Abstract:
A power supply for use in a UV curing lamp assembly is disclosed. The power supply is powered by two intermediate frequency (200-400 Hz) low voltage sinusoidal power sources that drive the primary windings of a dual laminated transformer. The low voltage sinusoidal power sources are configured to have different phases. The out-of-phase low voltage sine wave sources are converted to high voltage sine waves on the secondary windings of the dual laminated transformer having the same phase difference relationship. A single rectifier comprising six high voltage diodes, called a ladder rectifier, combine the two out-of-phase sine waves into a single, approximately DC output power source. By modulating a phase difference between two input sine wave power sources, the approximate DC output voltage exiting the ladder rectifier may be alternated between a low ripple mode of about a 13.84% ripple, a high current mode, a high voltage mode, and an intermediate mode with a ripple in the range of about 13.84% to about 100%.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional patent application No. 61/367,483 filed Jul. 26, 2010, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates generally to high voltage power supplies, and more particularly, to a dual transformer and ladder rectifier power supply for powering a magnetron in ultraviolet radiation (UV) curing lamp assemblies. 
       BACKGROUND OF THE INVENTION 
       [0003]    Radiant energy is used in a variety of manufacturing processes to treat surfaces, films, and coatings applied to a wide range of materials. Specific processes include, but are not limited to, curing (i.e., fixing, polymerization), oxidation, purification, and disinfection. Processes using radiant energy to polymerize or effect a desired chemical change is rapid and often less expensive in comparison to a thermal treatment. The radiation can also be localized to control surface processes and allow preferential curing only where the radiation is applied. Curing can also be localized within the coating or thin film to interfacial regions or in the bulk of the coating or thin film. Control of the curing process is achieved through selection of the radiation source type, physical properties (for example, spectral characteristics), spatial and temporal variation of the radiation, and curing chemistry (for example, coating composition). 
         [0004]    A variety of radiation sources are used for curing, fixing, polymerization, oxidation, purification, or disinfections due to a variety of applications. Examples of such sources include, but are not limited to, photon, electron, or ion beam sources. Typical photon sources include, but are not limited to, arc lamps, incandescent lamps, electrodeless lamps and a variety of electronic (i.e., lasers) and solid-state sources. 
         [0005]    An apparatus for irradiating a surface with ultraviolet light includes a lamp (e.g., a modular lamp, such as a microwave-powered lamp having a microwave-powered bulb (e.g., tubular bulb) with no electrodes or glass-to-metal seals), the lamp having reflectors to direct light (photons) on to the surface. The source of microwave power is conventionally a magnetron, the same source of microwaves typically found in microwave ovens. The microwave-powered bulb typically receives microwaves generated by the magnetron through an intervening waveguide. 
         [0006]    Conventional power supplies for magnetrons include a variety of designs. A typical design used for powering microwave ovens includes a one step-up resonant laminated transformer, a high voltage diode, and a high voltage capacitor. The transformer/capacitor combination takes a 50 Hz/60 Hz line voltage and outputs a 50/60 Hz half wave pulsed DC voltage or a 100% ripple DC voltage. It has the advantage of low cost, but includes the disadvantages of being large and heavy with a single level of output power. 
         [0007]    A second design employs a silicon-controlled rectifier (SCR) to control an amount of phase of an input power sine waveform that may be applied to a laminated transformer. The output windings of the laminated transformer steps up the input voltage which is applied to a full diode bridge. The output is a 50 HZ/60 Hz full wave rectified pulsed DC voltage or 100% ripple DC voltage. 
         [0008]    A third possible design is a switching mode power supply which provides a high power DC voltage with low ripple. Conventional high voltage, switching mode power supplies suffer from a number of problems. Because of a high working frequency (&gt;20 KHz), a high frequency, high power single output winding ferrit transformer is needed, along with a small number of high voltage, fast recovery diodes arranged in a diode bridge. The small number of high power, high frequency diodes dissipate a large amount of power. As a result, it is necessary to employ a ferrit transformer with multiple secondary windings coupled to a large number of diode bridges, each comprising 2 or 4 lower voltage diodes as shown in  FIG. 1 . 
         [0009]    Referring now to  FIG. 1 , a portion of a high voltage switching mode DC power supply  10  includes an AC pulsed input source  12  feeding a primary winding of a multiple output winding laminated transformer  14 . The multiple output windings  16   a - 16   l  feed a plurality of full-wave rectified diode bridge circuits  18   a - 18   l  (also labeled DB 1 -DB 12 ) requiring a total of 64 diodes. A rippled approximate DC output voltage is smoothed and high frequency components from the switching power supply are removed by a plurality of filter circuits  20   a - 20   l  each comprising at least a capacitor and an inductor, labeled C 1 -C 12  (references  22   a - 22   l ) and L 1 -L 12  (references  24   a - 24   l ), respectively. 
         [0010]    Since there is typically a long cable between a power supply and a magnetron in a UV curring lamp assembly, the outputs of the secondary windings  16   a - 16   l  of the multiple winding transformer  14  include a high level of high frequency components. For the power supply  10  to drive a magnetron with low frequency DC power with a long transmission cable (not shown), it is necessary to employ a large number of inductors  24   a - 24   l  and capacitors  22   a - 22   l , as well as 12 RC snubbers (not shown) employed as filters to remove high frequency components. Thus, a large number of diodes, inductors and capacitors need to be employed, which is expensive, consumes a large amount of board space, and reduces reliability. 
         [0011]    Accordingly, what would be desirable, but has not yet been provided, is an inexpensive high voltage and power output DC power supply having a low component count. 
       SUMMARY OF THE INVENTION 
       [0012]    The above-described problems are addressed and a technical solution achieved in the art by providing a high voltage, high power output power supply for driving a magnetron in a UV curing lamp assembly. The high voltage, high power output power supply includes two intermediate frequency (200-400 Hz) low voltage sinusoidal power sources that are configured to drive the primary windings of a dual laminated transformer. The low voltage sinusoidal power sources are configured to have different phases. The out-of-phase low voltage sine wave sources are converted to high voltage sine waves on the secondary windings of the dual laminated transformer having the same phase difference relationship. A single rectifier comprising six high voltage diodes, called a ladder rectifier, combines the two out-of-phase sine waves into a single, approximately DC output signal. 
         [0013]    The ladder rectifier rectifies the two sine wave AC output sources into one of various modes of DC power, which range from high output voltage to high output current depending on a predetermined phase difference between the two input sine wave sources. The approximate DC output signal exiting the ladder rectifier contains a ripple with intermediate frequencies, which cover the spectrum range of 400 Hz to 6.4 KHz. As a result, no filtering inductors or capacitors are needed following the ladder rectifier, thereby providing a low cost, low component count solution for driving a magnetron in a UV curing lamp assembly. The circuit is operable to supply high voltage, high power over a long cable between the power supply and the magnetron. 
         [0014]    By modulating a phase difference between two input sine wave power sources, the approximate DC output signal exiting the ladder rectifier may be alternated between a number of output modes: (1) a low ripple mode having an input power source phase difference of 60° and having an output voltage ripple as low as 13.84%; (2) a high current mode having an input power source phase difference of 0°; (3) a high voltage mode having an input power source phase difference of 180°; or (4) an intermediate mode with a ripple in the range of about 13.84% to about 100%. The mode changes may be implemented dynamically using hardware and/or software. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The present invention may be more readily understood from the detailed description of an exemplary embodiment presented below considered in conjunction with the attached drawings and in which like reference numerals refer to similar elements and in which: 
           [0016]      FIG. 1  depicts a portion of a conventional high voltage switching mode power supply for driving a magnetron in UV curing applications; 
           [0017]      FIG. 2  shows a high level block diagram of a power supply for driving a magnetron for UV curing applications, according to an embodiment of the present invention; 
           [0018]      FIG. 3  shows a detailed circuit schematic of the power supply of  FIG. 2  which employs a ladder rectifier circuit, according to an embodiment of the present invention; 
           [0019]      FIG. 4  is a graph of a set of voltage waveforms at both the inputs and output of the ladder rectifier circuit of  FIGS. 2 and 3  having a phase difference between V 1  and V 2  of 60° (minimum ripple mode); 
           [0020]      FIG. 5A  is a graph of a set of voltage waveforms at both the inputs and output of the ladder rectifier circuit of  FIGS. 2 and 3  having a phase difference between V 1  and V 2  of 30° (330°); 
           [0021]      FIG. 5B  is a graph of a set of voltage waveforms at both the inputs and output of the ladder rectifier circuit of  FIGS. 2 and 3  having a phase difference between V 1  and V 2  of 0° (360°); 
           [0022]      FIG. 5C  is a graph of a set of voltage waveforms at both the inputs and output of the ladder rectifier circuit of  FIGS. 2 and 3  having a phase difference between V 1  and V 2  of 90° (270°); 
           [0023]      FIG. 5D  is a graph of a set of voltage waveforms at both the inputs and output of the ladder rectifier circuit of  FIGS. 2 and 3  having a phase difference between V 1  and V 2  of 120° (240°); 
           [0024]      FIG. 5E  is a graph of a set of voltage waveforms at both the inputs and output of the ladder rectifier circuit of  FIGS. 2 and 3  having a phase difference between V 1  and V 2  of 150° (210°); 
           [0025]      FIG. 5F  is a graph of a set of voltage waveforms at both the inputs and output of the ladder rectifier circuit of  FIGS. 2 and 3  having a phase difference between V 1  and V 2  of 180°; 
           [0026]      FIG. 6A  depicts an equivalent circuit for the ladder rectifier of  FIG. 3  in a maximum current mode; 
           [0027]      FIG. 6B  depicts an equivalent circuit for the ladder rectifier of  FIG. 3  in a maximum voltage mode; 
           [0028]      FIG. 7  is a block diagram of a suitable circuit known in the art for providing each of the 200-400 Hz AC input power sources of  FIGS. 2 and 3  from an input 50/60 Hz power line; and 
           [0029]      FIG. 8  is a circuit schematic diagram of a suitable circuit for generating the 200-400 Hz AC power sources of  FIGS. 2 and 3  from an input 50/60 Hz power line. 
       
    
    
       [0030]    It is to be understood that the attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0031]      FIG. 2  shows a high level block diagram and  FIG. 3  shows a detailed circuit schematic of a power supply  30  for driving a magnetron for UV curing applications, according to an embodiment of the present invention. Referring now to  FIGS. 2 and 3 , the power supply  30  includes a pair of modulated AC power sources  32   a ,  32   b , each having the substantially the same predetermined amplitude and frequency, but having a variable phase relationship. The AC power sources  32   a ,  32   b , are electrically connected to a pair of input windings  34   a ,  34   b  of a dual laminated transformer  36 , or, alternatively, matched transformers  37   a ,  37   b  (also labeled T 1  and T 2 ), respectively. The dual laminated transformer  36  steps up the voltage of the AC power sources  32   a ,  32   b  on a pair of output windings  38   a ,  38   b . The output windings  38   a ,  38   b  are electrically connected to a pair of input ports  40   a ,  40   b  of a ladder rectifier circuit  42  to be described hereinbelow. The ladder rectifier circuit  42  comprises a total of six diodes  44   a - 44   f  (also labeled D 1 -D 6 , respectively), configured as shown. The ladder rectifier circuit  42  has a single DC output port  46 . 
         [0032]    As a non-limiting example of the operation of the power supply  30 , V DC  is defined as the voltage across output port  46 , V is the peak voltage present across either of the pair of output windings  38   a ,  38   b , and V 1  and V 2  are the instantaneous voltages across each of the pair of output windings  38   a ,  38   b , respectively. At any one moment, V 1 =V sine θ and V 2 =V sine (θ−Φ), where θ is an angle within one period of sinusoidal wave of V 1  and Φ is the phase difference between V 1  and V 2 . When Φ is a predetermined value, current may pass through the ladder rectifier circuit  42  in one of six different paths as follows: 
         [0033]    When V 1 &gt;0 and V 2 &lt;0, D 1 , D 4  and D 5  (i.e.,  44   a ,  44   d , and  44   e , respectively) are forward biased, while the diodes D 2 , D 3 , and D 6  (i.e.,  44   b ,  44   c , and  44   f , respectively) are reverse biased. As a result, current flows though D 1 , D 4  and D 5 , such that the output voltage is V DC =V 1 +|V 2 |=V sine θ+|V sine (θ−Φ)|=V [sine θ−sine (θ−Φ)]. 
         [0034]    When V 1 &gt;V 2 &gt;0, D 1 , D 4  and D 6  (i.e.,  44   a ,  44   d , and  44   f , respectively) are forward biased, while the diodes D 2 , D 3 , and D 5  (i.e.,  44   b ,  44   c , and  44   e , respectively) are reverse biased. As a result, current flows though D 1 , D 4  and D 6 , such that the output voltage is V DC =V 1 =V sine θ. 
         [0035]    When V 2 &gt;V 1 &gt;0, D 1 , D 3  and D 6  (i.e.,  44   a ,  44   c , and  44   f , respectively) are forward biased, while the diodes D 2 , D 4 , and D 5  (i.e.,  44   b ,  44   d , and  44   e , respectively) are reverse biased. As a result, current flows though D 1 , D 3 , and D 6 , such that the output voltage is V DC =V 2 =V sine (θ−Φ). 
         [0036]    When V 1 &lt;0 and V 2 &gt;0, D 2 , D 3  and D 6  (i.e.,  44   b ,  44   c , and  44   f , respectively) are forward biased, while the diodes D 1 , D 4 , and D 5  (i.e.,  44   a ,  44   d , and  44   e , respectively) are reverse biased. As a result, current flows though D 2 , D 3 , and D 6 , such that the output voltage is V DC =|V 1 |+V 2 =|V sine θ|+V sine (θ−Φ)=V [sine (θ−Φ)−sine θ]. 
         [0037]    When V 1 &lt;V 2 &lt;0, D 2 , D 3  and D 5  (i.e.,  44   b ,  44   c , and  44   e , respectively) are forward biased, while the diodes D 1 , D 4 , and D 6  (i.e.,  44   a ,  44   d , and  44   f , respectively) are reverse biased. As a result, current flows though D 2 , D 3  and D 5 , such that the output voltage is V DC =|V 1 |=|V sine θ|=−V sine θ. 
         [0038]    When V 2 &lt;V 1 &lt;0, D 2 , D 4  and D 5  (i.e.,  44   b ,  44   d , and  44   e , respectively) are forward biased, while the diodes D 1 , D 3 , and D 6  (i.e.,  44   a ,  44   c , and  44   f , respectively) are reverse biased. As a result, current flows though D 2 , D 4  and D 5 , such that the output voltage is V DC =|V 2 |=|V sine (θ−Φ)|=−V sine (θ−Φ). 
         [0039]    In different time intervals, the voltage across output port  46 , V DC , may be either V 1  or V 2  from one transformer (whichever amplitude is larger than that of the other) or the voltage summation |V 1 |+|V 2 | from two transformers together when V 1  and V 2  are inverted. The instantaneous phase difference between the sinusoidal waveforms of the voltages V 1  and V 2  across the pair of the output windings  38   a ,  38   b , is the factor that determines the DC output mode of the ladder rectifier circuit  42 . When the phase difference is fixed, the output mode (i.e., the RMS voltage and ripple voltage) is fixed. 
         [0040]      FIG. 4  is a graph of a set of voltage waveforms at both the inputs and output of the ladder rectifier circuit  42 , respectively. The waveform  50  is the voltage at the output winding  38   a  of the dual laminated transformer  36  (also labeled HV AC output I); the waveform  52  is the voltage at the output winding  38   b  of the dual laminated transformer  36  (also labeled HV AC output II); and the waveform  54  is a portion of the composite voltage at the a DC output port  46  of the ladder rectifier circuit  42 , V DC  (also labeled HV DC output). The waveform  54  exhibits a distinct ripple. When the phase difference between waveforms  50  and  52  is about Φ=60°, one period of the output waveform  54  may be divided into six time sections, S 1 , S 2 , S 3 , S 4 , S 5  and S 6 , each section covering 60 degrees of phase and described as follows: 
         [0041]    In time section S 1 , 0&lt;θ&lt;60°, V 1 &gt;0 and V 2 &lt;0, and output current passes through D 1 , D 4  and D 5 . Both of the output windings  38   a ,  38   b  provide power to the load and V DC =V [sine θ−sine (θ−60°)]. 
         [0042]    In time section S 2 , 60°&lt;θ&lt;120°, V 1 &gt;V 2 &gt;0, and output current passes through D 1 , D 4  and D 6 . Only the output windings  38   a  provides power to the load and V DC =V 1 =V sine θ. 
         [0043]    In time section S 3 , 120°&lt;θ&lt;180°, V 2 &gt;V 1 &gt;0, the output current passes through D 1 , D 3  and D 6 . Only the output windings  38   b  provides power to the load. V DC =V 2 =V sine (θ−60°). 
         [0044]    In time section S 4 , 180°&lt;θ&lt;240°, V 1 &lt;0 and V 2 &gt;0, the output current passes through D 2 , D 3  and D 6 . Both of the output windings  38   a ,  38   b  provide power to the load and V DC =V [sine (θ−60°)−sine θ]. 
         [0045]    In time section S 5 , 240°&lt;θ&lt;300°, V 1 &lt;V 2 &lt;0, the output current passes through D 2 , D 3  and D 5 . Only the output windings  38   a  provides power to the load. V DC =−V sine θ. 
         [0046]    In time section S 6 , 300°&lt;θ&lt;360°, V 2 &lt;V 1 &lt;0, the output current passes through D 2 , D 4  and D 5 . Only the output windings  38   b  provides power to the load. V DC =−V sine (θ−60°). 
         [0047]    For the waveforms of  FIG. 4 , Φ=60°, which corresponds to a minimum ripple mode, where the percentage DC output ripple is about 13.84% in theory. The output ripple is defined as the percentage of peak-to-peak voltage of ripple divided by the RMS voltage value of a corresponding DC output. In the example of minimum ripple, using a unity V value, i.e., V=1, the ripple may be calculated by the formula (1−sine (90°−60°/2))/RMS of V DC =(1−0.866)/0.968=13.84%. 
         [0048]      FIGS. 5A-5F  are graphs of a set of voltage waveforms at both the inputs and output of the ladder rectifier circuit  42 , respectively, for various phase differences between V 1  and V 2 , according to an embodiment of the present invention, wherein like reference numbers correspond to similar waveforms. In general, given an arbitrary phase difference between V 1  and V 2 , the time sections, S 1 -S 6  are not divided into six equal sections. The width of the time sections S 1 -S 6  depends upon the amplitude relationship between V 1  and V 2 . The only other equal size time sections occur when Φ=0° or 180°. 
         [0049]    At a phase difference of Φ=0°, the DC output voltage V DC =|V 1 |=|V 2 |=V |sine θ| as shown in  FIG. 5B . When |V 1 |=|V 2 | with zero phase difference, there are only two time sections, S 1  and S 2  of waveform  54 . In time section S 1 , 0&lt;θ&lt;180°, and V 1 =V 2 &gt;0. Output current passes through D 1 , D 3 , D 4  and D 6 , and V DC =V sine θ. In time section S 2 , 180&lt;θ&lt;360°, V 1 =V 2 &lt;0. Output current passes through D 2 , D 3 , D 4  and D 5 , and V DC =−V sine θ. The output current is provided by both T 1  and T 2 , and each transformer transmits half of the current all of the time. Therefore, when Φ=0°, the ladder rectifier circuit  42  is in a maximum current mode, which is equivalent to the circuit depicted in  FIG. 6A . 
         [0050]    At a phase difference Φ=180°, the DC output voltage is V DC =|V 1 |+|V 2 |=2 V |sine θ| as shown in  FIG. 5F . When |V 1 |=|V 2 | with 180° phase difference, there are only two time sections S 1  and S 2  of waveform  54 . In time section S 1 , 0&lt;θ&lt;180°, V 1 &gt;0 and V 2 &lt;0, V DC =V 1 −V 2 =V [sine θ−sine (θ−180)°]=2 V sine θ. In time section S 2 , 180&lt;θ&lt;360°, V 1 &lt;0 and V 2 &gt;0, V DC =−V 1 +V 2 =V [sine (θ−180°)−V sine θ]=−2 V sine θ. The output power is provided by both T 1  and T 2  transmit equal current and double the voltage of either V 1  or V 2 . Therefore, when Φ=180°, the ladder rectifier circuit  42  is in a maximum voltage mode, which is equivalent to the circuit depicted in  FIG. 6B . 
         [0051]    In summary, embodiments of the present invention may be developed as a power supply with multiple output features. Changing the phase difference between the modulated power sources AC  1  and AC  2 , may represent the following modes:
       Φ=60° phase difference providing a low ripple mode with 13.84% ripple.   Φ=0° phase difference providing a high current mode with 100% ripple.   Φ=180° phase difference providing a high voltage mode with 100% ripple.       
 
         [0055]    Other phase differences provide various modes ranging between high current mode and high voltage mode with ripple ranging between about 13.84% and about 100%. 
         [0056]    For high power applications, a block diagram of a suitable circuit  60  known in the art for providing each of the 200-400 Hz AC power sources  32   a ,  32   b  of  FIGS. 2 and 3  from an input 50/60 Hz power line is depicted in  FIG. 7 . The input 50/60 Hz power line voltage  62  is passed through a rectifier and filter circuit  64 , which converts the input power line voltage to an approximate DC power  66 . The approximate DC power  66  is chopped into a PWM (Pulse Width Modulation) sine wave  70 , which is a series of pulses resulting in a sine-like flux density waveform using a full bridge IGBT (Insulated Gate Bipolar Transistor) switch  68 . The chopping frequency of the full bridge IGBT switches is at least 100 times that of the PWM sine wave  70  frequency. For example, if the frequency of PWM sine wave is 300 Hz, the chopping frequency is more than 30 KHz. This PWM sine wave  70  in the form of a smooth sine wave  74  having a frequency in the range of 200 Hz to 400 Hz is produced by a low pass filter  72 . The output signal  74  is fed back to the full bridge switcher circuit  68  by a sine wave modulator circuit  76 . 
         [0057]    Alternatively, a suitable circuit for generating both of the 200-400 Hz AC power sources  32   a ,  32   b  of  FIGS. 2 and 3  from an input 50/60 Hz power line is depicted in  FIG. 8 . 
         [0058]    It is to be understood that the exemplary embodiments are merely illustrative of the invention and that many variations of the above-described embodiments may be devised by one skilled in the art without departing from the scope of the invention.