Method and apparatus for preventing boosting system bus when charging a battery

Abstract

A controllably alternating buck mode DC-DC converter conducts cycle by cycle analysis of the direction of inductor current flow to decide whether to operate in synchronous buck mode or standard buck mode for the next successive cycle. For each cycle of the PWM waveform controlling the buck mode DC-DC converter, a mode control circuit examines and latches data representative of the direction of inductor current flow relative to the chargeable battery. If the inductor current flow is positive, a decision is made to operate in synchronous buck mode for the next PWM cycle, which allows positive current to charge the battery; if the inductor current drops to zero, a decision is made to operate the converter in standard buck mode for the next PWM cycle, so as to prevent current from flowing out of the battery and boosting the system bus.

1. A controllably alternating buck mode DC-DC converter comprising:
 an upper switching stage and a lower switching stage having controlled current flow paths therethrough coupled between an input voltage terminal adapted to receive an input voltage, and a reference voltage terminal adapted to receive a reference voltage, a common node between said upper switching stage and said lower switching stage being coupled through an output inductor to an output port for charging a battery, said upper switching stage having an upper control terminal to which a first pulse width modulation (PWM) waveform is applied for controlling the conduction and non-conduction of said upper switching stage, and wherein said lower switching stage has a lower control terminal to which a second PWM waveform, referenced to said first PWM waveform, is selectively applied for controlling the conduction and non-conduction of said lower switching stage; and  
 a lower switching stage controller, which is operative, 
 in response to a positive inductor current flow from said common node to said output port at the end of one or more cycles including a respective ith cycle of said first PWM waveform, to allow said second PWM waveform to be applied to said lower control terminal of said lower switching stage during the (i+1)th cycle of said first PWM waveform, and thereby cause said buck mode DC-DC converter to operate in synchronous buck mode for the (i+1)th cycle of said first PWM waveform, and  
 in response to inductor current dropping to zero during said one or more cycles including said respective ith cycle of said first PWM waveform, to cause diode emulation of said lower switching stage during the (i+1)th cycle of said first PWM waveform, and thereby cause said buck mode DC-DC converter to operate in standard buck mode for the (i+1)th cycle of said first PWM waveform.  
 
 
     
     
       2. The DC-DC converter according to  claim 1 , wherein said lower switching stage controller is operative to store information representative of the direction of inductor current flow for said ith cycle of said first PWM waveform, and to selectively cause said buck mode DC-DC converter to operate in either synchronous buck mode or standard buck mode for the (i+1)th cycle of said first PWM waveform, based upon said information. 
     
     
       3. The DC-DC converter according to  claim 2 , wherein said lower switching stage controller comprises:
 a phase detector having inputs thereof coupled across the current flow path through said second switching stage, and an output coupled to a logic circuit,  
 a flip-flop having an input coupled to said output of said phase detector, a clock input coupled to receive said first PWM waveform, and an output coupled to said logic circuit,  
 said logic circuit being coupled to receive said first PWM waveform and having an output coupled to said lower control terminal of said lower switching stage.  
 
     
     
       4. The DC-DC converter according to  claim 3 , wherein said lower switching stage controller further comprises a blanking circuit which is operative to controllably disable said phase detector for a prescribed period of time following the termination of said first PWM waveform. 
     
     
       5. The DC-DC converter according to  claim 1 , wherein, in response to a positive inductor current flow from said common node to said output port at the end of said one or more cycles including said respective ith cycle of said first PWM waveform, said lower switching stage controller is operative to generate said second PWM waveform as the complement of said first PWM waveform, for application to said lower control terminal of said lower switching stage during the (i+1)th cycle of said first PWM waveform, and thereby cause said buck mode DC-DC converter to operate in synchronous buck mode for the (i+1)th cycle of said first PWM waveform. 
     
     
       6. The DC-DC converter according to  claim 1 , wherein, in response to a zero inductor current during said one or more cycles including said respective ith cycle of said first PWM waveform, said lower switching stage controller is operative to prevent said second PWM waveform from being applied to said lower control terminal of said lower switching stage during the (i+1)th cycle of said first PWM waveform, and thereby cause said buck mode DC-DC converter to operate in standard buck mode for the (i+1)th cycle of said first PWM waveform. 
     
     
       7. The DC-DC converter according to  claim 1 , wherein said upper switching stage comprises an upper MOSFET and said lower switching stage comprises a lower MOSFET, and wherein said lower switching stage controller is operative, in response to a positive inductor current flow from said common node to said output port at the end of said one or more cycles including said respective ith cycle of said first PWM waveform, to allow said second PWM waveform to be applied to a gate terminal of said lower MOSFET stage during the (i+1)th cycle of said first PWM waveform, and thereby turn on said lower MOSFET and cause said buck mode DC-DC converter to operate in synchronous buck mode for the (i+1)th cycle of said first PWM waveform and, in response to inductor current dropping to zero during said one or more cycles including said respective ith cycle of said first PWM waveform, to turn off said lower MOSFET during the (i+1)th cycle of said first PWM waveform, and thereby cause said buck mode DC-DC converter to operate in standard buck mode for the (i+1)th cycle of said first PWM waveform. 
     
     
       8. A method of operating a buck mode DC-DC converter comprised of an upper switching stage and a lower switching stage having controlled current flow paths therethrough coupled between an input voltage terminal adapted to receive an input voltage, and a reference voltage terminal adapted to receive a reference voltage, a common node between said upper switching stage and said lower switching stage being coupled through an output inductor to an output port for charging a battery, said upper switching stage having an upper control terminal to which a first pulse width modulation (PWM) waveform is applied for controlling the conduction and non-conduction of said upper switching stage, and wherein said lower switching stage has a lower control terminal to which a second PWM waveform, referenced to said first PWM waveform, is selectively applied for controlling the conduction and non-conduction of said lower switching stage, said method comprising the steps of:
 (a) in response to a positive inductor current flow from said common node to said output port at the end of each of one or more cycles including a respective ith cycle of said first PWM waveform, coupling said second PWM waveform to said lower control terminal of said lower switching stage during an (i+1)th cycle of said first PWM waveform, thereby causing said buck mode DC-DC converter to operate in synchronous buck mode for the (i+1)th cycle of said first PWM waveform; and  
 (b) in response to inductor current dropping to zero during said one or more cycles including said respective ith cycle of said first PWM waveform, producing diode emulation of said lower switching stage during the (i+1)th cycle of said first PWM waveform, thereby causing said buck mode DC-DC converter to operate in standard buck mode for the (i+1)th cycle of said first PWM waveform.  
 
     
     
       9. The method according to  claim 8 , wherein step (a) comprises storing information representative of the direction of inductor current flow for said ith cycle of said first PWM waveform, and selectively causing said buck mode DC-DC converter to operate in synchronous buck mode for the (i+1)th cycle of said first PWM waveform, in response to said information being representative of positive inductor current flow. 
     
     
       10. The method according to  claim 8 , wherein step (b) comprises storing information representative of the direction of inductor current flow for said one or more cycles including said ith cycle of said first PWM waveform, and selectively causing said buck mode DC-DC converter to operate in standard buck mode for the (i+1)th cycle of said first PWM waveform, in response to said information being representative of zero inductor current flow. 
     
     
       11. The method according to  claim 8 , further comprising the step (c) of controllably disabling steps (a) and (b) for a prescribed period of time following the termination of said first PWM waveform. 
     
     
       12. The method according to  claim 8 , wherein, in response to a positive inductor current flow from said common node to said output port during said one or more cycles including said respective ith cycle of said first PWM waveform, step (a) comprises generating said second PWM waveform as the complement of said first PWM waveform, for application to said lower control terminal of said lower switching stage during the (i+1)th cycle of said first PWM waveform, thereby causing said buck mode DC-DC converter to operate in synchronous buck mode for the (i+1)th cycle of said first PWM waveform. 
     
     
       13. The method according to  claim 8 , wherein, in response to a zero inductor current during said one or more cycles including said respective ith cycle of said first PWM waveform, step (b) comprises preventing said second PWM waveform from being applied to said lower control terminal of said lower switching stage during the (i+1)th cycle of said first PWM waveform, thereby causing said buck mode DC-DC converter to operate in standard buck mode for the (i+1)th cycle of said first PWM waveform. 
     
     
       14. A controller for a buck mode DC-DC converter comprised of an upper switching stage and a lower switching stage having controlled current flow paths therethrough coupled between an input voltage terminal adapted to receive an input voltage, and a reference voltage terminal adapted to receive a reference voltage, a common node between said upper switching stage and said lower switching stage being coupled through an output inductor to an output port for charging a battery, said upper switching stage having an upper control terminal to which a first pulse width modulation (PWM) waveform is applied for controlling the conduction and non-conduction of said upper switching stage, and wherein said lower switching stage has a lower control terminal to which a second PWM waveform, referenced to said first PWM waveform, is selectively applied for controlling the conduction and non-conduction of said lower switching stage, said controller comprising:
 a storage device which is operative to store information representative of the direction of inductor current flow for one or more cycles including an ith cycle of said first PWM waveform; and  
 a logic circuit coupled to said storage device and said lower switching stage and being operative to selectively cause said buck mode DC-DC converter to operate in one of synchronous buck mode and standard buck mode for an (i+1)th cycle of said first PWM waveform, based upon said information stored by said storage device.  
 
     
     
       15. The controller according to  claim 14 , wherein said logic circuit is operative, in response to a positive inductor current flow from said common node to said output port at the end of said one or more cycles including said ith cycle of said first PWM waveform, to allow said second PWM waveform to be applied to said lower control terminal of said lower switching stage during said (i+1)th cycle of said first PWM waveform, and thereby cause said buck mode DC-DC converter to operate in synchronous buck mode for the (i+1)th cycle of said first PWM waveform. 
     
     
       16. The controller according to  claim 14 , wherein said logic circuit is operative, in response to said inductor current being reduced to zero during one or more cycles including said ith cycle of said first PWM waveform, to cause diode emulation of said lower switching stage during the (i+1)th cycle of said first PWM waveform, and thereby cause said buck mode DC-DC converter to operate in standard buck mode for the (i+1)th cycle of said first PWM waveform. 
     
     
       17. The controller according to  claim 14 , further comprising a phase detector having inputs thereof coupled across the current flow path through said second switching stage, and an output coupled to said logic circuit, and wherein said memory device comprises a flip-flop having an input coupled to said output of said phase detector, a clock input coupled to receive said first PWM waveform, and an output coupled to said logic circuit, and wherein said logic circuit is coupled to receive said first PWM waveform and having an output coupled to said lower control terminal of said lower switching stage. 
     
     
       18. The controller according to  claim 17 , wherein said lower switching stage controller further comprises a blanking circuit which is operative to controllably disable said phase detector for a prescribed period of time following the termination of said first PWM waveform. 
     
     
       19. The controller according to  claim 14 , wherein, in response to a positive inductor current flow from said common node to said output port at the end of said one or more cycles including said ith cycle of said first PWM waveform, said logic circuit is operative to generate said second PWM waveform as the complement of said first PWM waveform, for application to said lower control terminal of said lower switching stage during the (i+1)th cycle of said first PWM waveform, and thereby cause said buck mode DC-DC converter to operate in synchronous buck mode for the (i+1)th cycle of said first PWM waveform. 
     
     
       20. The controller according to  claim 14 , wherein, in response to a zero inductor current during said one or more cycles including said ith cycle of said first PWM waveform, said logic circuit is operative to prevent said second PWM waveform from being applied to said lower control terminal of said lower switching stage during the (i+1)th cycle of said first PWM waveform, and thereby cause said buck mode DC-DC converter to operate in standard buck mode for the (i+1)th cycle of said first PWM waveform. 
     
     
       21. A controllably alternating buck mode DC- DC converter comprising:      an upper switching stage and a lower switching stage coupled in series between an input voltage terminal adapted to receive an input voltage and a reference voltage terminal adapted to receive a reference voltage, a common node between said upper switching stage and said lower switching stage being coupled through an output inductor to an output port for charging a battery, said upper switching stage having an upper control terminal to which a first pulse width modulation  ( PWM )  waveform is applied for controlling the conduction and non - conduction of said upper switching stage, and wherein said lower switching stage has a lower control terminal to which a second PWM waveform, referenced to said first PWM waveform, is selectively applied for controlling the conduction and non - conduction of said lower switching stage; and        a lower switching stage controller, which is operative,      in response to a positive inductor current flow from said common node to said output port at the end of one or more cycles including a respective ith cycle of said first PWM waveform, to allow said second PWM waveform to be applied to said lower control terminal of said lower switching stage during the  ( i+ 1   ) th cycle of said first PWM waveform, and thereby cause said buck mode DC - DC converter to operate in synchronous buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform, and        in response to inductor current dropping to approximately zero during said one or more cycles including said respective ith cycle of said first PWM waveform, to cause said buck mode DC - DC converter to operate in standard buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform.        
     
     
       22. The DC- DC converter according to    claim 21   , wherein said lower switching stage controller is operative to store information representative of the direction of inductor current flow for said ith cycle of said first PWM waveform, and to selectively cause said buck mode DC - DC converter to operate in either synchronous buck mode or standard buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform, based upon said information.    
     
     
       23. The DC- DC converter according to    claim 22   , wherein said lower switching stage controller comprises:      a phase detector having inputs thereof coupled across the current flow path through said second switching stage, and an output coupled to a logic circuit,        a flip - flop having an input coupled to said output of said phase detector, a clock input coupled to receive said first PWM waveform, and an output coupled to said logic circuit,        said logic circuit being coupled to receive said first PWM waveform and having an output coupled to said lower control terminal of said lower switching stage.      
     
     
       24. The DC- DC converter according to    claim 23   , wherein said lower switching stage controller further comprises a blanking circuit which is operative to controllably disable said phase detector for a prescribed period of time following the termination of said first PWM waveform.    
     
     
       25. The DC- DC converter according to    claim 21   , wherein, in response to a positive inductor current flow from said common node to said output port at the end of said one or more cycles including said respective ith cycle of said first PWM waveform, said lower switching stage controller is operative to generate said second PWM waveform as the complement of said first PWM waveform, for application to said lower control terminal of said lower switching stage during the  ( i+ 1   ) th cycle of said first PWM waveform, and thereby cause said buck mode DC - DC converter to operate in synchronous buck mode for the  ( i+I ) th cycle of said first PWM waveform.    
     
     
       26. The DC- DC converter according to    claim 21   , wherein, in response to an approximately zero inductor current during said one or more cycles including said respective ith cycle of said first PWM waveform, said lower switching stage controller is operative to prevent said second PWM waveform from being applied to said lower control terminal of said lower switching stage during the  ( i+ 1   ) th cycle of said first PWM waveform, and thereby cause said buck mode DC - DC converter to operate in standard buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform.    
     
     
       27. The DC- DC converter according to    claim 21   , wherein said upper switching stage comprises an upper MOSFET and said lower switching stage comprises a lower MOSFET, and wherein said lower switching stage controller is operative, in response to a positive inductor current flow from said common node to said output port at the end of said one or more cycles including said respective ith cycle of said first PWM waveform, to allow said second PWM waveform to be applied to a gate terminal of said lower MOSFET stage during the  ( i+ 1   ) th cycle of said first PWM waveform, and thereby turn on said lower MOSFET and cause said buck mode DC - DC converter to operate in synchronous buck mode for the  ( i+I ) th cycle of said first PWM waveform and, in response to inductor current dropping to approximately zero during said one or more cycles including said respective ith cycle of said first PWM waveform, to turn off said lower MOSFET during the whole  ( i+ 1   ) th cycle of said first PWM waveform, and thereby cause said buck mode DC - DC converter to operate in standard buck mode for the  ( i+I ) th cycle of said first PWM waveform.    
     
     
       28. A method of operating a buck mode DC- DC converter comprised of an upper switching stage and a lower switching stage coupled in series between an input voltage terminal adapted to receive an input voltage, and a reference voltage terminal adapted to receive a reference voltage, a common node between said upper switching stage and said lower switching stage being coupled through an output inductor to an output port for charging a battery, said upper switching stage having an upper control terminal to which a first pulse width modulation  ( PWM )  waveform is applied for controlling the conduction and non - conduction of said upper switching stage, and wherein said lower switching stage has a lower control terminal to which a second PWM waveform, referenced to said first PWM waveform, is selectively applied for controlling the conduction and non - conduction of said lower switching stage, said method comprising the steps of:    ( a )  in response to a positive inductor current flow from said common node to said output port at the end of each of one or more cycles including a respective ith cycle of said first PWM waveform, coupling said second PWM waveform to said lower control terminal of said lower switching stage during an  ( i+ 1   ) th cycle of said first PWM waveform, thereby causing said buck mode DC - DC converter to operate in synchronous buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform; and      ( b )  in response to inductor current dropping to approximately zero during said one or more cycles including said respective ith cycle of said first PWM waveform, causing said buck mode DC - DC converter to operate in standard buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform.      
     
     
       29. The method according to  claim 28 , wherein step ( a )  comprises storing information representative of the direction of inductor current flow for said ith cycle of said first PWM waveform, and selectively causing said buck mode DC - DC converter to operate in synchronous buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform, in response to said information being representative of positive inductor current flow.    
     
     
       30. The method according to  claim 28 , wherein step ( b )  comprises storing information representative of the direction of inductor current flow for said one or more cycles including said ith cycle of said first PWM waveform, and selectively causing said buck mode DC - DC converter to operate in standard buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform, in response to said information being representative of approximately zero inductor current flow.    
     
     
       31. The method according to  claim 28 , further comprising the step ( c )  of controllably disabling steps  ( a )  and  ( b )  for a prescribed period of time following the termination of said first PWM waveform.    
     
     
       32. The method according to  claim 28 , wherein, in response to a positive inductor current flow from said common node to said output port during said one or more cycles including said respective ith cycle of said first PWM waveform, step ( a )  comprises generating said second PWM waveform as the complement of said first PWM waveform, for application to said lower control terminal of said lower switching stage during the  ( i+ 1   ) th cycle of said first PWM waveform, thereby causing said buck mode DC - DC converter to operate in synchronous buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform.    
     
     
       33. The method according to  claim 28 , wherein, in response to a approximately zero inductor current during said one or more cycles including said respective ith cycle of said first PWM waveform, step ( b )  comprises preventing said second PWM waveform from being applied to said lower control terminal of said lower switching stage during the  ( i+ 1   ) th cycle of said first PWM waveform, thereby causing said buck mode DC - DC converter to operate in standard buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform.    
     
     
       34. A controller for a buck mode DC- DC converter comprised of an upper switching stage and a lower switching stage coupled in series between an input voltage terminal adapted to receive an input voltage, and a reference voltage terminal adapted to receive a reference voltage, a common node between said upper switching stage and said lower switching stage being coupled through an output inductor to an output port for charging a battery, said upper switching stage having an upper control terminal to which a first pulse width modulation  ( PWM )  waveform is applied for controlling the conduction and non - conduction of said upper switching stage, and wherein said lower switching stage has a lower control terminal to which a second PWM waveform, referenced to said first PWM waveform, is selectively applied for controlling the conduction and non - conduction of said lower switching stage, said controller comprising:      a storage device which is operative to store information representative of the direction of inductor current flow for one or more cycles including an ith cycle of said first PWM waveform; and        a logic circuit coupled to said storage device and said lower switching stage and being operative to selectively cause said buck mode DC - DC converter to operate in one of synchronous buck mode and standard buck mode for an  ( i+I ) th cycle of said first PWM waveform, based upon said information stored by said storage device.      
     
     
       35. The controller according to  claim 34 , wherein said logic circuit is operative, in response to a positive inductor current flow from said common node to said output port at the end of said one or more cycles including said ith cycle of said first PWM waveform, to allow said second PWM waveform to be applied to said lower control terminal of said lower switching stage during said ( i+ 1   ) th cycle of said first PWM waveform, and thereby cause said buck mode DC - DC converter to operate in synchronous buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform.    
     
     
       36. The controller according to  claim 34 , wherein said logic circuit is operative, in response to said inductor current being reduced to approximately zero during one or more cycles including said ith cycle of said first PWM waveform, to cause said buck mode DC- DC converter to operate in standard buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform.    
     
     
       37. The controller according to  claim 34 , further comprising a phase detector having inputs thereof coupled across the current flow path through said second switching stage, and an output coupled to said logic circuit, and wherein said memory device comprises a flip- flop having an input coupled to said output of said phase detector, a clock input coupled to receive said first PWM waveform, and an output coupled to said logic circuit, and wherein said logic circuit is coupled to receive said first PWM waveform and having an output coupled to said lower control terminal of said lower switching stage.    
     
     
       38. The controller according to  claim 37 , wherein said lower switching stage controller further comprises a blank blanking circuit which is operative to controllably disable said phase detector for a prescribed period of time following the termination of said first PWM waveform.  
     
     
       39. The controller according to  claim 34 , wherein, in response to a positive inductor current flow from said common node to said output port at the end of said one or more cycles including said ith cycle of said first PWM waveform, said logic circuit is operative to generate said second PWM waveform as the complement of said first PWM waveform, for application to said lower control terminal of said lower switching stage during the ( i+I ) th cycle of said first PWM waveform, and thereby cause said buck mode DC - DC converter to operate in synchronous buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform.    
     
     
       40. The controller according to  claim 34 , wherein, in response to a zero inductor current during said one or more cycles including said ith cycle of said first PWM waveform, said logic circuit is operative to prevent said second PWM waveform from being applied to said lower control terminal of said lower switching stage during the ( i+I)   th cycle of said first PWM waveform, and thereby cause said buck mode DC - DC converter to operate in standard buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform.    
     
     
       41. A controllably alternating buck mode DC- DC converter comprising:      an upper switching stage and a lower switching stage coupled in series between an input voltage terminal adapted to receive an input voltage and a reference voltage terminal adapted to receive a reference voltage, a common node between said upper switching stage and said lower switching stage being coupled through an output inductor to an output port for charging a battery, said upper switching stage having an upper control terminal to which a first pulse width modulation  ( PWM )  waveform is applied for controlling the conduction and non - conduction of said upper switching stage, and wherein said lower switching stage has a lower control terminal to which a second PWM waveform, referenced to said first PWM waveform, is selectively applied for controlling the conduction and non - conduction of said lower switching stage; and        a lower switching stage controller, which is operative,      in response to an inductor current flow of at least an output current threshold from said common node to said output port during an ith cycle of said first PWM waveform, to allow said second PWM waveform to be applied to said lower control terminal of said lower switching stage during the  ( i+ 1   ) th cycle of said first PWM waveform, and thereby cause said buck mode DC - DC converter to operate in synchronous buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform, and        in response to inductor current being approximately zero during said one or more cycles including said respective ith cycle of said first PWM waveform, to cause said buck mode DC - DC converter to operate in standard buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform.        
     
     
       42. A controllably alternating buck mode DC- DC converter comprising:      an upper switching stage and a lower switching stage coupled in series between an input voltage terminal adapted to receive an input voltage and a reference voltage terminal adapted to receive a reference voltage, a common node between said upper switching stage and said lower switching stage being coupled through an output inductor to an output port for charging a battery, said upper switching stage having an upper control terminal to which a first pulse width modulation  ( PWM )  waveform is applied for controlling the conduction and non - conduction of said upper switching stage, and wherein said lower switching stage has a lower control terminal to which a second PWM waveform, referenced to said first PWM waveform, is selectively applied for controlling the conduction and non - conduction of said lower switching stage; and        a lower switching stage controller, which is operative,      in response to a signal related to the inductor current flow above a first threshold from said common node to said output port during an ith cycle of said first PWM waveform, to allow said second PWM waveform to be applied to said lower control terminal of said lower switching stage during the  ( i+ 1   ) th cycle of said first PWM waveform, and thereby cause said buck mode DC - DC converter to operate in synchronous buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform, and        in response to the signal related to inductor current below a second threshold during said one or more cycles including said respective ith cycle of said first PWM waveform, to cause said buck mode DC - DC converter to operate in standard buck mode for the  ( i+ 1   ) th cycle of said first PWM waveform.