Abstract:
A driving circuit for switches of direct current fan motor is disclosed. The driving circuit includes a plurality of switches, a first control circuit, and a second circuit. The switches are driven by a first pulse width modulation signal and a second pulse width modulation signal, and they are electrically connected with the direct current fan motor in a bridge manner. A third pulse width modulation signal is used to drive the first control circuit connected to at least one of the switches driven by the first pulse width modulation signal. A fourth pulse width modulation signal is used to drive the second control circuit connected to at least one of the switches driven by the second pulse width modulation signal. Especially, either the first pulse width modulation signal or the second pulse width modulation signal is selected as the third pulse width modulation signal or the fourth pulse width modulation signal.

Description:
BACKGROUND OF THE INVENTION 
   (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 . 
   SUMMARY OF THE INVENTION 
   As described above, a prior direct current fan motor have the switches thereof be prone to malfunction caused by counter-electromotive force from shifting current directions of the motor. The malfunction may simultaneously damage the switches, and further lead to damages of the motor as well. 
   To overcome the issue, the invention provides a driving circuit for switches of direct current fan motor. The driving circuit is additionally provided with corresponding control circuits for controlling ON-OFF operations of individual switches to ensure the switches free from malfunction of being ON instead of OFF. 
   An object of the invention is to provide a driving circuit for switches of direct current fan motor, which uses driving signals for the same switches to drive a control circuit and forcibly lock the switches to be in an OFF state when the switches are shifted from an ON state to an OFF state, thereby avoiding malfunction. 
   The other object of the invention is to provide a driving circuit for switches of direct current fan motor, which uses driving signals for the opposite switches to drive a control circuit and forcibly lock the self switches to be in an OFF state when the self switches are shifted from an ON state to an OFF state, thereby avoiding malfunction. 
   The driving circuit for switches of a fan motor according to the invention has a plurality of switches, a first control circuit and a second control circuit. The switches are driven by a first pulse width modulation (PWM) signal and a second PWM signal, and are connected with the direct current fan motor in a bridge manner. In addition, the first control circuit is electrically connected with at least one switch driven by the first PWM signal, and is driven by a third PWM signal. The second control circuit is electrically connected with at least one switch driven by the second PWM signal, and is driven by a fourth PWM signal. 
   One characteristic of the invention is that, when the first PWM signal is at a low level, the switch driven by the first PWM signal is in an OFF state, and the third PWM signal drives the first control circuit and locks the switch driven by the first PWM signal to an OFF state, while when the second PWM is at a low level, the switch driven by the second PWM is in an OFF state, and the fourth PWM signal drives the second control circuit and locks the switch driven by the second PWM signal to an OFF state. 
   Another characteristic of the invention is that, either the first PWM signal or the second PWM signal is selected as the third PWM signal or the fourth PWM signal. 
   The invention is advantaged in that malfunction of the switches resulted from intrinsic counter-electromotive force is avoided, short circuit current between the switches is prevented, and damages to the fan motor by malfunction of the switches are avoided as well. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic view illustrating a structure of a prior driving circuit for switches of direct current fan motor. 
       FIG. 2  shows a schematic view illustrating a structure of a driving circuit for switches of direct current fan motor according to the invention. 
       FIG. 3  shows a schematic view illustrating a driving circuit for switches of direct current fan motor according to a first embodiment of the invention. 
       FIG. 4  shows a schematic view illustrating a driving circuit for switches of direct current fan motor according to a second embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 2 , a driving circuit for switches of direct current fan motor  10  according to the invention includes full-bridge connection switches  101 ,  102 ,  103  and  104 , and control circuits  105  and  106  that are electrically connected to the switches  103  and  104 , respectively. With respect to the operation of the invention, a pulse width modulation (PWM) signal A is used for driving the switches  102  and  103 , and a PWM signal B is used for driving the switches  101  and  104 , so as to conduct the direct current motor fan  10 . When the PWM signal A shifts from a high level to a low level, the PWM signal B shifts from a low level to a high level. At the instant that the PWM signal A shifts from a high level to a low level, the switches  102  and  103  change from an ON state to an OFF state. Meanwhile, a PWM signal C with an appropriate level (high or low) is used for driving the control circuit  105 , so that the control circuit  105  forcibly lock the switch  103  in an OFF state. Similarly, at the instant that the PWM signal B shifts from a high level to a low level, the switches  101  and  104  change from an ON state to an OFF state. In addition, a PWM signal D with an appropriate level (high or low) is also used for driving the control circuit  106 , so that the control circuit  106  forcibly lock the switch  104  in an OFF state. Especially, the PWM signal C is selected from either the PWM signal A or the PWM signal B while the PWM signal D is selected from either the PWM signal B or the PWM signal A. 
   According to the invention, the circuit structure of the direct current motor fan  10  for switches can be disposed with other control circuits based upon variations in duty cycles of the PWM signals A, B, C and D. For instance, when the PWM signal C is at a low level and is incapable of driving the control circuit  105 , a PWM signal E is used for driving a control circuit  107  and forcibly lock the switch  103  in an OFF state. Likewise, when the PWM signal D is at a low level and is incapable of driving the control circuit  106 , a PWM signal F is used for driving a control circuit  108  and forcibly lock the switch  104  in an OFF state. Especially, the PWM signals A and B are processed via a logic gate (not shown) to output the PWM signals E and F. 
   Therefore, it is ensured that malfunction as being ON instead of OFF of the switches  101  and  104 , or switches  102  and  103 , are not incurred by instantaneous counter-electromotive force generated by the direct current fan motor  10 . 
   Hereafter, detailed descriptions of preferred embodiments according to the invention shall be given with the accompanying drawings. Referring to  FIG. 3  showing a driving circuit for switches of direct current fan motor according to a first embodiment the invention, a coil  20  represents a direct current fan motor having an input voltage of V in . A PWM signal A is used to drive switches  202  and  203  while a PWM signal B is used to drive switches  201  and  204 . The switches  201  and  202  are p-channel enhancement-mode metal-oxide semiconductor field-effect transistors (MOSFET), and the switches  203  and  204  are n-channel enhancement-mode MOSFET. The characteristics of the embodiment are that, the PWM signal A, an applied voltage V 1  and a control circuit  205  are further used to control the switch  203 , and the PWM signal B, an applied voltage V 2  and a control circuit  206  are further used to control the switch  204 , to make the potential difference between gates and sources of the switches  203  and  204  in an OFF state be virtually zero. As a result, the switches  203  and  204  are forcibly locked in an OFF state for preventing malfunction of the switches  203  and  204 . In this embodiment, the control circuit  205  and control circuit  206  are identical, and have two n-channel enhancement-mode MOSFET switches  2051  and  2052 , and  2061  and  2062 , respectively. 
   The principles of driving the switches of fan motor are described as follows. 
   At first, the switches  202 ,  203  and  2051  are actuated by the PWM signal A with a high level to be in an ON state while the switch  2052  is kept in an OFF state, and the direct current fan motor is thus conducted with a current direction indicated as an arrow  207 . 
   Subsequently, the switches  202 ,  203  and  2051  are shut down by the PWM signal A with a low level to be in an OFF state while the switch  2052  is driven by the applied voltage V 1  to be in an ON state and forms a loop with the switch  203  to force a short circuit between a gate G and a drain D of the switch  203 . As for the switch  204 , since the PWM signal B is still at a low level, the switch  2061  is in an OFF state while the switch  2062  is driven by the applied voltage V 2  to be in an ON state and forms a loop with the switch  204  to force a short circuit between a gate G and a source S of the switch  204 . Consequently, at an instant that the switches  202  and  203  are shut down, the counter-electromotive force produced within the direct current fan motor does not lead to malfunction of being ON for the switches  203  and  204 . Therefore, it is ensured that the switches  203  and  204  are normal in operation. 
   Similarly, the switches  201 ,  204  and  2061  are actuated by the PWM signal B with a high level to be in an ON state while the switch  2062  is kept in an OFF state, and the direct current fan motor is thus conducted with a current direction indicated as an arrow  208 . 
   When the switches  201 ,  204  and  2061  are shut down by the PWM signal B with a low level to be in an OFF state while the switch  2062  is driven by the applied voltage V 2  to be in an ON state and forms a loop with the switch  204  to force a short circuit between a gate G and a drain D of the switch  204 . As for the switch  203 , since the PWM signal A is still at a low level, the switch  2051  is in an OFF state while the switch  2052  is driven by the applied voltage V 1  to be in an ON state and forms a loop with the switch  203  to force a short circuit between a gate G and a source S of the switch  203 . Consequently, at an instant that the switches  201  and  204  are shut down, the counter-electromotive force produced within the direct current fan motor does not lead to malfunction of being ON for the switches  204  and  203 . 
   It is to be noted that, the switches  203  and  204  in the embodiments adopt intrinsic driving signals, i.e., the PWM signals A and B, to drive the control circuits  205  and  206  to further lock the OFF state of itself. In other words, the driving circuit for switches in this embodiment is a self-locking circuit. 
   Referring to  FIG. 4  showing a driving circuit for switches of direct current fan motor according to a second embodiment of the invention, a coil  30  represents a direct current fan motor having an input voltage V in . A PWM signal A is used to drive switches  302  and  303  while a PWM signal B is used to drive switches  301  and  304 . The switches  301  and  302  are p-channel enhancement-mode metal-oxide semiconductor field-effect transistors (MOSFET), and the switches  303  and  304  are n-channel enhancement-mode MOSFET. The characteristics of the embodiment are that, the PWM signal B, a control circuit  305 ,  307 , and a PWM signal G are further used to control the switch  303 , and the PWM signal A, a control circuit  306 ,  308 , and the PWM signal G are further used to control the switch  304 , to make the potential difference between gates and sources of the switches  303  and  304  in an OFF state be virtually zero. As a result, the switches  303  and  304  are forcibly locked in an OFF state for preventing malfunction of the switches  303  and  304 . In this embodiment, the control circuit  305  and control circuit  306  are identical and have two n-channel enhancement-mode MOSFET switches  3051  and  3061 , respectively, and the control circuit  307  and control circuit  308  are identical and have two n-channel enhancement-mode MOSFET switches  3071  and  3081 , respectively. In addition, the PWM signal G is determined by using a NOR-logic gate  309  to process the levels of the PWM signals A and B. For instance, when the PWM signals A and B are both low-level signals, the PWM signal G is a high-level signal. 
   The principles of driving the switches of direct current fan motor in this embodiment shall be described. 
   When the switches  302  and  303  are actuated by the PWM signal A with a high level to be in an ON state, the direct current fan motor is thus conducted with a current direction indicated as an arrow  311 . 
   In a short period that the PWM signal A shifts from a high level to a low level to make the switches  302  and  303  be in an OFF state, the switch  3051  is in an OFF state for that the PWM signal B is still at a low level. At this point, the PWM signal G is mainly used to force short circuits between the gates and sources of the switches  303  and  304 . In other words, when the PWM signals A and B are both at a low level, the NOR logic gate  309  outputs a high-level signal to make the switches  3071  and  3081  be in an ON state and form loops with the switches  303  and  304 , thereby forcibly locking the switches  303  and  304  in an OFF state. 
   Similarly, When the switches  301  and  304  are actuated by the PWM signal B with a high level to be in an ON state, the direct current fan motor is thus conducted with a current direction indicated as an arrow  312 . In a short period that the PWM signal B shifts from a high level to a low level to make the switches  301  and  304  be in an OFF state, the switch  3061  is in an OFF state for that the PWM signal A is still at a low level. At this point, the PWM signal G is mainly used to force short circuits between the gates and sources of the switches  303  and  304 . 
   Consequently, at the instant that the switches  302  and  303  are shut down, or at the instant that the switches  301  and  304  are shut down, the counter-electromotive force produced within the direct current fan motor does not produce interference against the switches  303  or  304 , nor does it lead to malfunction of being ON for the switches  303  and  304 . 
   In this embodiment, the switches  303  adopts the PWM signal B of the switch  304  to drive the control circuits  305  and to further lock an OFF state of itself, and the switch  304  adopts the PWM signal A of the switch  303  to drive the control circuit  306  and to further lock an OFF state of itself. Therefore, the driving circuits for switches in this embodiment are mutual-locking circuits. 
   The embodiments and examples according to the invention are fully illustrated as in the above descriptions. For those who are skilled in this art, it is understood that the embodiments according to the invention are illustrative but not limitative. For instance, the direct current fan motor  10  may also be in semi-bridge connection with the switches  103  and  104 . Without departing from the true spirit and scope of the invention, various modifications and changes of the direct current fan motor shall be included by the appended claims of the invention.