WO2014197906A1

WO2014197906A1 - Switched lighting system and method of operation

Info

Publication number: WO2014197906A1
Application number: PCT/US2014/041587
Authority: WO
Inventors: Irwin Rudolph NEDERBRAGT; Steven Michael BARROW; Yan Yin; Craig Steven CAMBIER
Original assignee: Texas Instruments Japan Ltd; Texas Instruments Inc
Current assignee: Texas Instruments Japan Ltd; Texas Instruments Inc
Priority date: 2013-06-07
Filing date: 2014-06-09
Publication date: 2014-12-11
Anticipated expiration: 2015-12-07
Also published as: US20140361691A1; JP2016523430A; US9332605B2; JP6340419B2; CN105284187A; CN105284187B

Abstract

In described examples, a lighting system includes a switch (SW1, SW2, SW3) that is configured so that: when the switch is in a first state, current from a supply flows to a light emitter; and when the switch is in a second state, current from the supply flows through the switch bypassing the light emitter. A capacitor (C1, C2, C3) connected in parallel with the light emitter provides current to the light emitter, sufficient to cause the light emitter to emit light when the switch is in the second state.

Description

SWITCHED LIGHTING SYSTEM AND METHOD OF OPERATION

[0001] This relates in general to lighting systems, and in particular to a switched lighting system and method of operation.

BACKGROUND

[0002] Efficient (high lumens per watt) lighting systems may be powered directly by alternating current (AC) power mains (such as 120V_RMS, 60HZ, or 230V_RMS, 50 Hz). Examples include household and commercial indoor lighting, outdoor street lights, traffic lights, and signage. Light-emitting diodes (LEDs) are one example technology for efficient light emitters.

[0003] FIG. 1A shows an example of a conventional lighting system 100, in which LEDs 102 are connected in series and driven directly by a rectified AC supply voltage V_RAC- The system 100 may also include a current limiter or current regulator 104.

[0004] FIG. IB shows example timing for the lighting system 100, in which V_T is a threshold at which V_RAC exceeds the forward-biased voltage of the entire series of LEDs 102 plus the voltage drop across the current limiter 104. At time to, V_RAC starts increasing from zero. At time ti, V_RAC exceeds the threshold V_T, and the LEDs 102 emit light. At time t₂, V_RAC falls below the threshold V_T, and the LEDs 102 stop emitting light. Accordingly, the LEDs 102 are on during only the time period from ti until t₂ (shaded portion 106). In that manner, light is emitted for only a fraction of the time, and the light flickers at twice the frequency of the AC power mains. If the peak of V_RAC drops too far (such as during a "brown-out" or responsive to a dimming switch), then the lighting system 100 may fail to turn on.

[0005] FIG. 2 shows an example of an alternative conventional lighting system 200, in which current for LEDs is provided by electronic drivers. In the example of FIG. 2, a rectified AC supply voltage V_RAC provides power to driver/bypass circuits (204, 206, 208, 210) connected in series, and to a current limiter or current regulator 202. Each driver/bypass circuit (204, 206, 208, 210) drives a respective LED (212, 214, 216, 218). Each driver/bypass circuit (204, 206, 208, 210) includes a respective bypass switch that can bypass current around its LED. When the supply voltage (V_RAC) exceeds a voltage sufficient to power LED 212 (and the current limiter or current regulator 202, and accounting for the series voltage drops of the bypass switches), driver/bypass circuit 204 turns on, opens its bypass switch, and drives its LED 212. As the supply voltage (V_RAC) continues to increase, the driver/bypass circuits (206, 208, 210) sequentially turn on (and open their respective bypass switches) until all LEDs are being driven. When the supply voltage (V_RAC) decreases, the driver/bypass circuits (204, 206, 208, 210) sequentially turn off (and close their respective bypass switches). Accordingly, LEDs start turning on at a relatively low voltage. As the supply voltage (V_RAC) increases, more LEDs are driven, and the overall intensity increases. As the supply voltage (V_RAC) decreases, fewer LEDs are driven, and the overall intensity decreases.

SUMMARY

[0006] In described examples, a lighting system includes a switch that is configured so that: when the switch is in a first state, current from a supply flows to a light emitter; and when the switch is in a second state, current from the supply flows through the switch bypassing the light emitter. A capacitor connected in parallel with the light emitter provides current to the light emitter, sufficient to cause the light emitter to emit light when the switch is in the second state. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 A is a block diagram schematic of an example of a conventional lighting system.

[0008] FIG. IB is a timing diagram of example timing for the lighting system of FIG. 1A.

[0009] FIG. 2 is a block diagram schematic of an example of an alternative conventional lighting system.

[0010] FIG. 3 is a block diagram schematic of an example embodiment of an improved lighting system.

[0011] FIGS. 4A - 4D are timing diagrams of example timing for the lighting system of FIG. 3.

[0012] FIG. 5 is a block diagram schematic of a switch controller for the lighting system of FIG. 3.

[0013] FIG. 6 is a flowchart of an operation of the example embodiment.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0014] FIG. 3 shows an example embodiment of an improved lighting system 300. In FIG. 3, light emitters (306, 308, 310, 314, 316, 320) are divided into three segments (SEGMENT 1, SEGMENT2, SEGMENT3), and the segments are connected in series. The number of segments and the number of light emitters per segment may vary. For clarity, FIG. 3 shows one simplified example. In this example, the light emitters (306, 308, 310, 314, 316, 320) are LEDs, but the lighting system 300 is likewise applicable to other efficient low-voltage light emitters. The lighting system 300 is driven by a rectified AC supply voltage V_RAC- The lighting system 300 includes a current regulator 302. Each segment includes a respective electronic bypass switch (SW1, SW2, SW3), a respective isolation diode (304, 312, 318) connected in series with the segment's light emitter(s), and a respective capacitor (CI, C2, C3) connected in parallel with the segment's light emitter(s). Each electronic bypass switch (SW1, SW2, SW3) has associated switch control circuitry, as shown in FIG. 5.

[0015] Initial conditions exist when V_RAC is first turned on. After an initialization period (such as a few half-cycles of V_RAC), steady-state conditions exist. Initially, all bypass switches (SW1, SW2, SW3) are closed, and no current flows into the light emitters (306, 308, 310, 314, 316, 320). When V_RAC increases above a first threshold: (a) bypass switch SW3 opens; (b) light emitter 320 receives current through bypass switches SW1 and SW2 and isolation diode 318; (c) light emitter 320 emits light; and (d) capacitor C3 charges. Similarly, when V_RAC increases above other thresholds, additional segments turn on and off (depending on the available voltage), and additional capacitors (CI, C2) charge. Depending on their size, the capacitors may fully charge over a few half-cycles of V_RAC- After the capacitors (CI, C2, C3) are charged, they supply current in the steady-state to the light emitters (306, 308, 310, 314, 316, 320) when the bypass switches (SW1, SW2, SW3) are closed, so that the light emitters emit light continuously. The isolation diodes (304, 312, 318) prevent the capacitors from discharging through the bypass switches (SW1, SW2, SW3).

[0016] When V_RAC increases above a second threshold, bypass switch SW2 opens: (a) light emitters 314 and 316 receive current through bypass switch SW1 and isolation diode 312; (b) light emitters 314 and 316 emit light; and (c) capacitor C2 charges. As bypass switch SW2 opens, the voltage at the anode of isolation diode 312 in SEGMENT2 is near V_RAC, and the voltage at the anode of isolation diode 318 in SEGMENT3 then drops by the voltage across SEGMENT2. Depending on the magnitude of the thresholds and the voltage across the segments, the voltage at the anode of isolation diode 318 may then drop below the first threshold. If the voltage at the anode of isolation diode 318 drops below the first threshold, then bypass switch SW3 will close again. If bypass switch SW3 closes again, then it will open again when the voltage at the anode of isolation diode 318 again increases above the first threshold.

[0017] When the supply voltage increases above a third threshold, bypass switch SW1 opens, so current flows to light emitters 306, 308, and 310 and to capacitor CI . Light emitters 306, 308, and 310 then emit light, and capacitor CI charges. When bypass switch SWl opens, the voltage at the anode of isolation diode 304 is at V_RAC, and the voltage at the anode of isolation diode 312 in SEGMENT2 drops by the voltage across SEGMENT 1. Bypass switches SW2 and SW3 may then close again. If bypass switch S W3 closes again, then it will open again when the voltage at the anode of isolation diode 318 again increases above the first threshold. If bypass switch S W2 closes again, then it will open again when the voltage at the anode of isolation diode 312 again increases above the second threshold.

[0018] When the bypass switch SW3 opens, current from V_RAC flows to the light emitter 320 and to capacitor C3. When the bypass switch SW3 closes again, current flows from V_RAC through the bypass switch SW3, bypassing the light emitter 320 and the capacitor C3. When the bypass switch SW3 closes, current from capacitor C3 flows through the light emitter 320 until the bypass switch SW3 opens again. Depending on the size of capacitor C3, it may fully charge over multiple half-cycles of V_RAC- After capacitor C3 is fully charged, light emitter 320 emits light continuously, receiving current from V_RAC or capacitor C3, depending on the state of bypass switch SW3. Likewise, after capacitor C2 is charged, light emitters 314 and 316 emit light continuously, receiving current from V_RAC or capacitor C2, depending on the state of bypass switch SW2. After all capacitors (CI, C2, C3) are charged, all light emitters (306, 308, 310, 314, 316, 320) emit light continuously. Accordingly, the lighting system 300 emits light continuously and with almost constant intensity. Only a relatively small amount of intensity variation results from reducing voltage on the capacitors (CI, C2, C3) as they discharge. If the peak voltage of V_RAC falls below the third threshold but is above the second threshold (such as during a brown-out or as a result of a dimmer switch), then the light emitters in SEGMENT2 and SEGMENT3 will continue to emit light. If the peak voltage of V_RAC falls below the second threshold but is above the first threshold, then the light emitters in SEGMENT3 will continue to emit light.

[0019] FIGS. 4A-4D are timing diagrams of example timing, example segment voltages, and example thresholds for the lighting system 300 of FIG. 3. In the example of FIGS. 4A-4D, the voltage across SEGMENT3 when bypass switch SW3 is open is assumed to be 20V, the voltage across SEGMENT2 when bypass switch SW2 is open is assumed to be 40V, and the voltage across SEGMENT 1 when bypass switch SWl is open is assumed to be 80V. In the example of FIGS. 4A-4D, the headroom required for the current regulator 302 and switches is assumed to be 5V, the first threshold V_Ti is assumed to be 25V, the second threshold V_T2 is assumed to be 45V, and the third threshold V_T3 is assumed to be 85V. FIGS. 4A-4D respectively show V_RAC, the voltage across SEGMENT 1, the voltage across SEGMENT2, and the voltage across SEGMENT3.

[0020] At time t₀, V_RAC starts increasing from zero. At time t_ls V_RAC exceeds the first threshold V_TI (25V), and bypass switch SW3 opens. At time t₂, V_RAC exceeds the second threshold V_T2 (45V), and bypass switch SW2 opens. When bypass switch SW2 opens at time t₂, the voltage across SEGMENT3 drops by the voltage across SEGMENT2 (40V), and bypass switch SW1 closes. At time t₃, V_RAC exceeds 65V, the controller for bypass switch SW3 again senses 25V relative to ground, and bypass switch SW3 opens again. At time t₄, V_RAC exceeds the third threshold V_T3 (85V), and bypass switch SW1 opens. When bypass switch SW1 opens at time t4, the voltage across SEGMENT2 and SEGMENT3 drops by the voltage across SEGMENT 1 (80V), and bypass switches SW1 and SW2 close. At time t₅, V_RAC exceeds 105V, the controller for bypass switch SW3 again senses 25V relative to ground, and bypass switch SW3 opens again. At time t₆, V_RAC exceeds 125V (notably, peak voltage for a 120V_RMS mains is -170V), the controller for bypass switch SW2 again senses 45V relative to ground, and bypass switch SW2 opens again. When bypass switch SW2 opens at time t₆, the voltage across SEGMENT3 drops by the voltage across SEGMENT2 (40V), and bypass switch SW3 closes again. At time t₇, VRAC exceeds 145V, the controller for bypass switch SW3 again senses 25V relative to ground, and bypass switch SW3 opens again. At time t₈, V_RAC falls below 145V, and the switching sequence described above progresses in the reverse order.

[0021] Given the above assumed segment voltages and thresholds, the following Table 1 lists the states of the bypass switches (SW1, SW2, SW3) as a function of V_RAC-

TABLE 1

105 - 125 OFF ON OFF

125 - 145 OFF OFF ON

> 145 OFF OFF OFF

[0022] Many alternative choices exist for segment voltages and thresholds. The above assumed thresholds and segment voltages were chosen to improve efficiency. However, each switch transition from open-to-close or close-to-open generates a transient current on the AC mains. Alternatively, the segment voltages and thresholds may be chosen to reduce the number of switch transitions to reduce transient currents on the AC mains. Also, the thresholds may be adjusted to change the order in which segments turn on and off. The following example is for a lighting system with minimal current transients, which adjusts the order in which segments turn on and off. Assume a lighting system as in FIG. 3, but with four segments, with SEGMENT 1 closest to the AC mains, and SEGMENT4 closest to ground. Assume that V_RAC is 230V_RMS- Assume SEGMENT4 has a segment voltage of 40V, and the remaining three segments have segment voltages of 80V. Assume that the threshold for SEGMENT4 is 48V, the threshold for SEGMENT 1 is 88V, the threshold for SEGMENT2 is 172V, and the threshold for SEGMENT3 is 256V. In this example, the order of the thresholds is different than the order of the segments. The following Table 2 lists the states of the four bypass switches (SW1, SW2, SW3, SW4) as a function of V_RAC for these assumed values. For these assumed values, only bypass switch SW4 switches ON and OFF multiple times as V_RAC increases from zero to a peak voltage. The remaining switches only switch once, which reduces the transient currents on the AC mains.

TABLE 2

[0023] Referring to FIG. 3, for the assumptions leading to Table 1, the voltage across the current regulator 302 ranges from ~5V to ~25V. For example, when V_RAC is slightly below 65V, a 40V drop exists across SEGMENT2, and the voltage across the current regulator 302 is ~25V. When V_RAC slightly exceeds 65V, bypass switch SW3 opens, and a 20V drop exists across SEGMENT3 in addition to the 40V drop across SEGMENT2, so the voltage across the current regulator 302 drops to ~5V. Similarly, for the assumptions leading to Table 2, the voltage across the current regulator ranges from ~8V to -48 V for most of the range of V_RAC. However, because of the 172V threshold for SEGMENT2, the voltage across the current regulator varies from ~8V to ~52V when V_RAC is in the range of 128V to 172V, and the voltage across the current regulator varies from ~12V to -48 V when V_RAC is the range of 172V to 208V. Accordingly, selecting segment voltages and thresholds to reduce transient currents on the AC mains results in a slightly higher average voltage across the current regulator, resulting in a slightly reduced efficiency (slightly more heat loss occurs in the current regulator).

[0024] FIG. 5 shows an example embodiment of switch control circuitry 500 for one of the electronic bypass switches (SW1, SW2, SW3) of FIG. 3. Specifically, FIG. 5 shows switch control circuitry for bypass switch SW2 in SEGMENT2. For clarity, the switch control circuitry 500 in FIG. 5 is simplified. In the example of FIG. 5, the switch control circuitry 500 is driven by the voltage across capacitor C2 (Vi_N - Vs). A voltage regulator 502 provides a constant voltage Vcc for the electronics. In the example of FIG. 5, bypass switch SW2 is implemented as a MOS transistor Q2. Transistor Q2 is driven by a latch 506. The latch 506 is SET dominant (so that, if both SET and RESET are high, then the latch 506 is SET). The SET input of the latch 506 is driven by an amplifier 504. A current source ii is connected between the input of the amplifier 504 and Vs. A resistor Rl is connected between the input of the amplifier 504 and ground. The RESET input of the latch 506 is driven by an amplifier 508. The resistor Rl is also connected to the negative input of the amplifier 508, and a second resistor R2 is connected between the negative input of the amplifier 508 and Vm. A voltage source Vi is connected to the positive input of the amplifier 508. The voltage of V_RAC at which the RESET amplifier 508 changes states is slightly below the voltage at which the SET amplifier 504 changes states. This provides hysteresis to prevent the transistor Q2 from being affected by noise on V_RAC or ground. As V_RAC increases from zero, V_IN and Vs increase, and the SET amplifier 504 drives the SET input of the latch 506. As V_RAC increases above a RESET threshold, the RESET input of the latch 506 is also driven. Then, when the current through Ri exceeds the current source ii, the SET amplifier 504 ceases driving the SET input of the latch 506, so the latch 506 is RESET when the SET input is no longer driven. As V_RAC decreases from the peak voltage, the SET input of the latch 506 is again driven at the higher threshold of amplifier 504. Accordingly, the voltage at which the transistor Q2 switches from ON-to-OFF as V_RAC is rising is lower than the voltage at which Q2 switches from OFF-to-ON as V_RAC is falling.

[0025] FIG. 6 is a flowchart 600 of an operation of the example embodiment. At step 602, a switch control circuit senses a voltage at the switch control circuit. At step 604, when the voltage at the switch control circuit exceeds a threshold, the switch control circuit opens a switch, allowing current to flow to a light emitter and to a capacitor. At step 606, when the voltage at the switch control circuit is less than the threshold, the switch control circuit closes the switch, bypassing the light emitter and the capacitor. At step 608, the capacitor provides current to the light emitter when the switch is closed.

[0026] In summary, the system of FIGS. 3 and 5 emits light continuously and with almost constant intensity. The system does not require any power source other than the AC mains. The only active circuitry in the current path through the light emitters is a current regulator. The system self-senses when to bypass AC mains current around light emitters and when to allow AC mains current to flow through light emitters. No communications connections, other than local voltage sense connections, are required for the switch controllers. The system can be adjusted to improve efficiency by reducing the average voltage drop across the current regulator. Alternatively, the system can be adjusted to reduce current transients on the AC mains.

[0027] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.

Claims

CLAIMS What is claimed is:

1. A lighting system, comprising:

at least one light emitter;

a switch configured so that: when the switch is in a first state, current from a power source flows to the light emitter; and when the switch is in a second state, current from the power source flows through the switch bypassing the light emitter; and

a capacitor connected in parallel with the light emitter providing current to the light emitter, sufficient to cause the light emitter to emit light when the switch is in the second state.

2. The system of claim 1, wherein the light emitter is a light-emitting diode.

3. The system of claim 1, further comprising a diode connected so that the capacitor does not discharge through the switch when the switch is in the second state.

4. The system of claim 1 , wherein the switch is controlled to be in the first state when a voltage of the power source is above a predetermined threshold.

5. The system of claim 4, wherein the switch is controlled to be in the first state a plurality of times as the voltage of the power source increases from zero to a peak voltage.

6. The system of claim 1, further comprising a switch controller coupled to the switch.

7. The system of claim 6, wherein the switch controller is configured to sense a voltage at the switch controller relative to ground, the switch controller controlling the switch to be in the first state when the voltage sensed by the switch controller exceeds the predetermined threshold.

8. The system of claim 6, wherein the switch controller includes: an amplifier having an input; and a resistor coupled between the input and ground, so that the amplifier causes the switch to be in the first state when current through the resistor exceeds a predetermined threshold.

9. The system of claim 1, wherein the light emitter continuously emits light after an initialization period.

10. A lighting system, comprising:

a plurality of light emitters arranged into a plurality of segments, the segments connected in series between a rectified alternating current (AC) power mains and ground;

wherein each segment includes a respective capacitor connected in parallel with at least one light emitter in the segment; wherein each segment includes a respective electronic bypass switch that allows current to flow from the power mains through the light emitters in the segment and to the capacitor in the segment when the bypass switch is in a first state, and current from the power mains bypasses the light emitters and the capacitor in the segment when the bypass switch is in a second state; and wherein current to the light emitters in each segment is provided by the capacitor in the segment when the bypass switch is in the second state.

11. The system of claim 10, wherein the light emitters are light-emitting diodes.

12. The system of claim 10, wherein each segment further includes a respective diode connected so that the capacitor in the segment does not discharge through the bypass switch when the bypass switch is in the second state.

13. The system of claim 10, wherein the bypass switch is controlled to be in the first state when a voltage at the segment is above a predetermined threshold.

14. The system of claim 13, wherein the switch is controlled to be in the first state a plurality of times as the rectified AC power mains increases from zero to a peak voltage.

15. The system of claim 10, wherein each segment including a respective switch controller coupled to the bypass switch.

16. The system of claim 15, wherein the switch controller includes: an amplifier having an input; and a resistor coupled between the input and ground, so that the amplifier causes the bypass switch to be in the first state when current through the resistor exceeds a predetermined threshold.

17. The system of claim 10, wherein all of the light emitters continuously emit light after an initialization period.

18. A method, comprising :

sensing, by a switch control circuit, a voltage at the switch control circuit relative to ground;

opening, by the switch control circuit, a switch to allow current to flow to a light emitter and to a capacitor, when the voltage at the switch control circuit exceeds a threshold;

closing, by the switch control circuit, the switch to bypass the light emitter and the capacitor, when the voltage at the switch control circuit is less than the threshold; and

providing current to the light emitter, by the capacitor, when the switch is closed.

19. The method of claim 18, further comprising:

preventing, by a diode, current from the capacitor against flowing through the switch when the switch is closed.

20. The method of claim 18, further comprising:

closing, by the switch control circuit, the switch a plurality of times as a supply voltage increases from zero to a peak voltage.