Abstract:
A bleeder arrangement for a phase-cut circuit for a high-impedance load and having a leading-edge phase-cut device is disclosed, the bleeder arrangement comprising: a controllable current sink adapted to sink a latching current through the leading-edge phase-cut device, and a controller for controlling the controllable current sink, wherein the controller is configured to disable the current sink after the leading-edge phase phase-cut device has latched in at least two stages. A controller for use in such an arrangement is also disclosed, as is a method of controller such a bleeder arrangement.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to bleeder arrangements for phase-cut circuits, for high-impedance loads and having a leading-edge phase-cut device. It further related to controllers in and for such bleeder arrangements, and to methods of controlling such bleeder arrangements. 
       BACKGROUND OF THE INVENTION 
       [0002]    There is a very wide installed base of dimmer circuits in mains lighting arrangements. The vast majority of dimmer circuits use phase-cut dimmers, in which power is supplied to the lighting unit during only one part of the mains phase; the power being cut during the remainder. The cut part may either be the trailing part of the phase: such trailing-edge phase cut dimmers normally have a transistor to cut the phase; alternatively the cut part may be the leading part of the phase; such leading-edge phase cut dimmers are more common and normally use a triac to cut the phase. 
         [0003]    In order to operate properly, the triac requires a certain level of current (so-called sync or synchronisation current) through it during the first (cut) part of the phase, in order to determine when to switch on, at a predetermined switch-on voltage; also, and more significantly, the triac requires a (generally higher) latching current through it whilst it switches and settles to an on-state. In order to ensure that it remains on for the remainder of the phase, a third current called a holding current is required. 
         [0004]    This is illustrated in  FIG. 1  which shows two part-rectified mains half-cycles  110  (shown partially dotted), and the voltage shape  120  of the voltage supplied to the load. The voltage shape  120  includes an initially off period  122 , and a rising leading-edge  124 . The lower part of the figure shows the current  130  through the device which is required to properly operate the triac. This includes a synchronization current  132  during the initial (cut) part of the phase; a latching current  134  around the leading-edge; and a hold current  136  throughout remainder of the mains half cycle. 
         [0005]    Conventional incandescent lighting units generally have a low impedance, and thus the current through them is sufficiently high to meet the current requirements  130  of the dimmer. However, modern forms of lighting such a compact fluorescent (CFL) and light-emitting diode (LED) are generally more efficient, and in particular offer a high impedance, such that for lighting sources such as LEDs in particular, it is no longer the case that the lighting unit will sink sufficient to ensure that the triac properly operates. High impedance lighting sources are thus generally not compatible with conventional triac-based dimming units without modification. 
         [0006]    It is known to provide a so-called bleeder circuit in parallel with the lighting unit in order to sink current and ensure the triac operates correctly. Such bleeder circuits are known, for instance NXP™ SSL 210x series of LED lighting controllers. Since such a bleeder circuit does not directly contribute to the luminous output of the LEDs lighting, it represents a source of loss; it is known to disable the bleeder during part of the mains phase when it is not required. 
         [0007]    It will be appreciated that, although mains lighting is one example of an application which can be used with a phase cut dimmer, the invention is not limited thereto, but extends to other applications operable with a leading-edge phase cut dimmer, such as, for instance cooling fans. 
       SUMMARY OF THE INVENTION 
       [0008]    According to a first aspect there is provided a bleeder arrangement for a phase-cut circuit for a high-impedance load and having a leading-edge phase-cut device, the bleeder arrangement comprising: a controllable current sink adapted to sink a latching current through the leading-edge phase-cut device, and a controller for controlling the controllable current sink, wherein the controller is configured to disable the current sink after the leading-edge phase phase-cut device has latched in at least two stages. The leading-edge phase-cut device is latched by means of the latching current. Such an arrangement can provide an improved bleeder functionality, which may suffer to a lesser extent, or not at all, from inductive oscillations upon switch-off. 
         [0009]    In embodiments, the controller is configured to start to disable the current sink between 100 μs and 200 μs following the leading edge. This may provide sufficient time for the triac to settle and latch into its on-state. 
         [0010]    In embodiments, the at least two stages is two stages, and the first stage comprises an immediate reduction in current through the current sink to an intermediate level and the second stage comprises a gradual reduction in the current through the current sink to zero. The intermediate level is between 40% and 60% of the latching current. 
         [0011]    In embodiments, the gradual reduction is a linear reduction lasting between 100 μs and 300 μs. This may provide a particular simple arrangement to implement within a controller the range 100 μs to 300 μs is not exhaustive, and the reduction may last for a different time; in general this may depend on the value of the current initially. In other embodiments, the gradual reduction is an exponential reduction, and the current falls to less than 10% of the intermediate level over a time, which is for instance between 100 μs and 300 μs. 
         [0012]    According to another aspect there is provided a controller configured to be used as the controller comprised in a bleeder arrangement as described above, and according to a further aspect there is provided a lighting circuit comprising a bleeder arrangement as claimed in any previous claim, wherein the high impedance load is a lighting unit. 
         [0013]    According to a yet further aspect there is provided a method of controlling a bleeder for a phase-cut circuit for a high-impedance load and having a leading-edge phase-cut device, the method comprising sinking a controllable latching sink current through the leading-edge phase-cut device, and disabling the sink current after the leading-edge phase phase-cut device has latched in at least two stages. 
         [0014]    In embodiments, disabling the current sink is started between  100  μs and  200  μs following the leading edge. 
         [0015]    In embodiments, the at least two stages is two stages. In embodiments, the first of the two stages comprises reducing the sinking current to an intermediate level. In embodiments, the second stage is a gradual reduction from the intermediate level to zero. 
         [0016]    These and other aspects of the invention will be apparent from, and elucidated with reference to, the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]    Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which 
           [0018]      FIG. 1  shows voltages and required current for a leading-edge phase cut dimmer over mains phases; 
           [0019]      FIG. 2  is a schematic circuit diagram of a bleeder arrangement in a lighting circuit; 
           [0020]      FIG. 3  shows experimental results according to known bleeder circuits; 
           [0021]      FIG. 4  shows experimental results according to embodiments; 
           [0022]      FIG. 5  shows voltages and currents according to embodiments; and 
           [0023]      FIG. 6  is an illustrative schematic showing a controllable bleeder circuit. 
       
    
    
       [0024]    It should be noted that the Figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these Figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar feature in modified and different embodiments 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0025]      FIG. 2  shows a schematic circuit diagram of a bleeder arrangement in a lighting circuit. The circuit  200  shows a phase-cut mains input  205  which is rectified by rectifier  210 . The rectified voltage is converted by means of a switched mode power converter (SMPC)  220  to provide the DC current supply to lighting unit  230 . The SMPC is controlled by controller  240 . Controller  240  also controls the bleeder circuit  250 . The bleeder circuit  250  comprises a pair of resistors  261  and  262 , and a bipolar transistor T 1 . The gate of transistor T 1  is controlled by the controller such that the bleeder circuit comprises a controllable current sink. 
         [0026]    In conventional bleeder circuits, the bleeder is enabled during a first part of the phase. Before the triac latches on, the output voltage is low, and thus the bleeder current is correspondingly low; however, it is sufficient to establish the synchronisation current  132 . When the leading-edge is reached, the triac switches on, the output voltage rises, and the bleeder provides a higher current—the latch current. In practice, typically the latching current is set in advance of the leading edge, as shown in  FIG. 2 ; however, during the initial phase cut period the voltage across the current is low, so the current source is saturated and the current is limited, so that the full latching current only flows once the leading edge is reached. Once the triac has latched on, the latch current  134  is no longer required, and only a lower hold current  136  is needed. This hold current may be supplied by the LED lighting application itself, or may be supplied by a second, low-current bleeder, also known as a weak bleeder which remains permanently on. The bleeder may thus be switched off, once the triac has latched on. 
         [0027]    This is illustrated in  FIG. 3 , which shows experimental results according to known bleeder circuits. The bleeder current is shown at trace  310 , and the triac output current at  320 . The triac output voltage is shown at  330 . In this instance oscillations are shown at  325  due to the switching off of the bleeder current at  315 , after the leading edge  322 , following a fixed delay D. As shown, the oscillations do not quite reach zero, and so the operation of the triac, and in particular its latching, is not interrupted. However, it will be appreciated that depending on the specific circuit, the oscillations  325  may cause the current to fall to or below zero in which case the triac may become unlatched and switch off. 
         [0028]    In order to avoid the above problems, according to embodiments, the bleeder is switched off in at least two stages. This is illustrated in  FIG. 4  which shows experimental results according to embodiments. The bleeder current is shown at trace  410 , and the triac output current at  420 . The triac output voltage is shown at  430 . In this case, after the delay D, the bleeder current is not completely switched off at  425  but set to a lower value V; it is then gradually decreased as shown at  426  towards zero. (As shown on both  FIGS. 3 and 4 , at  317  and  417  respectively, in this implementation, once the bleeder current has reached zero, it gradually rises due to the fact that the current sunk by the converter is not strong enough to provide the holding current. A weak, or low current, bleeder is then activated to maintain the holding current. In other embodiments which sink more current, this weak bleeder could stay off). 
         [0029]      FIG. 5  shows voltages and currents according to embodiments, in order to achieve the experimental results shown in  FIG. 4 . The figure shows the same leading-edge dimmed voltage shape  120  as shown in  FIG. 1  with phase cut first part  122  followed by the rise at edge  124 . Curve  510  indicates when the enable bleeder is enabled and curve  520  shows a soft-stop enable signal. As can be seen, the bleeder is enabled from the start of the phase until a specific moment  530 , which will be considered in more detail below. 
         [0030]    At the end of a delay following the rising edge, a soft-stop enable function  520  is turned on. The delay may typically be between 100 and 300 μs although values outside this range are not excluded, and in particular may be 150 μs. Such a delay is helpful to ensure a good ignition of the triac and to ensure that the device has properly latched on. The soft stop enable signal  520  initiates the soft stop function whereby the bleeder current is reduced to 0 but at the same time oscillations due to a sudden turn off are prevented. The soft-stop is a multistage function. In the example shown on  FIG. 5  the soft-stop has two stages: the first stage occurs immediately soft-stop  520  is enabled, and this comprises a reduction in the bleeder current from its maximum to an intermediate value. In the example shown in  FIG. 5  the intermediate value is 50% of the maximum value I_bleed_Max, that is to say the intermediate value is 0.5* I_bleed_Max. By immediately reducing the bleeder current to an intermediate value, which may typically be between 40% and 60% of its maximum value, the losses associated with the bleeder current are kept low. Thereafter, in later stages of the soft stop, the bleeder current is gradually reduced to zero. In the example shown in  FIG. 5 , this is a single second stage, during which the current is linearly reduced from its intermediate value to zero. This provides for a particularly simple implementation, which is conveniently designed into the controller. However, there is no limitation to a linear reduction, and in other embodiments, other forms of reduction may be used: for example, an exponential reduction in the bleeder current may be provided or a part-exponential followed by a simple switch-off. Conveniently, the control is arranged so the specific moment  530  at the end of the bleeder enablement is coincident with the end of the soft-stop  520 . 
         [0031]      FIG. 6  is an illustrative schematic showing a controllable bleeder circuit. The circuit shows an IC  610 , which may comprise the controller  240 , which includes a digital control block  620 . Digital output from digital control block  620  is converted into an analog signal by digital-to-analog converter (DAC)  630 . The analogue signal is used to control the gate of bipolar transistor T 1  which forms part of the bleeder circuit  250 . It will be appreciated that, although the controller shown in  FIG. 6  is digital, the invention is not limited thereto, and the soft-stop functionality could be provided by means of analog circuitry and signals. 
         [0032]    It will be appreciated that although embodiments have been described with respect to the phase of the mains, the invention is not limited to mains NC supplies, but extends to other A/C supplies, such as an NC supply produced by an inverter for non-mains connected equipment and installations. 
         [0033]    From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of phase-cut dimmer-controlled circuits, and which may be used instead of, or in addition to, features already described herein. 
         [0034]    Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. 
         [0035]    Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. 
         [0036]    The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom. 
         [0037]    For the sake of completeness it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, a single processor or other unit may fulfil the functions of several means recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims.