Abstract:
Methods and apparatus are provided for driving a power amplifier load, such as a loudspeaker. The apparatus comprises a processor-controlled drive assembly configured as an H-bridge system. Groupings of half-bridge assemblies are connected to the load in an H-bridge structure, and are operated in pulse-width-modulation mode by the processor. The half-bridge assembly groupings receive time sliced commands from the processor in order to output a composite output signal to the load at a frequency higher than the operating frequency of a single half-bridge assembly. In general, the composite output frequency is the product of the individual half-bridge assembly operating frequency and the number of half-bridge assemblies in a grouping. As such, high frequency and high power output levels can be achieved using standard low-cost components.

Description:
TECHNICAL FIELD 
   The present invention generally relates to power amplifiers, and more particularly relates to high frequency high power audio amplifiers. 
   BACKGROUND 
   Power amplifiers are used in many application areas, including industrial and consumer electronics. One such application involves the amplification of audio power supplied to output devices, such as loudspeakers. For many low power applications, field effect transistors (FET&#39;s) are typically used in amplifier circuits due to their relatively high-speed switching capability. However, FET&#39;s generally become power limited in applications where the operating voltage is in the range of approximately 200 volts or higher. 
   One electronic switching device capable of higher power operation (i.e., in excess of 200 volts) is the insulated gate bipolar transistor (IGBT). While the IGBT can be used at power levels in the kilowatt range, the device has a relatively slow switching speed, with a typical upper frequency limit of approximately 20 kHz. As such, conventional power amplifier circuits using IGBT&#39;s would generally be limited in frequency to approximately 20 kHz. 
   Other switching devices having higher power and frequency capabilities may be considered for this type of power amplifier application, but the cost of such devices is typically many times higher than the cost of a standard IGBT, making the high-cost devices generally undesirable for production applications. Furthermore, as technology advances, it can be assumed that there will be an ongoing demand for power amplifiers that are capable of operating at higher frequencies and higher power levels. For example, there are current applications for a loudspeaker power amplifier requiring an operating frequency in the range of 60 kHz, with a minimum power level of 8 kilowatts. Therefore, there is a need for an audio power amplifier system capable of operating at high frequency and high power levels. Moreover, this type of power amplifier could be suitable for production applications if the switching elements were standard low-cost components, such as IGBT&#39;s. 
   Accordingly, it is desirable to provide a power amplifier system for applications requiring high (audio) frequency and high power capabilities. In addition, it is desirable to implement the power amplifier system with low-cost components for production applications. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
   BRIEF SUMMARY 
   According to various exemplary embodiments, devices and methods are provided for frequency scaling the output of a pulse-width-modulated (PWM) power amplifier drive assembly configured as an H-bridge system. One method comprises the step of sequentially activating a grouping of half-bridge assemblies in the H-bridge system to generate a composite output signal to a load, where the frequency of the composite output signal is greater than the frequency capability of a single one of the half-bridge assemblies. In general, the frequency of the composite output signal is equal to the product of the frequency capability of a single one of the half-bridge assemblies and the number of half-bridge assemblies in a grouping. 
   One exemplary embodiment comprises a control system for scaling the pulse-width-modulated (PWM) frequency output of a power amplifier connected to a load. The control system includes a drive assembly configured as an H-bridge structure electrically connected to the load. The drive assembly is configured as a number of half-bridge assembly groupings, with each half-bridge assembly in a grouping containing switching elements having a maximum individual operating frequency. The switching elements are controlled by a processor that provides time sliced commands in a manner that enables the composite output signal frequency from the half-bridge assembly grouping to be greater than the maximum operating frequency of an individual switching element. Typically, the control system is configured so that the composite output frequency is equal to the product of the individual switching element operating frequency and the number of half-bridge assemblies in a grouping electrically connected to the load. 
   To achieve high power levels in conjunction with frequency scaling, devices such as IGBT&#39;s can be used for the switching elements in half-bridge assembly groupings. Thus, an exemplary embodiment of half-bridge assemblies using IGBT switching elements in an H-bridge configuration can achieve high power amplification in the audio frequency range. A further benefit of using IGBT&#39;s in production applications is that they are typically available commercially as standard low-cost components. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
       FIG. 1  is a block diagram of an exemplary control system for a power amplifier; 
       FIG. 2  is a block diagram of an exemplary H-bridge structure; 
       FIG. 3  is a schematic diagram of an exemplary H-bridge structure; 
       FIG. 4  is a timing diagram for the exemplary H-bridge structure of  FIG. 3 ; 
       FIG. 5  is a block diagram of an exemplary composite H-bridge structure; 
       FIG. 6  is a schematic diagram of an exemplary composite H-bridge structure; and 
       FIG. 7  is a timing diagram for the exemplary composite H-bridge structure of  FIG. 6 . 
   

   DETAILED DESCRIPTION 
   The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
   Various embodiments of the present invention pertain to the area of scaling the frequency capability of a power amplifier connected to a load, such as a loudspeaker system. Combining multiple groupings of half-bridge assemblies in an H-bridge configuration enables the composite output frequency of the power amplifier to exceed the frequency capability of the individual switching elements used in the half-bridge assemblies. As such, frequency scaling of the power amplifier can be achieved with standard low-cost components. 
   According to an exemplary embodiment of a system  100  for scaling the frequency capability of a power amplifier  104  connected to a load  110 , as shown in  FIG. 1 , a processing element  108  provides command signals to a drive assembly  106  that is in electrical communication with the load  110 . Processing element  108  may be any type of microprocessor, micro-controller, or other computing device capable of executing instructions in any computing language. A conventional power supply  102  provides power for the various components of power amplifier  104 . Drive assembly  106  is generally configured to produce pulse-width-modulated (PWM) output signals to produce a desired current relationship in load  110 . 
   An exemplary embodiment of a basic power amplifier configuration is shown in  FIG. 2 . In this embodiment, drive assembly  106  is configured as an H-bridge structure with two half-bridge assemblies  202  and  204 . This configuration does not include frequency scalability, since the PWM output frequency to load  110  from drive assembly  106  would typically be limited to the maximum frequency capability of each half-bridge assembly ( 202 ,  204 ). 
   The H-bridge structure of  FIG. 2  is shown schematically in  FIG. 3 , where each half-bridge assembly  202 ,  204  is comprised of two switching elements  302 . Typically, switching elements  302  are triggered by commands from a controller, such as processing element  108  in  FIG. 1 , in order to produce a PWM output signal to load  110 . The timing diagrams in  FIG. 4  show a typical one-to-one frequency relationship between the individual switching elements  302  and the PWM output signal. 
   As shown in  FIG. 4 , lines a and b represent a typical half-bridge assembly timing arrangement, with an individual switching element operating frequency of f s . The resultant output, as shown in line c, has an effective frequency f p , which is essentially equal to f s . 
   As previously noted in the Background section, field effect transistors (FET&#39;s) are often used as switching elements for high frequency (i.e., audio) applications, and are depicted schematically in  FIG. 3 . However, FET&#39;s are typically limited to an operating voltage range of approximately 200 volts, which makes them generally unsuitable for higher power applications. One type of switching element that is capable of higher power operation is the insulated gate bipolar transistor (IGBT). Due to its insulated gate structure, however, the IGBT is generally limited in switching speed to a range of approximately 20 kHz. Therefore, to achieve the previously stated objectives of high frequency and high power, a multiple configuration of IGBT half-bridge assemblies can be structured to provide frequency scaling in an H-bridge arrangement. 
   Referring now to  FIG. 5 , an exemplary embodiment of a frequency scaled H-bridge structure is shown in block diagram form. In this embodiment, drive assembly  106  includes two groupings ( 501 ,  507 ) of three half-bridge assemblies ( 502 ,  504 ,  506 , and  508 ,  510 ,  512 , respectively) in electrical communication with load  110 . Each grouping  501 ,  507  is connected to a node ( 503 ,  509 , respectively), and the nodes  503 ,  509  connect each grouping  501 ,  507  of respective half-bridge assemblies to opposite sides of load  110 . As such, each grouping  501 ,  507  can provide the current of a single half-bridge assembly in the H-bridge structure, which is typically equivalent to the maximum continuous current capability of the individual switching elements in the half-bridge assemblies. In order to achieve the desired objective of frequency scaling, each half-bridge assembly ( 502 ,  504 ,  506 ,  508 ,  510 ,  512 ) in the exemplary configuration can be operated at a third of the desired output frequency to load  110 , by time slicing the commands to each half-bridge assembly in the groupings  501 ,  507 , as will be described below. 
   A schematic representation of the block diagram of  FIG. 5  is shown in  FIG. 6 . In this exemplary embodiment, each half-bridge assembly ( 502 ,  504 ,  506 ,  508 ,  510 ,  512 ) is comprised of a pair of switching elements  602 , which are depicted as IGBT&#39;s. As noted above, IGBT&#39;s can provide operational power capability at voltages in excess of 200 volts, but their switching speed is typically limited to 20 khz. With the exemplary multiple half-bridge assembly configurations shown in  FIGS. 5 and 6 , however, the output frequency to load  110  can be effectively three times the switching frequency of each half-bridge assembly, so that an output frequency of approximately 60 kHz can be achieved. While IGBT&#39;s are indicated herein as appropriate components for high power, high frequency applications, any suitable alternative device can be used, depending on the particular application requirements. For example, devices such as thyristors or mercury valves, among others, may also be considered as switching element possibilities for a high power frequency scaled type of H-bridge structure power amplifier. 
   The basic operation of the exemplary multiple half-bridge assembly configuration of  FIG. 6  is illustrated in the timing diagrams of  FIG. 7 . In the exemplary embodiment of  FIG. 6 , there are three half-bridge assemblies connected to a respective output node (e.g.,  502 ,  504 ,  506  to node  503 , and  508 ,  510 ,  512  to node  509 ) to provide a PWM output signal to load  110 . The half-bridge assembly switching elements  602  are typically triggered by commands from a controller, such as processing element  108  in  FIG. 1 , in order to produce a PWM output signal to load  110 . The 12 command signals used in this exemplary embodiment can be distributed to the 12 switching elements  602  via a device such as a Field Programmable Gate Array (FPGA), or the like. Other embodiments of half-bridge assemblies in H-bridge structures can be configured in various alternate but equivalent ways without departing from the general concepts set forth herein. 
   As illustrated in  FIG. 7 , the first and second switching elements  602  of each half-bridge assembly are operated in the same manner as previously described for a single half-bridge assembly  202 ,  204  in  FIG. 4 , where f s  is the operating frequency of each switching element  602 . However, in this exemplary embodiment, the switching elements of each half-bridge assembly are staggered in time relative to the switching elements in the other half-bridge assemblies of that grouping, as indicated in timing diagrams a through f. As a result, the composite frequency f p , as shown in timing diagram g, is typically generated from each grouping ( 501 ,  507 ) of three half-bridge assemblies ( 502 ,  504 ,  506 , and  508 ,  510 ,  512 , respectively). In this embodiment, therefore, it follows that f p =3×f s . Accordingly, if IGBT&#39;s are used as switching elements for this exemplary embodiment, a maximum frequency of approximately (3×20)=60 kHz can be achieved. 
   Moreover, the efficiency of a typical switching element can generally be increased if it is operated below its maximum frequency. As such, a grouping of half-bridge assemblies can be operated below the maximum switching frequency of its switching elements while still achieving a higher composite output frequency, since the output frequency is typically the product of the switching element operating frequency and the number of half-bridge assemblies in a grouping. Accordingly, the exemplary embodiments of  FIGS. 5 and 6  can be used for both frequency scaling and for efficiency improvement. 
   The various examples disclosed herein are merely illustrative of various methods of arranging the half-bridge assemblies. In this regard, the half-bridge assemblies can be physically located in any one of a number of different manners with respect to one another. Moreover, various multiple half-bridge assembly configurations are available commercially, such as the model 4357 3-phase motor drive including three half-bridge assemblies, manufactured by M.S. Kennedy Corp. of Liverpool, N.Y. 
   Accordingly, the shortcomings of the prior art have been overcome by providing an improved method and apparatus for scaling the frequency of a high power amplifier. Sequential triggering of multiple half-bridge assemblies configured as groupings in an H-bridge structure enables the frequency scaling of a composite output signal to a load. Moreover, the disclosed exemplary frequency scaling technique allows the use of standard low-cost components for the switching elements in the half-bridge assemblies, and also enables the switching elements to operate at relatively high efficiencies. 
   While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.