Abstract:
A dual-mode system for inexpensively operating electrically powered double-insulated devices ( 12 ), such as hand-held power tools and appliances. The system includes a cordless battery pack ( 14 ) that supplies the power and current demands of the device ( 12 ) in a cordless mode or a non-isolated corded voltage converter ( 16 ) that supplies the necessary power and current demands in a physical envelope commensurate in size and interchangeable with that of the battery pack ( 14 ). The corded voltage converter ( 16 ) is provided with a non-isolated high efficiency power supply that allows the converter ( 16 ) to generate the power and current required by the driven device ( 12 ). The double insulation of the driven device ( 12 ) negates the need for a transformer-isolated voltage converter. Eliminating the power transformer from the converter significantly reduces the cost of the module ( 16 ). Additionally, the need for multiple battery packs and fast rechargers is minimized by the availability of a low-cost converter. The voltage converter ( 16 ) includes an inrush current limiter ( 103 ) and power conditioner for filtering AC or DC input power. The filtered voltage is chopped by a transformerless buck-derived converter. The chopped voltage is rectified and filtered to provide low-voltage DC power to the drive motor of the powered double-insulated device ( 12 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of the filing date of U.S. provisional application No. 60/114,218 filed Dec. 30, 1998. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to electrically operated power tools and in particular, to portable hand-held power tools which can alternatively operate in either a cordless mode from a self-contained power source or a corded mode from a conventional AC/DC generator power source.  
         BACKGROUND OF THE INVENTION  
         [0003]    Electrically operated devices that function in a cordless mode typically include a housing which has a chamber for receiving and retaining a removable battery pack. The battery pack completely encloses one or more cells and provides the necessary DC power for operation of the device. Historically, cordless electrically powered devices have included relatively low power devices such as shavers and hand-held calculators. Recently, improvements in battery technology have led to the development of batteries that store more energy and are capable of driving higher power devices. These devices include for example, portable hand-held power tools and appliances operating at power levels from 50 watts up to hundreds of watts. A hand-held power tool is typically powered by a battery pack that comprises a number of batteries connected in series. To provide the higher power levels required by high power devices an increased number of batteries are connected in series resulting in higher input voltages and battery pack volumetric requirements.  
           [0004]    Cordless power devices permit work operations to be performed in areas where a conventional AC power source is not available or inconvenient to use. However, the effective charge capacity of the battery pack and the availability of replacement battery packs limit the use of cordless devices. When the battery pack is discharged, it must be recharged or replaced with a fully charged pack.  
           [0005]    Both batteries and battery chargers are expensive in comparison to the power device for which they are intended. Batteries for high power applications cost approximately 30% of the cost of the applicable power device. Additional batteries are required to permit cordless mode operation while a battery is recharged and to replace dead batteries. High power levels drawn from batteries during operation of the power tool, the depth of discharge of the battery, the number of charge/discharge cycles, and the speed with which a battery is recharged all contribute to shortening the usable lifetime of a battery. Fast chargers can cost more than the power tool or appliance that is powered by the battery. There are two basic types of battery chargers, trickle chargers and fast chargers. Trickle chargers are significantly less expensive than fast chargers, however a trickle charger requires approximately ½ day to recharge a battery pack. A fast charger on the other hand can recharge a battery pack within approximately one hour. Therefore, a trade off must be made between using a trickle charger with a large number of battery packs versus using a costly fast charger with very few replacement battery packs.  
           [0006]    It has recently been proposed to provide portable cordless power tools with an optional corded AC converter module that is connected to an AC power source and designed to replace the battery pack. The corded converter module converts power from the AC source to a regulated low-voltage DC level that is usable by the motor of the power device. Such a device allows a tool operator to use the tool in either the cordless battery mode or the corded AC mode as needed. Thus, the availability of such device enables the operator of a cordless tool to complete a project when the battery pack has been discharged, or to continue to use the tool while the battery pack is charging and a fully charged backup battery pack is unavailable. Hence, by using a corded converter module the need for extra battery packs is minimized.  
           [0007]    However, the prior art design of a corded converter module is constrained by a number of factors such as the physical envelope, the required output power level, the voltage conversion ratio of the converter, safety requirements to protect the operator from electrical shock, and cost. The envelope of the corded converter module must conform to the envelope of the battery pack with which it is interchangeable. With the increased volumetric requirements for battery packs there is increased volume available for housing a corded converter. The power output level of the converter is directly related to the available volume within the container envelope. The power output levels adequate to drive power devices such as hand held power tools are possible within the physical envelope of commercial battery packs. The voltage conversion ratio of the converter is the ratio between the rectified input voltage and the converter output voltage. The converter output voltage is set to a level roughly equivalent to the battery voltage. The greater the voltage conversion ratio the more difficult it is to accurately regulate the output voltage. The safety regulations are typically met by isolating the operator of the power device from the AC power source. Commercially available systems meet the safety regulations by employing a high frequency power transformer to isolate the output power of the converter module from the relatively high voltage AC input power source. Power transformers are custom devices that are expensive and bulky in comparison with the other electronic devices of the converter module. Attempts to minimize costs of corded converter modules have concentrated on optimizing the output power capability of the converter module for a given power device. By designing the converter module for the minimum output power required to satisfactorily drive the power device, lower cost electronic components can be chosen for the converter.  
           [0008]    Operators of cordless power tools already faced with the cost of battery packs and battery chargers must also invest in expensive corded converter modules for their power tools. As an alternative many purchase a corded power tool to use in lieu of the cordless tool when an AC power source is nearby. Attempts to minimize the cost of corded conversion modules have been constrained by the cost of using transformer isolation to meet the government safety requirements. Obtaining further cost reductions by reducing the output power level of a corded converter module would result in under-powered power devices. While the prior art can be used to provide corded converter modules for a handheld power tool, it has not proven capable of providing low cost modules that are convenient to use.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention decreases costs by meeting the government safety requirements in a unique manner. The invention uses a double insulated casing for the power tool rather than employing transformer isolation. Eliminating the power transformer from the corded converter module significantly reduces the cost and weight of the module. A low cost converter module provides operators of cordless power tools the low cost option of using a corded converter module when AC power sources are available. This eliminates the cost of purchasing a separate corded power device as well as reducing the number of battery packs that must be purchased.  
           [0010]    Corded power converters designed without power transformers are substantially less expensive than converters designed with power transformers. Additionally, eliminating the power transformer decreases the weight of the converter resulting in improved operator comfort.  
           [0011]    For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a three-dimensional view partially showing the manner of connecting a battery pack to the power device;  
         [0013]    [0013]FIG. 2 is a three-dimensional view partially showing the manner of connecting an AC/DC power converter module to the power device;  
         [0014]    [0014]FIG. 3A is a three-dimensional exploded view of the battery pack;  
         [0015]    [0015]FIG. 3B is a three-dimensional exploded view of the power converter module;  
         [0016]    [0016]FIG. 4 is an end view of the battery pack illustrating an attached terminal block;  
         [0017]    [0017]FIG. 5 is a three-dimensional view of the power tool terminal block that mates to the battery pack terminal block;  
         [0018]    [0018]FIG. 6 is a two-dimensional view of the interface between the battery pack terminal block and the power tool terminal block;  
         [0019]    [0019]FIG. 7 is a two-dimensional view of the interface between the AC/DC power converter module and the power tool terminal block;  
         [0020]    [0020]FIG. 8 is a block diagram of a power converter assembled and contained within the AC/DC power converter module of FIG. 2;  
         [0021]    [0021]FIG. 9 is a schematic diagram of the power stage of the power converter of FIG. 8;  
         [0022]    [0022]FIG. 10 is a schematic diagram of the control circuit of the power converter of FIG. 8;  
         [0023]    [0023]FIG. 11 is a signal diagram showing the voltage and current waveforms associated with the power converter;  
         [0024]    [0024]FIG. 12 is a cross-sectional view of an armature of a non-double insulated DC power tool motor;  
         [0025]    [0025]FIG. 13 is a cross-sectional view of an armature of DC power tool motor that employs a first method of double insulation;  
         [0026]    [0026]FIG. 14 is a cross-sectional view of an armature of DC power tool motor that employs a second method of double insulation;  
         [0027]    [0027]FIG. 15 is a cross-sectional view of an armature of DC power tool motor that employs a third method of double insulation;  
         [0028]    [0028]FIG. 16 is cross section through the center of the lamination stack of an armature for a DC power tool motor that employs double insulation; and  
         [0029]    [0029]FIG. 17 is a cross-sectional view of a housing for a DC power tool that employs double insulation. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    Referring to FIGS. 1 and 2, a dual-mode portable power tool  12  according to the present invention is shown. While the present invention is shown and described with a reciprocating saw  12 , it will be appreciated that the particular tool is merely exemplary and could be a circular saw, a drill, or any other similar portable power tool constructed in accordance with the teachings of the present invention.  
         [0031]    The power tool  12  includes a DC motor (not shown) that is adapted in the preferred embodiment to be powered by a source having a relatively low voltage such as a 24 volt DC source, although other low voltage DC systems, such as 12 volts or 18 volts, could be used. In a first operating mode shown in FIG. 1, the power tool  12  is powered by a removable battery power supply module  14 . Alternatively, as shown in FIG. 2, the power tool  12  may be powered from a source having a relatively high voltage such as common 115 volt AC line power via an AC/DC power converter module  16  which is adapted to be plugged into the power tool in place of the battery power supply module  14 . Additionally, the power tool  12  may be powered from a relatively high voltage DC generator (not shown) via the AC/DC power converter module  16 . As used in this specification and the accompanying claims, the term relatively high voltage means voltages of 40 volts or greater and the term relatively low voltage means voltages less than 40 volts.  
         [0032]    With particular reference to FIGS. 3A and 4, the rechargeable battery power supply module  14  of the present invention is illustrated to generally include a housing  18 , a battery  20  which in the exemplary embodiment illustrated is a 24 volt nickel-cadmium battery, and a battery pack terminal block  22 . To facilitate releasable attachment of the battery power supply module  14  to the tool  12 , the upper portion  25  of the housing  18  is formed to include a pair of guide rails  24 . The guide rails  24  are adapted to be slidably received into cooperating channels  13  (FIG. 1) formed in a housing  14  of the tool  12 . To further facilitate removable attachment of the battery power supply module  14  to the tool  12 , the upper portion  25  of the housing  18  further defines a recess  26 . The recess  26  is adapted to receive a latch (not shown) carried by the housing of the tool  12 . The latch is conventional in construction and operation and is spring biased to a downward position so as to engage the recess  26  upon insertion of the rechargeable battery power supply module  14 . Removal of the battery power supply module  14  is thereby prevented until the spring bias of the latch is overcome in a conventional manner insofar as the present invention is concerned.  
         [0033]    With continued reference to FIGS. 3A and 4, the battery pack terminal block  22  comprises a main body portion  28  constructed of rigid plastic or other suitable material and a plurality of blade-type terminals  30 . In the exemplary embodiment illustrated, the battery pack terminal block  22  includes four blade terminals  30 . Two of the blade terminals  30  comprise the positive and negative terminals for the battery  20 . A third terminal  30  may be used to monitor the temperature of the battery  20  and a fourth terminal may be used to identify the battery type (e.g., 24 volt NiCad). As best shown in FIG. 4, a pair of holes  32  are formed in the two guide rails  24  in the upper portion  25  of the battery pack housing  18  on either side of the row of blade terminals  30 . The function of these holes is described below.  
         [0034]    Turning now to FIG. 5, the terminal block  34  of the power tool  12  is shown. The main body of the tool terminal block  34  is also constructed of a rigid plastic material and is formed with a row of four U-shaped guideways  36  guiding the four corresponding blade terminals  30  of the battery power supply module  14  when the battery pack is inserted into the tool  12 . Located within the guideways  36  are female connectors  38  that are adapted to engage and make electrical contact with the blade terminals  30  of the battery power supply module  14 . Although the tool terminal block  34  shown is designed to accommodate four female connectors for each of the four battery pack blade terminals  30 , only two female connectors  38  adapted to engage the positive and negative blade terminals  30  of the battery power supply module  14  are used in the tool terminal block  34 , as the remaining two battery pack blade terminals  30  are only used when recharging the battery power supply module  14 .  
         [0035]    Also connected to the positive and negative female terminals  38  in the tool terminal block  34  are positive and negative male terminals  40  that project through openings  42  in the terminal block on either side of the row of guideways  36 . As will subsequently be discussed below, the male positive and negative terminals  40  are used to electrically connect the tool  12  to the AC/DC converter module  16 .  
         [0036]    With additional reference to FIG. 6, the interface between the battery terminal block  22  and the tool terminal block  34  is illustrated. As the guide rails  24  of the battery power supply module  14  are slid into the channels  13  in the tool housing, the battery pack terminal block  22  is guided into alignment with the tool terminal block  34  as shown. To further facilitate proper alignment between the two terminal blocks  22  and  34 , the main body portion of the tool terminal block  34  includes a pair of laterally spaced rails  44  that are adapted to be received within the grooves  46  provided in the battery pack housing  18  immediately below the guide rails  24 . Further insertion of the battery power supply module  14  onto the tool  12  results in the positive and negative blade terminals  30  of the battery power supply module  14  passing through the openings in the U-shaped guideways  36  and engaging the female connectors  38  in the tool terminal block  34 . Note that the male positive and negative terminals  40  from the tool terminal block  34  simultaneously project into the openings  32  formed in the rails  24  on the upper portion  25  of the battery pack housing  18 , but do not make electrical contact with any terminals in the battery power supply module  14 . Similarly, the remaining two blade terminals  30  from the battery terminal block  22  project into empty guideways  36  in the tool terminal block  34 .  
         [0037]    Returning to FIG. 2 with reference to FIG. 3B, the AC/DC converter module  16  according to the present invention is adapted to convert 115 volts AC house current to 24 volts DC. The housing  48  of the converter module  16  in the preferred embodiment is configured to be substantially similar to the housing  18  of the battery power supply module  14 . In this regard, the housing  48  includes first and second clam shell halves joined at a longitudinally extending parting line. An upper portion  50  of the housing  48  includes a pair of guide rails  52  similar to those of the battery power supply module  14  for engaging the channels  13  in the tool housing. The upper portion  50  also defines a recess (not shown) which includes a latch (not shown) for preventing the inadvertent removal of the converter module  16 . The housing  48  also defines a recess  51  in which a fan  45  is adapted for providing cooling airflow to the converter module  16 . Attached to the fan  45  is a fan cover  47  for preventing foreign objects from impeding the operation of the fan  45 . Within the housing  48  several heatsinks  43  provide heat spreading and cooling for selected power converter components.  
         [0038]    With additional reference to FIG. 7, the interface between the converter module  16  and tool terminal block  22  is shown. The converter module  16  includes a pair of female terminals  54  that are adapted to receive the male terminals  40  of the tool terminal block  22 . In a manner similar to that described above in connection with the installation of the battery power supply module  14  on the tool  12 , the guide rails  52  on the upper portion  50  of the converter housing  48  are adapted to engage the laterally spaced rails  44  on the tool terminal block  34  as the converter module  16  is installed on the tool  12  to ensure proper alignment between the female connectors  54  of the converter module  16  and the male connectors  40  of the tool  12 .  
         [0039]    Due to the non-isolated nature of the AC/DC converter module  16  in the present invention, the female terminals  54  are recessed within the upper portion  50  of the housing  48  of the converter module  16  to meet safety requirements. In the preferred embodiment, the female terminals  54  are recessed within the housing  48  of the converter module  16  by at least 8 mm. 115 volt AC power is converted to 24 volt DC power by the converter module  16  and delivered to the tool  12  through the female terminals  54 . When the converter module  16  is operatively installed on the tool  12 , the female terminals  38  of the tool terminal block  34  are electrically inoperative.  
         [0040]    The presently preferred embodiment of the AC/DC power converter module  16  is a fixed-frequency, non-isolated, buck-derived topology; however, the principles of the invention can be extended to variable-frequency converters and topologies other than buck-derived, such as Cük and flyback converters. The power converter module  16  is designed to convert an unregulated AC voltage to a regulated DC voltage that is usable by the power tool  12 . For example, the converter module  16  can convert an input of 120 volts, 60 Hz AC to any low-level DC voltage less than 42 volts that is required by the power tool  12 , such as 24 volts DC.  
         [0041]    As illustrated in block diagram form in FIG. 8, the power converter module  16  includes a fuse  101  in series with diode bridge  102 . A power plug and cord (refer to FIG. 2) connect from fuse  101  to the other input of diode bridge  102 . The output of diode bridge  102  is applied between high side line  104  and an inrush limiter  103  connected to ground reference line  106 . The rectified output voltage of diode bridge  102  is filtered by the input capacitor  108 . The resulting filtered voltage is nominally 165 volts DC. The input capacitor  108  connects to the drains of parallel power MOSFETs  110   a  and  110   b  that act as a voltage controlled switch. When MOSFETs  110   a  and  110   b  are in the ON state the impedance between the drain and source is low. When in the OFF state the impedance between drain and source is very high, effectively preventing current flow. The sources of MOSFETs  110   a  and  110   b  connect to the junction of output inductor  112  and the cathode of free-wheeling output diode  114 . The other side of output inductor  112  connects to output capacitor  116 . Current sense resistor  118  connects between the output capacitor  116  and the anode of the freewheeling diode  114 . The anode of output diode  114  also connects to ground reference line  106 . The voltage across output capacitor  116  is applied to the output of power converter module  16  across outputs VOUTHI  120  and VOUTLO  122 , which connect to the pair of female terminals  54 . Fan  123  is connected in parallel with output capacitor  116 . Diode bridge  102 , MOSFET  110 , and freewheeling output diode  114  all mount on heat sinks that provide heat spreading and a thermal path for dissipated power.  
         [0042]    [0042]FIGS. 8 and 10 illustrate the circuitry that provides control and protection functions for power converter module  16  which includes voltage regulated power supply  124 , PWM control  126 , voltage feedback  128 , current limit  130 , and temperature sense  134 . The voltage regulated power supply  124  connects across input capacitor  108  to provide a low power, regulated low voltage output to supply power to the internal circuitry of power converter module  16 . The regulated low voltage output as well as the remainder of the internal circuitry is referenced to ground reference line  106 . VOUTHI  120  connects to voltage feedback  128  which connects to PWM control  126 . The current sense resistor  118  connects to current limit  130  which also is connected to temperature sense  134 . The output of current limit  130  connects to PWM control  126 . The arrangement of components that comprise voltage regulated power supply  124 , PWM control  126 , voltage feedback  128 , current limit  130 , and temperature sense  134  are well known in the art.  
         [0043]    [0043]FIGS. 9 and 10 illustrate the circuitry that provides the power conversion function for power converter module  16  which includes high voltage driver  132  and power stage components. The output of PWM control  126  connects to high voltage driver  132  which level shifts the output of PWM control  126  to drive the gates of MOSFETs  110   a  and  110   b.  The arrangement of components that comprise high voltage driver  132  are well known in the art. In the presently preferred embodiment of the invention an SGS-Thomson L6381 high-side driver  172  with associated components comprises the high voltage driver  132 . However, other circuit configurations for level-shifting the PWM output are within the scope of the invention, such as discrete component configurations and Motorola high-side driver chips.  
         [0044]    Referring to FIG. 8, at initial power-on of power converter module  16 , the power plug and cord are connected to an AC power source. The AC voltage is rectified by diode bridge  102  and applied across input capacitor  108 . Current from the AC source surges as it flows through fuse  101 , inrush limiter  103 , diode bridge  102 , and begins to charge input capacitor  108 . The magnitude of the surge in current is limited to a safe level by the action of the inrush limiter  103  which is a high impedance initially, but rapidly changes to a low impedance. In the present embodiment the inrush limiter  103  consists of a triac  152  in parallel with a resistor  150  that is triggered by current flowing through output inductor  112 . However, other well known circuits are also envisioned, such as a series thermistor, and a high valued series resistor in parallel with a controlled semiconductor that is triggered by temperature, time, or current magnitude. As the voltage across input capacitor  108  rises towards its nominal value of 165 volts DC the voltage regulated power supply  124  becomes active and begins to supply voltage to the internal circuitry of the power converter module  16  including PWM control  126 . During the initial charging of input capacitor  108 , the triac  152  remains off forcing return current to flow through resistor  150 , thereby limiting the peak value of the inrushing current. The triac  152  remains OFF until the output of PWM control  126  becomes active driving the MOSFETs  110   a  and  110   b  to the ON state, at which time current flowing through output inductor  112  couples through a sense winding of inductor  112  to trigger the triac ON.  
         [0045]    The PWM control  126  in the present embodiment is a Texas Instruments TL494 with the associated components as depicted in FIG. 10. There are numerous other control chips which could be used, such as UC1845 and SG1625. The output of PWM control  126  is disabled until the regulated output of voltage regulated power supply  124  exceeds 6.4 volts, at which time soft-start mode is enabled. Prior to the beginning of soft-start the oscillator of PWM control  126  begins to operate. The present embodiment switches at a fixed frequency of 40 kHz, although higher or lower frequencies are within the scope of the invention. During steady-state operation of power converter module  16  the PWM control  126  output is a low-voltage square-wave signal having a variable pulse-width, where the pulse-width is adjusted to maintain a regulated output voltage at outputs VOUTHI  120  and VOUTLO  122 . During soft-start the pulse-width of the PWM control  126  output is initially zero, gradually increasing to a steady-state value that results in the output voltage being regulated at a desired voltage. The duration of soft-start mode is controlled by the selection of component values in PWM control  126  and is well known in the art. The purpose of soft-start is to limit the current and voltage stress of the power converter module  16  components during the time period when output capacitor  116  is being charged up to its nominal steady-state value. As the voltage across output capacitor  116  approaches its steady-state value the output of voltage feedback  128  rises towards its steady-state value, resulting in the pulse-width of PWM control  126  attaining a steady-value that regulates the voltage across output capacitor  116  at the desired value. The feedback network in the present embodiment is a lag-lead-lag-lead configuration with well known design requirements to maintain a stable operation of power converter module  16 . During steady-state operation the output from PWM control  126  which is level-shifted by the high voltage driver  132  repetitively drives the MOSFETs  110   a  and  110   b  into an ON state and an OFF state at the switching frequency.  
         [0046]    Referring to waveforms vs, iL, and vout of FIG. 11 in addition to FIG. 8, when MOSFETs  110   a  and  110   b  are in the ON state, the voltage from input capacitor  108  is passed through to the sources of MOSFET  110   a  and  110   b,  vs, and impressed on the input of output inductor  112  reverse biasing free-wheeling diode  114 . The voltage across output inductor  112  during the ON state is equal to the voltage across input capacitor  108  minus the voltage across output capacitor  116 , vout. The positive voltage across inductor  112  causes current, iL, through inductor  112  to increase at a linear rate. The current splits between VOUTHI  120  and output capacitor  116  with the DC component flowing to VOUTHI  120  and the AC component substantially flowing through output capacitor  116 . Current returning from load  121  flows from VOUTLO  122  through current sensor resistor  118  and input capacitor  108  thereby completing the current path.  
         [0047]    When the MOSFETs  110   a  and  110   b  are switched to the OFF state, they present a high impedance to the voltage from input capacitor  108  decoupling that voltage from the remainder of the circuit. During this period free-wheeling diode  114  is active. The current, iL, from output inductor  112  which previously flowed through MOSFETs  110   a  and  110   b  now flows through free-wheeling output diode  114 . With output diode  114  conducting, the voltage, vs, at the input to output inductor  112  is approximately one diode drop below ground reference line  106 . The voltage across output inductor  112  is equal to negative one volt minus the voltage across output capacitor  116 . The negative voltage across inductor  112  causes current through inductor  112  to decrease at a linear rate. The current again splits between VOUTHI  120  and output capacitor  116  with the DC component flowing through VOUTHI  120  and the AC component substantially flowing through output capacitor  116 . The current returning from load  121  flows from VOUTLO  122  through current sense resistor  118  and free-wheeling output diode  114 , thereby completing the current path. The MOSFETs  110   a  and  110   b  remain in the OFF state for the remainder of the cycle time period.  
         [0048]    Again referring to FIG. 8 with additional reference to waveforms vg and vpwm of FIG. 11, the output of PWM control  126  is level-shifted by high voltage driver  132  in order to drive power MOSFETs  110   a  and  110   b  to either the ON state or the OFF state. During the transition from the OFF state to the ON state, the PWM control  126  output voltage, vpwm, transitions low which causes the output of driver  172  to transition high, thus biasing the base emitter junction of PNP transistor  178  turning it OFF. At the same time NPN transistor  174  turns ON. Current flows through NPN transistor  174  and resistors  176   a  and  176   b  into the gates of power MOSFETs  110   a  and  110   b  charging up the internal gate-source capacitance, raising the MOSFETs  110   a  and  110   b  gate voltage, vg, above ground before returning from the sources of MOSFETs  110   a  and  110   b  to filter capacitor  168 . The increasing voltage across the gate-source of MOSFETs  110   a  and  110   b  causes the MOSFETs  110   a  and  110   b  to begin to turn ON, causing the source voltage of MOSFETs  110   a  and  110   b  to increase from minus one volt relative to ground reference line  106  to a value approaching the value of voltage across input capacitor  108  and additionally causing the MOSFETs  110   a  and  110   b  gate voltage, vg, to increase to the value of voltage across input capacitor  108  plus the MOSFETs gate-source voltage. As the source voltage of MOSFETs  110   a  and  110   b  increases, the decoupling diode  166  becomes reverse biased decoupling the diode  166  from the remainder of the high voltage driver  132 . Filter capacitor  168  remains referenced to the source of MOSFETs  110   a  and  110   b  and thereby provides the energy required to maintain the gate-source voltage of MOSFETs  110   a  and  110   b  during the remainder of the ON state.  
         [0049]    The PWM control  126  output voltage, vpwm, transitions from a low to a high value to initiate the start of the OFF state. The high-side driver  172  inverts and level shifts the signal which causes NPN transistor  174  to turn OFF and PNP transistor  178  to turn ON. The energy stored in the internal gate-source capacitance of MOSFETs  110   a  and  110   b  discharges through resistor  176  and PNP transistor  178 . When the gate-source voltage of MOSFETs  110   a  and  110   b  decreases to less than approximately four volts MOSFETs  110   a  and  110   b  turn OFF. Free-wheeling diode  114  becomes active which causes the voltage at the sources of MOSFETs  110   a  and  110   b  to decrease to minus one volt. Current then flows through decoupling diode  166  into filter capacitor  168  recharging the capacitor  168 . Parallel zener diode  170  clamps the voltage across filter capacitor  168  to a safe value that does not overstress the gate-source junctions of the MOSFETs  110   a  and  110   b.  The circuit remains in the OFF state until the output of PWM control  126  once again transitions low.  
         [0050]    In addition to controlling pulse width to maintain a constant output voltage, PWM control  126  also varies the pulse width in response to an output from current limit  130  to protect power converter module  16  from excessive output current loads. Output current flows through current sense resistor  118  causing a voltage to develop that is proportional to the output current. The voltage across resister  118  is compared to a reference voltage derived from the PWM control reference. When the output current is greater than a pre-determined maximum level the output of current limit  130  causes PWM control  126  to reduce the pulse width of the output. The reduced duty cycle causes the voltage at outputs VOUTHI  120  and VOUTLO  122  to decrease until the resulting output current is less than the pre-determined maximum level.  
         [0051]    Temperature sense  134  protects power converter module  16  from overtemperature stress of MOSFET  110  and output diode  114 . In the presently preferred embodiment a thermistor is employed as temperature sense  134  to monitor the temperature of heatsinks  43 . If the temperature rises due to overload, debris blocking an air intake, or other fault condition, temperature sense  134  modifies the current limit reference voltage, thereby causing the PWM control  126  to generate a shorter pulse width. The shorter pulse width results in a lower output voltage and output current that corresponds to a lower overall output power. The lower output power causes a reduction in the power dissipated in the components of power converter module  16 , resulting in lower component temperatures.  
         [0052]    Returning to FIG. 1, although the power tool  12  of the present invention is designed to be powered by a relatively low voltage DC power source (i.e., a DC source less than 50 volts), the housing  201  of the power tool in the preferred embodiment is nonetheless double insulated from the electrical system of the tool. As is well known to those skilled in the art, power tools designed to be operated by a high voltage power source, such as a conventional AC or corded power tool, are typically constructed so that the housing of the tool is double insulated from the electrical system of the tool for safety reasons. In this manner, the operator of the tool is protected against electrical shock in the event of a short in the electrical system of the tool. Cordless or DC powered tools are powered by low voltage power sources and therefore do not require such safety measures. Consequently, conventional DC powered tools do not insulate the housing from the electrical system of the tool.  
         [0053]    There are of course, many DC powered portable devices that are alternatively powered from high voltage AC house current. To enable this alternative operation, however, AC/DC powered devices universally employ transformers to step down the high AC voltage and thereby isolate the device from the high voltage AC power source.  
         [0054]    While this solution may be acceptable for relatively low powered devices, such as portable stereos, the power requirements of many portable power tools necessitates the use of large step-down transformers which are not only bulky, but also very heavy. Consequently, DC powered tools that can alternatively be powered from AC house current have rarely been offered commercially.  
         [0055]    The present invention solves this dilemma by providing a relatively light weight non-isolated AC to DC converter and then constructing the DC powered tool in a manner consistent with the double insulation safety requirements of a conventional AC powered tool. In other words, by eliminating transformer isolation in the present AC/DC power converter module  16 , the DC output voltage supplied to the motor of the power tool is referenced to the 115 volt AC input. Consequently, double insulation of the tool housing from the electrical system of the power tool is required.  
         [0056]    In addition, as discussed above in connection with the description of FIGS.  5 - 7 , the power tool terminal block  34  according to the present invention is provided with independent male connectors  40  uniquely adapted to make electrical contact with, and thereby receive electrical power from, specially recessed female connectors  54  in the AC/DC converter module  16 . Thus, despite the non-isolated construction of the present AC/DC converter module  16 , all applicable safety requirements for operating a power tool from a relatively high voltage power source are satisfied.  
         [0057]    [0057]FIGS. 12 through 17 depict the effect of employing double insulation within a motor and housing. Double insulation techniques are well known in the art. Double insulated tools are typically constructed of two separate layers of electrical insulation or one double thickness of insulation between the operator and the tool&#39;s electrical system. With specific reference to FIG. 12, a cross-sectional view of a non-double insulated DC motor armature  200  is illustrated. The armature  200  consists of a shaft  202  with a core built up over it. The core is composed of many laminations  206  with notches along the outer periphery to hold the armature windings  204 . A gear or chuck (not shown) is built onto the shaft at one end of the armature  206  to provide a means of transferring rotational energy to the working end  208  (see FIG. 1) of the power tool  12 . For example a gear mechanism would convert rotational energy to the forward and back motion used to drive a reciprocating saw. The path from the armature shaft  202  to the gear mechanism or chuck, and finally to the working end is electrically conductive. Therefore any electrical energy that exists on the armature shaft  202  is conducted to the working end, which is exposed to the operator of the power tool  12 . Locations  208 ,  210 , and  212  indicate areas of the rotor that could become energized through contact with electrically live assemblies if insulation is not employed. At location  208  the armature shaft  202  could be energized through contact with energized armature laminations  206 . At location  210  the armature shaft  202  could be energized through contact with end turns of the armature windings  204 . At location  212  the armature laminations  206  could be energized through contact to end turns of the armature windings  204 .  
         [0058]    Referring to FIG. 13, a first method of employing double insulation of the motor armature  220  of a power tool is illustrated. The armature  220  consists of a shaft  222  with a core built up over it. The core is composed of many laminations  226  with notches along the outer periphery to hold the armature windings  224 . A chuck  228  is built onto the shaft at one end of the armature laminations  206  to provide a means of affixing a device such as a drill, bit to the working end  208  (see FIG. 1) of the power tool  12 . A molded plastic insulator  230  provides basic insulation between the armature windings  224  and the laminations  226  as well as between the shaft  222  and the windings  224 . A press fit plastic tube insulator  232  encases the shaft  222  providing supplementary insulation to prevent the shaft from becoming energized if the basic insulation breaks down.  
         [0059]    Referring to FIG. 14, a second method of employing double insulation of the motor armature  220  of a power tool is illustrated. A paper insulator  240  provides basic insulation between the armature windings  224  and the laminations  226 . A second insulator  242  of double thickness, 2 mm, encases the shaft  222  providing reinforced insulation, which substitutes for supplementary insulation, to prevent the shaft from becoming energized through electrical shorts td the laminations  226  or the armature windings  224 .  
         [0060]    Referring to FIG. 15, a third method of employing double insulation of the motor armature  220  of a power tool is illustrated. An insulator  250  of either paper or molded plastic provides basic insulation between the armature windings  224  and the laminations  226 . An in situ molded thermoset plastic insulator  252  of double thickness encases the shaft  222  providing reinforced insulation, which substitutes for supplementary insulation, to prevent the shaft from becoming energized through electrical shorts to the laminations  226  or the armature windings  224 .  
         [0061]    Referring to FIG. 16, a cross-section through the center of the lamination stack of the motor armature  220  of a power tool is illustrated. A slot liner insulator  260  provides basic insulation between the armature windings  224  and the laminations  226 . The slot liner insulator is constructed of any suitable electrical insulator material such as paper, coated paper, polyester, and vulcanized fiber. Supplementary insulation is provided by a glass reinforced polyester insulator sleeve  262  which encases the shaft  222 . The insulator sleeve prevents the shaft from becoming energized if the basic insulation provided by slot liner  260  fails.  
         [0062]    Referring to FIG. 17, a double insulated housing  270  of a power tool is illustrated. As is known in the art, the double insulation methods employed are intended to prevent electrical energy within the housing  270  from energizing the outside surface of the housing  270 . The housing  270  is depicted with a hypothetical metal foil covering  272  on the outside surface to simulate interaction with an operator. Also illustrated are a ring terminal  274  and an insulated wire  276  that includes a conductive wire  278  and wire insulation  280 . Electrical energy exists on both the ring terminal  274  and the conductive wire  278 . Double insulation of the ring terminal  274  is provided by a double thickness, 2 mm, of housing material which serves as a reinforced insulator. The wire insulation  280  provides basic insulation for conductive wire  278 . Supplementary insulation is provided by the housing  270  which prevents electrical energy that breaks through the wire insulation from energizing the outside surface of the housing  270 .  
         [0063]    The power converter module  16  initially converts the low frequency AC input to a high level DC voltage, then to a high frequency voltage level that is thereafter filtered to the lower voltage supply level of power tool  12 . The power tool employs double insulation of the motor rather than transformer isolation of the power converter  16 , thereby significantly reducing the cost and weight of the power converter module  16 .  
         [0064]    In addition, the converter module  16  is designed with a comparatively small number of components while providing an efficient conversion process. This further enhances the lightweight, compact features of the converter module  16 . The size of the converter module  16  further permits the use of the converter in power-operated devices, such as the reciprocating saw  12 , which heretofore were too small to support and contain conversion units providing power in a range of at least 50 watts and higher.  
         [0065]    Further, while the preferred embodiment of the converter module  16  converts a low frequency, high voltage level to a low DC voltage level, the converter can be used to convert a high DC voltage level to a low voltage DC level by applying the high DC level directly to a suitable power cord and plug that connects to the input of converter module  16 . In this manner, the power tool  12  could be operated from the high DC voltage source instead of the low DC voltage of the cells  26  and thereby conserve the charge life of the cells.  
         [0066]    The converter module  16  could be designed to operate from external AC power sources other than 120 volts at 60 Hz. Without departing from the spirit and scope of the invention, the converter module  16  also could be designed to provide DC output voltage levels in a range of 3.6 to 48 volts. In a particular example, the converter could be adjusted to develop a DC output of 24 volts between the outputs VOUTHI  120  and VOUTLO  122  derived from an external AC source of 220 volts at 50 Hz as applied to a suitable power plug and cord. The converter module  16  could then be used to provide inexpensive dual mode capability for power-operated devices that operate at a DC voltage supply level of 24 volts.  
         [0067]    The reciprocating saw  12  is merely illustrative of one example of many power-operated, cordless-mode devices that become more versatile because of the inventive cost efficient dual-mode capability. Other examples of power-operated cordless devices which are enhanced by the inventive concept include, but are not limited to, drills, screwdrivers, screwdriver-drills, hammer drills, jig saws, circular saws, hedge trimmers, grass shears, as well as battery-operated household products and the like.  
         [0068]    Thus it will be appreciated from the above that as a result of the present invention, an inexpensive dual-mode corded/cordless system for power-operated devices is provided by which the principal objectives, among others, are completely fulfilled. It will be equally apparent and is contemplates that modification and/or changes may be made in the illustrated embodiment without departure from the invention. Accordingly, it is expressly intended that the foregoing description and accompanying drawings are illustrative of preferred embodiments only, not limiting, and that the true spirit and scope of the present invention will be determined by reference to the appended claims and their legal equivalent.