Abstract:
The Light Emitting Diode (LED) lamp is adapted for mounting in a fluorescent tube mount having a selectively activatable magnetic ballast powering two pairs of sockets with AC power upon activation. The LED lamp has a body having connectors at both opposite ends adapted to the corresponding pair of sockets, at least one LED string extending along at least a portion of the body, a driver inside the body connecting the connectors to the LED string, the driver including a transistor, an inductor, and a rectifier in a switch mode converter configuration, for converting the AC power of the sockets into a DC power of a voltage adapted to power the LED string; and a pulse mode controller for controlling the transistor in an intermittent manner upon activation of the multiple fluorescent tube mount, until conduction across the at least one LED string is achieved.

Description:
FIELD 
     The improvements generally relate to the field of Light Emitting Diodes (LED) lamp systems, but more particularly to providing a LED lamp which can replace a fluorescent tube in a conventional fluorescent tube mount having a dual magnetic ballast. 
     BACKGROUND 
     Fluorescent tube lamps are gas-discharge lamps. When a discharge occurs between electrodes of a fluorescent tube due to a voltage exceeding a certain threshold, a short is created from one electrode to the other and current rapidly rises. If the current is not controlled, the fluorescent tube can break. 
     Henceforth, fluorescent tube mounts are provided with a ballast which has the function of providing a satisfactory form of electric power to correctly drive the fluorescent tubes therein. Two of the most common forms of ballasts are magnetic ballasts and electronic ballasts. Magnetic ballasts were used for many decades. The most common forms of magnetic ballast are the lead-lag circuit and the series sequence circuit which can be used to simultaneously drive two fluorescent tubes held in corresponding sockets of a multiple fluorescent tube mount. The lead-lag circuit allows to offset the intermittent activation of the fluorescent tubes (each running at ˜50-60 Hz) by 180° in a manner to give the eye a lighting at ˜100-120 Hz and thus reduce the potentially stroboscopic effect. 
     In recent years electronic ballasts have tended to replace magnetic ballasts. Electronic ballast build up a high amount of voltage typically faster than magnetic ballasts (typically in less than 1 ms) and can thus lead to more instantaneous illumination. Moreover, independent electronic ballasts are typically provided for corresponding fluorescent tubes. Notwithstanding the above, dual magnetic ballasts are still widely present, especially in older constructions. 
     In even more recent years, Light Emitting Diodes (LEDs) lamps have received an increasing amount of attention, especially given the fact that their lighting efficiency and power consumption are now considered to compete with fluorescent tube lighting. There is thus an increased motivation for providing a LED lamp product which could simply replace a fluorescent tube on existing ballasts. 
     Although such LED lamp products have been provided which were satisfactory to a certain degree, there remained room for improvement. Particularly in addressing the issues specific to making the LED lamp adaptable to dual magnetic ballasts. 
     SUMMARY 
     Simply using a driver which drives a LED string with a typical switch mode converter led to unbalanced operation in dual magnetic ballasts where the two most common situations were alternating, or significantly uneven lighting of the two LED lamps replacing the fluorescent tubes. 
     Dual magnetic ballasts typically heat the filament of the fluorescent tubes they were designed for progressively (most commonly in terms of milliseconds) while building up the threshold voltage required for satisfactory discharge. Discharge progressively begins to occur, and current thus begins to flow partially, as the voltage reaches the threshold voltage of the tube. Fluorescent tubes thus represent a particular form of electric load when integrated in the overall dual magnetic ballast and fluorescent tube circuit. It was found that this particularity was relevant to allowing a balanced mode of operation to be set up by the magnetic ballast with the fluorescent tubes. 
     It was found that a balanced mode of operation of a LED string could be achieved in dual magnetic ballasts by providing the LED string with a driver which allows the LED lamps to mimic (somewhat) the electric load behaviour of fluorescent tubes. More specifically, by intermittently activating the transistor of a typical switch mode converter, the activation voltage ramped up progressively and when the LED string threshold voltage was reached, both LED lamps were driven in a satisfactorily balanced manner. 
     In accordance with one aspect, there is provided a Light Emitting Diode (LED) lamp for mounting in a corresponding one of at least two pairs of sockets of a fluorescent tube mount, the fluorescent tube mount having a selectively activatable magnetic ballast powering two of the pairs of sockets with AC power upon activation, the LED lamp comprising: a body having connectors at both opposite ends adapted to the corresponding pair of sockets; at least one LED string extending along at least a portion of the body; and a driver inside the body connecting the connectors to the LED string, the driver being configured to provide a progressive voltage increase to the at least one LED string during a period of time following activation of the magnetic ballast. 
     In accordance with another aspect, there is provided a method of driving at least one LED string using a transistor, an inductor and a rectifier in a switch mode converter configuration; the method comprising: intermittently activating the transistor until detecting illumination of the LEDs of the at least one LED string. 
     In accordance with another aspect, there is provided a Light Emitting Diode (LED) lamp for mounting in a corresponding one of at least two pairs of sockets of a fluorescent tube mount, the fluorescent tube mount having a selectively activatable magnetic ballast powering two of the pairs of sockets with AC power in a lead-lag configuration upon activation, the LED lamp comprising: a body having connectors at both opposite ends adapted to the corresponding pair of sockets; at least one LED string extending along at least a portion of the body; a driver inside the body connecting the connectors to the LED string, the driver including: a transistor, an inductor, and a rectifier in a switch mode converter configuration, for converting the AC power of the sockets into a DC power of a voltage adapted to power the LED string; and a pulse mode controller for controlling the transistor in an intermittent manner upon activation of the multiple fluorescent tube mount, until conduction across the at least one LED string is achieved. 
     Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure. 
    
    
     
       DESCRIPTION OF THE FIGURES 
       In the figures, 
         FIG. 1  is an oblique view of an example of a multiple fluorescent tube mount; 
         FIG. 2  is an oblique view showing an example of a LED lamp system formed with two LED lamps and the multiple fluorescent tube mount of  FIG. 1 ; 
         FIG. 3  is a block diagram of the LED lamp system of  FIG. 2 ; 
         FIG. 4  is a block diagram of a first example of a LED lamp driver; 
         FIG. 5  is a schematic of an example electrical circuit of a LED lamp driver; 
         FIG. 6  is an oblique view of an example of a LED lamp; and 
         FIG. 7  is a block diagram of a second example of a LED lamp driver. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of a dual fluorescent tube mount  10  as known in the art. The multiple fluorescent tube mount  10  generally has an elongated frame  12 . In the illustrated embodiment, the elongated frame  12  has a backing  14  and a facing  16 , and has two pairs of sockets  18 ,  18 ′ disposed thereon. In this embodiment, the fluorescent tube mount has two pairs of sockets, though it will be understood that in alternate embodiments, fluorescent tube mounts can have more than two pairs of sockets and be adapted to receive more than two fluorescent tubes. When the backing  14  is mounted to a surface, the pairs of sockets  18 ,  18 ′ are directed towards a room or other area to be lit. In this example, the multiple fluorescent tube mount  10  has a dual magnetic ballast  20  connected to the two pairs of sockets  18 ,  18 ′. An electricity mains can be provided to the magnetic ballast  20  using conventional electrical wiring for providing an AC current to the two pairs of sockets  18 ,  18 ′ of the multiple fluorescent tube mount  10 . 
       FIG. 2  is an example of a LED lamp system  22  where LED lamps are used to replace the fluorescent tubes the multiple fluorescent tube mount  10  was designed for. More specifically, two LED lamps  24 ,  24 ′ (only a first example of the LED lamps is shown) can be manually installed in corresponding pairs of sockets  18 ,  18 ′ following removal of fluorescent tubes. Each of the LED lamps  24 ,  24 ′ incorporates at least one LED string (multiple LEDs connected in series) driven by a driver (not shown in  FIG. 2 ) having a switch mode converter and a pulse mode controller which will be detailed further below, and which, when combined with the dual magnetic ballast  20  of the multiple fluorescent tube mount  10 , allows for a balanced mode of operation of the two LED lamps  24 ,  24 ′. In the balanced mode of operation, the two LED lamps  24 ,  24 ′ are operated continuously by the magnetic ballast  20  at a satisfactorily similar intensity. 
     As referred to above, using LED strings driven by corresponding switch mode converters but without the pulse mode controller led to unsatisfactory, unbalanced operation of the two LED lamps, e.g. alternating flickering or uneven intensity of the two LED lamps. More specifically, it appears that when using such LED lamps, the controller of the switch mode converter attempts to immediately power the LED upon activation, which allows current to flow across the controller, which could entrain voltage drop and its deactivation. This was repeated a given number of times until, for instance, one of the LED lamps was lit, which drained the power from the magnetic ballast preventing correct activation of the other one of the LED lamps. 
       FIG. 3  shows a block diagram illustrating the LED lamp system  22  which shows the drivers  26 ,  26 ′ of the LED Lamps  24 ,  24 ′ which incorporate the pulse mode controllers  32 ,  32 ′. More specifically, the mains  28  which supplies an AC power to a magnetic ballast  20  of the multiple fluorescent tube mount  10 . The AC power can be selectively activatable with an interrupter  30  to be actuated by a user, for instance. When the interrupter  30  is actuated, the AC power is provided, in a parallel fashion, to a first pulse mode controllers  32 , and to a second pulse mode controller  32 ′ via the dual magnetic ballast  20 , which act on corresponding first and second switch mode converters  34 ,  34 ′ to ultimately power corresponding first and second LED lamps  36 ,  36 ′. 
       FIG. 4  provides a more detailed illustration of an embodiment of a LED lamp driver  26 . In this embodiment, a transistor  58 , an inductor  56  and a rectifier  48  are arranged in a switch mode converter arrangement  41 . A typical switch mode converter also includes a controller chip  60  which is used to drive the transistor in a continuous manner. In this embodiment, the pulse mode controller has a current pulser  43  (which can, in this embodiment, also be referred to as a current pulser and limiter since it also limits/conditions the intensity of the electricity reaching the controller chip). The current pulser  43  provides intermittent power to the controller chip  60  entraining continuous driving of the transistor  58  during a starting period following activation of the magnetic ballast  20 . 
     In this embodiment, the magnetic ballast  20  provides AC power to the LED string(s)  36  via the switch mode converter arrangement  41 . The current pulser  43  can provide a pulsed and limited current to the controller chip  60 . Then, the controller chip can go on and off (on-state, off-state) repeatedly in accordance with the pulses of the pulsed current. At each on-state of the controller chip  60 , the latter has enough energy to reach a threshold of the transistor  58 , which can provide a increment of voltage to the LED string(s)  36 , progressively building voltage. Once sufficient increments of voltage have been provided to the LED string(s), it can start to conduct electricity, and emit light in a continuous fashion, at which point it also conducts electricity. When the LED string(s)  36  conduct, power is fed back to the controller chip  60  by a feedback loop which overrides the intermittent pulses of the current pulser and allows the controller chip to continuously drive the transistor. In this sense, the feedback loop can be considered to also provide the function of a LED string conduction sensor  53  in this embodiment. At this point, both LED lamps can operate in a balanced manner, since the incremental increase of voltage occurs in both LED lamps connected to the dual magnetic ballast. When such a progressive rise of current is achieved (incremental increase), the LED lamp somewhat mimics a conventional fluorescent tube. Consequently, LED lamps can be installed in conventional multiple fluorescent tube mounts having dual magnetic ballasts. 
     In this specific embodiment, the controller chip  60  can provide a satisfactory voltage buildup by pulsing on and off between 2 and 100 times within a period of 100 ms following activation of the dual magnetic ballast. 
     More generally, the driver  26  can force the magnetic ballast  20  to provide a progressive increase of voltage to each of the two LED string(s)  36 ,  36 ′, which allows balancing the buildup voltage and subsequent illumination of the first and second LED lamps  36 ,  36 ′. 
     As illustrated in  FIG. 7 , in another embodiment, the pulse mode controller  32  can be provided in the form of a microcontroller programmed to provide intermittent activation of the transistor  58  until LED string conduction is determined (e.g. via a sensor). 
     In still another example alternate embodiment, a progressively increasing current slope can otherwise be provided to the LED string  36  (e.g. without the use of a pulse mode converter arrangement altogether, and/or via a computer or device having satisfactory capabilities). It will be understood that in a preferred mode, each LED lamp has one or more LED strings having an voltage illumination threshold of over 500V and in other words, this voltage is built progressively to reach the illumination threshold. The duration of the building of the illumination voltage can vary depending on a plurality of factors, including the available power source (mains). However, it can be considered safe to assume that it will always more progressive than electronic ballasts, which can build up the voltage in less than one millisecond. The voltage buildup can thus be driven to take more than 1 ms, and more likely tens of ms, following activation. 
     Referring back to  FIG. 4 , a possible embodiment can be seen to further incorporate optional shunt capacitors  52  and high speed rectifier  46  (which can contribute to making the driver readily adapted to electronic ballast compatibility), output filter capacitors  50 , and safety circuit  45 . 
       FIG. 5  is an example electrical circuit in accordance with the LED lamp system which incorporates the pulse mode controller, and other optional electrical components. In this example circuit, the current pulser has a RC circuit generating the pulsed activation of the controller chip. In this specific example, the capacitance has 10 mF and the resistance has 6-7 kOhms. In alternate embodiments using a similar current pulser, the capacitance can range between 1 mF and 470 mF and the resistance can range between 1 kOhms and 3 kOhms, for instance. 
       FIG. 6  is a second example of a LED lamp  124 . In contrast with the first example of LED lamps  24 ,  24 ′ of  FIG. 2 , which each incorporates one LED string  36 , the LED lamp  124  of  FIG. 6  has a LED array  64  having two LED strings  36  adjacent one to the other. Of course, the LED lamp  124  is not limited to a rectilinear shape, and its LED array  64  is not limited only to two LED strings  36 . Moreover, the LED lamp  124  has a bi-pin connector  66  for mating with a pair of sockets of the multiple fluorescent tube mount. The driver  26  can be hidden in the LED lamp, under the LED array  64 , for instance. 
     As can be seen therefore, the examples described above and illustrated are intended to be exemplary only. The scope is indicated by the appended claims.