Capacitive drop power supply for lighting control

This application claims priority to U.S. Provisional Application No. 63/018,231, filed Apr. 30, 2020, which is hereby incorporated by reference.

Light bulbs implemented as light emitting diodes (LEDs) are becoming increasing more ubiquitous. An LED operates from a direct current (DC) voltage and current. However, the electrical power to a building (e.g., office budding, home, etc.) is an alternating current (AC) voltage and current. An AC-to-DC converter converts the AC voltage a DC voltage of a suitable voltage level to power the LED, A light control module may couple to the LED. The light control module may include a switch to permit the LED to be turned on or off. The AC-to-DC converter may be part of the light control module.

The AC wires often are designated “line” and “neutral,” The light control module and its switch may be put in-line with the “line” wire between the AC voltage source and the LED. Such applications are called “neutral-less” configurations because only the line wire is provided to the light control module, not the neutral wire. Without a neutral wire, the light control module does not have a voltage source to power any active electronics it may contain.

A circuit includes a capacitor-drop power supply including a series combination of a resistor and a first capacitor. The capacitor-drop power supply includes an output and is adapted to be coupled to a light source. The circuit also includes a second capacitor, a switch, and an active clamp circuit. The second capacitor couples to the output of the capacitor-drop power supply. The switch couples in parallel with the series combination of the resistor and the first capacitor. The switch is configured to cause the light source to illuminate. The active clamp circuit couples to the capacitor-drop power supply. The active clamp circuit has an output coupled to the capacitor-drop power supply. The active clamp circuit is configured to cause current to continuously flow through at least one of the switch or the series combination of resistor and first capacitor regardless of a magnitude of the voltage across the second capacitor.

FIG. 1 is a schematic circuit diagram showing an example of a light control module.

FIG. 2 includes waveforms that illustrate that the current through the light control module of FIG. 1 is discontinuous.

FIG. 3 is a schematic circuit diagram showing a second example of a light control module in which the current is continuous.

FIG. 4 is the schematic circuit diagram of FIG. 3 illustrating the direction of current flow through the light control module.

FIG. 5 includes waveforms showing an example of the current through the light control module of FIG. 3 illustrating that the current is continuous.

FIG. 6 is a schematic circuit diagram showing one example of an active clamp circuit usable with the light control module of FIG. 3 .

FIG. 7 is a schematic circuit diagram showing another example of an active clamp circuit usable with the light control module of FIG. 3 .

FIGS. 8 and 9 are is a schematic circuit diagrams illustrating the direction of current flow through the light control module of FIG. 3 when a switch within the light control module is closed and a light source is illuminated.

FIG. 10 is a schematic circuit diagram illustrating the switch as a solid-state relay in accordance with an example.

FIGS. 11 and 12 are waveforms illustrating techniques for modulating the on and off states of the switch to implement a dimming function for the light source.

In a neutral-less configuration, a light control module implements current stealing, in which a continuous current path through the light control module and the LED has enough current to power active electronics in the light control module, but not enough current to also illuminate the LED. In some power stealing configurations, however, the LED may undesirably flicker on and off as a result of current spikes that are unintentionally created. FIG. 1 shows such an example.

FIG. 1 shows a light source 110 coupled to a light control module 120 . The light source 110 may include an LED or other type of light source. An AC voltage source 105 provides power for the light source 110 . The AC voltage source 105 includes a line 106 and a neutral 107 . In this example, the neutral 107 is coupled to the light source 110 , and the line 106 is coupled to terminal T 1 of the light control module 120 . Terminal T 2 of the light control module 120 continues the line 106 to the light source 110 . The light control module 120 includes a switch SW 1 coupled across terminals T 1 and T 2 . When switch SW 1 is closed, a current path is formed thereby causing AC current to flow through the light source 110 , and the light source 110 illuminates.

The light control module 120 includes one or more active components that require electrical power for their operation. In the embodiments described herein, the active component includes a WiFi transceiver 125 (and identified by reference numeral 324 in other figures), but can include additional or different active components as well. The WiFi transceiver 125 may receive wireless signals from an external device (such as originating from a smart phone, tablet device, etc.). The wireless signals may include commands to turn the light source 110 on and off. For example, with the light source 110 off, the WiFi transceiver 125 may receive a wireless command to turn on the light source 110 . In response, the WiFi transceiver 125 may assert a signal 126 to cause a solid-state switch SW 1 to turn on (close). With switch SW 1 turned on, a current path for AC current completes through the light source 110 and the light source 110 illuminates. An “off” command can also be wirelessly received by the light control module 120 to cause the light control module 120 to deassert signal 126 to thereby turn off (open) switch SW 1 to turn off the light source 110 . The WiFi transceiver 125 toggles the on and off states of switch SW 1 .

FIG. 1 shows an example implementation of the light source 110 . In this example, the light source 110 includes one or more LEDs 114 , a rectifier 111 (to rectify the AC voltage to the light source to an approximate DC voltage), a capacitor C 1 , and an LED lamp driver 109 . The capacitor C 1 is charged to an approximate DC voltage by the rectifier 111 . The LED lamp driver 109 receives the capacitor's voltage and converts the voltage to an appropriate level for driving the LED 114 .

The light control module 120 also includes a rectifier 121 coupled to a capacitor C 2 and to a voltage regulator 122 . The rectifier 121 includes diodes D 1 -D 4 in a full bridge rectifier configuration and the rectified output voltage from the rectifier 121 charges capacitor C 2 . The voltage regulator 122 includes a low drop-out (LDO) regulator 123 and produces a regulated output voltage to power the WiFi transceiver 125 and to charge a battery BATT 1 . When switch SW 1 is on to turn on the light source 110 , switch SW 1 effectively shorts terminals T 1 and T 2 together and thus little or no current flows to the rectifier 121 and the voltage regulator 122 . For this reason, the battery BATT 1 has been included. Battery BATT 1 is charged when switch SW 1 is open and supplies the operating voltage to the WiFi transceiver 125 when switch SW 1 is closed, at least until the point that the battery's charge is depleted. When switch SW 1 is open, enough current flows through the rectifier 121 to power the WiFi transceiver 125 and to charge the battery BATT 1 through the voltage regulator 122 . The current that flows through the rectifier 121 when switch SW 1 is open also flows through the light source 110 , but the magnitude of the current should not be large enough to cause the light source 110 to illuminate. However, if the charge within battery BATT 1 becomes depleted, the output of the voltage regulator 122 directly powers the WiFi transceiver 125 (instead of the battery), and the current from line 106 powers the voltage regulator. Therefore, if the WiFi transceiver were to operate at a sufficiently high power state (e.g., to send and receive numerous wireless packets), the current through line 106 to the light control module 120 may be large enough in each cycle of the AC voltage to momentarily cause the light source 110 to illuminate thereby resulting in a flickering light source.

When switch SW 1 is open, the rectifier 121 produces a current to charge capacitor C 2 . The voltage across capacitor C 2 is labeled V_C 2 . When switch SW 1 is open, for current to flow through the rectifier 121 to charge capacitor C 2 , the instantaneous voltage across terminals T 1 and T 2 must be larger than the voltage V_C 2 . Accordingly, during one half-cycle of the AC voltage (when the voltage on neutral 107 is positive relative to line 106 ), diodes D 3 and D 2 are on and current flows from terminal T 2 , through diode D 3 , through capacitor C 2 , and back through diode D 2 to terminal T 1 . However, diodes D 3 and D 2 are on only when the voltage on terminal T 2 relative to terminal T 1 is more than two diode voltage drops greater than voltage V_C 2 . During the opposite half-cycle, diodes D 1 and D 4 are on to provide charge current to capacitor C 2 but only when the voltage on terminal T 1 relative to terminal T 2 is more than two diode voltage drops greater than voltage V_C 2 .

FIG. 2 shows the approximately sinusoidal AC voltage waveform 201 of line 106 relative to neutral 107 . Waveform 202 is the current that flows through the rectifier 121 when switch SW 1 is open. As is shown, waveform 202 is characterized by a current pulse 203 during each half-cycle as a result of none of the diodes of the rectifier 121 being on until the AC voltage is large enough (either positive or negative) relative to the voltage V_C 2 as described above. The current is not continuous in this example. As described above, it is possible that the WiFi transceiver 125 draws enough current that during each cycle of waveform 202 , the current magnitude exceeds a threshold at which the light source 110 illuminates. Accordingly, the light source 110 may flicker if the battery were to become depleted and the WiFi transceiver 125 were to draw too much current.

FIG. 3 shows an example embodiment of a light control module 320 that includes a capacitor-drop power supply containing one or more capacitors that, in part, function to limit the current magnitude to a low enough level to avoid the flickering problem described above. Light control module 320 includes a capacitor-drop power supply 310 , a capacitor Cdc, a WiFi transceiver 324 , a driver 322 , and an active clamp circuit (ACC) 330 . The capacitor-drop power supply 310 includes a resistor Rs coupled to a capacitor Cs. The capacitor-drop power supply 310 uses the capacitive reactance of capacitor Cs to reduce the voltage of the AC voltage source 105 (e.g., 110 VAC, 220 VAC, etc.) to a lower voltage. One terminal of resistor Rs is coupled to terminal T 31 of the light control module 320 , and the other terminal of resistor Rs is coupled to capacitor Cs. Resistor Rs and capacitor Cs are coupled in series. Capacitor Cs is coupled to an anode of diode D 3 . The anode of diode D 4 is coupled to terminal T 32 of the light control module 320 . The cathodes of diodes D 3 and D 4 are coupled together and to one terminal of capacitor Cdc.

Whereas the embodiment of FIG. 1 included diodes D 1 and D 2 to complete a full bridge rectifier including diodes D 1 -D 4 , the light control module embodiment of FIG. 3 has transistors M 1 and M 2 instead of diodes D 1 and D 2 . In this example, transistors M 1 and M 2 are metal-oxide semiconductor field-effect transistors (MOSFETs) which include parasitic body diodes. Transistor M 1 includes a body diode D 1 _M 1 and transistor M 2 includes a body diode D 2 _M 2 . In the example of FIG. 3 , transistors M 1 and M 2 are N-type MOSFETs. When transistors M 1 and M 2 are off, their respective body diodes D 1 _M 1 and D 2 _M 2 , combined with diodes D 3 and D 4 function as a full-bridge rectifier. However, when M 1 and M 2 are on, as described below, a current path is formed through the channels of the transistors and the full-bridge rectifier functionality is lost in favor of a continued current path to avoid the discontinuous current waveform with current pulses 203 as characterized by the embodiment of FIG. 1 . When the charge on capacitor Cdc becomes depleted, a continuous flow of current is available to recharge the capacitor Cdc without the charge current being so high as to cause the light source to illuminate.

The charge on capacitor Cdc provides the voltage source (voltage V_Cdc) for the WiFi transceiver 324 (which includes an antenna 325 ). The current from line 106 through resistor Rs and capacitor Cs charges capacitor Cdc, and does not flow directly to the WiFi transceiver 125 . Capacitor Cs limits the magnitude of the current from line 106 to a low enough level so as not to cause the light source 110 to illuminate while being sufficiently large to charge capacitor Cdc.

The active clamp circuit 330 has inputs 331 and 332 and an output 333 . Input 331 is coupled to the upper terminal of capacitor Cdc and thus receives the voltage V_Cdc. Input 332 is coupled to a reference voltage source, Vref. The output 333 is coupled to the control inputs (gates) of transistors M 1 and M 2 . The active clamp circuit 330 includes a comparator (as is described below) which compares voltages V_Cdc and Vref. Voltage Vref is set to a voltage level which generally corresponds to a voltage level for V_Cdc that is sufficiently high to adequately power the WiFi transceiver 324 . The active clamp circuit 330 generates an output signal 335 on its output 333 that is high enough to turn on turn on transistors M 1 and M 2 responsive to voltage V_Cdc being higher than voltage Vref and low enough to turn off transistors M 1 and M 2 responsive to voltage V_Cdc being lower than voltage Vref. Accordingly, responsive to capacitor Cdc transferring enough of its charge such that its voltage V_Cdc falls below voltage Vref, the active clamp circuit 330 turns off transistors M 1 and M 2 . With transistors M 1 and M 2 being off, diodes D 3 and D 4 and the transistors' body diodes D 1 _M 1 and D 2 _M 2 operate as a full-bridge rectifier to provide a current to charge capacitor Cdc.

Responsive to the voltage V_Cdc then increasing above the voltage Vref, the active clamp circuit 330 turns on transistors M 1 and M 2 . With capacitor Cdc being sufficiently charged to directly power the WiFi transceiver 324 , current from the capacitor-drop power supply is not needed to charge the capacitor. However, with transistors M 1 and M 2 being turned on in this state (i.e., capacitor Cdc being sufficiently charged to power the transceiver), a path continues to be available for the AC current to flow as described below. The path for current to flow between terminals T 31 and T 32 of the light control module 320 depends on (a) whether switch SW 2 is on or off and (b) whether the capacitor Cdc is sufficiently charged to power the WiFi transceiver 324 .

Rather than a switch being coupled between the terminals of the light control module as was the case for the embodiment of FIG. 1 , switch SW 2 is coupled in parallel with the series combination of resistor Rs and capacitor Cs as shown in the embodiment of FIG. 3 . Switch SW 2 is implemented as a solid-state switch (e.g., one or more transistors) which is turned on and off by a signal from driver 322 . Driver 322 receives a control signal 326 from the WiFi transceiver 324 and generates a suitable voltage to turn on or off switch SW 2 .

Responsive to voltage V_Cdc being less than voltage Vref, the active clamp circuit 330 turns off transistors M 1 and M 2 , thereby causing diodes D 3 , D 4 , and body diodes D 1 _M 1 and D 2 _M 2 to operate as a full-bridge rectifier. With switch SW 2 off, current flows during the half-cycle of the AC voltage in which the voltage on terminal T 31 is positive relative to the voltage on terminal T 32 as shown by arrows 351 . Accordingly, current 351 flows through resistor Rs, capacitor Cs, diode D 3 , and to capacitor Cdc and to the WiFi transceiver 324 . Current 351 continues flowing through body diode D 2 _M 2 and to terminal T 32 . The dashed arrows 352 show the direction of current flow during the other half-cycle of the AC voltage in which the voltage on terminal T 31 is negative relative to the voltage on terminal T 32 . Other than through capacitor Cdc and the WiFi transceiver 324 , the direction of current flow is generally in the opposite direction. Due to the full-bridge rectification afforded by the combination of diodes D 3 and D 4 and body diodes D 1 _M 1 and D 2 _M 2 , current flows in the same direction in both half-cycles through the capacitor Cdc to thereby charge the capacitor.

With switch SW 2 on/closed (e.g., as a result of a wireless signal received by the WiFi transceiver 324 to turn on the light source 110 ), the flow of current (voltage V_Cdc being less than voltage Vref) is largely the same as described above with the exception that current flows through switch SW 2 rather than through the series combination of resistor Rs and capacitor Cs of the capacitor-drop power supply 310 . Turning on switch SW 2 causes a sufficiently large current to flow through the light source 110 to thereby cause the light source 110 to be illuminated. With switch SW 2 off, the current draw by the WiFi transceiver 324 is limited by capacitor Cs to a sufficiently low level so as not to cause the light source 110 to illuminate.

FIG. 4 repeats the same circuit from FIG. 3 but indicates the flow of current when voltage V_Cdc is greater than voltage Vref. In this state, the active clamp circuit asserts its output signal 335 to a voltage level that turns on transistors M 1 and M 2 . During the half-cycle of the AC voltage (T 31 voltage greater than T 32 voltage), the current 441 flows from terminal T 31 through resistor Rs and capacitor Cs (when switch SW 2 is off), transistor M 1 , and transistor M 2 to terminal T 32 . During the half-cycle of the AC voltage (T 32 voltage greater than T 31 voltage), the current 441 flows in the opposite direction, accordingly, from terminal T 32 through transistor M 2 , transistor M 2 , and through resistor Rs and capacitor Cs (when switch SW 2 is off) to terminal T 31 . With switch SW 2 being on, the direction of current flow is the same with the exception that the current flows through switch SW 2 instead of through the series combination of resistor Rs and capacitor Cs. FIGS. 8 and 9 show the direction of current flow when switch SW 2 is on.

As described above regarding the embodiment of FIGS. 3 and 4 , current flow is continuous. If the voltage V_Cdc is below voltage Vref, the active clamp circuit 330 turns off transistors M 1 and M 2 , and their body diodes (combined with diodes D 3 and D 4 ) receive current to produce a rectified current to charge capacitor Cdc. If the voltage V_Cdc is greater than voltage Vref, the active clamp circuit 330 turns on transistors M 1 and M 2 to provide a continued current path despite capacitor Cdc not needing any charge current.

FIG. 5 illustrates the sinusoidal AC voltage 201 from AC voltage source 105 and that the current 502 through the light source module 320 continues to flow (not discontinuous as in the example of FIG. 2 ). As a result of continued current flow and at a level that is below (due to capacitor Cs) a threshold at which the light source 110 would be illuminated, the risk of the light source 110 flickering is reduced or eliminated. The capacitor Cdc has sufficient time to be charged to power the WiFi transceiver.

The embodiment of FIGS. 3 and 4 is an example that lacks a battery. A battery may not be necessary in the embodiment of FIGS. 3 and 4 because current can flow to charge the capacitor Cdc even if switch SW 2 is on (whereas in the embodiment of FIG. 1 , current could not flow to charge capacitor C 2 if switch SW 1 was on). When on, switch SW 2 does not short-circuit terminals T 31 and T 32 of the light source module 320 . In contrast, when on, switch SW 1 does short-circuit terminals T 1 and T 2 of the light source module 120 . Some embodiments may include a battery (across capacitor Cdc) to be charged by the capacitor Cdc.

FIG. 6 shows an example of an active clamp circuit 630 which implements the active clamp circuit 330 described above. Active clamp circuit 630 includes a comparator 640 that implements hysteresis. The comparator 640 includes a positive (+) input 331 and a negative (−) input 332 . The positive input is coupled to the upper terminal of capacitor Cdc and the negative input is coupled to the reference voltage source (Vref). The output 333 of the comparator 640 is coupled to the gates of transistors M 1 and M 2 .

FIG. 7 shows an example of an active clamp circuit 730 which implements the active clamp circuit 330 described above. Active clamp circuit 730 includes a comparator 740 that implements hysteresis. A reference voltage source 745 is shown which provides the reference voltage Vref to the negative input 332 of comparator 740 . FIG. 7 also shows a trim circuit 752 which is usable to trim the reference voltage source 745 .

FIGS. 8 and 9 show the same circuit as in FIG. 3 but indicate the direction of current flow when the light source 110 is illuminated. FIG. 8 shows the direction of current flow when voltage V_Cdc is less than Vref (and transistors M 1 and M 2 are off). FIG. 9 shows the direction of current flow when voltage V_Cdc is greater than Vref (and transistors M 1 and M 2 are on). In FIG. 8 , arrows 841 (during one half-cycle) show that, from terminal T 31 , the current flows through switch SW 2 and diode D 1 to both capacitor Cdc and the WiFi transceiver 324 , and then through body diode D 1 _M 1 to terminal T 32 . During the opposing half-cycle, the current flow 851 is in the opposite direction with the exception that current flows in the same direction through capacitor Cdc and the WiFi transceiver 324 due to the full-bridge rectification provided by the diodes D 1 and D 2 and the transistors' body diodes D 1 _M 1 and D 2 _M 2 .

In FIG. 9 , arrows 941 (during one half-cycle) show that, from terminal T 31 , the current flows through switch SW 2 and through transistors M 1 and M 2 (which are on), and to terminal T 32 . During the opposing half-cycle, the current flow 951 is in the opposite direction.

FIG. 10 illustrates an example embodiment in which switch SW 2 is implemented as a solid-state relay (SSR) 1011 . The SSR 1011 includes transistors M 3 and M 4 coupled in series. The series combination of transistors M 3 and M 4 is coupled in parallel with the series combination of resistor Rs and capacitor Cs. Transistors M 3 and M 4 are MOSFETs in this example and have their sources connected together. The output 3233 of driver 322 is coupled to the gates of transistors M 3 and M 4 . The body diodes D 3 _M 3 and D 4 _M 4 have opposite polarities as shown to reduce or avoid current flowing through the SSR 1011 during either half-cycle of the AC voltage when SSR 1011 is off.

Because switch SW 2 is a solid-state switch, its on and off states can be modulated to implement a dimming control of the average amount of light produced by the light source 110 . For example, the WiFi transceiver 324 may receive a wireless signal requesting, specifying, or otherwise indicating an increase or decrease of a dimming control. The WiFi transceiver 324 can modulate signal 326 to thereby turn on and off switch SW 2 (e.g., SSR 1011 ) at a desired level. FIG. 11 shows an example of the AC voltage waveform produced by the AC voltage source 105 in which during a first portion 1111 of each half cycle, the switch SW 2 is turned off and during a second portion 1112 of each half cycle, the switch SW 2 is turned on. FIG. 12 shows an example in which during a first portion 1211 of each half cycle, the switch SW 2 is turned on and during a second portion 1212 of each half cycle, the switch SW 2 is turned off. The modulation scheme can be different than those shown in FIGS. 11 and 12 .

In this description, the term “couple” may cover connections, communications, or signal paths that enable a functional relationship consistent with this description. For example, if device A generates a signal to control device B to perform an action: (a) in a first example, device A is coupled to device B by direct connection; or (b) in a second example, device A is coupled to device B through intervening component C if intervening component C does not alter the functional relationship between device A and device B, such that device B is controlled by device A via the control signal generated by device A.

Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.