Patent Publication Number: US-2021194393-A1

Title: Power tool having stamped brake resistor

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/949,900, filed Dec. 18, 2019, the entire content of which is hereby incorporated by reference. 
    
    
     FIELD 
     Embodiments described herein relate to a brake resistor. 
     SUMMARY 
     In a permanent magnet brushless motor having a permanent magnet rotor and an associated stator having stator windings, rotation of the rotor causes a changing magnetic field that induces a current within windings of the stator. This current, in turn, produces a voltage in the stator windings, known as a back-emf of a motor. When power ceases to be supplied to stator windings to stop driving the motor, the inertia will cause the rotor to continue to rotate for some time as the rotor decelerates. The still-rotating rotor, thus, continues to generate back-emf, effectively turning the motor into a generator. The back-emf also generates an opposing force that assists in slowing the motor. However, if unchecked, the back-emf may become excessive and damage components of the motor drive circuitry. To avoid excess back-emf, a braking resistor may be used. The braking resistor is coupled to the stator windings during braking to dissipate the energy generated by the still-rotating rotor as heat. 
     Typical, barrel-shaped resistors having sufficient resistance to serve as brake resistors may be large and undesirable in applications where space is limited. Additionally, dissipating heat from brake resistors can require large heat sinks or other heat sinking techniques that are undesirable in applications where space is limited. 
     Some embodiments described herein provide a power tool, and a method of using a power tool, having a stamped braking resistor. The stamped braking resistor may be coupled to one or more planar heat sinks enabling improved heat dissipation and a compact braking resistor assembly. Accordingly, in some embodiments, power tools and methods of using power tools described herein include improved braking and corresponding heat dissipation. 
     In some embodiments, a power tool is provided including a motor, a trigger configured to be actuated, a brake switch coupled to the motor, and a brake resistor assembly. The brake resistor assembly is selectively connected to the motor via the brake switch and includes a stamped brake resistor, which includes a terminal portion and a resistive portion. The resistive portion includes a planar serpentine path. The power tool further includes a controller including an electronic processor and a memory. The controller is connected to the trigger, the motor, and the brake switch. The controller is configured to control power delivered to the motor based on a position of the trigger, determine to brake the motor during operation of the motor, and activate, in response to determining to brake the motor, the brake switch to connect the stamped brake resistor to the motor. 
     In some embodiments, the brake resistor assembly further includes a first insulating pad and a second insulating pad that sandwich the stamped brake resistor. In some embodiments, the brake resister assembly further includes a first heat sink and a second heat sink that sandwich the first insulating pad and the second insulating pad. 
     In some embodiments, the resistive portion and the terminal portion are situated on the same plane. In some embodiments, the resistive portion includes a first leg, a second leg perpendicular to the first leg, a third leg, and a fourth leg perpendicular to the third leg. In some embodiments, the first leg and the third leg are separated via an opening in the resistive portion. In some embodiments, the second leg and the fourth leg are coupled via the planar serpentine path. In some embodiments, the stamped resistor further includes interlock gaps. In some embodiments, the stamped resistor is composed of at least one selected from the group consisting of zinc, aluminum, chromium, iron, brass, bronze, stainless steel, carbon, metal-oxide, phenol formaldehyde resin, tantalum, cermet, and a metallic alloy. 
     In some embodiments, a method of braking a motor in a power tool is provided. The method includes controlling, by a motor controller of the power tool, power delivered to the motor based on a position of a trigger. The method includes determining, with the motor controller, to brake the motor during operation of the motor. In response to determining to brake the motor, the method includes activating, with the motor controller, a brake switch to connect a stamped brake resistor with the motor. The stamped brake resistor includes a terminal portion and a resistive portion, wherein the resistive portion includes a planar serpentine path. In response to the brake switch being activated, the method includes receiving, with the stamped brake resistor, motor current from the motor to thereby brake the motor. 
     In some embodiments, the method includes determining, with the motor controller, to brake the motor in response to detecting a release of the trigger. In some embodiments, the motor current received by the brake resistor is a current resulting from a back EMF of the motor. In some embodiments, the method includes selectively activating, with the motor controller and to control power delivered to the motor based on a position of a trigger, switching elements coupled to phases of the motor. Additionally, in some embodiments, selectively activating the power switching elements connected to phases of the motor includes selectively activating the power switching elements based on a position of a rotor while the brake switch is activated. In some embodiments, the method includes opening, with the motor controller and in response to determining to brake the motor, a switch connected between a power source and the power switching elements. 
     In some embodiments, braking system for a power tool is provided. The system includes a stamped resistor having a terminal portion and a resistive portion. The resistive portion includes a planar serpentine path forming a plurality of leaves with a gap between each leaf. The system further includes a first insulating pad and a second insulating pad that sandwich the stamped resistor, and a first heat sink and a second heat sink that sandwich the first insulating pad and the second insulating pad. 
     In some embodiments, the terminal portion includes a terminal for connecting to a voltage source, the terminal being situated on a same plane as the stamped resistor. In some embodiments, each leaf of the plurality of leaves has a width of approximately 2.5 mm, and wherein each gap of the plurality of gaps has a width of at least 1.5 mm. In some embodiments, the stamped resistor further includes interlock gaps. In some embodiments, the serpentine path includes a filling substance located within each gap formed between adjacent leaves. 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Use of “including” and “comprising” and variations thereof as used herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Use of “consisting of” and variations thereof as used herein is meant to encompass only the items listed thereafter and equivalents thereof. 
     Also, the functionality described herein as being performed by one component may be performed by multiple components in a distributed manner. Likewise, functionality performed by multiple components may be consolidated and performed by a single component. Similarly, a component described as performing particular functionality may also perform additional functionality not described herein. For example, a device or structure that is “configured” in a certain way is configured in at least that way but may also be configured in ways that are not listed. 
     Furthermore, some embodiments described herein may include one or more electronic processors configured to perform the described functionality by executing instructions stored in a non-transitory, computer-readable medium. Similarly, embodiments described herein may be implemented as a non-transitory, computer-readable medium storing instructions executable by one or more electronic processors to perform the described functionality. As used in the present application, “non-transitory computer-readable medium” comprises all computer-readable media but does not consist of a transitory, propagating signal. Accordingly, non-transitory computer-readable medium may include, for example, a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a ROM (Read Only Memory), a RAM (Random Access Memory), register memory, a processor cache, or any combination thereof. 
     Many of the modules and logical structures described are capable of being implemented in software executed by a microprocessor or a similar device or of being implemented in hardware using a variety of components including, for example, application specific integrated circuits (“ASICs”). Terms like “controller” and “module” may include or refer to both hardware and/or software. Capitalized terms conform to common practices and help correlate the description with the coding examples, equations, and/or drawings. However, no specific meaning is implied or should be inferred simply due to the use of capitalization. Thus, the claims should not be limited to the specific examples or terminology or to any specific hardware or software implementation or combination of software or hardware. 
     Other aspects of the embodiments will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a power tool, according to some embodiments. 
         FIG. 2  illustrates a perspective view of the power tool shown in  FIG. 1  with a top housing portion removed, according to some embodiments. 
         FIG. 3  illustrates an exploded view of a brake resistor assembly as shown in  FIG. 2 , according to some embodiments. 
         FIG. 4  illustrates a partial side view of the brake resistor assembly as shown in  FIG. 3 , according to some embodiments. 
         FIGS. 5A-5B  illustrate a stamped brake resistor as shown in  FIG. 3 , according to some embodiments. 
         FIG. 6  illustrates a block diagram of the power tool as shown in  FIG. 1 , according to some embodiments. 
         FIG. 7  illustrates a circuit diagram of the power tool as shown in  FIG. 1 , according to some embodiments. 
         FIG. 8  illustrates a flow chart of a method that may be implemented by the power tool of  FIG. 1 , according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example of a power tool  100  with a battery pack  105 . In the illustrated embodiment, the power tool  100  is a chop saw having a housing  110  with a handle  115 . The housing  110  is coupled to a base  120 , which includes a clamp  125  and a blade housing  130 . The power tool  100  has a motor  630  (see  FIG. 6 ) configured to rotate an arbor holding a blade (not shown) of the blade housing  130 . The rotating blade is configured to cut a workpiece on the base  120  secured by the clamp  125 . Although the power tool  100  illustrated in  FIG. 1  is a chop saw, the present description applies also to other power tools having a motor such as, for example, an impacting wrench, an AC grinder, a hammer drill, an impact hole saw, an impact driver, a drill, a reciprocating saw, a nailer, a stapler, and the like. The present description also applies to brushed and brushless motors and controls. In other words, in some embodiments, the power tool  100  includes a brushed motor in place of the brushless motor  630 . 
       FIG. 2  illustrates a perspective view of the housing  110  with a top housing portion removed. The handle  115  of the housing  110  further includes a trigger  205 . The trigger  205  is electronically coupled to an electronic processor (see  FIG. 7 ). During operation, the trigger  205  is pulled to activate the power tool  100  and rotate a motor  630  (see  FIG. 6 ) of the power tool  100 . Releasing the trigger  205  stops the flow of power to the motor  630 , but may not immediately stop the motor  630 . A brake resistor assembly  200  (detailed further below) is situated in the top portion of the housing  110  and is coupled to a power switching network  700  and brake switch  640  (see  FIG. 7 ) via power terminals  215 . In some embodiments, the power terminals  215  couple to the power switching network  700  and brake switch  640  via terminal fasteners  225 . In some embodiments, the power terminals  215  may couple to the electronic processor via a welding, fusing, or soldering process. The housing  110  is configured to receive the battery pack  105  (see  FIG. 1 ) via power interface  220 . 
     In some embodiments, the power interface  220  is similar to, and thus the power tool battery pack  105  is also compatible with, power interfaces of a suite of two or more types of devices selected from power tools, fluid flow control devices, test and measurement devices, work site radios, and work lights. The battery pack  105  includes a battery housing within which are one or more battery cells, which may be lithium ion (“Li-ion”) cells, Nickel-Cadmium (“Ni-Cad”) cells, or cells of another chemical type. The cells, collectively, may provide nominal voltages of different values, depending on the pack. For example, the battery pack  105  may have a nominal output voltage of 4V, 12V, 18V, 28V, 36V, 40V, a voltage between levels, or other levels. In some embodiments, the power interface  220  is an alternating current (AC) power interface that is configured to be connected to a standard AC outlet that is further coupled to an AC power grid or AC generator. For instance, the AC source may include an approximately 120 V, 60 Hz power signal or an approximately 240 V, 50 Hz power signal. In other words, in some embodiments, the power tool  100  includes an AC power cord to receive AC power (e.g., from a wall outlet) to drive the motor  630 , rather than a battery interface to receive the battery pack  105  providing DC power. 
       FIG. 3  shows an exploded view of the brake resistor assembly  200 . The brake resistor assembly  200  includes a stamped brake resistor  300 , a first insulating pad  305   a , a second insulating pad  305   b , a first heat sink  310   a , a second heat sink  310   b , a plurality of fasteners  315 , and a plurality of insulating sleeves  318 . In the illustrated example, each of the stamped brake resistor  300 , the first insulating pad  305   a , the second insulating pad  305   b , the first heat sink  310   a , and the second heat sink  310   b  includes four holes  320 , one at each outer corner. The holes  320  at each respective corner align such that when the stamped brake resistor  300 , insulating pads  305   a,b , and heat sinks  310   a,b  are stacked, a passage  325  is formed at each corner (see, e.g.,  FIG. 4 ). Each of the fasteners  315  passes through aligned, respective holes  320  of the stamped brake resistor  300 , insulating pads  305   a,b , and heat sinks  310   a,b  (i.e., through one of the passages  325 ) to secure these elements together and to attach the brake resistor assembly  200  to the power tool  100 . In some embodiments, each of the passages  325  also receives one of the insulating sleeves  318 . More specifically, as illustrated in  FIG. 4 , each of the fasteners  315  passes through one of the passages  325  and one of the insulating sleeves  318 , and engages a threaded bore  405  of the housing  110  of the power tool  100  to attach the brake resistor assembly  200  to the power tool  100 . The insulating sleeve  318  restricts electrical current flowing through the stamped brake resistor  300  from entering the fastener  315 . While each of the stamped brake resistor  300 , insulating pads  305   a,b , and heat sinks  310   a,b  is illustrated as including four holes  320 , in some embodiments, they are provided with more or fewer holes  320 , with holes  320  in different locations than the corners, or a combination thereof. 
     When assembled, as shown in  FIG. 4 , the stamped brake resistor  300  is sandwiched between the first insulating pad  305   a  and the second insulating pad  305   b ; and, the collection of the first insulating pad  305   a , the stamped brake resistor  300 , and the second insulating pad  305   b  are sandwiched between the first heat sink  310   a  and the second heat sink  310   b . As a result, the first insulating pad  305   a  is positioned on a first side of the stamped brake resistor  300 , and the second insulating pad  305   b  is positioned on a second, opposite side of the stamped brake resistor  300 . Planes formed by the first insulating pad  305   a  and the second insulating pad  305   b  are substantially parallel to one another. The first heat sink  310   a  is positioned on a first side of the first insulating pad  305   a , and the second heat sink  310   b  is positioned on a second side of the second insulating pad  305   b . Planes formed by the first heat sink  310   a  and the second heat sink  310   b  are substantially parallel to one another. While  FIG. 4  illustrates a cross-section at one corner of the brake resistor assembly  200 , the other corners of the brake resistor assembly  200  may include similar configurations. 
     Returning to  FIG. 3 , each of the stamped brake resistor  300 , the first insulating pad  305   a , the second insulating pad  305   b , the first heat sink  310   a , and the second heat sink  310   b  includes one or two generally planar engaging surfaces  330  (labeled individually  330   a - j ) to engage an adjacent one of the engaging surfaces  330  of another of the stamped brake resistor  300 , the first insulating pad  305   a , the second insulating pad  305   b , the first heat sink  310   a , and the second heat sink  310   b . For example, the stamped brake resistor  300  includes an engaging surface  330   a  that engages an engaging surface  330   c  of the first insulating pad  305   a , and the first heat sink  310   a  includes an engaging surface  330   e  that engages an engaging surface  330   d  of the first insulating pad  305   a . Additionally, the engaging surface  330   a  faces an opposite direction as the engaging surface  330   b ; the engaging surface  330   c  faces an opposite direction as the engaging surface  330   d ; and the engaging surface  330   f  faces an opposite direction as the engaging surface  330   g . Further, the first heat sink  310   a  includes an outer facing surface  335   a  and the second heat sink  310   b  includes an outer facing surface  335   b . The outer facing surface  335   a  faces an opposite direction as the engaging surface  330   e , and the outer facing surface  335   b  faces an opposite direction as the engaging surface  330   h.    
     The insulating pads  305   a,b  are composed of a material that is electrically insulating and restricts current from traveling from the stamped brake resistor  300  to the heat sinks  310   a,b . In some embodiments, the insulating pads  305   a,b  are composed of a material that is also thermally conductive to conduct heat generated in the stamped brake resistor  300  to the heat sinks  310   a,b , respectively. The heat sinks  310   a,b  dissipate heat generated by the stamped brake resistor  300  away from the stamped brake resistor  300  and into the ambient environment within the housing  110 . In some embodiments, the housing  110  further includes one or more vents into the ambient environment around the power tool  100  and, accordingly, the heat dissipated by the heat sinks  310   a,b  is at least partially dissipated into the ambient environment around the power tool  100  via the one or more vents. 
     In some embodiments, the stamped brake resistor  300  is composed of zinc. In some embodiments, the stamped brake resistor  300  is composed of another conductive material, such as carbon, metal-oxide, phenol formaldehyde resin, tantalum, cermet, a metallic alloy including elements such as aluminum, copper, nickel, chromium, and/or iron, brass, bronze, stainless steel, or the like. In some embodiments, the insulating pads  305   a,b  are composed of silicon. In some embodiments, the insulating pads  305   a,b  are composed of another insulating material, such as glass, paper, Teflon, or porcelain. 
       FIG. 5A  shows a perspective view of the stamped brake resistor  300 . The stamped brake resistor  300  includes a resistive portion  500  and a terminal portion  530 . The resistive portion  500  includes a first leg  505 , a second leg  510 , a third leg  515 , and a fourth leg  520 . The first leg  505  is coupled to the terminal portion  530  at a first distal end and coupled to the second leg  510  at a second distal end, and the first leg  505  is substantially perpendicular to the second leg  510 . The third leg  515  is coupled to the terminal portion  530  at a first distal end and coupled to the fourth leg  520  at a second distal end, and the third leg  515  is substantially perpendicular to the fourth leg  520 . The first leg  505  and the third leg  515  are separated by a gap  540  of the terminal portion  530 . 
     The second leg  510  and the fourth leg  520  extend substantially parallel to one another, away from an end of the stamped brake resistor  300  having the first leg  505  and the third leg  510 . The second leg  510  has a first distal end connected to the first leg  505 , and a second distal end coupled to a first end  525   a  of a planar serpentine path  525 . The fourth leg  520  has a first distal end connected to the third leg  515 , and a second distal end coupled to a second end  525   b  of the planar serpentine path  525 . 
     The second leg  510  and the fourth leg  520  are coupled via the planar serpentine path  525 . In some embodiments, the planar serpentine path  525  is a winding path that is multi-turn and planar. For example, as illustrated, the planar serpentine path  525  includes a plurality of leaves  526  forming a plurality of gaps  527 . Each leaf  526  includes a first member  528   a  extending substantially parallel to a second member  528   b , where the first and second members  528   a,b  are separated by one of the plurality of gaps  527  at one distal end  529   a  and joined at a turn point at a second distal end  529   b . In the illustrated embodiment, the planar serpentine path  525  includes eight leaves  526 ; however, in some embodiments, more or fewer leaves  526  are included in the planar serpentine path  525 . 
     For clarity in the illustration, only some of the leaves  526 , gaps  527 , members  528   a,b , and ends  529   a,b  are labeled along the planar serpentine path  525 . In some embodiments, each leaf  526  of the planar serpentine path  525  has a width of at least 2.5 mm. In some embodiments, each gap of the plurality of gaps  527  has a width of at least 1.5 mm. In some embodiments, each of the legs  510  and  520 , taken individually, have a width that is greater than the width of each the leaves  526  taken individually; however, collectively, the width of the leaves  526  (i.e., the width of the planar serpentine path  525 ) is greater than the width of the legs  510  and  520  taken individually or together. Additionally, the gaps  527 , taken individually, have a width that is less than the width of the leaves  526 , taken individually, and less than the width of the members  528   a,b , taken individually. 
     In some embodiments, such as the embodiment of  FIG. 5A , the planar serpentine path  525  is bounded by the first leg  505 , the second leg  510 , the third leg  515 , and the fourth leg  520 . In some embodiments, the members  528   a,b  turn before extending further than the end of the stamped brake resistor  300  opposite the terminal portion  530  (i.e., the end at which the planar serpentine path  525  connects to the second leg  510  and the fourth leg  520 ). In other words, the second leg  510  and the fourth leg  520  extend distally as far or farther than the members  528   a,b  of the leaves  526  extend. 
     In some embodiments, the winding path, or the planar serpentine path  525 , includes a filling substance located within each gap  527 . For example, without a filling, the leaves  526  may begin to morph from the desired shape over time or during construction of the stamped brake resistor  300 . A polymethyl methacrylate (PMMA) filling may be placed in each gap  527  formed between each leaf  526  to maintain the correct shape of the stamped brake resistor  300 . Additionally, the stamped brake resistor  300  may include a plurality of interlock gaps  550   a - f  to assist with securing the stamped brake resistor  300  during tooling, as shown in  FIG. 5B . For example, the interlock gaps  550   a - g  may engage corresponding features on a mold that retains the stamped brake resistor  300  while the PMMA filling is injected into the mold to fill in the gaps  527 . 
     Terminal portion  530  includes a first terminal  535   a  and a second terminal  535   b  configured to connect the stamped brake resistor  300  to the power terminals  215  (see  FIGS. 2 and 7 ). The first terminal  535   a  and the second terminal  535   b  are separated via a gap  540 . In some embodiments, the resistive portion  500  is situated on a first plane. As illustrated in  FIG. 5A , the terminal portion  530  may include a bending portion that bends the terminal portion  530  such that the first terminal  535   a  and the second terminal  535   b  lie in second plane that is a separate parallel plane from the first plane. In other embodiments, such as illustrated in  FIG. 5B , the terminal portion  530  may lie in the same plane as the resistive portion  500  (e.g., the terminal portion  530  is flat with respect to the resistive portion  500 ). 
       FIG. 6  is a block diagram of the exemplary power tool  100  of  FIG. 1 . A system  600  of the power tool  100  includes, among other things, the power interface  605 , field effect transistors (FETs)  620 , a motor  630 , an output unit  635 , Hall sensors  625 , a motor controller  615 , user input  610 , a brake switch  640 , and other components  650  (battery pack fuel gauge, work lights (LEDs), current/voltage sensors, etc.). The power interface  605  may be, for example, the power interface  220  of  FIG. 2 . The motor controller  615  may also be referred to as an electronic motor controller or a motor microcontroller and includes, among other things, an electronic processor and a memory. In some embodiments, the memory stores instructions that are executed by the electronic processor to implement the functionality of the motor controller  615  described herein. 
     The Hall sensors  625  provide motor information feedback, such as motor rotational position information, which can be used by the motor controller  615  to determine motor position, velocity, and/or acceleration. The motor controller  615  receives user controls from user input  610 , such as by depressing the trigger  205  or shifting a forward/reverse selector of the power tool  100 . In response to the motor information feedback and user controls, the motor controller  615  transmits control signals (e.g., pulse width modulated signals) to control the FETs  620  to drive the motor  630 . For example, by selectively cyclically enabling and disabling the FETs  620 , power from the power interface  605  is selectively applied to stator windings of the motor  630  to cause rotation of a rotor of the motor  630 . The rotating rotor of the motor  630  drives the output unit  635 . Upon receiving an indication to stop the motor  630  from user input  610 , such as a depression of the trigger  205 , the electronic processor of the motor controller  615  may activate the brake switch  640  to enable a braking mechanism of the motor  630 . Although not shown, the motor controller  615  and other components of the power tool  100  are electrically coupled to and receive power from the power interface  605 . The FETs  620  may also be referred to as power switching elements. The FETs  620 , motor  630 , Hall sensors  625 , motor controller  615 , and output unit  635  may be referred to as electromechanical components  660  of the power tool  100 . 
       FIG. 7  illustrates a power switching network  700  of the electromechanical components  660 . In the illustrated embodiment, the motor  630  is a brushless DC motor including three phases. For example, the motor  630  may include an outer stator having six stator windings arranged in three phases, and an inner rotor with four permanent magnets. In other embodiments, however, different types of motors may be used, such as those with different number of phases, windings, and magnets. As shown in  FIG. 7 , the power switching network  700  includes three high side electronic switches  712 ,  716 ,  720  and three low side electronic switches  710 ,  714 ,  718 , which are examples of the FETs  620  shown in  FIG. 6 . In the illustrated embodiment, the electronic switches  710 - 720  include MOSFETs. In other embodiments, other types of electronic switches may be used, such as bipolar junction transistors (BJT), insulated-gate bipolar transistors (IGBT), and other electronic switch types. Additionally, each electronic switch  710 - 720  is connected in parallel to a body diode  730 ,  732 ,  734 ,  736 ,  738 ,  740 , respectively. In the diagram of  FIG. 7 , each phase of the motor  630  is represented by an inductor, a resistor, and a voltage source. Since the motor  630  is a three-phase motor,  FIG. 7  illustrates three inductors  704   a - c , three resistors  706   a - c , and three voltage sources  702   a - c . Each inductor  704   a - c  represents motor windings of each phase of the motor  630 . Each resistor  706   a - c  represents motor windings of each phase of the motor  630 . Each voltage source  702   a - c  represents the back electromagnetic force generated (i.e., back-electromotive force [emf]) in each phase. Back-emf is generated by the rotation of the rotor magnets inducing current in the stator windings. Additionally, the power switching network  700  includes the brake switch  640  connected in series with the stamped brake resistor  300 . The brake switch  640  and stamped brake resistor  300  are connected in parallel to the first phase  750 , the second phase  752 , and the third phase  754  of the brushless DC motor  630 . 
     The power switching network  700  receives power from the power interface  605 . In the illustrated embodiment, the power interface  605  receives power from the battery pack  105 . The battery pack  105  is represented by a power source  701  connected in series with a resistor  722  and inductor  724 , which represent internal resistance and inductance of the power interface  605 , the battery pack  105 , or both. Additionally, the power source  701  is connected in series with a parallel combination of a diode  726  and a switch  727 . The diode  726  and switch  727  control the flow of current from the power source  701 . For example, the switch  727  is switchable between a conducting state and a non-conducting state. The switch  727  may be controlled based on the trigger  205 . For example, in some embodiments, the electronic processor of the motor controller  615  controls the state of the switch  727  based on the condition of the trigger  205 . For example, in some embodiments, the switch  727  is closed by the motor controller  615  when the trigger  205  is depressed, and the switch  727  is opened when the trigger  205  is released. In some embodiments, the switch  727  and the trigger  205  are an electromechanical device whereby the switch  727  is closed by the trigger  205  when the trigger  205  is depressed (e.g., a contact is pivoted to make an electrical connection), and the switch  727  is opened when the trigger  205  is released. 
     When the switch  727  is in the conducting state, current can flow bidirectionally to and from the power source  701 . When the switch  727  is in the non-conducting state, however, current (e.g., regenerative current) can only flow to the power source  701  through the diode  726 . In some embodiments, the diode  726  is not provided. A capacitor  728  is connected in parallel to the power source  701 , as shown in  FIG. 7 . The capacitor  728  smooths the voltage from (and to) the power source  701 . Additionally, the electronic processor of the motor controller  615  controls the state of each of the electronic switches  710 - 720  in the power switching network  700 . 
     To drive the motor  630  forward, the electronic processor sets the switch  727  to be in the conducting state, and activates a high side electronic switch  712  and a low side electronic switch  714 . As shown in  FIG. 7 , the high side electronic switch  712  is on a first side of the first phase  750  of the motor  630  and the low side electronic switch  714  is on a second side of the first phase  750 . In such a configuration, the first phase  750  of the motor  630  is connected such that the back-emf has an opposite polarity with respect to the power source  701 . Accordingly, while the electronic processor maintains the first high side electronic switch  712  and the first low side electronic switch  714  in a conducting state, the back-emf detracts from the overall power provided to the motor  630 . In other words, the motor current is set by dividing the difference between the voltage from the power source  701  and the back-emf by the resistance of the motor  630 . That is, the motor current is set based on the equation: I motor =(V power source −V back-emf )/R motor . 
     The electronic processor of the motor controller  615  determines which high side electronic switches  712 ,  716 ,  720  and low side electronic switches  710 ,  714 ,  718  to place in the conducting state based on the position of the rotor in relation to the stator of the motor  630 . In particular, each activation of a pair of a high side electronic switch  712 ,  716 ,  720  and a low side electronic switch  710 ,  714 ,  718  rotates the motor  630  approximately 120 degrees. When the motor  630  rotates about 60 degrees, the electronic processor deactivates one pair of electronic switches and activates a different pair of electronic switches to energize a different phase of the motor  630 . In particular, the electronic processor activates the first high side electronic switch  712  and the first low side electronic switch  714  to drive the first phase  750  of the motor forward. The electronic processor activates the second high side electronic switch  716  and the second low side electronic switch  718  to drive the second phase  752  of the motor  630  forward, and the electronic processor activates the third high side electronic switch  720  and the third low side switch  710  to drive the third phase  754  of the motor  630  forward. The switches  710 - 720  may be driven by the motor controller  615  using a pulse width modulated (PWM) control signal. During each phase, the motor controller  615  can set the current provided to the motor  630  (and thus speed and torque) by adjusting the duty ratio of the PWM control signal to one or both of the active switches. 
     In the illustrated embodiment, the electronic processor of the motor controller  615  also implements electronic braking of the motor  630 .  FIG. 8  is a flowchart illustrating a method  800  of electronically braking the motor  630 . At block  801 , the motor controller  615  drives the motor  630  (e.g., delivers power to the motor  630 ) based on a position of the trigger  205 . For example, with reference to  FIG. 6 , a trigger signal is received via the user input  610  indicating to the motor controller  615  that the trigger  205  is depressed. In some embodiments, the trigger signal is binary indicating whether the trigger  205  is depressed or not depressed. In some embodiments, rather than a binary indication of whether the trigger  205  is depressed, a trigger sensor (e.g., potentiometer, Hall sensor, or the like) is provided as part of the user input  610  that indicates an amount of trigger depression (e.g., between 0-100%). In response to detecting that the trigger  205  is depressed, the motor controller  615  drives the motor  630  by providing control signals to the switches  710 - 720 , as described above with respect to  FIG. 7 . 
     At block  802 , the motor controller  615  determines to brake the motor  630 . For example, the motor controller  615  may detect that the trigger  205  has been released, may detect a fault in the system  600  of the power tool  100 , may detect a low state of charge of the battery pack  105  (e.g., using a voltage sensor to sense that battery voltage has dropped below a low voltage threshold), or the like. 
     At block  803 , the motor controller  615  activates the brake switch  640 , in response to determining to brake the motor  630 , to connect the stamped brake resistor  300  with the motor  630 . As illustrated in  FIG. 7 , the brake switch  640  is connected in series with the stamped brake resistor  300 , and the series-connected stamped brake resistor  300  and brake switch  640  are connected to the power supply line providing power to the high side switches  712 ,  716 , and  720 . 
     At block  804 , the motor controller  615  selectively activates power switching elements (e.g., high side electronic switches  712 ,  716 ,  720  and low side electronic switches  710 ,  714 ,  718 ) based on the position of the rotor in relation to the stator of the motor  630 . In some embodiments, the motor controller  615  activates a pair of power switching elements (including one high side electronic switch and one low side electronic switch) that provides the lowest impedance path from the motor to the brake resistor  300 , given the present position of the rotor of the motor  630 . In other words, each potential pair of power switching elements that is selectable to be activated has an associated impedance at a moment in time, and the pair that offers the lowest impedance path of the potential pairs is activated. For example, the motor controller  615  may activate the high side electronic switch  712  and low side electronic switch  714  to provide a path from the first phase  750  when the rotor is in a first position, may activate the high side electronic switch  716  and low side electronic switch  718  to provide a path from the second phase  752  when the rotor is in a second position, and may activate the high side electronic switch  720  and low side electronic switch  710  to provide a path from the third phase  754  when the rotor is in a third position. In one example of a three-phase, six stator coil brushless motor, the selected pair of power switching elements activated by the motor controller  615  may change approximately every 60 degrees of rotation of the rotor. The activation may be performed by the motor controller  615  by providing a respective PWM signal to each of the power switch elements of the pair. At least in some embodiments, an expected impendence of the selectable pairs of power switching elements for a given rotor position is known (e.g., from experimental testing) such that the motor controller  615  has mapped (in memory) various rotor positions relative to pairs of power switching elements that should be activated at a particular moment in time to result in connecting the lowest impedance path. By providing the lowest impedance for the energized phase, a maximum motor current can flow from the energized phase of the motor  630  to the stamped brake resistor  300 . 
     At block  805 , the stamped brake resistor  300  receives motor current from the motor  630  in response to the brake switch  640  being activated to thereby brake the motor  630 . The motor current may be, for example, current resulting from the back-emf of the motor  630 . 
     In some embodiments, the power tool  100  may continue to execute the method  800  by looping back from block  805  to block  803  to continue to activate the brake switch  640 , selectively activate a pair of the power switching elements, and direct motor current to the stamped brake resistor  300  while the motor is braked. Accordingly, as the rotor rotates during braking, the motor controller  615  continues to selectively activate the pair of power switching elements that provides the lowest impedance path based on the rotor position. Thus, the selected pair of power switching elements changes as the rotor rotates during braking of the motor. 
     In some embodiments, such as in the case of a brushed motor in the power tool  100 , block  804  may be bypassed, as power switching elements as shown in  FIG. 7  may not be present. For example, in some embodiments of the tool having a brushed motor, a single power switching element in series with the brushed motor is controlled by the motor controller via a PWM signal to regulate the motor speed or torque. In such embodiments, this power switching element may be held closed (e.g., with a 100% duty cycle PWM signal) while the brake switch  640  is closed in blocks  803  and  805 . 
     In some embodiments, the motor controller  615  may be coupled to a solid state disconnect (SSD) PCB assembly (not shown). The SSD PCB assembly includes a solid state disconnect switch between the power interface  605  and the motor  630 . The solid state disconnect switch is semiconductor-based, and may include a field effect transistor (FET), for example. When the solid state disconnect switch is closed, the solid state disconnect switch allows current to flow from the power interface  605  to the motor  630 , and, when the solid state disconnect switch is open, the solid state disconnect switch prevents power from flowing to or from the power interface  605 . In some embodiments, the motor controller  615  is configured to close the solid state disconnect switch in response to determining to drive the motor  630  (for example, in block  801 ). In some embodiments, the motor controller  615  is configured to open the solid state disconnect switch in response to determining to brake the motor  630  (for example, in block  803  before activating the brake switch  640 ). Opening the solid state disconnect switch in this manner prevents current resulting from the back-emf of the motor from flowing to the power interface  605  (e.g., power source  701 ). In some embodiments, the switch  727  of  FIG. 7  represents the solid state disconnect switch. 
     In some embodiments, the motor controller  615  may be configured to deactivate the brake switch  640  in response to determining that the motor  630  has stopped rotating or dropped below a low speed threshold. The electronic processor may determine that the motor  630  has stopped or slowed below the threshold based on motor position information from the Hall sensors  625 . For example, each Hall sensor  625  may output a pulse upon a rotor magnet of the motor  630  rotating across the face of the Hall sensor  625 . Accordingly, the frequency of the pulses is proportional and indicative of the rotation speed of the motor  630 , and the lack of pulses for a certain amount of time indicates that the motor  630  is not rotating. 
     Thus, the disclosure provides, among other things, a system and method of braking a motor of a power tool.