Low-cost fan manager for power-supply applications

Modern system ergonomics require an increasing amount of electronics to be crammed into ever tighter quarters. Consequently, space allocations for the ubiquitous power supply shrinks. These smaller power-supply form factors, in addition to greater power demands, are boosting the popularity of brushless dc (BDC) cooling fans. But while BDC fans adequately evacuate heat from the system enclosure, they add other more significant problems.

First, there’s the annoying acoustic roar of turbulent air flow from the fan’s normal full-speed operation, Then there’s the issue of the large current consumed by the fan itself—a significant problem in today’s microampstingy systems. In addition, fan reliability and service life are concerns. Because the fan is a complex electro-mechanical device, it probably will require replacement before the lifetime of its host system expires. Finally, even if all is well with the fan, trauma brought on by an obstruction of the fan air intake or an object lodged in its rotor, can completely incapacitate it. These issues are bringing ever increasing scrutiny to the fan and its well-being.

These concerns can be addressed with common system techniques. For example, it’s been known for some time that acoustic noise is greatly reduced when the fan is operated at lower speeds. Temperature-proportional fanspeed control, for example, runs the fan only as fast as required to keep the system cool. This not only greatly reduces acoustic noise, but extends fan service life because the fan isn’t subjected to the stress imposed by continuous fullspeed operation.

While speed control is a cure for several fan ills, monitoring the fan’s health can be a complex task. One very simple approach is to detect when system temperature is excessively high, indicating the fan has either lost capacity (possible air-intake obstruction or bearing failure), or has stopped running altogether.

The circuit shown in the figure is a complete solution to the problem of fan management as previously outlined. TelCom’s TC648 is a stand-alone, lowcost fan manager that integrates a pulse-width modulation (PWM) fan speed controller (with integrated startup timer) and an over-temperature detector. It drives any standard two-wire fan through a low-cost transistor (Q1). The input voltage range (VIN) is 1.5 V to 2.6 V (corresponding to 0% to 100% fan speed). The figure shows the temperature signal generated with a low-cost NTC thermistor and R1. The output duty cycle (and, therefore, fan speed) increases with increasing voltage on VIN. The over-temperature fault output (—OTF) is asserted when the voltage on VIN exceeds 2.6 V, indicating insufficient cooling due to a fan fault or system thermal runaway.

The circuit provides temperatureproportional fan-speed control and over-temperature detection. It also suspends fan operation (for even greater system efficiency) when temperature falls to a point (set by R2 and R3) where forced-air cooling is no longer required. This elegant little circuit is a fitting mate to any two-wire BDC fan, and rewards the end user with quieter system operation, improved system reliability, and more efficient system operation.

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