Patent Publication Number: US-2023155483-A1

Title: Low ground current ac-dc power supply for no-neutral electrical devices and fault protection therefor

Description:
BACKGROUND 
     In recent years, smart control technologies, such as wireless control, have been incorporated as features into consumer products. Example devices are those for lighting control, such as dimmers and other forms of wall switches. In this space, there are so-called “three-wire” wall switches and “two-wire” wall switches. A three-wire wall switch unit has connections for three wires usually referred to as “line”, “neutral” and “load”, in addition to an Earth ground connection. Presence of the neutral connection makes incorporation of smart control technology into a three-wire wall switch not particularly difficult insofar as powering the device&#39;s smart control electronics (circuit, processor, etc.). Around the world, consumer houses and other buildings commonly utilize two-wire wall switches, having connections for only two wires “line” and “load” (in addition to the Earth ground connection). Such devices are also referred to as “no-neutral” wall switches. Challenges exist in obtaining sufficient power for a smart control circuit in such two-wire devices. 
     SUMMARY 
     Shortcomings of the prior art are overcome and additional advantages are provided through the provision of an electrical device that includes a bridge rectifier having four diode legs, each diode leg of the four diode legs having a respective plurality of diodes electrically coupled in-series, and the bridge rectifier further having a positive directed current (DC) rail and a negative DC rail; a step-down switching DC-DC converter; and a fault-protection circuit, where the fault-protection circuit is configured to perform sensing (i) current from the step-down switching DC-DC converter, (ii) a first voltage from the step-down switching DC-DC converter, and/or (iii) a second voltage, the second voltage being an output voltage at an output of the step-down switching DC-DC converter. 
     In some embodiments, the fault-protection circuit is further configured to perform detecting (i) a fault inside the step-down switching DC-DC converter based on sensing the current or the first voltage, or (ii) a fault at the output of the step-down switching DC-DC converter based on sensing the second voltage; and based on detecting the fault inside the step-down switching DC-DC converter or the fault at the output of the step-down switching DC-DC converter, triggering opening a switch of the fault-protection circuit between a return of the output voltage from the step down switching DC-DC converter and the negative DC rail of the bridge rectifier. 
     In some embodiments, the fault-protection circuit is further configured to perform: detecting a fault at the output of the step-down switching DC-DC converter based on sensing the second voltage; and based on detecting the fault at the output of the step-down switching DC-DC converter, triggering opening a switch of the fault-protection circuit between a return of the output voltage from the step down switching DC-DC converter and the negative DC rail of the bridge rectifier. 
     In some embodiments, the electrical device further includes a pi filter that has (i) a first plurality of capacitors electrically coupled between the positive DC rail of the bridge rectifier and the negative DC rail of the bridge rectifier, (ii) a second plurality of capacitors electrically coupled between a positive DC input of the step-down switching DC-DC converter and a return of the fault-protection circuit, and (iii) an inductor electrically coupled between the first plurality of capacitors and the second plurality of capacitors. In some examples, the fault-protection circuit is coupled between a return rail of the pi filter and the negative DC rail of the bridge rectifier. 
     In some embodiments, the electrical device further includes a plurality of capacitors electrically coupled in-series between the positive DC rail of the bridge rectifier and the negative DC rail of the bridge rectifier. 
     In some embodiments, the electrical device includes a capacitor electrically coupled between a positive DC input and a negative DC input to the step-down switching DC-DC converter, where the fault-protection circuit is electrically coupled between a return of the output voltage and the negative DC rail of the bridge rectifier. 
     In some embodiments, the electrical device includes a power supply, and the fault-protection circuit provides fault-protection for the power supply of the electrical device. The power supply may be, for instance, a ground leakage power supply configured for using ground leakage current. 
     Further, a method is provided to facilitate fault-protection for a power supply. The method includes providing a bridge rectifier of the power supply, the bridge rectifier including, on each diode leg of the bridge rectifier, a respective first diode and a respective additional diode redundant to the first diode, and the bridge rectifier configured to generate and output direct current (DC) power from alternating current (AC); providing a filter circuit for the bridge rectifier, the filter circuit including at least one first capacitor and at least one additional capacitor redundant to the at least one first capacitor; and providing a fault-protection circuit having a switch, the fault-protection circuit being configured for electrical coupling between a return of input DC power to a step-down switching DC-DC converter and a return rail of rectified DC voltage of the output DC power generated by the bridge rectifier, and the fault-protection circuit being configured to perform opening the switch based on sensing a current fault or voltage fault. 
     In some embodiments, the power supply is a ground leakage power supply configured for using ground leakage current. 
     In some embodiments, providing the bridge rectifier provides each diode leg of the bridge rectifier with two diodes electrically coupled in-series. Additionally or alternatively, the filter circuit may have a respective additional capacitor redundant in-series to each first capacitor. 
     In some embodiments, the fault-protection circuit is further configured to perform sensing current from the step-down switching DC-DC converter; sensing a first voltage from the step-down switching DC-DC converter; sensing a second voltage at an output of the step-down switching DC-DC converter; and sensing the current or voltage fault as (i) a fault inside the step-down switching DC-DC converter based on the sensed current or sensed first voltage, or (ii) a fault at the output of the step-down switching DC-DC converter based on sensing the second voltage. 
     Further, in some embodiments, an electrical device is provided that includes a bridge rectifier, the bridge rectifier including, on each diode leg of the bridge rectifier, a respective first diode and a respective additional diode redundant to the first diode, and the bridge rectifier configured to generate and output DC power from alternating current (AC); a filter circuit for the bridge rectifier, the filter circuit including at least one first capacitor and at least one additional capacitor redundant to the at least one first capacitor; and a fault-protection circuit that includes a switch, the fault-protection circuit being configured for electrical coupling between a return of input DC power to a step-down switching DC-DC converter and a return rail of rectified DC voltage of the output DC power generated by the bridge rectifier, and the fault-protection circuit being configured to perform opening the switch based on sensing a current fault or voltage fault. 
     In some embodiments, the electrical device includes an alternating current to direct current (AC-DC) power supply, the power supply including the bridge rectifier, and the power supply being configured to receive the AC as a supply of input AC power and to output the DC power generated by the bridge rectifier. In some embodiments, the power supply is a ground leakage power supply configured for using ground leakage current. 
     In some embodiments, the bridge rectifier includes, on each diode leg of the bridge rectifier, two diodes electrically coupled in-series. 
     In some embodiments, the filter circuit includes a respective additional capacitor redundant in-series to each first capacitor. 
     In some embodiments, the fault-protection circuit is further configured to perform: sensing current from the step-down switching DC-DC converter; sensing a first voltage from the step-down switching DC-DC converter; sensing a second voltage at an output of the step-down switching DC-DC converter; and sensing the current or voltage fault as (i) a fault inside the step-down switching DC-DC converter based on the sensed current or sensed first voltage, or (ii) a fault at the output of the step-down switching DC-DC converter based on sensing the second voltage. 
     Additional features and advantages are realized through the concepts described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects described herein are particularly pointed out and distinctly claimed as examples in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG.  1    depicts an example three-wire wall switch device for smart lighting control; 
         FIG.  2    depicts an example two-wire wall switch that obtains control power via phase-cutting; 
         FIG.  3    depicts an example two-wire wall switch that obtains control power via Earth ground leakage current using a linear power supply; 
         FIG.  4    depicts an embodiment of an alternating current to direct current (AC-DC) power supply in accordance with aspects described herein; 
         FIGS.  5 A- 5 G  depict example embodiments of a barrier circuit for an AC-DC power supply in accordance with aspects described herein; 
         FIG.  6    depicts an example full bridge rectifier having a capacitor at the DC bus; 
         FIGS.  7 - 8    present example waveform plots of DC bus voltage and input AC current for power supplies with DC bus capacitors of varying capacitance; 
         FIG.  9    depicts an embodiment of a high power factor bridge rectifier with leg capacitors and an output filter in accordance with aspects described herein; 
         FIG.  10    depicts an example waveform plot of DC bus voltage and input AC for the example high power factor bridge rectifier of  FIG.  9   ; 
         FIG.  11    depicts another embodiment of a high power factor bridge rectifier with leg capacitors and an output filter in accordance with aspects described herein; 
         FIGS.  12 A- 12 E  depict example embodiments of a step-down switching DC-DC converter in accordance with aspects described herein; 
         FIG.  13    depicts an embodiment of an electrical device having an AC-DC power supply in accordance with aspects described herein; 
         FIG.  14    depicts an example two-wire wall switch device utilizing ground current to generate control power; 
         FIG.  15    depicts an example AC-DC power supply obtaining low-voltage control power from Earth ground current; 
         FIG.  16    depicts an embodiment of an AC-DC power supply incorporating fault-protection and limiting ground current in accordance with aspects described herein; 
         FIG.  17    depicts an embodiment of a fault-protection circuit in accordance with aspects described herein; and 
         FIGS.  18 - 20    depict additional embodiments of an AC-DC power supply incorporating fault-protection and limiting ground current in accordance with aspects described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein are example approaches for powering electronic devices, such as wall switches described herein that can provide smart lighting control, via AC-DC power supplies, and providing fault-protection for such devices. 
     As context for aspects described herein, many countries have an electric grid infrastructure that uses alternating current (AC) as a power source (referred to herein as an “AC source”). These systems can be either balanced or unbalanced and may include a phase line (“phase conductive path”) and a return path (usually referred to as a “neutral” line or “neutral conductive path”). The neutral conductive path can be used as a return path for the AC source supplied by a phase conductive path. A conductive path can also be referred to as a “wire”. The terms “conductive path”, “conductor”, and “wire” are considered herein to be synonymous. For safety reasons, the neutral wire is typically grounded at some juncture, for instance the main electrical panel. Although a ground wire is typically present at all electrical boxes, a neutral wire may not be present in some electrical boxes, such as switch boxes used to control a lighting load. In such instances, the electrical box typically contains a phase wire, a load wire, and a ground wire (or ground connection via a metal sheath of the electrical cable). As such, the lighting load is to be controlled by a device referred to as a “two-wire” device (examples of which are a switch or a dimmer), where the phrase “two-wire” refers to the phase wire and the load wire (e.g. the absence of a neutral wire). A two-wire device does not exclude the possibility of the device being connected to a third, ground wire. 
     As noted above, the presence of a neutral connection in a three-wire wall switch unit having connection to line, neutral and load, renders it easier to provide power necessary for smart control than it does for two-wire switches.  FIG.  1    depicts an example three-wire wall switch device for smart lighting control. The device  100  has respective connections for line (line wire  102 ), neutral (neutral wire  104 ), and load (load wire  106 ), in addition to its connection to Earth ground  108 . Device  100  provides power to lighting load  110  via line wire  106 , which also has a connection to neutral  111  as a return to the AC source. Device  100  includes an alternating current to direct current (AC-DC) power supply  112  (which may be isolated or non-isolated) that receives input AC power from the line  102  and neutral  104  connections, and generates a low voltage direct current (DC) power to supply to a controller, i.e. control circuit  114 . Control circuit  114  drives a signal provided to relay  116 , for instance a switch circuit, for switching AC power on the load connection and thereby provide power to the lighting load  110 . 
     Control circuit  114  may be a ‘smart’ control circuit that connects to a network, typically a wireless network. This enables wireless communication functions, such as obtaining information/data from sensors and/or receiving commands from a remote controller. Based on the received information and commands, the control circuit  114  drives the relay  116  on and off, thus selectively controlling power to the lighting load  110 . The power supply  112  is expected to provide enough DC power to the control circuit  114  to enable desired, and potentially complex, control over the relay  116 . Example wireless protocols with which the control circuit may be expected to work include, but are not limited to, WiFi (a trademark of the Wi-Fi Alliance, Austin, Tex.), Bluetooth Low Energy BLE, also referred to as Bluetooth LE, BLE, and Bluetooth Smart (some or all of which are trademarks of Bluetooth Special Interest Group, Kirkland, Wash.), and Zigbee (a trademark of Zigbee Alliance, Davis, Calif.). 
     The available neutral connection of a three-wire device enables the power supply to derive power from the connections to line and neutral. However, challenges exist in obtaining sufficient power for some control circuits, for instance smart controls, in two-wire devices. Various approaches exist that attempt to address these challenges. 
     One such approach to derive control power is to insert a phase cut circuit between the line and load connections.  FIG.  2    depicts an example two-wire wall switch that obtains control power via phase-cutting. Here, device  200  has a connection to line (line wire  202 ), load (load wire  206 ) and Earth ground  208 . Similar to the device  100  of  FIG.  1   , device  200  includes AC-DC power supply  212  that powers a control circuit  214 . Here, control circuit  214  controls phase cut circuit  218  electrically coupled between the line and load connections to provide power to lighting load  210 , with the return again being via neutral  211 . 
     During the ‘off-time’ of a phase cut, when a portion of the AC waveform is ‘cut’, i.e. power to the lighting load  210  along load wire  206  is cut, there is voltage between line and load. AC-DC power supply  212  is designed to utilize this voltage as input to generate a low voltage DC power to supply the control circuit  214 . This method has restrictions. For one, it requires a minimum load to operate the switch properly. If the load is too small, impedance of the load become very large so the AC-DC power supply may not have enough input voltage to use and therefor may not be able to generate a stable low voltage DC. Additionally, the conducted phase angle has an upper limit near 180 degrees; beyond this limit, the power supply does not have enough input energy to operate properly and thus it is not able to deliver full power to the load. Furthermore, this approach does not work with some smart bulbs. When the smart bulbs are turned off remotely and the circuit is shut off, the power supply has no energy to supply to the control circuit for subsequent operation. 
     Another approach to derive control power is to use a linear power supply circuit to drain a small amount of Earth ground current to power the control circuit.  FIG.  3    depicts an example two-wire wall switch that obtains control power via Earth ground leakage current using a linear power supply. Device  300  has a connection to line (line wire  302 ), load (load wire  306 ) and Earth ground  308 . Device  300  includes linear AC-DC power supply  312  that powers control circuit  314 , which drives a signal to relay  316  electrically coupled between the line and load connections to provide power to lighting load  310 , with the return again being via neutral  311 . The small amount of Earth ground current is available on account of the connection of the power supply  312  between line  302  and Earth ground  308 . 
     Safety standards and specifications, such as the National Electrical Code (NEC) published by the National Fire Protection Association or the UL Standards promulgated by Underwriters Laboratories, may dictate tolerable levels of ground current leakage. The tolerable levels are those that must not be exceeded and are generally ultra-small values, for example 0.7 milliamperes (mA) or 0.5 mA. Per the UL60730-1 specification, for instance, this limit is 0.7 mA and per the UL773, UL916 and UL1472 specifications this limit is 0.5 mA. For a linear regulator, the output current is equal to or smaller than the input current, which means under these standards the current of the low voltage DC power supply (e.g.  312 ) is to be smaller than 0.5 mA or 0.7 mA. Such a small current cannot support some complicated smart controller, such as those that communicate using wireless protocols such as WiFi, BLE, or Zigbee, as examples. 
     A switching AC-DC power supply may be more efficient to attaining low voltage DC utilizing an ultra-small ground current. However, insertion of a switching AC-DC power supply circuit between line and Earth ground comes with some critical compliance challenges. These include but not limited to:
         the current to the Earth ground cannot exceed the applicable ground leakage current limit (e.g. 0.5 mA or 0.7 mA as examples);   the circuit shall be able to withstand a high-potential test between the line and the Earth ground. An example such test is one with applied voltage of a few kilovolts (kV) that lasts 60 seconds; and   the circuit shall be able to survive many input voltage surge strikes between the line and Earth ground. Example such strikes can be a few kV, appear at different angles of the AC waveform, and last a few microseconds at each strike.       

     In some aspects described herein, a low ground current no-neutral AC-DC power supply is disclosed that supplies enough DC power for desired complicated smart control of a two-wire wall switch or other electrical device, and addresses the previous compliance challenges. Example levels of DC power provided by embodiments disclosed herein utilizing ultra-small ground leakage current (e.g. less than 0.7 or 0.5 mA) are between 5 mA and 15 mA, inclusive, to power a control circuit load that consumes a relatively small amount of power (levels in the tens of milliwatts). Such an AC-DC power supply could be incorporated into any electrical device having a control circuit of this power specification, for instance two-wire wall switches for smart lighting control as described herein. 
     In some aspects, an AC-DC power supply that receives AC input from connections to line and Earth ground and generates relatively low-voltage DC power for powering a control circuit includes an input resistor-capacitor barrier (referred to herein as a “barrier circuit”), a high power factor full bridge rectifier, and a step-down switching DC-DC converter able to reach desired ultra-low ground current (less than 0.7 mA) and meet other desired specifications, such as the ability to withstand high voltage short pulses or long duration strikes, such as high-potential tests, surges, electrical fast transient bursts, and electrostatic discharge, as examples. 
       FIG.  4    depicts an embodiment of an AC-DC power supply in accordance with aspects described herein. The AC-DC power supply is a no-neutral power supply utilizing suitably low ground current. AC-DC power supply  400  has a connection to a supply of input AC power (AC line  402 ), an output of DC power (low voltage DC output  404 ), a connection for Earth ground  408 , and a connection for DC ground  409  (which may be a floating ground). 
     The power supply  400  includes a barrier circuit  410  electrically coupled between the AC power  402  and a bridge rectifier  420  providing protected AC voltage to the bridge rectifier, and also includes a step-down switching DC-DC converter  430  that is electrically coupled to bridge rectifier  420 , taking high voltage DC output therefrom, stepping it down to low voltage DC, and outputting that DC power  404 . 
     Barrier circuit  410  can include one or more resistors and one or more capacitors, and have a connection to the AC power  402 , a connection to Earth ground  408 , a connection to a first input of the bridge rectifier  420 , and/or a connection to a second input of the bridge rectifier  420 , depending on varying scenarios described below. The barrier circuit  410  builds a protection barrier to protect the later stages ( 420 ,  430 , etc.) from damage by voltage spikes on the input side. 
     The protected AC voltage from the barrier circuit  410  is applied to the second stage—the bridge rectifier  420 . Bridge rectifier  420  can be a full bridge rectifier, and includes an output filter as described herein. It is configured to generate a high-voltage DC from the input protected AC and supply that DC to the third stage, the step-down switching DC-DC converter. The rectifier can have a high power factor. Power factor refers to the ratio of real power absorbed by the load to the apparent power flowing in the circuit. A higher power factor correlates to greater conformity between the waveform of input current and the input AC sine waveform. For a given level of power, a lower power factor correlates to higher input current, and a higher power factor correlates to lower input current. In the context of this description, a “high” power factor is defined to mean a power factor of at least  0 . 7 . Such a high power factor enables input current to the AC-DC power supply to be kept smaller, while delivering the desired level of power. 
     The step-down switching DC-DC converter  430  is electrically coupled to the bridge rectifier  420  and is configured to receive a first DC voltage from bridge rectifier  420  and output DC power of a second DC voltage that is lower than the first DC voltage. 
     In some examples, the output DC power  404  from the converter  430  has a current of between 5-15 mA, inclusive, and the output second voltage is between 1 and 5 volts (V), inclusive. More particularly, output second voltage can may typically be 1.8, 2.5, 3.0, 3.3 or 5.0 V. 
     The resistors and capacitors of the barrier circuit  410  are series-connected in any of varying configurations.  FIGS.  5 A- 5 G  depict example embodiments of a barrier circuit for an AC-DC power supply in accordance with aspects described herein. In each embodiment, the resistors can be one or more in quantity, are selected with proper physical size and resistance to withstand high-voltage, narrow pulse strikes, and allow enough current to flow to the next stage—the high power factor full bridge rectifier  420 . Additionally, in each embodiment the capacitors can be one or more in quantity, are selected with proper capacitance and voltage rating to withstand high-voltage, extended-duration strikes, and allow enough current to flow to the next stage—the high power factor full bridge rectifier  420 . 
     In each embodiment of  FIG.  5 A- 5 G , the respective example barrier circuit  510  is depicted with a connection to AC power (line  502 ) and connection to Earth ground  508 , and output AC connections  516  and  518 , as first and second AC inputs respectively, to the bridge rectifier (not pictured). Each barrier circuit  510  of  FIGS.  5 A- 5 G  includes at least one set of one or more resistors (connected in-series if multiple are present in each set), and at least one set of one or more capacitors (connected in-series if multiple are present in each set). 
     Referring to the barrier circuit  510  of  FIG.  5 A , one or more resistors  512  and one or more capacitors  514  are electrically coupled in-series between the connection to the AC power (line  502 ) and the connection to the first input ( 516 ) of the bridge rectifier. 
     Referring to the barrier circuit  510  of  FIG.  5 B , one or more resistors  512  and one or more capacitors  514  are electrically coupled in-series between the connection to Earth ground ( 508 ) and the connection to the second input ( 518 ) of the bridge rectifier. 
     Referring to the barrier circuit  510  of  FIG.  5 C , there are plurality of capacitors, in which (i) a first one or more capacitors  514   a  and one or more resistors  512  are electrically coupled in-series between the connection to the AC power (line  502 ) and the connection to the first input ( 516 ) of the bridge rectifier, and (ii) a second one or more capacitors  514   b  are electrically coupled between the connection to Earth ground ( 508 ) and the connection to the second input ( 518 ) of the bridge rectifier. 
     Referring to the barrier circuit  510  of  FIG.  5 D , there are plurality of resistors, in which (i) a first one or more resistors  512   a  and one or more capacitors  514  are electrically coupled in-series between the connection to the AC power (line  502 ) and the connection to the first input ( 516 ) of the bridge rectifier, and (ii) a second one or more resistors  512   b  are electrically coupled between the connection to Earth ground ( 508 ) and the connection to the second input ( 518 ) of the bridge rectifier. 
     Referring to the barrier circuit  510  of  FIG.  5 E , there are plurality of capacitors, in which (i) a first one or more capacitors  514   a  are electrically coupled between the connection to the AC power (line  502 ) and the connection to the first input ( 516 ) of the bridge rectifier, and (ii) one or more resistors  512  and a second one or more capacitors  514   b  are electrically coupled in-series between the connection to Earth ground ( 508 ) and the connection to the second input ( 518 ) of the bridge rectifier. 
     Referring to the barrier circuit  510  of  FIG.  5 F , there are plurality of resistors, in which (i) a first one or more resistors  512   a  are electrically coupled between the connection to the AC power (line  502 ) and the connection to the first input ( 516 ) of the bridge rectifier, and (ii) a second one or more resistors  512   b  and one or more capacitors  514  are electrically coupled in-series between the connection to Earth ground ( 508 ) and the connection to the second input ( 518 ) of the bridge rectifier. 
     Referring to the barrier circuit  510  of  FIG.  5 G , there are plurality of resistors and a plurality of capacitors, in which (i) a first one or more resistors  512   a  and a first one or more capacitors  514   a  are electrically coupled in-series between the connection to the AC power (line  502 ) and the connection to the first input ( 516 ) of the bridge rectifier, and (ii) a second one or more resistors  512   b  and a second one or more capacitors  514   b  are electrically coupled in-series between the connection to Earth ground ( 508 ) and the connection to the second input ( 518 ) of the bridge rectifier. 
     Referring back to  FIG.  4   , the high power factor full bridge rectifier  420  is provided with passive power factor improvements. As noted, a purpose of reaching a high power factor is to keep the input current small while supplying enough DC energy to the next stage, the step-down switching DC-DC converter  430 . 
     Two approaches may be used to boost the power factor. The first is to set the capacitance of a DC bus capacitor to be relatively small.  FIG.  6    depicts an example full bridge rectifier having a capacitor at the DC bus. AC input to the bridge rectifier  620  is provided via first and second AC voltage inputs  622 ,  624 . Outputs of the bridge rectifier  620  are positive DC rail  626  (high-voltage DC+) and negative DC rail  628  (high-voltage DC-). The positive and negative high-voltage DC are DC inputs to the step-down switching DC-DC converter (e.g.  430  of  FIG.  4   ). 
     Bridge rectifier  620  also includes four diode legs  605   a ,  605   b ,  605   c ,  605   d , each with a respective diode, and a bus capacitor  630  coupled between the positive and negative DC rails  626 ,  628 . In a conventional full bridge rectifier circuit, the bus capacitor is selected to be of relatively large capacitance so that ‘ripple’ on the rectified DC bus is less than, say, 10% of the DC voltage. This is relatively large-capacitance scenario is depicted by  FIG.  7   .  FIGS.  7 - 8    present example waveform plots of DC bus voltage and input AC current for power supplies with DC bus capacitors of varying capacitance. In particular,  FIG.  7    presents an example waveform plot of DC bus voltage and input AC current for a power supply with a DC bus capacitor of relatively large capacitance and  FIG.  8    presents an example waveform plot of DC bus voltage and input AC current for a power supply with a DC bus capacitor of relatively small capacitance. 
     Referring initially to  FIG.  7 ,  702    is the waveform plot of the input AC current and  704  is the corresponding waveform plot of the DC bus voltage. Waveform  702  would theoretically look like a normal sine waveform but for the bus capacitor of relatively large capacitance (in this example), which causes the conduct angle of the bridge diodes to be very small. The waveform  702  exhibits a spike in each half-cycle, sacrificing conduction angle (represented by the small width of each spike in the AC input) in order to keep the ripple in the DC voltage small, as shown. This relatively small conduction angle leads to a relatively high total harmonics distortion (THD) in the input current and a low power factor. For the current shown in  FIG.  7   , THD is about 198% and the power factor is less than 0.4. 
     If the capacitance of the DC bus capacitor is reduced, the ripple on the DC bus may be increased as depicted by  FIG.  8   .  802  is the waveform plot of the input AC current and  804  is the corresponding waveform plot of the DC bus voltage. A benefit of reducing the capacitance of the bus capacitor is that the conduction angle of the bridge diodes is boosted (as seen by the wider spikes in the input AC current  802 ) and THD is reduced, thus the power factor is improved. In the example of  FIG.  8   , the DC bus has a high ripple—about 40% of the DC voltage—but the conduction angle is advantageously dramatically increased, which largely reduces the THD in the input current and boosts the power factor. For the AC current in  FIG.  8   , the THD is 64% and the power factor is increased to 0.67. This improvement means at the same output power, the input current is reduced by 68% compared to that of the scenario of  FIG.  7   . 
     In embodiments of the AC-DC power supply described herein, a regulated step-down switching DC-DC converter ( 430  of  FIG.  4   ) is the next stage and high-low frequency ripple on the high voltage DC rail will not impact the low voltage DC (LVDC) output characteristics, such as regulation and ripple. 
     The power factor can be further boosted by adding two leg capacitors to two legs of the full bridge rectifier.  FIG.  9    depicts an example high power factor bridge rectifier with leg capacitors and an output filter, in accordance with aspects described herein. In  FIG.  9   , AC input to the bridge rectifier  920  is provided via positive and negative voltage inputs  922 ,  924 , and outputs of the bridge rectifier  920  are positive DC rail  926  (high-voltage DC+) and negative DC rail  928  (high-voltage DC−). The bridge rectifier includes four diode legs  905   a ,  905   b ,  905   c ,  905   d , each with a respective diode. Meanwhile, a filter capacitor  930  is electrically coupled between the positive and negative DC rails  926 ,  928 . The bridge rectifier  920  further includes (i) a first additional capacitor  932  electrically coupled between an AC connection terminal  924  of the bridge rectifier and the positive DC rail  926  and (ii) a second additional capacitor  934  electrically coupled between the AC connection terminal  924  of the bridge rectifier and the negative DC rail  928 . Capacitors  932 ,  934  are referred to as leg capacitors, and their capacitance can be selected to be smaller than the capacitance of the bus/filter capacitor  930 . In some examples, the capacitance of each leg capacitor  932 ,  943  is smaller than one-third a capacitance of the filter capacitor  930 . 
     The leg capacitors help produce an even larger conduction angle, conforming the current waveform more closely to the theoretical sine waveform and thus increasing the power factor and reducing input current even further.  FIG.  10    depicts an example waveform plot of DC bus voltage and input AC for the example high power factor bridge rectifier of  FIG.  9   , having a relatively small capacitance filter capacitor  930  as discussed above and two leg capacitors of even smaller capacitance. In  FIG.  10 ,  1002    is the waveform plot of the input AC current and  1004  is the corresponding waveform plot of the DC bus voltage.  FIG.  10    shows further improvement in the current waveform. Since the leg capacitors  932 ,  934  are of smaller capacitance compared to that of the bus capacitor  930 , ripple level is not affected but the leg capacitors cause a step-up (at example points  1006 ) at diode turn-on at each positive and negative half cycle, thus further increasing the conduction angle, which further reduces THD of the input current and further increases the power factor. In the example of  FIG.  10   , THD is about 45% and the power factor is over 0.78. At the same output power, the input current of this scenario is reduced by almost half compared to that of the scenario of  FIG.  7   . 
     In some examples, the output filter of the bridge rectifier, a single capacitor  930  in the example of  FIG.  9   , is instead a pi filter.  FIG.  11    depicts such an example of a high power factor bridge rectifier with leg capacitors and a pi filter as the output filter. The bridge rectifier  1120  of  FIG.  11    includes some components that are analogous to those of the bridge rectifier  920  of  FIG.  9    and are not repeated for purposes of this discussion, except that the output filter of  FIG.  9    (the single capacitor  930 ) is replaced in  FIG.  11    with a pi filter  1140  that includes (i) a first capacitor  1142  electrically coupled between the positive and negative DC rails  1126 ,  1128 , (ii) a second capacitor  1144  electrically coupled between those rails, as the first and second DC inputs to the next stage—the step-down switching DC-DC converter, and (iii) an inductor  1146  that is electrically coupled in-series between the first capacitor  1142  and the second capacitor  1144 . 
     The capacitance of capacitors  1142  and  1144  can be selected sufficient small that the power factor improvement is still effective, i.e. extending closer to unity (1.0). 
     A third stage of a no-neutral AC-DC power supply (e.g. AC-DC power supply  400  of  FIG.  4   ) as described herein is a step-down switching DC-DC converter (e.g.  430  of  FIG.  4   ). The DC-DC converter generates low-voltage DC (LVDC) from the high-voltage DC (HVDC) bus output of the second stage (bridge rectifier, e.g.  420  of  FIG.  4   ).  FIGS.  12 A- 12 E  depict example differing embodiments of a step-down switching DC-DC converter in accordance with aspects described herein. 
     Referring initially to  FIG.  12 A , step-down switching DC-DC converter  1230  has input  1240  for high-voltage DC and output  1242  for low-voltage DC power. It also includes a controlled switching device  1250  that is controlled by control circuit  1260 , a diode  1262  (e.g. a flywheel diode here), an inductor  1266  and a capacitor  1264  between the output DC rails. 
     As an alternative, the control circuit and switching device can be incorporated together.  FIG.  12 B  presents an identical DC-DC converter  1230  to that of  FIG.  12 A  except that the control circuit is incorporated into the switching device as component  1270 . 
     As yet another alternative, the diode can be incorporated into the switching component.  FIG.  12 C  presents an identical DC-DC converter  1230  to that of  FIG.  12 B  except that the diode is incorporated into the switching device as a switching integrated circuit component  1280 . 
     As another alternative, step-down switching DC-DC converter could be implemented as an isolated converter with a transformer inserted between HVDC and LVDC.  FIG.  12 D  presents an example such converter. As shown in  FIG.  12 D , converter  1230  has input  1240  for high-voltage DC and output  1242  for low-voltage DC power. As input on the primary side of transformer  1290  is the input for the high-voltage DC and the isolated converter components  1250  (controlled switching device) and  1260  (control circuit). As output on the secondary side of transformer  1290  is the output low-voltage DC power  1242 , with capacitor  1294  coupled between the output LVDC rails. 
     As an alternative to  FIG.  12 D , the control circuit and switching device can be incorporated together.  FIG.  12 E  presents an identical DC-DC converter  1230  to that of  FIG.  12 D  except that the control circuit is incorporated into the switching device as a switching power integrated circuit component  1280 . 
       FIG.  13    depicts an example electrical device having an AC-DC power supply in accordance with aspects described herein. This is an example implementation that provides smart control in a two-wire wall switch unit with a low-ground-current AC-DC power supply as described herein. The device  1300  has respective connections for line (line wire  1302 ) and load (load wire  1306 ), in addition to its connection to Earth ground  1308 . Device  1300  provides power to lighting load  1310  via line wire  1306 , which returns via neutral line  1311 . Device  1300  includes a low-ground-current AC-DC power supply  1313  as described herein, example embodiments of which are depicted and described with reference to  FIGS.  4 ,  5 A- 5 G,  9 ,  11  and  12 A- 12 E . The AC-DC power supply  1313  receives input AC power from line  1302  and generates LVDC power to supply to a controller, microcontroller, or the like (i.e. control circuit  1314 ), which drives a control signal to power/dimming circuit  1316  that incorporates a switch for switching AC power on the load connection  1306  and thereby provides controlled power to the lighting load  1310 . Advantageously, the power supply  1313  can generate sufficient power to power smart functionality, including network communication over, e.g., a wireless connection, of control circuit  1314 . 
     Device  1313  of  FIG.  13    therefore could be an electrical load controller for controlling conduction of the supply of input AC power to a load, where the load controller includes a line input terminal configured to be electrically coupled to the supply of AC power and a load output terminal configured to be electrically coupled to the load. With a switching circuit electrically coupled in series between the line input terminal and the load output terminal, which switching circuit has an ON state in which the switching circuit conducts the supply of AC power to the load and an OFF state in which the switching circuit does not conduct the supply of AC power to the load, a control circuit can be configured to control operation of the electrical load controller, including firing of the switching circuit, and an example AC-DC power supply as described herein can be provided to supply sufficient power to that control circuit for performing such functions. 
       FIG.  14    depicts an example two-wire wall switch device utilizing ground current to generate control power. Utilization of ground current to generate low-voltage DC power was described briefly above. Device  1400  has a connection to line (line wire  1402 ), load (load wire  1406 ) and Earth ground  1408 . Device  1400  includes AC-DC power supply  1412  that powers a smart control circuit  1414 , which drives a signal to relay  1416 . Device  1400  provides power to lighting load  1410  via line wire  1406 , which also has a connection to neutral  1411  as a return to the AC source. 
     Additional aspects are described herein that ensure an AC-DC power supply utilizing ground current remains compliant with ground current requirements (e.g. less than 0.7 or 0.5 mA) at single fault conditions. This makes it possible for example switched-mode power supplies with no neutral to meet full safety requirements. Aspects use a combination of component redundancy and an open-circuit to limit ground current to be desirably-small at any single component short-circuit condition. 
     An example AC-DC power supply  1500  that obtains low-voltage control power from Earth ground current is depicted in  FIG.  15   . The AC-DC power supply  1500  is located between AC line  1502  and the Earth ground  1508 , and includes input circuit  1503  a full bridge rectifier  1520  that includes four diode legs  1505   a ,  1505   b ,  1505   c , and  1505   d , each with a respective diode thereof, a filter circuit  1540  that in this example is a pi filter that includes capacitors  1542 ,  1544  and inductor  1546 , and a step-down DC-DC converter  1530 . The input circuit may include, but is not limited to, a barrier circuit such as that described herein, for instance above with reference to  FIG.  12    and elsewhere. The AC-DC power supply  1500  provides DC power to load  1510 , for instance a smart lighting control circuit described above that consumes the low-voltage DC power generated by the AC-DC power supply. 
     As noted, for safety reasons the utilization of ground current is constrained by way of safety standards, such as those of UL or NEC, specifying that ground current must not exceed some ultra-small limit. Per the UL60730-1 specification, for instance, this limit is 0.7 mA and per the UL773, UL916 and UL1472 specifications this limit is 0.5 mA. 
     Example implementations of AC-DC power supplies that can generate sufficient control power for a smart control circuit while meeting these ground current standards and other electromagnetic compatibility (EMC) and safety standards in normal operation are disclosed above. 
     An additional challenge related to ground current based power supplies is that the applicable ground current standard(s) also apply at single fault conditions, meaning the ground current cannot exceed (e.g.) 0.7 mA or 0.5 mA when any single component in the circuit is at short-circuit. For example, in the circuit of  FIG.  15   , the ground current (here the same as the line current) shall not exceed the same ultra-low current limit (0.5 mA or 0.7 mA) when any single component (such as any of the four leg diodes, capacitor  1542  or  1544 , inductor  1546 , or any component in input circuit  1503  or step-down DC-DC converter  1530 ) is at short circuit. Presented herein are approaches that limit ground current of a no-neutral AC-DC power supply at single fault of any component to the same or smaller level than its normal operating level (which by design must not exceed the applicable ultra-low current specification, e.g. 0.5 or 0.7 mA. “Fault” in this context is safety-relevant and is a failure (most likely a short-circuit) of or with a component. 
     In embodiments of limiting ground current at single-fault, aspects use (i) redundancy of components and (ii) fault event open-circuit protection to limit the ground current at single fault in an AC-DC power supply located between AC line and Earth ground. 
     More specifically, redundancy of components in front of the rectified DC bus to the step-down converter are provided with respect to the rectifier diodes and the filter capacitors sitting between the rectified DC positive rail and the rectified DC negative rail. For the bridge rectifier with four diode legs, each diode leg of the four diode legs is provided with respective diodes (two or more) electrically coupled in-series. Filter capacitors are also replicated to provide series-connected capacitors. 
     After the rectified DC bus, a fault event protection circuit is used to open the circuit on a fault condition. Such a fault-protection circuit, as described herein, is configured to sense (i) current from the step-down switching DC-DC converter, (ii) a first voltage from the step-down switching DC-DC converter, and/or (iii) a second voltage at an output of the step-down switching DC-DC converter, and trigger opening a switch when appropriate to prevent excessive current leakage, i.e. that would exceed the allowable leakage under the appropriate specification. 
       FIG.  16    depicts an embodiment of an AC-DC power supply  1600  incorporating fault-protection for and limiting ground current in accordance with aspects described herein. The AC-DC power supply sits between AC line  1602  and Earth ground  1608 . After input circuit  1603  is a cascade diode bridge rectifier  1620  with redundancy of diodes. Specifically, the rectifier includes four diode legs  1605   a ,  1605   b ,  1605   c , and  1605   d , with each diode leg of the four diode legs including a respective plurality of diodes electrically coupled in-series. 
     Along the DC output rails  1626  (positive) and  1628  (negative) is output filter  1640 . Output filter circuit  1640  in this example is a pi filter but with redundancy of capacitors. The filter  1640  includes (i) a first plurality  1642  of capacitors electrically coupled between the positive DC rail  1626  of the bridge rectifier  1620  and the negative DC rail  1628  of the bridge rectifier  1620 , (ii) a second plurality  1644  of capacitors electrically coupled between the positive DC input  1627  of the step-down switching DC-DC converter  1630  and a return  1629  of a fault-protection circuit  1680 , and (iii) an inductor  1646  electrically coupled between the first plurality  1642  of capacitors and the second plurality  1644  of capacitors. 
     Step-down switching DC-DC converter can be any desired DC-DC converter, such as one as described herein. 
     The fault-protection circuit  1680  includes a fault event trigger circuit  1682  that senses (i) current from the step-down switching DC-DC converter along line  1670  and via current sense line  1684 , (ii) a first voltage from the step-down switching DC-DC converter  1630  via line  1686 , and (iii) a second voltage via line  1688 , which second voltage is an output voltage at an output of the step-down switching DC-DC converter  1630 . The trigger circuit  1682  controls a switch  1690  which serially-coupled between the input  1627  of the step down converter  1630  and the negative DC rail  1628 / 1629 . 
     Load  1610  is not a part of the example AC-DC power supply of  FIG.  16    but is provided for context. 
     If a single fault (e.g. short circuit) occurs with any of the diodes on the diode legs of the bridge rectifier  1620 , the input current will not change because of the diode redundancy provided; a short circuit of a single diode will not bring an apparent input current change. If a single fault (e.g. short circuit) occurs with any one of the capacitors of  1642  or  1644 , the input current will not change because of the redundancy provided in the series-connected capacitors. 
     Because the input circuit  1603  and the inductor  1646  are serial-connected inside the AC-DC power supply, a short-circuit of either of them will not bring apparent input current change, though it may impact EMC. Therefore, in this example redundancy in either is not provided. 
     If the single fault occurs after the rectified DC bus, such as inside or at an output of the step-down DC-DC converter  1630 , the fault-protection circuit  1680  functions as the protection mechanism to open the switch to limit the input current within the leakage current standard. 
     Further details of the fault-protection circuit  1680  are provided with reference to  FIG.  17   , depicting an embodiment of such a fault-protection circuit. This circuit  1780  is connected in this example in-series between the low voltage DC return (DC ground)  1770  (e.g.  1670  of  FIG.  16   ) and the rectified high-voltage negative DC rail  1729  (e.g.  1629  of  FIG.  16   ). It includes a current sensing circuit  1783 , a voltage sense circuit  1787  (which has one or more voltage sense points), a fault event trigger circuit  1782  and an on/off switch  1790 . In some examples, the current and voltage sense circuits  1783 ,  1787  are sense points. 
     The mechanism of this fault-protection circuit  1780  is as follows: If a single fault (e.g. short-circuit) occurs inside or at the output of the step-down DC-DC converter, sensed current or voltage at the critical points of the converter will have an abnormal value. The sensed abnormal current or voltage is sent to the fault-protection circuit  1780 , which detects it as a fault, and the trigger circuit  1782  triggers the turn-off action of the on/off switch  1790 , thereby opening the circuit. When the circuit between the DC ground  1770  and high voltage negative rail  1729  is open, the input current (i.e. ground current) will drop to a level below the standardized maximum ground current limit. 
     The fault-protection circuit thus detects (i) a fault inside the step-down switching DC-DC converter based on sensing the current or the first voltage (from within the step-down switching DC-DC converter), and/or (ii) a fault at the output of the step-down switching DC-DC converter based on sensing the second voltage (at the output of the DC-DC converter), and, based on detecting the fault inside the step-down switching DC-DC converter or the fault at the output of the step-down switching DC-DC converter, triggers opening the switch of the fault-protection circuit between a return of the output voltage from the step down switching DC-DC converter and the negative DC rail of the bridge rectifier. 
       FIGS.  18 - 20    depict additional embodiments of an AC-DC power supply incorporating fault-protection and limiting ground current in accordance with aspects described herein. Referring initially to  FIG.  18   , presented is an AC-DC power supply  1800  identical in components to that of  FIG.  16    except that the output filter  1640  of  FIG.  16    (which was a pi filter with redundant capacitors) is replaced with filter  1840  having two capacitors  1842 ,  1844  electrically coupled between the positive DC rail  1826  of the bridge rectifier and the negative DC rail  1828  of the bridge rectifier  1820 . An advantage of this approach is cost savings achieved by eliminating additional capacitors and the inductor as seen in  FIG.  16   . 
     As yet another alternative to  FIG.  16   , the output filter (e.g. pi filter) may be relocated to after the fault-protection circuit, as shown in  FIG.  19   .  FIG.  19    includes components analogous to those of  FIG.  16    except the fault-protection circuit  1980  is coupled between the return rail  1969  of the output filter  1940  and the negative DC rail  1928  of the bridge rectifier  1920 . Meanwhile, redundant capacitors in the filter could be eliminated, as depicted in this example. 
     As an alternative to  FIG.  19   , the filter circuit  1940  (a pi-filter in  FIG.  19   ) may be replaced by a single capacitor. This is depicted in the example of  FIG.  20   , which is analogous to  FIG.  19    except that in the output filter  2040 , a capacitor  2042  is electrically coupled between the positive DC input  2027  and a negative DC input  2070  to the step-down switching DC-DC converter  2030 . Like in  FIG.  19   , the fault-protection circuit  2080  is electrically coupled between the return  2069  of the output voltage from the filter circuit  2040  (i.e. return of the input of the step-down DC-DC converter) and the negative DC rail  2028  of the bridge rectifier  2020 . 
     Although various examples are provided, variations are possible without departing from a spirit of the claimed aspects. Although various embodiments are described above, these are only examples. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below, if any, are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of one or more embodiments has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain various aspects and the practical application, and to enable others of ordinary skill in the art to understand various embodiments with various modifications as are suited to the particular use contemplated.