Abstract:
Typically, in a half-bridge converter A DC blocking capacitor and inductance on the primary side combines with the inductor and capacitor on the secondary side to produce complicated converter dynamic characteristics that make converter stabilization difficult. The disclosed system and methods enable converter stabilization by modulating, as a function of the voltage across the DC blocking capacitor, the feedback output error signal or the ramp signal for the PWM controller of the converter. The modulation removes the effect of the resonant circuit formed by the primary inductance and the DC blocking capacitor in the output voltage regulation loop. Further adjustment of the duty cycle of the power switches enables keeping the output voltage undisturbed by voltage variation across the DC blocking capacitor. Since, this inner compensation loop results in the voltage regulation feedback loop respond to only the output inductor-capacitor filter, the transfer function of this loop displays second order characteristics from the fourth order. Hence, stability issues are simplified to conventional second order compensation network whereby the converter can be optimized for fast transient response.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of power converter, in particular to the field of feedback compensation in complementary driven half-bridge converters. 
     BACKGROUND OF THE INVENTION 
     Zero voltage switching reduces switching losses significantly and greatly improves converter efficiency especially when the bridge voltage is high and capacitance loss is significant. Furthermore, Zero voltage switching allows higher switching frequency for the bridge switches as well as use of smaller components. 
     Placing a DC blocking capacitor on the primary side of a converter results in an improved half bridge topology that provides zero voltage switching. This enables use of the complementary driving method, together with inductance on the primary side, for zero voltage switching of half-bridge switches. 
     Despite its many advantages, the complementary driving method has not been widely employed in half-bridge converters due to the complexity encountered in actual implementations. Small signal analysis of a circuit for a typical half-bridge converter reveals that the primary DC blocking capacitor and magnetizing inductance of the main transformer produce a second order resonant circuit. On the secondary side, output filter inductor and capacitor produce another resonant circuit. Together these two circuits form a fourth order system that is complex and consequently difficult to stabilize. 
     A number of researchers have addressed this problem but without forwarding a satisfactory solution. For instance, Korotkov et al. showed that the phase shift approaches 360 degrees when the both resonant circuits take effect with the aid of the theoretical Bode plot and the phase plot on the transfer function. Korotkov et al., “Small-signal modeling of soft-switched asymmetrical half-bridge DC/DC converter” in the  Applied Power Electronics Conference and Exposition  on pages 707-11 in 1995. This phase shift can result in stability problems with use of negative feedback for regulating the output voltage. 
     While it is possible to solve the stability problem by rolling off the open loop gain to less than one before both of the two resonant circuits take effect this results in other undesirable consequences. For instance, this greatly impairs the dynamic response of the converter along with additional limitations in the design of the feedback compensation network. 
     Sebastian et al. suggested another solution in the paper titled “Small-signal modeling of the half-bridge complementary-control DC-to-DC converter” in the  Power Electronics Congress  of 1995. Accordingly, ensuring a much greater resonant frequency for the output LC filter than the circuit comprising the primary magnetizing inductance and the DC blocking capacitor reduces the phase shift of asymmetrical half-bridge converter to less than 180 degrees. However, this method has limited applicability due to the limitation placed on the output filter design resulting in incompatibility with many normal converter specifications. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system and method to reduce the aforementioned complexity enabling better regulation of the output voltage in a half-bridge converter with complementary drive. The system and method taught by the present invention do not require limitations on the choice of resonant frequencies in the half-bridge or a fall-bridge converter. In example embodiments of the invention, the voltage regulating feedback loop sees only the second order output inductor-capacitor filter. Therefore, ordinary second-order compensation can stabilize the loop and provide optimization more readily for better dynamic response. 
     An illustrative embodiment of the present invention includes modulation of a voltage ramp signal or an output error feedback voltage as a function of the voltage across the DC blocking capacitor. A comparator of the Pulse Width Modulation (PWM) controller controlling the power switches receives the modulated signal. This inner compensation loop eliminates the effect of voltage variation across the DC blocking capacitor. 
     Moreover, the present invention allows furher optimization of the dynamic response of half-bridge converters and full-bridge converters. This follows, in part, from many embodiments in accordance with the present invention employing simple second-order compensation and conventional techniques to stabilize the output voltage-regulating loop of half-bridge converters with complementary drive. 
     Thus, possible variations include, without limitation, a controlled current source providing a current as a function of the DC blocking capacitor voltage of a half-bridge converter to produce the voltage ramp signal for the PWM controller. Alternatively, the voltage of a winding coupled to the main transformer of the half-bridge converter modulates the output voltage error signal followed by feed the error signal into the PWM controller. The winding has voltage proportional to the voltage across the DC blocking capacitor and in a desired phase to provide inner loop compensation. Another embodiment illustrates the present invention in full-bridge converters. 
     These and other aspects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and from the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention is described herein with the aid of drawings to provide an understanding of the different aspects of the invention in a non-exhaustive manner. These drawings are illustrative rather than limiting as to the scope of the invention and should be interpreted accordingly. 
     FIG. 1 is an illustration of a half-bridge DC-DC power converter with an inner compensation feedback loop and the output voltage regulation feedback loop; 
     FIG. 2 illustrates a half-bridge converter, in an embodiment of the invention, using a voltage related to the DC blocking capacitor to modulate the output error signal feedback in an inner compensation loop; 
     FIG. 3 illustrates a half-bridge converter, in an embodiment of the invention, using a voltage related to the DC blocking capacitor to modulate the ramp signal in an inner compensation loop; 
     FIG. 4 illustrates an embodiment of the invention using a controlled current source to produce the modulation signal at the input of a PWM controller; 
     FIG. 5 illustrates a more detailed implementation of the power converter illustrated in FIG. 4; 
     FIG. 6 is the measured Bode plot and phase plot in an embodiment similar to that illustrated in FIG. 1, but with no inner loop compensation; 
     FIG. 7 shows the measured Bode plot and phase plot illustrating suppression of changes in phase and gain at 1.7 kHz, the resonant frequency of the DC blocking capacitor and the magnetizing inductance, relative to FIG. 6 by inner loop compensation; 
     FIG. 8 shows an embodiment of the present invention illustrating the inner loop compensation by the signal obtained via voltage in a winding coupled to the main transformer of a half-bridge converter; 
     FIG. 9 is an implementation illustrating additional design details in an embodiment similar to that depicted in FIG. 8; and 
     FIG. 10 shows a full bridge converter constructed in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The principle of operation of the present invention is described in FIG.  1 . The various figures illustrate several design variations for implementing various embodiments of the invention mi a non-exhaustive manner. Different figures employ the similar numbers to label similar components with the leading digit representing the particular figure. 
     Thus, in FIG. 1, modulator  102  senses voltage across DC blocking capacitor  100  as the control signal. Modulator  102  modulates the output error signal with the control signal. Comparator  104  produces the output error signal by comparing the output voltage to a voltage provided by reference  106  in conjunction with a feedback network. The modulated output error signal is, next, applied to comparator  108  of PWM controller  110 . The phase of the control signal is modified so that PWM controller  110  senses voltage variations across capacitor  100 . PWM controller  110  responds to the changes in control signal and the resultant modulated output error signal by adjusting the duty cycle of the half-bridge to control switches  112  and  114  in order to counteract variations in the output voltage. This feedback eliminates the effect of voltage variation across DC blocking capacitor  100  on the output voltage. Therefore, the effect of the resonant circuit formed by primary equivalent magnetizing inductance  116  and DC blocking capacitor  100  is removed from the voltage regulating loop. 
     In the design described above, the duty cycle controls the power converter. The PWM controller  110  determines the duty cycle d with the aid of the output of internal comparator  108 . Accordingly, the duty cycle is related to the difference of this internal comparator&#39;s inputs, the modulated output error signal and a ramp signal as shown in eqn. 1. 
       d ≈( V   control   +V   output     —     error )− V   ramp   eqn. 1 
     where V control  is the control signal modulating the output error signal V output     —     error , and V ramp  is the ramp signal. FIG. 2 illustrates such an embodiment with modulator  202  receiving the control signal and the output error signal. 
     Rearranging eqn. 1, an equivalent equation is obtained as shown in eqn. 2. 
     
       
           d≈V   output     —     error   −[V   ramp +(− V   control )]  eqn. 2 
       
     
     Eqn. 2 shows that following rearrangement the introduced control signal can be interpreted to modulate the ramp signal input of modulator  102  of FIG.  1 . Of course, in the alternative arrangement the phase of the control signal is inverted as well to ensure consistency. FIG. 3 shows such an embodiment with modulator  302  receiving the ramp signal and the control signal. 
     Furthermore, the control signal, V control , in an embodiment of the invention, also functions as a ramp signal. This is possible, for instance, when the slope of the ramp signal is a function of the voltage across the DC blocking capacitor  100 . Then removing the separate ramp signal further simplifies the system as is shown in FIG.  4 . 
     The embodiment shown in FIG. 4 consists of a half-bridge converter having output nodes  418  and  420  and a duty cycle (1−d) and (d) produced by PWM controller  410  driving switches  412  and  414 . The switching operation of switches  412  and  414  converts the DC supply voltage from input nodes  422  and  424  to a pulsating voltage across nodes  426  and  424 . Transformer  428  receives this pulsating voltage with DC blocking capacitor  400  blocking off the DC component. As shown, transformer  428 , having primary and secondary windings with N 1  and N 2  turns respectively, presents magnetizing inductance reflected to the primary side as inductor  416 . The aforesaid pulsating voltage is coupled to the secondary winding and rectified in half-wave configuration by diodes  430  and  432 . The rectified current/voltage is then filtered to remove ripples by output filter formed by inductor  434  and capacitor  436  to provide smooth DC voltage output across output nodes  418  and  420 . 
     The ramp signal received at comparator  408  in PWM controller  410  following modulation provides inner loop compensation as described previously. However, instead of using a constant ramp signal and like techniques, it is possible to employ a variable ramp signal. Several components, namely controlled current source  438 , capacitor  440  and switch  442  generate this variable ramp signal in an embodiment illustrated in FIG.  4 . 
     In FIG. 4, current source  438  has an amplitude proportional to the control signal sensed across DC blocking capacitor  400  with proportionality constant k. Switch  442  discharges capacitor  440  after duty cycle d is determined by the PWM controller. The following analysis although described in the context of FIG. 4 to illustrate the operation of the invention, is not intended to be limiting on the scope of the claimed invention. 
     Thus, the slope of the modulated voltage ramp signal M is              M   =       i     current                 _                 source         C     current                 _                 source                 eqn   .              3                                
     This slope is not constant and depends on the voltage across blocking capacitor  400 . Thus,              M   =       k   ·     v   control         C     current                 _                 source                 eqn   .              4                                
     where V control  is the voltage across blocking capacitor  400 , k is the proportionality constant and C current     —     source  is the capacitance of capacitor  440 . Node  424  is the reference node for measuring voltage. The PWM controller  410  compares the ramp signal and the output error signal to produce duty cycle d. It can be shown that                v     output                 _                 error       =       k   ·     v   control     ·   d         f   sw     ·     C     current                 _                 source                   eqn   .              5                                
     where v output     —     error  is the output error signal with node  424  being the reference node and f SW  is the switching frequency of the converter. 
     Averaged small signal analysis provides the following 6 equations. 
     
       
           s·L   induct   ·Δi   induct =(1 −D )·Δ v   input   −Δv   control   eqn. 6  
       
       
         
           
             
               
                 
                   
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     where v input  is the input voltage, v control  is the voltage across blocking capacitor  400 , v output     —     error  is the output error feedback voltage, and v o  is the output voltage across output nodes  418  and  420 . Δ represents the small perturbation of the parameter on its right hand side. D, I rectified  and V control  are the steady state values of d, i rectified  and v control  respectively. 
     Eqn. 10 is obtained directly by adding perturbation to eqn. 5 which is the effect produced by the inner compensation loop. 
     Solving eqn. 6-11 gives the transfer function from the output error feedback voltage, v output     —     error , to the output voltage v o . This result is shown in eqn. 12.                  Δ                   v   o         Δ                   v     output                 _                 error           =         N   2       N   1       ·         f   sw     ·     C     current                 _                 source         k     ·     1     1   +       s   2     ·     L     ripple                 _                 filter       ·     C     ripple                 _                 filter                       eqn   .              12                                
     Eqn. 12 shows that the transfer function from the output error feedback voltage, v output     —     error , to the output voltage v o  is second order, depending on the output filter formed by L ripple     —     filter  and C ripple     —     filter . This transfer function is independent of primary magnetizing inductance  434  and the DC blocking capacitor  400  due to the presence of the inner compensation loop. 
     The above calculations show the effect of inner loop compensation by using a voltage proportional to the voltage across the DC blocking capacitor, e.g., capacitor  400 , to produce the ramp signal. Since the modulation signal is already a ramp signal, no extra ramp signal is required. The voltage variation on the DC blocking capacitor  400  introduces corresponding change in the duty cycle for controlling switches  412  and  414  to regulate the output voltage. Thus, the effect of the resonant circuit formed by the primary reflected magnetizing inductance and the DC blocking capacitor is removed and the control loop is greatly simplified and made more stable. 
     FIG. 5 shows an embodiment of the invention. Resistor  548  and capacitor  540  produce the voltage ramp signal for the PWM controller  510  and switch  542 . Resistor  548  connected to the DC blocking capacitor  500  detects the voltage variation across capacitor  900 . The charging current injected to capacitor  540  is approximately equal to the voltage across capacitor  500  divided by the resistance of resistor  548  if the voltage of the ramp signal is much smaller than voltage across capacitor  500 . This is also an illustrative example of the controlled current source described previously. The duty cycle d responds to the voltage variation of capacitor  500  to cancel out or reduced the effect on the output voltage v o . This performs the inner loop compensation that removes or reduces the effect of the resonant circuit formed by the primary reflected magnetizing inductance  516  and the DC blocking capacitor  500 . 
     FIGS. 6 and 7 show comparison of the gain plot and phase plot of the first embodiment without and with inner loop compensation respectively. A constant voltage ramp signal is applied to the PWM controller for the one without inner loop compensation. The circuit parameters used are as follows, 
     L induct =180 uH, the primary magnetizing inductance; 
     C control =44 uF, the blocking capacitor; 
     L ripple     —     filter =1.2 uH, the inductance in output filter; and 
     C ripple     —     filter =1200 uF, the capacitance in the output filter. 
     The resonant frequency of the resonant circuit formed by the primary reflected magnetizing inductance  516  and the DC blocking capacitor  500  is around 1.7 KHz and the resonant frequency formed by the output filter elements  534  and  536  is around 4 KHz. Specifically, FIG. 7 shows significant suppression of the effect of the resonant circuit elements inductor  516  and capacitance  500  in comparison with the FIG. 6 in which no inner loop compensation is implemented. 
     FIG. 8 shows another embodiment of the present invention which employs an additional winding  854  to sample the voltage accross DC blocking capacitor  800 . Specifically, inner loop compensation is applied to a half-bridge converter with full wave rectification. Two switches  812  and  814  are driven with duty cycle (1−d) and (d) respectively by the PWM controller  810 . The switching operation of switches  812  and  814  converts the DC supply voltage from node  822  and  824  to a pulsating voltage coupled to the main transformer  828  through a path including the DC blocking capacitor  800 . Turning on switch  814  in accordance with duty cycle d applies the voltage across capacitor  800  to winding  856  of the main transformer  828 . Moreover, this voltage is reflected onto coupled winding  854  of transformer  828 . Thus, turning on switch  814  obtains, through winding  854 , a voltage that is a function of the voltage across the DC blocking capacitor  800 . Rectifier  852  and the phase of the winding  854  are arranged to produce the required voltage for the output error signal modulation. This voltage is a function of the voltage across the DC blocking capacitor  800  with the correct phase as described previously in order to ensure effective inner loop compensation. The modulation adjustment network  802  adjusts the level of modulation to suit PWM controller  810 . 
     FIG. 9 illustrates an embodiment employing two resistors  956  and  958  to implement modulation network  902  similar to modulation network  802 . To this end, the following equation shows the modulated error signal vmod output error at the input to comparator  908  of PWM controller  910  with the drop across diode  952  ignored.                v     mod                 _                 output                 _                 error       =       1       R   956     +     R   958         ·     (                    v     output                 _                 error       ·     R   956       -       v   control     ·       N   4       N   1       ·     R   958         )               eqn   .              13                                
     v output     —     error  is the output error signal, v control  is the voltage across the DC blocking capacitor  900 , N 4  and N 1  are the number of turns of winding  954  and the number of turns of winding  960  respectively while node  924  serves as the reference node in the above expressions. 
     Another embodiment extends the present invention to a full bridge converter as shown in FIG.  10 . FIG. 10 illustrates DC input applied to input nodes  1022  and  1024 . Switches  1012 ,  1013 ,  1014  and  1015  are connected in full bridge configuration and driven by duty cycles d 1 , d 2 , d 3  and d 4  respectively to convert the DC input to a pulsating voltage coupled to transformers  1028  and  1029 . The DC blocking capacitor  1000  prevents DC from entering transformers  1028  and  1029 . The AC pulsating voltage couples to the secondary windings followed by rectification at rectifiers  1030  and  1032 . A low pass output filter formed by inductor  1034  and capacitor  1036  provides a smooth DC output by reducing ripples. 
     The ramp signal for the PWM controller  1010  is modulated by the voltage across the DC blocking capacitor  1000  through the controlled current source i constant     —     current    1038 . The operational principles behind the functioning of this embodiment are similar to those illustrated previously except that the present embodiment extends to a full-bridge converter. The duty cycles d 1 , d 2 , d 3 , and d 4  obtained from the PWM controller  1010  for driving the four switches  1012 ,  1014 ,  1013  and  1015  are adjusted in response to the modulated ramp signal. Of course, in this arrangement, voltage variation across capacitor  1000  is reflected in the modulated ramp signal to produce the desired inner loop compensation. 
     Therefore, there are disclosed several designs and principles for designing a power converter for DC to DC voltage conversion to generate a desired and easily regulated output voltage. Such a power converter includes input terminals for receiving power, at least one switch coupled to the input terminals to generate a variable current in accordance with a duty cycle; a magnetic component suitable for transforming the variable current to generate a transformed current; an input capacitor coupled to magnetic component; a rectifying circuit for rectifying the transformed current to generate a rectified current; a filter for smoothing the rectified current at output terminals; a feedback network generating an error signal reflecting deviation of a voltage at the output terminals from a desired reference; and a controller receiving the error signal, sensing the voltage across the input capacitor and responsive to a ramp signal for generating driving signals to operate the at least one switch in accordance with the duty cycle whereby the power converter operates as a closed loop feedback system. Variations and enhancements include different modulation schemes, use of current sources, different methods for sampling the voltage across one or more blocking capacitors of interest and extension to full-bridge power converter design. 
     Although the preceding description of the invention is in the context of the embodiments described herein, this is not intended to be a limitation on the scope of the invention. As readily recognized by one of ordinary skill in the art, the disclosed invention encompasses the disclosed embodiments along with other embodiments providing different filter configurations, diodes, rectifiers and magnetic materials. These variations are intended to be within the scope of the following claims.