Abstract:
A multi-phase transformer type DC-DC converter. In one embodiment, the multi-phase transformer type DC-DC converter includes a plurality of DC-DC converters comprising a plurality of transformers, respectively, wherein the plurality of DC-DC converters are coupled in parallel between an input and an output. A circuit is coupled to the plurality of DC-DC converters and configured to generate a plurality of clock signals for use by the plurality of DC-DC converters, respectively, wherein the plurality of clock signals are phase shifted with respect to each other.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the domestic benefit under Title 35 of the United States Code §119(e) of U.S. Provisional Patent Application Ser. No. 61/875,143, filed on Sep. 9, 2013, entitled “Multi-Phase Transformer Type DC-DC Converter,” which is hereby incorporated by reference in its entirety and for all purposes as if completely and fully set forth herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Large data centers contain rows and rows of server racks, which consume substantial amounts of power at a high cost. Some data centers consume power more than 100 times that of a typical office building. For large power consuming data centers, electricity costs are a dominant operating expense and can account for over 10% of the total cost of ownership. 
         [0003]    Local utilities provide power to data centers via power lines that have resistive elements R, which consume power P as a function current I (i.e., P=I 2 R). Utilities prefer to transmit power at high voltage and low current in order to minimize resistive power consumption. Data centers distribute power they receive to server racks and other components via internal power transmission lines that also contain resistive elements. Like utilities, data centers seek to minimize resistive power consumption in their power distribution lines by transmitting power to server racks at high voltage, low current. At some point, however, power must be converted to low voltage (e.g., 1.2 volts DC) and high current for use by components such as CPUs within the servers. 
       SUMMARY OF THE INVENTION 
       [0004]    A multi-phase transformer type DC-DC converter is disclosed. In one embodiment, the multi-phase transformer type DC-DC converter includes a plurality of DC-DC converters comprising a plurality of transformers, respectively, wherein the plurality of DC-DC converters are coupled in parallel between an input and an output. A circuit is coupled to the plurality of DC-DC converters and configured to generate a plurality of clock signals for use by the plurality of DC-DC converters, respectively, wherein the plurality of clock signals are phase shifted with respect to each other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The present invention may be better understood in its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 
           [0006]      FIG. 1  is a block diagram illustrating an example power distribution system that could be employed in a data center. 
           [0007]      FIG. 2  is a block diagram illustrating an example DC-DC converter that could be employed in the example power distribution system of  FIG. 1 . 
           [0008]      FIG. 3  is a timing diagram illustrating control signals employed in the DC-DC converter of  FIG. 2 . 
           [0009]      FIG. 4  is a block diagram illustrating an example DC-DC converter that could be employed in the example power distribution system of  FIG. 1 . 
           [0010]      FIG. 5  is a timing diagram illustrating example control signals employed in the DC-DC converter of  FIG. 4 . 
           [0011]      FIG. 6  is a block diagram illustrating an example embodiment of the DC-DC converter shown in  FIG. 4 . 
           [0012]      FIGS. 7A and 7B  illustrate timing diagrams showing example control signals and voltages employed in the DC-DC converter of  FIG. 6 . 
           [0013]      FIG. 8  is a block diagram illustrating an example embodiment of the DC-DC converter shown in  FIG. 6 . 
           [0014]      FIG. 9  is a block diagram illustrating an example embodiment of the DC-DC converter shown in  FIG. 6 . 
           [0015]      FIG. 10  is a block diagram illustrating an example embodiment of the DC-DC converter shown in  FIG. 6 . 
           [0016]      FIG. 11  is a block diagram illustrating an example embodiment of the DC-DC converter shown in  FIG. 6 . 
           [0017]      FIG. 12  is a block diagram illustrating an example embodiment of the DC-DC converter shown in  FIG. 6 . 
       
    
    
       [0018]    The use of the same reference symbols in different drawings indicates similar or identical items. 
       DETAILED DESCRIPTION 
       [0019]      FIG. 1  illustrates a portion of an example system for distributing power to integrated circuits (e.g., CPUs) in server racks in a data center. For purposes of explanation only, the present invention will be described with reference to distribution of power to CPUs mounted on server printed circuit boards (PCBs) within server racks of a data center, it being understood the present invention should not be limited thereto. 
         [0020]    With continuing reference to  FIG. 1 , AC-DC converter  102  is configured to convert high voltage, low current AC power into high voltage, low current DC power. An intermediate power transmission line transmits the high voltage, low current DC power from converter  102  to at least one rack of servers. The intermediate power transmission line contains resistive elements. Because power is transmitted at low current, these resistive elements consume relatively small amounts of power. 
         [0021]    Server racks typically contain DC-DC converters, such as DC-DC converter  104 , for converting power before it is transmitted to one or more server PCBs via a rack power transmission line  112 . Like the intermediate transmission line, the rack power transmission line  112  contains resistive elements. 
         [0022]    Server PCBs, relevant aspects of one of which are shown in  FIG. 1 , typically include a DC-DC converter  106  for converting power into a form (e.g., 1.2 volts DC, 150 amps) needed by a CPU  110 . As an aside, CPUs can operate in different modes. For example, in a low data processing mode of operation, CPU  110  generally requires power at relatively low voltage (e.g., 1.2 volts DC). In a high data processing mode of operation, CPU requires power at higher voltage (e.g., 1.8 volts DC). DC-DC converter  106  should be able to provide a variable source of voltage Vout to the changing needs of CPU  110 . Also, DC-DC converter  106  should be capable of quickly responding to changes in current and voltage demands of a CPU  110  during operation thereof. 
         [0023]    As noted above, the rack power transmission line, like the intermediate power transmission line, includes resistive elements that consume power. If power is transmitted over the rack transmission line at high voltage and low current, the power costs associated with these resistive elements can be reduced. However, depending on the technology employed by DC-DC converter  106 , there may be an upper limit on the input voltage Vin that DC-DC converter  106  can convert. 
         [0024]      FIG. 2  illustrates one type of DC-DC converter  106 . More particularly,  FIG. 2  illustrates a non-isolated, multiphase step-down DC-DC converter  200 . Each phase  208  includes high-side and low-side transistors Q 1  and Q 2  coupled to an inductor  210 , which in turn is coupled to CPU  110  via an output node  204  as shown. For purposes of explanation, all transistors described herein will take form in n-channel or p-channel MOSFETs, it being understood the present invention should not be limited thereto. 
         [0025]    Each phase  208  includes a driver circuit  206  that generates complementary, high-side and low-side square waves (not shown) that control transistors Q 1  and Q 2 , respectively. Drivers  206  generate these square waves as a function of respective, phase shifted square wave inputs Vsw provided by PWM control logic  212 . The duty cycle D of the square wave inputs Vsw is t 1 /(t 1 +t 2 ).  FIG. 3  illustrates examples of phase shifted square waves Vsw. 
         [0026]    The pulses of high-side and low-side square waves activate Q 1  and Q 2 , respectively. The high-side square wave provided to Q 1  has a pulse width of t 1 , while the low-side square wave provided to Q 2  has a pulse width of t 2 . Q 1  transmits current to output node  204  via inductor  210  with each pulse of the high-side square wave, and Q 2  transmits current from ground to output node  204  via inductor  210  with each pulse of the low-side square wave. Since the high-side and low-side square waves are complementary, which means they do not have overlapping pulses, only one of Q 1  and Q 2  in each phase transmits current at any given time. One of ordinary skill in the art understands that the magnitude of the output voltage Vout provided by DC-DC converter  200  depends on the duty cycle D=t 1 /(t 1 +t 2 ) and Vin. More particularly, Vout=DVin for the non-isolated, multiphase step-down DC-DC converter  200 . 
         [0027]    Non-isolated, multiphase step-down DC-DC converter  200  is limited in its ability to convert a high voltage Vin to a low voltage Vout. For example, to convert Vin at 48 volts DC to Vout at 1.2 volts DC, the PWM control logic  212  must generate phase shifted square wave inputs Vsw having a very low duty cycle of D=0.025, which may be difficult if the frequency of Vsw is high. Additionally, the ability of converter  200  to quickly respond to changes in voltage and current demanded by CPU  110  may be difficult when Vsw has a small duty cycle. 
         [0028]      FIG. 4  illustrates relevant components of another type of DC-DC converter  106  that could be employed in  FIG. 2 .  FIG. 4  illustrates an example multiphase, transformer-type DC-DC converter  400  according to one embodiment of the present invention. DC-DC converter  400  is capable of converting a relatively high input voltage Vin (e.g., Vin=48 volts DC) into a low output voltage Vout (e.g., Vout=1.2 volts DC) using control signals having larger duty cycles when compared to the duty cycle employed in DC-DC converter  200 , while remaining very quick in responding to sudden changes in power demands of CPU  110 . 
         [0029]    DC-DC converter  400  includes three phases 1-3 coupled in parallel between input node  404  and output node  406 . DC-DC converter  400  shown in  FIG. 4  includes three phases, it being understood that alternative embodiments of the DC-DC converter  400  may include additional or fewer phases. Phases 1-3 should contain the same components in respective embodiments of DC-DC converter  400 . As such phases 1-3 should operate identically in respective embodiments. For ease of illustration, the relevant components of phase 1 will be shown in each embodiment of DC-DC converter  400 . 
         [0030]    DC-DC converter  400  can be embodied as an isolated DC-DC converter  400  or a non-isolated DC-DC converter  400 . In the isolated embodiment, CPU  110  is coupled to ground GND 1 , which is separate and electrically isolated from a second ground GND 2  that is provided to each phase of DC-DC converter  400 . In the non-isolated embodiment, a common ground (e.g., the first ground GND 1 ) is employed by CPU  110  and throughout DC-DC converter  400 . 
         [0031]    DC-DC converter  400  includes a phase controller  408  coupled to and configured to control phases 1-3 in accordance with digital voltage request Vreq generated by CPU  110 . Vreq can change as CPU  110  transitions between different modes of operation as will be more fully described. 
         [0032]    Each of the phases 1-3 contains a transformer (not shown in  FIG. 4 ) that includes primary and secondary windings. A transformer is a static electrical device that transfers energy by inductive coupling between its primary and secondary windings. Vp, the voltage across the primary winding is related to Vs, the voltage across the secondary winding. In general Vp/Vs is proportional to Np/Ns, where Np/Ns is the winding turns ratio between the primary and secondary windings. Vout, the output of DC-DC converter  400  is dependent on Vr, and thus Vp and the windings ratio. As will be more fully described below, DC-DC converter can change the output voltage Vout in response to a change in Vreq requested by CPU  110 . 
         [0033]    Phase controller  408  includes controller logic  409  that generates phase shifted clock signals CLK 1 -CLK  3  for controlling phases 1-3, respectively.  FIG. 5  illustrates an example timing diagram of clock signals CLK 1 -CLK 3  provided by controller logic  409 . In the embodiment shown, clock signals CLK 1 -CLK 3  are phase shifted by 60°. In the embodiment where DC-DC controller  400  contains more phases, phase controller will provide additional clock signals. For controllers with M phases, controller logic  409  will provide M clock signals CLK 1 -CLKM to respective phases, with the phase difference between them set to 180°/M in one embodiment. In the example shown in  FIG. 5 , the duty cycle D of each clock signal CLK 1 -CLK 3  is 0.50. In one embodiment, controller logic  409  can change the frequency of clock signals CLK 1 -CLK 3  in response to an externally received instruction from CPU  110  or other device. 
         [0034]    DC-DC converter  400  is capable of converting a large Vin (e.g., Vin=48 volts DC) to a small Vout (e.g., Vout=1.2 volts DC) with internally generated pulse width modulation (PWM) signals (not shown in  FIG. 4 ) having a relatively larger duty cycle. As will be more fully described below, the output voltage Vout is dependent on the windings ratio Np/Ns and the duty cycle of the internally generated PWM cycles. In one embodiment, a conversion of Vin=48 volts DC to Vout=1.2 volts DC can be accomplished with a windings ratio Np/Ns=0.167 and duty cycle of 0.15 for the internally generated PWM signals, which is substantially larger than the duty cycle of 0.025 that is needed by the DC-DC converter of  FIG. 2  to implement the same conversion (i.e., Vin=48 volts DC to Vout=1.2 volts DC). The larger duty cycles reduce or eliminate many of the problems that plague the PWM control logic  212  and other components in  FIG. 2 . 
         [0035]    Phase controller  408  receives Vreq from CPU  110  and Vout. Vreq is a digital signal that identifies a voltage level needed by CPU  110  for proper operation. Vreq can change over time depending on processing demands placed on CPU  110 . Phase controller  408  contains a digital-to-analog converter (DAC)  410  that directly or indirectly receives Vreq, and generates Vtarget, an analog equivalent of Vreq. 
         [0036]    Voltage adjust circuit  412  receives Vtarget and Vout, and generates a comparative voltage E as a function thereof. Comparative voltage E is provided to each phase of DC-DC converter  400 , and is used to control the magnitude of Vout as will be more described below. In one embodiment, Vout varies directly with comparative voltage E; if Vout is lower than Vtarget, voltage adjust circuit  412  increases comparative voltage E until Vout equals Vtarget, and if Vout is greater than Vtarget, voltage adjust circuit  412  decreases comparative voltage E until Vout equals Vtarget. 
         [0037]    Each of the phases 1-3 receives comparative voltage E from phase controller  408 . Each phase 1-3 increases Vout as comparative voltage E increases, and each phase 1-3 decreases Vout as comparative voltage E decreases. Since phases 1-3 are identically configured, each phase generates the same voltage Vout. 
         [0038]      FIG. 6  illustrates relevant components of DC-DC converter  600 , which is one embodiment of DC-DC converter  400 . The relevant components of only phase 1 are shown, it being understood that phases 2 and 3 are identically configured. As seen in  FIG. 6 , phase 1 includes a transformer circuit  612 , which includes a transformer. A primary winding  614  of the transformer is shown in  FIG. 6 . The secondary winding of the transformer is not shown in  FIG. 6 , but the secondary winding is contained within the secondary winding circuit  616 , which generates Vout. The output of secondary winding circuit is coupled to output node  406 . As will be more fully described, secondary winding circuit  616  rectifies the voltage across the secondary winding of the transformer. 
         [0039]    A full-bridge circuit consisting of MOSFETs  620 - 626  controls the flow of current in primary winding  614  based on control signals A 1 -D 1  generated by PWM generator  630 . PWM generator  630  in combination with MOSFETS  620 - 626  generates a PWM voltage across the primary winding  614  as will be more fully described. In some embodiments of DC-DC converter  600 , PWM generator  630  generates control signals F 1  and G 1  for controlling MOSFETs in secondary winding circuit  616  as will be more fully described below. The gates of MOSFETS  620 - 626  may be decoupled from PWM generator  630  via an optional decouple circuit  632  depending on whether DC-DC converter  600  is implemented as an isolated or non-isolated converter. Ground GND  1  is provided to MOSFETs  622  and  624  in the non-isolated version of DC-DC converter  600 , and ground GND 2  is provided to MOSFETs  622  and  624  in the isolated version of DC-DC converter  600 . The decouple circuit  632  is configured to isolate ground GND 2  provided to MOSFETs  620 - 626  and ground GND 1  provided to secondary winding circuit  616  when DC-DC converter  600  is implemented in the isolated version. 
         [0040]    Each phase includes a current sense circuit  636 , which generates a voltage Vcs that is proportional to current flow En from input node  404  to the phase&#39;s primary winding  614 . PWM controller  634  receives Vcs in addition to CLK 1  and comparative voltage E. PWM controller  634  controls PWM generator  630 , and thus control signals A 1 -D 1 , based on Vcs, CLK 1 , and comparative voltage E as will be more fully described below. 
         [0041]    With continuing reference to  FIG. 6 ,  FIG. 7A  is a timing diagram that illustrates example control signals A 1 -D 1  generated by PWM generator  630 . Control signals A 1 -D 1  control MOSFETS  620 - 626 , respectively, which in turn control current flow through primary winding  616 .  FIG. 7A  also shows comparative voltage E and CLK  1  provided by phase controller  408 , Vcs generated by current sense circuit  636 , and Vp, which is the voltage across primary winding  614 . 
         [0042]      FIG. 7A  shows t on , which is the time period during which MOSFETs  626  and  622  are activated by control signals D 1  and A 1 , respectively, or when MOSFETs  620  and  624  are activated by control signals B 1  and C 1 , respectively. During t on  current from input node  404  flows through primary winding  614  and induces voltage Vp. During the first half cycle of CLK 1  when MOSFETS  626  and  622  are activated, Vp is approximately equal to +Vin. During the second half cycle of CLK 1  when MOSFETs  620  and  624  are activated, Vp is approximately equal to −Vin. 
         [0043]    Current sense circuit  636  generates Vcs, which is proportional to current flow into primary winding  614  through MOSFET  636  or MOSFET  620 . As current flow into primary winding  614  increases, Vcs increases in proportion. PWM control  634  receives and compares Vcs with comparative voltage E. When Vcs equals comparative voltage E during the first half cycle of CLK 1 , PWM control  634  generates a signal that instructs PWM generator  630  to de-assert control signal D 1 , which in turn deactivates MOSFET  626 . When Vcs equals comparative voltage E during the second half cycle of CLK 1 , PWM control  634  generates a signal that instructs PWM generator  630  to de-assert control signal C 1 , which in turn deactivates MOSFET  626 . One of ordinary skill understands that the length of t on  can be adjusted by adjusting comparative voltage E; an increase in E results in a proportional increase in t on , and vice-versa. 
         [0044]    Secondary winding circuit  616  will generate Vout proportional to D t (Ns/Np)Vin, where D t =t on /(t on +t off ), and where t off  is the time period between t on  in respective cycles of CLK 1 . Since the length of t on  can be adjusted by adjusting comparative voltage E, Vout can be adjusted by adjusting comparative voltage E. In other words, Vout will increase with t on , which increases when comparative voltage E increase. And Vout will decrease with t on , which decreases when comparative voltage E decreases. As noted above, comparative voltage E compare will increase or decrease until Vout equals Vtarget. Vreq is the digital equivalent of Vtarget. Accordingly, Vout will increase or decrease with a corresponding increase or decrease in Vreq. 
         [0045]      FIG. 8  illustrates one example of an isolated version of DC-DC converter  600  shown in  FIG. 6 . Voltage adjust circuit  412  in  FIG. 8  includes amplifiers  802  and  804 . Additionally, voltage adjust circuit  412  includes a pair of resistors and a capacitor arranged as shown. Sense amplifier  802  receives Vout and ground GND 1  at its input terminals as shown. The output terminal of sense amplifier  802  is coupled to one input terminal of error amplifier  804  via resistor  806 . This input terminal of error amplifier  804  is coupled to the output terminal of error amplifier  804  via capacitor  810  and resistor  812 . The other input terminal of error amplifier  804  receives Vtarget, the analog equivalent of Vreq. Error amplifier  804  generates comparative voltage E at its output terminal. As noted above, comparative voltage E is provided to the PWM control circuit  634  in each phase 1-3. For purposes of explanation only, each version of DC-DC converter shown in the remaining figures will employ the same phase controller  408  that is shown in  FIG. 8 . 
         [0046]    With continuation reference to  FIG. 8 , PWM control circuit  634  includes an SR flip flop  814 , a voltage comparator  816 , and a pulse generator  818 . CLK 1  is coupled to the input of pulse generator  818 , which generates a set pulse with each rising or falling edge of CLK 1 . The output of comparator  816  is coupled to the R input terminal of flip flop  814 , while the output of pulse generator  818  is coupled to the S input terminal of flip flop  814 . The Q output of SR flip flop  814  is coupled to an input of PWM generator  630 . For purposes of explanation only, each version of DC-DC converter shown in the remaining figures will employ the same PWM control circuit  634  that is shown in  FIG. 8 . 
         [0047]    PWM generator  630  implements a state machine. With continuing reference to  FIGS. 7A and 8 , MOSFETs  622  and  624  are initially turned off or deactivated since control signals A 1  and C 1  are low, and MOSFETs  620  and  626  are initially turned on or activated since control signals B 1  and D 1  are high. In this state, no current flows from input node  404  to primary winding  614 . The output of flip flop  814  is also initially set low. With the rising edge of CLK 1 , pulse generator  818  generates a set pulse, which switches the Q output of flip flop  814  to high. In response to this change in Q, PWM generator  630  deactivates MOSFET  620  via control signal B 1 , and after a small time delay PWM generator  630  activates MOSFET  622  via control signal A 1  as shown. MOSFET  626  is active when PWM generator  630  activates MOSFET  622 , and as a result Vcs ramps up as current increasingly flows to primary winding  614  via current sense circuit  636 . Comparator  816  compares Vcs as it rises with comparative voltage E. When these two voltages are equal, the output of comparator  816  switches to low, which in turn switches the Q output of flip flop  814  to low. In response to this change in Q, PWM generator  630  deactivates MOSFET  626  via control signal D 1 , and after a small time delay PWM generator  630  activates MOSFET  624  via control signal C 1  as shown. When MOSFET  626  deactivates, current no longer flows through current sense circuit  636 , and Vcs falls, which in turn causes comparator  816  to quickly switch its output to low. The Q output of flip flop  814  should remain low when comparator  816  switches its output. With the falling edge of CLK 1 , pulse generator  818  generates another set pulse, which switches the Q output of flip flop  814  to high. In response to this change in Q, PWM generator  630  deactivates MOSFET  622  via control signal A 1 , and after a short time delay PWM generator  630  activates MOSFET  620  via control signal B 1 . MOSFET  624  is activated when PWM generator  630  activates MOSFET  620 , and as a result Vcs begins to rise as current increasingly flows to primary winding  614  via current sense circuit  636 . When Vcs equals comparative voltage E comparator  816  reasserts its output, which in turn switches the Q of flip flop  814  to low. In response PWM generator  630  deactivates MOSFET  624  via control signal C 1 , and after a short time delay PWM generator  630  activates MOSFET  626  via control signal D 1 . In this state, current does not flow through current sense circuit  636 , and Vcs falls. The process repeats with the next rising edge of CLK 1 . For purposes of explanation only, each version of DC-DC converter shown in the remaining figures will employ the same PWM generator  630  that is shown in  FIG. 8 . 
         [0048]    In the embodiment shown in  FIG. 8 , secondary winding circuit  616  includes a center-tapped, secondary winding  620  and diodes  622  and  624 . One of ordinary skill understands the combination of diodes and transformer in  FIG. 8  forms an example of a full wave rectifier. The secondary winding  620  is also coupled to inductor  626  via capacitor  628 , the combination of which is coupled to output node  406  as shown. One of ordinary skill in the art understands that Vout is proportional D t (Ns/Np)Vin when current flow through the primary winding  614  is controlled by the control signals A 1 -D 1  shown in  FIG. 7A . Further, current sense circuit  636  includes a transformer. Current flow between input node  404  and primary winding  614  induces Vcs. Since current sense circuit  636  includes a transformer in  FIG. 8 , current sense circuit  636  maintains electrical isolation of grounds GND 1  and GND 2 . 
         [0049]      FIG. 9  illustrates a non-isolated embodiment of the DC-DC converter  800  shown within  FIG. 8 . More particularly, as seen in  FIG. 9 , DC-DC converter  900  lacks the decouple circuit  632  of DC-DC converter  800 . Additionally, GND 1  is coupled to MOSFETs  622  and  624  in DC-DC converter  900 , as opposed to ground GND 2  in DC-DC converter  800 . The remaining components of DC-DC converter  900  shown in  FIG. 9  operate in the same manner as their equivalents in a DC-DC converter  800  and described above. 
         [0050]      FIG. 10  illustrates another example of a non-isolated DC-DC converter. Like the non-isolated version shown in  FIG. 9 , DC-DC converter  1000  shown in  FIG. 10  lacks decouple circuit  632 , and MOSFETs  622  and  624  are coupled to ground GND 1 . Further, current sense circuit  636  takes form in a current sensing and measuring circuit that includes MOSFETs  1002 - 1008  coupled to operational amplifiers  1010  and  1012  as shown. MOSFETs  1002  and  1004  are controlled by control signals D 1  and B 1 , respectively. In one embodiment, the current sensing and measuring circuit generates a current Isense as a function of phase 1 input current En provided via input node  404 . Since virtually no current flows into terminals of comparator  816 , Isense flow through resistor  1014  and generates voltage Vcs. 
         [0051]      FIG. 11  illustrates an isolated version of DC-DC converter  600  shown within  FIG. 6 . In this version, the secondary winding circuit  616 , however, is substantially different. Moreover, PWM generator  630  generates control signals E 1  and F 1  that control MOSFETs  1104  and  1106  as will be more fully described below. With continuing reference to  FIG. 11 , DC-DC converter  1100  includes a secondary winding  1102 , the terminals of which are coupled to MOSFETs  1104  and  1106  as shown. Moreover, respective terminals of secondary winding  1102  are coupled to inductors  1110  and  1012  as shown. Capacitor  1114  is coupled between ground GND 1  and output node  406 .  FIG. 7B  illustrates the timing diagram shown in  FIG. 7A  in addition to the control signals E and F generated by PWM generator  430 , and the voltage Vs across the secondary winding  1102 . In the embodiment shown, control signals E 1  and F 1  control MOSFETs  1104  and  1106 , respectively. Pulse generator  818  generates a set pulse with the rising edge of CLK 1  in the same manner as described with reference to  FIGS. 7A and 8 . The set pulse is received by flip flop  814  and in response, flip flop  814  switches its Q output to high as described above. PWM generator  630 , in response to receiving the change in the Q from flip flop  814 , deactivates MOSFET  1104  via control signal E 1  after a short time delay. During time t 0 , Vcs increases until Vcs equates comparative voltage E. When these two voltages are equal, PWM generator  630  activates MOSFET  1104  via control signal E 1  at the same time PWM generator  630  deactivates MOSFET  626  via control signal D 1 . In similar fashion, the PWM generator  630  deactivates MOSFET  1106  via control signal F 1  shortly after the falling edge of CLK 1  as shown in  FIG. 7B . MOSFET  1106  remains deactivated during time t 0  until comparative voltage E and Vcs equate with each other, at which point PWM generator  630  activates MOSFET  1106  via control signal F 1  and deactivates MOSFET  624  via control signal C 1 . This process continues with the next cycle of CLK 1  as shown in  FIG. 7B . This process results in the rectification of the secondary voltage Vs across capacitor  1114 . 
         [0052]      FIG. 12  illustrates a non-isolated version of the DC-DC converter shown within  FIG. 6 . DC-DC converter  1200  shown in  FIG. 12  is substantially similar to the DC-DC converter  1100  shown within  FIG. 11 . However, DC-DC converter  1200  lacks the decouple circuit  632 , and MOSFETs  624  and  622  are coupled to ground GND 1  as shown. Additionally, the current sense circuit  636  takes form in the current sensing and measuring circuit described in the DC-DC converter  100  shown in  FIG. 10 . PWM generator  630  generates control signals E 1  and F 1  in the same manner as described with reference to  FIG. 12 . 
         [0053]    Although the present invention has been described in connection with several embodiments, the invention is not intended to be limited to the specific forms set forth herein. On the contrary, it is intended to cover such alternatives, modifications, and equivalents as can be reasonably included within the scope of the invention as defined by the appended claims.