Patent Publication Number: US-7719137-B2

Title: DC-DC switching converter device

Description:
TECHNICAL FIELD 
   The invention relates to a DC-DC switching converter device, in particular a quarter-brick or eighth-brick device having an industry standard pin out, comprising a pulse-width modulation (PWM) circuit for driving a power converting switch and a trim connector for adjusting an output voltage of the device. The invention further relates to a DC-DC converter circuit and a method of converting a first DC voltage into a second DC voltage. 
   BACKGROUND ART 
   DC-DC switching converters, i.e. devices that accept a DC input voltage and produce a DC output voltage, in particular on another voltage level, have a wide range of applications in today&#39;s power electronics. DC-DC Switching converters are used e.g. for power supplies, DC motor control or battery management. Apart from converting the input DC voltage they provide noise isolation, power bus isolation etc. The switching regulators allow for step-up operation or voltage inversion and offer a higher efficiency compared to linear regulators. Using a transformer as the energy-storage element also allows the output voltage to be electrically isolated from the input voltage. 
   However, the switching operation of the converter creates noise that has to be suppressed in order to avoid electromagnetic interference (EMI) affecting other devices connected to the power converter. To achieve this, an EMI filter, i.e. a suitable low-pass filter, is commonly arranged on the input side of the converter. The EMI filter rejects the fixed frequency current ripple generated by the switching converter. 
   In cases where more power has to be delivered into different loads or where different output voltages have to be provided it is known to parallel a plurality of DC-DC converters. The converters share the same input bus and use one common EMI filter. However, this arrangement leads to frequency beating phenomena between different converters sharing the same line input as well as to additional low frequency interferences. Altogether, the peak to peak amplitude of the interference may be about five times higher than those of just the input current ripple created by a single converter. This greatly increases the performance demand for the system EMI filter. In principle, most of the additional interference due to the parallel arrangement of the converters might be avoided by externally synchronizing the converter&#39;s switching frequencies. 
   However, synchronization is not easily possible with basic types of switching converters that are widely used for output voltages of several volts and output currents of about 10-60 A, namely the so-called “quarter-brick” or “eighth-brick” converters. These types have predefined mechanical dimensions and feature an industry-standard pin out, including two input and two output connectors, an On/Off connector for remotely controlling the device, two output sensing connectors for line drop compensation as well as a trim connector (called sometimes the “trim pin” or “trim terminal”) for adjusting the output voltage of the device. 
   However, the industry standard quarter and eighth-brick formats do not allow for an extra pin available for external frequency synchronization. Therefore, a sophisticated EMI filter device has to be employed with prior art quarter or eighth-brick converters, or the DC-DC converting circuit has to be completely redesigned in order to utilize specific DC-DC converters that provide extra pins for synchronization. Both possibilities are costly and cumbersome. 
   SUMMARY OF THE INVENTION 
   It is therefore the object of the invention to create a DC-DC switching converter device pertaining to the technical field initially mentioned, that allows for the reduction of noise in the case of a plurality of DC-DC converters arranged in parallel and sharing a common input bus and EMI filter, the device being cost-efficient and easy to implement into usual converter circuits. 
   In one exemplary embodiment of the invention the solution of the invention is provided by a DC-DC switching converter having a pulse-width modulator circuit for driving a power converting switch, and a trim connector for adjusting an output voltage of the device, wherein the pulse-width modulation circuit is synchronizable to an external oscillator by an external synchronization signal applied to the trim connector. 
   The usual functionality of the trim connector is not lost, but the external synchronization signal is superimposed to the trim DC voltage (if present). With the inventive device, the trim connector serves two purposes, namely for adjusting the output voltage and additionally for externally synchronizing the pulse-width modulation circuit. Thereby, synchronization of a plurality of DC-DC converters having no extra sync pin becomes possible. This is particularly advantageous in the case of the widely used quarter or eighth-brick converters having the standard pin out because employing the inventive devices does not require an extensive redesign of the usual converter circuits, neither concerning the electrical circuit nor the mechanical arrangement. Due to the added possibility of external synchronization a rather simple EMI filter may cope with the residual interference generated by a plurality of the inventive devices arranged in a parallel fashion. 
   Preferably, the device comprises a low pass filter connected between the trim connector and a compensation loop of the device in order to avoid disturbances of the compensation loop by the external synchronization signal. The low pass filter rejects the high frequency external synchronization component of the combined signal but is passed by the trim component, which is a substantially constant DC voltage. The filter is chosen to have a cut off frequency well below the frequency of the external oscillator and an attenuation that effectively suppresses the high frequency component. 
   Advantageously, the device comprises a buffer and level shift circuit connected between the trim connector and the pulse-width modulation circuit for processing the external synchronization signal, in particular for processing a rectangular external synchronization signal on a 5 Vp-p amplitude TTL level. The buffer and level shift circuit processes the raw synchronization signal in such a way that it is directly useable in the pulse-width modulation circuit. Thereby, a simple external oscillator that is easily commercially available and of low cost may be used, in particular one that generates a common rectangular 5 Vp-p amplitude signal according to the TTL (“Transistor-Transistor Logic”) industry standard. Such a signal is easy to transmit, in particular because it is not susceptible to noise, and easy to filter out from the combined trim/synchronization signal. 
   Alternatively, the processing of the synchronization signal happens outside of the device and the preprocessed signal is fed to the trim pin. Inside the device it is only filtered out from the combined trim/synchronization signal and delivered directly to the PWM circuit. 
   Preferably, the buffer and level shift circuit is designed and connected such that an oscillator unit of the pulse-width modulation circuit is controlled by an output signal of the buffer and level shift circuit delivered to an R/C (remote control) pin of the oscillator unit. The internal PWM will be locked up and change the converter&#39;s switching frequency from the free running frequency to the external synchronization signal frequency. This allows for using the commercially available oscillator units commonly used in DC-DC power converters, such as commercially available low power current mode push-pull PWM units. 
   Preferably, a coupling capacitor is connected between the trim pin and the buffer and level shift circuit. The capacitor filters out the DC (trim) component of the combined trim/synchronization signal and thereby prevents it from disturbing the buffer and level shift circuitry. The capacity is chosen such that the high frequency synchronization signal is efficiently separated from the low frequency trim component. 
   In a further exemplary preferred embodiment of the invention a DC-DC converting circuit comprises
         a) a plurality of DC-DC switching converters featuring trim connectors for adjusting output voltages of the converters, whereby the converters are connected such that they share a same input bus;   b) a system EMI (electromagnetic interference) filter common to all the DC-DC switching converters;   c) an external oscillator delivering an external synchronization signal to the plurality of DC-DC switching converters;   d) where the external oscillator is designed such that a frequency of the external synchronization signal is higher than a free running frequency of each of the plurality of DC-DC switching converters; and   e) where the external oscillator is connected to the trim connectors of the DC-DC switching converters.       

   Thereby, the switching frequencies of all the converters connected to the external oscillator are synchronized by the external synchronization signal that is delivered in phase to the trim connectors. Thereby, the frequency beating phenomena between different converters are eliminated. 
   Advantageously, the external oscillator is designed such that the external synchronization signal is rectangular, and in particular on a 5 Vp-p amplitude TTL level. Such a signal is easy to transmit and easy to filter out from the combined signal delivered to the trim connector. 
   Preferentially, coupling capacitors are connected between the external oscillator and the trim connectors of the DC-DC switching converters. Thereby, impact of the (substantially DC) trim voltage or other low-frequency interference on the external oscillator is avoided. 
   Preferably, the DC-DC converting circuit comprises at least one fixed delay cell, connected between the external oscillator and one of the plurality of DC-DC switching converters for allowing phase interleaving between different DC-DC switching converters. 
   This allows for delaying the external synchronization signal applied to one or more of the plurality of DC-DC switching converters, such that phase interleaving between different DC-DC converters is achieved. Due to the interleaving, ripple effects of the different frequency synchronized converters will not be additive, such that the amplitude of the electromagnetic interference to be filtered out by the system EMI filter is further reduced. 
   A method of converting a first DC voltage to a second DC voltage in accordance with a preferred exemplary embodiment of the invention comprises the steps of
         a) providing a plurality of DC-DC switching converters featuring trim connectors for adjusting output voltages of the converters; and connecting the converters to a same input bus;   b) connecting a system EMI (electromagnetic interference) filter to the input bus;   c) providing an external synchronization signal having a frequency that is higher than a free running frequency of each of the plurality of DC-DC switching converters; and   d) applying the external synchronization signal to the trim connectors of the DC-DC switching converters.       

   Other advantageous embodiments and combinations of features will be apparent from the detailed description below and the totality of the claims as may be further objects and advantages. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings used to explain the embodiments are: 
       FIG. 1A-C  Schematic representations of an industry standard quarter-brick DC-DC converter and of its trim functionality; 
       FIG. 2  a schematic representation of a plurality of prior art DC-DC converters arranged in parallel, sharing a common input bus and system EMI filter; 
       FIG. 3A-C  schematic representations of interference effects with DC-DC conversion using prior art switching converters; 
       FIG. 4  a schematic representation of a DC-DC converting circuit according to the invention; 
       FIG. 5  a schematic representation of another DC-DC converting circuit according to the invention, featuring delaying of the synchronization signal; 
       FIG. 6  a schematic representation of a DC-DC converter device according to the invention; 
       FIG. 7  a schematic representation of the buffer and level shift circuit of the inventive DC-DC converter device as well as of its connection to the internal oscillator unit; and 
       FIG. 8A-D  schematic representations of the waveform of the initial and processed synchronization signal at different stages of the processing. 
   

   In the figures, the same components are given the same reference numerals. 
   PREFERRED EMBODIMENTS 
   The  FIGS. 1A-C  are schematic representations of an industry standard quarter-brick DC-DC converter and of its trim functionality. The  FIG. 1A  shows the pin out of a standard quarter-brick DC-DC converter  1 . It features eight pins, namely two input pins  2 ,  3  indicated by V in (+) and V in (−) respectively, two output pins  4 ,  5  V out (+) and V out (−), two sense pins  6 ,  7 , denoted by Sense(+) and Sense(−), a control pin  8  (On/Off) and finally a trim pin  9  (Trim). The dimensions of a quarter-brick device are 37 mm×58 mm (1.45″×2.3″), its height usually amounts to about 13 mm or less. The pins are on a spacing of 51 mm (2″). The sense pins  6 ,  7  allow for sensing the line drop and accordingly compensating the output voltage. 
   Note, that the invention is as well applicable to eighth-brick DC-DC converters, having the same pin out as described above. Their standard size is 23 mm×58 mm (0.90″×2.3″) with a typical width of about 9 mm. Again, the pins are on a spacing of 51 mm (2″). 
   The trim functionality is schematically represented in  FIGS. 1B ,  1 C. A load  10  is connected to the output pins  4 ,  5  by output lines  11 ,  12 , and the sense lines  13 ,  14  are connected to the respective output lines  11 ,  12  close to the load  10 .  FIG. 1B  shows the “trim up” configuration where the output voltage is increased by connecting a trim up resistor  15  between the trim pin  9  and the sense line  13  connected to the Sense(+) pin  6 . Internally, the trim voltage applied to the trim pin  9  is suitably combined with a reference voltage and fed to an error amplifier where the combined voltage is compared to the actual output voltage. Depending on the output of the error amplifier the duty cycle of the power switch is adjusted in order to equal the actual output voltage to the desired voltage set by the trim and reference voltages. 
   Similarly,  FIG. 1C  shows the “trim down” configuration, where the output voltage is decreased by connecting a trim down resistor  16  between the trim pin  9  and the sense line  14  connected to the Sense(−) pin  7 . Generally, the voltage applied to the trim pin  9  is substantially DC referred to the V out (−) output pin  5 . Voltage trim ranges of typical quarter-brick DC-DC converters amount to about ±10-20%. 
   The  FIG. 2  is a schematic representation of a plurality of prior art DC-DC converters arranged in parallel, sharing a common input bus and system EMI filter. The converters  1 . 1 ,  1 . 2 , . . . ,  1 . n  are connected to the input bus  17  which in turn is connected to a power source via a system EMI (electromagnetic interference) filter  18 . The EMI filter  18  is common to all the converters  1 . 1 ,  1 . 2 , . . . ,  1 . n  and its purpose is to reject the interference they generate. Various loads are connected to the converters  1 . 1 ,  1 . 2 , . . . ,  1 . n  between the respective output pins  4 . 1 ,  5 . 1 ;  4 . 2 ,  5 . 2 ; . . . ;  4 . n ,  5 . n . Each of the converters  1 . 1 ,  1 . 2 , . . . ,  1 . n  may be individually trimmed by arranging trim up and/or down resistors  15 . 1 ,  16 . 1 ;  15 . 2 ,  16 . 2 ; . . . ;  15 . n ,  16 . n  between the trim pins  9 . 1 ,  9 . 2 , . . . ,  9 . n  and the respective positive and/or negative sense lines  13 . 1 ,  14 . 1 ;  13 . 2 ,  14 . 2 ; . . . ;  13 . n ,  14 . n.    
   The  FIGS. 3A-C  are schematic representations of interference effects with DC-DC conversion using prior art switching converters. The  FIG. 3A  shows input current ripple generated by a single DC-DC converter. The fixed high frequency of this kind of interference corresponds to the switching frequency. The current ripple is injected into the input system EMI filter where it has to be rejected as completely as possible. 
   The  FIG. 3B  shows a waveform which is typical for frequency beating phenomena between different DC-DC converters sharing the same input bus. The signal, having e.g. a frequency of about 25 kHz is substantially the difference between the different free running frequencies of the converters. Its amplitude is substantially larger than that of the high frequency current ripple. 
   In the  FIG. 3C  an additional interference of the frequency beating signal is additionally represented, namely a parasitic low frequency interference, the DC-DC converters introduce into each other. Including the beating of the ripple effects and this kind of interference the peak to peak amplitude of the total interference is about five times higher than just the input current ripple generated by a single converter. Therefore, a highly sophisticated system input EMI filter is required in order to effectively filter out the interference in the case of a plurality of parallel prior art DC-DC converters sharing a common input bus. 
   The  FIG. 4  is a schematic representation of a DC-DC converting circuit according to the invention. It comprises a plurality of the inventive DC-DC converters  101 . 1 ,  101 . 2 , . . . ,  101 . n  sharing a common input bus  117  and a common system EMI filter  118 . Again, various loads may be connected to the converters  101 . 1 ,  101 . 2 , . . . ,  101 . n  between the respective output pins  104 . 1 ,  105 . 1 ;  104 . 2 ,  105 . 2 ; . . . ;  104 . n ,  105 . n , and each of the converters  101 . 1 ,  101 . 2 , . . . ,  101 . n  may be individually trimmed by arranging trim up/down resistors  115 . 1 ,  116 . 1 ;  115 . 2 ,  116 . 2 ; . . . ;  115 . n ,  116 . n  between the trim pins  109 . 1 ,  109 . 2 , . . . ,  109 . n  and the respective positive and/or negative sense lines  113 . 1 ,  114 . 1 ;  113 . 2 ,  114 . 2 ; . . . ;  113 . n ,  114 . n.    
   The switching frequencies of the converters  101 . 1 ,  101 . 2 , . . . ,  101 . n  are synchronized by an external oscillator  119  connected to the trim pins  109 . 1 ,  109 . 2 , . . . ,  109 . n  of the converters  101 . 1 ,  101 . 2 , . . . ,  101 . n  via coupling capacitors  120 . 1 ,  120 . 2 , . . . ,  120 . n . The external oscillator  119  generates a synchronization signal  121  having a frequency that is higher than each converter&#39;s free running frequency and that is on a 5 Vp-p amplitude TTL level. The signal  121  is delivered in phase to all the converters  101 . 1 ,  101 . 2 , . . . ,  101 . n  where it is internally processed to lock up the internal pulse-width modulation circuit such that the free running frequency is changed to the external synchronization signal frequency. Thereby, the frequency beating phenomena and the higher interferences between the converters  101 . 1 ,  101 . 2 , . . . ,  101 . n  are substantially alleviated such that a usual, plain and low-cost EMI filter  118  suffices for filtering out the input interference of the entire DC-DC converter circuit. 
   The  FIG. 5  is a schematic representation of another DC-DC converting circuit according to the invention, featuring delaying of the synchronization signal. The configuration substantially corresponds to that in  FIG. 4 , however a plurality of fixed delay cells  122 . 1 ,  122 . 2 , . . . ,  122 .( n− 1) are arranged between the external oscillator  119  and the coupling capacitors  120 . 1 ,  120 . 2 , . . . by which the synchronization signal  121  is delivered to the trim pins  109 . 1 ,  109 . 2 , . . . ,  109 . n . This configuration allows phase interleaving between different DC-DC converters, whereby it is avoided that ripple effects of the frequency synchronized converters  101 . 1 ,  101 . 2 , . . . ,  101 . n  become additive. Otherwise, the frequency synchronization of the converters  101 . 1 ,  101 . 2 , . . . ,  101 . n  could lead to increased input current ripple due to positive interference between the noise signals generated by the single converters  101 . 1 ,  101 . 2 , . . . ,  101 . n  which could partly vitiate the desired effects of the synchronization. 
   The delay imposed upon the signal  121  by each of the delay cells  122 . 1 ,  122 . 2 , . . . ,  122 .( n− 1) is chosen such that the resulting constant phase differences suppress the occurrence of constructive interference and preferably even lead to destructive interference of the current ripple. The configuration of the delay cells may be chosen as represented in  FIG. 5  by serially arranging the cells along the synchronization signal bus, or the external oscillator may be star connected to the trim pins, whereby in each of the connections a single delay cell is arranged, the different cells imposing different delays to the synchronization signal. 
   The  FIG. 6  is a schematic representation of a DC-DC converter device according to the invention; of the internal circuits only a few components relevant to the invention are represented. For trim functionality, the trim pin  109  of the DC-DC converter  101  is internally connected to an error amplifier  123  of the converter&#39;s voltage compensation loop. The error amplifier  123  serves for comparing a desired voltage to the actual output voltage between the output pins  104 ,  105  or the sense lines  113 ,  114  respectively. An internal voltage reference V ref  is provided for the purpose of voltage compensation. Without external trim up/down resistors  115 ,  116 , the DC voltage applied on the minus pin of the error amplifier  123  is 1.24 V with ±1% accuracy, whereby the source of the voltage reference V ref  is connected to the minus pin through serially arranged internal resistors  124 ,  125 . A by-pass capacitor  126  is connected between the two internal resistors  124 ,  125  and diverts high-frequency noise to ground potential, in order to suppress noise on the minus pin of the error amplifier  123 . 
   The DC output voltage of the converter  101  may be adjusted by the user through utilizing external trim up and/or down resistors  115 ,  116  as described above. The middle point between the internal resistors  124 ,  125  and thus accordingly the potential on the minus pin of the error amplifier  123  is tuned by externally adjusting the trim DC voltage by providing corresponding trim resistors  115 ,  116  (which may be potentiometers for easier adjustment). 
   To avoid the internal compensation loop being disturbed by the synchronization signal which is also applied on the trim pin  109 , a low pass filter  127  is provided. The first cell of the low pass filter  127  is constituted by the by-pass capacitor  126  and a further internal resistor  128 , connected between the trim pin  109  and the connection between the other two internal resistors  124 ,  125 . The second cell is built up by the internal resistor  125  connected to the minus pin of the error amplifier  123 , by a further capacitor  129  connected between the minus pin and the output of the error amplifier  123  and by the error amplifier  123  itself. Altogether, the low-pass filter  127  has a cut-off frequency of about 6 kHz, and it provides an attenuation of about −40 dB/decade. As a result, the common mode signal present at the differential input of the error amplifier  123  is reduced below 50 mVp-p. Furthermore, the low pass filter  127  helps for rejecting noise delivered to and from the trim pin  109 . 
   For providing the synchronization functionality, a buffer and level shift circuit  131  is connected to the trim pin  109  via a coupling capacitor  130 . The buffer and level shift circuit  131  processes the external synchronization signal and delivers the processed signal to the converter&#39;s pulse-width modulation (PWM) circuit  132 . The coupling capacitor  130  rejects low frequency components of the combined trim/synchronization signal, i.e. primarily the DC trim voltage. Thereby the buffer and level shift circuit  131  is not disturbed by the trim component of the combined signal. 
   The  FIG. 7  is a schematic representation of the buffer and level shift circuit of the inventive DC-DC converter device as well as of its connection to the internal oscillator unit. The buffer and level shift circuit  131  is arranged internally of the DC-DC converter  101  and connected to the trim pin  109  via the entrance coupling capacitor  130 . The base of a pnp transistor  133  is connected to the input of the buffer and level shift circuit  131  via a resistor  134 . The collector of the transistor  133  is connected with the RC pin (No. 6) of an oscillator unit  135  via a high-speed switching diode  136 . In the example represented in  FIG. 7 , the oscillator unit  135  is a low power current mode push-pull PWM of the type UCC 2808A-2PW (available from Texas Instruments/Unitrode Products) having the following pin assignment: 
   
     
       
         
             
             
             
           
             
                 
             
             
               pin 
               name 
               function 
             
             
                 
             
           
          
             
               1 
               OUTA 
               output #1 
             
             
               2 
               VDD 
               power input 
             
             
               3 
               COMP 
               output of error amplifier/input of PWM comparator 
             
             
               4 
               FB 
               inverting input to error amplifier 
             
             
               5 
               CS 
               input to PWM, peak current and overcurrent comparators 
             
             
               6 
               RC 
               oscillator programming pin 
             
             
               7 
               GND 
               ground 
             
             
               8 
               OUTB 
               output #2 
             
             
                 
             
          
         
       
     
   
   The buffer and level shift circuit  131  further features a resistor  137  connected between the collector of transistor  133  and ground, as well as a resistor  138  connected between the base and the emitter of transistor  133 . Power is supplied from a power input  139  to the emitter of the transistor  133 , whereby a grounded by-pass capacitor  140  is as well connected to the emitter for suppressing high-frequency noise. 
   Another grounded by-pass capacitor  141  is connected to the RC pin of the oscillator unit  135 . A resistor  142  is connected between the RC pin and the power input  139 . In the represented example the above mentioned resistors and capacitors may have the following parameters: 
   
     
       
         
             
             
             
             
           
             
                 
             
             
               resistors 
               resistance 
               capacitors 
               capacity 
             
             
                 
             
           
          
             
               134 
               1.21 kΩ 
               130 
                56 nF 
             
             
               137 
                 10 kΩ 
               140 
               330 nF 
             
             
               138 
               1.21 kΩ 
               141 
               100 pF 
             
             
               142 
                 20 kΩ 
             
             
                 
             
          
         
       
     
   
   The further circuitry represented in  FIG. 7  further connects the oscillator unit  135  to the power input  139 , the voltage loop  143 , the comparator  144  and to output circuits  145 ,  146 . The corresponding configuration is predetermined by the type of oscillator unit used. 
   The  FIGS. 8A-D  are schematic representations of the waveform of the initial and processed synchronization signal at different stages of the processing. The places where the represented voltages V 1  . . . V 4  appear in the circuit are marked in  FIG. 7 . The  FIG. 8A  represents the initial rectangular synchronization signal having a duty cycle of 50% and an amplitude of 5 Vp-p TTL as it is delivered by the external oscillator  119 . 
   The  FIG. 8B  represents the differentiated double polarity AC signal produced by the entrance coupling capacitor  130  of the buffer and level shift circuit  131 . This signal is used for controlling the transistor  133  where the positive component is chopped off and where the negative component is inverted and amplified to yield the signal represented in  FIG. 8C , having an amplitude of again 5 V. The signal finally delivered to the RC pin of the oscillator unit  135  is depicted in  FIG. 8D . It is linearly rising until it reaches a potential of about 4 V, another linear rise follows, having a steeper slope until the amplitude of 5 V is reached. Subsequently, the signal decays linearly and rapidly until the 0 V potential is reached whereupon the first linear rise starts anew. The frequency of the peaks equals the frequency of the initial rectangular signal. 
   The invention is applicable to DC-DC switching converters that are not in a quarter or eighth-brick package but that nevertheless feature a trim pin and do not allow for an extra synchronization pin. The details of the implementation of the invention, especially concerning the low-pass filter arranged in front of the compensation loop and the buffer and level shift circuit, may be different from the described example. In particular, they may depend from the overall design and from the components used for the DC-DC converter circuitry. 
   In summary, it is to be noted that the invention creates a DC-DC switching converter device, that allows for the reduction of noise in the case of a plurality of DC-DC converters arranged in parallel and sharing a common input bus and EMI filter, the device being cost-efficient and easy to implement into usual converter circuits. The invention further creates a DC-DC converting circuit comprising a plurality of DC-DC converters where the generation of noise is highly suppressed. 
   Whereas particular preferred exemplary embodiments of the invention have been described, these should not be construed as limiting, since modifications, additions and revisions may be made, as will be apparent to one of ordinary skill in the art without departure from the spirit and scope of the invention as set forth in the appended claims.