Abstract:
A method and apparatus for providing electrical power through two terminals to a load wherein a portion of a LC filter circuit is connectable to each of the terminals. A feedback circuit having a sense capacitor is operably couplable to both portions of the LC output filter. The feedback circuit provides a feedback signal when a frequency of AC voltage across the sense capacitor substantially reaches the resonant frequency of the LC output filter so as to actively damp the voltage across the load.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application is a continuation of and claims priority of Ser. No. 11/287,146, filed Nov. 23, 2005, which is based on and claims the benefit of U.S. provisional patent application Ser. No. 60/630,888, filed Nov. 24, 2004, the contents of which are hereby incorporated by reference in their entirety. 

   BACKGROUND OF THE INVENTION 
   The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. 
   It is common to provide filtering of power electronic amplifiers in order to remove high frequency elements therein. One common approach is to use a LC (inductor-capacitor) in the output stage across which the output power for the load is obtained. Although such a filter is effective in removing high frequency components, a problem arises if the resonant frequency of the LC filter is in the operational range of the power amplifier. In such cases, an undesirable large voltage can develop across the load at the resonant frequency. Thus, a method of damping the natural response of the LC filter to prevent unwanted and excessive load voltage is necessary. 
   Various approaches of damping have been used. In a first form, damping is provided by using a dissipative approach, generally in the form of a resistor or a combination of a resistor and a capacitor. However, this approach results in unnecessary and potentially high levels of power dissipation. In an alternative approach, active damping is used. However, active damping requires the use of a control loop and therefore, a method of sensing the output voltage. 
   Two known methods of sensing output voltage have been used. The first method requires the use of high impedance resistors and a differential operational amplifier. However, this method does not provide galvanic isolation, and therefore can result in limited or even prohibited use in circuits requiring a high level of electrical isolation. A second known method requires the use of relatively expensive and electrically complex Hall Effect, or a similar type close-loop current sensor. In this method, the current sensor is configured to measure the current flowing through a resistor disposed across the terminals of a voltage signal to be measured. The current measurement is proportional to the voltage of interest. Drawbacks of this second approach include high cost and complexity. 
   SUMMARY OF THE INVENTION 
   This Summary and the Abstract are provided to introduce some concepts in a simplified form that are further described below in the Detailed Description. This Summary and the Abstract are not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. In addition, the description herein provided and the claimed subject matter should not be interpreted as being directed to addressing any of the short-comings discussed in the Background. 
   An aspect of the present invention includes a method and apparatus for actively damping a resonant LC filter using a control loop combined with a transformer-coupled voltage feedback element. The LC filter circuit is connectable to each of the terminals of power electronics configured to provide power to a load based in part on a command signal. A feedback circuit having a sense capacitor is operably couplable to both portions of the LC filter circuit. The feedback circuit provides a feedback signal when a frequency of AC voltage across the sense capacitor substantially reaches the resonant frequency of the LC filter circuit so as to actively damp the voltage across the load. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic circuit diagram having an active LC damping circuit with galvanic isolation. 
       FIG. 2  illustrates plots of various signals. 
       FIG. 3  illustrates plots of various signals. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates an active LC damping circuit  10  with galvanic isolation. The schematic/block diagram is provided in  FIG. 1  because as appreciated by those skilled in the art, components therein can be implemented in hardware (digital and/or analog) and or software modules as is well known in the art. However, it should be noted that the schematic diagram of  FIG. 1  is typically used to model circuit  10  using analytical tools such as SPICE modeling techniques. Thus,  FIG. 1  includes additional electrical components generally used to model parasitic elements of actual components. 
   Power amplifier electronics, herein represented by gain stage  24 , provide output power to an LC filter indicated at  14 , which in turn, provides power to the desired load  16 . 
   Power amplifier electronics include modules (hardware and/or software) for receiving a command input signal and controlling power control elements to provide output power. Power amplifier electronics are well known and can take many forms, the design of which is not important for purposes of providing this description. 
   In an aspect of the present invention, a voltage control loop or feedback circuit  18  provides feedback for active damping. The voltage control loop  18  includes an error amplifier  20  (represented herein with summer  22  and gain stage  24 ), a voltage sensor  30 , and a compensation network  32 . In particular, the voltage sensor  30  comprises a current transformer  36 , which provides galvanic isolation. As illustrated, the current transformer  36  is operably coupled to sense current flowing through a sense capacitor  38  where the output terminals, or secondary terminals, of the current transformer  36  are coupled to a burden resistor  40 . A voltage signal across the burden resistor  40  is in proportion to the sensed current flowing through capacitor  38 . 
   A particular advantageous feature of the present invention, in one embodiment, is that the voltage feedback of the control loop  18  becomes noticeably active at least, or only, when the frequency range of the AC voltage across the sense capacitor  38  corresponds to the resonant frequency of the LC circuit  14 , which is substantially higher than the corner frequency determined by the current transformer  36  and the burden resistor  40 . The corner frequency is thus selectable. When the voltage feedback becomes “active” (i.e. no longer negligible and accurate or proportional with respect to the current flowing in the LC filter  14 ), the voltage feedback signal leads the output voltage across load  16  by approximately 90°.  FIG. 2  illustrates the feedback signal at  41 , the output voltage at  42  and the current in the sense capacitor  38  at  44 . The feedback control loop  18  operates over a wide range, but its active influence on the output voltage  42  occurs in a narrow range of frequencies resulting from, and centered about, the resonant frequency of the LC output filter  14 . 
   The feedback or voltage signal across burden resistor  40 , is scaled by feedback compensation circuit  32  and is summed with a desired command signal provided at  50  by the error amplifier  20  in order to provide a system error signal  52 . In a preferred embodiment, the gain of the error amplifier  20  is configured so as to provide unity gain in the command path. In this manner, the signal by the control loop  18  is negligible at low frequencies. With the voltage feedback provided as above, attenuation or damping of the power amplifier electronic output voltage signal is achieved specifically at the point of resonance of the LC output filter  14 . This is illustrated in  FIG. 3  where the amplifier output voltage is indicated at  60  and the voltage across the load is indicated at  62 . 
   It is important to note that in  FIG. 1 , both of the terminals  24 A and  24 B from the power electronics  24  include unwanted high frequency electrical activity so filtering is provided for each of the output terminals  24 A and  24 B. 
   The LC output filter  14  includes two LC circuits  70  and  72  connected to ground, where one circuit is provided on each terminal of the power electronics  24 . Both LC circuits  70  and  72  include a capacitor  73  and an inductor  74 . In operation, each of the LC circuits  70  and  72  can go into resonance. However, sense capacitor  38  spans across the LC circuits  70  and  72 , in particular, in parallel with the series combination of capacitors  73  thereof, and thus provides a single current sense signal for current transformer  36 . In this manner, the sensed signal is that of the current between the LC circuit  70  and  72  rather than sensing the current of any one LC circuit. Thereby, an advantageous indication of the state of resonance of the output filter  14  as a whole is provided rather than just one portion thereof, while using a minimal amount of components. 
   It should also be noted that scaling of the feedback voltage may not be necessary in some applications, for example, a simple voltage divider may be used, if desired, in combination with the burden resistor  40  to provide the desired feedback voltage. Furthermore, a low pass filter can be added to the feedback signal to compensate, or further attenuate, the feedback signal at high frequencies since the gain of the feedback signal increases with frequency due to the reduction in impedance of capacitor  38  with frequency. 
   A particular advantageous feature of the present invention is that the forward gain provided by the error amplifier  20  can be unity. In this manner, at low frequencies, the feedback signal has a very low amplitude and thus a negligible effect on the voltage command signal. This is due at least in part to the nature of the current transformer  36  used with the sense capacitor  38 . Stated another way, the compensation network gain is adjusted such that at the desired frequency (i.e., the resonant frequency of the LC filter), or the other components of the voltage feedback signal, are adjusted so that the voltage feedback signal is strong enough to provide compensation. However, at low frequencies, it is as if the voltage feedback signal does not exist and the voltage command signal is passed with unity gain through the error amplifier  20 . Thus, damping is provided for the output voltage at the resonant frequency, while allowing the power electronics to have an operable range across and including the resonant frequency. 
   Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not limited to the specific features or acts described above as has been held by the courts. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.