Abstract:
A conventional single-ended, primary-inductance converter (SEPIC) has its switching frequency determined by a controller, which determines the duty cycle at which the switch operates by measuring differences between the SEPIC output voltage and a reference voltage. Output voltage overshoot and undershoot are reduced.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 14/089,955, which was filed Nov. 26, 2013, and entitled, “PWM Generation for DC/DC Converters with Frequency Switching.” The content of that application is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The single-ended primary inductance convertor or SEPIC is a well-known DC-to-DC voltage convertor that can provide an output voltage greater or less than an input voltage. It also does not change or reverse an input voltage polarity. A detailed explanation of SEPICs is available from Daniel W. Hart, Power Electronics, McGraw-Hill Companies, Inc., 2011, pages 231-235, the content of which is incorporated herein by reference. 
         [0003]    Referring now to  FIG. 1 , a SEPIC  100  needs a switch  24 , typically embodied as a metal oxide semiconductor field effect transistor (MOSFET) to charge and discharge inductors  22 ,  28  and capacitors  26  and  32  as described by Hart, supra. The SEPIC output voltage i.e., V out  is the function of the input voltage, V in  and a duty ratio of the switch in Continuous Conduction Mode (CCM) and is the function of the input voltage, Vin, output power and a duty ratio of the switch in Discontinuous Conduction Mode (DCM). 
         [0004]    In Automotive applications good dynamic response to changes of the input voltage and output power is desired. This is only possible, if SEPIC is running in DCM. When SEPIC input voltage decreases, at some point it is necessary, as known in previous art, to reduce SEPIC operating frequency in order to keep it running in DCM. The change of the operating frequency may cause the overshoot or undershoot of the SEPIC output voltage. 
         [0005]    A power supply that can minimize or at least reduce the overshoot and undershoot of prior art SEPIC convertors would be an improvement over the prior art. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0006]      FIG. 1  depicts a prior art single-ended primary inductance convertor or SEPIC; 
           [0007]      FIG. 2  depicts a prior art SEPIC and a controller, which outputs a pulse width modulated signal to the SEPIC&#39;s control switch and which minimizes overshoot and undershoot of the output voltage; 
           [0008]      FIG. 3  is a graph of a saw tooth signal and a pulse width modulation (PWM) output signal for two operating frequencies implemented in prior art. 
           [0009]      FIG. 4  is a graph of a saw tooth signal and a pulse width modulation (PWM) output signal for two operating frequencies, which eliminates SEPIC output voltage overshoot or undershot, when operating frequency changes; and 
           [0010]      FIG. 5  depicts steps of a method. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 2  depicts a power supply comprising a prior art SEPIC  100  and a controller  200 . The SEPIC  100 , which is also shown in  FIG. 1 , includes an input filter capacitor  204  across an input voltage terminal  206 , to which an input voltage, V in , (not shown) is provided. The magnitude of the input voltage, V in , which is typically provided by a vehicle battery, is measured or determined relative to a reference potential node  208 . The reference potential voltage in a vehicle is typically at zero or near zero volts and commonly referred to as “ground” potential. 
         [0012]    A series-connected “primary” inductor  210  connects an input voltage, V in  to a semiconductor switch  214 , preferably embodied as a MOSFET the gate or control input  216  of which is connected to an output node  202  of the controller  200 . As is well known, the MOSFET switch  214  “opens” and “closes” responsive to voltages on the control input  216 . When the switch  214  is “closed,” i.e., the MOSFET is conducting, the switch  214  shunts current flowing through a first inductor  210 , i L  to ground  208  and current from the capacitor  218  into the second inductor  28 . When the switch  214  is open, i.e., not conducting, it forces the first inductor current, i L  to flow through a coupling capacitor  218  to a steering diode  220  and the second inductor current to a steering diode  220 , which directs the current to a load, R L    205 . 
         [0013]    The output voltage, V out , of the SEPIC  100 , is measured across an output voltage terminal  222  and reference potential node  208 . The duty cycle ratio of the switch open time, DT to its closed time, 1-DT, determines the output voltage, V out . The output voltage is thus a function of the duty cycle of the switch  214 . That duty cycle is determined by the controller  200  to maintain the output voltage Vout constant. 
         [0014]    The preferred embodiment of the controller  200  comprises two comparators  230 ,  236 , an oscillator  232 , a ramp generator  234 , a D flip-flop  238 , an error amplifier  242  and a voltage reference source  240 . As described more fully below, the output of the flip-flop  238  is a pulse train having a duty cycle determined by changes in the SEPIC output voltage, V out  relative to a reference voltage  240 . 
         [0015]    The first comparator  230 , which is known, has two inputs  244  and  246 . The first input  244  is connected to the input voltage terminal  206  of the SEPIC  100 . The other input terminal  246  is connected to the output terminal  248  of the voltage reference source  240 . 
         [0016]    The first comparator  230  has an output terminal  244  coupled to the oscillator  232 . The oscillator  232  is constructed to provide output signals  233  of two frequencies synchronizing the operation of SEPIC from an output terminal  235  responsive to the voltage input to the oscillator by the first comparator  230 . Stated another way, the oscillator  232  produces an output signal, the frequency of which changes between two frequencies or values responsive to the input signals to the oscillator  232  from the comparator  230 . 
         [0017]    The oscillator output signal  233  is input to a conventional ramp generator  234 . The ramp generator  234  is also known as a “saw tooth” generator because the shape of its output signal  237  resembles a saw tooth: it increases continuously and linearly until a peak value is reached at the end of the given operating cycle, then drops to zero. 
         [0018]    The oscillator output  233  is also provided into a set terminal  239  of a conventional, D-type flip-flop  238 . The “reset” input  241  of the flip-flop  238  is connected to the output of a second comparator  236 . 
         [0019]    The second comparator  236  has one input connected to the ramp generator  234  output. A second input is connected to the output of an error amplifier  242 . 
         [0020]    The error amplifier  242  receives two input signals: the output voltage  222  of the SEPIC and the reference voltage  248 . The error amplifier  242  produces an output signal  245  that is proportional to the magnitude of the difference between the reference voltage  248  and the SEPIC output voltage  222 . As the output voltage  222  of the SEPIC  100  changes relative to the reference  208 , the output voltage  245  of the error amplifier  242  changes accordingly. The error amplifier output  245  thus determines when the second comparator  236  changes its output signal, thereby, “toggling” the D flip-flop  238  and producing a pulse train input to the gate  216  of the MO SFET  214 . 
         [0021]      FIG. 3  depicts the saw tooth wave form  310  output from the ramp generator  234  and a pulse train  322  output from the D flip-flop  228  for two operating frequencies as known in prior art and  FIG. 4 , depicts the saw tooth wave form provided by the circuit shown in  FIG. 2 , and which essentially eliminates output voltage overshoot or undershoot when operating frequency changes. 
         [0022]    In  FIG. 3 , the saw tooth wave form  310  between t 0  and t 1  has a first frequency, denominated as F 1 . The output pulse train  322  during that same time period has a first duty cycle denominated as DC 1 . 
         [0023]    At a later time denominated as t 1 , at which the input voltage, V in  to the SEPIC  100  decreases, the input voltage to the controller  200  also drops below the reference voltage  240 . The drop in the input voltage V in  causes the first comparator  202  to change its output state, which in turn changes the oscillator frequency to a lower frequency value F 2 , not shown per se in  FIG. 3  and  FIG. 4 , but clearly depicted by the lower frequency saw tooth waveform  310  that begins at t 1 . 
         [0024]    At time t 1 , the SEPIC output voltage does not change instantaneously. The error voltage, V e , therefore does not change instantaneously and together with the new slope of the ramp generator will define the duty cycle DC 2 . Those of ordinary skill in the art should thus realize that as operating frequency of the SEPIC running in DCM changes, the output voltage will stay constant only if the output power will stay constant, i.e. the product of cycle energy and operating frequency before and after frequency change is constant. 
         [0025]    Still referring to  FIG. 3 , since the peak voltage of the saw tooth signal does not change, the duty cycle DC 1  at operating frequency F 1  is equal to duty cycle DC 2  at operating frequency F 2 . If K is the ratio of two operating frequencies, i.e. K=F1/F2, then to eliminate the SEPIC output voltage overshoot or undershoot when operating frequency changes, one need to have E2/E1=K, where E 1 , E 2  are cycle energy at respective operating frequency F 1 , F 2 . Cycle energy of the SEPIC converter is proportional to the second power of the current change in SEPIC inductors. Current change in the SEPIC inductors is proportional to the switch ON time Switch ON time is proportional to the duty cycle and inversely proportional to operating frequency. Thus cycle energy is proportional to the square of the ratio of duty cycle and operating frequency. In prior art the duty cycle stays the same, so the product of the cycle energy and operating frequency will change, when operating frequency is changed, and in turn, produce the overshoot or undershoot of the SEPIC output voltage. 
         [0026]    In  FIG. 4 , the duty cycle DC 1  at operating frequency F 1  is not equal to the duty cycle DC 2  at operating frequency F 2 . When operating frequency is switched from F 1  to F 2 , the saw tooth peak voltage is increased by the factor of square root of K, or saw tooth slope is decreased by the factor of square root of K and duty cycle D2=D1/SQR (K). In that case the product of cycle energy and operating frequency is not changing when operating frequency is changed and output voltage of the SEPIC converter running in DCM stays constant. 
         [0027]    The foregoing description is for purposes of illustration only. The true scope of the invention is set forth in the following claims.