Abstract:
A method for providing non-resonant zero-voltage switching in a switching power converter. The switching power converter converts power from input power to output power during multiple periodic switching cycles. The switching power converter includes a main switch and an auxiliary capacitor adapted for connecting to the main switch, and an inductor connectible to the auxiliary capacitor. When the main switch is on, a previously charged (or previously discharged) auxiliary capacitor is connected to the main switch with auxiliary switches. The main switch is switched off with zero voltage while discharging non-resonantly (charging) the auxiliary capacitor by providing a current path to the inductor. The auxiliary capacitor is disconnected from the main switch. The voltage of the auxiliary capacitor is charged and discharged alternatively during subsequent switching cycles. The voltage of the auxiliary capacitor stays substantially the same until the subsequent turn off of the main switch during the next switching cycle with substantially no energy loss in the auxiliary capacitor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation application of pending U.S. patent application Ser. No. 12/409,604 filed on Mar. 24, 2009 by the present inventors. 
         [0002]    The present application claims priority from U.S. provisional application 61/039046 filed on Mar. 24, 2008 by the present inventors. 
     
    
     BACKGROUND 
     1. Technical Field 
       [0003]    The present invention relates to switching converters and to specifically a method and devices for zero voltage switching for reducing switching losses in switching converters. 
       2. Description of Related Art 
       [0004]      FIG. 1  shows a typical conventional buck-boost DC-to-DC converter circuit  20 . The buck circuit of buck-boost DC-to-DC converter  20  has an input voltage V in  with an input capacitor C 1  connected in parallel across V in . Two switches are implemented as field effect transistors (FET) with integral diodes: a high side buck switch Q 1  and a low side buck switch Q 2  connected in series by connecting the source of Q 1  to the drain of Q 2 . The drain of Q 1  and the source of Q 2  are connected parallel across an input capacitor C 1 . A node A is formed between switches Q 1  and Q 2  to which one end of an inductor  206  is connected. The other end of inductor  206  is connected to the boost circuit of buck-boost DC-to-DC converter  20  at a node B. Node B connects two switches: a high side boost switch Q 4  and a low side boost switch Q 3  together in series where the source of Q 4  connects to the drain of Q 3  to form node B. The drain of Q 4  and the source of Q 3  connect across an output capacitor C 2  to produce the output voltage V out  of buck-boost DC-to-DC converter  20 . 
         [0005]      FIG. 2 a    illustrates the buck phase or on-state circuit of DC-to-DC converter circuit  20  shown in  FIG. 1 , the input voltage source V in  is directly connected to inductor  206  and the load is isolated from V in  because Q 1  is on, Q 2  is off, Q 3  is on and Q 4  is off. These switch positions: Q 1  on, Q 2  off, Q 3  on and Q 4  off; result in accumulating energy in inductor  206  since source V in  is directly connected to inductor  206 . In the on-state, output capacitor C 2  supplies energy to the load. 
         [0006]      FIG. 2 b    illustrates the boost phase or off-state circuit of DC-to-DC converter circuit  20 , Inductor  206  is connected in parallel across the load and capacitor C 2  because Q 1  is off, Q 2  is on, Q 3  is off and Q 4  is on. Q 1  being off isolates inductor  206  from the input voltage (V in ) and capacitor (C 1 ). The stored energy in inductor  206  (as a result of the previous On-state) is transferred from inductor  206  to C 2  and the load. 
         [0007]    Two common methods of operating DC-to-DC converter circuit  20  are in either continuous mode or discontinuous mode. If the current through the inductor  206  never falls to zero during a commutation cycle (i.e. the time period to perform both the on-state and the off-state), DC-to-DC converter circuit  20  is said to operate in continuous mode and typically the on-state operates for a shorter period of time when compared to the off-state. Discontinuous mode of operation for DC to DC converter circuit  20  occurs when the amount of energy required by the load is small enough to be transferred in a time period smaller than the whole commutation cycle. Typically, the current through inductor  206  falls to zero for a short time period after the off-state period and therefore inductor  206  is completely discharged at the end of the commutation cycle. The commutation cycle therefore includes the on-state, the off-state and the short time period during which the inductor current is zero. 
         [0008]    A conventional “resonant” method for achieving virtually zero power loss when switching a switch is to apply a direct current voltage input voltage V in  across a switch (with a diode connected across the switch, the diode is reverse biased with respect to V in ) in series with an inductor L and a capacitor C. The output voltage of the circuit is derived across the capacitor. The output voltage of the circuit could then in principle be connected to the input of a power converter, for example a buck-loaded series tank circuit with load. The resonant frequency of the series inductor L and capacitor C is given by Eq. 1 and the corresponding resonant periodic time T given in Eq. 2. 
         [0000]        f   o =½π( LC ) 1/2   Eq.1
 
         [0000]        T=l/f   o   Eq.2
 
         [0000]    A pulse response of the circuit means that when the switch turns on, there is both zero current in the inductor and zero voltage across the capacitor (Power=Volts X Current=0×0=zero power loss at turn on). During steady state operation of the circuit, the inductor current and capacitor voltage are sinusoidal and have a 90 degrees phase shift with respect to each other. When the switch turns off (the on period of the switch corresponds to half of the resonant periodic time) there is zero current in the inductor and maximum positive voltage (i.e. V capacitor =V in ) across the capacitor (Power=Volts X Current=V in ×0=zero power loss at turn off). 
       BRIEF SUMMARY 
       [0009]    According to an embodiment of the present invention there is provided a method for providing non-resonant zero-voltage switching in a switching power converter. The switching power converter converts power from input power to output power during multiple periodic switching cycles. The switching power converter includes a main switch and an auxiliary capacitor adapted for connecting to the main switch, and an inductor connectible to the auxiliary capacitor. When the main switch is on, a previously charged (or previously discharged) auxiliary capacitor is connected to the main switch with auxiliary switches. The main switch is switched off with zero voltage while discharging non-resonantly (charging) the auxiliary capacitor by providing a current path to the inductor. The auxiliary capacitor is disconnected from the main switch. The voltage of the auxiliary capacitor is charged and discharged alternatively during subsequent switching cycles. The voltage of the auxiliary capacitor stays substantially the same until the subsequent turn off of the main switch during the next switching cycle with substantially no energy loss in the auxiliary capacitor. The switch may include a: silicon controlled rectifier (SCR), insulated gate bipolar junction transistor (IGBT), bipolar junction transistor (BJT), field effect transistor (FET), junction field effect transistor (JFET), switching diode, electrical relay, reed relay, solid state relay, insulated gate field effect transistor (IGFET), diode for alternating current (DIAC), and/or triode for alternating current TRIAC. 
         [0010]    According to the present invention there is provided a switching converter including a buck stage and/or a boost stage including a main switch connecting an input voltage terminal to a first node; an auxiliary capacitor adapted for connecting to the main switch and an inductor adapted for connecting to the first node. The first node is connectible to the auxiliary capacitor by at least two current paths. When the main switch is on, a previously charged (or previously discharged) auxiliary capacitor is connected across the main switch typically with auxiliary switches. The main switch is switched off with zero voltage while discharging (charging) the auxiliary capacitor by providing a current path to the inductor. The auxiliary capacitor is disconnected from the switch. The voltage of the auxiliary capacitor is charged and discharged alternatively during subsequent switching cycles. The voltage of the auxiliary capacitor stays substantially the same until the subsequent turn off of the main switch during the next switching cycle with substantially no energy loss in the auxiliary capacitor. The switching converter may include a single buck stage or a single boost stage. 
         [0011]    According to the present invention there is provided a switching converter included a plurality of main switches interconnected in a full bridge topology, the main switches including a first switch, a second switch, a third switch and a fourth switch. A pair of input voltage terminals are attachable at a first node connecting the first and third switches and at a second node connecting the second and fourth switches. A first output voltage terminal is operatively attached at a third node connecting the third and fourth switches. A second output voltage terminal is operatively attached at a fourth node connecting the first and second switches. Bidirectional switches are interconnected in a full bridge topology. The bidirectional switches include a first bidirectional switch, a second bidirectional switch, a third bidirectional switch and a fourth bidirectional switch. The third node connects the first and third bidirectional switches and the fourth node connects the second and fourth bidirectional switches. An auxiliary capacitor connects at one end at a node connecting the first and second bidirectional switches and at the other end at a node connecting the second and fourth bidirectional switches. The main switches are preferably configured to be periodically switched on and off during a plurality of switching cycles. One or more of the main switches is on. A previously charged (or previously discharged) auxiliary capacitor is connected across the main switch typically with auxiliary switches. The main switch is switched off with zero voltage while discharging (charging) the auxiliary capacitor by providing a current path to the inductor. The auxiliary capacitor is disconnected from the switch. The voltage of the auxiliary capacitor is charged and discharged alternatively during subsequent switching cycles. The voltage of the auxiliary capacitor stays substantially the same until the subsequent turn off of the main switch during the next switching cycle with substantially no energy loss in the auxiliary capacitor. A first inductor is typically attachable between the first output voltage terminal and the third node. A second inductor is typically attachable between the second output voltage terminal and the fourth node. The first and second inductor is optionally a single split inductor or inductor is a single inductor connected in series to a transformer primary or other circuitry. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
           [0013]      FIG. 1  shows a typical conventional buck-boost DC-to-DC converter circuit. 
           [0014]      FIG. 2 a    illustrates the buck phase or on-state circuit of conventional DC-to-DC converter circuit. 
           [0015]      FIG. 2 b    illustrates the boost phase or off-state circuit of DC-to-DC converter circuit  20 ; 
           [0016]      FIG. 3  ( FIGS. 3 a -3 d   ) illustrate a buck-boost DC-to-DC converter, according to an embodiment of the present invention; 
           [0017]      FIG. 4  shows a flow diagram of a method for zero voltage switching, running in either continuous or discontinuous mode during the turn off of main switches Q 1  and/or Q 3 , according to embodiments of the present invention; 
           [0018]      FIG. 5  shows another embodiment of present invention as applied to a full bridge switched DC-to-DC converter. 
           [0019]      FIG. 6  shows a timing diagram of selected voltages and currents in the embodiment of  FIG. 5 . 
       
    
    
       [0020]    The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures. 
       DETAILED DESCRIPTION 
       [0021]    Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures. 
         [0022]    It should be noted, that although the discussion herein relates to buck, boost, buck-boost full bridge switching topologies, the present invention may, by non-limiting example, alternatively be configured as well using other types of switching power DC-DC converters including half bridge, flyback, Cuk, as well as DC-AC inverters for both power supply and regulation applications. 
         [0023]    Before explaining embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of design and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
         [0024]    The term “switch” as used herein refers to any type of switch known in the art of electronics switches such as silicon controlled rectifier (SCR), insulated gate bipolar junction transistor (IGBT), bipolar junction transistor (BJT), field effect transistor (FET), junction field effect transistor (JFET), switching diode, electrical relay, reed relay, solid state relay, insulated gate field effect transistor (IGFET), DIAC, and TRIAC. 
         [0025]    The term “zero voltage switching” (or “ZVS”) as used herein is that the peak voltage across a switch, is reduced to substantially zero volts when the switch is being turned either on or off. 
         [0026]    The term “cycle” or “commutation cycle” refers to the periodicity of main switch positions in a circuit which performs a process of electrical power conversion or inversion. 
         [0027]    The term “power converter” as used herein applies to DC-to-DC converters, AC-to-DC converters, DC-to-AC inverters, buck converters, boost converters, buck-boost converters, buck/boost converters, full-bridge converters and half-bridge converters or any other type of electrical power conversion/inversion known in the art. 
         [0028]    The term “buck-boost” converter as used herein refers to a buck converter followed by a boost converter. 
         [0029]    The term “non-resonant” as used herein to exclude resonant and quasi-resonant circuits and methods as arc known in the prior art for achieving zero voltage switching. 
         [0030]    The terms “charging” and “discharging” in the context of the present invention in reference to charging and discharging a capacitor, are used herein interchangeably except that current flow while charging and discharging is usually in the opposite direction. 
         [0031]    Reference is now made to  FIG. 3  ( FIGS. 3 a -3 d   ) showing a buck-boost DC-to-DC converter  40  according to an embodiment of the present invention. A buck circuit  42  of buck-boost DC-to-DC converter  40  has an input voltage V in  with an input capacitor C 1  connected in parallel across V in . Two switches Q 1  and Q 2  are connected in series at node A by connecting the source of Q 1  to the drain of Q 2 . The drain of Q 1  and the source of Q 2  are placed in parallel across capacitor C 1 . A zero-voltage switching feature according to embodiments of the present invention is provided using components: switches Q bu , Q abu , auxiliary (AUX) capacitor C bu , and diodes D 2bu  and D abu  in buck circuit  42 . The cathode of diode D 2bu  is connected to one end of capacitor C bu . The other end of C bu  is connected to the drain of switch Q bu . The drain of Q abu  and the anode of diode D 2bu  are connected in parallel across capacitor C 1 . The cathode of diode D abu  is connected to the source of switch Q bu . The anode of diode D abu  and the source of switch Q abu  are connected across with capacitor C bu . Node A shared by the cathode of diode D abu  and the source of switch Q bu  is connected to the buck end of inductor  206 . 
         [0032]    The other end of inductor  206  is connected to a boost circuit  44  of buck boost DC-to-DC converter  40  at node B. Two switches Q 4  and Q 3  are connected in series. The source of Q 4  connects to the drain of Q 3  at node B. The drain of Q 4  and the source of Q 3  connect across capacitor C 2 , across capacitor C 2  is the output voltage V out  of buck-boost DC-to-DC converter  40 . Additional components: switches Q bo , Q abo , auxiliary (AUX) capacitor C bo , and diodes D 1bo , D abo  are added to achieve zero-voltage switching in boost circuit  44 . The anode of diode D 1bo  is connected to one end of capacitor C bo . The other end of C bo  is connected to the source of switch Q bo  and the drain of Q abo . The source of Q abo  and the cathode of diode D 1bo  are connected in parallel across capacitor C 2 . The anode of diode D abo  is connected to the drain of switch Q bo . The node between the anode of diode D abo  and the drain of switch Q 3  are connected to node B. The cathode of diode D abo  and the source of switch Q bo  are connected across with capacitor C bo . 
         [0033]    Reference is still made to buck-boost DC-to-DC converter  40  shown in  FIGS. 3 a  -3 d    which illustrate operation of buck-boost DC-to-DC converter  40 . Reference is now also made to  FIG. 4  showing a flow diagram of a method for zero voltage switching, in boost and/or buck topologies during the turn off of main switches Q 1  and/or Q 3 , according to embodiments of the present invention. 
         [0034]    Step A. Before switching phase: In  FIG. 3 a   , current flow in buck circuit  42  and boost circuit  44  is indicated by arrow markings and gray shaded line. In buck circuit  42 , switch Q 1  is on (step  400 ), switch Q 2  is off and switch Q bu  is on. Auxiliary (AUX) capacitor C bu  is previously charged to V in  and connected to the source of Q 1  (step  402 ). I L  current flows from input, through Q 1  through node A to inductor  206 . 
         [0035]    Meanwhile, in boost circuit, Q 3  is on, Q 4  is off, Q bo  is on. C bo  is previously charged to V out . I L  current flows from inductor  206  through node B. 
         [0036]    Step B. Switching off phase: In  FIG. 3 b   , current flow in buck circuit  42  and boost circuit  44  is indicated by the arrow markings and gray shaded lines. Switch Q 1  turns off at substantially zero voltage (step  404 ). Switch Q bu  is still on. I L  current (of inductor  206 ) discharges capacitor C bu  to zero voltage through node A through diode D 2bu . Q 2  turns on and Q bu  turns off disconnecting (step  406 ) auxiliary (AUX) capacitor C bu . 
         [0037]    Meanwhile, in boost circuit  44  Q 3  turns off at substantially zero voltage. I L  current (of inductor  206 ) discharges capacitor C bo  to zero voltage through node B through diode D 1bo . Q 4  turns on and Q bo  turns off disconnecting auxiliary (AUX) capacitor C bo . 
         [0038]    Thus ends one switching cycle. Now, for the next switching cycle: 
         [0039]    Step C. Before switching phase: In  FIG. 3 c    current flow in buck circuit  42  and boost circuit  44  is indicated by the arrow markings and Grey shaded line. Switch Q 1  is on, Switch Q 2  is off (step  400 ). Switch Q abu  turns on. C bu  remains discharged from the previous switching cycle and still connected to the source of Q 1  (step  402 ). I L  current flows from input through node A, through Q 1  to inductor  206 . 
         [0040]    Meanwhile in boost circuit  44 : Q 3  is on, Q 4  is off, Q abo  turns on. C bo  remains discharged from the previous switching cycle. 
         [0041]    Step D. Switching off phase: In  FIG. 3 d   , current flow in buck circuit  42  and boost circuit  44  is indicated by the arrow markings and Grey shaded line. Switch Q 1  turns off with substantially zero voltage (step  404 ). Switch Q bu  is still on. I L  current (of inductor  206 ) charges capacitor C bu  to V in  voltage through node A through diode D abu . Switch Q 2  turns on. Q abu  is turned off and auxiliary (AUX) capacitor C bu  disconnected from the source of Q 1  (step  406 ). 
         [0042]    Meanwhile in the boost circuit  44 , Q 3  turns off at substantially zero voltage. I L  current (of inductor  206 ) charges capacitor C bo  to V out  voltage through node B through diode D abo . Q 4  turns on and Q abo  turns off. 
         [0043]    Thus ends the second switching cycle. Now, for the next switching cycle the sequence starts again at Step A. 
         [0044]      FIG. 5  shows a further embodiment of present invention as applied to a full bridge DC to DC converter  50 . Full bridge DC to DC converter  50  has four main switches S m,1 , S m,2 , S m,3  and S m,4  connected together in a full bridge configuration. Each of the four main switches (S m,1 , S m,2 , S m,3  and S m,4 ) have respective diode shunts connected in parallel thereto. The diodes placed across switches S m,1  and S m,2  are in both the same direction similarly the diodes of S m,3  and S m,4  are both in the same direction. All diodes connected across switches S m,1 S m,2 , S m,3  and S m,4  are reverse biased with respect to the input voltage V in . An input voltage (V in   − ) of full bridge DC-to-DC converter  50  is connected across the node between switches S m,2  and S m,4  and an input voltage (V in   + ) is connected at the node between switches S m,1 , and S m,3 . An output voltage (V out   − ) of full bridge DC-to-DC converter  50  is connected across the node between switches S m,1  and S m,2  connected through a split inductor  500   a  and output voltage V out + is connected at the node between switches S m,3  and S m,4  through a split inductor  500   b  . A bi-directional switch unit  502  includes four bidirectional switches. Each bidirectional switch includes has two switches in series, e.g. (S a,1 , S a,2 ) each with a diode connected across each switch with the diodes connected in opposite directions. Bi-directional switch unit  502  is connected at X 1  to the node between switches S m,1  and S m,2  and at X 2  to the node between switches S m,3  and S m,4 . Bidirectional auxiliary switches are formed between nodes Y 1  and X 1  using switches S a,1  and S a,2 , between nodes Y 1  and X 2  using switches S a,5  and S a,6 , between nodes Y 2  and X 1  using switches S a,3  and S a,4  and between nodes Y 2  and X 2  using switches S a,7  and S a,8 . An auxiliary capacitor C aux  is connected between nodes Y 1  and Y 2 . 
         [0045]    In different embodiments the present invention may be configured to operate in either continuous or discontinuous current mode. The operation of full bridge circuit  50 , according to a feature of the present invention and with reference again to  FIG. 5  and  FIG. 6  which shows a timing diagram of selected voltages and currents for steps A to H is as follows:
       A) Switches S m,2 , S m,3 , S a,3 , S m,4 , S m,5 , and S a,6  are turned on, all other switches are off. Current flows from V out − to V in − through inductor  500   a,  and through main switch S m,2 . Current flows from V in   +  to V out + through S m,3  and through inductor  500   b.  Capacitor C aux  is charged so that node Y 1  approaches V in   +  and node Y 2  approaches V in   − .   B) S m,2  and S m,3  are switched open (off). Switches S a,3 , S m,4 , S a,5 , and S a,6  remain on. During the switching open of S m,2  and S m,3  current from inductor  500   a  and  500   b  is diverted respectively through bidirectional switches (S a,3  S a,4 ) and (S a,5  S a,6 ) with voltage across switches S m,2  and S m,3  substantially zero. All inductor current flows through C aux  from node Y 2  to node Y 1  which during a period of time dependent on the current in inductors  500   a  and  500   b  and the capacitance of C aux  inverts the voltage across C aux  so that node Y 1  is charged to a voltage level equal to V in − and node Y 2  is charged to a voltage level equal to V in   +  via auxiliary capacitor C aux      C and D) Once C aux  is fully charged and inverted, node Y 1  is charged to V in   −  and Y 2  is charged to V in   + , current now flows from V out − through inductor  500   a  through parallel connected diode of S m,1  and current flows from V in   −  through the parallel connected diode of S m,4 , and through inductor  500   b  to V out   + .   E) Auxiliary switches S a,3 , S a,4 , S a,5 , and S a,6  are now turned off with no current flowing through them nor a voltage across them.   F) Main switches S m,1  and S m,4  are turned on with substantially zero voltage across them, diverting most of the current from flowing through their parallel connected diodes to flowing through switches S m,1  and S m,4  themselves.   G) Before the turn on of Sm, 2  and Sm, 3 , main switches Sm, 1  and Sm, 4  are turned off with zero voltage across them so that current flow is diverted again through their parallel connected diodes.   H) Main switches S m,2  and S m,3  turn on to begin the next switching cycle. Current of inductor  500   a  flows from V out   −  through inductor  500   a,  through switch S m,2  to V in   − ; and current of inductor  500   b  flows from V in   +  through S m,3  to V out   + .   I) Auxiliary switches S a,1 , S a,2 , S a,7  and S a,8  are turned on with zero voltage and zero current.   J) S m,2  and S m,3  open at zero voltage. All inductor current flows now through from V out   −  through inductor  500   a,  through bidirectional switch (S a,1 ,S a,2 ), through C aux , through bidirectional switch (S a,1 ,S a,2 ) from node Y 1  to node Y 2  which over a period of time (dependent on the current in inductors  500   a  and  500   b  and the capacitance of C aux ) inverts again the voltage across C aux  so that node Y 1  is charged to a voltage level equal to V in   +  and node Y 2  is charged to a voltage level equal to V in   −  (as it was in (1)).   K) Current flows from V out   −  through inductor  500   a,  through the parallel connected diode of S m,1  to V in   + . Current flows from V in   −  through the parallel connected diode of S m,4  and through inductor  500   b  to V out   − .   L) Auxiliary switches S a,1 , S a,2 , S a,7 , and S a,8  are turned off with zero voltage and zero current.   M) Main switches S m,1  and S m,4  are turned on at zero voltage forcing the current through themselves from V out   −  through inductor  500   a,  through S m,1 , and from V in   −  through S m,4  and through inductor  500   b  to V out   + .   N) Before S m,2  and S m,3  are turned on, main switches S m,1  and S m,4  are turned off with zero voltage, the current flowing again from V out   −  through inductor  500   a,  through the diode of S m,1 , through V in + and from V in − through the diode of S m,4  and through inductor  500   b  to V out +.   O) Main switches S m,2  and S m,3  turn on to begin the next switching cycle. The current flows again from V out   −  through inductor  500   a,  through S m,2  to V in   −  and from V in   + , through S m,3  and through inductor  500   b  to V out   + .   P) S a,3 , S a,4 ,S a,5 , and S a 6  are turned on with zero voltage and zero current.       
 
         [0061]    During the two switching cycles as shown, auxiliary capacitor C aux  is charged and discharged by the inductor current with substantially no energy loss due to switching. 
         [0062]    The definite articles “a”, “an” is used herein, such as “a converter”, “a switch” have the meaning of “one or more” that is “one or more converters” or “one or more switches”. 
         [0063]    Although selected embodiments of the present invention have been shown and described, it is to be understood the present invention is not limited to the described embodiments. Instead, it is to be appreciated that changes may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and the equivalents thereof.