Abstract:
A power system is provided comprising an electric universal motor including an armature rotatable coupled to an armature shaft and a commutator disposed on an armature shaft, a pair of brushes engaging the commutator, and a field having at least two field windings electrically coupled in series with the pair of brushes. The power system includes a power line having two terminals arranged to provide alternating-current (AC) power from a power supply, and a power switch provided in series with the field windings on a power line to provide AC power from the terminals to the motor when the power switch is closed. An electronic brake module is provided in the power system and configured to generate a braking force to stop the motor when the switch is opened, the electronic brake module comprising: a solid-state semiconductor switch arranged across the motor armature and the pair of brushes, a first diode arranged between a first node of the power line and the semiconductor switch, and a second diode arranged between a second node of the power line and the semiconductor switch, wherein the first node is arranged between one of the terminals and the power switch, and the second node is arranged between the power switch and the armature. A controller is provided in the power system and configured to initiate a braking mode of operation to close the semiconductor switch when the power switch is opened.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application 62/000,758 filed May 20, 2014, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an electric motor in a power system and particularly to electronic braking of a universal electric motor in a power tool. 
     BACKGROUND 
     Alternating Current (AC) universal electric motors are used in a wide variety of applications involving power tools such as drills, saws, sanding and grinding devices, yard tools such as edgers and trimmers, just to name a few. These devices all make use of electric motors having an armature and a field, such as a stator. The armature is typically formed from a lamination stack or core around which a plurality of windings of magnet wires are wound. The lamination stack is formed to have a plurality of poles around which the magnet wires are wound. In this regard, the lamination stack may be formed with a plurality of slots in which the magnet wires are wound. The magnet wires are coupled at their ends to a commutator, such as to tangs when the commutator is a tang type commutator, disposed on an armature shaft extending coaxially through the lamination stack. The commutator is in contact with one or more brushes, which energize the magnet wires to cause rotation of the armature inside the stator. 
     In conventional power tools, when the user stops operating the tool by, for example, releasing the tool trigger switch or turning off the power switch, the electric motor is disconnected from the power source and allowed to coast down. Coasting often takes a long time and is undesirable to the user. 
     As an alternative method to coasting, braking mechanisms have been offers to bring the motor into a halt. One such mechanism is a mechanical brake, which engages the motor shaft and/or tool transmission to stop the rotation of the motor. Alternatively, electronic braking mechanisms may be employed to brake the motor in a controlled fashion. Electronic brake modules often include switching mechanism to short the armature windings and use the current generated by the back electromotive force (EMF) of the motor armature to slow down the armature. This may be done by running current through dedicated brake windings provided in the proximity of the armature windings in the opposite direction of the back EMF current. Alternatively, current from the AC mains may be directed to one or more of the motor field windings in the opposite direction of the back EMF current to slow down the armature. 
     Conventional electronic brake modules typically utilize multiple mechanical switch or relays for braking. Such circuits tend to be complex and costly. What is needed is a braking circuit arrangement that minimizes switch contacts in order to reduce cost and space. 
     SUMMARY 
     In an embodiment of the invention, a power system is provided comprising an electric universal motor including an armature rotatable coupled to an armature shaft and a commutator disposed on an armature shaft, a pair of brushes engaging the commutator, and a field having at least two field windings electrically coupled in series with the pair of brushes. The power system further comprises a power line having two terminals arranged to provide alternating-current (AC) power from a power supply, and a power switch provided in series with the field windings on a power line to provide AC power from the terminals to the motor when the power switch is closed. In an embodiment, the power system comprises: an electronic brake module configured to generate a braking force to stop the motor when the switch is opened, the electronic brake module comprising: a solid-state semiconductor switch arranged across the motor armature and the pair of brushes, a first diode arranged between a first node of the power line and the semiconductor switch, and a second diode arranged between a second node of the power line and the semiconductor switch, wherein the first node is arranged between one of the terminals and the power switch, and the second node is arranged between the power switch and the armature. In an embodiment, the power system comprises a controller configured to initiate a braking mode of operation to close the semiconductor switch when the power switch is opened. 
     In an embodiment, the controller is configured open the semiconductor switch when the power switch is closed to initiate a normal mode of operation. 
     In an embodiment, in the braking mode of operation, current from the AC power source flows via a first current path through the first diode, the semiconductor switch, and at least one of the field windings. In an embodiment, in the braking mode of operation, current associated with the motor armature voltage flows via a second current path through the second diode and the semiconductor switch. 
     In an embodiment, one of the field windings is arranged between the armature and the power switch such that, in the braking mode of operation, current from the AC power source flows through the other field winding. In an alternative embodiment, neither of the field windings is arranged between the armature and the power switch such that, in the braking mode of operation, current from the AC power source flow through both field windings. 
     In an embodiment, the controller configured to monitor voltage across the power switch to determine if the power switch is open or close. 
     In an embodiment, the power system further comprises a phase-controlled switch disposed in series with the field windings on the power line to control the supply of AC power from the terminals to the motor. In an embodiment, the controller is configured to control a phase of the phase-controlled switch according to a desired speed level of the motor in a normal mode of operation. 
     In an embodiment, the controller is configured to monitor voltage across the controllable switch to determine if the power switch is open or closed. 
     In an embodiment, the controller is configured to control a phase of the phase-controlled switch to optimize at least one of a baking time or braking torque associated with the motor in the braking mode of operation. In an embodiment, the controller is configured to control a phase of the phase-controlled switch to provide a conduction angle of forty degrees or lower. In an embodiment, the controller is configured to control the phase of the phase-controlled switch at a first conduction band within a first braking cycle and at a second conduction band different from the first conduction band within a second braking cycle. 
     In an embodiment, the controller is configured to introduce a delay period between the power switch being opened and the semiconductor switch getting closed. 
     In an embodiment, the electronic brake module comprises a gate driver configured to drive a gate of the semiconductor switch to close the semiconductor switch in the braking mode of operation based on a control signal from the controller. 
     In an embodiment, the power system is a power tool. In an embodiment, the power tool is a grinder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a side view of an exemplary power tool, in this case an angle grinder, with its housing partially removed, according to an embodiment; 
         FIG. 2  depicts a perspective view of the angle grinder of  FIG. 1 , according to an embodiment; 
         FIG. 3  depicts a block circuit diagram for a braking mechanism of a universal motor, according to an embodiment; 
         FIG. 4  depicts the circuit diagram of  FIG. 3  during the normal mode of operation of the power tool, according to an embodiment; 
         FIG. 5  depicts the circuit diagram of  FIG. 3  during the braking mode of operation of the power tool, according to an embodiment; 
         FIG. 6  depicts a waveform diagram of the various voltage and current measurements in the circuit of  FIGS. 3-5 , according to an embodiment; 
         FIG. 7  depicts more complete circuit diagram of  FIG. 3  including a block diagram of a gate driver, according to an embodiment; 
         FIG. 8  depicts a circuit diagram of a power supply and storage unit, according to an embodiment; 
         FIG. 9  depicts a circuit diagram of a level shifter, according to an embodiment; 
         FIG. 10  depicts a circuit diagram of a gate drive unit, according to an embodiment; 
         FIG. 11  depicts a circuit diagram of an under-voltage protection unit, according to an embodiment; 
         FIG. 12  depicts a circuit diagram of a power switch on/off sensor, according to an embodiment; 
         FIG. 13  depicts a modified version of block circuit diagram of  FIG. 3 , where both field windings are arranged on side of the armature, according to an embodiment; and 
         FIG. 14  depicts a perspective view of a brake module, according to an embodiment. 
     
    
    
     DESCRIPTION 
     With reference to  FIGS. 1 and 2 , an embodiment of an angle grinder  10  is shown. The embodiments of the present disclosure describe various features of an angle grinder and it will be readily appreciated that the described features may be applied to any angle grinder known in the art, including large angle grinders (LAG), medium angle grinders (MAG), and small angle grinders (angle grinder). It is noted that while angle grinder  10  is depicted as an example of a power tool in which aspects of the invention may be employed, the electronic Brake module and mechanism of the invention may be used in any power tool, including a drill, impact driver, hammer drill, hammer, chain saw, chop saw, circular saw, nailer, jig saw, concrete cutter, etc. 
     According to an embodiment, the angle grinder  10  preferably includes a housing  12  having a handle portion  14 , a field case  16 , and a gear case  18 . The handle portion  14  is preferably fixedly attached to a first end  20  of the field case  16  and the gear case  18  is preferably fixedly attached to a second end  22  of the field case  16 . The handle portion  14  preferably supports a switch  24  and associated components. The handle portion  14  also preferably supports a particle separation assembly  26 . The field case  16  preferably supports a motor  28  having a motor spindle  30  that extends into the gear case  18  for driving gearset  32  supported therein. A wheel spindle  34  preferably extends from gear case  18  and is driven by the motor spindle  30  through the gearset  32 . The axis of rotation of motor spindle  30  is generally perpendicular to the axis of rotation of the wheel spindle  34 . A grinder wheel  36  is preferably selectively attachable to the wheel spindle  34  and is rotatably driven thereby. The motor  28  may also have a second spindle  38  that extends into the handle portion  14  for rotatably driving a fan  40 , associated with the particle separation assembly  26 . 
     In an embodiment, the motor  28  preferably is in electrical communication with the switch  24  through wires (not shown). Preferably, the switch  24  is further in electrical communication with a power source via a cord  42  including a plug (not shown). The handle portion  14  preferably includes an opening  44 , opposite the connection end, through which the cord  42  runs. A trigger  46  preferably is in mechanical communication with the switch  24  for selectively supplying power to the motor  28 . Mechanical actuation of the trigger  46  preferably results in actuation of the switch  24  thereby resulting in operation the angle grinder  10 . 
     Referring to  FIG. 3 , a block circuit diagram for a braking mechanism of a universal motor in a power system, such as motor  28 , is depicted, according to an embodiment of the invention. 
     In this embodiment, motor  28  is a universal series-wound brushed motor including two field windings  102   a  and  102   b , brushes  104   a  and  104   b , and an armature  105 . Motor  28  is coupled to nodes GND and VS of an AC power source (not shown). In an embodiment, provided in series with field winding  102   a  on the AC power line is an ON/OFF power switch  106 . Power switch  106  may be a mechanical switch coupled to an actuator that is turned on or off by the user. Alternatively, power switch  106  may be coupled to a variable-speed trigger switch, release of which opens the power switch  106 . It is noted that power tool  10  of the present invention may be a variables-speed tool having a trigger switch, a speed dial, etc., or a constant-speed tool having an on/off switch. It is further noted that power switch  106  may be provided anywhere on the AC power line. 
     According to an embodiment, provided in series with field winding  102   b  is a phase-controlled power switch, in this case a triac  110 , on the AC power line. Triac  110  is controlled by a controller  120 . Controller  120  is a speed controller that generates a speed control signal based on, for example, a trigger on line  112  that determines the firing angle of the triac  110 . The firing point of the triac  110  correlates to the conduction band of the AC power supplied to the motor  28 . In this manner, the controller  120  controls the speed of the motor  28 . It is noted that triac  110  may be provided anywhere on the AC power line in series with the field windings  102   a ,  102   b . It is also noted that triac  110  herein is provided as an example of a phase-controlled switch and any other type of phase-controlled switch such as one or more thyristors, silicon-controlled rectifier (SCR) switch, etc. may be used in place of triac  110 . 
     According to an embodiment, controller  120  may monitor the voltage across the power switch  106  to determine if power switch  106  is open or closed. Alternatively, controller  120  may monitor the voltage across triac  110  to determine if power switch  106  is open or closed. In the latter embodiment, since one node of the triac is couple to the GND terminal, some voltage develops across the triac  110  when power switch  106  is closed. Conversely, when power switch  106  is opened, no voltage is detected across the triac  110 . 
     According to an embodiment, an electronic brake module  200  is provided to electronically brake the motor  28  when power switch  106  is opened, i.e., the tool is turned off by the user or variable speed trigger is released. Brake module  200  includes a semiconductor switch Q 1  provided across the motor armature  105  and the brushes  104   a ,  104   b . In an embodiment, Q 1  may be any solid-state semiconductor power device. In an embodiment, switch Q 1  may be a MOSFET, although in high power applications operating with high voltage power sources (e.g., 230V), an insulated-gate bipolar transistor (IGBT) may be used instead. A MOSFET provides some advantages over an IGBT, namely, high switching capabilities. An IGBT has better power handling capability. In this disclosure, references are made to a gate, a source, and a drain as nodes of switch Q 1 . Those skilled in the art will appreciate that source and drain commonly refer to nodes for a MOSFET, and emitter and collector are commonly used to refer to nodes of an IGBT. It should be understood that any reference to a source or drain of Q 1  in this disclosure is exemplary and these terms respectively correspond and refer to an emitter and a collector where Q 1  is an IGBT. 
     In an embodiment, brake module  200  is controlled and activated by controller  120 . When controller  120  detects that power switch  106  has been opened, it initiates a brake control signal on line  122 . Brake module  200  includes a gate driver module  210  that is activated by line  122 . Gate driver module  210  in turn provides a voltage to the gate of Q 1  sufficient to turn Q 1  ON, as discussed below in detail. 
     In addition, in an embodiment, braking circuit  200  further includes two diodes D 1  and D 2 . In an embodiment, cathodes of D 1  and D 2  are both coupled together with the source of Q 1 . In an embodiment, the anode of D 1  is coupled to node  132 , which is located between power switch  106  and Vs terminal. In an embodiment, the anode of D 2  is coupled to node  134  located between field winding  102   a  and brush  104   a . The source of Q 1  is coupled to node  136  located between field winding  102   b  and brush  104   b.    
       FIG. 4  depicts the circuit diagram of  FIG. 3  during the normal operation of the power tool, according to an embodiment. In this mode, power switch  106  is closed, and motor current follows current path  302  and flows through the motor winding  102   a , the armature  105 , motor winding  102   b , and triac  110 . The Controller  120  controls the firing angle of the triac  110 , thereby regulating the conduction band of each AC half cycle flowing through the motor  32 . Meanwhile, current flowing through D 1  and D 2  through current paths  304  and  306  are cut off at Q 1 , which is turned OFF during normal motor run mode. 
       FIG. 5  depicts the circuit diagram of  FIG. 3  during motor brake mode, according to an embodiment. In this mode, power switch  106  is opened, and current no longer flows from the AC mains power line through field windings  102   a  and into the armature  105 . Instead, current from the AC mains power line flows along current path  304 , though D 1 , Q 1  (which has been turned ON by the controller  120  and the gate driver  210 ), field winding  102   b , and triac  110 . Meanwhile, the continued rotation of the armature  105  inside the field winding  102   b  generates a back EMF voltage as a result of the relative motion of the armature and a magnetic field. This voltage is positive at node  134 . After Q 1  is turned ON, it shorts nodes  134  and  136 , thus creating a current path along  306  though diode D 1  and switch Q 1 . This current provides the braking torque for field winding  102   b  to brake the rotation of armature  105 . 
     In this embodiment, the current through Q 1  is the sum of the currents in current paths  304  and  306  through diodes D 1  and D 2 . Specifically, contrary to some conventional designs where multiple power switches are utilizes to carry out electronic braking of the motor, the present design utilizes a single power switch Q 1  to carry both the field current supplied by the AC mains power line and the armature current generated by the back EMF voltage of the armature. 
     According to an embodiment, during the braking mode of the motor, controller  120  optimizes the braking time and braking torque by controlling the AC current running through the triac  110 . For example, in some applications it may be desirable to execute motor braking over a time span of 2-5 seconds to ensure that the motor doesn&#39;t come to a sudden halt, which may damage the tool or create a kickback for the user. Thus, controller  120 , according to an embodiment, is configured to fire the traic  110  at, for example, 140 to 160 degree firing angles (i.e., 20 to 40 conduction angles) to provide a smooth braking operation. 
     Two aspects and advantages of this embodiment are discussed herein. 
     First, when a single field winding  102   b  is utilized during the braking cycle as described above, controller  120  can execute braking over a longer conduction angle of the AC waveform. Specifically, when utilizing both field windings  102   a  and  102   b  during braking, the controller has to fire the triac  110  within narrow conduction angles in order to extend the total braking period. By way of example, when using a single field winding  102   b  as show in the illustrative figures, the controller  120  controls the triac  110  to conduct at 20 to 40 degree conduction angles in order to executing motor braking in 2 to 5 seconds. By comparison, in a circuit arrangement where both field windings  102   a  and  102   b  are used to brake the motor, as described later in this disclosure, the controller has to control the triac  110  to conduct at, for example, approximately 10-20 conduction angles to accomplish the same braking time. Although the latter arrangement may be advantageous in some embodiments, as discussed later, the former arrangement provides a greater conduction angle for the triac  110  to be fired, thus make it easier to obtain an accurate amount of conduction current. 
     Second, according to an embodiment, controller  120  may be configured to dynamically modify the firing angle of the triac  110  to execute a steady and smooth braking operation. Specifically, when braking mode is first initiated, armature rotational speed is relatively high, which causes higher amount of armature current through current path  306  and diode D 2 . As the armature slows down, armature current through current path  306  gradually decreases. In order to maintain a steady amount of total current through the power switch Q 1 , and moreover to create a smooth braking operation, according to an embodiment, the controller  120  is configured to incrementally increase the conduction angle of the AC power line via the triac  110 . In an embodiment, the controller  120  is configured to break the total brake period into segments and incrementally increase the conduction angle from one segment to the next. In an exemplary embodiment, where the total desired braking time is 3 seconds, the controller  120  may modify the conduction band from 15 degrees within the 1st second, to 20 degrees within the 2nd second, and 25 degrees within the 3rd second. 
       FIG. 6  depicts a waveform diagram of the various voltage and current measurements in the circuit of  FIGS. 3-5 , according to an embodiment of the invention. As shown herein, when the power tool  10  is off and the power switch  106  is open, there is no current or voltage across the triac  110 . In an embodiment, once the power switch is closed at  400 , controller  120  initially controls the triac  110  to conduct at a low conduction angle in order to provide a tool soft-start. Soft-start decreases the motor in-rush current that results from the lack of rotational momentum in the armature  105  during this time. After soft-start, the tool operates normally at  402  until the power switch  106  is opened again at  404 . In an embodiment, the controller  120  introduces a delay period between  404 , which is when the controller  120  senses the opening of the power switch  106 , and  406 , when the controller  120  initiates braking. This delay period may be, for example, 100 ms, and is introduced to ensure that the current through the triac  110  is dropped to zero before braking is initiated. Furthermore, in an embodiment where Q 1  is an IGBT, this delay period may be utilized to turn the gate of the IGBT ON. Depending on Q 1  timing requirements, the brake control signal at line  122  may be activated by the controller  120  during or at the end of the delay period. 
     In an embodiment, braking of the motor is initiated at  406  when or after the brake control signal is activated by the controller  120 . Q 1  gate control signal is also activated by the gate driver  210  on line  124 . Controller  120  controls and optimizes braking between  406  and  408 , when the braking is terminated. In one embodiment, the controller  120  may monitor the motor speed and determine when to terminate braking dynamically once the motor speed reaches zero. In another embodiment, the total braking period between  406  and  408  may be configured to be long enough, for example, 2-5 seconds, to ensure motor stoppage regardless of motor speed at the start of braking. 
     According to an embodiment, as discussed above, the controller  120  may optimize and control the triac  110  during the braking period between  406  and  408  to limit the current through the field winding  102   b  in order to compensate for the high armature current through the armature  105  in the beginning of the braking period. In the shown embodiment, the controller breaks down the braking period into three segments or cycles, and fires the triac at a different conduction angle during each cycle. In an exemplary embodiment, the controller fires the triac at 15-30 degree conduction during the first cycle, 20-35 degree conduction during the second cycle, and 25-40 degree conduction during the third cycle. 
     Turning now to  FIG. 7 , a more complete circuit diagram of  FIG. 3  is depicted, including a block diagram of the gate driver  210 , and connections to the gate driver  210  from nodes  132 ,  134  and  136  of the power tool, according to an embodiment. In an embodiment, gate driver  210  includes a power supply and storage unit  502 , a level shifter  504 , a gate drive unit  506 , an under-voltage protection unit  508 , and a power switch ON/OFF sensor  510 . 
     In an embodiment, the power supply and storage  502  is coupled to nodes  134  and  136  of the motor  28  (via lines  144  and  146  respectively), which allow its storage unit (discussed below) to be charged by the voltage across the armature  105 . In an embodiment, the power supply and storage  502  is coupled to the gate drive unit  506  (via line  512 ), which includes the switching mechanism to provide power from the storage unit in the power supply and storage  502  to the gate of Q 1 . 
     In an embodiment, the gate drive unit  506  is controlled by level shifter  504  (via line  514 ), which is in turn coupled to the brake control signal  122  from the controller  120 . In an embodiment, the level shifter  504  enables the gate drive unit  506  to supply power to the gate of Q 1  when the brake control signal is activated by the controller  120 . 
     In an embodiment, the under-voltage protection unit  508  is coupled to the gate drive unit  506  and the power supply and storage  502 . In an embodiment, the under-voltage protection unit  508  monitors the storage unit in the power supply and storage  502  to ensure that it stores sufficient voltage to drive Q 1 . If the voltage of the storage unit is below a certain threshold, the under-voltage protection unit  508  disables the output of the gate drive unit  506 . 
     In an embodiment, the power switch ON/OFF sensor  510  detects the state of the power switch  106  and overrides the brake control signal on line  122  if the power switch  106  is closed. 
       FIG. 8  depicts a circuit diagram of the power supply and storage unit  502 , according to an embodiment. The power supply and storage unit  502  is provided to supply sufficient power to drive the gate of Q 1  throughout the braking period. In an embodiment, the power supply and storage unit  502  should be configured to maintain power supply to Q 1  for 3 seconds or more, preferably 4 second or more, even more preferably 5 second or more. 
     According to an embodiment, as shown in this figure, with continued reference to  FIG. 7 , power supply and storage unit  502  includes a power terminal  602  coupled to one end of the armature  105  at node  134  via line  144  and a ground terminal  604  coupled to another end of the armature  105  at node  136  via line  146 . In an embodiment, the power supply and storage unit  502  includes an R-C circuit including a capacitor C 1  that is coupled to terminals  602  and  604  and is charged by the voltage developed across the armature  105  during the normal run time of the power tool. In an embodiment, the R-C circuit further includes resistors R 1  and R 2  in series with a diode D 1  to direct flow of charging current to the capacitor C 1 , and a zener diode D 2  that sets the power supply voltage to the capacitor C 1  at a maximum threshold, e.g., 33V. In an embodiment, the power supply and storage unit  502  includes an output terminal  606  that couples the capacitor C 1  to line  512  to supply power from the capacitor C 1  to the gate drive unit  506  during the braking period. 
       FIG. 9  depicts a circuit diagram of the level shifter  504 , according to an embodiment. Level shifter  504  is arranged to transfer the logic control brake signal  122  from the controller  120  to a high-voltage signal provided by the capacitor C 1  sufficient to enable or disable the gate drive unit  506 . In other words, the level shifter  504  enables the gate drive unit  506  to supply power to the gate of Q 1  when the brake control signal is activated by the controller  120 . 
     According to an embodiment, as shown in this figure, with continued reference to  FIG. 7 , level shifter  504  includes a brake terminal  612  coupled to the control brake signal  122  from the controller  120 . This signal is coupled to a gate of a switch, such as a FET or BJT transistor, Q 4 . Switch Q 4  is arranged between ground terminal  614  and output terminal  616 , which are respectively coupled to lines  126  (Gnd) and  514  (coupled to the gate drive unit  506 ). In an embodiment, when the controller  120  activates the brake control signal on line  122 , switch Q 4  grounds line  514 , which as discussed below, enables the gate drive unit  506 . 
       FIG. 10  depicts a circuit diagram of the gate drive unit  506 , according to an embodiment. The gate drive unit  506  is arranged as a switching mechanism between the power supply and storage unit  502  and the gate of Q 1 . The switching operation of the gate drive unit  506  is controlled by the output  514  of the level shifter  504 . 
     According to an embodiment, as shown in this figure, with continued reference to  FIG. 7 , gate drive unit  506  includes a power terminal  622 , which is coupled to capacitor C 1  of the power supply and storage unit  502 , and a control terminal  624 , which is coupled to line  514  from the level shifter  504  discussed above. When line  514  is grounded by the level shifter  504  in response to activation of the control brake single  122 , it creates a voltage difference between terminals  622  and  624 , causing current to flow through resistor R 5  (controlled by zener diode D 3 , resistors R 6 , R 8 , R 9 , and diode D 9 . This current path pulls the voltage at the gate of switch Q 3  to Gnd. Q 3 , in this embodiment, is a p-channel MOSFET (PMOS), which is activated by a negative voltage at its gate. Thus, in an embodiment, when line  514  is grounded by the level shifter  504  in response to activation of the brake control single  122 , it connects line  512  from the capacitor C 1  to its output terminal  626 , which is connected to a brake drive signal at line  516 . In an embodiment, line  516  is coupled to the under-voltage protection unit  508 . 
     The level shifter  504  and gate drive unit  506  discussed herein utilize a switching arrangement to cut off power from the capacitor C 1  when the control brake signal is at a logic level ‘1’. It is noted that a variety of other circuits may be used to accomplish the same task. For example, in an embodiment, the level shifter  504  and gate drive unit  506  may be combined into a single unit including an optical-isolator (also referred to as a photo-coupler) including a light emitting diode that turns on when the brake control signal on line  122  is a logic ‘1’ and a photo-transistor arranged between the capacitor C 1  and the gate of Q 1  to cut off power from the capacitor C 1  when the light emitting diode is turned off. 
       FIG. 11  depicts a circuit diagram of the under-voltage protection unit  508 , according to an embodiment. The under-voltage protection unit  508  is arranged to monitor the voltage level of the capacitor C 1  in the power supply and storage unit  502 . If the voltage of C 1  falls below a certain level, it can potentially damage the IGBT switch Q 1 . If an under-voltage condition at C 1  is detected by under-voltage protection unit  508 , it disables the brake drive signal. 
     According to an embodiment, as shown in this figure, with continued reference to  FIG. 7 , under-voltage protection unit  508  includes two terminals  632  and  634  coupled respectively to line  512  from the power supply and storage unit  502  and line  516  from the gate drive unit  506 . The under-voltage protection unit  508  also includes a Gnd terminal  636  coupled to line  146 , which is in turn coupled to node  136  of the armature  105 , and an output terminal  638 , which is coupled via line  124  to the gate of Q 1 . Arranged between terminals  634  and  636  is a zener diode D 4 . If the voltage at terminal  634  (i.e., capacitor C 1  voltage) falls below a certain threshold, e.g., 15V, zener diode D 4 , together with resistors R 7  and R 4  disable switch Q 2 A. Switch Q 2 A in turn controls the gate of Q 2 B to short terminal  638  to the Gnd terminal  636 . Accordingly, if the voltage of capacitor C 1  is at its normal operation range, line  124  is driven by the brake drive signal from line  156  to activate Q 1 . If the voltage of capacitor C 1  falls below the threshold, this circuit grounds the brake drive signal of the gate of Q 1  at line  124  to shut off Q 1 , thus stopping the braking operation. 
     An aspect of the invention is discussed herein. As discussed above, in an embodiment, the controller  120  typically initiates braking of the motor  28  and the braking continues for a predetermined amount of time, e.g., 2-5 seconds, until the capacitor C 1  is fully discharged. A conflict occurs when a user action inadvertently opens the power switch (for example, the user&#39;s finger slips of the trigger switch). If the user attempts to use the tool again while the tool is in brake mode, it is inconvenient to the user to have to wait for the braking mode to complete, which as discussed above may take 2-5 seconds. Thus, in an embodiment, a mechanism is provided to stop the braking of the motor  28  if the power switch  106  becomes closed during the brake mode. 
     During the normal operation of the circuit shown in  FIGS. 3-7 , the controller  120  may sense whether the power switch  106  is open or close by sensing the voltage across the triac  110 . Lack of voltage across the triac  110  is indicative of the power switch  106  being open. During the brake mode, however, the voltage across the triac  110  is no longer indicative of the state of the power switch  106 . Thus, a different mechanism has to be utilized. 
     In one embodiment, the controller  120  may sense the state of the power switch  106  either directly or through an auxiliary sensor. In another embodiment, as depicted in  FIGS. 7 and 12 , a power switch on/off sensor  510  may be utilizes, according to an embodiment of the invention. 
       FIG. 12  depicts a circuit diagram of the power switch on/off sensor  510 , according to an embodiment. In an embodiment, the on/off sensor  510  includes a first terminal  642  coupled via line  144  to node  134  between the armature  105  and the first field winding  102   a . In series with terminal  642  are a diode D 11 , resisters R 16  and R 17 , and a photo-coupler U 1 . The other end of the photo-coupler is coupled via a ground terminal  644  to the AC Gnd terminal. When the power switch  106  is closed, a current path is created though the photo-coupler U 1 , resisters R 16  and R 17 , the diode D 11 , field winding  102   a , and the power switch  106 . This current turns on the photo-coupler U 1 , which in turn grounds the brake control signal  122  (coupled to terminal  646 ). Grounding the brake control signal  122  disables the gate driver  210  circuit. Furthermore, in an embodiment, the controller  120  is configured to sense the grounding of the brake control signal  122  and resume normal operation. 
       FIG. 13  depicts a modified version of block circuit diagram of  FIG. 3 , where both field windings  102   a  and  102   b  are arranged on side of the armature  105  between the armature  105  and the power supply, e.g., between the GND node of the AC power source and the armature  105 . In other words, none of the field windings  102   a  and  102   b  are arranged between the armature  105  and the power switch  106 . In this manner, both field windings  102   a  and  102   b  are used to apply braking force on the motor. As previously discussed, this arrangement may provide the controller  120  with a smaller time frame to fire the triac  110  in order to obtain a smooth braking operation. However, it was found by the inventors that this effect may be offset by the combined mutual inductance of field windings  102   a  and  102   b.    
       FIG. 14  depicts a perspective view of the brake module  200 , according to an embodiment. As shown herein, the brake module  200  includes a circuit board  700  on which the diodes D 1  and D 2 , switch Q 1 , and capacitor C 1  are mounted. The brake module  200  also includes a heat sink  702  extending from the circuit board  700  and folded over o the aforementioned components for effective heat transfer. The folded portion of the heat sink  702  particularly covers the switch Q 1 , which is an IGBT in an exemplary embodiment. In an exemplary embodiment, the brake module  200  may be placed in a mold designed to overmold the circuit board  700  but not the heat sink  702 . 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure. 
     Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.