Abstract:
A controller for reducing acoustic noise produced during use of a leading edge dimmer, includes a leading edge controller responsive to an input voltage fed thereto for producing a control signal upon detection of a leading edge, and a linear switch coupled to the leading edge controller and responsive to the control signal for linearly switching the input voltage so that a rate of rise of the leading edge is decreased. A trailing-edge controller may be coupled to a leading-trailing edge detector and responsive to detection of a trailing edge dimmer for disabling the leading edge controller and decreasing a rate of decline of the trailing edge of the input voltage.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to noise detection and reduction when electronic converters/transformers or magnetic transformers are used together with lamp dimmers.  
         BACKGROUND OF THE INVENTION  
         [0002]    As is known lamp dimers reduce the power fed to a lamp by reducing the average voltage applied to the lamp. To this end, two types of dimmer are known: leading edge dimmers that switch the voltage on after the zero crossing point in the AC cycle; and trailing edge dimmers that switch the voltage off before the zero crossing point in the AC cycle. Typically, lamp dimmers employ very fast triacs or thyristors such that the rate of switch-on in a leading edge dimmer and the rate of switch-off in a trailing edge dimmer is substantially instantaneous, typically 15-25 μs.  
           [0003]    One of the known problems with dimmers, be they trailing or leading edge types, is that they may give rise to acoustic noise, and this noise is exacerbated when the dimmer is used in conjunction with a transformer, particularly magnetic transformers or electronic converters/transformers providing DC output. The noise may be at least partially caused by the vibration of the ferromagnetic laminates forming the core of the choke/s (in dimmer and transformer) and/or the vibration of certain capacitors. It is also known that the actual lamp filaments are caused to vibrate, and this is the case particularly with DC output electronic converters/-transformers, giving rise to an additional source of acoustic noise or hum. This noise/hum is disturbing and unpleasant and it is obviously desirable to reduce it as much as possible.  
           [0004]    There exist both magnetic and electronic transformers on the market. Electronic transformers (or “converters”) may be AC or DC (input and) output units. In the case of DC output transformers, a chopper is used to convert the DC voltage to a pulsating voltage, which may then be transformed to a different voltage, and ultimately rectified and smoothed. Regardless of the dimming technology used, there is always inherent noise associated with a dimmer/transformer combination that cannot be avoided, and the problem is particularly acute with DC output transformers.  
           [0005]    Prior art approaches require additional components to be installed in the dimmer itself in order to reduce noise. Thus, in the case of leading-edge dimmers, the prior art uses a de-buzzing coil (choke), which is bulky, expensive, reduces efficiency and must be tuned to the dimmer circuitry. Coil selection can be a time-consuming task.  
           [0006]    For trailing edge dimmers, designed for use with electronic transformers/-converters, the conventional noise reduction solution is to add a large capacitor for reducing the rate of decline of the trailing edge. When used in conjunction with an electronic transformer/converter, the capacitor may be placed before or after the diode bridge. However, if such a modified transformer is used with a leading edge dimmer, it actually increases the noise. Thus, known electronic lamp transformers/-converters employing a capacitor as explained above are intended for use with trailing edge dimmers only.  
           [0007]    This means that it has not so far been possible to take a commercially available leading edge dimmer and use it in conjunction with a lamp transformer without avoiding noise, unless a “debuzzing coil” is selected and used. Conversely, common practice for noiseless dimming with the use of electronic transformers has been the use of electronic transformers incorporating a capacitor as described above, in conjunction with trailing edge dimmers.  
           [0008]    JP 6089784 published Mar. 29, 1994 describes a low noise dimmer that attempts to reduce noise by smoothing a supply voltage waveform by controlling rise and fall curves of the lamp voltage. To this end, prior to the supply voltage crossing to zero from the negative half cycle, a forward power switch element is turned on, and an output voltage is detected. When an effective voltage approaches a target value, a microcomputer enters a fall motion and gradually lowers the output voltage within a predetermined time to the zero line smoothly. Also, for the negative side voltage, a reverse side power switch element and the microcomputer perform the same operation so that a rise motion is smooth. Thereby, hum noise production is prevented without applying an electromagnetic surge to an incandescent lamp.  
           [0009]    This patent appears to relate to the known problem of switching a triac or thyristor during zero crossing of the AC voltage, since triacs are known to stop conducting when the current is zero and require a trigger signal to initiate conduction, such conduction being possible only if the anode voltage of the triac is larger than the cathode voltage thereof. Therefore, two switching devices are employed and are controlled to conduct during opposite halves of the AC cycle. The switching between the two thyristors itself causes noise and JP 6089784 appears to relate to a smoother mechanism for effecting the required switching so as to reduce the acoustic noise.  
           [0010]    U.S. Pat. No. 5,319,301 issued Jun. 7, 1994 to Callahan M. et al. discloses an inductor-less light dimmer with semiconductor power devices coupled between an alternating current supply and a lamp load. Undesirable effects of the high current demands of cold lamp filaments are reduced by initially increasing the conductive portion of half-cycles, relative to the proportion required to produce the desired amount of average power, while avoiding transitions at phase angles that would produce excessive losses. A transition shape may be employed in this mode and in normal operation that maximizes audible lamp noise suppression for a given level of thermal losses.  
           [0011]    WO 91/06047 published May 2, 1991 to Bayview Technology Group, Inc. and entitled “Reverse phase control switching circuit and method without zero crossing detection” discloses a method and circuit, in a dimmer, for reverse phase control of alternating current being delivered to a load wherein voltage-controlled semiconductor switches such as MOSFET&#39;s and IGBT&#39;s are used as electronic switches to conduct voltage during the leading edge of the AC voltage cycle and conduction is terminated when the desired phase angle of the current flow has been reached. The disclosed method and circuit eliminate the need for zero crossing detection of the AC waveform and ensure that the voltage-controlled switches are always turned on before the zero crossing thereby minimizing radiated interference and incandescent lamp hum.  
           [0012]    Both U.S. Pat. No. 5,319,301 and WO 91/06047 appear to relate to leading edge dimmers only, to a noise solution inside the dimmer itself only, and appear only to address problems associated with switching at zero crossing of the AC voltage so as to reduce lamp hum.  
           [0013]    It would therefore be desirable to provide a universal electronic lamp transformer that may be used with any commercially available dimmer, be it a leading edge or a trailing edge type, without requiring modification to the dimmer or the addition of external components in order to reduce noise, and also to provide a module, with the same dimming noise reduction effect, that may be added to conventional magnetic transformers.  
         SUMMARY OF THE INVENTION  
         [0014]    It is an object of the invention to provide a lamp transformer (or standalone module) and method that are suitable for use with both leading and trailing edge dimmers so as to reduce lamp hum.  
           [0015]    According to one aspect of the invention, there is provided a method for reducing acoustic noise produced during use of a dimmer, the method comprising:  
           [0016]    (a) detecting that a dimmer is present, and if so:  
           [0017]    i) detecting whether the dimmer is a leading edge dimmer or a trailing edge dimmer,  
           [0018]    ii) if the dimmer is a leading edge dimmer, reducing the rate of rise of the leading edge, and  
           [0019]    iii) if the dimmer is a trailing edge dimmer, reducing the rate of fall of the trailing edge.  
           [0020]    According to a further aspect of the invention, there is provided a controller for reducing acoustic noise produced during use of a leading edge dimmer, the controller comprising:  
           [0021]    a leading edge controller responsive to an input voltage fed thereto for producing a control signal upon detection of a leading edge, and  
           [0022]    a linear switch coupled to the leading edge controller and responsive to the control signal for linearly switching the input voltage so that a rate of rise of the leading edge is decreased.  
           [0023]    Preferably, such a controller is adapted for reducing acoustic noise produced during use of a trailing edge electronic dimmer, and further comprises:  
           [0024]    a leading-trailing edge detector responsive to an input voltage fed thereto for detecting whether the input voltage results from a leading edge dimmer or a trailing edge dimmer, and  
           [0025]    a trailing edge controller coupled to the leading-trailing edge detector and responsive to detection of a trailing edge dimmer for disabling the leading edge controller and decreasing a rate of decline of the trailing edge of the input voltage;  
           [0026]    said leading edge controller being coupled to the leading-trailing edge detector and responsive to detection of a leading edge dimmer for disabling the trailing edge controller. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:  
         [0028]    [0028]FIG. 1 is a block diagram showing a conventional prior art lamp transformer;  
         [0029]    [0029]FIGS. 2 a ,  2   b ,  2   c  and  2   d  show graphically typical voltage waveforms appearing at different stages in a conventional prior art lamp transformer using a leading edge dimmer;  
         [0030]    [0030]FIGS. 3 a ,  3   b ,  3   c  and  3   d  show corresponding voltage waveforms when a trailing edge dimmer is used;  
         [0031]    [0031]FIG. 4 is a block diagram showing a modified lamp transformer in accordance with a first embodiment of the invention for reducing noise in a leading edge dimmer;  
         [0032]    [0032]FIGS. 5 a ,  5   b ,  5   c ,  5   d  and  5   c  show voltage waveforms corresponding to those of FIGS. 2 and 3 for the lamp transformer shown in FIG. 4;  
         [0033]    [0033]FIG. 6 is a block diagram showing a modified lamp transformer in accordance with a second embodiment of the invention for reducing noise in a leading or trailing edge dimmer;  
         [0034]    [0034]FIGS. 7 a ,  7   b ,  7   c ,  7   d  and  7   e  show voltage waveforms corresponding to those of FIG. 5 for the lamp transformer shown in FIG. 6;  
         [0035]    [0035]FIG. 8 is a schematic circuit diagram of a leading/trailer edge controller that may be incorporated in a conventional prior art lamp transformer for reducing noise;  
         [0036]    [0036]FIGS. 9 a ,  9   b  and  9   c  show typical voltage waveforms associated with the lamp transformer according to the invention when used with a trailing edge dimmer; and  
         [0037]    [0037]FIGS. 10 a ,  10   b  and  10   c  show typical voltage waveforms associated with the lamp transformer according to the invention when used with a leading edge dimmer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]    [0038]FIG. 1 is a block diagram showing a conventional prior art DC output lamp transformer  10  comprising an input  11  for coupling to a mains electricity supply, typically 120 or 230 VAC. Connected to the input  11  is a dimmer  12  for reducing the RMS voltage. The dimmer  12  may be a leading edge or a trailing edge dimmer whose output is connected to an RFI filter  13  coupled to a bridge rectifier  14 , producing a rectified AC voltage that is fed to an inverter  15 . The inverter  15  includes an output transformer and optionally also a rectifier for rectifying the output voltage. Neither of these components is shown in the figure. The output of the inverter is thus AC (or DC) voltage typically in the range 0-30 V, suitable for powering low voltage tungsten halogen lamps and the like.  
         [0039]    [0039]FIG. 2 a  shows graphically the sinusoidal input voltage waveform. FIG. 2 b  shows graphically the voltage appearing at the output of the dimmer  12  when a leading edge dimmer is used, it being noted that the leading edges in both positive and negative half cycles are chopped. FIG. 2 c  shows graphically the voltage appearing at the output of the bridge rectifier  14  and FIG. 2 d  shows graphically the voltage appearing at the output of the inverter  15 . FIGS. 3 a ,  3   b ,  3   c  and  3   d  show corresponding voltage waveforms when the dimmer  12  is a trailing edge dimmer.  
         [0040]    [0040]FIG. 4 is a block diagram showing a modified lamp transformer  20  in accordance with a first embodiment of the invention for reducing noise in a leading edge dimmer. An input  21  is coupled to a mains electricity supply, typically 120 or 230 VAC. Connected to the input  21  is a leading edge dimmer  22  whose output is connected to an RFI filter  23 . The RFI filter  23  is connected to a bridge rectifier  24 , producing a rectified AC voltage that is fed via a leading edge controller  25  to a linear switch  26  connected to an inverter  27 . The output of the inverter is a DC voltage typically in the range 0-30 V DC, suitable for powering low voltage tungsten halogen lamps and the like.  
         [0041]    [0041]FIG. 5 a  shows graphically the sinusoidal input voltage waveform. FIG. 5 b  shows graphically the voltage appearing at the output of the dimmer  22 . Again, it will be noted that the leading edges in both positive and negative half cycles are chopped. FIG. 5 c  shows graphically the voltage appearing at the output of the bridge rectifier  14 . FIGS. 5 d  and  5   e  show graphically the voltages appearing at the output of the linear switch  26  and the inverter  27 , respectively. In particular, it will noted from the output of the inverter  27  that the leading edge climbs much more slowly than that of the conventional transformer, as depicted by the waveform shown in FIG. 2 d.    
         [0042]    [0042]FIG. 6 is a block diagram showing a modified lamp transformer  30  in accordance with a second embodiment of the invention for reducing noise in a leading or trailing edge dimmer. An input  31  is coupled to a mains electricity supply, typically 110 or 220 VAC. Connected to the input  31  is a dimmer  32  whose output is connected to an RFI filter  33 . The dimmer  32  may be leading edge or a trailing edge device and, as will now be explained, suitable control circuitry is employed to control the leading edge and trailing edge, as required. The RFI filter  33  is connected to a bridge rectifier  34 , producing a rectified AC voltage that is fed via a leading edge controller  35  to a linear switch  36  connected to an inverter  37  via a switch capacitor  38 . The output of the inverter is a DC voltage typically in the range 0-30 V DC, suitable for powering low-voltage tungsten halogen lamps and the like. The output of the bridge rectifier  34  is also coupled via a leading-trailing edge detector  39  to a switch capacitor controller  40 .  
         [0043]    The leading edge controller  35  is responsive to a leading edge being detected by the edge detector  39  for operating in conjunction with the linear switch  36  as shown in FIG. 4 for slowing down the rate of rise of the leading edge as shown in the voltage waveforms depicted in FIGS. 5 d  and  5   e . When the edge detector  39  detects a trailing edge, the switch capacitor controller  40  operates in conjunction with the switch capacitor  38  for slowing down the rate of decline of the trailing edge.  
         [0044]    [0044]FIG. 7 a  shows graphically the sinusoidal input voltage waveform. FIG. 7 b  shows graphically the voltage appearing at the output of the dimmer  32 . Again, it will be noted that the leading edges in both positive and negative half cycles are chopped. FIG. 7 c  shows graphically the voltage appearing at the output of the bridge rectifier  14 . FIGS. 7 d  and  7   e  show graphically the voltages appearing at the output of the linear switch  36  and the inverter  37 , respectively. In particular, it will noted from the output of the inverter  37  that the trailing edge declines much more slowly than that of the conventional transformer, as depicted by the waveform shown in FIG. 3 d.    
         [0045]    [0045]FIG. 8 is a schematic circuit diagram of a leading/trailer edge controller  50  that may be incorporated in a conventional prior art lamp transformer  51  for reducing noise. Thus, the lamp transformer  51  comprises an input  52  for coupling to a mains electricity supply, typically 120 or 230 VAC. Connected to the input  52  is a dimmer  53  for reducing the average voltage. The dimmer  53  may be a leading edge or a trailing edge dimmer whose output is connected to an RFI filter  54  coupled to a bridge rectifier  55 , producing a rectified AC voltage that is fed to an inverter  56  via the controller  50 . The output of the inverter  56  is a DC voltage typically in the range 0-30 V DC, suitable for powering low-voltage tungsten halogen lamps and the like.  
         [0046]    The controller  50  comprises an inductor (choke)  60  connected at one end to the output of the bridge rectifier  55  and connected at its opposite end to the drain of a first N-channel-type MOSFET  61  whose gate is connected via a resistor  62  to the positive terminal of a DC voltage source  63 . The junction of the gate of the MOSFET  61  and the resistor  62  is connected to the anode of a first rectifier diode  64  whose cathode is connected to the anode of a second rectifier diode  65  whose cathode is connected to the positive terminal of the DC voltage source  63 . The junction between the rectifier diodes  64  and  65  is connected to a first end of a capacitor  66  whose second end is connected to the anode of a zener diode  67  in parallel with a resistor  68 . The cathode of the zener diode  67  is connected to GND. The second end of the capacitor  66  is also connected to the cathode of a rectifier diode  69  whose anode is connected via a resistor  70  to the input of a voltage divider comprising a first resistor  71  coupled to GND and a second resistor  72 , and across which is connected a DC voltage source  73  whose negative terminal is connected to GND. The junction of the voltage divider resistors  71  and  72  is connected to the base of an NPN bipolar junction transistor  74  whose emitter is connected to GND and whose collector is connected via a resistor  75  to the positive terminal of the DC voltage source  73 . A decoupling capacitor  76  is connected between the collector and the emitter of the bipolar junction transistor  74 .  
         [0047]    The junction between the anode of the rectifier diode  69  and the resistor  70  is connected via a capacitor  77  to GND. The collector of the bipolar junction transistor  74  is connected to the gate of a second N-channel-type MOSFET  78  whose source is connected to GND and whose drain is connected to a first end of a capacitor  79  (corresponding to the switch capacitor  38  in FIG. 6) whose second end is connected to the source of the first N-channel-type MOSFET  61  and constitutes also an output  80  of the controller  50  that is connected to the inverter  56 . A capacitor  81  is connected across the output  80  and GND.  
         [0048]    Operation of the circuit is as follows. When a leading edge dimmer is detected, the MOSFET  61 , resistor  62 , voltage source  63 , rectifier diode  64  and capacitor  66  function as the linear switch  36  (in FIG. 6). Whenever there is a positive change in the output voltage of the dimmer, the linear switch brings the output voltage linearly up to its peak value in about 500 μsec. The MOSFET  61  operates as a voltage follower and the voltage source  63  follows the gate voltage of the MOSFET  61 , which changes linearly since a constant current flows through the resistor  62  and the rectifier diode  64  for charging the capacitor  66 , thus forcing the voltage across the capacitor  66  to rise linearly. The current magnitude is determined by the values of the resistor  62 , the voltage source  63  and the threshold voltage of the MOSFET  61 . The rectifier diode  65  serves to discharge the capacitor  66  at the end of each cycle.  
         [0049]    The resistor  68 , rectifier diode  69  and capacitor  77  function as the trailing edge detector  39  (in FIG. 8). The voltage across the capacitor  77  is proportional to the negative slope of the dimmer: the higher the slope, the higher is the magnitude of the negative voltage. The resistors  70 ,  71 ,  72  and  75 , the bipolar junction transistor  74 , the capacitor  76 , the MOSFET  78  and the DC voltage source  73  serve as a controller to the switch capacitor  79 . When the voltage across the capacitor  77  is sufficiently negative, the bipolar junction transistor  74  cuts off and the MOSFET  78  starts to conduct. When no dimmer is present, the MOSFET  78  is cutoff. The MOSFET  78  thus serves as a switch capacitor control switch for switching the switch capacitor  79  on and off. When the switch capacitor  79  is switched on, the negative slope is decreased since the discharge time is longer.  
         [0050]    In an actual embodiment of the controller reduced to practice, values and types of the components were as follows:  
                                                   Component   Type/Value                           60    2 mH           61   IRF740           62    2 kΩ           63    15 VDC           64, 65   D1N4148           66   6nF           67   D1N750           68    10 kΩ           69   D1N4148           70, 72   300 kΩ           71    33 kΩ           73    10 VDC           74, 76, 77, 79    1 μF           75   100 kΩ           78   IRF470           81   220 nF                      
 
         [0051]    [0051]FIGS. 9 a ,  9   b  and  9   c  depict typical voltage waveforms associated with the lamp transformer according to the invention when used with a trailing edge dimmer. FIG. 9 a  shows the voltages at the input to the controller  50 , referenced VIN in FIG. 8 and the source voltage of the MOSFET  61 , referenced V S  in FIG. 8. Thus, it is seen that where the input voltage falls abruptly, the source voltage of the MOSFET  61  declines much less sharply, taking about 500 μs before reaching zero.  
         [0052]    [0052]FIG. 9 b  shows the output voltage of the inverter  56  and follows the source voltage of the MOSFET  61 .  
         [0053]    [0053]FIG. 9 c  depicts the voltage V CAP  across the switch capacitor  79  when the MOSFET  78  is conducting, which it does only when a trailing edge dimmer is detected. When either no dimmer or a leading edge dimmer is detected, the MOSFET  78  is “OFF” and the switch capacitor  79  is disconnected.  
         [0054]    [0054]FIGS. 10 a ,  10   b  and  10   c  depict typical voltage waveforms associated with the lamp transformer according to the invention when used with a leading edge dimmer. FIG. 10 a  shows the voltages at the input to the controller  50 , referenced V IN  in FIG. 8 and the source voltage of the MOSFET  61 , referenced V S  in FIG. 8. Thus, it is seen that where the input voltage climbs abruptly, the source voltage of the MOSFET  61  climbs much less sharply, taking about 500 μs before reaching its maximum value.  
         [0055]    [0055]FIG. 10 b  shows the output voltage of the inverter  56  and follows the source voltage of the MOSFET  61 .  
         [0056]    [0056]FIG. 10 c  depicts the difference between the gate voltage V G  and the source voltage V S  of the MOSFET  61  when a leading edge dimmer is detected. V G −V S  remains constant until the leading edge is detected, whereupon it falls to the threshold voltage V T  of the MOSFET (typically about 3.5V). This corresponds to the linear region of the MOSFET, which thus operates as a linear switch as explained above.  
         [0057]    It will be appreciated that the circuit described above and the specimen and component values as tabulated are by way of illustration only and are not intended to limit the scope of the attached claims.  
         [0058]    It will also be appreciated that while the invention has been described with particular reference to a controller that may be used with any off-the-shelf electronic transformer as described, the invention also contemplates within its scope an electronic transformer having the controller integral therewith.