Patent Description:
This disclosure relates to power tools, particularly, a discrete battery disconnect circuit for current protection of cordless power tools.

As demand for greater output power from a power tool increases, the total current requirements increase to meet such demand. Conventional mechanical switches with metallic power contacts and conventional fuses having commonplace fusing elements have limitations that make them either unable to meet the current demand or too large for usage in cordless power tools. Such mechanical switches and fuses are also highly inefficient and produce significant heat. <CIT> discloses a motor driven appliance comprising a battery, a motor, at least one switch, a control unit and an abnormality detection unit which detects abnormality of the appliance to switch off switching elements of a driver circuit. <CIT> discloses a power tool comprising:a housing; a brushless direct-current (BLDC) electric motor disposed inside the housing;a battery pack;a battery receptacle configured to receive the battery pack, the battery pack including a plurality of battery cells; a protection against an overcurrent being associated with a first current profile; a plurality of power switches disposed between the battery receptacle and the electric motor, the plurality of power switches including a plurality of high-side switches and a plurality of low-side switches, the plurality of power switches being associated with a second current profile; a control unit configured to control a switching operation of the plurality of power switches to operate the electric motor; and wherein a battery disconnect circuit disposed between the battery receptacle and the plurality of power switches to disconnect the supply of power from the battery pack if the current draw from the battery pack exceeds a current threshold.

In the accompanying drawings which form part of the specification:.

Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings.

In an embodiment, a power tool is provided comprising: a housing; a brushless direct-current (BLDC) electric motor disposed inside the housing; a battery pack; a battery receptacle configured to receive a battery pack, the battery pack including a plurality of battery cells and a fuse being associated with a first current profile; a plurality of power switches disposed between the battery receptacle and the electric motor, the plurality of power switches including a plurality of high-side switches and a plurality of low-side switches, the plurality of power switches being associated with a second current profile; a control unit configured to control a switching operation of the plurality of power switches to operate the electric motor; and a battery disconnect circuit disposed between the battery receptacle and the plurality of power switches to disconnect the supply of power from the battery pack if, when the battery pack is received in the battery receptacle, the current draw from the battery pack exceeds a current threshold for a given duration of time, wherein the current threshold is smaller than a first current level associated the first current profile for the given duration of time but larger than a second current level associated with the second current profile for the given duration of time.

In an embodiment, the battery disconnect circuit includes a main solid-state switch disposed on a current path from the battery receptacle to the plurality of power switches, and a shunt resistor disposed in series with the main solid-state switch.

In an embodiment, the battery disconnect circuit further includes a switch controller separate from the control unit that is configured to monitor a current across the shunt resistor and drive the main solid-state switch accordingly.

In an embodiment, the switch controller is configured to deactivate the switch controller if the current across the shunt resistor exceeds the current threshold for the given duration of time.

In an embodiment, the switch controller is configured to store a value associated with the current across the shunt resistor in a non-volatile memory unit and continue to deactivate the power switch even if the battery pack is decoupled and recoupled to the battery receptacle.

In an embodiment, the battery disconnect circuit further includes a comparator configured to compare a voltage drop across the shunt resistor to a reference voltage value, and a latch configured to drive the main solid-state switch according to an output of the comparator.

In an embodiment, the current threshold is determined based on a current-time profile that at no point of time exceeds the first current profile or falls below the second current profile.

In an embodiment, the current-time profile includes a linear portion. Alternatively, and/or additionally, the current-time profile includes a stepped-profile or a flat portion.

In an embodiment, a power tool is provided comprising: a housing; a brushless direct-current (BLDC) electric motor disposed inside the housing; a battery receptacle configured to receive a battery pack, the battery pack including a plurality of battery cells and a fuse being associated with a first current profile; a plurality of power switches disposed between the battery receptacle and the electric motor, the plurality of power switches including a plurality of high-side switches and a plurality of low-side switches, the plurality of power switches being associated with a second current profile; a control unit configured to control a switching operation of the plurality of power switches to operate the electric motor; and a battery disconnect circuit including: a main solid-state switch disposed on a current path from the battery receptacle to the plurality of power switches, a shunt resistor disposed in series with the main solid-state switch between the battery receptacle and the plurality of power switches, and a switch controller separate from the control unit that is configured to monitor a current across the shunt resistor and deactivate the main solid-state switch to disconnect the supply of power from the battery pack if the current draw from the battery pack exceeds a current threshold for a given duration of time.

The following description illustrates the claimed invention by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the disclosure, describes several embodiments, adaptations, variations, alternatives, and uses of the disclosure, including what is presently believed to be the best mode of carrying out the claimed invention. Additionally, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. The disclosure is capable of other embodiments and of being practiced or being carried out in various ways.

Referring to <FIG>, a circuit block diagram of power tool <NUM> including a motor <NUM> and a motor control module <NUM> is depicted, according to an embodiment. In an embodiment, motor control module <NUM> includes a power unit <NUM> and a control unit <NUM>. In <FIG>, power tool <NUM> received DC power from a DC power source such as a battery pack <NUM> via B+ and B- terminals.

In an embodiment, power unit <NUM> may include a power switch circuit <NUM> coupled between the power source B+/B- terminals and motor windings to drive BLDC motor <NUM>. In an embodiment, power switch circuit <NUM> may be a three-phase bridge driver circuit including six controllable semiconductor power devices (e.g. FETs, BJTs, IGBTs, etc.).

In an embodiment, control unit <NUM> may include a motor controller <NUM>, a gate driver <NUM>, a power supply regulator <NUM>, and a contact switch <NUM>. In an embodiment, motor controller <NUM> is a programmable device arranged to control a switching operation of the power devices in power switching circuit <NUM>. In an embodiment, motor controller <NUM> receives rotor rotational position signals from a set of position sensors <NUM> provided in close proximity to the motor <NUM> rotor. In an embodiment, position sensors <NUM> may be Hall sensors. It should be noted, however, that other types of positional sensors may be alternatively utilized. It should also be noted that motor controller <NUM> may be configured to calculate or detect rotational positional information relating to the motor <NUM> rotor without any positional sensors (in what is known in the art as sensorless brushless motor control). motor controller <NUM> may also receive a variable-speed signal from variable-speed actuator or a speed-dial. Based on the rotor rotational position signals from the position sensors <NUM> and the variable-speed signal, motor controller <NUM> outputs drive signals UH, VH, WH, UL, VL, and WL through the gate driver <NUM>, which provides a voltage level needed to drive the gates of the semiconductor switches within the power switch circuit <NUM> in order to control a PWM switching operation of the power switch circuit <NUM>.

In an embodiment, power supply regulator <NUM> may include one or more voltage regulators to step down the power supply to a voltage level compatible for operating the motor controller <NUM> and/or the gate driver <NUM>. In an embodiment, power supply regulator <NUM> may include a buck converter and/or a linear regulator to reduce the power voltage of battery pack <NUM> down to, for example, 15V for powering the gate driver <NUM>, and down to, for example, <NUM>. 2V for powering the motor controller <NUM>.

In an embodiment, contact switch <NUM> may be provided between the power supply regulator <NUM> and the gate driver <NUM>. Contact switch <NUM> may be an ON/OFF switch coupled to the ON/OFF trigger or the variable-speed actuator to allow the user to begin operating the motor <NUM>. Power switch <NUM> in this embodiment disables supply of power to the motor <NUM> by cutting power to the gate drivers <NUM>. It is noted, however, that contact switch <NUM> may be provided at a different location, for example, within the power unit <NUM> between the battery pack <NUM> and the power switch circuit <NUM>. It is further noted that in an embodiment, power tool <NUM> may be provided without a contact switch <NUM>, and the motor controller <NUM> may be configured to activate the power devices in power switch circuit <NUM> when the ON/OFF trigger (or variable-speed actuator) is actuated by the user.

In an embodiment, battery pack <NUM> is provided with at least one string of cells <NUM>, which includes a series of battery cells, for example, lithium or lithium-ion cells, connected in series. A fuse <NUM> is coupled in series with the string of cells <NUM>. In an embodiment, two or more strings of cells <NUM> may be connected together in parallel, thus increasing the ampere-hour capacity of the battery pack. When two or more strings of cells <NUM> are provided, in an embodiment, two or more fuses <NUM> corresponding to the number of parallel strings of cells <NUM> may be provided. The fuse <NUM> has a current rating above which level the fuse opens and disconnects supply of current from the battery pack <NUM>. In an embodiment, the fuse <NUM> may have a current-time curve (e.g., a |^<NUM>. t curve) that defines the current bounds of the fuse <NUM>.

In an embodiment, a battery disconnect circuit <NUM> is provided on the current path of the battery pack <NUM>, e.g., between the B+ terminal of the battery pack <NUM> and the power switch circuit <NUM>. Battery disconnect circuit <NUM> is provided as a placement for a conventional fuse to ensure disconnection of the battery pack <NUM> from the motor <NUM> in the event that an electrical shortage or other system failure causes high current draw from the battery pack <NUM>.

<FIG> depicts a circuit diagram of the battery disconnect circuit <NUM>, according to an embodiment. In an embodiment, battery disconnect circuit <NUM> includes a main solid-state switch <NUM>, for example, a MOSFET, provided on the current path between B+ and the power switch circuit <NUM>. In an embodiment, the battery disconnect circuit <NUM> further includes a shunt resistor <NUM> provided in series with the main switch <NUM>. The voltage drop across the shunt resistor <NUM> corresponds to the amount of current being drawn by the power switch circuit <NUM>. An amplifier <NUM> is coupled to the two sides of the shunt resistor <NUM> as a differential amplifier. The output of the amplifier <NUM> is received by a switch controller <NUM>. The amplifier <NUM> amplifies the voltage drop across the shunt resistor <NUM> by gains controlled via resistors RGAIN_1 AND RGAIN_2 to a level operable by the switch controller <NUM>.

The switch controller <NUM>, similarly to motor controller <NUM>, is a programmable device, though, in an embodiment, the switch controller <NUM> has less processing speed than the motor controller <NUM>. In an embodiment, the switch controller <NUM> is configured to receive the signal from the amplifier <NUM> and activates a drive signal via the gate driver <NUM> accordingly to turn the main witch <NUM> ON or OFF. In an embodiment, voltage drop across the shunt resistor <NUM> exceeds a reference threshold, the switch controller <NUM> determines that the current draw is larger than a reference current threshold and turns the main switch <NUM> OFF. This reference current threshold, as will be described later, is typically associated with current levels seen in the event of an electrical shortage or system failure within the power tool <NUM> that may lead to damage to power tool <NUM> components. In an embodiment, switch controller <NUM> activates the drive signal to maintain the main switch <NUM> ON as long as the current draw does not exceed the reference current threshold. If the current draw is larger than the reference current threshold, the switch controller <NUM> activates the drive signal to turn OFF the main switch <NUM>.

Furthermore, in an embodiment, the switch controller <NUM> stores a value associated with the signal from the comparator <NUM> in storage <NUM>, which may be a non-volatile memory unit. Accordingly, if current exceeds the reference current threshold and the value is stored in storage <NUM>, the switch controller <NUM> will not allow the main switch <NUM> to close even if the battery pack <NUM> is removed and reattached to the power tool <NUM>. This ensured that the system failure or electrical shortage within the power tool <NUM> that caused the high current will not be repeated and that the power tool <NUM> cannot be used until the tool is serviced and the switch controller <NUM> is reset by an authorized technician.

In an embodiment, switch controller <NUM> is in communication with motor controller <NUM>. This communication allows the motor controller <NUM> to know the state of the battery disconnect circuit <NUM>, i.e., if a "fuse event" takes place, and provide a secondary means to disable the main switch <NUM> of the battery disconnect circuit <NUM> for added redundancy. In an embodiment, upon detection of the current being larger than the reference current threshold, the switch controller <NUM>, in addition to disabling the main switch <NUM> via the gate driver <NUM>, sends a signal to the motor controller <NUM>. The motor controller <NUM> sends a signal back to the switch controller <NUM> signifying that the main switch <NUM> is to be disabled. Additionally, and/or alternatively, the motor controller <NUM> sends a signal to the gate driver <NUM> to disable the main switch <NUM>. In this manner, the motor controller <NUM> provides a secondary level of redundancy to ensure that the main switch <NUM> is disabled upon detection of a "fuse event.

In an embodiment, a power supply regulator <NUM> may be provided to supply power from the battery pack <NUM> to the switch controller <NUM>. Further, in an embodiment, a contact switch <NUM> may be provided between the battery pack <NUM> and the power supply regulator <NUM>. Contact switch <NUM> may be provided in addition to, or in place of, contact switch <NUM> described above. In an embodiment, contact switch <NUM> may be actuated via the power tool trigger switch.

<FIG> depicts a circuit diagram of the battery disconnect circuit <NUM>, according to an alternative embodiment. In this embodiment, the battery disconnect circuit <NUM> includes, instead of a switch controller <NUM>, a comparator <NUM>, a latch <NUM>, and a gate switch <NUM>. Comparator <NUM> compares the output of the amplifier <NUM>, which signifies the delta of the voltage across the resistor <NUM>, with a reference voltage VREF. If the output voltage of the amplifier <NUM> exceeds the reference voltage VREF at any time, latch <NUM> drives the gate of gate switch <NUM>, which causes the gate driver <NUM> to shut down the main switch <NUM>.

<FIG> depict graphs of current-time profiles associated with the battery disconnect circuit in comparison to profiles associated with the battery pack and the power tool, according to an embodiment. In these graphs, plot <NUM> represents the current-time rating of the motor control module <NUM>. A current level above this plot for a given duration of time is likely to cause damage to the control and power components of the motor control module <NUM>. Similarly, plot <NUM> represents the current-time rating of battery packs <NUM>. A current level above this plot for a given duration of time is likely to cause damage to the battery pack <NUM> components. In an embodiment, plot <NUM> correspond to the current-time rating of the fuse <NUM> within the battery pack <NUM>. Dots <NUM> represent sample applied currents for corresponding durations of time performed for UL testing.

In an embodiment, plots <NUM>, <NUM> and <NUM> represent the current-time rating of the battery disconnect circuit <NUM>, according to three exemplary embodiments. In particular, the reference current threshold discussed above is set as a function of time according to plots <NUM>, <NUM> and <NUM>. Each plot <NUM>, <NUM> and <NUM> includes at least a significant portion that is located between plots <NUM> and <NUM>. This ensures that high current levels that may occur as a result of electrical shortage or damage within the motor control module <NUM> (i.e., current levels exceeding the rated current of the motor control module <NUM>) neither damage the battery pack <NUM> nor activate the fuse within the battery pack <NUM>.

In an embodiment, plot <NUM>, as shown in <FIG>, includes at least a linear portion (e.g., between <NUM> to <NUM> seconds) located between plots <NUM> and <NUM>. The plot <NUM> further includes a flat portion at which the current is clamped at <NUM> amps for short durations of time less than <NUM> seconds.

In an embodiment, plot <NUM>, as shown in <FIG>, is a linear plot fully contained between plots <NUM> and <NUM>. In an embodiment, plots <NUM> and <NUM> may include linear portions, non-linear (curved) portions, or combinations thereof.

In an embodiment, plot <NUM>, as shown in <FIG>, is a stepped plot, where discrete current thresholds are set corresponding to discrete time intervals (e.g., every <NUM> seconds). This may be done according to a look-up table.

In an embodiment, main controller <NUM> is configured to enforce a separate current control scheme from the battery disconnect circuit <NUM>. In this embodiment, the current limit enforced by the main controller <NUM> is lower than the current limit enforced by the battery disconnect circuit <NUM> as well as the current rating of the motor control module <NUM>. Additionally, and/or alternatively, the current-time profile enforced by the main controller <NUM> is positioned lower than the current-time profile enforced by the battery disconnect circuit <NUM> as well as the current-time profile corresponding to the current rating of the motor control module <NUM>.

Claim 1:
A power tool comprising:
a housing;
a brushless direct-current (BLDC) electric motor (<NUM>) disposed inside the housing;
a battery pack (<NUM>);
a battery receptacle configured to receive the battery pack (<NUM>), the battery pack including a plurality of battery cells and a fuse being associated with a first current profile;
a plurality of power switches disposed between the battery receptacle and the electric motor, the plurality of power switches including a plurality of high-side switches and a plurality of low-side switches, the plurality of power switches being associated with a second current profile;
a control unit (<NUM>) configured to control a switching operation of the plurality of power switches to operate the electric motor; and
a battery disconnect circuit (<NUM>) disposed between the battery receptacle and the plurality of power switches to disconnect the supply of power from the battery pack if the current draw from the battery pack exceeds a current threshold for a given duration of time, wherein the current threshold is smaller than a first current level associated the first current profile for the given duration of time but larger than a second current level associated with the second current profile for the given duration of time.