Patent Publication Number: US-2016226265-A1

Title: Method and power converter for predictive discontinuous charge mode control

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and power converter for predictive charge mode control. 
     BACKGROUND OF THE INVENTION 
     Switched DC-DC converters comprise a switchable power stage, wherein an output voltage is generated according to a switching signal and an input voltage. The switching signal is generated in a control circuit that adjusts the output voltage to a reference voltage. A buck converter is shown in  FIG. 1 . The switched power stage  11  comprises a dual switch consisting of a high-side field effect transistor (FET)  12  and a low-side FET  13 , an inductor  14  and a capacitor  15 . During a charge phase, the high-side FET  12  is turned on and the low-side FET  13  is turned off by the switching signal to charge the capacitor  15 . During a discharge phase the high-side FET  12  is turned off and the low-side FET  13  is turned on to match the average inductor current to the load current. The switching signal is generated as digital pulse width modulation signal with a duty cycle determined by a control law by the controller  16 . 
     The power converter can be operated either in continuous-conduction-mode (CCM) or in discontinuous conduction mode. (CCM) means that the current in the energy transfer inductor substantially never goes to zero between switching cycles, although it may momentarily go through zero while transitioning from a positive to negative current or negative to positive current. In DCM, the current goes to zero during a substantial part of the switching cycle. In buck derived converters as shown in  FIG. 1  the major effect is that when it changes from CCM to DCM, it goes from one control law to another. In boost and buck-boost derived systems there is a right-half-plane zero in CCM which is not present in the DCM. This makes it much more difficult to stabilize these converters with good dynamic response. 
     DCM regulation therefore typically requires compensation that is different from CCM. Thus, transition from discontinuous to continuous conduction mode requires a rapid controlled change in compensation. 
     DISCLOSURE OF THE INVENTION 
     It is an objective of the present disclosure to provide a control method for a power stage of a power converter that improves the transition from discontinuous to continuous conduction mode and vice versa. 
     This objective is achieved with a method for controlling a power stage according to the independent method claim and a power converter according to the independent apparatus claim. Dependent claims relate to further aspects of the present invention. 
     The present invention relates to method for controlling a power stage of a power converter configured to generate an output voltage from an input voltage according to a control law controlling a switchable power stage. The method comprises generating a pulsed control signal for switching the power stage by varying a pulse width of the pulsed control signal so that a square of the pulse width of the pulsed control signal yields a charge to be delivered in a cycle in dependence of a voltage error, wherein the charge to be delivered in a cycle depends on the voltage error and the square of the pulse width. 
     Thus, the square of the pulse width of the pulsed control signal varies in dependence of the voltage error to increase or decrease a charge to be delivered in a cycle. The voltage error is derived from a difference between a reference voltage and the output voltage. The pulse control signal may be cyclic periodic. 
     This is a predictive method of charge mode control. 
     Past attempts at charge control have tried to measure the charge as it was delivered. The pulse would be terminated when the measured charge equaled the required value. In this invention, the charge to be delivered is predicted by the system parameters and the programmed pulse width. This simplifies the process because no charge needs to be measured and no fast decisions need to be made about terminating a pulse except the apriori decision to terminate a pulse as predicted by this technique. 
     The method is for a modulation scheme that does not require compensation for the discontinuous conduction mode. 
     Thus the requirement of a rapid controlled change in compensation is relieved in that the discontinuous conduction mode does not require compensation. 
     Specifically, the method may comprise generating the pulsed control signal such that a resulting charge Q, i.e. the charge to be delivered, in a cycle is given by 
     
       
         
           
             
               Q 
               = 
               
                 
                   
                     
                       V 
                       in 
                     
                     - 
                     
                       V 
                       out 
                     
                   
                   
                     2 
                      
                     
                         
                     
                      
                     L 
                   
                 
                  
                 
                   ( 
                   
                     
                       V 
                       in 
                     
                     
                       V 
                       out 
                     
                   
                   ) 
                 
                  
                 
                   t 
                   p 
                   2 
                 
               
             
             , 
           
         
       
     
     wherein V in  is the input voltage, V out  is the output voltage, L is an inductance of the switchable power stage and t p  is the pulse width of the pulsed control signal. 
     The skilled person will appreciate that the equation above is idealized and can be expanded to account for higher order effects and parasitic elements. 
     When a steady pulse width t ss  is determined otherwise, the method may comprise generating the pulse control signal by augmenting the steady state pulse width t ss  by an additional on-time t d  such that an additional charge Q d  in a cycle is given by 
     
       
         
           
             
               Q 
               d 
             
             = 
             
               
                 
                   
                     
                       V 
                       in 
                     
                     - 
                     
                       V 
                       out 
                     
                   
                   
                     2 
                      
                     
                         
                     
                      
                     L 
                   
                 
                  
                 
                   ( 
                   
                     
                       V 
                       in 
                     
                     
                       V 
                       out 
                     
                   
                   ) 
                 
                  
                 
                   
                     t 
                     d 
                   
                    
                   
                     [ 
                     
                       
                         2 
                          
                         
                             
                         
                          
                         
                           t 
                           ss 
                         
                       
                       - 
                       
                         t 
                         d 
                       
                     
                     ] 
                   
                 
               
               ≈ 
               
                 
                   
                     
                       V 
                       in 
                     
                     - 
                     
                       V 
                       out 
                     
                   
                   
                     2 
                      
                     
                         
                     
                      
                     L 
                   
                 
                  
                 
                   ( 
                   
                     
                       V 
                       in 
                     
                     
                       V 
                       out 
                     
                   
                   ) 
                 
                  
                 
                   t 
                   d 
                 
                  
                 
                   
                     t 
                     ss 
                   
                   . 
                 
               
             
           
         
       
     
     The method may further comprise determining the steady state pulse width t ss  prior to generating the pulse control signal. 
     The present invention further relates to a power converter comprising a switched power stage configured to generate an output voltage form an input voltage and being controlled by a control law implemented by a controller wherein the controller is configured to generate a pulsed control signal for switching the power stage by varying a pulse width of the pulsed control signal so that square of the pulse width of the pulsed control signal yields a charge to be delivered in a cycle in dependence of a voltage error, wherein the charge to be delivered in cycle depends on the voltage error and the square of the pulse width. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will be made to the accompanying drawings, wherein 
         FIG. 1  shows a prior art switchable buck converter; 
         FIG. 2  shows a diagram showing an inductor current and a pulse width modulation (PWM) switching signal of a switchable power stage operated in DCM; and 
         FIG. 3  shows a diagram showing an inductor current and a pulse width modulation (PWM) switching signal of a switchable steady state duty cycle is determined otherwise. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A power converter as shown in  FIG. 1  is operated in DCM. As a predictive method of charge mode control, the controller  16  generates a PWM control signal for switching the switchable power stage, wherein the pulse control signal is forwarded to the high-side FET  12  and the complement of the control signal is forwarded to the low side FET  13 . The controller  16  generates the pulsed control signal such that a resulting charge Q of the capacitor  15  in a cycle of the PWM signal is given by 
     
       
         
           
             
               Q 
               = 
               
                 
                   
                     
                       V 
                       in 
                     
                     - 
                     
                       V 
                       out 
                     
                   
                   
                     2 
                      
                     
                         
                     
                      
                     L 
                   
                 
                  
                 
                   ( 
                   
                     
                       V 
                       in 
                     
                     
                       V 
                       out 
                     
                   
                   ) 
                 
                  
                 
                   t 
                   p 
                   2 
                 
               
             
             , 
           
         
       
     
     wherein the pulse width t p  of the PWM signal is shown versus the resulting inductor current in  FIG. 2 . 
       FIG. 3  relates to an operation of the power converter as shown in  FIG. 1  when a steady state pulse width t ss  is determined otherwise. The controller augments the steady state pulse width t ss  of the PWM signal by an additional on-time t d  as indicated by the dotted line such that an additional charge Q d  in a cycle is given by 
     
       
         
           
             
               Q 
               d 
             
             = 
             
               
                 
                   
                     
                       V 
                       in 
                     
                     - 
                     
                       V 
                       out 
                     
                   
                   
                     2 
                      
                     
                         
                     
                      
                     L 
                   
                 
                  
                 
                   ( 
                   
                     
                       V 
                       in 
                     
                     
                       V 
                       out 
                     
                   
                   ) 
                 
                  
                 
                   
                     t 
                     d 
                   
                    
                   
                     [ 
                     
                       
                         2 
                          
                         
                             
                         
                          
                         
                           t 
                           ss 
                         
                       
                       - 
                       
                         t 
                         d 
                       
                     
                     ] 
                   
                 
               
               ≈ 
               
                 
                   
                     
                       V 
                       in 
                     
                     - 
                     
                       V 
                       out 
                     
                   
                   
                     2 
                      
                     
                         
                     
                      
                     L 
                   
                 
                  
                 
                   ( 
                   
                     
                       V 
                       in 
                     
                     
                       V 
                       out 
                     
                   
                   ) 
                 
                  
                 
                   t 
                   d 
                 
                  
                 
                   
                     t 
                     ss 
                   
                   . 
                 
               
             
           
         
       
     
     The effect on the inductor current is also shown in  FIG. 3 . It can be observed that the charge increases in the cycle to an extent which is proportional to the area bounded by the dotted line and the solid line of the inductor current. 
     As in DCM no compensation is necessary, the present invention reduces time and effort needed to compensate. It improves the transition from DCM to CCM and thus results in a more robust power converter.