The temperature controlled pulse-width modulator (PWM) boost converter shown in here allows operation of a 12V brushless DC fan from a +5V supply. The circuit is based on the Unitrode UCC2805, a single chip BiCMOS PWM controller which contains all the necessary circuitry (voltage reference, error amplifier, comparator, MOSFET gate drive, and oscillator) for closed loop PWM power supply control. At maximum boost, a voltage of +10V is provided to the fan, which operates at about 80% of its rated (12V supply) speed.
Fan speed is controlled by sensing the ambient temperature in the system enclosure, and reducing the fan supply voltage whenmaximum cooling is not necessary. Reducing the operating speed of the fan not only saves power but also extends operating life and reduces acoustic noise.
To use this circuit with a load requiring a higher operating current, it's necessary to change the values of C1 and L1.The value of C1 needs to be increased in proportion to the increase in current, to keep the same ripple amplitude. The value of L1 needs to be decreased (changed in inverse proportion to the increased current), since the maximum output current is inversely proportional to the inductance.For large increases in current,Q1 and D1 may need to changed as well.
Courtesy: John McNeill, Worcester Polytechnic Institute, Worcester, MA
