LED lamp with controlled inductors

Abstract

LED lamp comprising two or more connectors for receiving an electrical current from a luminaire for supply to one or more LEDs, and a rectifier circuit connected to receive the electrical current from one or more of the connectors and configured to generate a rectified current. The LED lamp includes a first variable impedance circuit comprising a first inductive element and a first bypass switch connected to the first inductive element for reducing inductance of the first variable impedance circuit, wherein the first inductive element is connected in series between one of the connectors and the rectifier circuit, and the first bypass switch is connected to receive a first control signal for controlling the first switch. The LED lamp further includes a second variable impedance circuit comprising a second inductive element and a second bypass switch connected to the second inductive element for reducing inductance of the first variable impedance circuit, wherein the second inductive element is connected in series between the rectifier circuit and the one or more LEDs, and the second bypass switch is connected to receive a second control signal for controlling the second switch.

Description

Technical Field

[0001] The invention relates generally to light emitting diode (LED) lamps and LED lighting, and more particularly to LED lamps suitable to replace a fluorescent lamp in a luminaire having a ballast for use with fluorescent lamps.Background

[0002] Fluorescent lamps generally comprise a tube filled with an inert gas and a small amount of mercury, capped at both ends with double pinned end caps. Both end caps contain a filament (glow wire). Before ignition the filaments are preheated to enable thermionic emission of electrons. After the user turns on a main switch (e.g. a wall switch or a cord switch on the ceiling), the fluorescent lamp is ignited and heat generated by the conducted current vaporize more mercury in order to increase the luminous output of the fluorescent lamp and keeps the fluorescent lamp in a stable operational condition. To facilitate these starting conditions and to limit current through the fluorescent lamp during operation, and thus limit the power consumed, a ballast is usually fitted in the fluorescent luminaire, connected between the mains power supply and the fluorescent lamp, and power is supplied to the lamp via the ballast.

[0003] When first introduced, the only available ballasts were simple inductive or reactive elements placed in series with the power supply to the fluorescent lamp, which limit consumed power by limiting the AC current as a result of the frequency dependent impedance of the inductor. An undesirable result is a relatively low power factor and relatively high reactive power. These types of ballasts are usually referred to as magnetic ballasts.

[0004] More recently other types of ballasts have been introduced, such as electronic ballasts. These ballasts usually first convert AC mains power into DC power, and subsequently convert the DC power into high frequency AC power to drive the fluorescent lamp.

[0005] Electronic ballasts can further be categorized into two main types: constant current ballasts and constant power ballasts. Most electronic ballasts are constant current ballasts, designed to deliver current at a substantially constant amplitude. These ballasts can be modelled as a constant AC current source. These ballasts typically comprise a self-protection / self-correcting mechanism to avoid potential problems of maintaining a constant current. A constant power ballast is designed to deliver substantially constant power and the output current will vary depending on the load to attempt to maintain the design power output. If the operating power is below the design output level, constant power ballasts usually try to increase the output current to come closer to the design power level.

[0006] LED lamps are more efficient than fluorescent lamps, do not require mercury, provide more directional light, are more easily controlled or regulated, and have a longer lifetime. Thus, replacing fluorescent lamps with LED lamps is often desirable, and it is also desirable to be able to fit replacement LED lamps into existing luminaires designed for fluorescent lamps without needing to modify the luminaire. However, an LED lamp typically operates differently when used with different types of ballasts, and simple replacement of a florescent lamp by an LED lamp in a fluorescent luminaire may result in unsatisfactory performance or failure of the lamp or the entire luminaire.

[0007] An LED lamp arrangement compatible with all three types of ballasts mentioned above (magnetic ballasts, constant current ballast, and constant power ballast) is described in the applicant's US patent no. 9,832,837, herewith incorporated by reference in its entirety. An LED lamp arrangement additionally compatible with luminaries having no ballast is described in the applicant's US patent no. 10,342,079, herewith incorporated by reference in its entirety. These lamp arrangements comprise an inductive element and a switch which can be closed to short the inductive element, depending on whether the electronic ballast is a constant power ballast or a constant current ballast.Summary of the Invention

[0008] It is therefore an object of the invention to provide an LED lamp which has good lighting performance when fitted in a variety of luminaires, regardless of the type of ballast used in the luminaire, e.g. whether the luminaire has a constant current ballast or constant power ballast.

[0009] According to one aspect of the invention, an LED lamp comprises two or more connectors for receiving an electrical current from a luminaire for supply to one or more LEDs, and a rectifier circuit connected to receive the electrical current from one or more of the connectors and configured to generate a rectified current. The LED lamp includes a first variable impedance circuit comprising a first inductive element and a first bypass switch connected to the first inductive element for reducing inductance of the first variable impedance circuit, wherein the first inductive element is connected in series between one of the connectors and the rectifier circuit, and the first bypass switch is connected to receive a first control signal for controlling the first switch. The LED lamp further includes a second variable impedance circuit comprising a second inductive element and a second bypass switch connected to the second inductive element for reducing inductance of the first variable impedance circuit, wherein the second inductive element is connected in series between the rectifier circuit and the one or more LEDs, and the second bypass switch is connected to receive a second control signal for controlling the second switch.

[0010] The LED lamp that may be used on a wide variety of electronic (high frequency) ballasts. The LED lamp is designed to provide improved turn-on compatibility during the ignition period, including turn-on of a cold or hot LED lamp operated with a cold or hot ballast; stable operation without noticeable light flickering after turn-on and without overheating of the LED lamp and / or the ballast; and operation within a defined power window (e.g. -10% < lamp power < +5%); and Stroboscopic Visibility Measure (SVM) below 0.4 (e.g. light flicker measurement according to CIE TN006 and IEC TR 63158.

[0011] Additional features are defined in the dependent claims.Brief Description of the Drawings

[0012] The features and advantages of the invention will be appreciated upon reference to the following drawings, in which: FIG. 1 is a diagram of an LED lamp configured to operate in a luminaire designed for a fluorescent tube; FIG. 2A is a simplified block diagram of an embodiment of an LED lamp comprising variable impedance circuits; FIG. 2B is an alternative embodiment of the LED lamp of FIG. 2A; FIG. 3 is a simplified schematic diagram of an embodiment of an LED lamp having a variable impedance circuit comprising a variable inductive element arranged on a AC-side of the LED lamp circuit; FIG. 4 is a simplified schematic diagram of an embodiment of an LED lamp having a variable impedance circuit comprising a variable inductive element arranged on a DC-side of the LED lamp circuit; FIG. 5 is a simplified schematic diagram of an embodiment of an LED lamp having a variable impedance circuit comprising a variable inductive element arranged across an input of the LED lamp circuit; FIG. 6 is a simplified schematic diagram of an embodiment of an LED lamp having a combination of variable impedance circuits each comprising a variable inductive element; FIG. 7 is a simplified schematic diagram of an alternative embodiment of the LED lamp of FIG. 6; FIG. 8 is a simplified schematic diagram of an embodiment of an LED lamp having a combination of variable impedance circuits each comprising an inductive element and bypass switch; FIG. 9 is a simplified schematic diagram of an alternative embodiment of the LED lamp of FIG. 8; FIG. 10 is a schematic diagram of two coupled inductors; FIG. 11 is a circuit model of a coupled inductor; FIG. 12 shows the circuit model of FIG. 11 with the secondary leakage inductance reflected onto the primary side; FIG. 13 shows the circuit model of FIG. 12 with the secondary side of the circuit shorted; FIG. 14 is a simplified schematic diagram of an embodiment of a variable impedance circuit comprising a coupled inductor with a bypass switch on the secondary-side of the coupled inductor; FIG. 15 is a simplified schematic diagram of another embodiment of a variable impedance circuit comprising a coupled inductor with a bypass switch on the secondary-side of the coupled inductor; and FIG. 16 is a simplified schematic diagram of a third embodiment of a variable impedance circuit comprising a coupled inductor with a bypass switch on the secondary-side of the coupled inductor. Description of Illustrative Embodiments

[0013] The following is a more detailed explanation of exemplary embodiments of the invention. Components having the same reference number are the same in the various drawings except as described herein. Note that the term "connected" is used herein to indicate an electrical connection between circuit elements, which may be a direct connection or may be a connection made via one or more other circuit elements.

[0014] The embodiments of the LED lamp described herein may be implemented in a variety of forms and configurations. FIG. 1 is a diagram of an LED lamp 1 which is configured so that it can operate in a luminaire 2 designed for a fluorescent tube. The LED lamp 1 preferably has a same or similar length and shape as a standard fluorescent tube to enable the LED lamp 1 to fit into the luminaire 2 without modification. Two electrical connectors 1a, 1b (usually in the form of conductive pins) are provided on end caps at each end of the LED lamp 1, for releasably connecting to corresponding connectors 4 of the luminaire 2. The luminaire 2 may include a ballast 5, which may be for example an electronic ballast which operates as a constant current ballast, or an electronic ballast which operates as a constant power ballast.

[0015] The luminaire 2 provides electrical power to the LED lamp 1 via the connectors 4. The electrical power provided by the luminaire 2 which is input to the LED lamp 1 will vary depending on the design of the luminaire, i.e. whether the luminaire has a ballast and if so, what type of ballast. The LED lamp 1 receives electrical power (an electrical current) from luminaire 2 via the connectors 1a, 1b that are connected to corresponding input terminals (nodes) 3 of the lamp circuit. In the embodiment shown, the LED lamp 1 comprises connectors 1a, 1b at each end of the lamp and may have filament circuits (not shown in FIG. 1) connected across the conductive pins 1a, 1b at each end cap, which connect to one of the terminals 3 at each end of the lamp circuit.

[0016] The luminaire 2 is typically connected to receive AC mains voltage, e.g. 120Vac or 230Vac at 50Hz or 60Hz, and supplies power to the LED lamp 1 via the luminaire's connectors 4. A magnetic ballast regulates the electrical current supplied by the luminaire using an inductive element, typically providing electrical power at the connectors 4 at mains frequency and voltage. An electronic ballast typically converts AC mains to a DC voltage and then back to a variable frequency AC voltage, providing a high frequency electrical power at the connectors 4, e.g. typically in the range 40-80Vac at a high frequency typically in the range 20kHz to 200kHz. Electronic ballasts are generally designed as constant current ballasts (i.e. current controlled ballasts) or constant power ballasts. Constant current ballasts are designed to supply electrical power at a substantially constant current. Constant power ballasts are designed to deliver substantially constant power and their output current will vary depending on the load impedance (e.g. the impedance of LED lamp 1) to try to maintain the design power output. If the load voltage (e.g. the voltage across LED lamp 1) is below the expected fluorescent lamp voltage, constant power ballasts usually try to increase the output current to supply a level of power to the LED lamp 1 closer to the design power level.

[0017] Note that in the context of the present application, the term "AC voltage" or "AC current" generally refers to a signal that changes its polarity (alternating between positive and negative) over time and is not limited to a sine wave or a fixed periodicity; outputs from magnetic ballasts and electronic ballasts are also to be understood as AC signals.

[0018] FIG. 2A is a simplified schematic diagram of one embodiment. In this embodiment the LED lamp comprises two or more terminals 3 for receiving an electrical current from a luminaire 2 for supply to one or more LEDs D1, and a rectifier circuit 6 connected to receive the electrical current from one or more of the terminals 3 and configured to generate a rectified current. The LED lamp includes a first variable impedance circuit 11 which is connected in series between one of the terminals 3 and the rectifier circuit 6, and a second variable impedance circuit 12 which is connected in series between the rectifier circuit 6 and the one or more LEDs D1. The LED lamp in FIG. 2A further includes an optional third variable impedance circuit 13 which is connected in parallel across the input to the LED lamp 1, e.g. connected between two of the terminals 3 at one end of the lamp circuit.

[0019] The rectifier circuit 6 may be arranged to receive power from the terminals 3 via connectors 1a, 1b at both ends of the LED lamp 1. The rectifier circuit 6 preferably forms a full-wave rectifier although other configurations may also be used, and may be implemented using diodes or other components such as transistors. Terminals 3 receive AC power from the luminaire connectors 4 when the LED lamp 1 is installed in the luminaire 2. The rectifier circuit 6 has an AC input for receiving an AC voltage from the terminals 3, and rectifies the AC voltage to generate a DC voltage at its DC output for powering the LEDs D1.

[0020] FIG. 2B illustrates an alternative embodiment in which the rectifier circuit 6 is formed in two parts, comprising a first rectifier receiving power from one of the terminals 3 via connectors 1a at one end of the circuit of LED lamp 1, and a second rectifier receiving power from one of the terminals 3 via the connectors 1b at the other end of the lamp circuit. Preferably, the two rectifier circuits 6 together form a full-wave rectifier, e.g. with two diodes at one end of the lamp circuit and two diodes at the other end of the circuit, although other configurations may also be used and other components such as transistors may also be used.

[0021] LED lamp 1 includes one or more LEDs D1 for generating light. Any number of LEDs may be used as appropriate considering the specification of LEDs used, the amount of light to be generated, and the overall design of the LED lamp. The LEDs D1 may be connected in series or parallel, or multiple series strings of LEDs connected in parallel, or any other suitable combination of connections.

[0022] The LED lamp 1 also has a control circuit 10 and variable impedance circuits 11 and 12. The first variable impedance circuit 11 is connected on the AC-side of the LED lamp circuit, i.e. connected in series between the terminals 3 and rectifier circuit 6, to receive current from the terminals 3 and supply current to the AC input of rectifier circuit 6 via the variable impedance circuit 11. In FIG. 2A there is one variable impedance circuit 11, connected to receive AC current from the terminals 3 at one end or both ends of the LED lamp and supply the current to an AC input of the rectifier circuit 6. In FIG. 2B there are two variable impedance circuits 11, each connected to receive AC current from the terminals 3 at each respective end of the LED lamp and supply the current to an AC input of a corresponding rectifier of the rectifier circuit 6. As a result, the current on the AC-side of the rectifier 6 passes through the variable impedance circuit(s) 11 for supplying the current (via the rectifier 6) to the LEDs D1. By increasing the impedance of the variable impedance circuit 11, the current drawn by the LED lamp 1 and supplied to the LEDs D1 can be reduced.

[0023] The second variable impedance circuit 12 is connected on the DC-side of the LED lamp circuit, i.e. connected to the DC output of rectifier circuit 6, between the rectifier 6 and LEDs D1. As a result, the current on the DC-side of the rectifier 6 passes through the variable impedance circuit 12 for supply to the LEDs D1. By increasing the impedance of the variable impedance circuit 12, the current drawn by the LED lamp 1 and supplied to the LEDs D1 can be reduced.

[0024] Note that FIGs. 2A and 2B are simplified diagrams and do not show other circuits that may be included in the LED lamp 1, such as LED driver circuits, filter circuits, switching circuits, protection circuits, etc. which are omitted for clarity.

[0025] The embodiments in FIGs. 2A and 2B may also include a third variable impedance circuit 13 on the AC-side, connected in parallel across an input of the LED lamp 1 between two of the terminals 3, i.e. before the first variable impedance circuit 11 (e.g. with one end connected between one connector 3 and first variable impedance circuit 11 and the other end connected to the other connector 3, as shown in the FIGs. 2A and 2B).

[0026] The embodiments in FIGs. 2A and 2B may alternatively or additionally include a fourth variable impedance circuit 14 on the AC-side, connected in parallel across an input of the rectifier circuit 6, i.e. after the first variable impedance circuit 11 (e.g. with one end connected between the first variable impedance circuit 11 and the rectifier circuit 6, and the other end connected to the other connector 3, as shown in the FIGs. 2A and 2B).

[0027] The variable impedance circuits 11-14 are designed to facilitate operation of the LED lamp 1 when operated with an electronic ballast regardless of whether the ballast is a constant current ballast or a constant power ballast, to provide improved compatibility of the LED lamp for use with different types of ballast. A constant power ballast is designed to deliver a (nominally) constant amount of power to a fluorescent lamp. The nominally constant amount of power may for example be derived from multiplication of the voltage drop across the lamp and the amount of current flowing through the lamp. When a fluorescent lamp is replaced by an LED lamp designed for operating at a lower power and the power consumed by the LED lamp is below the designed power of the ballast, these ballasts are designed to increase the current supplied to the LED lamp to reach the designed power. These ballasts are hereinafter called "constant power" ballasts for distinguishing from electronic ballasts which are designed to maintain a substantially constant current output, hereinafter called "constant current" ballasts.

[0028] Constant power ballasts are typically designed to operate with fluorescent tubes at a power that is significantly higher than the designed operating power of an energy saving LED lamp (e.g. designed to save 50% energy). Consequently, when fitting such an LED lamp in a luminaire designed for a fluorescent lamp, the constant power ballast typically determines that the power is too low (e.g. by determining that the voltage across the LED lamp is too low), so the ballast increases the current supplied to the LED lamp to reach the designed power output of the ballast. This can result in the ballast driving the LED lamp with a current that is too high which results in too much light output from the LED lamp 1 and shortening of the life or failure of the LED lamp and / or the ballast.

[0029] The variable impedance circuits 11-14 are designed to address this problem by presenting a relatively high load to the ballast (i.e. a relatively high input impedance of the LED lamp 1) when the voltage, current or derivative (e.g. power) supplied to the LED lamp 1 rises too high, e.g. above a predetermined threshold. Such a threshold may be set at a value that effectively distinguishes between operation with a constant current ballast versus with constant power ballast. In operation the variable impedance circuits 11, 12 add only a (very) small impedance to the circuit supplying current to the LEDs D1 when operating below the threshold, and add a larger impedance to the circuit supplying current to the LEDs D1 when operating above the threshold. This effect can be achieved by incorporating a variable impedance circuit 11 on the AC-side of the circuit, or a variable impedance circuit 12 on the DC-side of the circuit, but it has been found that adding variable impedance circuits on both AC- and DC-sides provides a much better solution, permitting flexibility in the operation of the LED lamp to achieve more efficient operation and better compatibility.

[0030] Thus for constant power ballasts, the variable impedance circuits 11-14 provide an effective way to reduce the power level of the LED lamp 1 by providing a higher impedance in the LED lamp circuit. For constant current ballasts, the impedance of the variable impedance circuits 11-14 may be reduced to avoid inefficient operation and unwanted reduction in current, and avoid causing turn-on problems. The impedance of the variable impedance circuits 11-14 may also be controlled to adjust the forward voltage of the LEDs D1 as an effective means to control the power of the LED lamp 1.

[0031] The variable impedance circuits may be implemented as variable inductances. FIG. 3 illustrates a simplified circuit diagram of an embodiment having a first variable impedance circuit 11 comprising a variable first inductive element L1, wherein the first inductive element L1 is connected in series between one of terminals 3 and rectifier circuit 6, shown as full-wave rectifier DB1. FIG. 4 illustrates an embodiment having a second variable impedance circuit 12 comprising a variable second inductive element L2, wherein the second inductive element L2 is connected in series between rectifier circuit 6 and LEDs D1. FIG. 5 illustrates a simplified circuit diagram of an embodiment having a third variable impedance circuit 13 comprising a variable third inductive element L3, wherein the third inductive element L3 is connected in parallel across an input of the LED lamp between two of the terminals 3.

[0032] Thus, the variable impedance circuits may each have a variable inductance. The variable inductive elements L1 and / or L2 may be adjusted to insert a high inductance into the circuit of the LED lamp 1 when the voltage, current or derivative (e.g. power) supplied to the LED lamp 1 rises above a predetermined threshold, and to insert a low inductance into the circuit when the voltage or current supplied to the LED lamp 1 is below the predetermined threshold. This threshold may be set at a value that effectively distinguishes between operation with a constant current ballast versus with constant power ballast.

[0033] FIGs. 6 and 7 illustrate embodiments combining the variable inductive elements of FIGs. 3-5 in the LED lamp 1. In FIG. 6, the first and second variable inductive elements L1 and L2 are used, in combination with an optional third variable inductive element L3 connected in parallel across an input of the LED lamp between two of the terminals 3. In FIG. 7, the first and second variable inductive elements L1 and L2 are used, in combination with an optional fourth variable inductive element L4 connected in parallel across an input of the rectifier circuit 6. These embodiments combine a variable impedance circuit 11 (with inductive element L1) arranged on the AC-side of the LED lamp circuit with a variable impedance circuit 12 (with inductive element L2) arranged on the DC-side of the LED lamp circuit, and optionally a third or a fourth variable impedance circuit 13 and / or 14 (with inductive element L3 and / or L4) connected on the AC-side in parallel across an input of the LED lamp or an input of the rectifier circuit 6.

[0034] The variable impedance circuits may be implemented using variable inductive elements which comprise an inductive element and a bypass switch connected to the inductive element. The variable impedance circuits are configured so that the bypass switch switches between a first (e.g. open) position and a second (e.g. closed) position. In the first position the inductive element is connected into the circuit of the LED lamp, and in the second position the inductive element is effectively removed from the circuit.

[0035] FIG. 8 is a simplified schematic diagram of an embodiment of the LED lamp 1 including first variable impedance circuit 11 implemented as a variable inductance comprising first inductive element L1 and first bypass switch S1. The first bypass switch S1 is connected across the first inductive element L1 for bypassing the first inductive element L1. When the first bypass switch is in an open position, the inductance of the first inductive element L1 is placed into the circuit in series between the connector 3 and rectifier circuit 6 (e.g. bridge rectifier DB1), so that the AC current supplied to the rectifier 6 passes through the first inductive element L1. This presents a higher impedance to the flow of AC current to the rectifier, reducing the current drawn by the LED lamp and supplied to the LEDs.

[0036] When the first bypass switch S1 is in a closed position, the inductance of the first inductive element L1 is effectively short-circuited by the first bypass switch S1 and the impedance placed into the circuit in series between the connector 3 and rectifier circuit 6 is reduced to a very low value (e.g. the low impedance value of the closed first bypass switch S1), so that the AC current supplied to the rectifier 6 passes (almost entirely) through the first bypass switch S1 rather than the first inductive element L1. This presents a lower impedance to the flow of AC current to the rectifier, so that the current drawn by the LED lamp and supplied to the LEDs is largely unaffected by the first inductive element L1.

[0037] Similarly, the second variable impedance circuit 12 is implemented as a variable inductance comprising second inductive element L2 and second bypass switch S2. The second bypass switch S2 is connected across the second inductive element L2 for bypassing the second inductive element L2. When the second bypass switch S2 is in an open position, the inductance of the second inductive element L2 is placed into the circuit in series between the rectifier circuit 6 (e.g. bridge rectifier DB1) and LEDs D1, so that the DC current supplied to the LEDs D1 passes through the second inductive element L2. This presents a higher impedance to the flow of DC current to the LEDs, reducing the current drawn by the LED lamp and supplied to the LEDs.

[0038] When the second bypass switch is in a closed position, the inductance of the second inductive element L2 is effectively short-circuited by the second bypass switch S2 and the impedance placed into the circuit in series between rectifier circuit 6 and LEDs D1 is reduced to a very low value (e.g. the low impedance value of the closed second bypass switch S2), so that the DC current supplied to the LEDs D1 passes (almost entirely) through the second bypass switch S2. This presents a lower impedance to the flow of DC current to the LEDs, so that the current drawn by the LED lamp and supplied to the LEDs is largely unaffected by the second inductive element L1.

[0039] The first bypass switch S1 is connected to receive a first control signal 15 for controlling the switch S1 to switch between the open and closed positions, and the second bypass switch S2 is connected to receive a second control signal 16 for controlling the switch S2 to switch between the open and closed positions, as shown in FIG. 2A.

[0040] The embodiment of FIG. 8 may optionally include a third variable impedance circuit 13 implemented as a variable inductance comprising third inductive element L3 and third bypass switch S3. The third bypass switch S3 is connected in series with third inductive element L3 for bypassing the third inductive element L3. When the third bypass switch S3 is in an open position, the inductance of the third inductive element L3 is removed from the circuit. When the third bypass switch S3 is in a closed position, the inductance of the third inductive element L3 is placed into the circuit across the input of the LED lamp 1 in series with the low impedance of the closed third bypass switch S3. The third bypass switch S3 is connected to receive a third control signal 17 (see e.g. FIG. 2A) for controlling the switch S3 to switch between the open and closed positions.

[0041] The embodiment of FIG. 9 is the same as the embodiment of FIG. 8 except that it includes fourth variable impedance circuit 14 implemented as a variable inductance comprising fourth inductive element L4 and fourth bypass switch S4. The arrangement and operation of the fourth inductive element L4 and fourth bypass switch S4 is the same as the third inductive element L3 and third bypass switch S3, except that when the fourth bypass switch S4 is in a closed position, the inductance of the fourth inductive element L4 is placed into the circuit across the input of the rectifier 6 (e.g. rectifier DB1) in series with the low impedance of the closed fourth bypass switch S4. The fourth bypass switch S4 is connected to receive a fourth control signal 18 (see e.g. FIG. 2A) for controlling the switch S4 to switch between the open and closed positions.

[0042] The bypass switches S1-S4 may be implemented in any of the embodiments as an electromechanical switch, or more preferably as an electronic switch such as transistor, e.g. an FET such as a MOSFET. Thus, the "open" position of a switch should be understood to indicate an off or deactivated position of an electronic switch when the switch does not conduct. Conversely, the "closed" position of a switch should be understood to indicate an on or activated position of an electronic switch when the switch is in a conductive state.

[0043] Any one or more of the variable impedance circuits 11-14 may be implemented using a coupled inductor to produce a variable impedance. The variable impedance circuit may be switched between a low impedance and a high impedance on a first (primary) winding side of the coupled inductor, making use of a small low-power switching circuit operating on the second (secondary) winding side of the coupled inductor. This is explained by reference to FIGs. 10-13.

[0044] FIG. 10 is a schematic diagram of a coupled inductor comprising a primary winding having number of turns N 1 with primary self-inductance L 1 and a secondary winding having number of turns N 2 with secondary self-inductance L 2 . The winding may be arranged with polarity aligned or opposed. The associated primary and secondary voltages v 1 (t) and v 2 (t) and currents i 1 (t) and i 2 (t), and mutual inductance M between the windings are also shown.

[0045] FIG. 11 is a circuit model of the coupled inductor, showing an ideal coupled inductor with inductive elements modelling the primary and secondary leakage inductances L 11 and L 12 and the magnetization inductance L M and current i M (t). The primary leakage inductance L 11 is computed as (1-k) × L 1 and the secondary leakage inductance L 12 is computed as (1-k) × L 2 where k is the coupling coefficient between the windings.

[0046] FIG. 12 shows the circuit model of FIG. 11 with the secondary leakage inductance L 12 reflected onto the primary side as N 2< × L 12 , taking into account the turn ratio of the windings N, equal to N 1 / N 2 .

[0047] FIG. 13 shows the situation when the secondary side of the circuit is shorted, by connecting the two terminals on the secondary side together with a low impedance connection so that V2 (t) is zero or close to zero. The total impedance seen from the primary side becomes the primary leakage inductance Ln in series with the secondary leakage L 12 in parallel with the magnetization inductance L M . With a coupling coefficient k close to one, then the leakage inductances Ln and L 12 will be much smaller self-inductances L 1 and L 2 . For example, if L 1 = L 2 = 150 µH and k = 0.97, then Ln = L 12 = 4.5µH. When the secondary winding is not shorted, the inductance seen on the primary side is L 1 + Ln or approximately 154.5 µH in the above example. When the secondary winding is shorted, the inductance seen on the primary side also becomes much lower, approximately L 11 + L 12 = 9µH in the above example. This results in the inductance of the primary winding becoming low when the secondary winding is shorted (i.e. a low impedance connection is made across the secondary winding).

[0048] FIGs. 14-16 are simplified schematic diagrams of embodiments of a variable impedance circuit implemented as a coupled inductor 20 comprising a first (primary) winding 21 magnetically coupled to a second (secondary) winding 22. The first winding 21 is connected into the circuit of the LED lamp 1 to provide the variable inductance. The coupled inductor 20 comprises a bypass switch arranged on the secondary side of the coupled inductor 20 to control the impedance of the first winding 21.

[0049] The coupled inductor 20 may be used, for example, as any one or more of the variable impedance circuits 11-14 in FIG. 2A or 2B, operating as any one or more of the variable inductive elements L1-L4 in the embodiments shown in FIG. 6 or FIG. 7. The coupled inductor 20 may be used, for example, on the AC-side of the LED lamp circuit as the first variable inductive element L1 with the first winding 21 connected in series between one of the terminals 3 and the rectifier circuit 6, so that the AC current supplied to the rectifier 6 passes through the first winding 20. The coupled inductor 20 may be used on the DC-side of the LED lamp circuit as the second variable inductive element L2 with the first winding 21 connected in series between the rectifier circuit 6 the LEDs D1, so that the DC current supplied to the LEDs D1 passes through the first winding 20. The coupled inductor 20 also may be used as the third variable inductive element L3 with the first winding 21 connected in parallel across the input to the LED lamp 1, and / or as the fourth variable inductive element L4 with the first winding 21 connected in parallel across the input to the rectifier circuit 6.

[0050] In FIGs. 14-16, the bypass switch Q1-Q4 is adapted to switch between an open (off / nonconducting) position and a closed (on / conducting) position based on a control signal 25 via resistor R1-R3. In the closed position, the bypass switch Q1-Q4 provides a low impedance across (i.e. shorts) the second winding 22, while in the open position, the switch Q1-Q4 does not short the second winding 22. The bypass switch Q1-Q4 is preferably an electronic switch such as transistor, e.g. an FET such as a MOSFET, but an electromechanical switch may also be used. In the embodiments shown in FIGs. 14-16, the bypass switch is a MOSFET, with gate terminal connected to receive the control signal 25, and source and drain terminals connected across the second winding 22.

[0051] When the coupled inductor 20 is used for one of the variable impedance circuits 11-14, it will be understood that the relevant bypass switch Q1-Q4 operates as the corresponding bypass switch S1-S4, and the control signal 25 operates as the corresponding control signal 15-18 for the relevant variable impedance circuit.

[0052] In the embodiment in FIG. 14, an optional diode D2 and bypass switch Q1 are connected in series across the second winding 22. The diode D2 (which may be a Schottky diode) may be used with a MOSFET switch Q1. This diode functions to block conduction via the internal diode in the MOSFET (due to the PN junction between the base and drain terminals of the MOSFET forming an effective diode between the source and drain terminals).

[0053] In the FIG. 15 embodiment, a rectifier DB2 is connected across the second winding 22 for receiving AC current from the second winding 22, and bypass switch Q2 is connected on the DC side of the rectifier DB2.

[0054] In the FIG. 16 embodiment, the bypass switch comprises two transistors Q3, Q4 connected in series across the second winding 22. The transistors Q3, Q4 are configured to conduct an AC current under control of a single control signal 25. The embodiments in FIGs. 15 and 16 are particularly suitable for use on the AC-side of the LED lamp, e.g. as variable impedance circuit 11, 13 or 14, but they may also be used on the DC-side as variable impedance circuit 12.

[0055] As explained in the discussion of the coupled inductor above, when the bypass switch Q1-Q4 conducts (i.e. switch is closed or turned on) to provide a low impedance connection across the second winding 22 (i.e. the second winding is shorted), then the inductance of the first winding 20 becomes low. Conversely, when the bypass switch Q1-Q4 is not conducting (i.e. switch is open or turned off), there is a high impedance (e.g. open circuit) across the second winding 22, and the inductance of the first winding 20 becomes higher.

[0056] When LED lamp 1 is installed in a luminaire with an electronic ballast, the terminals 3 receive a high frequency AC voltage generated by the electronic ballast. When the electronic ballast is started, it usually first generates a high output voltage typically around 400Vac designed to ignite a fluorescent lamp. After the ignition period the ballast output voltage drops and is dependent on the load impedance, i.e. the input impedance of the LED lamp 1. The ballast output will typically have a voltage lower than AC mains voltage, e.g. in the range from 40-80Vac depending on the load, and having a frequency much higher than AC mains voltage, e.g. typically in the range from 20kHz to 200kHz. Accordingly, the output from rectifier 6 is a high frequency pulsating DC voltage with low maximum voltage (after the ignition period) typically in the range 40-80Vac (depending on the load) and with a high frequency ripple of typically 40kHz to 400kHz for full-wave rectification.

[0057] This output from rectifier 6 can be regarded as a combination of a constant DC voltage and a high frequency AC ripple voltage. The waveform of the electronic ballast output supplied to the LED lamp 1 (e.g. whether it is a sine wave or square wave or other type of waveform) and the design of the rectifier (e.g. whether it is a half-wave or full-wave rectifier and whether it provides smoothing) will determine the relative size of the DC and AC parts of the output of rectifier 6. The inductive element L2 (and first winding 21 of the coupled inductor 20) of variable impedance circuit 12 provides only a small impedance for the DC part of the rectified current, but provides a much higher impedance to the AC part of the rectified current. As the impedance of an inductance is proportional to the frequency, the higher frequency due to full-wave rectification in the rectifier 6 provides twice as much as impedance compared to half-wave rectification, or compared to an inductance used on the AC-side of the LED lamp circuit. Thus a relatively small inductance may add a significant impedance into the DC-side of the circuit supplying current to the LEDs D1.

[0058] When the voltage or current supplied to the LED lamp 1 from a constant power ballast is above a predetermined threshold, the bypass switch S1 of first variable impedance circuit 11 may be set into the open or off position so that it does not conduct and the inductive element L1 is not shorted. Additionally or alternatively, the bypass switch S2 of second variable impedance circuit 12 may be set into the open or off position so that it does not conduct and the inductive element L2 is not shorted. If variable impedance circuit 11 and / or 12 is implemented by one of the coupled inductor circuits 20, the bypass switch Q1-Q4 may be set into the open or off position so that it does not conduct so that the second winding 22 is not shorted and the inductance of the first winding 21 is not reduced. In this way, variable impedance circuit 11 and / or variable impedance circuit 12 add sufficient impedance into the LED lamp circuit to reduce the current drawn by the LED lamp 1 and supplied to the LEDs D1 to an appropriate level.

[0059] When the voltage or current supplied to the LED lamp 1 is below the predetermined threshold (e.g. supplied by a constant current ballast), the bypass switch S1 of second variable impedance circuit 11 and / or bypass switch S2 of second variable impedance circuit 12 may be set into the closed or on position so that it conducts and shorts the inductive element L1 and / or L2. If variable impedance circuit 11 and / or 12 is implemented by one of the coupled inductor circuits 20, the bypass switch Q1-Q4 may be set into the closed or on position so that it conducts so that the second winding 22 is shorted and the inductance of the first winding 21 is reduced. In this position, the inductance of the first variable impedance circuit 11 and / or second variable impedance circuit 12 is reduced so that it adds very little impedance into the circuit supplying current to the LEDs D1. This prevents a loss of efficiency of the LED lamp 1 or an undesired drop in current when the extra impedance is not needed, and also prevents that the ballast detects a high load impedance and does not turn on (due to protection circuits built into the ballast designed to prevent turn-on when there is no lamp present in the luminaire).

[0060] Note that the term "short" is used to denote that the two ends of the second winding 22 are connected (by the secondary winding circuit 23 via the conducting bypass switch Q1-Q4) by a very low impedance. For example, when bypass switch Q1 is conducting in the embodiment of FIG. 14, the voltage across the second winding 22 will be the drain-source voltage Vds of switch Q1 and the diode voltage of diode D2.

[0061] For example, assuming a coupled inductor 20 with self-inductances of the first winding 21 (L 1 ) and second winding 22 (L 2 ) = 150 µH, a coupling coefficient k = 0.97, and leakage inductances L 11 = L 12 = 4.5 µH, when bypass switch Q1 is in the open or off position and the second winding 22 is not shorted, the inductance of the first winding 21 will be 150 µH. When bypass switch Q1 is in the closed or on position and the second winding 22 is shorted, the inductance provided by the variable impedance circuit will be approximately 9 µH. When the output of the rectifier 6 has a frequency of 100 kHz, this results in an impedance provided by the variable impedance circuit varying from a high impedance of about 97 Ohms to a low impedance of about 5.6 Ohms.

[0062] The design of the variable impedance circuit as a coupled inductor 20 places the bypass switch Q1-Q4 in the second winding circuit 23 on the secondary side of the coupled inductor 20. With this arrangement, the current supplied to the LEDs D1 passes through the first winding 21, and this current does not pass through the second winding circuit 23 and bypass switch Q1-Q4. This design enables the use of a bypass switch Q1-Q4 with lower current-carrying capacity, which is smaller and less expensive, and generates less heat and losses. The design using the coupled inductor 20 also enables the use of other components on the secondary side, such as diode D2 in FIG. 14, with lower current-carrying capacity, so that these components may be smaller and less expensive, and generate less heat and losses. Another advantage of the coupled inductor design is that one end of the second winding 22 can be grounded so that the control signal 25 can be referenced to ground. This avoids the use of a level-shifter for the control signal which may be needed for a switch S1-S2 connected across an inductance L1-L2 which is not grounded.

[0063] The design of variable impedance circuits 11-12 using a bypass switch S1-S2 arranged to provide a low impedance path short-circuiting the inductive element L1-L2, such as shown in FIGs. 8 and 9, provide a simpler design but have the disadvantage that the current supplied to the LEDs passes through the short-circuit path. This requires a design in which the bypass switch (and any other components in the short-circuit path) needs sufficient current-carrying capacity to conduct the LED current, and increases the size and expense of the circuit and heat and loss generated by the circuit.

[0064] Returning to FIGs. 2A and 2B, a control unit 10 is shown for controlling the variable impedance circuits. The control circuit 10 may be configured to measure an amount of electrical current or power received by the LED lamp 1 from the luminaire 2 or supplied to the one or more LEDs D1, and configured to generate, based on the measurement, one or more control signals for control of the variable impedance circuits. These may include a first control signal 15 for control of bypass switch S1 of inductive element L1, the second control signal 16 for control of bypass switch S2 of inductive element L2, the third control signal 17 for control of bypass switch S3 of inductive element L3, and the fourth control signal 18 for control of bypass switch S4 of inductive element L4.

[0065] The control circuit 10 may be configured to generate the first control signal 15 to switch the first bypass switch S1 to reduce inductance of the first variable impedance circuit 11 during a starting phase of the LED lamp, and / or to generate the second control signal 16 to switch the second bypass switch S2 to reduce inductance of the second variable impedance circuit 12 during a starting phase of the LED lamp. This addresses the high output voltage usually generated by the ballast during the ignition period at lamp start-up.

[0066] The control circuit 10 may also be configured to generate the second control signal 16 having pulses to control an activation time of the second bypass switch S2 to regulate inductance of the second variable impedance circuit 12 for at least a portion of time during operation of the LED lamp.

[0067] The control circuit 10 may be configured to generate the third control signal 17 to switch the third bypass switch S3 to increase impedance of the third variable impedance circuit 13 during a starting phase of the LED lamp, and / or to generate the fourth control signal 18 to switch the fourth bypass switch S4 to increase impedance of the fourth variable impedance circuit 14 during a starting phase of the LED lamp.

[0068] The control circuit 10 (for example as shown in FIGs. 2A and 2B) may be configured to receive input signals 24, 25, e.g. from a sensing circuit configured to measure one or more parameters representing an amount of current received by the LED lamp 1 from the luminaire 2 (e.g. measurement signal 24) or supplied to the plurality of LEDs D1 (e.g. measurement signal 25). The current or voltage (current is often measured by measuring voltage across a known resistance) may be measured at one or more points in the circuit of the LED lamp 1, e.g. measuring an AC current or voltage supplied to the LED lamp 2, or a rectified DC current or voltage at the output of the rectifier 6, or a current or voltage as supplied to the LEDs D1.

[0069] The control circuit 10 may be configured with one or more predetermined threshold values, and may include a logic circuit configured to compare the measured current or voltage with one of the predetermined thresholds to generate a control signal 11-14, 25 for control of the bypass switch of one or more of the variable impedance circuits 11-14. The predetermined threshold may be set at a value sufficiently above the design LED driving current of the LED lamp 1 that the threshold will not be exceeded by normal variations in the current which may be due to manufacturing variations of the LED lamp 1 or ballast, or expected operational variation in the mains supply voltage. The threshold may set at a value taking into account the expected current supplied by a constant current ballast. The control circuit may thus operate to detect whether a constant current or constant power ballast is supplying power to LED lamp 1, and to control the bypass switch to switch the variable impedance circuits between a low impedance mode and a high impedance mode in dependence on the detection.

[0070] The control circuit 10 may be configured to generate a pulse-width modulated (PWM) signal having a duty cycle which varies in dependence on the measured current or voltage. The PWM signal may be used as the control signal 15-18, 25 for control of the bypass switch S1-S4, Q1-Q4 of one or more of the variable impedance circuits 11-14. By varying the on time of the control signal (e.g. by varying the duty cycle of PWM signal), the proportion of time that bypass switch is in the closed or on position relative to the open or off position can be varied. This provides a means to control the proportion of time during which the inductive element L1-L4 or second winding 22 is shorted and the inductance provided by the variable impedance circuit is reduced, relative to the time during which the inductive element L1-L4 or second winding 22 is not shorted and the variable impedance circuit provides a high impedance into the LED lamp circuit. In this way, the variable impedance circuit(s) 11-14 may be used for power control of the LED lamp 1, varying the time-averaged impedance of the variable impedance circuit in accordance with the control signal. The preferred frequency used for the PWM control signal(s) are in the range of 20kHz - 400kHz. Synchronization of the switching frequency with the ballast operation frequency can be an option. For example, the switching circuit may be configured to adjust the switching frequency in accordance with an integer multiple of the ballast operation frequency (including 1× the ballast operation frequency) or vice versa.

[0071] In one embodiment, one or more of the variable impedance circuits 11-14 are bypassed during a starting phase of the LED lamp 1, i.e. during a predetermined time period when the LED lamp is turned on. This bypass comprises, for example, closing bypass switch S1 and / or S2 and / or opening bypass switch S3 and / or S4. This configuration may be used to maximize compatibility of the LED lamp 1 with ballasts, some of which may have a safety circuit which prevents turn-on if the load impedance is too high.

[0072] In one embodiment, the impedance added to the LED lamp circuit by one or more of the variable impedance circuits 11-14 is controlled during operation of the LED lamp 1, e.g. after a starting phase of the LED lamp 1. This control may be achieved by varying the duty cycle of the relevant control signal 15-18, e.g. using a PWM control signal. In one embodiment, the impedance of the series-connected variable impedance circuits 11, 12 is controlled during operation of the LED lamp 1 by varying the duty cycle of the relevant control signal 15-16, and the impedance of a parallel-connected variable impedance circuit 13, 14 is controlled by connecting or disconnecting the variable impedance circuit in the LED lamp circuit.

[0073] In one embodiment, the impedance added to the LED lamp circuit by one or more of the series-connected variable impedance circuits 11-12 is controlled during operation of the LED lamp 1 by measuring the voltage across the LEDs D1 and / or the current supplied to the LEDs D1, to regulate the power to the LEDs D1.

[0074] The variable impedance circuit 11 can thus be used to adjust the load impedance of the LED lamp 1 to prevent excessive current (e.g. when the LED lamp is driven by a constant power ballast) and / or to adjust the load impedance to implement power control in the LED lamp 1. In this way, the variable impedance circuit 11 may implement both power control and protection against excessive current, e.g. as a result of operation with a constant power ballast.

Claims

1. An LED lamp (1) comprising: two or more connectors (3) for receiving an electrical current from a luminaire (2) for supply to one or more LEDs (D1); a rectifier circuit (6) connected to receive the electrical current from one or more of the connectors (3) and configured to generate a rectified current; a first variable impedance circuit (11) comprising a first inductive element (L1) and a first bypass switch (S1) electrically coupled to the first inductive element (L1) for reducing inductance of the first variable impedance circuit (11), wherein the first inductive element (L1) is connected in series between one of the connectors (3) and the rectifier circuit (6), and the first bypass switch (S1) is connected to receive a first control signal (15) for controlling the first switch (S1); and a second variable impedance circuit (12) comprising a second inductive element (L2) and a second bypass switch (S2) electrically couped to the second inductive element (L2) for reducing inductance of the first variable impedance circuit (12), wherein the second inductive element (L2) is connected in series between the rectifier circuit (6) and the one or more LEDs (D1), and the second bypass switch (S2) is connected to receive a second control signal (16) for controlling the second switch (S2).

2. The LED lamp (1) of claim 1, wherein the first bypass switch (S1) is connected across the first inductive element (L1) for bypassing the first inductive element (L1),3. The LED lamp (1) of claim 1, wherein the first inductive element (L1) comprises a coupled inductor (20) comprising a first winding (21) magnetically coupled to a second winding (22), and the first bypass switch (S1) is connected to the second winding (22) via a second winding circuit (23).

4. The LED lamp (1) of claim 3, wherein the first bypass switch (S1) is configured to switch between a high impedance and a low impedance connection across the second winding (22).

5. The LED lamp (1) of any one of claims 1-4, wherein the second bypass switch (S2) is connected across the second inductive element (L2) for bypassing the second inductive element (L2).

6. The LED lamp (1) of any one of claims 1-4, wherein the second inductive element (L2) comprises a coupled inductor (20) comprising a first winding (21) magnetically coupled to a second winding (22), and the second bypass switch (S2) is connected to the second winding (22) via a second winding circuit (23).

7. The LED lamp (1) of claim 6, wherein the second bypass switch (S2) is configured to switch between a high impedance and a low impedance connection across the second winding (22).

8. The LED lamp (1) of any one or the preceding claims, further comprising a third variable impedance circuit (13) comprising a third inductive element (L3) and a third bypass switch (S3) connected in series, wherein the third variable impedance circuit (13) is connected in parallel across an input of the LED lamp between two of the connectors (3), and the third bypass switch (S3) is connected to receive a third control signal (17) for controlling the third switch (S3).

9. The LED lamp (1) of any one or the preceding claims, further comprising a fourth variable impedance circuit (L4) comprising a fourth inductive element (L4) and a fourth bypass switch (S4) connected in series, wherein the fourth variable impedance circuit (14) is connected in parallel across an input of the rectifier circuit (6), and the fourth bypass switch (S4) is connected to receive a fourth control signal (18) for controlling the fourth switch (S4).

10. The LED lamp (1) of any one or the preceding claims, further comprising a control circuit (10) configured to measure an amount of electrical current or power received by the LED lamp (1) from the luminaire (2) or supplied to the one or more LEDs (D1), and configured to generate, based on the measurement, at least one of the first control signal (15), the second control signal (16), the third control signal (17), and the fourth control signal (18).

11. The LED lamp (1) of claim 10, wherein the control circuit (10) is configured to generate the first control signal (15) to switch the first bypass switch (S1) to reduce inductance of the first variable impedance circuit (11) during a starting phase of the LED lamp.

12. The LED lamp (1) of claim 10 or claim 11, wherein the control circuit (10) is configured to generate the second control signal (16) to switch the second bypass switch (S2) to reduce inductance of the second variable impedance circuit (12) during a starting phase of the LED lamp.

13. The LED lamp (1) of any one of claims 10-12, wherein the control circuit (10) is configured to generate the second control signal (16) to switch the second bypass switch (S2) to reduce inductance of the second variable impedance circuit (12) for at least a portion of time during operation of the LED lamp.

14. The LED lamp (1) of any one of claims 10-12, wherein the control circuit (10) is configured to generate the second control signal (16) having pulses to control an activation time of the second bypass switch (S2) to regulate inductance of the second variable impedance circuit (12) for at least a portion of time during operation of the LED lamp.

15. The LED lamp (1) of any one of claims 10-14, wherein the LED lamp comprises: a third variable impedance circuit (13) according to claim 8, and the control circuit (10) is configured to generate the third control signal (17) to switch the third bypass switch (S3) to increase impedance of the third variable impedance circuit (13) during a starting phase of the LED lamp, and / or a fourth variable impedance circuit (14) according to claim 9, and the control circuit (10) is configured to generate the fourth control signal (18) to switch the fourth bypass switch (S4) to increase impedance of the fourth variable impedance circuit (14) during a starting phase of the LED lamp.

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