Abstract:
An input current shaping AC to DC converter with PFC front end that reduces input current harmonics is provided. In one embodiment, an AC to DC converter connectable with an alternating current source and operable to output a direct current has a PFC front end followed by a DC/DC converter. The PFC front end reduces harmonic components present in an input current waveform received by the PFC front end from the alternating current source and includes current steering circuitry and, optionally, valley filling circuitry. The DC/DC converter is one that presents pure resistive input impedance to the PFC front end. The DC/DC converter outputs the direct current to a load connected thereto.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to power converters, and more particularly to alternating current (AC) to direct current (DC) converters with power factor corrective (PFC) requirements. 
   BACKGROUND OF THE INVENTION 
   AC to DC power converters with PFC capability are desirable in a number of applications including, for example, in laptop and desktop computers. However, conventional AC to DC power converters have high harmonic input currents and their efficiency is not as good as desired for many applications. In this regard,  FIG. 1  is a schematic diagram of one prior art AC to DC converter with PFC front end  110 . As shown, the PFC front end  110  includes a valley filling circuit  140  (inductor  142 , diode  144  and transistor  146 ) and a current steering network  160  having two capacitors  162 ,  166  and three diodes  170 ,  176 ,  178  arranged in a network with four nodes.  FIG. 2  is a plot of a simulated input current waveform  202  for the PFC front end  110  shown in  FIG. 1 . As shown in the plot of  FIG. 2 , the input current waveform includes significant harmonics. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is providing an AC to DC converter with a PFC front end. 
   Another object of the present invention is reducing input current harmonics in an AC to DC converter with a PFC front end. 
   These objects and others are achieved by various aspects of the high efficient input current shaping AC to DC converter with PFC front end of the present invention. According to one aspect, an AC to DC converter connectable with an alternating current source and operable to output a direct current comprises a PFC front end followed by a DC/DC converter. The PFC front end includes current steering circuitry that reduces harmonic components present in an input current waveform received by the PFC front end from the alternating current source. The DC/DC converter comprises one that presents pure resistive input impedance to the PFC front end. The DC/DC converter outputs the direct current to a load. By connecting the objects in the aforementioned ways, the PFC front end does not have a power switch which is operating all the time or even a power switch which is only operating for a short time period around the input current zero crossing. As a consequence, the switching loss is greatly reduced while at the same time keeping a high power factor and low harmonics. 
   The current steering circuitry may be configured in various manners. In one embodiment, the current steering circuitry comprises three capacitors and six diodes (3C&amp;6D). The three capacitors and six diodes may, for example, be arranged in a network having six nodes. For example, a first capacitor may be connected between a first node and a second node, a second capacitor may be connected between a third and a fourth node, a third capacitor may be connected between a fifth node and a sixth node, a first diode may be connected between the first node and the fifth node, a second diode may be connected between the first node and the third node, a third diode may be connected between the second node and the third node, a fourth diode may be connected between the fourth node and the fifth node, a fifth diode may be connected between the fourth node and the sixth node, and a sixth diode may be connected between the second node and the sixth node. In another embodiment, the current steering circuitry comprises two capacitors and three diodes (2C&amp;3D). The two capacitors and three diodes may, for example, be arranged in a network having four nodes. For example, a first capacitor may be connected between a first node and a second node, a second capacitor may be connected between a third and a fourth node, a first diode may be connected between the first node and the third node, a second diode may be connected between the second node and the third node, and a third diode may be connected between the second node and the fourth node. 
   In addition to current steering circuitry, the PFC front end of the converter may also include valley filling circuitry that reduces the presence of discontinuities in the input current waveform. In one embodiment, the valley filling circuitry comprises an inductor, a diode, and a switching element. The diode, inductor, and switching element may, for example, be arranged in a network having four nodes. For example, the inductor may be connected between a first node and a second node, the diode may be connected between the second node and a third node, and the switching element may be connected between the second node and a fourth node. 
   The PFC front end may include other components in addition to the aforementioned current steering circuitry and valley filling circuitry. Further, the PFC front end may be implemented with different embodiments of the current steering circuitry in combination with valley filling circuitry or without valley filling circuitry. For example, the PFC front end may be configured with a 3C&amp;6D current steering network and valley filling circuitry or with a 3C&amp;6D current steering network but no valley filling circuitry. By way of further example, the PFC front end may be configured with a 2C&amp;3D current steering network and valley filling circuitry or with a 2C&amp;3D current steering network and no valley filling circuitry. 
   According to another aspect, AC to DC conversion means connectable with an alternating current source and operable to output a direct current comprise first stage means for correcting a power factor and second stage means for outputting the direct current to a load connected to the second stage means. The first stage means include current steering means for reducing harmonic components present in an input current waveform received by the first stage means from the alternating current source. The second stage means present pure resistive input impedance to the first stage means. The current steering means may, for example, be current steering circuitry such as, for example, a 3C&amp;6D current steering circuit or a 2C&amp;3D current steering circuit. The first stage means may optionally include valley filling means for reducing the presence of discontinuities in the input current waveform around the zero crossing such as, for example, a valley filling circuit. The second stage means may, for example, be a constant power DC/DC converter. 
   According to one more aspect, a current shaping AC to DC converter comprises a valley filling circuit, a current steering circuit connected with the valley filling circuit, and a constant power DC/DC converter connected with the current steering circuit and the valley filling circuit. In one embodiment, the current steering circuit comprises three capacitors and six diodes arranged in a network having six nodes with a first capacitor connected between a first node and a second node, a second capacitor connected between a third and a fourth node, a third capacitor connected between a fifth node and a sixth node, a first diode connected between the first node and the fifth node, a second diode connected between the first node and the third node, a third diode connected between the second node and the third node, a fourth diode connected between the fourth node and the fifth node, a fifth diode connected between the fourth node and the sixth node, and a sixth diode connected between the second node and the sixth node. In another embodiment, the current steering circuit comprises two capacitors and three diodes arranged in a network having four nodes with a first capacitor connected between a first node and a second node, a second capacitor connected between a third and a fourth node, a first diode connected between the first node and the third node, a second diode connected between the second node and the third node, and a third diode connected between the second node and the fourth node. In one embodiment, the valley filling circuit comprises an inductor, a diode, and a switching element arranged in a network having four nodes with the inductor connected between a first node and a second node, the diode connected between the second node and a third node, and the switching element connected between the second node and a fourth node. 
   Various nodes of the current steering network and the valley filling circuit may coincide thereby connecting the valley filling circuit with the current steering circuit. For example, the first and sixth nodes of the 3C&amp;6D current steering network may coincide with the third and fourth nodes, respectively, of the valley filling circuit. In a further example, the first and fourth nodes of the 2C&amp;3D current steering network may coincide with the third and fourth nodes, respectively, of the valley filling circuit. Additionally, input terminals of the DC/DC converter may be connected to various nodes of the current steering network and valley filling circuit. For example, one input terminal of the DC/DC converter may be connected with the coincident first/third nodes of the 3C&amp;6D current steering network/valley filing circuit and the other input terminal of the DC/DC converter may be connected with the coincident sixth/fourth nodes of the 3C&amp;6D current steering network/valley filling circuit. In a further example, one input terminal of the DC/DC converter may be connected with the coincident first/third nodes of the 2C&amp;3D current steering network/valley filing circuit and the other input terminal of the DC/DC converter may be connected with the coincident fourth/fourth nodes of the 2C&amp;3D current steering network/valley filling circuit. 
   These and other aspects and advantages of the present invention will be apparent upon review of the following Detailed Description when taken in conjunction with the accompanying figures. 

   
     DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and further advantages thereof, reference is now made to the following Detailed Description, taken in conjunction with the drawings, in which: 
       FIG. 1  is a schematic diagram showing a prior art AC to DC converter with a prior art PFC front end; 
       FIG. 2  is a plot of a simulated input current waveform for the AC to DC converter PFC front end shown in  FIG. 1 ; 
       FIG. 3  is a schematic diagram of one embodiment of an AC to DC converter having a PFC front end combining 3C&amp;6D current steering circuitry with valley filling circuitry, followed by a dc/dc converter with pure resistive input impedance; 
       FIG. 4  is a plot showing a simulated input voltage waveform and corresponding input current waveform for the AC to DC converter of  FIG. 3 ; 
       FIG. 5  is a schematic diagram of one embodiment of an AC to DC converter having a PFC front end with 3C&amp;6D current steering circuitry and without valley filling circuitry, followed by a dc/dc converter with pure resistive input impedance; 
       FIG. 6  is a plot showing a simulated input voltage waveform and corresponding input current waveform for the AC to DC converter of  FIG. 5 ; 
       FIG. 7  is a schematic diagram of one embodiment of an AC to DC converter having a PFC front end combining 2C&amp;3D current steering circuitry with valley filling circuitry, followed by a dc/dc converter with pure resistive input impedance; 
       FIG. 8  is a plot showing a simulated input voltage waveform and corresponding input current waveform for the AC to DC converter of  FIG. 7 ; 
       FIG. 9  is a schematic diagram of one embodiment of an AC to DC converter having a PFC front end with 2C&amp;3D current steering circuitry and without valley filling circuitry, followed by a dc/dc converter with pure resistive input impedance; and 
       FIG. 10  is a plot showing a simulated input voltage waveform and corresponding input current waveform for the AC to DC converter of  FIG. 9 . 
   

   DETAILED DESCRIPTION 
     FIG. 3  shows a schematic diagram of one embodiment of a power converter  300 . The power converter  300  includes a valley fill circuit  340 , a current steering network  360 , and a DC/DC converter  390 . The valley fill circuit  340 , the current steering network  360 , and the DC/DC converter  390  are connected to one another at node  302 . The valley fill circuit  340 , the current steering network  360  and the DC/DC converter  390  are also connected to common node  304 . Common node  304  may be referred to herein as the ground reference or simply ground. The power converter  300  is connectable to an alternating current source  306  (e.g., an electrical outlet) and operates to convert an input alternating current to direct current that may be supplied to a load  308 . Together, the valley filling circuit  340  and the current steering circuit  360  comprise a PFC front end  310 . In other embodiments, such as described herein in connection with  FIGS. 5 and 9 , valley filling circuitry is not included in the PFC front end  310 . 
   In addition to the valley fill circuit  340  and the current steering network  360 , the PFC front end  310  of power converter  300  may also include various additional components such as diodes (D 1 , D 2 , D 3 , D 5 , D 7 )  312 - 320 , resistors (R 7 , R 13 )  322 ,  324 , and capacitor (C 2 )  326 . Diode (D 1 )  312  is connected to diode (D 3 )  316 , diode (D 7 )  320 , resistor (R 13 )  324  and capacitor (C 2 )  326  at node  330  and to source  306  through EMI  328  and diode (D 2 )  314  at node  332 . Diode (D 2 )  314  is connected to diode (D 1 )  312  and source  306  through EMI  328  at node  332  and to common node  304 . Diode (D 3 )  316  is connected to diode (D 1 )  312 , diode (D 7 )  320 , resistor (R 13 )  324  and capacitor (C 2 )  326  at node  330  and to diode (D 5 )  318  and source  306  through EMI  328  at node  334 . Diode (D 5 )  318  is connected to diode (D 3 )  316  and source  306  through EMI  328  at node  334  and to common node  304 . Diode (D 7 )  320  is connected to diode (D 1 )  312 , diode (D 3 )  316 , resistor (R 13 )  324  and capacitor (C 2 )  326  at node  330  and to valley fill circuit  340 , current steering network  360  and DC/DC converter  390  at node  302 . Resistor (R 7 )  322  is connected between capacitor (C 2 )  326  and common node  304 . Resistor (R 13 )  324  is connected between node  330  and valley fill circuit  340 . Capacitor (C 2 )  326  is connected between node  330  and resistor (R 7 )  322 . The various components included in the power converter  300  in addition to the valley fill circuit  340 , current steering network  360  and DC/DC converter  390  and the arrangement thereof are exemplary, and in other embodiments, it may be possible to employ different components arranged in similar or in different configurations. 
   The valley fill circuit  340  includes an inductor (L 1 )  342 , a diode (D 6 )  344 , and a switching element (S 3 )  346  arranged in a network having four nodes  302 ,  304 ,  336  and  350 . Inductor (L 1 )  342 , diode (D 6 )  344  and switching element (S 3 )  346  are connected to one another at node  350 . More particularly, inductor (L 1 )  342  is connected between node  336  (a terminal of resistor (R 13 )  324 ) and node  350 , diode (D 6 )  344  is connected between node  350  and node  302 , and switching element (S 3 )  346  is connected between node  350  and common node  304 . When closed, switching element (S 3 )  346  provides a zero-resistance path from node  350  to common node  304 . In this regard, switching element (S 3 ) may comprise various components including, for example, one or more transistors (e.g., MOSFET(s) and/or BJT(s) and/or IGBT(s)). 
   The current steering network  360  includes three capacitors (C 9 , C 10 , and C 11 )  362 - 366  and six diodes (D 8 , D 9 , D 10 , D 11 , D 12 , D 13 )  368 - 378  arranged in a network having six nodes  302 ,  304 ,  380 - 386 . Capacitor (C 9 )  362  is connected to diode (D 8 )  368  and diode (D 9 )  370  at node  302  and to diode (D 12 )  376  and diode (D 13 )  378  at node  380 . Capacitor (C 10 )  364  is connected to diode (D 9 )  368  and diode (D 11 )  374  at node  382  and to diode (D 10 )  372  and diode (D 13 )  378  at common node  304 . Capacitor (C 11 )  366  is connected to diode (D 9 )  370  and diode (D 12 )  376  at node  384  and to diode (D 10 )  372  and diode (D 11 )  374  at node  386 . Diode (D 8 )  368  is connected to diode (D 9 )  370  and capacitor (C 9 )  362  at node  302  and to diode (D 11 )  374  and capacitor (C 10 )  364  at node  382 . Diode (D 9 )  370  is connected to diode (D 8 )  368  and capacitor (C 9 )  362  at node  302  and to diode (D 12 )  376  and capacitor (C 11 )  366  at node  384 . Diode (D 10 )  372  is connected to diode (D 11 )  374  and capacitor (C 11 )  366  at node  386  and to capacitor (C 10 )  364  and diode (D 13 )  378  at common node  304 . Diode (D 11 )  374  is connected to diode (D 10 )  372  and capacitor (C 11 )  366  at node  386  and to diode (D 8 )  368  and capacitor (C 10 )  364  at node  382 . Diode (D 12 )  376  is connected to diode (D 13 )  378  and capacitor (C 9 )  362  at node  380  and to diode (D 9 )  370  and capacitor (C 11 )  366  at node  384 . Diode (D 13 ) is connected to diode (D 12 )  376  and capacitor (C 9 )  362  at node  380  and to capacitor (C 10 )  364  and diode (D 10 )  372  at common node  304 . 
   The DC/DC converter  390  may be configured in a number of different manners. In this regard, DC/DC converter  390  may, for example, be configured to step-up or step-down the output voltage that is output to load  308 . Regardless of its configuration, it is desirable that DC/DC converter  390  be of a constant power type. Stated another way, DC/DC converter  390  desirably presents pure resistive input impedance to the PFC front end  310 . A constant power/pure resistive input impedance DC/DC converter  390  is desirable to avoid introducing a I/R negative impedance typical of many DC/DC converters. 
     FIG. 4  is a plot showing a simulated input voltage waveform  402  and corresponding input current waveform  404  for the power converter  300  of  FIG. 3  that combines the 3C&amp;6D current steering network  360  with the boost valley filling circuit  340 . As can be seen by comparing the plot of  FIG. 4  with the plot of  FIG. 2  for the prior art device, not only are the harmonic components of the input current waveform improved relative to the prior art device shown in  FIG. 1 , but the peak value of the current is suppressed. Here the DC/DC converter  390  is a constant power load and appears as a pure resistive impedance for the PFC stage. The 3C&amp;6D current steering network  360  of the power converter  300  of  FIG. 3  generates a less harmonic input current wave shape than the prior art 2C&amp;3D network of  FIG. 1 . 
     FIG. 5  shows another embodiment of a power converter  500  configured differently than in the embodiment of  FIG. 3 . The PFC front end  510  of power converter  500  of  FIG. 5  includes a 3C&amp;6D current steering network  360  but does not implement the valley filling circuit. In this regard, switch (S 3 ), and diode (D 6 ) are not included in power converter  500 . Instead, inductor (L 1 )  342  is connected directly with node  302 . 
     FIG. 6  is a plot showing a simulated input voltage waveform  602  and corresponding input current waveform  604  for the power converter  500  of  FIG. 5  with the 3C&amp;6D current steering network  360  without a boost valley filling circuit. As can be seen by comparing the plot of  FIG. 6  with the plot of  FIG. 4 , the input harmonics are slightly increased but are still acceptable for many applications and represent an improvement over the prior art device of  FIG. 1  that employs a 2C&amp;3D current steering circuit rather than a 3C&amp;6D current steering network and a constant power load which has a negative input impedance following the PFC stage. However, the absence of the valley filling circuit means that discontinuities  606  around the zero crossing points of the input current waveform  604  are not filled in as is the case with the power converter  300  of  FIG. 3 . Nevertheless, the presence of such discontinuities  606  may be acceptable for a number of applications. 
     FIG. 7  shows another embodiment of a power converter  700  configured differently than in the embodiment of  FIG. 3 . The PFC front end  710  of power converter  700  of  FIG. 7  includes a 2C&amp;3D current steering network  760  (instead of 3C&amp;6D current steering circuit) along with the valley filling circuit  340 . In this regard, the 2C&amp;3D current steering circuit includes two capacitors (C 9  and C 11 )  362  and  366  and three diodes (D 9 , D 12  and D 13 )  370 ,  376  and  378  arranged in a network having four nodes  302 ,  304 ,  380  and  384 . Capacitor (C 9 )  362  is connected to diode (D 9 )  370  at node  302  and to diode (D 12 )  376  and diode (D 13 )  378  at node  380 . Capacitor (C 11 )  366  is connected to diode (D 9 )  370  and diode (D 12 )  376  at node  384  and to diode (D 13 )  378  at common node  304 . Diode (D 9 )  370  is connected to capacitor (C 9 )  362  at node  302  and to diode (D 12 )  376  and capacitor (C 11 )  366  at node  384 . Diode (D 12 )  376  is connected to diode (D 13 )  378  and capacitor (C 9 )  362  at node  380  and to diode (D 9 )  370  and capacitor (C 11 )  366  at node  384 . Diode (D 13 ) is connected to diode (D 12 )  376  and capacitor (C 9 )  362  at node  380  and to capacitor (C 11 )  366  at common node  304 . 
     FIG. 8  is a plot showing a simulated input voltage waveform  802  and corresponding input current waveform  804  for the power converter  700  of  FIG. 7  with the 2C&amp;3D current steering network  760  and the boost valley filling circuit  340 . As can be seen by comparing the plot of  FIG. 8  with the plot of  FIG. 4 , the input harmonics are slightly increased but are still acceptable for many applications and represent an improvement of the input harmonics as compared with the prior art device of  FIG. 1  that lacks a DC/DC converter following the PFC stage. 
     FIG. 9  shows another embodiment of a power converter  900  configured differently than in the embodiment of  FIG. 7 . The PFC front end  910  of the power converter  900  of  FIG. 9  includes a 2C&amp;3D current steering network  760  similar to that of power converter  700  but does not implement a valley filling circuit. In this regard, switch (S 3 ), and diode (D 6 ) are not included in power converter  900 . Instead, inductor (L 1 )  342  is connected directly with node  302 . 
     FIG. 10  is a plot showing a simulated input voltage waveform  1002  and corresponding input current waveform  1004  for the power converter  900  of  FIG. 9  with the 2C&amp;3D current steering network  760  and without a boost valley filling circuit. As can be seen by comparing the plot of  FIG. 10  with the plot of  FIG. 8 , the input harmonics are slightly increased but are still acceptable for many applications and represent an improvement over the prior art device of  FIG. 1  that employs a 2C&amp;3D current steering circuit with a constant power load which has a negative input impedance following the PFC stage. However, the absence of the valley filling circuit means that discontinuities  1006  around the zero crossing points of the input current waveform  1004  are not filled in as is the case with the power converter  700  of  FIG. 7 . Nevertheless, the presence of such discontinuities  1006  may be acceptable for a number of applications. 
   The plots of  FIGS. 4 ,  6 ,  8  and  10  are based on various exemplary components having specified electrical characteristics that may be employed in the differently configured power converters  300 ,  500 ,  700  and  900 . However, the various embodiments are not limited to the exemplary capacitance, inductance, resistance, and threshold voltage (V TH ) values for the various capacitors, inductors, resistors and diodes included in the power converters  300 ,  500 ,  700  and  900  and such values may be varied as appropriate for different applications. 
   While various embodiments of the present invention have been described in detail, further modifications and adaptations of the invention may occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.