Abstract:
A DC to DC converter comprising a zigzag transformer is provided. The transformer operates at higher frequency with integrated magnetics and does not provide isolation. The multiphase converter has gate inputs with PWM signals appropriately phase-shifted depending on the number of phases to make balanced phase voltages across the transformer windings. The switching frequency of the converter is slightly lower but fast transient response can be achieved by adding an integrated zigzag transformer. The disclosed converter improves overall efficiency, reduces current ripple and simplifies current control.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     The invention relates to a DC to DC converter, and particularly to a DC to DC converter with a high frequency zigzag transformer.  
         [0003]     2. Related Art  
         [0004]     Dc-dc converters are widely used for battery-powered electronic equipment, renewable energy systems, and voltage regulator modules (VRM) to produce a regulated voltage or current derived from an unregulated power supply. Most converters require a higher switching frequency to improve the transient response and to reduce the size of passive components. However, such a high switching frequency of hard switching converter beyond 1 MHz is not available nowadays. Therefore, multiple operations with an interleaved switching on-off control are preferable for low voltage, high current converters. The paralleled interleaving operation of the switching converter is gaining popularity because it is more efficient. The I 2 R conduction power loss associated with power components of each module is greatly reduced. The interleaving converter also provides ripple cancellation and improved transient response. However, parallel modules must share current equally. Current imbalance may occur due to component tolerances and/or parameter variations.  
         [0005]      FIGS. 1 and 2  show boost and buck converter topologies with 3-phase interleaving, respectively. In  FIG. 1 , the boost converter, which boosts the power source  10  to the load  20 , includes a first inductor L 1  connected to a first diode D 1 , a second inductor L 2  connected to a second diode D 2 , and a third inductor L 3  connected to a third diode D 3 . The other terminals of the diodes D 1 , D 2  and D 3  are connected to one terminal of the load  20 . A first transistor T 1 , a second transistor T 2  and a third transistor T 3  are also included. The drain terminal of the first transistor T 1  is connected between the first inductor L 1  and the first diode D 1 , while the source terminal of the first transistor T 1  is connected to the other end of the load  20 . The connection of the second transistor T 2  and the third transistor T 3  are similar to the transistor T 1 . A capacitor C is connected to the load  20  in parallel.  
         [0006]     In  FIG. 2 , the buck converter, which bucks the power source  10  to the load  20 , includes a first transistor T 1  connected to a first inductor L 1 , a transistor T 2  connected to a second inductor L 2 , and a third transistor T 3  connected to a third inductor L 3 . The other terminals of the inductors L 1 , L 2  and L 3  are connected to one terminal of the load  20 . A fourth transistor T 4 , a fifth transistor T 5  and a sixth transistor T 6  are also included. The drain terminal of the fourth transistor T 4  is connected between the first transistor T 1  and the first inductor L 1 , while the source terminal of the fourth transistor T 4  is connected to the other end of the load  20 . The connections of the fifth transistor T 5  and the sixth transistor T 6  are similar to the fourth transistor T 4 . A capacitor C is connected to the load  20  in parallel.  
         [0007]     However, there are some technical problems in these conventional converters. For example, they require three currents sensed for current sharing purposes, and thus suffer from higher ripple current on semiconductor devices. Furthermore, the conventional converter requires many magnetic cores. For the foregoing reasons, there is need for a converter with simpler circuitry and higher efficiency.  
       SUMMARY OF THE INVENTION  
       [0008]     In view of the foregoing problems, a DC to DC converter with a high frequency zigzag transformer is provided to substantially eliminate the problems of the conventional topology. The disclosed DC to DC converter with a high frequency zigzag transformer is capable of reducing current ripple, simplifying current control, and achieving better transient response. Current ripples are reduced on all switching devices and passive components by adjusting the operating point near boundary conditions. So conduction losses are minimized at the operating points near boundary conditions. Current sharing control is not necessary and transient response is faster because all phase currents flow equally through the transformer windings. Phase-shifted PWM switching signals offer balanced voltages in the transformer windings.  
         [0009]     In accordance with one aspect of the invention, the boost DC to DC converter with a high frequency zigzag transformer of the invention includes a transformer having plurality-of-legged cores; a plurality of diodes, each P side of the diodes connected to each leg of the cores of the transformer; and a plurality of transistors, each drain terminal of the transistors connected to each winding of the core legs of the transformer.  
         [0010]     In accordance with the other aspect of the invention, the buck DC to DC converter with a high frequency zigzag transformer is also provided. The buck DC to DC converter includes a transformer having plurality-of-legged cores; and a plurality pairs of transistors, each of the pair of transistors connected in series, and each leg of the transformer connected between the transistors connected in series.  
         [0011]     According to the principle of the invention, the disclosed DC to DC converter has the advantage of not requiring current-sharing technique.  
         [0012]     According to the principle of the invention, the disclosed DC to DC converter has another advantage of having no current ripples on passive and switching components.  
         [0013]     According to the principle of the invention, the disclosed DC to DC converter has another advantage of lower conduction loss.  
         [0014]     According to the principle of the invention, the disclosed DC to DC converter has another advantage of fast transient response even with lower switching frequency.  
         [0015]     Further scope of applicability of the invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:  
         [0017]      FIG. 1  is a circuit diagram of the boost converter of the prior art;  
         [0018]      FIG. 2  is a circuit diagram of the buck converter of the prior art;  
         [0019]      FIG. 3  is a circuit diagram of the boost converter in accordance with the invention;  
         [0020]      FIG. 4  is a circuit diagram of the buck converter in accordance with the invention;  
         [0021]      FIG. 5A  shows the voltage vector diagram of the zigzag transformer employed by the invention;  
         [0022]      FIG. 5B  shows the winding connections of the zigzag transformer employed by the invention;  
         [0023]      FIGS. 6A  to  6 D show the operational waveforms of the 3-phase boost converter in accordance with the invention;  
         [0024]      FIG. 7  shows the operational waveforms of the 3-phase buck converter in accordance with the invention;  
         [0025]      FIGS. 8A and 8B  show normalized ripple currents of the inductor in the converters in accordance with the invention;  
         [0026]      FIG. 9  shows an isolated DC to DC converter with a zigzag transformer in accordance with the invention;  
         [0027]      FIG. 10A  shows a current double rectifier with a two-phase zigzag transformer in accordance with the invention;  
         [0028]      FIG. 10B  shows the winding connection of the two-phase zigzag transformer included in the current double rectifier in accordance with the invention;  
         [0029]      FIG. 11  shows the N-legged core structure of multiple zigzag transformers for multiphase DC to DC converter in accordance with the invention;  
         [0030]      FIGS. 12A and 12B  show the voltage vectors for the converters with 4 and 5 phases in accordance with the invention;  
         [0031]      FIG. 13  shows the operational waveforms of the converter with 4 phases in accordance with the invention;  
         [0032]      FIG. 14A  shows the current waveforms of the experimental results in which V s =36V and V o =48V;  
         [0033]      FIG. 14B  shows the transformer winding voltages of the experimental results in which V s =36V and V o =48V; and  
         [0034]      FIG. 15  shows the experimental results at D=33.3% in which V s =32V, V o =48V. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]     The invention will become more fully understood from the detailed description given in the illustration below only, and is thus not limitative of the present invention. Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0036]     Refer to  FIG. 3 , which illustrates the circuit diagram of a boost converter in accordance with the invention. The boost converter, which boosts the power source  10  to the load  20 , is composed of a transformer  30 , an input inductor LS, a first diode D 1 , a second diode D 2 , a third diode D 3 , a first transistor T 1 , a second transistor T 2  and a third transistor T 3 . A capacitor C is connected in parallel with the load  20 .  
         [0037]     The transformer  30  is a kind of autotransformer that does not provide isolation. For example, a zigzag transformer having three terminals may be adopted as the transformer  30 . The core material of the transformer  30  should be ferrite without any airgaps. The input inductor LS is connected to the central terminal of the transformer  30 . The P sides of the diodes D 1 , D 2 , and D 3  are connected to each of the three terminals of the transformer  30 , respectively. The drain terminals of the transistors T 1 , T 2 , and T 3  are connected to each of the three terminals of the transformer, while the source terminals of the transistors T 1 , T 2 , and T 3  are connected to the ground end of the power source  10 . The gate terminals of the transistors T 1 , T 2 , and T 3  receive the PWM signals, which are appropriately phase-shifted.  
         [0038]     Refer to  FIG. 4 , which illustrates the circuit diagram of a buck converter in accordance with the invention. The buck converter, which bucks the power source  10  to the load  20 , is composed of a transformer  30 , an output inductor LO, a first transistor T 1 , a second transistor T 2 , a third transistor T 3 , a fourth transistor T 4 , a fifth transistor T 5 , and a sixth transistor T 6 . A capacitor C is connected in parallel with the load  20 .  
         [0039]     Similarly, the transformer  30  is a kind of autotransformer that does not provide isolation. For example, a zigzag transformer may be adopted as the transformer  30 . The core material of the transformer  30  should be ferrite without any airgaps. The output inductor LO is connected to the central terminal of the transformer  30 . The drain terminals of the transistors T 1 , T 2 , and T 3  are connected to the three terminals of the transformer  30 , respectively, while the source terminals of each of the transistors T 1 , T 2 , and T 3  are connected to the ground end of the power source  10 . The drain terminals of the transistors T 4 , T 5 , and T 6  are connected to the power source  10 , while the source terminals of the transistors T 4 , T 5 , and T 6  are connected to the three terminals of the transformer  30 , respectively. The gate terminals of the transistors T 1 , T 2 , T 3  receive the PWM signals, which are appropriately phase-shifted.  
         [0040]     The transformer  30  is a kind of autotransformer that does not provide isolation. The voltage vectors and winding connections on the three-legged core of the zigzag transformer are shown in  FIGS. 5A and 5B , respectively. Each leg of the transformer  30  has two windings. The winding connection is made according to the voltage vector, as shown in  FIG. 5A . The converters in  FIGS. 3 and 4  can operate with a zigzag coupled inductor for other applications. The zigzag coupled inductor has the same core and winding connection as the zigzag transformer, but needs airgaps in the core. In other words, the transformer  30  may be substitute with a zigzag coupled inductor. In the present invention, the transformer  30  and the zigzag coupled inductor are defined as a zigzag device.  
         [0041]     The disclosed converter has three gate inputs for PWM signals, which are appropriately phase-shifted with 120 degrees to balance the three-phase voltages (V za +V zb +V zc =0) across the transformer windings. The three gate inputs are the gate terminals of the transistors T 1 , T 2 , T 3 . The input inductor LS and the output inductor LO are used to reduce the ripple current, since the leakage inductor of the transformer  30  may not be enough or higher leakage inductance may not be favorable.  
         [0042]     Based on the balanced winding voltages, for all winding currents to meet the requirement for current flow into a three-phase transformer, they must be equally distributed as i za =i zb =i zc =−i* za =−i* zb =−i* zc , where i zj (j=a,b,c) is the transformer winding current and * denotes secondary winding. The current of the power source  10  includes dc, is three times the switching frequency, and is three times the winding current as i s =3i za .  
         [0043]     The winding voltage ν za  neglecting resistances and leakage inductances is written by  
           v   za     =         L   za     ⁢       ⅆ     i   za         ⅆ   t         +       L   zb     ⁢       ⅆ     i   za         ⅆ   t         -       M   ab     ⁢       ⅆ     i   zc         ⅆ   t         -       M   ab     ⁢       ⅆ     i   zb         ⅆ   t             ,       
 
 where L za  and L zb  are self inductances of a and b phases, respectively and M ab  is mutual inductance between a and b phases. The impedance Z AN  for the equal currents becomes Z AN =2ω(L za −M)≈0, where L za =L zb ≈M, ω=2πƒ and f is switching frequency. 
 
         [0044]     Since the leakage coefficient is small, the transformer offers substantially lower impedance when the equal currents flow via all the transformer windings. On the other hand, the impedance considering the balanced three-phase currents at switching frequency is obtained by Z AN =3ωL za .  
         [0045]     Therefore, the switching frequency current component flowing into the zigzag transformer is negligible due to higher impedance path. The sum of all identical fluxes on three legs is not equal to zero so that the flux can flow through a small impedance path. However, there is no magnetomotive force (MMF) in all three legs since the currents entering the transformer are in phase. As a result, no flux except leakage flux occurs without providing excess losses.  
         [0046]     The operation of the boost DC to Dc converter and the buck DC to DC converter in accordance with the invention is illustrated in detail in the following paragraphs. The boost converter with a high-frequency zigzag transformer is first illustrated.  
         [0047]     Depending on overlapping periods between phase-shifted switching signals, the operation of the boost converter is divided into three different modes: (1) D&lt;33.3%; (2) 33.3%&lt;D&lt;66.7%; and (3) 66.7%&lt;D&lt;100%. Similar to the conventional converter, the transfer function of the disclosed boost type is expressed by defining a conversion ratio M as the ratio of the output voltage to the input voltage,  
         M   =         V   O       V   S       =     1     1   -   D           ,       
 
 where D is the duty ratio of the boost switch, V s  is the dc input voltage of the power source  10 , and V o  is the output voltage. The duty ratio is the on-off time ratio of the boost switch. All the transformer winding currents are identical if the magnetizing inductance is high enough. Key operational waveforms are shown in  FIGS. 6A  to  6 C according to three different modes. 
 
         [0048]     Current ripples appear at three times the switching frequency and 3-phase voltage balancing is obtained by interleaving 3-gate signals. Basic equations are derived as follows.  
         [0049]     (1) D&lt;33.3%:  
         [0050]     In this mode, overlap between gate signals never exists. To obtain ripple current on the input inductor, the voltage across the inductor is V Ls,neg =V s −V o , where V Ls,neg  is the negative voltage magnitude across the inductor as defined in  FIG. 5A . Also, V Ls,pos  denotes the positive inductor voltage. The areas A and B of the inductor voltage must be equal, therefore,  
         V     Ls   ,   pos       =           1   -     3   ⁢   D         3   ⁢   D       ⁢     V     Ls   ,   neg         =         1   -     3   ⁢   D         3   ⁢     (     1   -   D     )         ⁢       V   S     .             
 
         [0051]     Finally, the ripple current is obtained as  
         Δ   ⁢           ⁢     I   S       =           V     Ls   ,   pos         L   S       ⁢     DT   S       =         D   ⁡     (     1   -     3   ⁢   D       )         3   ⁢     (     1   -   D     )         ⁢           T   S     ⁢     V   S         L   S       .             
 
         [0052]     (2) 33.3&lt;D&lt;66.7%:  
         [0053]     As shown in  FIG. 6B , overlapping only occurs between two gate signals. The inductor positive and negative magnitudes are expressed as V Ls,neg −V Ls,pos =V o −2V s , and  
         V     Ls   ,   pos       =           2   -     3   ⁢   D           3   ⁢   D     -   1       ⁢     V     Ls   ,   neg         =         2   -     3   ⁢   D         3   ⁢     (     1   -   D     )         ⁢     V   S             
 
 respectively. 
 
         [0054]     The ripple current in this mode is  
               Δ   ⁢           ⁢     I   S       =           V     Ls   ,   pos         L   S       ⁢       (       3   ⁢   D     -   1     )     3     ⁢     T   S       =           (       3   ⁢   D     -   1     )     ⁢     (       3   ⁢   D     -   2     )         9   ⁢     (     D   -   1     )         ⁢           T   S     ⁢     V   S         L   S       .                                 66.7   &lt;   D   &lt;     100   ⁢   %   ⁢     :               (   3   )             
 
         [0055]     As shown in  FIG. 6C , overlapping between three switching signals exists in this mode. The voltage across the inductor is equal to the input voltage if all three switches are turned on. In this mode, V Ls,pos =V s .  
         [0056]     The ripple current is linearly increased by  
         Δ   ⁢           ⁢     I   S       =           V     Ls   ,   pos         L   S       ⁢     (     D   -     2   3       )     ⁢     T   S       =         D   -   2     3     ⁢           T   S     ⁢     V   S         L   S       .             
 
         [0057]      FIG. 6D  shows the boost converter operation at D=33.3%. Input current and the sum of 3-diode currents contain only dc quantities. These currents provide less stress on both the input filter and the output dc capacitor. In the conventional approach in  FIG. 1  and  FIG. 2 , each phase current has its own ripple at the boundary point. After interleaving together, the ripples of total input current cancel each other. The ripple current appears on all switching devices. However, the topology with a zigzag transformer does not show any current ripple on all switching devices and passive components at boundary conditions. All phase currents are dc constants, as shown in  FIG. 6D . Therefore, conduction losses of the switching and passive components can be lower than the conventional approach. Furthermore, since 3-winding currents flow equally, a current sharing scheme is not necessary and output transient response is faster than the conventional scheme.  
         [0058]     The multiple interleaved synchronous buck topologies with a zigzag transformer are introduced in the following.  
         [0059]     Similar to the above boost converter, the operation of the buck converter is also divided into three different modes: (1) D&lt;33.3%, (2) 33.3%&lt;D&lt;66.7%, and (3) 66.7%&lt;D&lt;100%. The relationship between input and output voltages is V o =DV s .  
         [0060]     Basic equations for ripple currents on the inductor Lo are given as follows. Key operational waveforms are shown in  FIG. 7 .  
             D   &lt;     3.33   ⁢   %             (   1   )                 V     Lo   ,   neg       =       DV   S     .                               Δ   ⁢           ⁢     I   Lo       =           V     Lo   ,   neg         L   O       ⁢     (       1   3     -   D     )     ⁢     T   S       =         D   ⁡     (     1   -     3   ⁢   D       )       3     ⁢           T   S     ⁢     V   S         L   O       .                                   33.3   ⁢   %     &lt;   D   &lt;     66.7   ⁢   %             (   2   )                 V     Lo   ,   neg       =           3   ⁢   D     -   1     3     ⁢       V   S     .                                 Δ   ⁢           ⁢     I   Lo       =           V     Lo   ,   neg         L   O       ⁢     {     D   -     2   ⁢     (     D   -     1   3       )         }     ⁢     T   S       =           (       3   ⁢   D     -   1     )     ⁢     (     2   -     3   ⁢   D       )       9     ⁢           T   S     ⁢     V   S         L   o       .                                   66.7   ⁢   %     &lt;   D   &lt;     100   ⁢   %             (   3   )                 V     Lo   ,   pos       =       (     1   -   D     )     ⁢       V   S     .                                 Δ   ⁢           ⁢     I   Lo       =           V     Lo   ,   neg         L   O       ⁢     (     D   -     2   3       )     ⁢     T   S       =           (     1   -   D     )     ⁢     (       3   ⁢   D     -   2     )       3     ⁢           T   S     ⁢     V   S         L   O       .                               
 
         [0061]     The inductor ripple currents of the boost converter in accordance with the invention are normalized by T s V s /L s  and plotted according to a duty cycle as shown in  FIG. 8A . Nearly ripple-free operation can be achieved at the boundary conditions such as D=33.3% and D=66.7% in a 3-interleaved converter. Therefore, to minimize the ripple current, the operating points of the boost converter design need to be set near those boundaries.  
         [0062]     The normalized ripple currents of the buck converter are shown in  FIG. 8B . The buck converter in accordance with the invention also provides ripple-free currents near the boundary points. Such buck converters can be widely used for voltage regulator modules (VRMs) with high current and low voltage.  
         [0063]     A current tripler as shown in  FIG. 9  is the isolated version with a zigzag transformer in accordance with the invention. This topology requires 2 three-phase magnetic cores.  
         [0064]     The isolated DC to DC converter with a zigzag transformer in  FIG. 9  include an integrated zigzag transformer  30  having 3-legged cores. The connection, function and operation of the transformer  30 , and the first transistor T 1 , the second transistor T 2  and the third transistor T 3  are similar with the above mentioned embodiment. Besides, the converter includes a three-phased transformer  40  having a primary winding and a second winding, wherein the second winding is connected to the integrated zigzag transformer. Six transistors  51 ˜ 56  are also included in the converter shown in  FIG. 9 . The six transistors  51 ˜ 56  form three pairs, each pair has two transistors connected in series. Each phase of the primary winding of the transformer  40  is connected between the two transistors in each pair.  
         [0065]      FIG. 10A  shows a current doubler rectifier with an integrated magnetrics in accordance with the invention. A two phase zigzag transformer without airgaps is included in the current doubler. The winding connection of the transformer is illustrated in  FIG. 10B . The current doubler has a transformer  70  having a primary winding and a second winding, and a zigzag transformer with two phases  60 . The two-phase windings are connected to the second windings of the transformer  70 .  
         [0066]     Besides, an output inductor LO is connected to the central terminal of the transformer  60 . A capacitor C is connected in parallel with the load  20 . Two transistors  57 ˜ 58  are connected to each winding of the transformer  60 .  
         [0067]     In the previous illustration, only 3-interleaved converter topologies have been shown. However, the 3-phase version can be extended to a multiple-phase structure more than 2-phases by adding the number of legs on the magnetic core, as shown in  FIG. 11 . Core material should be ferrite without any airgaps. Available topologies include boost converters, buck converters, Cuk converter, Sepic converter, and isolated converter. A zigzag coupled inductor may also be employed in the converter. Also, any current smoothing inductors in other dc-dc converters may be replaced by zigzag transformers. With balanced winding voltages, all winding currents are equally distributed as i 1 =i 2 = . . . =i n-1 , (n≧2) , where n is the number of phases. Overall input/output frequency occurs at n times switching frequency. The duty cycles D n  at the boundaries are obtained according to the number of interleaving phases  
           D   n     =       100   n     ⁢     h   ⁢           [   %   ]         ,       (       h   =   1     ,   2   ,   …   ⁢           ,     n   -   1       )     .         
 
         [0068]      FIGS. 12A and 12B  show the voltage vectors for the converters with 4 and 5 phases. From the vector diagrams shown in  FIG. 3  and  FIG. 12 , the winding voltage rating is estimated as,  
           V   z1     =         1     3       ⁢     V     1   ⁢   N         =     0.577   ⁢     V     1   ⁢   N             ,     (     n   =   3     )                     V   z1     =         1     2       ⁢     V     1   ⁢   N         =     0.707   ⁢     V     1   ⁢   N             ,     (     n   =   4     )                     V   z1     =         1     2   ⁢           ⁢   cos   ⁢           ⁢   18   ⁢   °       ⁢     V     1   ⁢   N         =     0.526   ⁢     V     1   ⁢   N             ,     (     n   =   5     )           
         [0069]     The winding voltages depend on the angle between two windings of the transformer.  FIG. 13  shows the operational waveforms for a 4-phase example.  
         [0070]     The proposed 3-phase boost converter is implemented with 200 W at 48 V output voltage.  
         [0071]     The switching frequency is set to 167 kHz. A very small input inductor compared with the conventional approach is used. Only a single current is sensed instead of 3 different signals. Experimental results are shown in  FIGS. 14 and 15 .  FIG. 14A  shows the three identical currents without current sharing. The duty cycle is about 25%. The frequency of the input current and the sum of diode currents is three times the switching frequency (500 kHz).  FIG. 14 B  shows both primary and secondary winding voltages.  FIG. 15  shows the waveforms when the converter operating point is adjusted at boundary condition near D=33.3%. Input voltage  32 V is transferred to 48 Vdc. All three input currents are dc constants with no ripples (small). Still all equal currents flow through transformer windings. Also, no ripple currents flow through other switching components.  
         [0072]     According to the multiple dc-dc converters with high frequency zigzag transformers in accordance with the invention, the transformer automatically makes all multi-phase currents identical. Therefore, current control can be simplified by sensing a single current for multiple converters without current sharing. Besides, current ripples are eliminated near the boundary conditions where the converter operating point was set. Furthermore, transient response of the converter is improved.  
         [0073]     While the preferred embodiment of the invention has been set forth for the purpose of disclosure, modifications of the disclosed embodiment of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments which do not depart from the spirit and scope of the invention.