Abstract:
A system and method for operating an inverted-based power source includes a power input configured to receive alternating current (AC) power and a rectifier configured to convert the AC power to direct current (DC) power. The inverter-based power source also includes an inverter configured to receive the DC power from the rectifier and convert the DC power to AC power and a controller configured to generate switching signals according to a pattern of offsets from a regular half period and communicate the switching signals to the inverter or rectifier control operation of the inverter or rectifier.

Description:
REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a welding-type system and, more particularly, to a system and method for controlling the harmonics injected onto an input power line during the operation of a welding-type system. 
     Welding-type systems, such as welders, plasma cutters, and induction heaters, often include an inverter-based power source that is designed to condition high power to carrying out a desired process. These inverter-based power sources, often referred to as switched-mode power supplies, can take many forms. For example, they may include a half-bridge inverter topology, a full-bridge inverter topology, a forward-converter topology, a flyback topology, a boost-converter topology, a buck-converter topology, and combinations thereof. 
     Regardless of the specific inverter topology employed, referring to  FIG. 1 , an inverter-based power source systems  10  typically includes a variety of components, such as an input filter  12 , first rectifier  14 , an inverter  16 , an analog controller  18 , and a second rectifier  20  that is connected to an output  22 . While  FIG. 1  is a simplified overview of common components of an inverter-based power source system  10 , it is contemplated that additional components may be included, such as filtering components, feedback and control loops, and transformers or other converters designed to provide a desired output power characteristic. 
     During operation, the system  10  is connected to a supply of power  24  that provides alternating-current (AC) power, for example, as received from a utility grid over transmission power lines  26 . The rectifier  12  is designed to receive the AC power from the supply of power  24  and convert the AC power to DC power that is delivered to a DC bus  28 . Specifically, the rectifier  20  includes a plurality of switches that rectify the AC power received from the supply of power  24 . 
     The DC power is delivered from the rectifier  12  over the DC bus  28  to the inverter  16 . The inverter  16  includes a plurality of switching devices (e.g., IGBTs or other semiconductor switches) that are positioned between the positive and negative buses  28 . The inverter  16  is controlled by the analog controller  18  to open and close specific combinations of the switches to sequentially generate pulses that are delivered to the second rectifier  20  and, ultimately, to the output  22  with the desired voltage and current characteristics. Specifically, the above-described inverter-based power source system  10  is specifically designed for delivering high-power to the output  22  to drive a process such as welding, plasma cutting, and induction heating. 
     As described, inverter-based power sources include at least one active switching device, the inverter  16 . The switching characteristics of the inverter  16  are controlled by the analog controller  18  to, along with the second rectifier  20 , produce the desired output power having the desired voltage and current characteristics. 
     Specifically, referring now to  FIG. 2 , the components of a half-bridge inverter and associated analog controller are shown. However, as addressed above, it is contemplated that additional topologies, such as a full-bridge inverter topology, a forward-converter topology, a flyback topology, a boost-converter topology, a buck-converter topology, and combinations thereof, may be employed. Furthermore, it is contemplated that additional components, such as transformers and various power conditioning components, are typically employed but have not been shown in order to simply the illustrated inverter configuration. 
     The analog controller  18  typically includes a waveform generator  30  and a comparator  32 . The waveform generator  30  generates a carrier signal or waveform having a first frequency and a first period that is passed to the comparator  32  to be compared to a sinusoidal command or modulating voltage waveform having a second, typically lower, frequency and a second, typically longer, period. Responsive thereto, the comparator  32  generates a first trigger signal and a second trigger signal corresponding to a comparison and identification of intersections of the command waveform and carrier waveform. The first trigger signal is provided to a first or “upper” switch  34  of the inverter  16  and the second trigger signal is provided to a second or “lower” switch  36  of the inverter  16 . In this regard, the switches  34 ,  36  of the inverter  16  are caused to open and close in alternating fashion to generate a high-frequency AC signal that is provided to the rectifier  20  to produce the desired output power. 
     Therefore, the comparator  32  generally functions by comparing a time varying analog signal to a ramp type signal, to generate timing pulses to the switches  34 ,  36  of fixed frequency but with variable pulse width or “ON” time. In this regard, the comparator  32  controls the pulse width modulation (PWM) or the ON/OFF ratio of the switches  34 ,  36  to effectively control the output voltage and/or current as required by a feedback control loop and a commanded output level. 
     Unfortunately, the operation of active switching devices, such as the inverter  16 , can inject high-frequency harmonics onto the power lines  26 . These injected harmonics can adversely affect operation of other systems connected to the supply of power  24 . Additionally, the rectifiers  14 ,  20  may be actively controlled as well and may, likewise, inject harmonics onto the power lines  26 . 
     As a result, one or more filters  12  are often arranged between the supply of power  24  and the inverter-based power source  10 . For example, the filter  12  may include passive filter components designed to suppress the harmonics injected onto the power lines  26  by operation of the inverter-based power source  10 . 
     However, as power regulations become more and more stringent, the amount of high-frequency harmonics tolerated under the regulations decrease. Furthermore, since these regulations vary between countries, the amount and concentration of harmonics tolerated under such regulations varies by country. 
     Therefore, it would be desirable to have a system and method for accurately controlling the harmonics injected onto power supply lines during the operation of welding-type systems. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention overcomes the aforementioned drawbacks by providing a system and method for controlling the half period and frequency of switching signals generated by an actively controlled switching device in an inverter-based power source. Specifically, the present invention is designed to control the amplitude of harmonic peaks injected onto an input power line by an inverter-based, welding-type device employing a digital control system. 
     In accordance with one aspect of the present invention, an inverter-based power source is disclosed that includes a power input configured to receive alternating current (AC) power, and a rectifier configured to convert the AC power to direct current (DC) power. The inverter-based power source also includes an inverter configured to receive the DC power from the rectifier and convert the DC power to AC power and a controller configured to generate switching signals according to a pattern of offsets from a regular half period and communicate the switching signals to the inverter or rectifier control operation of the inverter or rectifier. 
     In accordance with another aspect of the present invention, an inverter-based power source is disclosed that includes a power input configured to receive AC power, a rectifier configured to convert the AC power to DC power, and an inverter having a plurality of switches configured to receive the DC power from the rectifier and convert the DC power to AC power. The inverter-based power source also includes a clock configured to generate a clock signal having a regular frequency and a set of programmable logic configured to receive the clock signal and generate a series of control signals based on the clock signal configured to cause the plurality of switches to switch at an irregular frequency. 
     In accordance with yet another aspect of the present invention, a welding-type device is disclosed that includes a power input configured to receive AC power, a first rectifier configured to convert the AC power to DC power, and an inverter configured to receive the DC power from the rectifier. The welding-type device also includes a digital controller configured to generate switching signals according to a pattern of offsets from a regular half period and communicate the switching signals the inverter to control the inverter to convert the DC power to AC power and a second rectifier configured to receive the AC power from the inverter and convert the AC power to a welding-type power. 
     Various other features of the present invention will be made apparent from the following detailed description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
         FIG. 1  is a schematic illustration of an inverter-based power source of a welding-type device and associated connections; 
         FIG. 2  is a schematic illustration of the inverter and analog controller of  FIG. 1 ; 
         FIG. 3  is a schematic illustration of an inverter-based power source of a welding-type device including an inverter and discrete controller in accordance with the present invention; 
         FIG. 4  is a schematic illustration of a switching control algorithm in accordance with the present invention; and 
         FIG. 5  is a schematic illustration of another switching control algorithm in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 3 , the analog controller  18  of  FIGS. 1 and 2  have been replaced with a discrete, programmable controller  40 . Again, the components of a half-bridge inverter and associated analog controller are shown. However, as addressed above, it is contemplated that additional topologies, such as a full-bridge inverter topology, a forward-converter topology, a flyback topology, a boost-converter topology, a buck-converter topology, and combinations thereof, may be employed. Furthermore, it is contemplated that additional components, such as transformers and various power conditioning components, are typically employed but have not been shown in order to simply the illustrated inverter configuration. Furthermore, although explanation of the present invention will be made with respect to controlling the switching of an inverter, it is contemplated that the present invention is equally applicable to actively controlling other switching devices other than inverters. For example, as will be described, the present invention is equivalently applicable to controlling operation of active rectifiers, boost and/or buck converters, and the like. 
     The programmable controller  40  includes a clock  42  and a set of programmable logic  44 . In accordance with one embodiment, the programmable logic  44  includes a field-programmable gate array (FPGA), but may include a wide variety of other programmable logic systems. 
     The clock  42  is configured to generate a regular, periodic clock signal that is delivered to the programmable logic  44 . For example, in accordance with one embodiment, the clock  42  may generate a clock signal having a period of 12.5 nanoseconds (ns). The programmable logic  44  receives the clock signal and, in a manner similar to the above-described analog controller  18 , generates alternating switching signals to the first switch  34  and the second switch  36  to convert the DC power received from the rectifier  14  to AC power. However, unlike the above-described analog controller  18 , the discrete controller  40  is designed for extremely accurate switching frequency control. 
     Specifically, the discrete controller  40  is inherently capable of generating the alternating switching signals with a very high degree of precision and repeatability that an analog controller  18  is incapable of achieving. While this high degree of precision is generally advantageous, the highly periodic switching of the switches  34 ,  36  results in precise alignment of harmonics associated with the fundamental switching frequency that can stretch well into the megahertz (MHz) range. 
     Referring now to  FIGS. 3 and 4 , the present invention provides a system and method for controlling the harmonics associated with the fundamental switching frequency of an inverter-based power source  10  employing a discrete controller  40 . Specifically, as illustrated in  FIG. 4 , the discrete controller  40  is configured to generate a series of switching signals that alternate to cause the first switch  34  to switch and then the second switch  36  to switch and so on. In this regard, the switching signals have a period, T, that extends between switching signals for one switch  46 ,  48 ,  50  and a half period, ½ T, that extends between the switching signals  46 ,  48 ,  50  for the first switch  34  and the switching signals  52 ,  54 ,  56  for the second switch  36 . 
     Since these switching signals  46 - 56  are generated by the programmable logic  44  based on the clock signal received from the clock  42 , they have a highly regular period and half period. For example, if the discrete controller  40  is selected to operate at a frequency of 40 kilohertz (kHz), the fundamental switching period, T, is 25 microseconds (μs) and the half period, ½ T, is 12.5 μs. Using a clock period of 12.5 ns means that the switching signals  46 - 56  are generated every 12.5 μs with an accuracy of greater than 12.5 ns. Unfortunately, as addressed above, this high degree of accuracy aligns the harmonics generated by the switching well into the MHz range, for example, at the 428 th  and 636 th  harmonic of the base frequency of 40 kHz. 
     To overcome this concentration of harmonics, the present invention introduces a dither or offset into the each half period. Specifically, the programmable logic  44  is configured to include a pattern of offsets from the regular half-period calculation for generating the switching signals  46 - 56 . When creating a pattern of offsets or algorithm for generating the offsets, the offset selection must be selected to avoid introducing a long-term DC component into the primary switching waveform that would cause transformer saturation. As will be described, to do so, the offset pattern includes an odd number of offsets or steps. Additionally, when creating a pattern of offsets or algorithm for generating the offsets, the offset selection must be selected to avoid introducing an audible tone in the open arc generated by the inverter-based power source  10 , such as when using the inverter-based power source to perform a welding process. As will be shown below, to do so, the offset pattern is asymmetric. However, in some cases, it is contemplated the offset pattern may be symmetric. For example, when employing an operational frequency significantly greater than 40 kHz, such as 100 kHz, a symmetric offset pattern may be utilized without inducing substantial audible tones in the output power. 
     Referring to  FIG. 4 , a three-step, asymmetric, offset pattern is utilized to shift the switching signals  46 - 56 . Specifically, the three-step, asymmetric, offset pattern takes the form of a −1, 0, 1 pattern. That is, the first switching signal  46  is shifted back by one clock signal, in this example, 12.5 ns, represented by  46 ′. Following the three-step, asymmetric, offset pattern of −1, 0, 1, the next switching signal  52  is shifted by an offset of 0, and, therefore, remains unchanged. The following switching signal  48  is shifted forward by an offset of 1, represented by  48 ′. Thereafter, the −1, 0, 1 pattern repeats asymmetrically, such that the following switching signal  54  is shifted back by one clock signal, represented by  54 ′. The next switching signal  50  remains unchanged and the following switching signal  56  is shifted forward by one clock signal, represented by  56 ′. 
     The shift of one clock signal (i.e., 12.5 ns) when using a half period of 12.5 μs is nominal. However, it is sufficient to spread the spectrum of harmonics and avoid the focusing of harmonics to create a highly pronounced peak. Furthermore, using an asymmetric offset pattern (e.g., −1, 0, 1, −1, 0, 1) as opposed to a symmetrical offset pattern (e.g., −1, 0, 1, 0, −1) causes a higher “ripple” frequency that is less likely to result in a sub-harmonic tone appearing in the output power signal, for example, in a welding arc generated by the inverter-based power source  10 . 
     Referring now to  FIG. 5 , it is contemplated that other offset patterns may be employed. Specifically,  FIG. 5  illustrates the switching signals for a five-step, asymmetric offset sequence of −2, −1, 0, 1, 2, −2, and so on. In this case, a first switching signal  58  is shifted back two clock signals ( 58 ′), a second switching signal  60  is shifted back one clock signal ( 60 ′), a third switching signal  62  is not shifted, a fourth switching signal  64  is shifted forward by one clock signal ( 64 ′), a fifth switching signal  66  is shifted forward by two clock signals ( 66 ′), a sixth switching signal  68  is back two clock signals ( 68 ′), and so on. 
     While both the three-step, asymmetric offset pattern of  FIG. 4  and the five-step, asymmetric offset pattern of  FIG. 5  are suitable, in some cases, the five-step, asymmetric offset pattern of  FIG. 5  is more effective at distributing the harmonics. Furthermore, while three- and five-step offset patterns were described above, it is contemplated that more than five steps may be utilized. Additionally, while a base offset size of one clock signal is used in both of the above examples, it is contemplated that larger base offsets of multiple clock signals may be utilized. Further still, it is contemplated that the modulation or offset may be selected not based on steps formed by the clock signal but based on steps formed as a percentage of the switching frequency. As described above, the modulation of the fundamental switching frequency may follow a pattern, for example, a linear pattern or the asymmetric pattern. However, it is contemplated that the modulation of the switching frequency may be randomized. 
     Though the present invention has been described with respect to a controlling a half-bridge inverter, it is contemplated that the present invention is applicable to controlling other inverter topologies and/or components other than the inverter. For example, it is contemplated that additional inverter topologies, such as a full-bridge inverter topology, a forward-converter topology, a flyback topology, a boost-converter topology, a buck-converter topology, and the like, may be controlled using the above-described invention to control the harmonics injected by the active switching of such devices. Additionally, while the above-described system and method is effective at controlling injected harmonics attributed to inverter switching, it is contemplated that these systems and methods may also be applied to rectifier switching to control the concentration of harmonics associated with the fundamental switching frequency of an actively controlled rectifier. 
     Similarly, referring again to  FIG. 3 , many inverter-based welding-type power sources include an additional power processing stage located between the rectifier  14  and the inverter  16 . This additional power processing stage is typically referred to a “preregulator,” and is usually implemented as a boost converter. Similar to the inverter  14 , the boost converter has an active switch that is controlled by the controller  40  or another controller. It is contemplated that the boost converter could be operated in a fixed-frequency modulation mode. In this case, the above-described dithering techniques can be utilized to reduce harmonics injected by the operation of the boost converter of the preregulator or other similar system. In addition, it is contemplated that the above-described systems and methods may be combined with traditional filtering, grounding, loop minimization, and other harmonic control techniques. 
     Therefore, the above-described system and method is capable of controlling the half period and frequency of switching signals generated by an actively controlled switching device in an inverter-based power source. A pattern of offsets from a regular half period is used to switch the actively controlled switching device at an irregular frequency. The pattern of offsets is selected to unfocus harmonics injected at the power input due to switching the actively controlled switching device and generate an output power signal substantially free of audible tones. As such, the present invention is designed to control the amplitude of harmonic peaks injected onto an input power line by an inverter-based welding-type device employing a digital control system. 
     The present invention has been described in terms of the various embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention. Therefore, the invention should not be limited to a particular described embodiment.