Patent Publication Number: US-2016233761-A1

Title: Systems and Methods for Providing a Transformerless Power Supply

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 62/113,576, filed Feb. 9, 2015, entitled “Systems and Methods for Providing a Transformerless Power Supply,” the entirety of which is herein incorporated by reference. 
    
    
     FIELD 
     The technology described in this patent document relates generally to power supplies and more specifically to power supplies for lighting with reduced to eliminated transformer counts. 
     BACKGROUND 
     There are a wide variety of power supplies that are readily available for use in applications such as for providing power to lighting systems (e.g., lighting systems that provide LED light). Such power supplies often include components such as step-up or step-down transformers, DC-to-DC converters, AC-to-DC converters, buck and/or boost converters, and flybacks. In such power supplies, transformers tend to play a key role in providing the desired power supply voltage. But, the transformer is one of the single cost components of such power supplies. Systems and methods as described herein seek to reduce the number of transformers present in power supplies to reduce size and cost. 
     SUMMARY 
     Systems and methods are provided for a transformerless power supply. A first capacitor is positioned between an input node and an intermediate node. A second capacitor is positioned between an output node and a ground node. A first switch is positioned between the intermediate node and the output node, a second switch is positioned between the intermediate node and the ground node, and a third switch is positioned between the input node and the output node. A controller is configured to control the first switch, the second switch, and the third switch to provide output power within a prespecified range. 
     As another example, a method of providing power includes controlling a set of three switches based on an input voltage and a threshold voltage, a first switch being positioned between an intermediate node and an output node, a second switch being positioned between the intermediate node and a ground node, and a third switch being positioned between an input node and the output node, where a first capacitor is positioned between the input node and the intermediate node and a second capacitor is positioned between the output node and a ground node. The set of three switches is controlled by opening the first switch and closing the second switch and the third switch when the input voltage is less than the threshold voltage, and closing the first switch and opening the second switch and the third switch when the input voltage is greater than the threshold voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram depicting a schematic for a transformerless power supply. 
         FIG. 2  is a diagram depicting an example DC power source voltage generated from a rectified voltage. 
         FIG. 3  is a block diagram depicting a voltage crossing detector configured to control the switches SW 1 , SW 2 , SW 3  of  FIG. 1 . 
         FIG. 4  depicts a truth table indicating the states commanded of the switches by the voltage crossing detector based on the relation of the input signal voltage to the threshold voltage. 
         FIG. 5  is a flow diagram depicting a method of providing power. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram depicting a schematic for a transformerless power supply for use in an application such as providing lights via LED light bulbs. The power supply  100  includes an AC input  102  (e.g., a 120 V rms  or 240 V rms  voltage at a light socket) rectified by bridge rectifier  104  to generate a DC voltage (e.g., a 170V or 339V DC voltage). A load  106  is configured to use a lower DC voltage than is provided by the bridge rectifier  104 . In one embodiment, the load  106  is an LED light source that utilizes a 38V DC voltage. 
     In the example of  FIG. 1 , a circuit that includes a plurality of capacitors is utilized to generate the necessary voltage for the load  106  at node  108 . A capacitive divider is formed by a first capacitor C 1   110  and a second capacitor C 2   112 . A diode  114  isolates an input node  116  of the capacitive circuit from the bridge rectifier  104 . The first capacitor  110  is positioned between the input node  116  and an intermediate node  118 . The second capacitor is positioned between the output node  108  and a ground node  120 . The capacitive circuit includes a plurality of switches. A first switch SW 1   122  is positioned between the intermediate node  118  and the output node  108 . A second switch SW 2   124  is positioned between the intermediate node  118  and the ground node  120 . A third switch SW 3   126  is positioned between the input node  116  and the output node  108 . By controlling the three switches  122 ,  124 ,  126 , the power supply  100  of  FIG. 1  provides DC power within a desired range (e.g., ˜38V DC) to operate the load  106 . 
       FIG. 2  is a diagram depicting an example DC power source voltage generated from a rectified voltage. A rectified voltage measurement, taken at the output of the bridge rectifier  104  in  FIG. 1  at  128  indicates the voltage that is provided to the input node  116  via the isolating diode  114 . Using that input voltage signal  202 , the capacitive divider circuit provides the output signal depicted at  204  at output node  108 . That DC voltage provided at  108  can be utilized to power a load, such as load  106 . While the voltage indicated at  204  varies slightly around an average voltage level, it is sufficiently stable for many loads  106 . Additional circuitry can be incorporated into the capacitive circuit of  FIG. 1  to lessen the variation and provide a more stable DC output voltage. 
     The switches SW 1 , SW 2 , SW 3  can be operated via a variety of mechanisms to generate the output voltage depicted in  FIG. 2  at  204 .  FIG. 3  is a block diagram depicting a voltage crossing detector configured to control the switches SW 1 , SW 2 , SW 3  of  FIG. 1 . The voltage crossing detector  302  generates output signals  304 ,  306 ,  308  to switches SW 1 , SW 2 , SW 3 , respectively based on two input signals. A first input to the voltage crossing detector  302  is based on an input voltage (e.g., from  116  or  128  of  FIG. 1 ) to the capacitive circuit. A second input is a threshold input (e.g., a threshold voltage based on the output voltage at  108  or a user selected threshold voltage). As the time-varying input voltage (e.g., as depicted in  FIG. 2  at  202 ) crosses the threshold voltage to a voltage higher than the threshold voltage, the voltage crossing detector  302  is configured to: close the first switch SW 1   122  such that the intermediate node  118  is connected to the output node  108 ; open the second switch SW 2   124  such that the intermediate node  118  is disconnected from the ground node  120 ; and open the third switch SW 3   126  such that the input node  116  is disconnected from the output node  108 . As the time-varying input voltage then crosses the threshold voltage to a voltage lower than the threshold voltage, the voltage crossing detector  302  is configured to: open the first switch SW 1   122  such that the intermediate node  118  is disconnected from the output node  108 ; close the second switch SW 2   124  such that the intermediate node  118  is connected to the ground node  120 ; and close the third switch SW 3   126  such that the input node  116  is connected to the output node  108 .  FIG. 4  depicts a truth table indicating the states commanded of the switches  122 ,  124 ,  126  by the voltage crossing detector  302  based on the relation of the input signal voltage to the threshold voltage. 
     The example of  FIG. 3  depicts an example switch control circuit that receives the first input based on the input voltage  128  to the capacitive circuit, received at  305 , and two user-selectable options for threshold voltages. A first potential threshold voltage is based on the voltage at the output node  108  that is received at  307 , and a second potential threshold voltage is provided by a reference generator  309 , such as based on a user-selectable parameter. A voltage decimator  310  proportionally reduces the input signals received at  305 ,  307  to produce corresponding inputs  312 ,  314  to the voltage crossing detector  302  that are within an acceptable operating range of the detector. A threshold selector input  316  to the voltage crossing detector  302  enables user selection of either the output node voltage  307  or the reference generator  309  voltage as the basis for the voltage crossing detector threshold  302 . As discussed in detail above, the voltage crossing detector  302  provides control signals  304 ,  306 ,  308  to switches SW 1 , SW 2 , SW 3 , respectively based on the directions of crossings of the input signal  312  with respect to the selected threshold signal  309  or  314 . 
     In one example, with reference to  FIG. 1 , V_rect  128  is an unfiltered rectified voltage, as depicted in  FIG. 2  at  202 . When V_rect  128  is at its peak value, C 1   110  and C 2   112  are connected in series, and the output voltage div_out  108  is based on the ratio of the values of capacitors C 1   110  and C 2   112 . This is accomplished by closing switch SW 1   122  and opening switches SW 2   124  and SW 3   126 . As V_rect  128  falls below the threshold value (e.g., based on div_out  108 ), capacitor C 1   110  is disconnected from capacitor C 2   112  by opening switch SW 1   122 . The intermediate node  128  is connected to the ground node  120  by closing switch SW 2   124 . The output terminal div_out  108 , which is the high voltage terminal of capacitor C 2   112  is connected to the input node VDD_hiV  116  by closing switch SW 3   126 . Because the high voltage input of capacitor C 1   110  is also connected to the input node VDD_hiV  116 , C 1   110  and C 2   112  are then in a parallel configuration. The charge stored on C 1   110  is thus shared by C 2   112 . This configuration helps maintain the output voltage div_out  108  at the required level while V_rect  128  is less than the threshold voltage (e.g., div_out  108 ). As time elapses, the value of V_rect  128  increases until it surpasses the threshold voltage (e.g., div_out  108 ). At that point, C 1   110  and C 2   112  are returned to a series configuration by closing switch SW 1   122  and opening switches SW 2   124  and SW 3   126 . 
       FIG. 5  is a flow diagram depicting a method of providing power, such as to a smart lighting system where LED light bulbs are networked and configured to monitor light levels and adjust accordingly to provide a user-specified level of light. A set of three switches are controlled at  502  based on an input voltage and a threshold voltage, a first switch being positioned between an intermediate node and an output node, a second switch being positioned between the intermediate node and a ground node, and a third switch being positioned between an input node and the output node, where a first capacitor is positioned between the input node and the intermediate node and a second capacitor is positioned between the output node and a ground node. The set of three switches is controlled by opening the first switch and closing the second switch and the third switch at  504  when the input voltage is less than the threshold voltage, and closing the first switch and opening the second switch and the third switch at  506  when the input voltage is greater than the threshold voltage. 
     While the disclosure has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the embodiments. Thus, it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents. For example, power supplies as described herein can be configured to power smart lighting applications, such as those described in U.S. patent application Ser. No. 14/288,911, entitled “Systems and Methods for Providing a Self-Adjusting Light Source,” the entirety of which is herein incorporated by reference.