Abstract:
A switching power supply includes a bridge rectifier, a switching device, and a controller. The bridge rectifier is operatively coupled to an alternating current (ac) signal, and outputs a rectified signal. The switching device is electrically connected in parallel with the bridge rectifier. The controller is responsive to the rectified signal and selectively controls the switching device to divert at least a portion of the rectified signal therethrough in response to the rectified signal. A method of supplying power includes coupling a bridge rectifier operatively to an alternating current (ac) signal, the bridge rectifier outputting a rectified signal, coupling a switching device electrically in parallel with the bridge rectifier, and controlling the switching device to divert at least a portion of the rectified signal therethrough in response to the rectified signal.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/635,472, filed on Dec. 13, 2004, the disclosure of which is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to switching power supplies. In particular, the present invention is directed to a switching power supply that converts an alternating current (ac) input signal having a wide range of voltages to a direct current (dc) output signal having a substantially constant voltage and/or current. 
   2. Description of the Prior Art 
   Switching power supplies have certain properties that make them desirable. For instance, there is very little power dissipation, making switching supplies efficient even if there is a large voltage drop required from input to output. Switching power supplies can also generate output voltages that exceed the input voltage, which enables these supplies to run directly from a rectified power line without requiring an alternating current (ac) power transformer. The result is a compact, lightweight, and efficient power supply that is desirable for universal use in a variety of applications. 
   Direct current (dc) power supplies must often adapt to wide variations in input voltages. A multiple source power supply may be required in certain applications where the output of the supply depends on the device being supplied. A multiple source power supply may derive its input from multiple ac lines, one or more batteries, or even solar collectors to power the load connected to the output terminals. 
   Two conventional solutions for implementing multiple input power supplies are provided in  FIGS. 1 and 2 . As shown in  FIG. 1 , one such solution uses a transformer  10  to establish electrical connection of a multi-input transformer ac supply to a dc regulator. The primary of the transformer  10  includes several taps  12  to accommodate different values of AC input voltage (V AC1 , V AC2 , V AC3 ) and maintain approximately the same value at the secondary voltage V IN . The secondary winding  16  may supply this voltage V IN  to a power supply for rectification and generation of the dc power signal. A second solution that provides a similar result is shown in  FIG. 2 , in which power sources  12  are capacitively coupled to generate an ac voltage that is subsequently input to the power supply or load. 
   In each of the above configurations, it is necessary that the user manually select the connections for the ac input voltage during installation. Even in applications where power supplies have only two ac input connections, manual selection of the input connections is still required through incorporation of a multi-position switch or “intelligent” power supply having automated voltage selection capability. Since these solutions incorporate a multi-input circuit to accommodate various ac voltages, an undesirable selection must be made between manual setup for a particular voltage or the use of sophisticated and costly automatic voltage selection components. 
   Additional cost is incurred due to the expense of the purchase, installation, and maintenance of such voltage selection techniques. Inherent risks associated with the selection of an improper voltage can also result in improper functioning of the circuit and possible damage to the system within which it is implemented, thereby incurring additional financial cost and safety hazards. 
   It is therefore desirable to retain the range of ac input voltages that may be applied to a power supply circuit while minimizing the number of required inputs. Such a power supply circuit advantageously avoids restrictions on ac input voltage values. 
   As indicated above, some conventional solutions use a step-down transformer to convert the ac input signal to a lower voltage required by load circuitry. When the supply must be able to adapt to more than one ac input voltage, most conventional implementations use transformers with multi-tap inputs and fixed step-down ratios. As disclosed in U.S. Pat. No. 6,563,721, which is incorporated herein by reference, an alternative solution uses a single-tap (two-wire) input transformer in conjunction with a wide voltage-range switching regulator. In this instance, the power supply is essentially a voltage-to-voltage converter. That is, the supply converts a variable ac input voltage from a transformer secondary or battery into a constant dc output voltage required by the load. 
   As shown in  FIG. 3 , another solution incorporates a capacitor  18  electrically connected in series with a rectifier bridge  20  and a load  22 . The impedance of the capacitor  18  limits the load current I LOAD  to a predetermined value, and thus functions as a constant ac current generator. Since the ac input voltage V IN  is typically much greater that the dc output load voltage V LOAD , the ac current is fixed and essentially determined by the ac input voltage V IN  and the value of capacitor  18 . However, it is difficult to achieve high efficiency in such a power supply when using a constant-current ac generator to drive the voltage-to-voltage regulator. 
   As discussed above, to adapt the capacitor-input power supply to different ac input voltages, such as 120V, 277V, 437V, and the like, the multi-input design shown in  FIG. 2  may be used, which requires separate capacitors  24 ,  26 , and  28  and input connections for each ac voltage. However, this implementation increases the material cost, overall dimensions, and the risk of faulty installation. 
   It is therefore desirable to provide a power supply based on a single capacitor input that converts a wide range of input ac signals to a regulated dc output signal having substantially constant voltage and/or current. 
   SUMMARY OF THE INVENTION 
   A switching power supply formed in accordance with one form of the present invention, which incorporates some of the preferred features, includes a bridge rectifier, a switching device, and a controller. The bridge rectifier is operatively coupled to an alternating current (ac) signal, and outputs a rectified signal. The switching device is electrically connected in parallel with the bridge rectifier. The controller is responsive to the rectified signal and selectively controls the switching device to divert at least a portion of the rectified signal therethrough in response to the rectified signal. 
   The controller selectively controls the switching device with a control signal, having a substantially constant frequency with a modulated pulse width and/or a modulated frequency with a substantially constant pulse width. The controller selectively controls the switching device in response to a voltage or current at the output. 
   The power supply may include a first capacitor electrically coupled in parallel with the output of the power supply, a second capacitor electrically coupled in series with the alternating current (ac) signal, a fuse electrically coupled in series with the alternating current (ac) signal, a resistor responsive to the alternating current (ac) signal and electrically coupled in series with the bridge rectifier, and a diode electrically coupled in series with the bridge rectifier. 
   The switching device may include a bipolar transistor, field effect transistor (FET), metal oxide semiconductor field effect transistor (MOSFET), discrete device, relay, and/or integrated circuit. The controller may selectively control the switching device in response to a voltage and/or current at an output of the power supply. The controller may include a microprocessor, microcontroller, application specific integrated circuit (ASIC), programmable logic array (PLA), hybrid circuit, and/or discrete logic device. 
   The controller may be electrically coupled in parallel with the output and adapted to selectively control the switching device such that a substantially constant voltage is maintained at the output. The power supply may include a sensing resistor electrically coupled in series with the output and outputting a sensing signal. The controller may be responsive to the sensing signal and adapted to selectively control the switching device such that that a substantially constant current is maintained at the output in response to the sensing signal. 
   A method of supplying power in accordance with one form of the present invention, which incorporates some of the preferred features, includes coupling a bridge rectifier operatively to an alternating current (ac) signal, the bridge rectifier outputting a rectified signal, coupling a switching device electrically in parallel with the bridge rectifier, and controlling the switching device to divert at least a portion of the rectified signal therethrough in response to the rectified signal. 
   The method may include controlling the switching device with a control signal including a substantially constant frequency and a modulated pulse width, and/or a modulated frequency and a substantially constant pulse width. The method may control the switching device in response to a voltage and/or current at an output of the power supply. 
   These and other objects, features, and advantages of this invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a conventional transformer-based multiple input circuit for a power supply. 
       FIG. 2  is a schematic diagram of a conventional capacitively coupled multiple input circuit for a power supply. 
       FIG. 3  is a schematic diagram of a conventional capacitively coupled single input power supply. 
       FIG. 4  is a schematic diagram of a power supply formed in accordance with the present invention. 
       FIG. 5   a  is a graph of rectifier current I BR  and load current I LOAD  as a function of time for the power supply shown in  FIG. 4 . 
       FIG. 5   b  is a graph of gate voltage VG 1  or a control signal as a function of time for the power supply shown in  FIG. 4 . 
       FIG. 5   c  is a graph of diverted current I Q1  as a function of time for the power supply shown in  FIG. 4 . 
       FIG. 6  is a schematic diagram of an alternative embodiment of a power supply formed in accordance with the present invention. 
       FIG. 7   a  is a graph of rectifier current I BR  and load current I LOAD  as a function of time for the power supply shown in  FIG. 6 . 
       FIG. 7   b  is a graph of gate voltage VG 1  or a control signal as a function of time for the power supply shown in  FIG. 6 . 
       FIG. 7   c  is a graph of diverted current I Q1  as a function of time for the power supply shown in  FIG. 6 . 
       FIG. 8  is a schematic diagram of an alternative embodiment of a power supply formed in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is directed to a low-power direct current (dc) power supply that provides a dc output signal having a substantially fixed voltage and/or current given a wide range of alternating current (ac) input voltages, such as 100Vac to 400Vac, applied to only a single input. The power supply preferably includes a capacitor electrically coupled in series with a low-voltage ac/dc switching power supply circuit. 
   The switching power supply formed in accordance with the present invention regulates a constant-voltage dc output by periodically shorting the output of a rectifier bridge. This enables the regulator to transfer only the amount of energy required by a load at the constant-voltage output while interfacing with a constant-current ac generator, that is, the series capacitor. 
   When compared with conventional transformer-input power supplies, the capacitor-input power supply formed in accordance with the present invention provides energy efficiency with very low power losses or heat dissipation, a leading power factor, and may be implemented without the use of inductive components. In addition the subject power supply is smaller and more cost efficient than transformer-based designs, which make it ideal for low-power applications. 
     FIG. 4  shows a schematic diagram of a first embodiment of the power supply  24  formed in accordance with the present invention. The power supply  24  preferably includes a non-polarized capacitor  26  having a rated voltage that is equal to or greater than the maximum ac input voltage to be applied. The rated value of the capacitor  26  must also be large enough to supply the current required by a load  66 , the load current I LOAD , at the minimum ac input voltage to be applied. 
   The power supply  24  also preferably includes a bridge rectifier  28 , a switching component or field effect transistor (FET)  30 , an isolation diode  32 , an energy-storage or filtering capacitor  34 , and a constant-voltage switching control circuit  36 . The capacitor  26  is preferably connected electrically in series with a fuse  38  between a positive terminal  40  of the ac input signal and the rectifier bridge  28 . A current limiting resistor  42  is preferably connected electrically in series between a negative terminal  44  of the ac input signal and the rectifier bridge  28 . 
   The control circuit  36  preferably outputs a control signal  46  to a gate terminal of the FET  30 , which is adapted for turning the FET  30  on or off. The control signal  46  preferably exhibits a constant frequency, such as from 20 kHz and 100 kHz, with a variable or modulated pulse length or duty cycle. 
   The rectifier bridge preferably includes diodes  48 ,  50 ,  52 ,  54  and nodes  56 ,  58 ,  60 ,  62 . Fuse  38  is preferably connected to node  56  and the anode of diode  48 . The cathode of diode  48  is connected to node  58  and the cathode of diode  50 . The anode of diode  50  is connected to node  60 , the cathode of diode  52 , and resistor  42 . The anode of diode  52  is connected to node  62  and the anode of diode  54 . The cathode of diode  54  is connected to node  56 . 
   Source and drain terminals of the FET  30  are preferably connected electrically in parallel across nodes  58  and  62  of the rectifier bridge  28 . The anode of diode  32  is connected to node  58  of the rectifier bridge  28 , and its cathode is connected to the control circuit  36 , an anode of capacitor  34 , and a first terminal  64  of a load  66 . Node  62  of the rectifier bridge  28  is preferably connected to the control circuit  36 , the cathode of capacitor  34 , and a second terminal  68  of the load  66 . 
   The current required by the load I LOAD  is preferably regulated in order to keep the load voltage V LOAD  constant. This is preferably achieved by splitting the total average dc current from the bridge I BR  between two components: the current through the diode I D1  and the diverted current I Q1  through the FET  30 . The diode current I D1  preferably has an average value that is slightly larger than the load current I LOAD , which is primarily due to the current consumed by the control circuit  36  and other secondary energy conversion factors, such as thermal losses. As the current supplied by the rectifier bridge I BR  varies widely from zero to the peak of the ac input voltage V AC , the control circuit  36  preferably switches the FET  30  on and off with a variable duty factor in order to keep the diode current I D1  substantially constant, or as required by the load  66 . 
   In  FIG. 5   a , line  70  represents the rectifier current I BR  as a function of time and line  72  represents the load current I LOAD  as a function of time.  FIGS. 5   b  and  5   c  show graphs of the control signal  46  or gate voltage VG 1  and diverted current I Q1 , respectively, as functions of time, which correspond to  FIG. 5   a . Preferably, the greater the difference between I BR  and I LOAD , the longer the conduction time of the FET  30  and the greater the value of the diverted current I Q1 . That is, the control circuit  36  preferably controls the FET  30  such that only the average current needed to supply the control circuit  36  and the load  66  passes through the diode  32 . The remainder is diverted through the FET  30  during its conduction cycle and short-circuited to the rectifier bridge  28 . 
   For instance, during the period of time  74  shown in  FIGS. 5   a - c , the control signal  46  or gate voltage V G1  is high, which turns the FET  30  on and causes it to conduct. When the FET  30  is on, current is diverted from the load  66  (as the diverted current I Q1 ) which increases to compensate for the difference between the rectifier current I BR  and the current required by the load I LOAD . The diode  32  functions to isolate the load  66  from the effects of the FET  30  switching on and off. 
   An advantage of the power supply formed in accordance with the present invention is that, during the conduction phase of FET  30 , although the rectifier bridge  28  is short-circuited, the ac input current I AC  is limited by the impedance of capacitor  26  to a value not much larger than its average value. In addition, the active or real power consumption of the power supply is negligible and limited to the residual power losses of the components in the ac portion of the power supply, which include the fuse  38 , limiting resistor  42 , FET  30 , and bridge diodes  48 ,  50 ,  52 ,  54 . This enables the entire power supply to function with high efficiency (low power losses) while supplying the load  66  with a constant voltage and/or current. 
   Conventional switching power supplies, such as that described in U.S. Pat. No. 6,351,073, which is incorporated herein by reference, use a switching element electrically connected in series with the ac input signal and an inductive element as shown in  FIGS. 1-4  of the &#39;073 patent. However, the power supply formed in accordance with the present invention utilizes a switching element, such as the FET  30 , electrically connected in parallel with the rectifier bridge  28 . 
   Thus, the power supply formed in accordance with the present invention provides energy efficiency with very low power loss or heat dissipation, and a leading power factor rather than the lagging power factor inherent in conventional transformer-based input power supplies. The subject power supply also does not require inductive components, such as transformers and inductors. In addition, the only components required to be rated for high ac voltage are the capacitor  26 , rectifier bridge  28 , and protection components, such as the fuse  38  and limiting resistor  42 . This results in a smaller and more cost-effective approach than that offered by prior art solutions. 
   It is to be noted that the FET  30 , although shown in  FIG. 4  and described as a field effect transistor, could as well be replaced or supplemented by any type of switching component  31  shown in  FIG. 8 , such as a bipolar transistor, metal oxide semiconductor field effect (MOSFET) transistor, discrete component, relay, and/or one or more integrated circuits while remaining within the scope of the present invention. In addition, the switching power supply may be implemented with discrete components or manufactured as an integrated circuit while remaining within the scope of the present invention. 
   Further, the control circuit  36  may be implemented using a microprocessor, microcontroller, application specific integrated circuit (ASIC), programmable logic array (PLA), hybrid circuit, and/or one or more discrete logic devices. Yet further, the control circuit  36  may be adapted for supplying a constant dc output voltage, as shown in  FIG. 4 , or for supplying a constant dc output current, as shown by the control circuit  36  that monitors a sensing resistor  37 , which provides a voltage representing I LOAD , as shown in  FIG. 6 . Still further, the control circuit may be adapted to output the control signal  46  with a constant frequency and modulated pulse width, as shown in  FIG. 5   b , and/or a constant pulse width frequency modulated control signal  47 , as shown in  FIG. 7   b.    
   Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention.