Conversion circuit between fluorescent ballast and LED

Abstract

A tubular LED lamp designed for connection to a high frequency ballast. An electrically conductive screen is provided with an electrical connection between an internal node, such as an internal LED ground, and the screen. This provides a low impedance path5 for leakage currents which bypasses the LED string and thus prevents glow.

Claims

1. A tubular LED lamp, comprising: an LED arrangement electrically connected between a voltage bus and an internal ground; a driver circuit for driving the LED arrangement, wherein the driver circuit is adapted for connection to a high frequency ballast; an electrically conductive screen enclosed within an electrically insulating outer cover; and an electrical connection between a node of the LED arrangement and the screen for preventing glow of the LED arrangement when the tubular LED lamp is in an off state.

2. A lamp as claimed in claim 1, wherein the electrical connection is between the internal ground and the screen.

3. A lamp as claimed in claim 1, wherein the screen comprises a heatsink.

4. A lamp as claimed in claim 1, wherein the electrical connection between the node and the screen comprises a solder pad.

5. A lamp as claimed in claim 4, wherein the electrical connection between the node and the screen comprises a plurality of solder pads at multiple positions along the LED arrangement.

6. A lamp as claimed in claim 1, wherein the driver circuit comprises a dimming unit for providing a dimmable drive signal to the LED arrangement; and wherein the lamp further comprises a wireless receiver for receiving a command to implement said dimmable drive signal within the lamp itself.

7. A lamp as claimed in claim 1, comprising an outer casing, wherein the screen is electrically isolated from the outer casing.

8. A lamp as claimed in claim 7, wherein the outer casing is electrically insulating, and wherein the screen is mounted to the outer casing using screws which are sunk in recesses of at least 5 mm depth.

9. A lamp as claimed in claim 1, wherein the LED arrangement comprises an elongate printed circuit board on which a series string of LEDs is arranged.

10. A lamp as claimed in claim 1, wherein the driver circuit comprises: a filament emulation circuit; a pin safety and startup circuit; a rectifier; an output capacitor at the output of the rectifier; and an LED driver.

11. A lighting installation comprising: a fluorescent ballast; and a lamp as claimed in claim 1.

12. A lighting installation as claimed in claim 11, wherein the fluorescent ballast comprises a half-bridge resonant converter.

13. A method of preventing glow from a tubular LED which is driven by a fluorescent ballast, wherein the method is for preventing glow when the tubular LED is driven to an off state, the method comprising: coupling a node of an LED arrangement of the tubular LED to an electrically conductive screen; and, electrically isolating the screen from an outer housing of the lamp.

14. A method as claimed in claim 13, comprising forming the coupling by providing an electrical connection between an internal ground and the screen.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Examples of the invention will now be described with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a typical block diagram of a TLED that is compatible with a fluorescent ballast;

(3) FIG. 2 shows in a simplified block diagram how parasitic capacitances result in a leakage current which causes a glow even when the TLED is fully dimmed;

(4) FIG. 3 shows in a simplified block diagram how the problem explained with reference to FIG. 2 is addressed;

(5) FIG. 4 shows how connections can be made between the LED string and the heatsink; and

(6) FIG. 5 shows a casing design to prevent the heatsink being touched by a user.

DETAILED DESCRIPTION OF EMBODIMENTS

(7) The invention provides a tubular LED lamp designed for connection to a high frequency ballast. An electrically conductive screen is provided with an electrical connection between an internal node such as an internal ground and the screen. This provides a low impedance path for leakage currents which bypasses the LED string and thus prevents glow.

(8) FIG. 2 shows in a simplified block diagram how parasitic capacitances in the circuit of FIG. 1 result in a leakage current which causes a glow even when the TLED is fully dimmed.

(9) The same reference numbers are used as in FIG. 1. The internal driver circuit components (such as the filament emulation unit 14, the pin safety and start-up circuit 16, the matching circuit 18, the rectifier 20, the LED driver 22 and the smoothing capacitor 23 of FIG. 1) are simply represented as circuit blocks 30 at each end of the tubular LED lamp. The LED string 24 again extends between an internal ground 25 and a high voltage DC bus 26. The LED string comprises a series of nodes, including the internal ground at one end and the DC string voltage at the other end, and the interconnections between the LEDs in the string.

(10) The tubular LED is dimmable. A full (i.e. maximum depth) dimming setting is for example a standby mode and corresponds to an off state of the tubular LED.

(11) To implement a dimming functionality, a tubular LED lamp (without internal dimming control) may be connected to a dimmable ballast, and the driver circuit thus delivers a dimmable drive signal to the LED string. The glow problem explained above may be avoided because such a ballast will typically switch off the output when the system is in standby mode.

(12) The invention is of particular interest for implementing a dimmable LED lamp using a non-dimmable ballast. In this case, the tubular LED circuit blocks include wireless dimming functionality. For this purpose, they include a wireless receiver (or transceiver) circuit for receiving dimming commands from a wireless remote control device. The non-dimmable ballast remains turned on even when the tubular LED is dimmed to its maximum dimming setting. The dimming functionality is in this way implemented locally within the lamp itself, and the lamp itself has a dimming unit as part of its driver circuit.

(13) In this case, there is likely to be a glow issue because the ballast does not switch off the high frequency output when in the standby mode; instead it keeps generating the high frequency voltage. A leakage current via the LED string to the environment then causes the glow issue.

(14) The invention relates to overcoming this leakage current problem.

(15) The invention may however also find application for use with dimmable ballasts. One possible use case is that an installer can set the existing dimmable ballast system to operate at 100% all the time, thus no longer using the existing dimming functionality. The lamp control is then taken over by the wireless interface of the dimmable lamp. In this case, the existing dimmable ballast is just used as a non-dimmable ballast. The same glow effect then arises.

(16) Furthermore, when providing very deep dimming, a glow issue may still arise even with a dimmable ballast, meaning that lamps may have different brightness levels at a very deep dimming settings.

(17) Furthermore, if an injected current has a low frequency modulation, it can cause flicker even at relatively low injected currents. This issue arises with dimmable or non-dimmable ballasts.

(18) The dimming functionality is primarily used to enable brightness control, in known manner. In addition to providing general light output control, the dimming capability may also be used to set dynamic lighting features, such as a soft shut down, by which the light output is changed gradually over time in response to requested changes in dimming setting (or full on or full off commands).

(19) The ballast 10 generates a high voltage at a high frequency, for driving previous fluorescent lamps. The lamp has a heatsink 32, typically made of aluminum. The heatsink 32 carries the LED circuit board or boards.

(20) The outer casing of the lamp may be electrically insulating or it may be a metal outer casing. If a metal outer case is used, it may be connected to a protective earth terminal 34 for safety reasons or, more preferably, it is insulated from the live parts inside the lamp and then is electrically floating. Even when plastic outer casings are used, there will be internal metal planes to ensure proper high frequency fluorescent lamp operation and EMI requirements.

(21) The LED driver circuit ground creates the internal ground 25 and it has a common mode voltage to the protective earth 34 for the example of a metal outer casing. The common mode voltage is generally the potential between the LED string voltages (i.e. the ballast output wires) and earth, for example between the internal ground and earth. There are other components between, such as a plastic tube housing, and this will impact the parasitic capacitance Cp. As long as there is a parasitic capacitance and a common voltage is present, there will be a leakage current to earth.

(22) The high common mode voltage of several hundred volts at the ballast frequency (e.g. 50 kHz) in combination with the parasitic capacitance Cp between the LED string and the protective earth 34 injects current in the LED string and results in glow.

(23) FIG. 3 shows in a simplified block diagram how the problem explained with reference to FIG. 2 is addressed.

(24) The LED arrangement 24 is again electrically connected between a voltage bus 26 and an internal ground 25 (or another voltage bus). The driver circuit 30 drives the LED arrangement, and is connected to the high frequency ballast 10.

(25) The heatsink 32 forms an electrically conductive screen. Some designs may not require a heatsink, in which case a dedicated electrically conductive screen may be provided.

(26) The invention makes use of an electrical connection 40 between the LED string and the screen 32. The connection is for example from the internal ground 25 to the screen 32, but it may be from the string voltage level to the screen or from an internal node along the LED string to the screen.

(27) The internal ground is the cathode connection of the LED arrangement 24 and the string voltage level is the anode connection of the LED arrangement.

(28) The connection has a low impedance compared to the parasitic capacitance impedance to enable a reduction in charge injection.

(29) The parasitic capacitance Cp is in this way divided into two parts. A first part Cp1 exists between the LED string and the conductive screen 32. A second part Cp2 exists between the conductive screen 32 and the protective earth 34.

(30) The connection of the screen 32 to the node, for example to the internal ground, means the common mode voltage Vcm is only across the second parasitic capacitor part Cp2. As a result, no currents are injected into the LED string, resulting in no LED glow when the LED is set to its lowest brightness setting.

(31) In particular, a low impedance path is provided for leakage currents which bypasses the LED string. The screen 32 functions as a floating current sink.

(32) FIG. 4 shows the heatsink 32 surrounding the LED printed circuit board, on which the LEDs 24 are mounted. The electrical connection between the node (e.g. the internal ground) and the screen comprises a solder pad or a series of solder pads 50 between pads, e.g. internal ground pads, of the printed circuit board and the heatsink (or other screen). The solder pads provide an electrical connection between the screen and the node, e.g. the internal ground. By distributing the connections along the length of the LED arrangement a uniform charge distribution in the screen (or heatsink) is enabled. Other connections may instead be used, such as soldered wires. A solder connection additionally provides mechanical connection.

(33) The LED board may instead be connected to the heatsink via a friction force which also provides the desired electrical connections, so that soldering is not essential.

(34) The screen (or heatsink) now functions as an electrically floating charge store. It therefore should not be electrically contacted by a user. In particular, the design needs to satisfy safety contact tests.

(35) FIG. 5 shows how this may be achieved for a design with an electrically insulating outer cover 60, from which the connection pins 61 extend.

(36) The screen 32 is mounted to the outer casing using screws 62 which are sunk in recesses 64 of at least 5 mm depth. This avoids user contact with the mounting screws which mount the screen (e.g. heatsink).

(37) The invention thus provides a method of preventing glow from a tubular LED which is driven by a fluorescent ballast which is based on coupling an internal node of an LED arrangement of the tubular LED to a screen.

(38) Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.