Abstract:
An over-voltage protection coil control circuit has at least one coil driving circuit, wherein an over-voltage protection circuit is connected to the at least one coil driver circuit. The at least one coil driver circuit consists of a transistor. The over-voltage protection circuit is composed of a Zener diode connected to a power supply and the at least one coil driver circuit. When the coil generates a high inverse emf (electromotive force) due to the inversion of its polarity, the Zener diode conducts and guides the high voltage inverse emf to the power supply.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The invention relates to an over-voltage protection coil control circuit and, in particular, to a control circuit having at least one coil driver circuit that can change the voltage threshold in an over-voltage protection circuit according to the voltage of the power supply.  
         [0003]     2. Related Art  
         [0004]     To keep the high inverse emf (electromotive force) produced when the polarities of the coils in a brushless DC fan or DC motor reverse from damaging the driver circuit, a method currently used connects a simple protection circuit consisting of a Zener diode to the coil. The protection circuit shunts the sudden high inverse emf to ground and thus protects the coil driver circuit.  
         [0005]     With reference to  FIG. 4 , a conventional over-voltage protection circuit  60  is used in the control circuit of a brushless DC fan. The control circuit includes a Hall sensor  50 , an amplifier  51 , a pulse generator  52 , one or more driver circuits  53  and over-voltage protection circuits  60  corresponding to each driver circuit  53 . The Hall sensor  50  detects the polarity changes on the two coils L 1 , L 2  in the brushless fan. The amplifier  51  connects to the output terminal of the Hall sensor  50  to amplify the detection signal of the Hall sensor  50 . The pulse generator  52  connects to the output terminal of the amplifier  51 . Each driver circuit  53  consists of transistors Q 1 , Q 2 . The base of each transistor Q 1 , Q 2  is connected to a corresponding output terminal of the pulse generator  52  and the collector connects to the corresponding coil L 1 , L 2  of the brushless fan. Each of the over-voltage protection circuits  60  connects between the corresponding collector of the transistors Q 1 , Q 2  in the driver circuits  53  and ground. The over-voltage protection circuit  60  is a Zener diode Z 1 , Z 2 .  
         [0006]     At the instant the two coils L 1 , L 2  interchange, the coil L 1  through which the current stops passing effectively discharges and generates a high inverse emf, which drives the current to the collector of the transistor Q 1  in the driver circuit  53 . Due to the operation of the Zener diode Z 1  in the protection circuit when the inverse emf reaches the Zener diode&#39;s Z 1  breakdown voltage, the inverse emf is guided through the Zener diode Z 1  to ground. The operation of the Zener diode Z 1  ensures that the full force of the inverse emf is not applied to the transistor Q 1 , which would cause it to burn out. Since the breakdown voltage of the Zener diodes Z 1 , Z 2  in the over-voltage protection circuits  60  is a characteristic of the particular diode, the reverse emf applied to the transistors Q 1 , Q 2  does not decrease even when the power supply voltage is lowered.  
         [0007]     With reference to  FIGS. 5A and 5B , the output voltage at the collector of the transistor Q 1  in the driver circuit  53  is shown when the control circuit is under different work voltage. In  FIG. 5A , the power supply voltage of the control circuit is 12V and the breakdown voltage of the Zener diode Z 1  is VZ. When a high inverse emf is produced, Z 1  transmits immediately due to the instantaneous high voltage, limiting the collector voltage of the transistor Q 1  to a constant VZ. In  FIG. 5B , when the power supply voltage of the control circuit is lowered to 5V, the high inverse emf is produced when the current through the coils L 1 , L 2  is switched. Because the Zener diode Z 1  still breaks down at the same voltage, the peak voltage at the collector of the transistor Q 1  is still VZ.  
         [0008]     Most current brushless DC fans are equipped with many controls. For example, the fan speed can be adjusted by changing the power supply voltage. As previously described however, the control protection circuit  60  limits the coil generated high inverse emf to a constant value VZ but cannot change the limit voltage value according to different power supply voltages. Therefore, such control protection circuits are not completely satisfactory.  
       SUMMARY OF THE INVENTION  
       [0009]     The objective of the invention is to provide an over-voltage protection coil control circuit, wherein the coil control circuit has an over-voltage protection circuit. The over-voltage protection circuit can limit the inverse emf voltage on the coil according to the power supply voltage variation.  
         [0010]     To achieve the foregoing objective, the main technique of the invention is to connect the over-voltage protection circuit to one or more than one sets of driver circuits. Each driver circuit consists of a transistor and is connected to a coil. When the coil produces a high inverse emf due to the change in polarity, the over-voltage protection circuit can guide the high inverse emf to ground. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a block diagram of an over-voltage protection coil control circuit in accordance with the present invention;  
         [0012]      FIG. 2  is a circuit diagram of an embodiment of an over-voltage protection coil control circuit in accordance with the present invention;  
         [0013]      FIGS. 3A and 3B  are voltage response graphs illustrating the output voltage of the transistor N 1  in the driver circuit in  FIG. 1 ;  
         [0014]      FIG. 4  is a circuit diagram of a conventional coil control circuit with a power-voltage protection circuit in accordance with the prior art; and  
         [0015]      FIGS. 5A and 5B  are voltage response graphs illustrating the output voltage of the transistor Q 1  in the drive circuit in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     With reference to  FIG. 1 , an over-voltage protection coil control circuit comprises a Hall sensor  10 , an amplifier  11 , a pulse generator  12 , an over-voltage protection circuit  20  and a driving unit  9  and two coils L 1 , L 2 . When the Hall sensor  10  detects a polarity change in coils L 1  or L 2 , a Hall signal is output and amplified by the amplifier  11 . The amplified signal causes the pulse generator  12  to generate HIGH and LOW level voltage signals to control the driving unit  9  to drive coils L 1  or L 2 . When the coils L 1  or L 2  change its polarity, the coils L 1  or L 2  generate a high inverse emf (electromotive force), and then the over-voltage protection circuit  20  immediately limits the high inverse emf so as to protect the driving unit  9  and the coils L 1  and L 2 .  
         [0017]     With reference to  FIG. 2 , the detailed circuit of the driving unit  9  and the over-voltage protection circuit  20  is clearly shown. The driving unit  9  comprises two driver circuit  13 ,  14 . The Hall sensor  10  detects the polarity changes on the two coils L 1 , L 2  in the DC fan. The amplifier  11  connects to the output terminal of the Hall sensor  10  to amplify the detection signal from the Hall sensor  10 . The pulse generator  12  connects to the output terminal of the amplifier  11 . Each of the driver circuits  13 ,  14  is comprised of an FET (Field Effect Transistor) N 1 , N 2 . The drains of the FETs N 1 , N 2  connect to the over-voltage protection circuit  20  and connect to the respective coil L 1 , L 2  of the brushless fan. The gates of the FETs N 1 , N 2  connect to the respective output terminal VA, VB of the pulse generator  12 .  
         [0018]     Each of the FETs N 1 , N 2  can be replaced by a BJT (Bipolar Junction Transistor). If the BJT is used, its base is connected to the pulse generator  12 , and its collector is connected to the corresponding coil.  
         [0019]     The over-voltage protection circuit  20  includes an Zener diode Z 3 , and two diodes D 1 , D 2 . The drains of the FETs N 1 , N 2  in the driver circuits  13 ,  14  connect to the negative poles of diodes D 1 , D 2 , respectively. Both of the positive poles of the two diodes D 1 , D 2  connect to the negative pole of the Zener diode Z 3 . The positive pole of the Zener diode Z 1  connects to the power supply VDD.  
         [0020]     The circuit operation will be explained by using the over-voltage protection circuit  20  and FET N 1  in the driver circuit  13  as an example. When the Hall sensor  10  detects a polarity change in coil L 1 , a Hall signal is output and amplified by the amplifier  11 . The amplified signal causes the pulse generator  12  to generate HIGH and LOW level voltage signals to control the FET N 1 . When the gate of the FET N 1  receives the HIGH level voltage signals, the FET N 1  is driven to ON.  
         [0021]     When the output terminal VA of the pulse generator  12  turns from HIGH to LOW, the original conducting FET N 1  changes to OFF. When current stops moving through the coil L 1 , the coil L 1  generates a high inverse emf on the drain of the FET N 1  at the instant the current is switched. The circuit in the current embodiment adjusts the limit voltage value for the inverse emf according to the breakdown voltage VZ of the Zener diode Z 1 . The potential of the inverse emf is greater than the voltage level of the limit voltage (VDD+VZ), thus the Zener diode Z 3  and the diode D 1  conduct so as to lead the high voltage away from the power supply when the coil L 1  generates a high inverse emf due to the polarity changes.  
         [0022]     Analogously, when the coil L 2  changes its polarity, the actions of the FET N 2 , the diode D 2  and the Zener diode Z 3  are the same as above, and thus the description will not be repeated.  
         [0023]     In  FIG. 3A , the power supply voltage VDD of the control circuit is 12V and the breakdown voltage VZ is 5.5V. When a high inverse emf is produced, the voltage at the drain of the FET N 1  is limited to 17.5V. In  FIG. 3B , when the power supply voltage of the control circuit is lowered to 5V and the breakdown voltage VZ is still 5.5V, the high inverse emf at the drain of the FET N 1  is limited to 10.5V. In comparison with the voltage response of the conventional over-voltage protection circuit as reflected in  FIGS. 3A and 3B , it is clear that the over-voltage protection circuit  20  in accordance with the present invention adapts the peak voltage of the high inverse emf based on the power supply voltage VDD.  
         [0024]     As described, the over-voltage protection circuit uses a simple circuit to achieve the object of protecting the coil driver circuit in the control circuit of a fan. It can change and fix the limit voltage as the power supply voltage is changed. Thus, the protection circuit can adjust the limit reference voltage according to the needs of various types of coil driver circuits (e.g. DC motors and coil driver circuits in brushless DC fans). Through the adjustment of the limit reference voltage, the invention achieves the goal of tracking the limit voltage of the power supply variation and elongating the life of a driver circuit.  
         [0025]     The invention may be varied in many ways by a person skilled in the art. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following claims.