Patent Publication Number: US-2009237044-A1

Title: Bidirectional interleaved DC to DC converter utilizing positively coupled filter inductors

Description:
TECHNICAL FIELD 
     Generally, the present invention relates to DC to DC converters. Particularly, the present invention relates to a DC to DC converter that utilizes mutually coupled filter inductors. More particularly, the present invention relates to a DC to DC converter that utilizes mutually coupled filter inductors with a positive coupling coefficient, allowing a filter capacitor maintained by the converter to be reduced in size, such that the converter converts DC voltage (current) from one level to another with reduced voltage ripple. 
     BACKGROUND ART 
     Bidirectional DC to DC converters are typically used to convert DC voltage (current) from a given level to another, such that the converted DC voltage has a time-varying AC component or ripple that is compatible with the requirements or specifications of various electrical components that require DC power. That is, DC to DC converters are able to efficiently convert DC voltage (current) from one level, such as a high-voltage level, to another DC voltage level, such as a low-voltage level without the addition of a significantly large AC component or ripple. In addition to being able to convert DC power to various voltage levels, a DC to DC converter that is capable of bidirectional operation is able to convert DC power to different voltage (current) levels depending on the direction of the flow of electrical current through the converter. Thus, because of these features, bidirectional DC to DC converters are utilized in various applications where different DC voltage (current) power levels are needed. 
     Bidirectional DC to DC converters have been utilized by airships, or other lighter-than-air platforms, which utilize DC power from batteries to power various propulsion systems maintained by the platform. One type of bidirectional DC to DC converter used with an airship utilizes single interleaved converters with filter inductors that are not mutually coupled. However, because the charging and discharging cycles of the batteries used for powering the various propulsion systems of the airship differ in time for duty cycles other than 50%, the waveform of the current ripple passing through the converter is asymmetrical. Unfortunately, the non-coupled DC to DC converter is unable to cancel the asymmetrical ripple currents of the asymmetrical current waveform, which results in an output voltage from the converter that has significant ripple, which is not desirable for the optimal operation of DC powered electrical components, such as those aboard the airship. 
     To reduce the amount of ripple, or sometimes referred to as an AC component imposed on the DC power, various attempts have been made to incorporate supplemental filter capacitors into the design of the DC to DC converter. While the use of the additional filter capacitors in conjunction with the DC to DC converter provides an output voltage with reduced ripple, the added capacitors substantially increase the size of the DC to DC converter. This increased size is undesirable in an airship, as it has limited payload area, inasmuch as the airship relies upon its buoyancy to navigate efficiently. Moreover, the addition of supplemental filter capacitors also adds significantly to the cost of manufacturing the DC to DC converter. 
     Therefore, there is a need in the art for a bidirectional DC to DC converter that converts electrical voltage from one level to another level with reduced voltage ripple or AC component, without utilizing large, bulky supplemental filtering capacitors. In addition, there is a need for a bidirectional DC to DC converter that utilizes mutually-coupled filter inductors with a positive coupling coefficient. Furthermore, there is a need for a bidirectional DC to DC converter that utilizes an output capacitor with reduced size and weight, while maintaining a reduced voltage ripple. 
     SUMMARY OF INVENTION 
     In light of the foregoing, it is a first aspect of the present invention to provide a bidirectional DC to DC converter utilizing positively coupled filter inductors. 
     Another aspect of the present invention is to provide a bidirectional interleaved DC to DC converter for converting DC power from one voltage level to another voltage level comprising a switching network having a first input/output node, a pair of positively coupled inductors coupled to the switching network to form a second input/output node, and a controller coupled to the switching network, the controller actuating the switching network, such that power supplied to the first input/output node at one voltage level is converted into another voltage level at the second input/output node, and vice versa, the positively coupled inductors reducing the voltage ripple associated with converted voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings wherein: 
         FIG. 1  is a schematic diagram of an energy generation system utilizing an interleaved bidirectional DC to DC converter in accordance with the concepts of the present invention; 
         FIGS. 2A and 2B  are schematic views of the bidirectional DC to DC converter in accordance with the concepts of the present invention; 
         FIG. 3  is a graph showing the electrical currents that are processed by the interleaved positively coupled inductors of the converter to reduce the voltage ripple output by the converter in accordance with the concepts of the present invention; 
         FIG. 4  is a graph showing an output voltage of the converter using interleaved positively coupled inductors of the present invention having approximately the same amount of output voltage ripple as compared to a converter that uses interleaved non-coupled inductors and an output filtering capacitor that has twice the capacitance as that provided by the converter using interleaved positively coupled inductors; and 
         FIG. 5  is a graph showing an output voltage of the converter using interleaved positively coupled inductors of the present invention having approximately half of the output voltage ripple as compared to a converter that uses interleaved non-coupled inductors and an output filtering capacitor that has the same amount of capacitance as that provided by the converter using interleaved positively coupled inductors. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Before setting forth the structural and functional details associated with the present invention, a brief discussion of the particular context in which the present invention may be utilized will be provided below, as it is believed it will assist the reader in understanding the invention. 
     A bidirectional DC to DC converter, generally referred to by the numeral  10 , used in association with an energy generation system  20  maintained aboard an airship, or other lighter than air platform  30 , is shown in  FIG. 1 . Of course, the converter  10  could be used in any application. In particular, the energy generation system  20  is comprised of a fuel cell  40  and an electrolyzer  42  that are coupled together by an H 2 O storage tank  44  and an H 2 /O 2  storage tank  46 . The bidirectional DC to DC converter  10  is configured such that an input/output node  52  maintained by the converter  10  is coupled to a DC bus  54 , while another input/output node  56  maintained by the converter  10  is coupled to a switch  70 . The switch  70  maintains two positions designated A and B, and as such, when the switch  70  is in position A, the converter  10  is coupled to the fuel cell  40 , and when the switch  70  is in position B, as shown, the converter  10  is coupled to the electrolyzer  42 . 
     A battery  100 , as well as any other power source or electrical load  120  to be driven, may be coupled to the DC bus  54 . For example, the load  120  may be an electric powered propulsion unit that propels the airship  30  may be coupled to the DC bus  54 . As such, DC power may be bidirectionally transferred to and from the DC to DC converter  10  depending on whether power is being supplied via the DC bus  54  to the energy generation system  20 , or whether power is being supplied from the energy generation system  20  to the DC bus  54 . 
     For example, when the energy generation system  20  is actively generating power, the switch  70  is placed in position A, and the operation of the fuel cell  40  is initiated. Next, H 2  (hydrogen gas) and O 2  (oxygen gas) are supplied to the fuel cell  40  from the H 2 /O 2  storage tank  46 . The fuel cell  40  then acts on the H 2  and O 2  gases to generate low-voltage, high-current electrical power and H 2 O (water). The generated H 2 O is stored in the H 2 O storage tank  44 , while the electrical power generated is supplied to the bidirectional DC to DC converter  10 . The converter  10  converts the low-voltage, high-current power into high-voltage, low-current power for delivery to the DC bus  54 . Once the converted power is supplied to the DC bus  54  it may be supplied to the battery  100  for charging or the converted power may be delivered to the electrical load  120  also coupled to the bus  54 . 
     Alternatively, when the power generation system  20  is not generating power, the switch  70  is placed into position B, such that the battery  100  is coupled to the electrolyzer  42  via the DC to DC converter  10 . During this mode of operation, the DC to DC converter  10  converts the high-voltage, low-current power delivered from the battery  100  into low-voltage, high-current power that is supplied to the electrolyzer  42 . Utilizing the power from the battery  100 , the electrolyzer  42  disassociates H 2  (hydrogen gas) and O 2  (oxygen gas) from the water supplied from the H 2 O storage tank  44 . The H 2  (hydrogen gas) and O 2  (oxygen) generated therefrom are then stored at the H 2 /O 2  storage tank  46  for subsequent use by the fuel cell  40 . As such, it is evident that the use of the bidirectional DC to DC converter  10  enables the airship  30  to efficiently make use of the energy generating aspects of the fuel cell  40  and the electrolyzer  42 . With the context of the operation of the bidirectional DC to DC converter  10  set forth, a detailed discussion of the particular aspects of the bidirectional DC to DC converter  10  in accordance with the concepts of the present invention will now be presented. 
     Continuing to  FIGS. 2A-B , the bidirectional DC to DC converter  10  according to the concepts of the present invention, includes bidirectional input/output nodes  52  and  56  that are configured, such that when one node  52  or  56  is receiving power, the other node  56  or  52  is supplying power, and vice versa. Specifically, the converter  10  includes a switching network  230  that comprises a plurality of insulated gate bipolar transistors (IGBT)  240 A-D that are coupled together in an interleaved bridge-type configuration. Coupled between the input/output node  56  and the switching network  230  is a pair of inductors  250  and  252 , whereby the inductors  250  and  252  are integrated as a single, coupled inductor with a positive mutual coupling coefficient. 
     Coupled at the input/output node  56  at one end is a filtering capacitor  260 , while the other end is coupled to a ground terminal  262 . Additionally, a filtering capacitor  270  is coupled between the switching network  230  and the input/output node  52 . In order to control the operation of the switching network  230 , a controller  280  is provided by the converter  10 . The controller  280  analyzes the voltages and electrical current levels at several nodes of the converter  10 , and supplies suitable control signals to each of the IGBT&#39;s  240 A-D to turn them on and off, or otherwise actuate them, as needed to achieve a desired level of DC power conversion between the input/output nodes  52  and  56 , while maintaining a reduced amount of voltage ripple. Moreover, the positive coupling coefficient of the positively coupled inductors  250 , 252  allows the DC to DC converter  10  to output power with reduced voltage ripple at its nodes  52  and  56 , without the need for large, costly filtering capacitors. 
     Specifically, the switching network  230  of the DC to DC converter  10  comprises insulated gate bipolar transistors (IGBT)  240 A-D that are coupled together in a bridge configuration. While the use of IGBTs  240 A-D are contemplated, such should not be construed as limiting, as any other suitable switching devices, such as a BJTs (Bipolar Junction Transistor), or MOSFETs (metal oxide semiconductor field effect transistor), may be used to comprise the switching network  230 . The transistors  240 A and  240 C are coupled by their collector terminals (C) to the input/output node  52 , while their gate terminals (G) are coupled to the controller  280 . Coupled between the collector terminals (C) and the emitter terminals (E) of the transistors  240 A, 240 C are respective diodes  400  and  410 , such that the cathodes of the diodes  400 , 410  are coupled to node  52  and the anodes of the diodes  400 , 410  are coupled to the respective emitter terminals (E) of the transistors  240 A, 240 C. The transistors  240 B and  240 D are arranged such that their collector terminals (C) are respectively coupled to the emitter terminals (E) of the transistors  240 A and  240 B, so as to form respective nodes  420  and  430 . Coupled between the collector terminals (C) and the emitter terminals (E) of the transistors  240 B, 240 D are respective diodes  440  and  450 . The diodes  440 , 450  are arranged, such that the cathode of the diodes  440 , 450  are coupled to the collector terminal (C) of the respective transistors  240 B, 240 D, while the anode of the diodes  440 , 450  are coupled to the emitter terminal (E), which are also coupled to ground terminals  452  and  454 , of the respective transistors  240 C, 240 D. Finally, the gate terminals (G) of the transistors  240 B, 240 D are coupled to the controller  280 . 
     The inductors  250  and  252  are coupled to the switching network  230 , such that inductor  250  is coupled between node  430  and the input/output node  56 , whereas the inductor  252  is coupled between node  420  and the input/output node  56 . 
     To monitor the voltage at the input/output node  56 , a voltage sensor  600  is coupled between the input/output node  56  of the converter  10  and a ground terminal  610 . The voltage sensor  600  may also coupled to the controller  280  via monitor line  612  to allow the controller  280  to monitor the voltage level at the input/output node  56 . In particular, the voltage sensor  600  serves as the primary sensor for providing control feedback to the switching network  230  maintained by the DC to DC converter  10 . It should be appreciated that the voltage control is a critical function, as the various charging circuits and propulsion loads coupled to the bus  54  are strongly affected by voltage variations. 
     The controller  280  provided by the converter  10  may comprise any suitable general purpose or application specific computing device configured with the hardware and software needed to carry out the functions to be set forth. As discussed above, the controller  280  is coupled to the gate terminals (G) of the transistors  240 A-D via respective interface circuits, such as gate drivers, through control lines  700 ,  702 ,  704 , and  706  and is configured to supply suitable control signals thereto to actuate the IGBTs  240 A-D between their on and off states so as to effectuate the DC power conversion functions provided by the converter  10 . In addition, the controller  280  is coupled to nodes  52 ,  420 , and  430  via respective monitor lines  710 ,  712 , and  714 , thereby enabling the controller  280  to monitor the voltage levels present at the associated nodes. It should also be appreciated, the monitoring of the voltage levels of the nodes allows the converter  10  to maintain the voltage of the bus  54  in a narrow range to facilitate the efficient operation of the propulsion units or other electrical load that may be coupled thereto. 
     Thus, due to the arrangement of the IGBTs  240 A-D maintained by the switching network  230 , and the inductors  250 , 252 , the input/output node  52  is configured as the high-voltage node, while the input/output node  56  is configured as the low-voltage node. In other words, the input/output node  56  is configured to receive low-voltage, high-current power, which the switching network  230  converts into high-voltage, low-current power for supply to the input/output node  52 . Alternatively, the input/output node  52  is configured to receive high-voltage, low-current power, which the switching network  230  converts into low-voltage, high-current power for supply to the input/output node  56 . 
     With the structural relationships between the components of the bidirectional DC to DC converter  10  set forth above, the discussion that follows will be directed to the utilization of the converter  10  in association with the energy generation system  20  maintained by the airship  30 . However, it should be understood that the use of the converter  10  is not limited to its use with an airship  30 , as it is contemplated that the bidirectional DC to DC converter  10  may be utilized to convert power in any desired context or application. 
     During the forward operation (from the input/output node  52  to the input/output node  56 ) of the converter  10 , when input power from the battery  100  is received at the input/output node  52 , the IGBTs  240 A and  240 C are controlled on and off, and the IGBTs  240 B and  240 D are turned off, and remain off. Specifically, the IGBTs  240 B and  240 D are turned on and off in accordance with control signals sent from the controller  280  to the gate terminal (G) via respective control lines  700  and  704 , such that the pulses of the respective control signals are 180 degrees out of phase with one another. The high-voltage, low-current power supplied from the battery  100  is switched by the IGBTs  240 A and  240 C and diodes  440  and  450  via the controller  280 . When the IGBTs  240 A and  240 C are turned on, current flows through the positively coupled inductors  250  and  252 . And when the switches  240 A and  240 C are turned off, the voltage maintained by the inductors  250 , 252  oppose the voltage of the battery  100 , to thereby reduce the voltage supplied by the battery  100 . Thus, to effectuate the desired level of power conversion, the IGBTS  240 A and  240 C are turned on and off in accordance with a duty cycle calculated by the controller  280 . The particular duty cycle controls the voltage that is subtracted from the voltage supplied by the battery  100  to generate low-voltage, high-current power with some amount of voltage ripple. After processing by the switching network  230 , the low-voltage, high-current power is processed by the inductors  250  and  252 , whereby due to their positive coupling, they are able to cancel the phase currents that form the voltage ripple. For example,  FIG. 3  shows the opposing phase currents, designated as I 1  and I 2 , as they move through the positively coupled inductors  250  and  252 , such that the sum of I 1  and I 2  produces an output voltage at node  56  that has a reduced amount of ripple, as shown by identifier A in  FIG. 4 . That is, the filtering capacitor  260  also serves to reduce the amount of voltage ripple that is found on the voltage output by the DC to DC converter  10 , before being delivered to the electrolyzer  42 . 
     Returning to  FIGS. 2A and 2B , the voltage (current) supplied to the electrolyzer  42  at node  56  is sensed directly by the voltage sensor  600 , and processed through a PID (proportional-integral-derivative) controller  800 , which is subsequently fed back to the controller  280  via monitor line  612 . The controller  280  utilizes the processed voltage values to ascertain or otherwise calculate the duty cycle (on/off states) of the phase shifted control signals to be supplied to the IGBTs  240 A and  240 C, as previously discussed. In addition, the current supplied to the electrolyzer  42  via the converter  10  is also monitored by the controller  280  via monitor lines  712  and  714 . As such, the controller  280  ensures that the current delivered to the electrolyzer  42  via node  56  does not exceed an allowed threshold. 
     During reverse operation (from the input/output node  56  to the input/output node  52 ) of the converter  10 , low-voltage, high-current power is provided by the fuel cell  40  to the input/output node  56  of the converter  10 . Initially, the filtering capacitor  260  removes an initial amount of ripple associated with the power supplied by the fuel cell  40 . Next, IGBTs  240 B and  240 D are actively controlled on and off by the controller  280 , and IGBTs  240 A and  240 C remain off. Specifically, the IGBTs  240 B and  240 D are turned on and off in accordance with control signals sent from the controller  280  to the gate terminal (G) via respective control lines  702  and  706 , such that the pulses of the respective control signals are 180 degrees out of phase with one another. When IGBTs  240 B and  240 D are turned off, the voltage maintained by the coupled inductors  250 , 252  is added to the voltage provided by the fuel cell  40 . The switching of the IGBTs  240 B and  240 D on and off along with the operation of the diodes  400 , 410  controls the voltage that is added from the inductors  250 , 252  to the voltage from the fuel cell  40 . And as such, the coupled inductors  250 , 252  serve to convert the low-voltage, high-current power into a high-voltage, low-current power source, which is delivered by the IGBT&#39;s  240 B and  240 D to the filtering capacitor  270 . Furthermore the inductors  250  and  252  due to their positive coupling, cancel the phase currents that may form the voltage ripple at the input/output node  52 . The filtering capacitor  270  serves to filter the converted high-voltage, low-current power to reduce the amount of voltage ripple that may be present, before delivering the power to the battery  100  via the input/output node  52  for charging. 
     The voltage (current) supplied to the battery  100  at node  52  is sensed and processed by a PID (proportional-integral-derivative) controller  850 , which is subsequently fed back to the controller  280  via monitor line  710 . The controller  280  utilizes the processed voltage values to ascertain or otherwise calculate the duty cycle (on/off states) of the phase shifted control signals to be supplied to each of the IGBTs  240 B and  240 D, as previously discussed. In addition, the current supplied to the battery  100  is also monitored by the controller  280  via monitor lines  712  and  714 . As such, the controller  280  ensures that the current delivered to the battery  100  via node  52  does not exceed an allowed threshold. 
     It should be appreciated that the amount of phase shift between the control signals is determined by the formula  360 /N, where “N” is equal to the number of interleaved circuit branches established by the switching network  230 . For example, the converter  10  shown in  FIGS. 2A and 2B  has a value of “N” that is equal to 2, which is equal to the number of interleaved circuit branches defined by IGBTs  240 A/ 240 C and IGBTs  240 B/ 240 D, however such should not be construed as limiting as the as the switching network  230  may be readily modified to accommodate additional interleaved circuit branches. 
     Thus, the converter  10  is configured to produce a voltage output at output node  56  that has reduced ripple, but does not require a filtering capacitor  260  that is large, heavy, and bulky. For example, as shown in  FIG. 4 , the converter  10 , indicated by identifier A, is able to output a voltage with approximately the same amount of voltage ripple as a converter, indicated by the identifier B, that is structurally equivalent to the converter  10 , but utilizes a filtering capacitor that has twice the capacitance of the filtering capacitor  260 , and uses interleaved non-coupled inductors in place of the interleaved coupled inductors  250  and  252 . As such, the use of the interleaved positively coupled inductors  250  and  252  allows the converter  10  to output the same amount voltage ripple, while using a physically smaller filtering capacitor  260  due to the use of the interleaved coupled inductors  250  and  252 . Alternatively, as shown in  FIG. 5 , the converter  10 , indicated by identifier A, is able to output a voltage at output node  56  that has approximately half the amount of voltage ripple, as a converter, indicated by the identifier B, that is structurally equivalent to the converter  10  and utilizes a filtering capacitor of equal capacity to the filtering capacitor  260 , but is makes use of interleaved non-coupled inductors in place of the interleaved positively coupled inductors  250  and  252 . 
     It will, therefore, be appreciated that one advantage of one or more embodiments of the present invention is that a bidirectional DC to DC converter converts voltage from one level to another level with reduced ripple. Still another advantage of the present invention is that a bidirectional DC to DC converter converts voltage from one level to another level with reduced ripple, without the need for large, bulky filtering capacitors. Yet another advantage of the present invention is that a bidirectional DC to DC converter utilizes mutually-coupled filter inductors with a positive coupling coefficient to convert voltage from one level to another level with reduced ripple. 
     Although the present invention has been described in considerable detail with reference to certain embodiments, other embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein.