Abstract:
A step voltage controlling device and method for High Intensity Discharge (HID) light dimming applications includes an autotransformer, voltage sensing block, current sensing block, pulse forming block, microprocessor control unit (MCU) block and plural three switch blocks which allows an instantaneous variation of the voltage applied to the load which is controlled to be synchronized with the load current zero crossing point.

Description:
FIELD OF THE INVENTION  
       [0001]     This disclosure relates to alternating current voltage controllers, more particularly to controllers for varying the AC voltage applied to a large variety of lighting loads, including, but not limited to all types of gas discharge lamps as well as incandescent lamps. This application claims priority under the Paris Convention to U.S. Provisional Application for Patent Ser. No. 60/481,804 filed on Dec. 17, 2004, the entire contents of which are incorporated herein by reference.  
       BACKGROUND  
       [0002]     A number of electronic high frequency switching AC voltage controllers have been devised for delivering a variable sinusoidal output voltage. U.S. Pat. No. 5,018,058 discloses how to obtain a pure sinusoidal output voltage waveform from the AC line voltage using switchmode electronic circuitry. Furthermore, U.S. Pat. Nos. 5,500,575 and 5,714,847 show other methods of obtaining a variable AC output voltage having a sinusoidal waveform. Although existing electronic high frequency switchmode AC voltage controller have several major advantages, such as a small size and weight and a relatively low manufacturing cost, their main disadvantage is the lack of capacity to correct the output voltage waveform when a heavy inductive and non-linear load, such as an electromagnetic ballast and a HID lamp are present. The result is a relatively large distortion of the output voltage that decreases the overall system efficiency.  
       BRIEF SUMMARY  
       [0003]     The above described current/voltage distortions are at least reduced, if not eliminated by various embodiments of the invention by continuously magnetically coupling a version of the AC line voltage to the output voltage.  
         [0004]     Although adjusting the output voltage in steps may be generally known for some applications, there are a number of loads, such as a High Pressure Sodium (HPS) Ballast, that do not accept this type of voltage control. Indeed, a HPS lamp will simply extinguish if the applied voltage sharply decreases. Embodiments of the invention described herein eliminates this inconvenience by providing a novel voltage step control algorithm, among other features.  
         [0005]     In various embodiments of the invention, a step sinusoidal voltage controlling device and method for HID, fluorescent and incandescent light dimming applications includes use of a multiple tap step down autotransformer, multiple switch block, voltage sensing block, current sensing block, pulse forming block, and microprocessor control unit (MCU) block. In this context, step voltage-controlling means controlling an instantaneous variation of the voltage applied to the load that is synchronized with the load current zero crossing point. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  represents the basic block diagram of the step sinusoidal voltage controlling device for HID, fluorescent and incandescent light dimming applications, for example;  
         [0007]      FIG. 2  represents the internal structure of the voltage sensing block  1 , current sensing block  2  and pulse forming block  3 ;  
         [0008]      FIG. 3  describes the internal structure of the three identical switch blocks  5 ,  6  and  7 , respectively;  
         [0009]      FIG. 4  describes the automatic elimination of the effect caused by the temperature coefficient of comparators  301  and/or  302  contained by the pulse forming block  3 ;  
         [0010]      FIG. 5  describes the how the MCU block  4  automatically determines the ideal moment for a step voltage change based on the voltage/current trend evaluation; and  
         [0011]      FIG. 6  shows in detail die step voltage variation algorithm for avoiding any load anomaly, such as a HPS lamp being extinguished by the instantaneous change of the applied voltage. 
     
    
     DETAILED DESCRIPTION  
       [0012]     An embodiment of the invention is illustrated in  FIG. 1 , and this device includes a multiple tap step-down autotransformer  8 , voltage sensing block  1 , current sensing block  2 , pulse forming block  3 , MCU block  4  and switch blocks  5 ,  6  and  7 . As this embodiment is energized, MCU block  4  generates a command signal on line  60  for turning on switch block  5 . This means that the step voltage controlling device always starts at full output voltage measured between lines  11  and  21 , a voltage that is equal to the voltage measured between lines  10  and  20 . It is generally known that a HID ballast must be first energized at its nominal voltage, and it must continue to operate at the nominal voltage until it reaches its normal operating parameters. This period of time when the HID ballast must first operate at nominal voltage is known as “lamp warm-up time”.  
         [0013]      FIG. 5  indicates the voltage/current phase angle for a HID ballast. When the ballast is first energized, the voltage current phase angle has a relatively high negative value. As the lamp warms-up, the phase angle decreases. When the phase angle stabilizes, it means that the ballast has reached its normal operating parameters, and therefore the ballast is capable to support an instantaneous variation of the applied voltage on line  11 , from the level on line  10  to the level on line  30 . MCU block  4  will remove the command signal on line  60 , therefore turning off the switch block  5 . The sequence to turn off switch block  5  is associated with a command signal generated on line  61  by MCU block  4  for turning on switch block  6 . After this instantaneous applied voltage variation, the voltage/current phase angle sharply deteriorates; it drops at a low and stable value as the lamp is re-adjusting its temperature, as determined by the lower level of voltage on line  30 . After the phase angle becomes stable, the ballast is, again, capable to support a new instantaneous variation of the applied voltage, from the level on line  30  to the level on line  40 .  
         [0014]     MCU block  4  will remove the command signal on line  61 , therefore turning off the switch block  6 . Turning off the switch block  6  sequence is associated with a command signal generated on line  62  by the MCU block  4  for turning on switch block  7 .  
         [0015]      FIG. 2  describes the voltage sensing block  1 , current sensing block  2  and pulse forming block  3 . Resistors  101  and  102  perform the voltage divider function for delivering to pulse forming block  3  of a low amplitude signal via line  50 . The current sensing block  2  includes current sensing resistor  201  but, alternatively, a current transformer may be-also used, without departing from the spirit of the inventive concept.  
         [0016]     A signal equivalent to the ballast current on lines  10 ,  11 ,  21  and  20  is applied to the pulse forming block  3  by the current sensing block  2  via line  22 . Pulse forming block  3  contains the two comparators  301  and  302  for converting the positive half cycles of signals on lines  20  and  22  in square wave pulses to be delivered to the MCU block  4  via lines  51  and  52 .  
         [0017]      FIG. 3  describes each of the switch blocks  5 ,  6  or  7 . It consists of a power triac Tr, a triac conventional snubber network consisting of resistor R and capacitor C, and a metal oxide varistor MOV for absorbing any possible damaging high voltage transients. For safe interface with the MCU block  4  via line  60 ,  61  or  62 , the triac Tr gate is energized by an optically isolated triac driver Tr Drv. There are many available triac gate driver opto-couplers available on the market; their use is known in the art, and is not described herein.  
         [0018]      FIG. 4  shows the automatic compensation of an eventual input offset voltage drift caused by its inherent temperature coefficient. Graph ( 1 ) shows the AC line voltage AC IN on lines  10  and  20  equivalent signal on line  50 . Graph ( 2 ) shows the ballast current on lines  10 ,  11 ,  21  and  20  equivalent signal on line  22 . It is assumed that neither of comparators  301  and  302  of the pulse forming block  3  have any input voltage offsets. Graph ( 3 ) and graph ( 4 ) show the pulses on lines  51  and  52 . There are two methods to determine the phase of the pulses on lines  51  and  52 . The first method is to compare the time interval between the fronts (rising front) of pulses on lines  51  and  52 , while the second method is to compare the time interval between the middle of pulses on lines  51  and  52 . The middle of each pulse on lines  51  and  52  is determined by the MCU block  4  by dividing in half the P and R number of externally generated high frequency pulses counted during the duration of each pulse on lines  51  and  52 . The middle so determined of the pulses on lines  51  and  52  should be named “medians”. Indeed, as all graphs ( 1 ), ( 2 ), ( 3 ) and ( 4 ) show, in the absence of any input voltage offset of comparators  301  and  302  contained by the pulse forming block  3 , both methods are error free. However, as graphs ( 5 ), ( 6 ), ( 7 ) and ( 8 ) indicate, when one of the comparators  301  and  302  contained by the pulse forming block  3  is experiencing a non-zero input offset voltage, the only method that eliminates any voltage/current phase angle measurement requirement is when measuring the time interval between the medians of pulses on lines  51  and  52 , rather than when measuring the time interval between the fronts of these pulses on lines  51  and  52 .  
         [0019]      FIG. 6  describes in detail the mechanism of instantaneous voltage variation applied to a ballast for avoiding extinguishing the lamp. Assuming that all conditions described in  FIG. 5  for the instantaneous voltage changes are met, the dimming sequence is now described. At the current median described in  FIG. 4 , MCU block  4  is removing the command signal on line  60 . It is known that a triac continues to conduct current until the current value becomes zero; this is happening at the next current zero crossing. Based on this well-known triac property, the current via switch block  5 , therefore the current through the ballast on lines  10 ,  11 ,  21  and  20  becomes zero. When the ballast current becomes zero, the MCU block  4  instantaneously generates a command signal on line  61 , thereafter turning on switch block  6 . As a result, the ballast current through lines  10 ,  11 ,  21  and  20  continues to flow, following a non-discontinued sinusoidal waveform. Next, MCU block  4  decides to once more reduce the ballast-applied voltage. It will then remove the command signal on line  61  at the ballast current median. At the next ballast current zero crossing, switch block  6  turns off. During this ballast current zero crossing, MCU block  4  generates a command signal on line  62 , turning on switch block  7 .  
         [0020]     The ballast-applied voltage could suffer instantaneous variations in reverse order, in two increments, or by simply applying the voltage on line  10  in one step, if the algorithms described in  FIG. 5  and  FIG. 6  are observed by MCU block  4 .