Patent Publication Number: US-2023134538-A1

Title: Power Converter Apparatus and A Method of Modulating Thereof

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of electric power conversion, and particularly, but not exclusively, to a power conversion apparatus for converting an alternating current (AC) input to a direct current (DC) output with improved efficiency. 
     BACKGROUND OF THE INVENTION 
     Various electronic components including power converters for converting an alternating current (AC) power input to a direct current (DC) power output are known in the field. Particularly, a power supply with AC to DC (AC/DC) conversion at a light power load demonstrates low conversion efficiency which is attributed to the power consumption by other electronic components such as the digital control, the integrated circuit (IC), the magnetic core, etc. and because the driving power as well as the capacitive loss of these components are irrespective of the load power. For example, traditional power converters are generally configured with isolated converters connected in parallel, but this configuration has relatively low power efficiency, especially at a light power load. On the other hand, providing non-isolated converters in parallel may provide higher conversion efficiency but the arrangement suffers from circulating currents which is not desirable. 
     For example, PCT patent application publication no. WO 2017/191245 A1 discloses a converter system for converting a three-phase or a single-phase AC voltage into a DC voltage. The converter system comprises three converter branches each comprising a first input and a second input to be supplied with a single-phase AC voltage and a first output and a second output providing a DC voltage; wherein each converter branch comprises an AC-to-DC stage and a DC-to-DC stage connected between the first and second input and the first and second output. 
     Chinese patent no. CN 105827120B discloses a single phase, staggered power factor correction (PFC) circuit for use in an air conditioner. The circuit includes three branches arranged in parallel each including an inductor, a switching tube and a diode. The circuit obtains input current of an outdoor unit in the air conditioner or phase current of a compressor in the air conditioner; and controls the switching tube in each branch according to the input current of the outdoor unit or the phase current of the compressor, so that the staggered PFC circuit switches among three working states including three branches working in a staggered mode with a phase shift of 120 degrees, any two of the three branches working in a staggered mode with a phase shift of 180 degrees and any one of the three branches working in a Boost PFC mode to effectively improve the operating efficiency of a light load or no load. 
     U.S. Pat. No. 8,476,879 B2 discloses a method of controlling a power factor correction (PFC) converter having a single-phase system comprising a first PFC sub-circuit and a second PFC sub-circuit to determine when to transition the PFC converter between an interleaved mode and a saving energy mode (SEM). The method includes generating an amplified error signal based on a monitored output voltage of the PFC converter. The second PFC sub-circuit is disabled in response to the amplified error signal being less than a first threshold value and enabled in response to the amplified error signal exceeding a second threshold value. 
     US patent no. U.S. Ser. No. 11/011,992 B2 discloses a method and system for reducing a circulating current between a plurality of non-isolated modules operating in parallel. The input terminals and the output terminals of the plurality of non-isolated modules are respectively connected in parallel, and each of the non-isolated modules comprises a first stage converter, a bus capacitor and a second stage converter, which are electrically connected in sequence. 
     U.S. Pat. No. 7,948,222 B2 discloses a method of operating an asymmetric phase circuit topology comprising a power converter circuit operating a first phase switch circuit portion using a first number of switch devices when the power converter circuit is configured in a first mode of operation, with the first number being greater than zero; and operating a second phase switch circuit portion using a second number of switch devices when the power converter circuit is configured in the second mode of operation, with the second number being greater than the first number. 
     US patent no. U.S. Ser. No. 11/043,891 B1 discloses a controller for an AC to DC or a DC to AC multi-phase power converter of a type having N power converter phases arranged in parallel, where N is greater or equal to 2. The controller comprises a control module configured to change or vary a phase shift angle of the input current or output current for each of the N power converter phases such that an average phase shift value for each of said N power converter phases over a control module AC line cycle is about, near or substantially the same value. 
     There is therefore a need for a power converter which is capable of offering improved power efficiency. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is to provide a novel power converter capable of offering improved power efficiency particularly at light power load. 
     Another object of the present invention is to mitigate or obviate to some degree one or more problems associated with known power converters, or at least to provide a useful alternative. 
     The above objects are met by the combination of features of the main claim; the sub-claims disclose further advantageous embodiments of the invention. 
     One skilled in the art will derive from the following description other objects of the invention. Therefore, the foregoing statements of object are not exhaustive and serve merely to illustrate some of the many objects of the present invention. 
     SUMMARY OF THE INVENTION 
     In a first main aspect, the invention provides a power converter apparatus for converting an alternating current (AC) power input to a direct current (DC) power output. The apparatus comprises a plurality of n single-phase power converting circuits arranged in parallel, where n is equal to or greater than 2, wherein one first single-phase power converting circuit of said n single-phase power converting circuits comprises a single-stage AC/DC converter module having an operating AC/DC converter, and each of remaining n−1 second single-phase power converting circuits comprises a two-stage converter module having an AC/DC converter as an input stage and a DC/DC transformer as an output stage. 
     In a second main aspect, the invention provides method of modulating a power converting apparatus. The apparatus comprises a plurality of n power converting circuits in parallel, wherein n is equal to or greater than 2. The method comprises providing one first single-phase power converting circuit comprising a one-stage AC/DC converter module having an AC/DC converter; and providing n−1 second single-phase power converting circuits each comprising a two-stage converter module having an AC/DC converter as an input stage and a DC/DC transformer as an output stage; automatically by-passing one or more of the output stage DC/DC transformers of the n−1 second single-phase power converting circuits when a load power of the apparatus is less than or equal to a predetermined, selected or calculated power threshold. 
     In a third main aspect, the invention provides a controller for use with the power converter apparatus of the first aspect. The controller is adapted to automatically by-pass one or more of the output stage DC/DC transformers of the second single-phase power converting circuits when load power of the apparatus is less than or equals to a predetermined, selected or calculated power threshold. 
     The summary of the invention does not necessarily disclose all the features essential for defining the invention; the invention may reside in a sub-combination of the disclosed features. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further features of the present invention will be apparent from the following description of preferred embodiments which are provided by way of example only in connection with the accompanying figure, of which: 
         FIG.  1    is a circuit diagram showing a general structure of the power converter apparatus in accordance with an embodiment of the present invention; 
         FIG.  2    is a circuit diagram showing the power converter apparatus having a two-phase topology in accordance with an embodiment of the present invention; 
         FIG.  3    is a circuit diagram showing the power converter apparatus having a three-phase topology in accordance with an embodiment of the present invention; 
         FIG.  4    is a flow diagram showing operation of the power converter apparatus of  FIG.  3   ; 
         FIG.  5    shows the waveforms generated by the power converter apparatus of  FIG.  3   ; and 
         FIG.  6    shows the power efficiencies of the power converter apparatus of  FIG.  3    in comparison to a conventional power converter apparatus. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following description is of preferred embodiments by way of example only and without limitation to the combination of features necessary for carrying the invention into effect. 
     Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments. 
     The present invention relates to an electric power converter apparatus, and particularly, but not exclusively, to a power converter apparatus for converting an alternating current (AC) power input to a direct current (DC) power output. The power converter apparatus is configurable to automatically adjust and/or reduce the number of operating phases of the apparatus, for example, by converting a multi-phase operation such as a three-phase operation into a single-phase operation at light power load to thereby improve power conversion efficiency of the apparatus. 
     Referring to  FIG.  1   , shown is a power converter apparatus  10  for converting an alternating current (AC) power input to a direct current (DC) power output. The power converter apparatus  10  can be connected with a single-phase or a multi-phase grid such as an AC mains power grid. The power converter apparatus  10  may comprise a plurality of n single-phase power converting phases or circuits  20  arranged in parallel, where n is a natural number equal to or greater than 2. The power converter apparatus  10  preferably comprises an asymmetric circuit topology, which will be described further below. 
     In one embodiment, the n single-phase power converting phases  20  comprise alone first single-phase power converting circuit  20   a  having a/one single-stage AC/DC converter module  22 . Preferably, the single stage AC/DC converter module  22  comprises one AC/DC converter  24  connected between the power input  12  and the power output  16 . Preferably, the single stage AC/DC converter module  22  comprises only AC/DC converter  24  as a single input/output stage. The n single-phase power converting phases  20  further comprises a remaining n−1 number of second single-phase power convening circuits  20   b , each of which has a two-stage converter module  26  connected between their respective power inputs  13 ,  14  and the power output  16 . Preferably, the two-stage converter module  26  comprises an AC/DC converter  24  as an input stage connected with a DC/DC transformer  30  as an output stage. Each of the AC/DC converters  24  is adapted to convert a single-phase AC voltage into an intermediate DC voltage, and each of the DC/DC transformers  30  is adapted to convert the intermediate DC voltage into a preferably different DC voltage provided to the power output  16 . In one embodiment, the AC/DC converters  24  may comprise a rectifier, a power factor corrector, or the like; and the DC/DC transformers  30  may comprise an inverter, a transformer, a rectifier, or the like. 
     As shown in  FIG.  2   , for example, the power converter apparatus  10  may comprise a two-phase circuit having two (n=2) single-phase power converting phases  20 , with one single-phase power converting circuit  20   a  having a single-stage AC/DC converter module  22 ; and another second single-phase power converting circuit  20   b  having a two-stage converter module  26 . The apparatus  10  as shown in  FIG.  2    can be connected with a 2-phase grid which may comprise two power inputs  12 ,  13  supplied with respective single-phase AC voltages. The power converter apparatus  10  preferably has one power output  16  providing a DC voltage. 
     In another embodiment as shown in  FIG.  3   , the power converter apparatus  10  may comprise a three-phase circuit having three (n=3) single-phase power converting phases  20 , with one single-phase power converting circuit  20   a  having a single-stage AC/DC converter module  22 ; and two (n−1=2) second single-phase power converting circuits  20   b  each having a two-stage converter module  26 . In this embodiment, the apparatus  10  can be connected with a 3-phase grid which may comprise one power input  15  supplied with an AC voltage The power converter apparatus  10  preferably has one power output  16  providing a DC voltage. 
     Preferably, the power converter apparatus  10  may further comprise a controller or control module  40  for an adaptive modulation on the AC to DC power conversion. More preferably, the controller  40  is adapted to automatically adjust or modulate the number of operating power converting circuits  20  by disabling or by-passing one or more of the second single-phase power converting circuits  20   b  when a load power (P) of the power converter apparatus  10  is less than or equal to a predetermined, selected or calculated power threshold. In one specific embodiment, the power threshold can be determined by dividing a maximum load power (Pmax) of the apparatus  10  by the number n, i.e., the total number of single-phase power converting circuits  20  in the apparatus  10 . The controller  40  is adapted to continuously monitor the load power (P) to thereby adaptively control the number power converting circuits  20  and more specifically, the number of second single-phase power converting circuits  20   b  operating to minimize power loss at light load. 
     For example, one or more of the second single-phase power converting circuits  20   b  can be configured with a switch  32  actuatable to by-pass the output stage DC/DC transformer  30  of the two-stage converter module  26  thereby converting said two-stage converter module  26  into a single stage AC/DC converter module. This is advantageous in that it obviates or mitigates power loss associated with the output stage DC/DC transformer  30 . The switch  32  may be activated when the power converter apparats  10  is connected to a one-phase grid. The switch  32  may comprise any traditional electronic switch or circuit switch operable to by-pass the output stage DC/DC transformer  30  of the two-stage converter module  26 . In one embodiment, the switch  32  can be provided in the form of a relay actuatable to bypass one or more of the DC/DC transformers  30  of the respective two-stage converter modules  26  in the second single-phase power converting circuits  20   b  to convert said two-stage converter modules  26  into single stage AC/DC converter modules. The number of second single-phase power converting circuits  20   b  converted in this way can be determined by the load power of the apparatus  10 , that is, for the number of operating second single-phase power converting circuits  20   b  be dependent on the load power of the apparatus  10 . For example, the controller  40  may convert one or more of the two second single-phase power converting circuits  20   b  of a 3-phase power converting circuit  20  when a reduced power load is detected. 
     In one embodiment, the single-stage converter module  22  of the first single-phase power converting circuit  20   a  may comprise a plurality of modules  22  connected in parallel, and each of the two-stage converter modules  26  of the second single-phase power converting circuits  20   b  may comprise a plurality of modules  26  connected in parallel. The numbers of said pluralities of single-stage converter modules  22  and two-stage converter modules  26  in the respective circuits can be the same or different. For example, the first single-phase power converting circuit  20   a  may comprise any number N of single-stage AC/DC converter modules  22 , with N being any natural number equal to or greater than 2; while each of the second single-phase power converting circuits  20   b  may comprise any number N of two-stage converter modules  26 . Yet in another embodiment, the respective number of single-stage converter modules  22  at the first single-phase power converting circuit  20   a  and two-stage converter modules  26  at the second single-phase power converting circuits  20   b  could be different. Furthermore, the number of two-stage converter modules  26  at each of the plurality of second single-phase power converting circuits  20   b  may also be the same or different. For example, one second single-phase power converting circuit  20   b  may comprise a number Ni of two-stage convert modules  26  and another second single-phase power converting circuit  20   b  may comprise a number N ii  of two-stage convener modules  26 , for example. Without being limited by any specific embodiments herein described and/or illustrated, a person skilled in the art will appreciate that any variations in the numbers of the converter modules  22 ,  26  and/or the numbers of the second single-phase power converting circuits  20   b  at the power circuit  20  shall be encompassed by the present invention, as long as the apparatus comprises an asymmetric topology with one first single-phase power converting circuit  20   a  having a single-stage power converter module  22  and at least one second single-phase power converting circuit  20   b  having a two-stage power converter module  26 . 
     Preferably, a plurality of switches  32  such as relays can be arranged respectively one each at the plurality of two-stage converter modules  26  of the second single-phase power converting circuits  20   b . The controller  40  may then actuate selectively one or more of said relays at the plurality of modules  26  of the same or different second single-phase power converting circuit  20   b  to bypass one or more of the respective DC/DC transformers  30  thereby converting one or more modules  26  and/or one or more second single-phase power converting circuits  20   b  into single stage AC/DC converter modules. 
     More preferably, the controller  40  is adapted to adjust or modulate the number of the single-stage AC/DC converter modules  22  of the first single-phase power converting circuit  20   a  based on a detected reference current (I ref ). For example, for the first single-phase power converting circuit  20   a  having N single-stage AC/DC converter modules  22  where N can be any natural number equal to or greater than 2, the controller  40  is adapted to reduce the number of the operating single stage AC/DC converter modules  22  from N to N−1 when the reference current (I t r) is detected to be less than or equal to a product (multiplication) of N−1 and a maximum current (I max ) of the power converter module  22 . The controller  40  is therefore adapted to continuously monitor the reference current (I ref ) to thereby adaptively control the number of operating single-stage converter modules  22  so as to minimize power loss at light load. 
       FIG.  4    further illustrates an exemplified operation of the apparatus  10  having a 3-phase circuit topology such as that as shown in  FIG.  3   . To start with, the apparatus  10  may be provided with a user defined and/or system detected or recorded power load profile and optionally, a preset delay time information such as a preset response time of the controller  40 , depending on the application requirements of the apparatus  10 . For example, based on a detected or user defined load power of the apparatus  10 , the controller  40  may decide whether to execute a 3-phase operation or a single-phase operation. If a load power (P) which is larger than a threshold power, such as being determined by dividing the maximum load power (P max ) of the apparatus  10  by the total number of single-phase power converting circuits  20  (i.e. n=3), the apparatus  10  will be operated under all three phases and the controller  40  will enable all N—of the single-stage converter modules  22  of the one first single-phase power converting circuit  20   a  and the two-stage converter modules  26  of the two second single-phase power converting circuits  20   b . On the other hand, if a load power (P) which is smaller than the threshold power is detected, the controller  40  will preferably disable the two second single-phase power converting circuits  20   b  of the power converting circuit  20 , effectively converting the 3-phase operation into a single-phase operation to minimize power loss or actuate the switches  32  to bypass one or more of the respective DC/DC transformers  30  thereby converting the one or more modules  26  and thus the one or more second single-phase power converting circuits  20   b  into single stage AC/DC converter modules. 
     Preferably, a time delay module  42  can be provided which can be a part of the controller  40  or a separate component of the apparatus  10 . Once a load power of less than the threshold power is detected and prior to the action of disabling or converting the second single-phase power converting circuits  20   b , the time delay module  42  will determine a delay time for the controller  40  to respond. If the detected power load power (P) is low enough and/or the detected reference current (I ref ) is low enough, after the delay time has elapsed, the controller  40  will disable one or more of the modules  22 ,  26 . If the delay time is determined to be greater than or equal to a predetermined, selected or calculated delay time threshold, the controller  40  will preferably automatically disable one or more of the two (i.e., n−1) second single-phase power converting circuits  20   b . This is to ensure a slow response time for the controller  40  to respond to any load decrease detected to prevent disabling of one or more of the two (i.e., n−1) second single-phase power converting circuits  20   b  in response to momentary decreases in load, i.e., low load inverse power spikes. Alternatively, if the delay time is less than the delay time threshold, the controller  40  will allow continued operation of the second single-phase power converting circuits  20   b  until a delay time which meets or exceeds the delay time threshold is subsequently detected. The determining of the delay time is important in that it effectively slows down the response of the controller  40  to load decrease for a more stable control of the operation and thus power conversion by the apparatus  10 . 
     In one embodiment, it is preferred that all of the n−1 second single-phase power circuits  20   b  be bypassed or disabled, leaving only the/one first single-phase power converting circuit  20   a  at light power load to thereby improve conversion efficiency of the apparatus  10 . However, it is also possible that only one or more but not all of the n−1 second single-phase power circuits  20  be disabled or by-passed, depending on the load decrease detected. 
     After a light power load is detected and the apparatus is effectively converted to a single-phase operation, the controller  40  will then modulate the number of the plurality of single-stage converter modules  22  based on an adaptive modulation control (AMC), as shown in  FIG.  4   . For example, for a first single-phase power convening circuit  20   a  having N single-stage converter modules  22  each having one AC/DC converter  24 , where N is any natural number equal to or greater than 2, the controller  40  is adapted to reduce number of the single-stage AC/DC converter modules  22  from N to N−1 when a reference current (I ref ) of less than or equal to a multiplication of N−1 with a maximum current (In) of the converter module  22 , i.e. N−1*I mas ≥I ref  is detected; and more preferably, when the reference current (I ref ) detected might be greater than a multiplication of N−2 with the maximum current (I max ), that is, it falls within the range of N−1*I mas ≥I ref &gt;N−2*I mas . The controller  40  will continue to monitor the reference current (I ref ) and adjust the number of the operating single-stage converter modules  22  by a consecutive reduction on the number, i.e., one at a time during the iteration until the lowest operable number of the single-stage converter modules  22  is reached. On the other hand, if a reference current (I ref ) which is greater than (N−1*I mas ) is detected, the controller  40  will continue to enable all N number of the single-stage converter modules  22 . 
     Preferably, prior to reducing the number of the plurality of single-stage modules  22 , the time delay module  42  will be arranged to determine the delay time and if the delay time is greater than or equal to the predetermined, selected or calculated delay time threshold, the controller  40  executes the disabling or by-passing of one/each of the single-stage modules  22 . Again, the determination of the delay time slows down the response of the controller  40  to load decrease such that a more stable operation can be achieved. Optionally, an adjustment to the phase angle will also be conducted prior to the execution of the disabling of one/each of the single-stage modules  22 . The converter modules are preferably interleaved in operation such that each convert module has a respective phase angle in switching signal different to other converter modules. The phase angle difference is dependent on the pulse wave modulated (PMW) signal of the converter apparatus  10 . The phase angle difference is dependent on the result of 360°/N where N is the number of converter modules. Any ripple on the input AC current can be mitigated or minimized if the phase angles are properly adjusted. 
     Preferably, the AC/DC converter  24  of the one-stage converter module  22  of the first single-phase power converting circuit  20   a  is non-isolated; whereas the DC/DC transformers  30  of the two-stage converter modules  26  of the n−1 second single-phase power converting circuits  20   b  are preferably isolated, for example, in the form of galvanically separating transformers. 
     In another not shown embodiment of the power converter apparatus  10 , the first single-phase power converting circuit  20   a  may comprise a two-stage converter module comprising an AC/DC converter  24  as an input stage connected with a DC/DC transformer  30  as an output stage, but where the DC/DC transformer  30  is preferably permanently by-passed through, for example, permanent actuation of one or more by-pass switches or at least temporarily by-passed through actuation of said one or more by-pass switches. In this embodiment, the concept of the invention can be realized without requiring two different converter module configurations, namely all of the converter modules have the same two-stage configuration but a first one of said modules is configured to perform only as a single stage converter module. 
     In another aspect of the present invention there is provided a method of modulating a power converting apparatus  10 . The apparatus  10  preferably comprises a plurality of n power converting circuits  20  in parallel, wherein n is a natural number equals to or greater than 2. The method comprises the providing of one first single-phase power converting circuit  20   a  having a one-stage AC/DC converter module  22  with preferably only an AC/DC converter  24 , and n−1 second single-phase power converting circuits  20   b  each comprising a two-stage converter module  26  having an AC/DC converter  24  as an input stage and a DC/DC transformer  30  as an output stage. The method further comprises an automatic by-passing of one or more of the output stage DC/DC transformers of the n−1 second single-phase power converting circuits  20   b  when a load power of the apparatus  10  is less than or equal to a predetermined, selected or calculated power threshold. 
     Preferably, the one-stage AC/DC converter module  24  of the first single-phase power converting circuit  20   a  comprises a plurality of N one-stage AC/DC converter modules  24 , where N is equal to or greater than 2. The method further comprises the determining of an operating current as a reference current (I ref ) of the first single-phase power converting circuit  20   a , and adjusting number of the one-stage AC/DC converter modules  24  from N to a N−1 when the reference current (I ref ) determined is less than or equal to a multiplication of N−1 with a maximum current (I max ) of the apparatus, i.e. N−1*I mas ≥I ref ; and more preferably, when the reference current (I ref ) determined might as well greater than a multiplication of N−2 with the maximum current (I max ), that is, falls within the range of N−1*I mas ≥I ref &gt;N−2*I mas . 
     In one embodiment, the method further comprises the step of determining a delay time prior to the by-passing and/or adjusting steps and, if the determined delay time is greater than or equal to a predetermined, selected or calculated delay time threshold, executing the by-passing and/or adjusting steps. 
     In yet a further aspect of the present invention, there is provided a controller or control module  40  for use with the power converter apparatus  10  as described above. The controller  40  is adapted to automatically by-pass one or more of the output stage DC/DC transformers of the second single-phase power converting circuits  20   b  when the load power of the apparatus  10  is less than or equals to a predetermined, selected or calculated power threshold. Preferably, when the one-stage AC/DC converter module  22  of the first single-phase power converting circuit  20   a  comprises a plurality of N one-stage AC/DC converter modules  22 , where N is equal to or greater than 2, the controller is adapted to adjust number of the plurality of one-stage AC/DC converter modules  22  from N to N−1 when a detected reference current (I ref ) of the apparatus is less than or equal to a multiplication of N−1 with a maximum current (I max ) of the apparatus; and more preferably, when the reference current (I ref ) determined might as well greater than a multiplication of N−2 with the maximum current (I max ), that is, falls within the range of N−1*I mas ≥I ref &gt;N−2*I mas . 
     In one further embodiment, the controller  40  is further provided with a time delay module  42  for determining a delay time. Preferably, the controller  40  is adapted to by-pass one or more of the second single-phase power converting circuits  20   b  and/or adjust number of the plurality of one-stage AC/DC converter modules  22  of the first single-phase power converting circuit  20   a  from N to N−1 based on the above-described operating conditions only when the determined delay time is greater than or equals to a predetermined, selected or calculated delay time threshold. 
       FIG.  5    illustrated a plurality of waveforms showing the phase voltage, phase current, output voltage and output current under the power conversion process by a 3-phase power converter according to an embodiment of the present invention. Particularly, in response to a change of power load from 6 kW to 400 W, i.e., from a high power load to a light power load, the phase current is converted accordingly from a 3-phase current to a single phase current; and when the power load is changed from 400 W to 6 kW, i.e. from a light power load to a high power load, the phase current is converted accordingly from the single phase current to a 3-phase current. It is demonstrated that at the light load where only a single phase is operating, a power conversion efficiency of higher than 98% is achievable. It is further illustrated in  FIG.  6    that an increase of about 5% to about 13% in efficiency is achievable by converting a 3-phase operation to a single-phase operation of the present invention at a light load of below 1000 W when compared with the prior art technology. 
     The present invention is therefore advantageous in that it provides an electric power converter apparatus for converting an alternating current (AC) power input to a direct current (DC) power output. The power converter apparatus is configured with an asymmetric topology having preferably one first single-phase power converting circuit having preferably only one single-stage AC/DC converter module, and at least one second single-phase power converting circuit having a two-stage AC/DC and DC/DC convener module. Preferably, the apparatus is adapted to disable or by-pass one or more of the second single-phase power converting circuits at a light power load, i.e. when the power of apparatus is determined to be less than or equals to P max /n, where P max  being the maximum power operable at the apparatus and n being any number greater than or equals to 2, so as to reduce power loss and thus improves power efficiency of the apparatus at light power load. More preferably, the first single-phase power converting circuit may comprise a plurality of N single-stage AC/DC converter modules, where N being any number greater than or equals to 2. The number of the single-stage AC/DC converter modules are scalable by way of consecutively reducing the number N of operating single-stage modules based on a reference current (I ref ) when I ref  falls within the condition of N−1*I mas ≥I ref &gt;N−2*I mas , where I max  being the maximum current operable at the apparatus. The scalability or adjustability on the number of operating single-stage AC/DC converter modules enables a further reduction on the phase current when the apparatus is under a single-phase operation to minimize power loss and thus, enhances power conversion efficiency of the apparatus. 
     The present description illustrates the principles of the present invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. 
     Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only exemplary embodiments have been shown and described and do not limit the scope of the invention in any manner. It can be appreciated that any of the features described herein may be used with any embodiment. The illustrative embodiments are not exclusive of each other or of other embodiments not recited herein. Accordingly, the invention also provides embodiments that comprise combinations of one or more of the illustrative embodiments described above. Modifications and variations of the invention as herein set forth can be made without departing from the spirit and scope thereof, and, therefore, only such limitations should be imposed as are indicated by the appended claims. 
     In the claims hereof, any element expressed as a means for performing a specified function is intended to encompass any way of performing that function. The invention as defined by such claims resides in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the claims call for. It is thus regarded that any means that can provide those functionalities are equivalent to those shown herein. 
     In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e., to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 
     It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art.