Patent Publication Number: US-8970067-B2

Title: Hybrid DC/DC converters and methods

Description:
TECHNICAL FIELD 
     The present invention relates to isolated dc/dc converters and methods, and more particularly, to isolated dc/dc converters and methods employing a first stage and a second stage connected in cascade, wherein the output of the second stage can be regulated by adjusting an auxiliary power source of the first stage. 
     BACKGROUND 
     A telecommunication network power system usually includes an ac/dc stage converting the power from the ac utility line to a 48V dc distribution bus and a dc/dc stage converting the 48V dc distribution bus to a plurality of voltage levels for all types of telecommunication loads. Both stages may comprise isolated dc/dc converters. Isolated dc/dc converters can be implemented by using different power topologies, such as flyback converters, forward converters, half bridge converters, full bridge converters and the like. As known in the art, bridge converters generally are employed when the power of a dc/dc converter is more than 100 watts. 
     As shown in  FIG. 1 , a full bridge converter  100  is a conventional full bridge converter having a full wave rectifier coupled to a center-tapped secondary winding. The full bridge converter  100  includes four switches Q 1 , Q 2 , Q 3  and Q 4  at a primary side of a transformer Tx. The four switches Q 1 , Q 2 , Q 3  and Q 4  form a bridge having two legs. Q 1  and Q 3  in series connection have a junction point, referred to as A. Q 2  and Q 4  in series connection have a junction point, referred to as B. The primary winding of the transformer Tx is connected to A and B. A dc supply Vin is connected to the two legs to provide power to the full bridge converter  100 . According to the operating principle of a hard switching full bridge converter, the switches Q 1  and Q 4  are turned on simultaneously for an adjustable time during a first half cycle. After a period of dead time, the switches Q 2  and Q 3  are turned on simultaneously for an equal time during the second half cycle. As a result, Vin and −Vin are applied to the primary side of the transformer Tx in alternate half periods. 
     In a fixed duty cycle control scheme, the turn-on time of the switches Q 1  and Q 4  is equal to the turn-on time of the switches Q 2  and Q 3 . When all four switches are turned off, both S 1  and S 2  are turned on. The load current flows through S 1  and S 2 . This interval is referred to as a freewheeling period. The output voltage of the bridge converter  100  is proportional to the turn-on time of the switches. A controller (not shown) may detect the output voltage Vo and adjust the turn-on time via a negative feedback control loop (not shown). The secondary side of the transformer Tx is center-tapped. Such a center-tapped secondary and two switches S 1  and S 2  can form a full wave rectifier, which can convert the primary voltage having double polarities (Vin and −Vin) of the transformer Tx to a secondary voltage having a single polarity. Then, the secondary voltage having a single polarity is fed to an output filter including an inductor Lo and an output capacitor Co. The output filter averages the square voltage pulses at the output of the full wave rectifier and generates a dc voltage at Vo, which is then supplied to a load represented by a resistor RL. 
     A phase shift full bridge converter is capable of reducing switching losses by means of the zero voltage switching control technique. As shown in a dashed rectangle  120  of  FIG. 1 , instead of turning on two primary switches (e.g., Q 1  and Q 4 ) simultaneously, the turn-on time of these two switches are shifted by a period of time. More particularly, as depicted in the dashed rectangle  120 , a waveform  1061  and a waveform  1101  show Q 1  is on for a period of time before Q 4  is turned on. There is an overlap between Q 1 &#39;s turn-on time and Q 4 &#39;s turn-on time. After Q 1  is turned off, Q 4  stays on for a period of time. Likewise, a waveform  1081  and a waveform  1121  show there is a phase shift between Q 2  and Q 3 &#39;s turn-on time. A waveform  1021  shows the on-time of switches S 2  and S 3 . A waveform  1041  shows the on-time of switches S 1  and S 4 . 
     The phase shift full bridge can achieve a zero voltage switching by utilizing the L-C resonance between transformer leakage inductance and MOSFET (e.g., Q 1 ) output capacitance. For example, Q 3  has a parasitic capacitor (not shown) across its drain and source. During the period when both Q 1  and Q 4  are on, the voltage across Q 3 &#39;s parasitic capacitor is charged to a voltage approximately equal to Vin. According to the basic principle of the phase shift control technique, Q 1  is off prior to Q 4 . After Q 1  is off, the primary side current cannot change instantaneously. As a result, the primary side current will flow through the parasitic capacitors of Q 1  and Q 3 . The flow of the primary side current through both parasitic capacitors may cause the voltage at the junction between Q 1  and Q 3  to be discharged to zero, enabling a zero voltage switching when Q 3  is turned on without substantial power loss. Similarly, the phase shift operation may enable lossless turn-on of other switches, namely Q 1 , Q 2  and Q 4 . 
     SUMMARY OF THE INVENTION 
     These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention which provide a system, apparatus and method for reducing the current and voltage stress of a bridge converter. 
     In accordance with an embodiment, an apparatus comprises a first power source, a low power converter having an input coupled to the first power source and an output connected in series with the first power source, a selection network coupled to the first power source and the output of the low power converter and a main isolated power converter coupled to the selection network. 
     In accordance with an embodiment, a system comprises a first power source, a low power converter having an input coupled to the first power source and an output connected in series with the first power source, a selection network coupled to the first power source and the output of the low power converter, a first main isolated power converter coupled to the selection network and a second main isolated power converter coupled the selection network. 
     In accordance with another embodiment, a method comprises providing power from a first power source coupled to a low power converter, converting an output of the first power source to a first auxiliary voltage source connected in series with the first power source, applying a first combination of the first power source and the first auxiliary voltage source to an unregulated power converter during a first half cycle and applying a second combination of the first power source and the first auxiliary voltage source to the unregulated power converter during a second half cycle. 
     An advantage of a preferred embodiment of the present invention is reducing the switching and conduction losses of a hybrid dc/dc converter so as to improve the hybrid dc/dc converter&#39;s efficiency. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a full bridge converter having a full wave rectifier; 
         FIG. 2A  illustrates a block diagram of a hybrid dc/dc converter in accordance with an embodiment; 
         FIG. 2B  illustrates a block diagram of a hybrid dc/dc converter in accordance with another embodiment; 
         FIG. 3A  illustrates a block diagram of a first half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 2A  in accordance with an embodiment; 
         FIG. 3B  illustrates a block diagram of a second half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 2A  in accordance with an embodiment; 
         FIG. 3C  illustrates a block diagram of a second half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 2A  in accordance with another embodiment; 
         FIG. 4A  illustrates a schematic diagram of the hybrid dc/dc converter shown in  FIG. 2A  in accordance with an embodiment; 
         FIG. 4B  illustrates a schematic diagram of the hybrid dc/dc converter shown in  FIG. 2A  in accordance with another embodiment; 
         FIG. 5A  illustrates a schematic diagram of a first half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 4B ; 
         FIG. 5B  illustrates a schematic diagram of a second half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 4B ; 
         FIG. 6A  illustrates in detail a schematic diagram of the hybrid dc/dc converter shown in  FIG. 4B  in accordance with an embodiment; 
         FIG. 6B  illustrates in detail a schematic diagram of the hybrid dc/dc converter shown in  FIG. 4B  in accordance with another embodiment; 
         FIG. 7  illustrates a block diagram of a hybrid dc/dc converter in accordance with another embodiment; 
         FIG. 8  illustrates a schematic diagram of the selection network shown in  FIG. 7  in accordance with an embodiment; 
         FIG. 9  illustrates a schematic diagram of the dual output hybrid dc/dc converter shown in  FIG. 7  in accordance with an embodiment; 
         FIG. 10  illustrates a block diagram of a hybrid dc/dc converter in accordance with another embodiment; 
         FIG. 11A  illustrates a block diagram of a first half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 10  in accordance with an embodiment; 
         FIG. 11B  illustrates a block diagram of a second half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 10  in accordance with an embodiment; 
         FIG. 12  illustrates a schematic diagram of the hybrid dc/dc converter shown in  FIG. 10  in accordance with an embodiment; 
         FIG. 13A  illustrates in detail a schematic diagram of the hybrid dc/dc converter shown in  FIG. 12  in accordance with an embodiment; 
         FIG. 13B  illustrates waveforms of the hybrid dc/dc converter shown in  FIG. 13A  in soft switching operation according to an embodiment; 
         FIG. 14  illustrates a block diagram of a hybrid dc/dc converter in accordance with another embodiment; 
         FIG. 15  illustrates a schematic diagram of the hybrid dc/dc converter shown in  FIG. 14  in accordance with an embodiment; 
         FIG. 16  illustrates in detail a schematic diagram of the hybrid dc/dc converter shown in  FIG. 15  in accordance with an embodiment; 
         FIG. 17  illustrates a block diagram of a hybrid dc/dc converter in accordance with another embodiment; 
         FIG. 18  illustrates a schematic diagram of the hybrid dc/dc converter shown in  FIG. 17  in accordance with an embodiment; 
         FIG. 19A  illustrates in detail a schematic diagram of the hybrid dc/dc converter shown in  FIG. 18  in accordance with an embodiment; 
         FIG. 19B  illustrates in detail a schematic diagram of the hybrid dc/dc converter shown in  FIG. 18  in accordance with another embodiment; 
         FIG. 20A  illustrates a timing diagram of gate drive signals of  FIG. 19A  in accordance with an embodiment; 
         FIG. 20B  illustrates a timing diagram of gate drive signals of  FIG. 19B  in accordance with an embodiment; 
         FIG. 21  illustrates a block diagram of a hybrid dc/dc converter in accordance with another embodiment; 
         FIG. 22A  illustrates a block diagram of a first half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 21  in accordance with an embodiment; 
         FIG. 22B  illustrates a block diagram of a second half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 21  in accordance with an embodiment; and 
         FIG. 23  illustrates a schematic diagram of the hybrid dc/dc converter shown in  FIG. 21  in accordance with an embodiment. 
     
    
    
     Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     The present invention will be described with respect to preferred embodiments in a specific context, namely a hybrid full bridge converter. The invention may also be applied, however, to a variety of isolated dc/dc converters including half bridge converters, full bridge converters, push-pull converters and the like. In addition, the invention may be applied to power conversion topologies with a variety of control mechanisms such as PWM duty cycle control, symmetrical PWM, asymmetrical PWM, phase shift PWM, variable frequency PWM control, a combination of phase shift PWM and variable frequency PWM, constant on time control, constant off time control or any combination thereof. 
     Referring initially to  FIG. 2A , a block diagram of a hybrid dc/dc converter is illustrated in accordance with an embodiment. The hybrid dc/dc converter  200  comprises a first dc voltage source  102 , a low power converter  104 , a second dc voltage source  106 , a selection network  108  and a main power converter  110 . As shown in  FIG. 2A , the first dc voltage source  102  is coupled to the selection network  108  as well as the low power converter  104 . The low power converter  104  converts the first dc voltage source  102  to a different voltage level so as to generate the second dc voltage source  106 . In accordance with an embodiment, the low power converter  104  may be a non-isolated dc/dc converter such as buck dc/dc converters, boost dc/dc converters, buck-boost dc/dc converters and the like. Alternatively, the low power converter  104  may be an isolated dc/dc converter such as forward converters, flyback converters, half bridge converters, full bridge converters, push-pull converters and the like. 
     It should be noted that when the low power converter  104  may employ an isolated power topology (e.g., forward converters), the output voltage of the low power converter  104  is floating from the first dc voltage source  102 . As such, the second dc voltage source  106  can be either added into or subtracted from the first dc voltage source  102  by a connection network provided by the selection network  108 . On the other hand, when the low power converter  104  employs a non-isolated power topology (e.g., a buck dc/dc converter or a boost dc/dc converter), the low power converter  104  can only increase or decrease the first dc voltage source  102  by connecting the first dc voltage source  102  and the second dc voltage source  106  in series. More particularly, if the low power converter  104  is a buck dc/dc converter, the total output of combining the first dc voltage source  102  and the second dc voltage source  106  may be less than the output voltage of the first dc voltage source  102 . In contrast, if the low power converter  104  is a boost dc/dc converter, the total output of combining the first dc voltage source  102  and the second dc voltage source  106  may be greater than the output voltage of the first dc voltage source  102 . 
     It should further be noted that the low power converter  104  may process a small amount of power delivered to the main power converter  110 . As described above with respect to the operation of the hybrid dc/dc converter, the output voltage of the low power converter  104  is used as an auxiliary power source to adjust the output voltage of the main power converter  110 , which is an unregulated converter operating in a 50% duty cycle mode. Because the low power converter  104  may only process a fraction of the total power delivered to the load, the low power converter  104  may employ switches with smaller die size. In addition, when the low power converter  104  is an isolated power converter, a soft switching control mechanism may be employed to further reduce the switching losses so as to make the low power converter  104  operating at a higher frequency. Such a higher frequency helps to further reduce the size of the low power converter  104 . 
     The selection network  108  may comprise a plurality of switches. In accordance with the operation principles of the hybrid dc/dc converter  200 , the selection network  108  may connect the first dc voltage source  102  with the second dc voltage source  106  and further connect the combination of the first dc voltage source  102  and the second dc voltage source  106  with the main power converter  110 . Alternatively, the selection network  108  may connect the first dc voltage source  102  directly to the main power converter  110  to magnetically reset the transformer winding of the main power converter  110 . The detailed description of an embodiment of the selection network  108  will be described below with respect to  FIG. 3A  and  FIG. 3B . 
     In accordance with an embodiment, the main power converter  110  may be an unregulated isolated dc/dc converter. In other words, the main power converter  110  may operate in a fixed duty cycle mode. More particularly, in order to magnetically reset the transformer of the isolated power converter, the main power converter  110  may have the same turn-on period in the first half cycle and the second half cycle. For example, the main power converter  110  may operate in a 50% duty cycle mode to achieve an unregulated output voltage as well as a magnetic reset of the transformer. One advantageous feature of an unregulated main power converter  110  is that the 50% duty cycle mode helps to reduce the magnetic losses from the transformer of the main power converter  110 . Another advantageous feature is that the secondary synchronous rectifier may be better driven when the main power converter  110  operates in a 50% duty cycle mode. In sum, an unregulated main power converter  110  helps to reduce switching and conduction losses as well as electromagnetic interference (EMI) noise. 
     The main power converter  110  may comprise a transformer  152 , a rectifier  154  and an output filter  156 . In addition, depending on the topology, the main power converter  110  may further comprise a soft switching network formed by an auxiliary inductor and a resonant capacitor. Alternatively, for some isolated topologies such as a PWM switching full bridge dc/dc converter, an anti-saturation dc current blocking capacitor may be included in the main power converter  110 . It should be noted that through the description, various embodiments are described based upon a full bridge dc/dc conversion topology. However, a person skilled in the art will recognize that the various embodiments described below are further applicable to other isolation topologies such as forward converters, flyback converters, half bridge converters, push-pull converters and the like. 
     The transformer  152  provides electrical isolation between the primary side and the secondary side of the main power converter  110 . In accordance with an embodiment, the transformer  152  may be formed of two transformer windings, namely a primary transformer winding and a secondary transformer winding. Alternatively, the transformer  152  may have a center tapped secondary so as to have three transformer windings including a primary transformer winding, a first secondary transformer winding and a second secondary transformer winding. It should be noted that the transformers illustrated herein and throughout the description are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the transformer  108  may further comprise a variety of bias windings and gate drive auxiliary windings. 
     The rectifier  154  converts an alternating polarity waveform received from the output of the transformer  152  to a single polarity waveform. The rectifier  154  may be formed of a pair of switching elements such as n-type metal oxide semiconductor (NMOS) transistors. Alternatively, the rectifier  154  may be formed of a pair of diodes. Furthermore, the rectifier  154  may be formed by other types of controllable devices such as metal oxide semiconductor field effect transistor (MOSFET) devices, bipolar junction transistor (BJT) devices, super junction transistor (SJT) devices, insulated gate bipolar transistor (IGBT) devices and the like. The detailed operation and structure of the rectifier  154  are well known in the art, and hence are not discussed herein. 
     The output filter  156  is used to attenuate the switching ripple of the main power converter  110 . According to the operation principles of isolated dc/dc converters, the output filter  156  may be an L-C filter formed by an inductor and a plurality of capacitors. One person skilled in the art will recognize that some isolated dc/dc converter topologies such as forward converters may require an L-C filter. On the other hand, some isolated dc/dc converter topologies such as LLC resonant converters may include an output filter formed by a capacitor. One person skilled in the art will further recognize that different output filter configurations apply to different power converter topologies as appropriate. The configuration variations of the output filter  156  are within various embodiments of the present disclosure. 
       FIG. 2B  illustrates a block diagram of a hybrid dc/dc converter in accordance with another embodiment. The hybrid dc/dc converter  210  is similar to the hybrid dc/dc converter  200  shown in  FIG. 2A  except that the first dc voltage source  102  and the second dc voltage source  106  are swapped. More particularly, in  FIG. 2A , the selection network  108  may connect the first dc voltage source  102  and the second dc voltage source  106  together in a configuration that the second dc voltage source  106  is on top of the first dc voltage source  102 . In contrast, by swapping the first dc voltage source  102  and the second dc voltage source  106 , the first dc voltage source  102  may be on top of the second dc voltage source  106  through a connection path provided by the selection network  108 . 
       FIG. 3A  illustrates a block diagram of a first half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 2A  in accordance with an embodiment. During a first half cycle, the selection network  108  is configured such that the first dc voltage source  102  and the second dc voltage source  106  are connected in series. In addition, the series connected power sources (e.g.,  102  and  106 ) are further applied to the primary side of the transformer  152  of the main power converter  110 . As indicated by the curved arrow, the current flows from the positive terminal of the second dc voltage source  106  to the main power converter  110 , and then flows back to the negative terminal of the first dc voltage source  102 . It should be noted that the low power converter  104  has a control loop independent from the main power converter  110 . In other words, the low power converter  104  is used to maintain a regulated voltage at the second dc voltage source  106 . As a result, although the main power converter  110  is unregulated, the output of the main power converter  110  may be regulated accordingly by regulating the output voltage of the second dc voltage source  106 . 
       FIG. 3B  illustrates a block diagram of a second half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 2A  in accordance with an embodiment. During the second half cycle, the selection network  108  is configured such that the first dc voltage source  102  is applied to the primary side of the main power converter  110 . As indicated by the curved arrow, the current flows from the negative terminal of the first dc voltage source  102  to the positive terminal of the main power converter  110 , and then flows back to the positive terminal of the first dc voltage source  102 . 
       FIG. 3C  illustrates a block diagram of a second half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 2A  in accordance with another embodiment. During the second half cycle, the selection network  108  is configured such that the first dc voltage source  102  and the second dc voltage source  106  are connected in series and further applied to the primary side of the transformer  152  (not shown) of the main power converter  110 . As indicated by the curved arrow, the current flows from the positive terminal of the second dc voltage source  106  to the negative terminal of the main power converter  110 , and then flows back to the negative terminal of the first dc voltage source  102 . 
       FIG. 4A  illustrates a schematic diagram of the hybrid dc/dc converter shown in  FIG. 2A  in accordance with an embodiment. As shown in  FIG. 4A , the selection network  108  is actually the primary side switching network of a full bridge power converter. The main power converter  110  in  FIG. 4A  can be considered having two portions. Along the isolation barrier provided by the transformer  152  (not shown), the left side of the transformer  152  into which power flows is called the primary side and the circuit connected to the primary side winding is called the primary circuit of the hybrid dc/dc converter  402 . In accordance with an embodiment, the primary side switching network of the hybrid dc/dc converter  402  can be alternatively used as the selection network  108 . On the other hand, the right side of the transformer  152  from which power flows is called the secondary side, and the circuit connected to the secondary side winding is called the secondary circuit of the hybrid dc/dc converter  402 . The function of the primary side&#39;s circuit has been described in detail with respect to  FIG. 2 , and therefore not discussed herein. 
       FIG. 4B  illustrates a schematic diagram of the hybrid dc/dc converter shown in  FIG. 2A  in accordance with another embodiment. As shown in  FIG. 4B , the selection network  108  is similar to the primary side switching network of  FIG. 4A . In comparison with  FIG. 4A , switch S 46  is tied to the joint point between C 26  and Vsource. Such a configuration change makes Vsource as the resetting source during the second half cycle. As a result, the current stress of the low power converter  104  may be reduced. The operation details of the hybrid dc/dc converter  404  shown in  FIG. 4B  will be described below with respect to  FIG. 5A  and  FIG. 5B . 
       FIG. 5A  illustrates a schematic diagram of a first half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 4B . During the first half cycle, the series connected voltage sources are applied to the primary transformer winding of the main power converter through the turn-on of switch S 50  and switch S 48 . During the first half cycle, the lower power converter is enabled and the energy from the first dc voltage source is delivered to the capacitor C 26  so as to maintain a regulated voltage at C 26 . Moreover, by adjusting the voltage across the capacitor C 26 , the hybrid dc/dc converter  502  can regulate the output voltage of the main power converter. 
       FIG. 5B  illustrates a schematic diagram of a second half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 4B . During the second half cycle, the first dc voltage source is applied to the primary transformer winding through the turn-on of switch S 46  and switch S 52 . As a result, the transformer of the main power converter is magnetically reset and energy is transferred from the primary side to the secondary side. It should be noted that during the second half cycle, the low power converter is inactive because the capacitor C 26  is disconnected from the main power converter. As a result, the current stress of the low power converter is reduced. It should be noted that the system configurations shown in  FIG. 5A  and  FIG. 5B  is not a two-stage power conversion topology. Instead, there may be one single stage during the second half cycle. Therefore, the power topology shown in  FIG. 5A  and  FIG. 5B  may be alternatively referred to as a one and half stage power conversion topology. 
     The low power converter shown in  FIG. 5A  and  FIG. 5B  may process a small amount of power delivered to the main power converter. In accordance with an embodiment, the power processed by the low power converter may be described as follows: 
               P   LPC     =         V     IN   ⁢           ⁢   2       ·     P   O           2   ·     V   IN       +     V     IN   ⁢           ⁢   2                 
where P LPC  is the power processed by the low power converter and P O  is the output power of the main power converter. Moreover, the current flowing through the low power converter is reduced accordingly. In accordance with an embodiment, the current flowing through the low power converter may be described as follows:
 
     
       
         
           
             
               I 
               
                 IN 
                 ⁢ 
                 
                     
                 
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                 2 
               
             
             = 
             
               
                 
                   V 
                   
                     IN 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                 
                 · 
                 
                   P 
                   O 
                 
               
               
                 
                   V 
                   IN 
                 
                 · 
                 
                   ( 
                   
                     
                       2 
                       · 
                       
                         V 
                         IN 
                       
                     
                     + 
                     
                       V 
                       
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                         2 
                       
                     
                   
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       FIG. 6A  illustrates in detail a schematic diagram of the hybrid dc/dc converter shown in  FIG. 4B  in accordance with an embodiment. As shown in  FIG. 6A , the main power converter is implemented by a PWM switching full bridge converter. The transformer is a center tapped transformer coupled between the primary side switching network and the second side switching network. The primary side switching network includes four switches, namely S 234 , S 236 , S 238  and S 240 . In addition, the primary side switching network includes a dc current block capacitor C, which is connected in series with the primary winding of the transformer T x4  having a magnetizing inductance Lm. The secondary side employs a synchronous rectifier formed by switches S 242  and S 244 . In fact, the main power converter is a full bridge dc/dc converter. The operation of a full bridge dc/dc converter has been described in detail with respect to  FIG. 1 , and hence is not discussed in further detail herein. 
     It should be noted that the power topology of the hybrid dc/dc converter  602  may be not only applied to a full bridge converter having a center-tapped secondary winding as shown in  FIG. 6 , but also applied to a full bridge converter having a non center-tapped secondary winding. Furthermore, the power topology of the hybrid dc/dc converter  602  is applicable to bridge converters having other secondary configurations, such as voltage doubler rectifiers and current doubler rectifiers. In sum, the hybrid dc/dc converter can be applied to all types of isolated converters, including full bridge converters having different types of secondary configurations, and half bridge converters having different types of secondary configurations. Furthermore, different control techniques of bridge converters have no impact on the application of the hybrid dc/dc converter. For example, the hybrid dc/dc converter can be applied to a hard switching full bridge converter as well as a phase shift full bridge converter. 
     The low power converter shown in  FIG. 6A  employs a boost dc/dc conversion topology comprising an inductor L and switches S 230  and S 232 . The control of the low power converter is independent from the control of the main power converter. In accordance with an embodiment, the clock signal for generating PWM signals of the low power converter may be in sync with the clock signal for generating PWM signals of the main power converter so that the ripple voltage as well as the input filter can be reduced. The operation principles of boost dc/dc converters are well known in the art, and hence are not discussed in further detail herein. 
       FIG. 6B  illustrates in detail a schematic diagram of the hybrid dc/dc converter shown in  FIG. 4B  in accordance with another embodiment. The power topology shown in  FIG. 6B  is similar to the power topology shown in  FIG. 6A  except that the main power converter in  FIG. 6B  employs a soft switching full bridge power conversion topology. As known in the art, the resonant tank formed by Lr and Cr helps to reduce the switching losses of the main power converter so that the efficiency of the hybrid dc/dc converter can be further improved. The operation principles of soft switching full bridge converters are well known in the art, and hence are not discussed in further detail herein. 
       FIG. 7  illustrates a block diagram of a hybrid dc/dc converter in accordance with another embodiment. The hybrid dc/dc converter  700  shown in  FIG. 7  is similar to the hybrid dc/dc converter  200  shown in  FIG. 2A  except that the main power converter employs a multiple output structure. As shown in  FIG. 7 , the selection network  702  is coupled to the first main power converter  110  and the second main power converter  210  respectively. The system configuration of the first main power converter  110  and the second main power converter  210  is similar to the system configurations shown in  FIG. 6A  and  FIG. 6B , and hence is not discussed in further detail herein to avoid unnecessary repetition. 
     It should be noted that there may be a variety of isolated power conversion topologies. The first main power converter  110  and the second main power converter  210  may share the same power conversion topology. Alternatively, the first main power converter  110  and the second main power converter  210  may employ different power topologies. For example, the first main power converter  110  may employ a PWM switching full bridge dc/dc converter. In contrast, the second main power converter  210  may employ a soft switching full bridge dc/dc converter. 
       FIG. 8  illustrates a schematic diagram of the selection network shown in  FIG. 7  in accordance with an embodiment. The selection network  702  comprises a first switch S 402  and a second switch S 404  connected in series. The selection network  702  is coupled to the input dc source Vsource 1  as well as the capacitor C 6 . By controlling the turn-on and turn-off of S 402  and S 404 , the output voltages of the converter  374  and the converter  376  are regulated respectively. The detailed operation of the dual output hybrid dc/dc converter will be described below with respect to  FIG. 9 . 
       FIG. 9  illustrates a schematic diagram of the dual output hybrid dc/dc converter shown in  FIG. 7  in accordance with an embodiment. As shown in  FIG. 9 , the low power converter employs a boost dc/dc converter, which maintains a higher voltage at the positive terminal of the capacitor Cin. The power topology of the main power converter have been described above with respect to  FIG. 6A  and  FIG. 6B , and hence is not discussed in detail herein. It should be noted that the first output may be regulated by adjusting the duty cycle of the low power converter. In addition, the first output may be regulated by adjusting the duty cycles of S 464  and S 466 . In order to independently regulate the second output, a variable frequency control mechanism may be employed to adjust the output voltage of the second output. 
       FIG. 10  illustrates a block diagram of a hybrid dc/dc converter in accordance with another embodiment. The hybrid dc/dc converter  1000  shown in  FIG. 10  is similar to the hybrid dc/dc converter  200  shown in  FIG. 2A  except that one more power supply is employed. As shown in  FIG. 10 , a third dc input voltage source  202  is coupled to the selection network  1002 . In accordance with an embodiment, the third dc input voltage source  202  is similar to the first dc input voltage source  102 . The detailed operation of the third dc input voltage source  202  will be described below with respect to  FIG. 11A ,  FIG. 11B  and  FIG. 12 . 
       FIG. 11A  illustrates a block diagram of a first half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 10  in accordance with an embodiment. During a first half cycle, the system configuration of the selection network is similar to that shown in  FIG. 3A , and hence is not discussed in further detail to avoid unnecessary repetition.  FIG. 11B  illustrates a block diagram of a second half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 10  in accordance with an embodiment. The magnetic reset method shown in  FIG. 11B  is similar to that shown in  FIG. 3B  except that a third dc input voltage source  202  is used to magnetically reset the transformer of the main power converter  110 . It will be appreciated that using a same power source during the first half cycle and the second half cycle or using a different power source during two cycles is choice of design and depends on different needs of various applications. 
       FIG. 12  illustrates a schematic diagram of the hybrid dc/dc converter shown in  FIG. 10  in accordance with an embodiment. The system configuration of  FIG. 12  is similar to the system configuration shown in  FIG. 5A . In the first half cycle, the system configuration of  FIG. 12  is identical to that of  FIG. 5A . In the second half cycle, a different dc voltage source Vsource 2  is used to reset the transformer winding of the power converter  124 . In contrast, in  FIG. 5A  and  FIG. 5B , the first cycle and the second cycle share the same input voltage source. 
       FIG. 13A  illustrates in detail a schematic diagram of the hybrid dc/dc converter shown in  FIG. 12  in accordance with an embodiment. As shown in  FIG. 13 , the low power converter employs two switches S 214  and S 216  and adjusts the voltage at the positive terminal of Cin by adjusting the duty cycles of S 214  and S 216  respectively. ZVS and ZCS can be achieved for S 214  and S 216  through a resonant process between the inductor L and two resonant capacitors Cr 1  and Cr 2 . The detailed operation of ZVS and ZCS are well known in the art, and hence is not discussed in further detail herein. The ZVS and ZCS of the main power converter can also be achieved by controlling the turn-on and the turn-off of the main switches, S 218 , S 220 , S 222  and S 224 . More particularly, the ZVS of the main switches can be achieved at any input voltage and output current conditions. Likewise, the ZCS can be achieved at the secondary side synchronous rectifier formed by S 226  and S 228 . 
       FIG. 13B  illustrates waveforms of the hybrid dc/dc converter shown in  FIG. 13A  in soft switching operation according to an embodiment. As shown in  FIG. 13B , the switch S 216  can achieve a ZVS and ZCS transition by using the resonant process between the inductor L and the resonant capacitor Cr 2 . Likewise, the main switch S 220  may achieve a ZVS and ZCS transition by using the resonant process between the resonant process between Lr and Cr. One advantageous of having a soft switching transition at the lower power converter and the main power converter is that such a soft switching transition helps to reduce switching losses so as to improve the efficiency of the hybrid dc/dc converter shown in  FIG. 13A . 
       FIG. 14  illustrates a block diagram of a hybrid dc/dc converter in accordance with another embodiment. The hybrid dc/dc converter  1400  shown in  FIG. 14  is similar to the hybrid dc/dc converter  1000  shown in  FIG. 10  except that the low power converter  104  in  FIG. 14  not only maintains a regulated VIN 1 , but also provides energy to the load  112 . The detailed operation of the hybrid dc/dc converter  1400  will be described in further detail with respect to  FIG. 15  and  FIG. 16 . 
       FIG. 15  illustrates a schematic diagram of the hybrid dc/dc converter shown in  FIG. 14  in accordance with an embodiment. The system configuration of  FIG. 15  is similar to the system configuration shown in  FIG. 12  except that the low power converter  112  is coupled to the load directly. As such, the low power converter  112  is able to deliver energy to the capacitor C 28  as well as the output of the main power converter  126 .  FIG. 16  illustrates in detail a schematic diagram of the hybrid dc/dc converter shown in  FIG. 15  in accordance with an embodiment. As shown in  FIG. 16 , the low power converter employs a multiple output isolated dc/dc converter to maintain the voltage at the positive terminal of Cin. In addition, through the second transformer winding of the transformer T x1 , the lower power converter delivers energy to the output capacitor Co. 
     One advantageous feature of having the lower power converter coupled to the output capacitor directly is that the lower power converter can deliver energy to the load directly during some special operation conditions such as start-up, current limit and holdup time operations. It should be noted that the main power converter  0004  shown in  FIG. 16  may operate in an unregulated mode. More particularly, the main power converter  0004  may operate in a 50% duty cycle. The output voltage of the hybrid dc/dc converter  1600  is regulated by controlling the voltage across the capacitor Cin. Alternatively, in order to adjust the electrical characteristics of the hybrid dc/dc converter  1600  during special operation conditions such as startup, current limit and holdup time operations, the main power converter  0004  may employ a regulated dc/dc converter such as a soft switching full bridge converter controlled by a PWM controller (not shown). 
       FIG. 17  illustrates a block diagram of a hybrid dc/dc converter in accordance with another embodiment. The hybrid dc/dc converter  1700  shown in  FIG. 17  is similar to the hybrid dc/dc converter  1000  shown in  FIG. 10  except that main power converter in  FIG. 17  includes two separate isolated power converters to form a multiple output power system. Alternatively, the isolated power converters shown in  FIG. 17  may operate in an interleaving manner. As shown in  FIG. 17 , the first power converter  110  and the second power converter  210  may operate in a 50% duty cycle but having a phase shift between the leading edge of one PWM signal of the first power converter  110  and the leading edge of a corresponding PWM signal of the second power converter  210 . The detailed operation of multi-phase power converters is well known in the art, and hence is not discussed herein. 
       FIG. 18  illustrates a schematic diagram of the hybrid dc/dc converter shown in  FIG. 17  in accordance with an embodiment. The system configuration of  FIG. 18  is similar to the system configuration shown in  FIG. 12  except that the main power converter shown in  FIG. 12  may be replaced by a plurality of power converters. The power converters (e.g., converter  350  and converter  352 ) may form a multiple output power system by connecting loads (e.g., Load  1  and Load N) to their corresponding power converters. Alternatively, there may be a plurality of multi-phase isolated converter operating in an interleaving manner. Such an interleaving manner helps to reduce ripple while improving transient response. 
       FIG. 19A  illustrates in detail a schematic diagram of the hybrid dc/dc converter shown in  FIG. 18  in accordance with an embodiment. As shown in  FIG. 19A , the low power converter employs a boost dc/dc converter to maintain the voltage at the positive terminal of Cin. The first main power converter employs a PWM full bridge dc/dc converter. The output voltage of the first main power converter can be regulated by adjusting the output voltage of the low power converter. Alternatively, the output voltage of the first main power converter can be adjusted by adjusting the duty cycle of the PWM full bridge dc/dc converter. 
     The second main power converter is implemented by a soft switching half bridge dc/dc converter. The output voltage of the second main power converter may be regulated by adjusting the switching frequency of the soft switching half bridge dc/dc converter. An embodiment timing diagram of the gate drive signals of  FIG. 19A  will be discussed below with respect to  FIG. 20 .  FIG. 19B  illustrates in detail a schematic diagram of the hybrid dc/dc converter shown in  FIG. 18  in accordance with another embodiment. The system configuration in  FIG. 19B  is similar to that in  FIG. 19A  except that the low power converter employs an isolated dc/dc converter. As such,  FIG. 19B  is not discussed in detail herein. 
       FIG. 20A  illustrates a timing diagram of gate drive signals of  FIG. 19A  in accordance with an embodiment. As shown in  FIG. 20A , instead of turning on two primary switches (e.g., S 358  and S 364 ) simultaneously, the turn-on time of these two switches are shifted by a period of time. More particularly, the gate drive waveform of S 358  and the gate drive waveform  364  show S 358  is on for a period of time before S 364  is turned on. There is an overlap between S 358 &#39;s turn-on time and S 364 &#39;s turn-on time. After S 358  is turned off, S 364  stays on for a period of time. The phase shift full bridge can achieve a zero voltage switching by utilizing the L-C resonance between transformer leakage inductance and MOSFET (e.g., S 358 ) output capacitance.  FIG. 20B  illustrates a timing diagram of gate drive signals of  FIG. 19B  in accordance with an embodiment. The phase shift mechanism of  FIG. 20B  is similar to that of  FIG. 20A , and hence is not discussed in detail herein to avoid unnecessary repetition. 
       FIG. 21  illustrates a block diagram of a hybrid dc/dc converter in accordance with another embodiment. The hybrid dc/dc converter  2100  shown in  FIG. 21  is similar to the hybrid dc/dc converter  1000  shown in  FIG. 10  except that one more low power converter is employed. As shown in  FIG. 21 , a second low power converter  204  is coupled between the third dc input voltage source  202  and a fourth dc voltage source  206 . In accordance with an embodiment, the second low power converter  204  is similar to the first low power converter  104 . In addition, the fourth dc voltage source  206  may employ a same power topology as the second dc voltage source  106 . The detailed operation of the second low power converter will be described below with respect to  FIG. 22A ,  FIG. 22B  and  FIG. 23 . 
       FIG. 22A  illustrates a block diagram of a first half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 21  in accordance with an embodiment. During a first half cycle, the system configuration of the selection network is similar to that shown in  FIG. 11A , and hence is not discussed in further detail to avoid unnecessary repetition.  FIG. 22B  illustrates a block diagram of a second half cycle system configuration of the hybrid dc/dc converter shown in  FIG. 21  in accordance with an embodiment. The magnetic reset method shown in  FIG. 22B  is similar to that shown in  FIG. 11B  except that a combination of a third dc input voltage source  202  and the fourth dc input voltage source  206  is used to magnetically reset the transformer of the main power converter  110 . It will be appreciated that using a same power source during the first half cycle and the second half cycle or using a different power source during two cycles is choice of design and depends on different needs of various applications. 
       FIG. 23  illustrates a schematic diagram of the hybrid dc/dc converter shown in  FIG. 21  in accordance with an embodiment. The system configuration of  FIG. 23  is similar to the system configuration shown in  FIG. 13  except that a second low power converter is employed to provide further flexibility. As shown in  FIG. 23 , both the first low power converter and the second low power converter employ an isolated flyback converter to the voltages across Cin 1  and Cin 2  respectively. It will be appreciated that while  FIG. 23  illustrates each low power converter employs a flyback converter, each low power converter may employ a same power conversion topology such as buck, boost, buck-boost, flyback, forward and the like. Alternatively, a person skilled in the art will recognize that it is within the scope of various embodiments that the first low power converter and the second low power converter may employ different power conversion topologies. 
     Although embodiments of the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.