Abstract:
A method and apparatus for dynamically adjusting a dead time of a BLDC motor during a phase change detect the winding current of the BLDC motor during the dead time, and terminate the dead time when the winding current is detected to be substantially or close to zero. Thus, the method and apparatus can optimize the dead time and switch the BLDC motor between two phases at a zero-current point, without reducing the maximum rotation speed of the BLDC motor.

Description:
FIELD OF THE INVENTION 
     The present invention is related generally to a brushless direct-current (BLDC) motor and, more particularly, to a driving circuit and method for a BLDC motor. 
     BACKGROUND OF THE INVENTION 
     A brushless direct-current (BLDC) motor uses semiconductor switches to accomplish electronic phase changes, and thus has advantages, such as less mechanical wear and lower noise, as compared to a motor using a mechanical rectifier that is established by carbon brushes and commutators. 
     As shown in  FIG. 1 , a driving circuit for a BLDC motor  10  includes an H-bridge circuit, which is established by four MOSFETs Q 1 -Q 4  that act as switches and have body diodes D 1 -D 4 , respectively, and has two output terminals A and B to be connected to the BLDC motor  10 , a pulse width modulation (PWM) controller  12 , which has four output terminals AH, AL, BH, and BL to provide PWM signals to control the MOSFETs Q 1 -Q 4 , respectively, to thereby generate an operational voltage as required between the output terminals A and B to adjust the winding current Im of the BLDC motor  10  and accordingly the rotation speed of the BLDC motor  10 , and an over-current protection (OCP) circuit  14  connected between the H-bridge circuit and a ground terminal to detect the winding current Im for providing the system an over-load protection. When the winding current Im is so large to indicate that the system is at an over-load state, the OCP circuit  14  will signal the PWM controller  12  to stop providing the PWM signals, or to directly turn off the lower-side MOSFETs Q 2  and Q 4 , thereby stopping the BLDC motor  10 . 
     For the sake of convenient illustration, the MOSFETs Q 1 -Q 4  are called the first upper-side switch, the first lower-side switch, the second upper-side switch, and the second lower-side switch, respectively. The driving process of the BLDC motor  10  is shown in  FIG. 2 . For example, as shown in  FIG. 2(   a ), when the rotor of the BLDC motor  10  is at one of the phases, the PWM controller  12  maintains the second upper-side switch Q 3  off and the second lower-side switch Q 4  on, and alternatively switches the first upper-side switch Q 1  and the first lower-side switch Q 2  by a PWM signal, so that the winding current Im flows from the output terminal A through the BLDC motor  10  to the output terminal B; when the rotor is at another phase, as shown in  FIG. 2(   b ), the PWM controller  12  maintains the first upper-side switch Q 1  off and the first lower-side switch Q 2  on, and alternatively switches the second upper-side switch Q 3  and the second lower-side switch Q 4 , so that the winding current Im flows from the output terminal B through the BLDC motor  10  to the output terminal A. For the sake of convenient illustration, the phase depicted in  FIG. 2(   a ) is referred to as the first phase, and that depicted in  FIG. 2(   b ) is referred to as the second phase. When the BLDC motor  10  is switched between the first and the second phases, if the switch timing is not properly controlled, a phase-change surge current will occur due to the residual current Im in the windings of the BLDC motor  10  and thereby induce a reactive electromotive force to boost the voltage at the power input terminal Vin, as shown in  FIG. 3  for example, which may damage corresponding components. 
     In order to prevent the voltage Vin from instantly being boosted during a phase change, a dead time is inserted when the BLDC motor  10  is switched between different phases, for the winding current Im to decay to zero before the BLDC motor  10  is switched from the current phase to the next phase. For example, referring to  FIG. 4 , to switch from the first phase to the second phase, during the dead time inserted therebetween, the PWM controller  12  maintains the second lower-side switch Q 4  on and the other switches Q 1 -Q 3  off, thereby establishing a current loop to allow the winding current Im to be consumed by the second lower-side switch Q 4  and the body diode D 2 , or maintains the first lower-side switch Q 2  and the second lower-side switch Q 4  on and the other switches Q 1  and Q 3  off, thereby establishing a current loop to allow the winding current Im to be consumed by the first lower-side switch Q 2  and the second lower-side switch Q 4 . However, the winding current Im varies with the rotation speed of the BLDC motor  10  and thus requires different time periods to decay to zero at different rotation speeds, i.e., different rotation speeds require different dead times. For instance, if the dead time is such set that the residual current Im can be completely consumed at the rotation speed of 50%, then for the rotation speed of 70%, the residual current Im will be still high enough to cause a phase-change surge current after the dead time terminates. On the contrary, if the dead time is set longer, the maximum rotation speed of the BLDC motor  10  will be adversely limited. 
     Therefore, it is desired a method and apparatus for dynamically adjusting a dead time of a BLDC motor during a phase change. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a method and apparatus for dynamically adjusting a dead time of a BLDC motor during a phase change. 
     Another objective of the present invention is to provide a method and apparatus for optimizing a dead time of a BLDC motor during a phase change. 
     A further objective of the present invention is to provide a method and apparatus for switching a BLDC motor between two phases at a zero-current point. 
     According to the present invention, the winding current of a BLDC motor during a phase change is detected, and the dead time for the phase change is terminated when the winding current is detected to be equal to or smaller than a zero-current threshold. Since the dead time is dynamically adjusted depending on the winding current, the dead time can be optimized and the phase can be switched at a zero-current point. In addition, since the dead time is dynamically adjusted, it will not reduce the maximum rotation speed of the BLDC motor, and is adaptive to BLDC motors of various rotation speeds. 
     In one embodiment, during a dead time, the lower-side switch that is conventionally maintained on will be temporarily turned off for the winding current to be detected. 
     In one embodiment, during a dead time, the lower-side switch that is conventionally maintained on will be temporarily turned off, and an upper-side switch at the same side of this temporarily turned off lower-side switch will be temporarily turned on during a time interval of temporarily turning off this lower-side switch, for the winding current to be detected. 
     In one embodiment, during a dead time, the lower-side switch that is conventionally maintained on will be temporarily turned off at regular time intervals, for the winding current to be detected. 
     In one embodiment, during a dead time, the lower-side switch that is conventionally maintained on will be temporarily turned off at regular time intervals, and an upper-side switch at the same side of this temporarily turned off lower-side switch will be temporarily turned on during a time interval of temporarily turning off this lower-side switch, for the winding current to be detected. 
     In one embodiment, during a dead time, the PWM controller applies a short pulse to temporarily turn off the lower-side switch that is conventionally maintained on. 
     In one embodiment, during a dead time, the PWM controller applies a short pulse to temporarily turn off the lower-side switch that is conventionally maintained on and turn on an upper-side switch at the same side of this temporarily turned off lower-side switch. 
     In one embodiment, during a dead time, a detection voltage is generated depending on the winding current under detection, and is then compared to a zero-voltage threshold for asserting a control signal to terminate the dead time when the detection voltage is equal to or greater than the zero-voltage threshold. 
     In one embodiment, a current detector detects the winding current during a dead time, and signals a PWM controller to terminate the dead time when the winding current is detected to be equal to or smaller than a zero-current threshold. 
     In one embodiment, the current detector includes a resistor for generating the detection voltage depending on the winding current during a dead time, and a comparator for comparing the detection voltage to the zero-voltage threshold to assert a control signal for the PWM controller to terminate the dead time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objectives, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram of a conventional driving circuit for a BLDC motor; 
         FIG. 2  is a diagram showing the current paths of a BLDC motor at two different phases; 
         FIG. 3  is a waveform diagram of phase-change surge currents of a BLDC motor during phase changes; 
         FIG. 4  is a diagram showing a dead time that is conventionally inserted when a BLDC motor is switched between two phases; 
         FIG. 5  is a circuit diagram of an embodiment according to the present invention; 
         FIG. 6  is a waveform diagram of the circuit shown in  FIG. 5  during a phase change; 
         FIG. 7  is a timing diagram for the circuit shown in  FIG. 5  during a phase change; and 
         FIG. 8  is another timing diagram for the circuit in  FIG. 5  during a phase change. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     To dynamically adjust a dead time of a BLDC motor  10  during a phase change, in an embodiment as shown in  FIG. 5 , an apparatus according to the present invention includes a current detector  16  connected to the H-bridge circuit and the PWM controller  12 , to detect the winding current Im of the BLDC motor  10  during the dead time and to signal the PWM controller  12  to terminate the dead time when the winding current Im is detected to be substantially or close to zero. To determine a time point for the phase change, a zero-current threshold is set as a reference to indicate that the winding current Im is equal to or close to zero for the current detector  16 . When the current detector  16  detects that the winding current Im is equal to or smaller than the zero-current threshold, it is confirmed that the operation of the next phase can be started. The zero-current threshold may be set with consideration to the tolerance for surge currents. If the zero-current threshold is set with a value larger than zero, a surge current may be induced at the beginning of the next phase, while a higher rotation speed will be achieved due to the shorter dead time. Preferably, the zero-current threshold is set to be slightly larger than zero. In an optimal case, a phase change with zero current can be achieved. 
     In the embodiment shown in  FIG. 5 , the current detector  16  includes a resistor R connected between the H-bridge circuit and a ground terminal, and comparators  18  and  20  to compare the voltage V_R across the resistor R to thresholds V_ZCD and V_OCP, respectively, to determine control signals S 1  and S 2  for the PWM controller  12 . This embodiment integrates the apparatus for dynamically adjusting the dead time of the BLDC motor  10  during a phase change into the existing OCP circuit. Similar to prior arts, the threshold V_OCP represents a setting value of over-current protection, and when V_R≧V_OCP, the signal S 2  is asserted so that the PWM controller  12  will stop providing the PWM signals or directly turn off the lower-side MOSFETs Q 2  and Q 4 , thereby stopping the BLDC motor  10 . For switching from the first phase to the second phase, alike the mentioned prior art, during the dead time, the PWM controller  12  maintains the MOSFETs Q 1 -Q 3  off and the MOSFET Q 4  on, to establish a current loop as shown in  FIG. 4  for consuming the winding current Im. Additionally, however, for dynamically adjusting the dead time, the PWM controller  12  temporarily turns off the MOSFET Q 4  during the dead time, which will make the current Im flow to a power input terminal Vin from the output terminal B through the body diode D 3  of the MOSFET Q 3 , and flow to the body diode D 2  of the MOSFET Q 2  from the ground terminal through the resistor R, so that a negative detection voltage is generated at the detection node V_R, by which the winding current Im during the dead time is detected. Various approaches may be used for temporarily turning off the MOSFET Q 4 . For example, the PWM controller  12  may apply a short pulse to the gate of the MOSFET Q 4 . Preferably, as shown in  FIG. 6 , a short pulse Ts is sent to the gate of the MOSFET Q 4  at regular time intervals. In another embodiment, during each short pulse Ts, the MOSFET Q 3  is turned on during the time interval of temporarily turning off the MOSFET Q 4 , to allow the current Im to flow therethrough. Since the embodiment shown in  FIG. 5  detects the winding current Im by detecting the detection voltage V_R, a zero-voltage threshold V_ZCD corresponding to the zero-current threshold is set for the comparator  18 . Preferably, the zero-voltage threshold V_ZCD is set with a value slightly smaller than zero. During the short pulse Ts, when V_R≧V_ZCD, the signal S 1  is asserted and indicates that the winding current Im is substantially or close to zero, and according thereto, the PWM controller  12  may start to operate the BLDC motor  10  with the second phase. 
     Since the dead time is dynamically adjusted and independent of the rotation speed, the disclosed method and apparatus are suitable for use at different rotation speeds. 
     Generally, the PWM controller of the conventional driving circuit of a BLDC motor has a shortest pulse setting for defining the minimum on time or the minimum off time of the PWM signal. This shortest pulse may be used to generate the disclosed short pulse Ts for detection of the winding current Im. For example, referring to  FIG. 7 , for the first phase, the PWM controller  12  maintains the MOSFET Q 3  off and the MOSFET Q 4  on, and determines the duty cycle of the PWM signal according to the required rotation speed for switching the MOSFETs Q 1  and Q 2 . For the dead time, the PWM controller  12  maintains the MOSFETs Q 1 -Q 3  off, and uses the PWM signal with the minimum off time to control the MOSFET Q 4 ; while terminating the dead time once detecting the winding current Im equal to or smaller than the zero-current threshold. For the second phase, the PWM controller  12  maintains the MOSFET Q 1  off and the MOSFET Q 2  on, and determines the duty cycle of the PWM signal according to the required rotation speed for switching the MOSFETs Q 3  and Q 4 . Alternatively, during the dead time, the MOSFET Q 3  may be maintained on to allow the current Im to flow therethrough, and thus, in another embodiment as shown in  FIG. 8 , during the dead time, the PWM signal with the minimum on time is used to control the MOSFET Q 3 , and the PWM signal with the minimum off time is used to control the MOSFET Q 4 . 
     As illustrated by the embodiment shown in  FIG. 5 , according to the present invention, only a simple circuit is enough to dynamically adjust the dead time, and moreover, the simple circuit can be incorporated into an existing OCP circuit. In other embodiments, the current detector  16  may be realized by different circuits. 
     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.