Speed-control drive circuit for a D.C. brushless fan motor

A D.C. brushless fan motor having a speed-control drive circuit for two sets of motor windings, said speed-control drive circuit comprising:

• a pulse width modulation input end for receiving a pulse width modulation signal;
• and
• a motor winding drive circuit, said motor winding drive circuit and said pulse width modulation input end being connected to two sets of motor windings;
• said pulse width modulation signal controlling said motor winding drive circuit to cause excitation/unexcitation of said two sets of motor windings and to alternately change a direction of current such that said two sets of motor windings adjust a speed of the fan motor in response to excitation of said two sets of motor windings responsive to said pulse width modulation signal;
• wherein the speed of said fan motor increases when an operational period of said pulse width modulation signal increases, and wherein the speed of said fan motor decreases when the operational period of said pulse width modulation signal decreases.