(a) Field of the Invention
The invention relates to a control circuit of a direct current fan motor, and more particularly, to a driving circuit for switches of direct current fan motor.
(b) Description of the Related Art
Referring to FIG. 1, it is well known that two pulse width modulation (PWM) signals A and B are generally used within a circuit for driving four full-bridge switches 11, 12, 13 and 14 in order to control a rotational speed of the direct current fan motor 1. In practical applications, enhancement-mode metal-oxide semiconductor field-effect transistors (MOSFET) are usually selected as the switches 11, 12, 13 and 14.
With respect to the operations of the direct current motor fan 1, the PWM signals A and B drive the switches 11, 12, 13 and 14 with a high level, and stop driving the same with a low level. To be more precise, when the PWM signal A drives the switches 12 and 13 to be ON with a high level, the PWM signal B keeps the switches 11 and 14 to be OFF with a low level. As a result, the direct current fan motor 1 is conducted with a current having a direction as indicated by an arrow 15. Contrarily, when the PWM signal A is switched to a low level such that the switches 12 and 13 are OFF, the PWM signal B keeps the switches 11 and 14 to be ON with a high level. As a result, the direct current fan motor 1 is conducted with a current having a direction as indicated by an arrow 16. In other words, when the switches 12 and 13 are in an ON state, the switches 11 and 14 are in an OFF state; when the switches 11 and 14 are in an ON state, the switches 12 and 13 are in an OFF state.
However, there are problems during the switch of the ON/OFF state to the OFF/ON state for the above-mentioned switches because the direct current fan motor 1 is considered as an inductive load and these switches are mainly transistors. In other word, at the time when the switches 11 and 14, or the switches 12 and 13, are shifted from an ON state to an OFF state, counter-electromotive force produced within the direct current motor fan 1 is likely to shift the switches 11, 12, 13 and 14 at an OFF state back to an ON state, and thus results in malfunction of being ON instead of being OFF. The malfunction not only causes short circuit between the switches and then burn down switches, but also seriously damages the direct current fan motor 1.