Abstract:
A power switch includes a bridge-connected switching circuit with (1) a pair of MOSFETs back-to-back in series between line and load connections, an interconnection point of the MOSFETs being a first common connection, and (2) a pair of diodes back-to-back between the line and load connections, an interconnection point of the diodes being a second common connection. A line/load controller has supply inputs connected to the first and second common connections for receiving operating power, and (1) places both MOSFETs in the ON state to deliver normal operating current to the load for normal operation, and (2) places both MOSFETs in the OFF state to deliver (by body diode conduction) a substantially reduced leakage current to the load when the load is not powered for normal operation, the leakage current providing the operating power to the line/load controller.

Description:
BACKGROUND 
       [0001]    The present invention relates to the field of power circuitry, and in particular to power switches (e.g., light switches) providing for selectable delivery of power to a load (e.g., electric lamp). 
       SUMMARY 
       [0002]    One of the challenges in creating a smart light switch is generating an auxiliary power source to power local control logic and communication of the light switch. To power a small local microcontroller to implement basic on/off control and/or dimming by means of a simple mechanical or touch interface, the amount of power required is quite small (e.g., 100 mW or less) and creating a power source is less challenging. Given the availability of more sophisticated interface devices, various transducers to implement smart functions and the widespread use of wireless communication, the power requirements for a more advanced smart switch may be much higher (e.g., 1 W or more). This means that a power source needs to be created capable of providing energy to these advanced circuits either directly or by helping to trickle charge an energy storage device such as a battery or super capacitor that can periodically provide the required power. 
         [0003]    As known, a power switch is typically housed in an enclosure, referred to herein as a “switch box”. The switch box serves not only as a packaging element, but also as a wiring point. In building wiring that has been installed after 2011, it is likely that each switch box contains both the line and neutral feeds from the circuit panel in addition to the load wire that needs to be controlled. In this case it may be fairly straightforward to realize an auxiliary power source with inputs connected between line and neutral. In older switch box wiring, it is common that only the line and load connections are present, and the lack of a return (neutral) means that the straightforward approach cannot be used. Furthermore, this scenario can be complicated by the variability of lamp technologies, i.e., incandescent, LED, CFL, etc. An incandescent lamp presents essentially a resistive characteristic which can be exploited to generate a small leakage current, while other technologies generally implement front-end conditioning/conversion circuitry that may not provide the same ability. 
         [0004]    Disclosed herein is a power switch that includes a line connection to a line side of an AC supply, and a load connection to a load, the load having a return connection (i.e., neutral) to a return side of the AC supply. The power switch further includes a bridge-connected switching circuit including (1) a pair of MOSFETs connected back-to-back in series between the line connection and the load connection, an interconnection point of the MOSFETs being a first common connection, and (2) a pair of diodes connected back-to-back between the line connection and the load connection, an interconnection point of the diodes being a second common connection, each MOSFET having an ON state in which the MOSFET provides bidirectional channel conduction and an OFF state in which the MOSFET provides unidirectional body diode conduction. The power switch further includes a line/load controller having a pair of supply inputs connected respectively to the first and second common connections for receiving operating power. The line/load controller is arranged and operative to (1) place both MOSFETs in the ON state to deliver normal operating current to the load when the load is to be powered for normal operation, and (2) place both MOSFETs in the OFF state to deliver a substantially reduced leakage current to the load when the load is not to be powered for normal operation, the leakage current providing the operating power delivered to the line/load controller via its supply inputs. 
         [0005]    The disclosed power switch provides for efficient harvesting of a small amount of power through the load itself, by its use of the bridge-connected switching circuit including the diodes and series-connected MOSFETs. It thus enables flexible deployment including in installations in which the neutral conductor of a power circuit is not wired into the switch enclosure. 
         [0006]    Other aspects of the disclosure are directed to using only a “skirt” part of the conduction cycle (in a small neighborhood of zero) for harvesting power; using wireless charging of a separate switch module, which may be a higher level controller in a smart power system, for example; use of a wave-shaping inductor; use of an optically isolated, low-power OFF circuit for a startup circuit; and a managed AC source providing leakage current for a power-harvesting power switch. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The foregoing and other objects, features and advantages will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. 
           [0008]      FIG. 1  is a block diagram of a power converter; 
           [0009]      FIG. 2-4  are schematic diagrams of power converters; 
           [0010]      FIG. 5  is a block diagram of a load circuit with current bleeding; 
           [0011]      FIG. 6  is a schematic depiction of a lamp with an adapter base; 
           [0012]      FIG. 7  is a block diagram of a current bleeding circuit; 
           [0013]      FIGS. 8-13  are schematic diagrams of power converters; 
           [0014]      FIG. 14  is a schematic diagram of a portion of a power converter with a startup circuit; 
           [0015]      FIG. 15  is a schematic diagram of a startup circuit; 
           [0016]      FIGS. 16-17  are schematic diagrams of power converters; 
           [0017]      FIG. 18  is a schematic diagram of a current bleeding circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  shows the general environment of the presently disclosed circuits and techniques. A load  10  such as a lamp is powered from an AC source  12  (e.g., an AC distribution panel) via a user-controlled switch  14 , such as a wall-mounted switch in a bedroom, office, etc. The switch  14  selectively makes/breaks a connection between a Line conductor (from source  12 ) and a Load conductor (to load  10 ). Current flows from the source  12  via the Line conductor, switch  14 , and Load conductor to the load  12 , then returns via the Return conductor, also referred to herein as “Neutral”. As indicated at  16 , the Return conductor may or may not pass through the switch  14 , which as explained above can complicate the harvesting of power for local circuitry within the switch  14 . Approaches for addressing this complication are described herein. 
         [0019]    As outlined above, described herein is a technique for harvesting energy from leakage current that can be passed through the load and using this harvested energy to power local circuits and provide an energy source to either directly power a higher level controller or charge an energy storage component. 
         [0020]      FIG. 2  shows a first example circuit  20 , which would be part of the switch  14  of  FIG. 1 . This circuit requires no connection to the system Return or Neutral, which is shown as N and connected between the source  12  and load  10 . Diodes D 1  and D 2  and MOSFETs Q 1  and Q 2  are arranged in a bridge configuration across four circuit nodes including the Line node, Load node, and first and second local supply nodes  22 ,  24 . In the illustrated embodiment, the local supply nodes are a high-side node  22  and a low-side node  24 , which serves as a local neutral or return. 
         [0021]    When Q 1  and Q 2  are ON, the primary switch action of connecting Line to Load is achieved for normal operation, i.e., illumination of the lamp. Current flows from the source  12  through the two MOSFETs Q 1 , Q 2  and to the load  10 , returning to the source  12  by the external neutral connection. When Q 1  and Q 2  are off, their associated body diodes along with D 1  and D 2  create a full wave bridge rectifier and charge the capacitor C 1  by means of leakage current that flows through the load. The action of turning on/off Q 1 /Q 2  is managed by a controller  26 , which may employ certain timing as described below for harvesting energy and providing normal load current. The controller  26  receives its operating power from the bridge circuit via the supply nodes  22 ,  24 . An additional capacitor, C 2 , is used to provide holdup for the line/load controller  26  and is peak charged through a resistor R 1 . A Zener diode Z 1  can provide voltage clamping if necessary. 
         [0022]      FIG. 3  shows an expanded version of the circuit of  FIG. 2 , including certain additional circuitry for additional functionality as now described. In one respect it includes support for a separate auxiliary (Aux) power supply, not shown. Once C 1  is charge the stored energy can be pumped to s separate auxiliary power supply rail by means of inductor L 1  and transistor Q 3 . When Q 3  is turned on some of the energy in C 1  is transferred to L 1 , and when Q 3  is subsequently turned off this energy is then transferred to C 3  through D 4  by the flyback action defined by the polarity of the L 1  windings. The action of modulating Q 3  on/off for several switching cycles makes up the energy pumping function. The line/load controller  20  receives isolated feedback via an opto-coupler  30 . 
         [0023]    The circuit includes a linear regulator L/R that may have input voltage limits, in which case the Zener diode Z 1  provides clamping as a means of not exceeding those voltage limits. 
         [0024]    An opto-coupler  32  connects to the Line/Load controller  20  and is used to send control information from an interface controller and monitor  34 . When Q 1  and Q 2  are turned on to implement the intended switching function (turning on the light in this case), the low-side node  24  becomes connected to the AC line. This node needs to be considered floating with respect to the AC line, and as a result the isolation elements shown in this diagram are necessary. 
         [0025]      FIG. 3  also illustrates the timing of the above-described operations within the half-cycles of the AC waveform. When the line voltage is above a threshold shown as “bulb on threshold”, the transistors Q 1  and Q 2  are ON and normal operating current is being provided to the load (lamp). Late in the half-cycle when the line voltage falls below the threshold, energy is pumped to the auxiliary supply from C 1  as described above. Early in the half-cycle when the line voltage has not yet reached the threshold, C 1  is charged by the bridge rectifier operation as described above. 
         [0026]      FIG. 4  shows elements of a more complete system, including the elements of  FIGS. 2 and 3  as well as others as now described.  FIG. 4  shows the auxiliary power supply  40  powered from line and neutral that is used to create a reliable and predictable power source when neutral is present (i.e., connected to the switch box housing the illustrated circuitry, as mentioned above). If neutral is absent, all of the power for the local circuits is derived from the energy harvesting and energy pumping circuits as explained above. In this case there may also be a need to provide a power source for a separate higher level controller  42 , which may be a modular pluggable (“snap-in”) device as indicated. The higher level controller  42  communicates with the interface controller and monitor  34  via a serial interface. In the case that the only energy that is available is that that is harvested from the load connection. As such the amount of power that can be delivered to the higher level controller may be limited. In that case the higher level controller  42  may contain an energy storage device that is charged using the residual energy form the harvesting circuitry. The residual energy is what is available in excess of that used by the local circuits. In this case that status can be communicated to the higher level controller  42  through the serial interface protocol. A programmable current limit circuit may be added to prevent the attached higher level controller  42  from taking more power than can be afforded, with this being managed by the local interface controller and monitor  34 . 
         [0027]      FIG. 4  also shows connections to the separate load  10  and source  12  via circuitry  44  that includes a connection resolver, line voltage monitor and zero cross detector. Information from these circuits is provided to the interface controller and monitor  34  via respective digital output signals as shown. 
         [0028]    The ability to harvest energy by means of current leakage through the load is not always a predictable and reliable means of bleeding energy into a storage element such as C 1 . Incandescent light bulbs look resistive and thus lend themselves to such an application. Many LED light bulbs have switching power supply front ends, although many of these circuits contain elements that force a certain amount of leakage current through the load wire for the purpose of providing holding current for triacs that are used in traditional triac dimmer circuits. Such LED lamps will provide a current source that can be used to charge C 1  through the load as described above. 
         [0029]      FIGS. 5-7  depict an accessory device (or “current bleeding circuit”)  50  that may be used when the lighting device that is present as the load  10  does not afford a reasonable source of leakage current to charge C 1 . In this case the accessory device  50  may be retrofitted with the bulb or may reside in the light fixture or wiring box. In theory only one of these circuits would be required per switch branch.  FIG. 6  shows a retrofit example in which the circuit  50  is realized within an adapter  52  that receives a bulb  54  and includes a compatible base for mating with a light fixture. The accessory device  50  includes circuitry  56  that monitors line voltage and phase and controls a current sourcing element or switch  58  in parallel with the lamp in such a way to provide a leakage current during certain portions of the voltage waveform applied across the lamp. This arrangement guarantees a source of current that can be used to harvest energy in a switch box that is void of a neutral wire, such as described above with reference to  FIGS. 2-4 . 
         [0030]      FIG. 8  shows a circuit similar to that of  FIG. 4  and including an additional transistor QT for switching current to a separate “traveler” conductor such as used in typical 3-way wiring scenarios. 
         [0031]      FIG. 9  shows another concept that is a possible solution for harvesting energy through the load connection. In this implementation, a wireless charging function is established using a transmitter coil  60  and closely coupled receiver coil  62 . With proper magnetic core materials this may be modulated with the 60 Hz line frequency current delivered through the load. If high frequency operation is required, the current could be chopped at a higher frequency by means of switching Q 1 /Q 2  on and off at a high frequency. In this case additional circuit elements can be added to handle proper steering of any inductor currents flowing in the wiring. As shown, wireless charging may also be accomplished using a transformer  64  instead of separate coils  60 ,  62 . 
         [0032]      FIGS. 10-13  show a detailed example of a commercial product incorporating the disclosed switch technique. 
         [0033]      FIG. 10  shows an example line/neutral isolated auxiliary supply, a realization of the auxiliary supply  40  of  FIG. 8  for example. 
         [0034]      FIG. 11  shows a more detailed implementation of switch interface an energy harvesting circuitry such as described above with reference to  FIGS. 2-4  for example. This circuit also includes diodes D 7  and D 14  as well as resistor R 25  and inductor L 3 , which is the device labeled as “wave shaping inductor” in  FIG. 4 . Diode D 14  is used to rectify any voltage transients across L 3  and move the resultant energy into the energy harvesting capacitor shown as C 17 . R 25  provides a means to dissipate a small portion of that energy to help dampen any ringing for the purpose of reducing electrical noise (EMI). When the switching devices Q 2  and Q 5  are on, current flows from the line connection through the load connection. If Q 2  and Q 5  are turned off when current is still flowing in the inductor L 3 , D 14  provides a means to capture that residual energy. Since current through an inductor cannot change instantaneously, if the path which current flows through L 3  is abruptly interrupted the voltage across L 3  increases until it reaches the voltage across C 17  and the rectifier D 14  provides a current path allowing the energy stored in L 3  to transfer to C 17 . 
         [0035]      FIG. 12  shows an example of a harvested energy pumping circuit, a more detailed realization of the above-described energy pumping technique. 
         [0036]      FIG. 13  shows an example of microcontroller and related circuitry including a microcontroller U 11  that realizes the Line/Load controller  20  of  FIGS. 2-4  for example. 
         [0037]      FIG. 14  shows a portion of a switching power supply using a startup circuit. It includes switching and control (SW/CNTL) circuitry  110 , a power transformer T 1 , a startup circuit  112 , and optionally an input voltage source  114 . The transformer T 1  has main primary and secondary windings Wpri and Wsec, as well as a third or “auxiliary” winding Waux connected to a capacitor Caux. The switching and control circuitry  110  receives a DC voltage Vin as well as a voltage Vaux developed on the capacitor Caux. The voltage Vin is provided by the input voltage source  114  when present, and otherwise it may be an input from a separate voltage source. 
         [0038]    Pertinent operation of the power supply is divided into two periods, an initial startup period in which Vin is rising from zero to a normal operating value, and a subsequent steady-state operating period in which Vin is at its normal operating value and the power supply is providing a steady DC output voltage to separate powered circuitry (not shown). The switching and control circuitry  110  includes circuitry (not shown) that receives its operating power from the Vaux input; examples are described below. During steady-state operation, the combination of the winding Waux and capacitor Caux function as a simple power source for this circuitry. During at least an initial part of the startup period, no or little current is provided to the main primary winding Wpri and therefore no or little power is available via the winding Waux. The startup circuit  112  operates during this period along with Caux as the power source, until operation has proceeded to the point that the normal steady-state mechanism employing winding Waux is available and becomes operative. 
         [0039]      FIG. 15  shows the startup circuit  112  according to one embodiment. Its main purpose is to generate an unregulated supply voltage Vaux usable by the switching and control circuitry  110  ( FIG. 14 ) during an initial startup period of operation before all normal operating voltages have been established. Vaux is generated by supplying a charging current Ic(Q 4 ) to the capacitor Caux, which occurs in response to another current Ic(Q 2 ) that flows during an initial part of the startup period. Detailed operation is described below. One important feature is provided by a normally on transistor Q 1 , which may be implemented as a depletion-mode junction FET (J-FET) for example. Q 1  conducts during startup to allow generation of Ic(Q 2 ), and at the end of startup it is rendered non-conducting by application of an inhibitory control signal in the form of a positive gate voltage Vg(Q 1 ) from a separate Vg generator (not shown). This effectively disables the startup circuit  112 , reducing its power dissipation and improving overall efficiency of the power supply accordingly. 
         [0040]    Overall, the transistors Q 1 -Q 4  and related circuitry form a startup current source that pulls power from the input source to generate the charging current Ic(Q 4 ) for the storage capacitor Caux. In the illustrated configuration the startup current source includes two sub-level current sources—an emitter-switched current source formed by Q 2 , Q 1  and related circuitry that generates Ic(Q 2 ), and a second current source (referred to as an output current source) that responds to Ic(Q 2 ) to generate the charging current Ic(Q 4 ). In this configuration the current Ic(Q 2 ) may be seen as an enabling current that enables Q 4  to conduct the charging current Ic(Q 4 ). 
         [0041]    In the illustrated arrangement, Q 1  is a P-Channel depletion mode J-FET. A depletion mode FET is on (conducting) when zero volts is applied to its gate, and is turned off when a voltage in excess of a cutoff voltage is applied to its gate. At the very beginning of startup operation when Vin is equal to zero, Vg(Q 1 ) has zero volts applied and Q 1  behaves as if it were a resistor connected from the emitter of Q 2  to the return potential. Once the voltage on the base of Q 2  becomes high enough to establish current flow through Q 2 &#39;s base-emitter junction, it begins conducting. This will establish current flow through voltage-creating (V-C) elements U 1 , Q 3  connected between Vin and the base of Q 4 . Once the voltage created by the V-C elements is sufficient to establish base-emitter current in Q 4 , then collector current flows in Q 4 . This collector current is proportional to the voltage across R 4 , which is equal to the voltage across the V-C elements minus the base-emitter voltage drop (V BE ) for the conducting Q 4 . The Q 4  collector current Ic(Q 4 ) flows in a path that allows it to charge Caux. 
         [0042]    When Caux is charged to a sufficiently high voltage that allows startup of the power converter, current flows in the primary winding Wpri ( FIG. 14 ) and by magnetic coupling in the auxiliary winding Waux as well. This current maintains the voltage Vaux across Caux as part of steady state operation of the power supply. Additionally, at this point the Vg generator generates an inhibitory control signal in the form of a non-zero gate voltage Vg(Q 1 ), which is supplied to the gate of Q 1  in order to turn Q 1  off. When Q 1  is off, no current flows through Q 2 , and thus Q 4  is off and the startup current used to charge Caux is terminated. With the startup current disabled, no power loss from the startup circuit exists other than small losses from the input voltage monitoring circuitry R 2  and D 3 -D 5 . 
         [0043]    The cascade configuration of Q 2 , D 6  and Q 1  are used to switch on/off the current source formed by Q 4 , R 2 , Q 3  and U 1 . This current source starts the PWM controller U 2 . Once the PWM controller is started, the signal U 2 _ 8  turns off Q 1 , turning off this start up current source. The opto-coupler U 1  provides for disabling the power supply from an external signal in a way that dissipates very little bias power. When the LED in U 1  is turned on by applying a voltage to the signal IO_ 3 , the transistor in the opto-coupler U 1  is also turned on. This is the mechanism for entering the low power shutdown condition. In this condition, through D 1 , the collector of the opto-coupler&#39;s transistor pulls the base of Q 2  to a voltage low enough to turn Q 2  off, disabling the startup current source and thus preventing a start of the PWM controller. In the case that the PWM controller is running at the time of being disabled, its operation is terminated by additional circuitry. In particular, a pull down path through D 2  pulls the collector of Q 5  to a low enough voltage to disable the PWM&#39;s soft start circuit and force its error voltage to a low enough value to stop PWM operation. 
         [0044]      FIG. 16  shows a configuration that provides complete isolation of the logic voltage from the line voltages which may be needed in order to meet safety requirements depending on the mechanical design of the air gap switch. In this case the transformer needs two windings in addition to the primary and secondary in order to generate two isolated auxiliary voltages. One of these voltages is used to power the PWM controller. The other voltage is used to power the drivers to the switch devices, Q 1  and Q 2 . If the switch devices shared the same common as the PWM IC one leg of the bridge rectifier would be shorted when Q 1  and Q 2  are on creating a short during one of the half cycles of the AC line relative to the neutral. The configuration in  FIG. 16 a    prevents this condition by providing the additional isolation elements as well as this additional isolated bias voltage. 
         [0045]      FIG. 17  shows an alternate configuration for sharing the rectifier for the power supply circuit with the switching device used to control the switched load. This configuration trades off complexity of the bias power supply transformer, T 1 , for an additional circuit element Q 4 . In this case the auxiliary power supply&#39;s transformer has one less winding which would simplify construction relative to safety requirements. In this configuration Q 1  and Q 2  make up the switch that controls the attached load and they are simultaneously turned on to energize said load. However, if neutral was connected without the addition of Q 4 , Q 2  would short one of the legs of the bridge rectifier when on. Q 4  is added to break the path of the short by timing when it is on relative to Q 1  and Q 2 . Essentially Q 4  becomes a controlled rectifier to keep the integrity of the bridge rectifier operation in place. The signal shown as Make_Aux in the timing diagram accomplishes this control function. 
         [0046]      FIG. 18  is a practical implementation of a controlled current source used as an accessory device to shunt current around an attached load in order to provide a means to harvest energy in certain applications. Incandescent and magnetic low voltage loads provide a means to harvest energy which are generally well behaved. However, many LED light bulbs have internal circuitry that either does not allow sufficient current to be passed though the bulb or that will turn on the LED during small portions of the line cycle if used as a means to harvest energy. This controlled energy source is intended to shunt current around the bulb in a controlled manner. 
         [0047]    The microcontroller U 2  runs software that determines the phase of the AC line and the condition of the voltage applied across terminals  1  &amp;  2  and enables the current source formed by Q 1  and R 2  to shunt current around the bulb in a controlled fashion. If the bulb is intended to be on, the current source is disabled or set to a very low value. The rectifier D 1  allows bi-directional operation of this current source. U 1  is used to control the current though Q 1  to a maximum value in the situation that U 2  is not powered. When U 2  is powered the internal operational amplifier can take control of this current source pulling it to a lower value by using firmware to adjust the DAC used as the current source reference. This DAC is internal to U 2  in this implementation as shown. The comparator in U 2  is used to detect the phase of the voltage applied and this can also be detected by the zero cross circuit internal to this microcontroller as an alternative approach. R 8 , C 2  and U 3  make up a simple shut regulator to power U 2 . An option to set different modes of operation is provided by SW 1  and LED 1  is used to indicate modes of operation. SW 1  and LED 1  are optional features. Some microcontrollers also contain temperature sensing elements that are internal and in the mechanical implementation U 2  can be placed so that it has reasonable thermal coupling to Q 1 . In this case the firmware can be designed to enable the temperature sensing and adjust the current source vale to prevent excessive power dissipation in Q 1  as a protection feature. 
         [0048]    While various embodiments of the invention have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.