Patent Publication Number: US-2023134008-A1

Title: Bi-directional dc-dc converter

Description:
TECHNICAL FIELD 
     The present subject matter relates to a power supply system. More particularly, a bi-directional DC-DC converter integrated into the power supply system of a powered device. 
     BACKGROUND 
     An energy storage device collects and stores energy by charging itself from an electrical power source, and supplies the stored power to the loads by discharging. The charging and the discharging process are to be precisely managed to ensure safe, reliable, and long life of the energy storage device. In most applications, the charging and the discharging function are typically controlled by two separate powertrains to implement the different control targets such as a smaller charge current, smaller discharge current for low voltage electrical loads, and a larger discharge current for high voltage electrical load from the energy storage device. The powertrains comprise electrical and electronic components involved in charging and discharging of the energy storage device. 
     The powertrains in charging and discharging of the energy storage device include one or more rectifiers to convert AC power supply at the electrical power source to a DC voltage, one or more DC-DC converters to convert the DC voltage to the DC voltages needed by the energy storage device and the high or low voltage electrical loads. The DC-DC converters are unidirectional DC converters where transmission of current is unidirectional and the reverse breaks down or damages the switching devices within the DC-DC converters. However, for compact design of the power supply systems of powered devices, the powertrains need to be combined. Also, the combined powertrain needs to cater for rapid charging and discharging of the energy storage device using the DC-DC converters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is described with reference to the accompanying figures. The same numbers are used throughout the drawings to reference like features and components. 
         FIG.  1   (Prior Art) exemplarily illustrates a block diagram of an existing implementation of a power supply system; 
         FIG.  2    exemplarily illustrates a block diagram of a power supply system, in accordance with an embodiment of the present invention; 
         FIGS.  3 A- 3 D  exemplarily illustrate circuit diagrams of the bi-directional DC-DC converter, in accordance with an embodiment of the present invention; 
         FIGS.  4 A- 4 E  exemplarily illustrate circuit diagrams of the bi-directional DC-DC converter, in accordance with another embodiment of the present invention; 
         FIG.  5    exemplarily illustrates a flowchart depicting a method for converting a first voltage to a second voltage in the on-board charger; 
         FIG.  6    exemplarily illustrates a flowchart depicting steps of operation of the bi-directional DC-DC converter, in accordance with an embodiment of the present invention; and 
         FIG.  7    exemplarily illustrates a flowchart depicting steps of operation of the bi-directional DC-DC converter, in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     By combining the powertrains for charging and discharging of the energy storage device, a compact design of the power supply system with reduced cost may be achieved. However, one DC-DC converter that is bi-directional is needed to cater to both the charging of the energy storage device from the power source and the discharging of the energy storage device to supply the low voltage and high voltage loads in a combined powertrain. Thus, there exists a need for a bi-directional DC-DC converter that eliminates multiple DC-DC converters and operates seamlessly and efficiently both during the charging as well as the discharging cycles of the energy storage device in a power supply system. 
     Few examples of the power supply systems of powered devices employing DC-DC converters are chargers, photovoltaic systems, battery backup systems, or any power distribution system. The chargers may be charger of a laptop or a cellphone, an on-board/offline charger of a vehicle, etc. The on-board charger may be required in powered devices, such as, an electric vehicle or a hybrid electric vehicle. 
     Electric vehicles or hybrid electric vehicles have gained popularity in recent years as the potential replacement for internal combustion vehicles, since they have zero emission from electric drive system, and does not have oil dependency. The hybrid electric vehicles are configured to be powered either by an internal combustion engine or electric motor or both. The electric vehicles have rechargeable battery modules as the power source of the vehicle. These rechargeable battery modules are charged using AC chargers and separate DC/DC converters. There are many low voltage loads in the electric vehicles powered by the rechargeable battery modules. These low voltage loads have low voltage rating typically between 5V to 12V. The sourced voltage from the battery modules needs to be further stepped down to power such low voltage loads. In some existing implementations, separate DC-DC converters, for stepping-down the voltage to supply lower voltage loads, are integrated with the AC chargers. 
       FIG.  1    (Prior Art) exemplarily illustrates a block diagram of an existing implementation of a power supply system  100  in a powered device In an embodiment, the powered device is an electric vehicle or a hybrid electric vehicle. Such a vehicle may have a power supply system  100  to drive the motor of the vehicle, to power electrical loads in the vehicle, to start the engine of the vehicle, etc. The power supply system  100  of the vehicle includes an on-board charger  101 , one or more battery packs, such as,  104 , one or more DC-DC converters  105 , etc. The battery pack  104  may be rechargeable and charged using the on-board charger  101 . The on-board charger  101  includes a rectifier, a power factor correction stage  102 , and a DC-DC converter  103 . The on-board charger  101  may be connected to an AC outlet that is in turn connected to an electric grid at charging stations. The rectifier converts the AC voltage to DC voltage. The power factor correction stage of the on-board charger corrects power factor to unity and regulates the input DC voltage to a standard DC voltage for the next stage. The DC-DC converter  103  in the on-board charger  101  adjusts the level of the DC voltage to the levels required by the battery pack  104  and charges the battery pack  104 . 
     However, the DC-DC converter  103  is operational only on AC power supply and does not function during running condition of the vehicle. However, the low voltage loads, such as, the turn signal lamps, head light, horn, etc., need to be operated while the vehicle is in motion. Such low voltage loads require another DC/DC converter  105  to supply low voltage from the stored charge in the battery pack  104 . The low voltage loads in the electric vehicle, such as, the turn signal lamps, head light, horn etc., are supplied current from the battery pack  104  via one or more DC/DC converters  105 . 
     In an embodiment, the low voltage loads may be supplied from a low energy battery that is charged from the battery pack  104  in the vehicle via the DC/DC converter  105 . The low energy battery may source the required current to the low voltage loads. The other set of DC/DC converters  105  are used to reduce voltage levels, leading to increase in number of parts in the vehicle, weight of the power supply system of the vehicle, resulting in space crunch in the vehicle, hindrance in assembly and serviceability of the vehicle, and increase in overall cost of the vehicle. 
     Thus, there exists a need for an improved &amp; effective bi-directional DC-DC converter integrated with an on-board charger of a powered device, such as, the vehicle that functions during both the stationary condition and the running condition of the vehicle. 
     With the above objectives in view, the prevent invention discloses a bi-directional DC-DC converter that converts a first voltage to a second voltage in applications with combined powertrains for charging and discharging of energy storage devices. 
     It is an object of the invention to provide an on-board charger of a powered device, e.g. a vehicle with an integrated bi-directional DC-DC converter. As per an aspect of the present invention, the DC-DC converter is active during running condition of the vehicle and powers the low voltage electrical loads. 
     It is another objective of the invention to provide a bi-directional DC-DC converter in the charger of the powered device e.g. a vehicle with minimal heat dissipation in the power supply system. 
     Another objective of the invention is to provide a bi-directional DC-DC converter in the charger whose power level can be increased to cater for increased electrical loads in the powered device in future. 
     The present subject matter disclosed herein relates to a combined powertrain with a bi-directional DC-DC converter for charging and discharging of an energy storage device, for example, a battery pack in a powered device. More particularly, a bi-directional DC-DC converter integrated into an on-board charger of the powered unit is disclosed. 
     In an embodiment, a bi-directional DC-DC converter for converting a first voltage to a second voltage is disclosed in  FIG.  2   . The bi-directional DC-DC converter comprises a primary circuit, a transformer, a high voltage power source, and a low voltage power source. The primary circuit electrically receives the first voltage, that is, the output of a power factor correction (PFC) stage. The transformer magnetically couples the primary circuit on a primary side with a rectification circuit on a secondary side. The high voltage power source is electrically connected to the rectification circuit for supplying a high voltage to one or more high voltage loads. The low voltage power source is electrically coupled to the high voltage power source through a secondary circuit for supplying a second voltage to one or more low voltage loads. The high voltage power source charges the low voltage power sources using the same components of the bi-directional DC-DC converter as disclosed in the detailed description of  FIGS.  3 A- 3 D  and  FIGS.  4 A- 4 E . 
     In another embodiment of the present invention, an on-board charger of a powered device with a bi-directional DC-DC converter is disclosed as exemplarily illustrated in  FIG.  2   . The bi-directional DC-DC converter converts a first voltage to a second voltage to supply one or more high voltage loads and one or more low voltage loads. The bi-directional DC-DC converter is operational during a stationary condition and/or a running condition of the of the powered device. 
       FIG.  2    illustrates a block diagram of an exemplary implementation of a power supply system  200 , in accordance with an embodiment of the present invention. The power supply system  200  of the powered device includes an on-board charger  101 , one or more rechargeable battery pack  104 , and a low voltage power source, that is, a secondary battery  202 . The power supply system  200  illustrated is a combined circuit for charging of the battery pack  104  and discharging of the battery pack  104  towards charging of the secondary battery  202 . The on-board charger  101  includes a rectifier, a power factor correction stage  102 , and a bi-directional DC-DC converter  201 . The on-board charger  101  may be connected to an AC outlet that is in turn connected to an electric grid at charging stations. 
     The rectifier converts the AC voltage to DC voltage and the power factor correction stage  102  corrects the power factor and regulates to the DC voltage to a standard DC voltage. The bi-directional DC-DC converter  201  adjusts the level of the DC voltage to the levels required by the battery pack  104  and charges the battery pack  104 . The bi-directional DC-DC converter  201  converts the first voltage V 1  to the second voltage V 2 . The first voltage V 1  is the DC voltage output of the power factor correction stage  102 . The second voltage V 2  is the voltage of the secondary battery  202 . The low voltage loads in the powered device like a vehicle are the turn signal lamps, the head light, the horn etc., are supplied current from the battery pack  104  via the same bi-directional DC-DC converter  201 . While when the vehicle is in running condition, the bi-directional DC-DC converter  201  supplies low voltage from the battery pack  104 . In the present invention, compared to the on-board charger  101 , exemplarily illustrated in  FIG.  1   , the part count is reduced as the DC-DC converter  105  for supplying DC voltage to the low voltage electrical loads is completely eliminated &amp; an improved dual load supply system is configured to meet the requirements in an efficient manner. In an embodiment, the bi-directional DC-DC converter  201  is integrated within the casing of the on-board charger  101  of the powered device. In an embodiment, the bi-directional DC-DC converter  201  is located outside the casing of the on-board charger  101 . The operation of the bi-directional DC-DC converter  201  is controlled by a control unit  203  of the power supply system  200 . 
     The two modes of operation of the bi-directional DC-DC converter  201  are as follows: Charging of the battery pack  104  from the DC voltage of the power factor correction stage  102  and charging of the secondary battery  202  by the charged battery pack  104 . With respect to the embodiment of the present invention in a vehicle, the two modes of operation of the bi-directional DC-DC converter  201  are as follows: In stationary condition of the powered device, when the powered device is plugged for charging the battery pack  104 , the bi-directional DC-DC converter  201  functions as the unidirectional DC-DC converter  105  incorporated in the on-board charger  101  to charge the battery pack  104 . When the powered device is in the running condition or disconnected from a charging station, the bi-directional DC-DC converter  201  regulates the battery voltage from the battery pack  104  to a low voltage and stores in a low voltage or a secondary battery  202  to function as an auxiliary power supply. The secondary battery  202  supplies to the loads, such as, the LED head lamp, horn, side indicators, charging points for electronic devices, etc. 
     Thereby, using the power supply system  200 , the number of parts is reduced and space required is reduced or optimized. Further, manufacturing cost of the overall power supply system  200  is reduced. Also, the heat generated by the additional DC-DC converter  105  is reduced and risk involved in the failure of the additional electrical and electronic components in the powered device is reduced with the removal of the additional DC-DC converter  105 . Further, heat dissipation from the bi-directional DC-DC converter  201  integrated into the on-board charger  101  is taken care by the aluminium casing of the on-board charger  101 , and additional heatsinks for cooling are avoided. The bi-directional DC-DC converter  201  may be configured for higher power levels to meet increased load demands in the powered device. 
     In an embodiment, the bi-directional DC-DC converter  201  comprises a primary circuit that receives a first voltage, that is, the output of the PFC stage  102 . The first voltage on a primary side of a transformer is magnetically coupled to a rectification circuit on a secondary side. The battery pack  104  are electrically connected to the rectification circuit for charging the battery pack  104 , and later supply to one or more high voltage loads, such as, a traction motor. The secondary battery  202  is electrically coupled to the battery pack  104  through a secondary circuit and the secondary battery  202  supplies the second voltage to the low voltage loads. 
     In an embodiment, the bi-directional DC-DC converter  201  may be an isolated converter as exemplarily illustrated in  FIG.  3 A  or a non-isolated converter as exemplarily illustrated in  FIG.  4 A . The detailed construction is provided in  FIGS.  3 B- 3 D  and  FIGS.  4 B- 4 E  respectively. The modes of operation of each embodiment of the bi-directional DC-DC converter  201  is provided in  FIG.  6    and  FIG.  7    respectively. 
       FIGS.  3 A- 3 D  exemplarily illustrate circuit diagrams of an embodiment of the bi-directional DC-DC converter  201  in a power supply system. The bi-directional DC-DC converter  201  is an isolated integrated buck converter that supplies low voltage to the auxiliary loads, such as, horn, instrument cluster, turn signal lamps, and head lamp through an auxiliary secondary battery  202 , exemplarily illustrated in  FIG.  2   , during regular running condition. The primary circuit  300   a  electrically receives a first voltage V 1 , that is, the primary circuit is connected to the regulated voltage source V 1   307 . The transformer  300   b  magnetically couples the primary circuit  300   a  on the primary side with the rectification circuit  300   c  on the secondary side. The high voltage power source, that is, the battery pack  104  is electrically connected to the rectification circuit  300   c  for supplying high voltage, e.g., 48V to high voltage loads. The secondary battery  202  is electrically coupled to the battery pack  104  through the secondary circuit  300   d  for supplying a second voltage to the low voltage loads, such as, a horn, an instrument cluster, turn signal lamps, and a head lamp. 
     The primary circuit  300   a  of the bi-directional DC-DC converter  201  is a full bridge configuration of electronic switches MOSFETs Q 1   301 , Q 2   302 , Q 3   303 , and Q 4   304 . The rectification circuit  300   c  includes diodes D 1   308 , D 2   309 , D 3   311 , and D 4   310  and a filter circuit including an inductor L 1 , C 2 , and Q 5   305  connected to the battery pack  104 . The primary circuit  300   a  is on the primary side of the transformer  300   b  and the rectification circuit  300   c  is on the secondary side of the transformer  300   b . The rectification circuit  300   c  with the additional MOSFET Q 5   305  and the diodes D 3   311  and D 4   310  perform synchronous rectification on the secondary side of the transformer  300   b . A secondary circuit  300   d  includes a gating circuit, such as the MOSFET switch  306  that electrically connects the battery pack  104  to the secondary battery  202 . 
     The modes of operation of the bi-directional DC-DC converter  201  are exemplarily illustrated in  FIGS.  3 B- 3 D . The operation of the MOSFETs Q 1   301 , Q 2   302 , Q 3   303 , and Q 4   304  for respective switching periods is controlled by the control unit  203  of the on-board charger  101 . The control unit  203  operates the MOSFETs Q 1   301 , Q 2   302 , Q 3   303 , and Q 4   304  in PWM mode. The MOSFETs Q 3   303  and Q 4   304  are switched at 50% duty and 180 degrees out of phase with each other and the MOSFETs Q 1   301  and Q 2   302  are switched at 50% duty and 180 degrees out of phase with each other. 
     As exemplarily illustrated in  FIG.  3 B , during a first switching period, when MOSFETs Q 3   303  and Q 4   304  conduct, a positive voltage is applied to the dotted terminal of the primary winding of the transformer  300   b . A secondary voltage with same positive polarity at dotted terminals of the secondary winding of the transformer  300   b  is generated. According to the polarity, the diode D 2   309  in the secondary side of the transformer  300   b  is forward biased and current flows through the secondary side inductor L 1  and diode D 3   311  towards the battery pack  104  as exemplarily illustrated in  FIG.  1   . During this first switching period, the diode D 4   310  is reverse biased and stays open circuited. The MOSFETs Q 5   305  and Q 6  are  306  open circuited, as the gating signal to Q 5   305  and Q 6   306  are not supplied by the control unit  203  of the power supply system  200 . During the first switching period when the battery pack  104  is getting charged, gating signal to the MOSFETs Q 5   305  and Q 6   306  are not supplied by the control unit  203  and the circuit till the secondary battery  202  is isolated. 
     As exemplarily illustrated in  FIG.  3 C , during a second switching period when the MOSFETs Q 1   301  and Q 2   302  conduct, a negative voltage is applied to the dotted terminal of the transformer  300   b . At dotted terminals of the secondary winding of the transformer  300   b , a secondary voltage with same negative polarity is generated. According to the polarity, the diode D 1   308  in the secondary side is forward biased and current flows through the secondary side inductor L 1  and diode D 3   311  towards the battery pack  104 . During this second switching period, the diode D 4   310  is reverse biased and stays open circuited. The MOSFETs Q 5   305  and Q 6   306  are also open circuited since gating signal from the control unit  203  of the on-board charger  101  is not provided. The battery pack  104  continues to charge during this duration. The MOSFETs Q 1   301 , Q 2   302 , Q 3   303 , and Q 4   304  are operated at higher frequencies to charge the battery pack  104 . During the first switching period and the second switching period, the vehicle is connected to the AC power supply and a regulated voltage is available as an input to the bi-directional DC-DC converter  201 . 
     When the battery pack  104  discharges to charge the auxiliary secondary battery  202  via the bi-directional DC-DC converter, the mode exemplarily illustrated in  FIG.  3 D  is active. With respect to the powered device, for example, the vehicle, when the vehicle is in running condition or in stationary condition and not plugged-in for charging, the mode exemplarily illustrated in  FIG.  3 D  is active as the charging operation is not occurring. During this mode, the MOSFETs Q 1   301 , Q 2   302 , Q 3   303 , and Q 4   304  are open circuited by not providing the gating signal. The MOSFET Q 5   305  is the main buck switch, and the MOSFET Q 6   306  is ON all the time during this mode by the control unit  203 . During ON time of the switch Q 5   305 , current flows through the following viz. Battery pack-Q 5 -L 1 -Q 6 -secondary battery. The current flows from the battery pack  104  and charges the secondary battery  202  that powers the low voltage electrical and electronic loads in the vehicle. During OFF time of the switch Q 5   305 , current completes its path through L 1 -Q 6 -secondary battery-D 4 . 
     Thus, when there is isolation between the primary circuit with the MOSFETs Q 1   301 , Q 2   302 , Q 3   303 , and Q 4   304  and the secondary circuit with the secondary battery  202 , the battery pack  104  charges, while the secondary battery  202  is not charged by the full bridge configuration on the primary side of the transformer  300   b . That is, when the powered device is plugged-in for charging, the battery pack  104  charges and supplies current to the electrical and electronic loads. When the powered device, that is, the vehicle is disconnected from charging, the battery pack  104  charges the secondary battery  202  and the secondary battery  202  supplies to the low voltage electrical and electronic loads in the vehicle. As a result of the present invention, the need for another DC-DC converter that down-converts the voltage of the battery pack  104  to supply to the low voltage electrical and electronic loads is eliminated. Therefore, the powered device (e.g. a vehicle) with one or more onboard energy storage devices like a battery pack  104  can be charged by connecting it to an external charger unit or can also be directly plugged into a conventional power supply point for further discharging viz. an improved bi-directional DC-DC charger  201 . 
       FIGS.  4 A- 4 E  exemplarily illustrate circuit diagrams of an embodiment of the bi-directional DC-DC converter  201  integrated into the on-board charger  101  of the vehicle, in accordance with another embodiment of the present invention. The bi-directional DC-DC converter  201  as exemplarily illustrated in  FIG.  4 A  is a non-isolated integrated buck converter that supplies the auxiliary loads, such as, a horn, an instrument cluster, turn signal lamps, and a head lamp through an auxiliary secondary battery  202  for regular vehicle running applications. The primary circuit  400   a  electrically receives a first voltage V 1 , that is, the primary circuit is connected to the regulated voltage source V 1   407 . The transformer  400   b  magnetically couples the primary circuit  400   a  on the primary side with the rectification circuit  400   c  on the secondary side. The high voltage power source, that is, the battery pack  104  is electrically connected to the rectification circuit  400   c  for supplying high voltage VHIGH, e.g. 48V to high voltage loads. The secondary battery  202  is electrically coupled to the battery pack  104  through the secondary circuit  400   d  for supplying a second voltage V 2  to the low voltage loads, such as, a horn, an instrument cluster, turn signal lamps, and a head lamp. 
     The primary circuit  400   a  of the bi-directional DC-DC converter  201  is a full bridge center tapped transformer configuration of electronic switches MOSFETs Q 1   401 , Q 2   402 , Q 3   403 , and Q 4   404  with an additional secondary winding. Such a configuration facilitates charging of secondary battery  202 , as exemplarily illustrated in  FIG.  2   , while charging the main battery pack  104 , as exemplarily illustrated in  FIG.  2   , as well as while not charging main battery pack  104 . The rectification circuit  400   c  includes diodes D 1   408 , D 2   409 , a filter circuit including an inductor L 1 , C 2 , and switches Q 5   405 , and Q 6   406  electrically connecting the secondary side of the transformer  400   b  to the battery pack  104 . The primary circuit  400   a  is on the primary side of the transformer  400   b  and the rectification circuit  400   c  is on the secondary side of the transformer  400   b . The secondary circuit  400   d  is also on the secondary side of the transformer  400   b . The secondary circuit  400   d  comprises a bridge circuit magnetically coupled to the primary circuit  400   a  and the rectification circuit  400   c.    
     The secondary circuit  400   d  comprises four additional diodes D 3   410 , D 4   411 , D 5   412 , and D 6   413 . In an embodiment, the diodes D 3   410 , D 4   411 , D 5   412 , and D 6   413  may be replaced with controllable switches like MOSFETs for better controlling. The modes of operation of the bi-directional DC-DC converter  201  are exemplarily illustrated in  FIGS.  4 B- 4 D . The operation of the MOSFETs Q 1   401 , Q 2   402 , Q 3   403 , and Q 4   404  for respective switching periods is controlled by the control unit  203  of the on-board charger  101 . The control unit  203  operates the MOSFETs Q 1   401 , Q 2   402 , Q 3   403 , and Q 4   404  in PWM mode. 
     As exemplarily illustrated in  FIG.  4 B , the powered device is connected to the AC power supply and a regulated voltage V 1  is available as an input to the bi-directional DC-DC converter  201 . In this mode, the battery pack  104  and the secondary battery  202  charges using the regulated voltage V 1  in this mode. The MOSFETs Q 3   403  and Q 4   404  are conducting when a gating signal is applied by the control unit  203 . When the MOSFETs Q 3  and Q 4  are conducting, a positive voltage is applied to the dotted terminal of the transformer  400   b , and same positive polarity applies at the dotted terminals of the secondary winding of the transformer  400   b . According to the polarity, the diode D 2   409  in the secondary side is forward biased and current flows through the secondary side inductor L 1  to charge the battery pack  104 . The dotted terminal of the additional third winding of the transformer  400   b  also sees voltage with positive polarity and the corresponding diodes D 3   410  and D 6   413  are forward biased and charge the secondary battery  202 , till it reaches its 100% state of charge (SOC) and stops. The MOSFETs Q 5   405  and Q 6   406  are open circuited by not providing gating signal from the control unit  203  of the on-board charger  101 . 
     As exemplarily illustrated in  FIG.  4 C , the powered device is connected to the AC power supply and a regulated voltage V 1  is available as an input to the bi-directional DC-DC converter  201 . In this mode, the battery pack  104  and the secondary battery  202  charges using the regulated voltage V 1  in this mode. The MOSFETs Q 1   401  and Q 2   402  are conducting when a gating signal is applied by the control unit  203 . When the MOSFETs Q 1   401  and Q 2   402  are conducting, the dotted terminal of the transformer  400   b  as shown in  FIG.  4 C  will see a voltage with negative polarity, and same negative polarity applies at dotted terminals of the secondary winding of the transformer  400   b . According to the polarity, Diode D 1  in the secondary side is forward biased and current flows through the secondary side inductor L 1  to charge the battery pack  104 . The dotted terminal of the third additional winding of the transformer  400   b  as shown in  FIG.  4 C  also sees a voltage with negative polarity and the corresponding diodes D 4   411  and D 5   412  are forward biased and charge the secondary battery  202 , till it reaches its 100% SOC and stops. The MOSFETs Q 5   405  and Q 6   406  are also open circuited by not supplying gating signal by the control unit  203 . 
     When the battery pack  104  discharges to charge the secondary battery  202  via the bi-directional DC-DC converter  201 , the mode exemplarily illustrated in  FIG.  4 D  is active. With respect to the powered device, for example, the vehicle, when the vehicle is in running condition or the vehicle is in stationary condition and not charging, the mode exemplarily illustrated in  FIG.  4 D  is active. In this mode, the bi-directional DC-DC converter  201  functions as a push-pull converter by using the main battery pack  104  as a source of power. In this mode, the MOSFET Q 5   405  is turned ON by the control unit  203  and current flows in the path viz. Battery-L 1 -Q 5 . The current flow creates negative polarity at a dotted terminal of the corresponding secondary winding of transformer  400   b . Further, voltage with negative polarity is generated at all the other dotted terminals. The third additional winding of the transformer  400   b  forward biases the diodes D 4   411  and D 5   412  and current flows to the secondary battery in the path: D 4 -secondary battery-D 5 . During this time, the MOSFETs Q 1   401 , Q 2   402 , Q 3   403 , and Q 4   404  are open circuited by not supplying the gating signal. 
     When the powered device is in running condition or in stationary condition and not charging, the mode exemplarily illustrated in  FIG.  4 E  is active. In this mode, the DC-DC converter  201  functions as a push-pull converter by using main battery pack  104  as a source of power. In this mode, the MOSFET Q 6   406  is turned ON by the control unit  203  and current flows in the path defined by Battery-L 1 -Q 6 . The current flow creates positive polarity at a dotted terminal of the corresponding secondary winding of transformer  400   b , thereby creating a positive polarity at all other dotted terminals. The third additional winding of the transformer  400   b  forward biases the diodes D 3   410  and D 6   413  and current flows to the secondary battery  202  in the path viz. D 6 -secondary battery-D 3 . The current flow charges the secondary battery  202  that powers the low voltage electrical and electronic loads in the vehicle. During this time the MOSFETs Q 1   401 , Q 2   402 , Q 3   403 , and Q 4   404  are open circuited by not providing the gating signal. Thus, the battery pack  104  for traction and the secondary battery  202  for the low voltage loads both are charged by the full bridge configuration of the MOSFETs Q 1   401 , Q 2   402 , Q 3   403 , and Q 4   404  of the primary circuit  400   a.    
     The bi-directional DC-DC converter  201  exemplarily illustrated in  FIGS.  3 A and  4 A  facilitates the charging of the secondary battery  202  using the DC current and the secondary battery  202  powers the low voltage electrical and electronic loads in the powered device, in the running condition and/or stationary condition of the powered device as described above. 
       FIG.  5    exemplarily illustrates a flowchart depicting a method for converting a first voltage to a second voltage in an on-board charger  101 . At step  501 , a control unit  203  and a bi-directional DC-DC converter  201  as exemplarily illustrated in  FIGS.  3 A and  4 A  are connected. The control unit  203  at step  502  determines availability of the first voltage. Based on the availability of the first voltage, the control unit  203  at step  503  senses battery parameters of the high voltage power source, that is, the battery pack  104  and the low voltage power source, that is, the secondary battery  202 . Based on the sensed battery parameters, the control unit  203  at step  504  applies a gating signal to the components of the primary circuit  300   a  or  400   a , the rectification circuit  300   c  or  400   c , and the secondary circuit  300   d  or  400   d  for converting the first voltage V 1  to the high voltage VHIGH and the second voltage V 2  as disclosed in the detailed description of  FIG.  6    and  FIG.  7   . 
     For converting the first voltage V 1  to the high voltage VHIGH as exemplarily illustrated in  FIG.  6   , the control unit  203  applies the gating signal to the primary circuit  300   a  based on the sensed battery parameters of the high voltage power source  104 . Further, for converting the high voltage VHIGH to the second voltage V 2  comprises determining availability of the low voltage power source  202  and applying the gating signal to the rectification circuit  300   c  and the secondary circuit  300   d , based on sensed battery parameters of the high voltage power source  104  and the low voltage power source  202 . 
     For converting the first voltage V 1  to the high voltage VHIGH based on the sensed battery parameters of the high voltage power source  104  as exemplarily illustrated in  FIG.  7   , the control unit  203  determines availability of the low voltage power source  202 . Further, the control unit  203  applies the gating signal to the primary circuit  400   a , the rectification circuit  400   c , and the secondary circuit  400   d . Further, for converting the high voltage VHIGH to the second voltage V 2 , the control unit  203  determines availability of the low voltage power source  202  and applies the gating signal to the rectification circuit  400   c  and the secondary circuit  400   d , based on the sensed battery parameters of the high voltage power source  104  and the low voltage power source  202 . 
       FIG.  6    exemplarily illustrates a flowchart showing steps for converting a first voltage to a second voltage by an embodiment of the bi-directional DC-DC converter  201  exemplarily illustrated in  FIG.  3 A . The control unit  203  of the on-board charger at step  602  determines if input power to the bi-directional DC-DC converter  201  is available. If the input power is available, the control unit  203  at step  603  senses the parameters of the battery pack  104 . The control unit  203  at step  604  determines if the state of charge (SOC) of the battery pack  104  is less than 99%, that means the battery pack  104  needs to be charged. The control unit  203  at step  605  initiates charging process of the battery pack  104 . The control unit  203  at step  606  provides gating signals to the MOSFETs Q 1   301 , Q 2   302 , Q 3   303 , and Q 4   304  alternatively as described in the detailed description of  FIGS.  3 A- 3 D . The control unit  203  again determines if the SOC of the battery pack  104  is greater than 99% and indicates the charging of the battery pack  104  is complete. 
     Once the charging of the battery pack  104  is complete, the battery pack  104  can discharge and charge a secondary battery  202 . The control unit  203  at step  607  determines if a secondary battery  202  is available. If the secondary battery  202  is available, the control unit  203  at step  608  initiates charging of the secondary battery  202 . The control unit  203  then at step  609  provides a gating signal to Q 5   305  and Q 6   306  as described in the detailed description of  FIGS.  3 A- 3 D . During the process, the control unit  203  at step  610  determines if the SOC of the battery pack  104  is less than 10%. If incase this is true, the control unit  203  disrupts the discharging of the battery pack  104  and initiates the charging of the battery pack  104 . If the SOC of the battery pack  104  is not less than 10%, the control unit  203  at step  611  determines if the SOC of the secondary battery  202  is greater than 99%. If true, the control unit  203  stops the charging of the secondary battery  202 . If false, the control unit  203  continues the charging of the secondary battery  202  from the battery pack  104  via the MOSFETs Q 5   305  and Q 6   306 . 
       FIG.  7    exemplarily illustrates a flowchart showing steps for converting a first voltage to a second voltage by an embodiment of the bi-directional DC-DC converter  201  exemplarily illustrated in  FIG.  4 A . The control unit  203  at step  702  determines if input power, that is, V 1   407  to the bi-directional DC-DC converter  201  is available. If the input power, that is V 1   407  is available, the control unit  203  at step  703  senses the parameters of the battery pack  104 . The control unit  203  at step  704  determines if the state of charge (SOC) of the battery pack  104  is less than 99%, that means the battery pack needs to be charged. If yes, the control unit  203  at step  705  determines if a secondary battery  202  is available. 
     If the secondary battery  202  is available, the control unit  203  initiates charging of the battery pack  104  and the secondary battery  202 . The control unit  203  at step  706  provides gating signals to the MOSFETs Q 1   401 , Q 2   402  and Q 3   403 , and Q 4   404  alternatively as disclosed in the detailed description of  FIGS.  4 A- 4 E . Accordingly, the diodes D 2   409 , D 6   413 , D 3   410 , and diodes D 1   408 , D 4   411 , and D 5   412  are forward biased and reverse biased. If the secondary battery  202  is not available, the control unit  203  at step  706  provides gating signals to the MOSFETs Q 1   401 , Q 2   402  and Q 3   403 , and Q 4   404  alternatively and only the diodes D 1   408  and D 2   409  will be forward biased and reverse biased accordingly. 
     However, if the input power, that is V 1   407  is not available, the control unit at step  708  provides if the secondary battery  202  is available. If the secondary battery  202  is available, the control unit at step  709  senses the parameters of the secondary battery  202 . The control unit  203  at step  710  determines if the state of charge (SOC) of the secondary battery  202  is less than 99%, that means the secondary battery  202  needs to be charged. If yes, the control unit  203  initiates charging of the secondary battery  202  from the battery pack  104 . The control unit  203  at step  711  provides a gating signal to Q 5   405  and Q 6   406  as described in the detailed description of  FIGS.  3 A- 3 D  and the diodes D 4   410 , D 5   413  and D 3   411 , D 6   412  will be forward biased or reverse biased accordingly. During the process, the control unit  203  at step  712  determines if the SOC of the battery pack  104  is less than 10%. If incase this is true, the control unit  203  disrupts the discharging of the battery pack  104  and initiates the charging of the battery pack  104 . If the SOC of the battery pack  104  is not less than 10%, the control unit  203  at step  710  determines if the SOC of the secondary battery  202  is not less than 99%. If true, the control unit  203  stops the charging of the secondary battery  202 . If false, the control unit  203  continues the charging of the secondary battery  202  from the battery pack  104  via the MOSFETs Q 5   405  and Q 6   406 . 
     Improvements and modifications may be incorporated herein without deviating from the scope of the invention.