Abstract:
A method and circuitry are disclosed that provide for linear operation of a flyback converter through zero output. Broadly, the preferred embodiment enforces a minimum control pulse width thereby isolating energy derived thereby from the eventual load, and dissipating the energy from the minimum control pulse width. The net effect is linear operation inclusive of zero output.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority from U.S. Provisional Patent Application Serial No. 60/380,437, filed May 13, 2002, the entire content of which is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to switch-mode power supplies and DC-DC converters and, more particularly, to flyback converters.  
         BACKGROUND OF THE INVENTION  
         [0003]    A DC-to-DC converter is a device that accepts a DC input voltage and produces a DC output voltage. Typically the output produced is at a different voltage level than the input. In addition, DC-to-DC converters are used to provide noise isolation, power bus regulation, etc. Many popular DC-to-DC topologies are based upon flyback converters, which incorporate an inductor to provide voltage boost and usually implemented in boost or buck/boost topologies.  
           [0004]    Although flyback converters can be controlled in a linear fashion through a broad range, this range does not include the regime approaching zero output. As switching delays of the switching element used become significant in relation to the commanded pulse width, severe deviations from linear operation occur. Linear operation of flyback converters is, however, sometimes desired inclusive of zero output. A need exists to thus extend the operating range.  
         SUMMARY OF THE INVENTION  
         [0005]    A method and circuitry are disclosed that provide for linear operation of a flyback converter through zero output. Broadly, the preferred embodiment enforces a minimum control pulse width thereby isolating energy derived thereby from the eventual load, and dissipating the energy from the minimum control pulse width. The net effect is linear operation inclusive of zero output. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 shows a typical flyback converter in a buck/boost configuration;  
         [0007]    [0007]FIG. 2 shows control, as well as voltage and current waveforms of the inductor and capacitor in the circuit of FIG. 1;  
         [0008]    [0008]FIG. 3 shows a preferred embodiment of the present invention; and  
         [0009]    [0009]FIG. 4 shows control, as well as voltage, and current waveforms of the inductor and capacitor in the circuit of FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]    Referring now to FIG. 1, prior-art pulse-width modulation (PWM) control circuit  100  provides a control pulse width  101  to switching device  102 , which, when energized, charges inductor  103 . At release of pulse width  101 , switching device  102  ceases sinking current into inductor  103 , which then, in attempting to maintain its previous current, sources a voltage to the anode of diode  104 . Diode  104  conducts this voltage into one terminal of both capacitor  105  and load resistance  106 . The second terminals of capacitor  105  and load resistor  106  are connected to the positive supply rail. Capacitor  105  serves to limit the peak of the flyback voltage thus generated, while resistor  106  dissipates the majority of its energy.  
         [0011]    Referring now to FIG. 2, voltage trace  201  shows control pulse width  101  of FIG. 1, voltage trace  202  and current trace  204  show their respective characteristics at the lower terminal of inductor  103  of FIG. 1, and voltage trace  203  and current trace  205  show their respective characteristics at the lower terminal of capacitor  105  of FIG. 1.  
         [0012]    At time marker  206 , control voltage  201  can be seen to go high, energizing switching device  102 , which sinks current in inductor  103 , indicated by voltage  202  and current  204 . At time marker  207 , switching device  102  is released, at which point inductor  103  flies back through diode  104  into capacitor  105  and load resistance  106 . This flyback action continues until time marker  208 , at which point current from inductor  103  into capacitor  105  is depleted, as shown in current traces  204  and  205 . Charge transferred into capacitor  105  is now dissipated in resistance  106 , until time marker  209 , at which point all charge is depleted. This action is visible in voltage  203  and current  205 .  
         [0013]    At time marker  210 , switching device  102  is again energized, but for a much shorter duration than at time marker  206 . Although de-energized at time marker  211 , switching device does not physically turn off until time marker  212 , augmenting voltages  202  and  203  and currents  204  and  205  over those expected. The output  203  is thus becoming non-linear to the control voltage  201 . At time marker  213 , control voltage  201  is turned on and off before switching device  102  is capable of response. No resultant output is then seen in voltages  202 / 203  or currents  204 / 205 . The sign of non-linear response is then seen to reverse from time marker  210  to time marker  213 , compounding non-linearity approaching zero.  
         [0014]    Referring now to FIG. 3, circuitry similar to that of FIG. 1 is seen, with the addition of switching device  308  under the control of control voltage  307 , constant current sink comprised of transistor  309  and resistors  310 ,  311 , and  312 , under the control of control voltage  314 , and operational amplifier  313 . Switching device  308 , by position, serves to selectively isolate load resistance  306 , the eventual output, from the remainder of the circuitry, under control of PWM Controller  300 . Transistor  309 , in conjunction with resistors  310 ,  311 , and  312 , and under control of PWM Controller  300 , sinks a constant current from capacitor  305 . Operational amplifier  311  provides feedback from the eventual output to PWM Controller  300 .  
         [0015]    Referring now to FIG. 4, traces  401 ,  407 , and  408  show control voltages  301 ,  307 , and  314 , respectively, of FIG. 3. Voltage trace  402  and current trace  405  show their respective characteristics at the lower terminal of inductor  303  of FIG. 3, and voltage trace  403  and current trace  406  show their respective characteristics at the lower terminal of capacitor  305  of FIG. 3. Voltage trace  404  shows the ultimate output across load resistor  306  of FIG. 3.  
         [0016]    At time marker  409 , switching device  302  is energized by PWM Controller  300 , at which point current in inductor  303  builds, indicated in trace  405 . At time marker  410 , switching device  302  is released, resulting in voltage flyback from inductor  303  into capacitor  305 , through diode  304 . Note that switching device  308  is disabled during the inductor charge and flyback periods, as indicated in trace  407 . The flyback voltage imposed on capacitor  305  is therefore isolated from load resistance  306 , transferring no energy thereto. From time marker  410  until time marker  411 , energy can be seen to transfer from inductor  303  to capacitor  305  in voltage trace  403  and current traces  405  and  406 .  
         [0017]    At time marker  411 , when charge transfer is complete (indicated by zero inductor current in trace  405 ), transistor  309  is energized by control voltage  312  from PWM Controller  300 . This initiates a constant current sink against the charge previously imparted on capacitor  305 , indicated in trace  406 . Resultant voltage drop can be observed after time marker  411  in voltage trace  403 . After a predetermined time indicated at time marker  412 , PWM Controller  300  deasserts control voltage  408 , ending the current sink on capacitor  305 . Concurrently, at time marker  412 , switching device  308  is reasserted by PWM Controller  300  via control voltage  307 , (indicated in trace  407 ) to apply the charge in capacitor  305  to load resistance  306 .  
         [0018]    The net effect of the foregoing sequence is to isolate circuit operation into inductor charge (between markers  409  and  410 ), inductor discharge/capacitor charge (between markers  410  and  411 ), capacitor discharge (between markers  411  and  412 ), and load dissipation of residual capacitor charge (between markers  412  and  413 ). Separation of inductor discharge from capacitor discharge is necessary to avoid the increased L/R time constant that would result from concurrent actions. Isolation of the output from load resistance  306  until charge reduction is effected is necessary to facilitate operation to zero, with assistance from filter inductance  313 .  
         [0019]    Feedback from operational amplifier  311  to PWM controller  300  is shown in FIG. 3 in order to illustrate that the time duration between time markers  411  and  412  of FIG. 4 may be controlled to produce a specific output (presumed zero in the example).  
         [0020]    By the method and circuitry disclosed herein, linear operation of a flyback converter is extended to zero.