Abstract:
Over-voltage protection circuits are provided for power supplies that may limit input voltage spikes to a power converter and amplifiers including a damping loop that may damp filter resonance. Active damping systems incorporating the over-voltage protection circuits and/or damping loops of the present invention are also provided which may be particularly advantageous in applications such as aircraft or other vehicle applications where size and power consumption are important design considerations. The over-voltage protection circuit in various embodiments senses AC and/or DC voltage levels above a detection threshold level and momentarily disconnects the voltage input to thereby reduce the maximum voltage level input transients seen by the power converter. A capacitor may be provided to maintain the input voltage to the power converter while the input is switched out. The amplifier circuit damping loop in various embodiments is nested with a control circuit current feedback loop which may compensate for feedback problems such as instability which may result when an inductor-capacitor-inductor (L-C-L) filter is included on the output of the amplifier.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/132,778, entitled “Amplifier Including Over-voltage Protection Pre-regulator and Damping Loop” filed on May 6, 1999, which is hereby incorporated herein in its entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to voltage regulation in power supplies for electrical circuits, such as in amplifiers. The invention further relates to control of resonance in an electronic circuit, such as an amplifier. More particularly, the invention relates to amplifiers included in active noise and/or vibration control systems. 
     BACKGROUND OF THE INVENTION 
     Generic amplifiers are known for use with active noise and/or vibration control systems. Moreover, active noise and/or vibration control systems (e.g., U.S. Pat. Nos. 5,845,236, 5,754,662, 5,619,581, 5,551,650, 5,526,292, 4,715,559) including error sensors, reference sensors, controllers, amplifiers (e.g., U.S. Pat. No. 5,802,184), and inertial actuators (e.g., U.S. Pat. No. 5,884,736) are known. 
     SUMMARY OF THE INVENTION 
     The present invention provides over-voltage protection circuits for power supplies that may limit input voltage spikes to a power converter and amplifiers including a damping loop that may damp filter resonance. Active damping systems incorporating the over-voltage protection circuits and/or damping loops of the present invention are also provided which may be particularly advantageous in applications such as aircraft or other vehicle applications where size and power consumption are important design considerations. In various embodiments, the over-voltage protection circuit in various embodiments senses AC and/or DC voltage levels above a detection threshold level and momentarily disconnects the voltage input to thereby reduce the maximum voltage level input transients seen by the power converter. A capacitor may be provided to maintain the input voltage to the power converter while the input is switched out. The amplifier circuit damping loop in various embodiments is nested with a control circuit current feedback loop which may compensate for feedback problems, such as instability, which may result when an inductor-capacitor-inductor (L-C-L) filter is coupled to the output of the amplifier. 
     In embodiments of the present invention, a power supply includes a voltage source input and a power converter coupled to the voltage source input. A switch is coupled between the voltage source input and the power converter. A controller is coupled to the voltage source input and the switch. The controller opens the switch responsive to detection of a voltage on the voltage source input that exceeds a prescribed limit and closes the switch responsive to detection of a voltage on the voltage source input that does not exceed the prescribed limit to limit over-voltage transients seen by the power converter. The power converter may be a DC to DC converter. The voltage source input may be a DC rail. A capacitor may be coupled between a high side and a low side of the voltage source input. 
     In further embodiments of the present invention, the voltage source input comprises an AC input and the protection circuit further includes an AC to DC converter coupled between the AC input and the switch. The switch is coupled to the power converter by a DC rail. The controller may be further configured to open the switch responsive to detection of a voltage on the DC rail that exceeds a second prescribed limit to limit over-voltage transients seen by the power converter. 
     In other embodiments of the present invention, the power supply includes an input voltage sense circuit, having a sense input coupled to the AC input, that outputs an input voltage signal to the controller. The power supply also includes a DC voltage sense circuit having a sense input coupled to the DC rail that outputs a DC voltage signal to the controller. The controller includes a first threshold comparator coupled to the input voltage signal and a second threshold comparator coupled to the DC voltage signal. A switch drive circuit opens and closes the switch responsive to the first and second threshold comparators. A voltage reference signal may be coupled to a reference input of the second threshold comparator and the switch drive circuit may open the switch responsive to the second threshold comparator when the DC voltage signal exceeds the voltage reference signal. The first and second threshold comparators may be hysteretic comparators. 
     In further embodiments of the present invention, active noise/vibration control systems are provided including at least one reference sensor having a reference signal output representative of a source of disturbance and at least one error sensor having an error signal output representative of a residual disturbance. A controller is coupled to the reference signal output and the error signal output. The controller generates at least one output signal responsive to the reference signal output and the error signal output based on a control method. The control system further includes an amplifier having at least one amplifier channel that amplifies the at least one output signal to provide at least one drive signal. The amplifier includes a voltage source input and a DC to DC power converter coupled to the voltage source input. A switch is coupled between the voltage source input and the power converter. A controller is coupled to the voltage source input and the switch. The controller opens the switch responsive to detection of a voltage on the voltage source input that exceeds a prescribed limit and closes the switch responsive to detection of a voltage on the voltage source input that does not exceed the prescribed limit to limit over-voltage transients seen by the power converter. 
     In other embodiments of the present invention, an amplifier channel is provided including an L-C filter having an input side and an output side coupled to an output of the amplifier channel. A current loop feedback circuit is coupled between the L-C filter and an input of the amplifier channel. A damping loop is nested within the current loop, the damping loop being configured to dampen resonance from the L-C filter in the current feedback circuit. The L-C filter may be an L-C-L filter. The damping loop may be responsive to a voltage at the output side of the L-C filter or to a main current feedback signal. 
     In further embodiments of the present invention, the damping loop further includes an amplifier coupled to the main current feedback to provide an active damping loop. The damping loop may also include a differentiating input circuit coupling the main current feedback to the amplifier. The differentiating input circuit may include a resistor and a capacitor in series. The damping loop may also include a current feedback summer coupled to the input side of the L-C filter and a MOSFET H-bridge circuit coupled between the current feedback summer and the differentiating input circuit. 
     In other embodiments of the present invention, an over-voltage protection circuit adapted for interconnection to an input power line is provided. The circuit includes a DC rail voltage for powering a power device and an input switch for selective connecting or disconnecting the input power line from the DC voltage rail. A switch drive is operatively coupled to the input switch for opening and closing the switch at commanded times. A comparator compares the input voltage of the input power line or DC voltage rail to a prescribed limit set by a voltage reference and upon exceeding said limit, commanding the switch drive to open the input switch thereby disconnecting the input power line from the DC voltage rail and protecting a power device from over-voltage transients. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram illustrating an active control system including an amplifier according to embodiments of the present invention. 
     FIG. 2 is a block diagram illustrating a pre-regulator over-voltage protection circuit according to embodiments of the present invention. 
     FIG. 3 is a block diagram illustrating a pre-regulator over-voltage protection circuit according to further embodiments of the present invention. 
     FIG. 4 is a schematic circuit diagram illustrating an amplifier channel according to embodiments of the present invention. 
     FIG. 5 is schematic circuit diagram illustrating an amplifier channel according to further embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. In the drawings, layers and regions may be exaggerated for clarity. 
     Active Noise And/Or Vibration Control System 
     Embodiments of an active system are shown in FIG. 1 generally at  20  which may be useful, for example, for actively controlling noise in vehicle cabins or other closed structures or for controlling vibration in aircraft or other structures. The active system  20  comprises at least one sensor  24 , a system controller  22 , an amplifier  23  including one or more amplifier channels, and at least one actuator (e.g., inertial actuator or speaker). The electronic controller  22  receives sensor inputs from the at least one sensor  24 , such as microphones or accelerometers or any other suitable sensor. The controller  22  may also receive reference input(s) for example, representative of a tonal disturbance) from a reference sensor(s)  21 , such as from an engine(s) tachometer or an accelerometer mounted to a source of disturbance (engine(s)). The controller  22  processes the inputs according to a control method, such as the filtered-x LMS method (e.g., U.S. Pat. No. 5,627,896), and generates at least one output signal  26 . The at least one output signal  26  is provided to the amplifier  23  which may include multiple amplifier channels  28 . 
     The amplifier  23  according to embodiments of the present invention may comprise a power supply  25  and one or more (e.g., eight), preferably identical, amplifier channels  28 . The amplifier channels  28  output an amplified signal  33  to drive the at least one, and preferably a plurality of actuators  30  (e.g. inertial actuators—otherwise referred to as active tuned vibration absorbers or speakers). The power supply  25  comprises an AC/DC converter, such as a rectifier section  27 , a novel over-voltage protection pre-regulator  29  in accordance with a first aspect of the invention, and a power device  31 , such as a DC/DC converter. The novel amplifier channel  28  in embodiments of the present invention may be a current feedback system with a class D, or PWM, output stage and includes a resonant output filter and a novel damping loop in accordance with another aspect of the invention to be described later herein. 
     Therefore, in accordance with a first aspect of the invention a novel pre-regulator  29  is provided in the power supply section  25  that may accommodate the occasional surge voltages present on the incoming power  32 . In accordance with a second aspect of the invention, a novel amplifier  28  including a damping loop is provided. The damping loop is preferably nested inside the current feedback loop of the amplifier channel to actively damp the resonance associated with an L-C-L filter in the output network portion of the amplifier circuit  28  driving the actuator  30 . 
     Power Supply Over-voltage Protection Pre-Regulator 
     As shown in FIGS. 2 and 3, the pre-regulator over-voltage protection circuit  29  in accordance with embodiments of the present invention may be a low-cost pre-regulator that is intended to protect commercially available or custom power supply  25  and/or power amplifier channels  28  from transient over-voltage conditions that can occur on the incoming power distribution busses. The invention may be applicable to AC or DC input systems. These transient over-voltage conditions can result in DC voltages that exceed the input voltage specifications of power devices (e.g., the DC/DC converter or amplifier channels), and thus may result in either reduced life or catastrophic failure of the devices. In operation, the pre-regulator  29  in accordance with the invention includes a control circuit  34  which senses the input voltage on the AC or DC power bus  32 , or both, and will disconnect the line voltage from DC rail voltage at  43  which powers the power device (converter)  31  input until the over-voltage condition clears. Typically a hold-up capacitor  39  on the power device  31  input will continue to feed the power device  31  while it is disconnected from the input supply bus, or until the over-voltage transient condition clears. 
     The over-voltage pre-regulator  29  may provide a very simple low-cost solution to protect power components from transient over-voltage conditions. Without the over-voltage pre-regulator  29 , the power components, such as converter  31 , would generally have to be specified to accept higher input voltages. These higher voltage components are often larger, more expensive, or less efficient than lower voltage versions for the same output power rating. The end result is that a power supply  25  or power amplifier which takes advantage of the over-voltage pre-regulator circuit  29  can be less expensive, weigh less, and dissipate less power than a power supply that is sized to handle the specified input voltage transients. 
     The commercial value of over-voltage pre-regulator circuit  29  is illustrated by the example of the amplifier power supply  25  in the block diagram of FIG.  2 . In aircraft systems, for example, the specification for the aircraft AC power bus typically calls out normal AC operating voltages between 97 Vac and 134 Vac. These voltages typically are rectified to a maximum DC voltage of approximately 190 Vdc. Expected AC surge voltages of 180 Vac for 100 mS and 148 Vac for up to one second must be accommodated. This AC input voltage rectifies to a maximum of 255 Vdc. 
     In one example, the power supply  25  includes commercial off-the-shelf modules manufactured by Vicor Corporation. The power supply included two AIM modules (AC/DC Converter  27 ) and two VI-25-LU modules (DC/DC Converter  31 ). Each VI-25-LU module is rated to deliver 200 Watts of output, for a total of 400 Watts of output power at 60 Vdc. The VI-25-LU modules are rated for 200 Vdc of input, and up to 215 Vdc for very short transients by Vicor. Other available modules are rated to handle up to 255 Vdc but these units were only rated to deliver 100 Watts each when provided in the same size package as the VI-25-LU devices. Thus, the cost, size, and weight of the power supply would be increased if the VI-25-LU modules were replaced by these other units. Moreover, the enclosure size would have to be significantly increased to accommodate the large power supply. With the addition of the over-voltage pre-regulator circuit  29  in accordance with the invention, the higher power density modules may be used, with the associated potential savings in cost, weight, and volume. It is to be understood, however, that the over-voltage conditions described here are general problems for many applications and are not just limited to aircraft or aerospace nor is the present invention. 
     Another example of the utility of this invention can be taken from the power supply/power amplifier system for another inertial actuation system. The amplifier for the inertial actuation system is required to deliver up to 1200 Watts through a four channel switching amplifier. The power supply is a simple three phase half wave rectified arrangement. In this case the amplifier is a switching amplifier manufactured by Apex Microtechnology Corporation. The preferred Apex device is an SA04 module which has a 200 Volt maximum input power rating. Again, the specification on the power bus requires that the unit withstand transient conditions that result in rectified voltages in excess of 200 Volts. Apex modules with higher voltage ratings are less efficient, and the efficiency of the amplifier units may be critical to the success of the performance of the active system. The over-voltage pre-regulator circuit  29  on the rectified DC power rail adjusts the voltage cut-off for the 200 Volt maximum condition. 
     A block diagram of embodiments of an over-voltage pre-regulator circuit  29  is given in FIG.  3 . The over-voltage pre-regulator circuit  29  senses the AC (or DC) line voltage, the DC rail voltage, or both, and opens the input switch  37  if either of these voltages exceed a resistor-programmed limit in the control circuit. The over-voltage pre-regulator circuit  29  is intended to operate during relatively short duration transients. In these cases, the hold-up capacitor  39  may continue to supply the power device (e.g., a DC to DC converter)  31  with power during the transient. Thus, the power supply  25  may continue to function normally during the transient. The output  64  of power device  31  provides power to the various amplifier channels  28 . 
     The illustrated embodiments of the over-voltage pre-regulator circuit  29  includes the following components and inputs/outputs. The pre-regulator  29  receives input power in input line  32 . This power input can be an AC voltage (but may be DC as well). An optional line input filter  35  filters the input lines to filter out any high frequency noise present. The line input filter  35  is optional and can be omitted if system requirements permit. An AC to DC converter or rectifier unit  27  receives the output of the input filter  35 , if used. This rectifier unit  27  is generally only necessary in systems powered by an AC input voltage and converts the AC input to a DC input. Input voltage sense circuit  36  is configured to sense either instantaneous AC voltages or the DC input power voltage. The circuit  36  generally low-pass filters and divides down the input voltage for a suitable input to the hysteretic threshold comparator  45 . The input switch  37 , either a field effect transistor (FET), or Bipolar device, is on (closed) during normal circuit operation, and, thus, the over-voltage pre-regulator circuit  29  may be transparent to the operation of the power supply  23 . When either the AC input voltage in  32  and/or the DC rail voltage at  43  exceeds a prescribed value, the switch  37  is opened and the input line voltage at  32  is disconnected from the DC rail voltage at  43 . Thus, the over-voltage pre-regulator circuit  29  may perform its regulatory function. 
     The input switch  37  is expected to be the primary power consuming element in the pre-regulator circuit  29 . The dissipation of the switch  37  can be reduced by paralleling one or more low resistance devices. In this manner, the pre-regulator  29  can be made to be highly efficient. This switch  37  can also be placed on the other side of the hold-up capacitor  39 . In this case, the RMS currents can be reduced through the switch  37  in the AC input case, but the power device  31  may be starved for power during over-voltage transients. A high side switch drive circuit  40  powers the input switch  37  when the DC rail voltage at  43  is high. A boot-up bypass circuit  41  may be used to allow partially charge the DC rail voltage at  43  during power-up conditions. Once the DC rail voltage at  43  reaches a certain voltage, the low voltage power supply  42  powers up and turns the input switch  37  on to allow the DC rail  43  to fully charge. The DC rail voltage at  43  is the main power rail that is being regulated. This power rail  43  powers the power device  31  that is being protected from the high voltage transients. A hold-up capacitor  39  operates as the main storage capacitor(s) for the DC power rail at  43 . This capacitor(s)  39  continues to supply power to the power device  31  while the switch  37  is open. The power device  31  can be one or more DC/DC converter(s) (as shown in FIG.  1 ), one or more power amplifier channels, or other electrical component. 
     Very often, a lower voltage version of a power device  31  will be less expensive, more efficient, more available, or less bulky or heavy. The over-voltage pre-regulator circuit  29  may allow a lower voltage power device  31  to be used during normal operation and while protecting the power device  31  from transient conditions that might otherwise force the use of a more costly higher voltage power device. 
     A DC voltage sense circuit  44  senses the DC power rail voltage at  43 . The circuit  44  generally low-pass filters and divides down the input voltage for a suitable input to the hysteretic threshold comparator  45 . The low voltage power supply  42  powers the small signal circuitry in the voltage sense  44 , voltage reference  46 , and comparator circuits  45 . The low voltage power supply  42  is powered from the DC power rail at  43 . The voltage reference  46  provides an accurate voltage reference that is used by the hysteretic comparator circuits  45  to determine the input switch shut-off levels for the power input voltage and the DC power rail. The hysteretic threshold comparators  45  compare the scaled-down input voltage and DC rail voltage to the voltage reference. When the input voltage or DC rail voltage  43  exceeds the resistor programmed limits, the comparator  45  outputs command the high side switch drive circuit to open the input switch  37  to disconnect the power input from the DC rail hold-up capacitor  39 . Thus, during transient conditions, the DC power rail may be prevented from exceeding the resistor programmed limits. As a result, the power device  31  may be protected from the over-voltage transient conditions and a lower voltage device can be used which may save cost, weight, size, or reduce power dissipation. 
     Active Damping Loop 
     An amplifier channel  28  according to embodiments of the present invention is shown in FIG.  4 . An amplifier channel  28  according to embodiments of the present invention is further illustrated in the circuit diagram of FIG.  5 . The elements of the amplifier channel  28  as shown in the FIGS. includes a front-end section  47  which conditions the inputs to the current loop  48  which controls the output current to the actuator  30  (FIG.  2 ), a current loop  48  which includes a current loop closure stage  63  (including an operational amplifier  49 ) and the damping loop  50 , and a damping loop  50  including a damping loop closure stage (an operational amplifier (op amp))  51  (included on PWM  52  in the illustrated embodiment as an unused op amp was available on PWM  52  in this case), the PWM  52 , bridge controller  53 , the MOSFET H-bridge  54  (including Q 3 , Q 4 , Q 5  and Q 6  of FIG.  4  and  5 ), the filter  56 , the cable  55 , the actuator  30 , the resistors R 40 , R 41 , current feedback summer  58 , and differentiating input circuitry  60 . 
     The damping loop  50  as shown in FIGS. 4 and 5 is nested within the current loop  48 . The channel  28  ultimately drives the actuator  30  at the end of a cable  55  attached to the output port  33 . 
     An L-C-L filter  56 , comprising inductor L 1  (including two very low loss inductors) and capacitor C 31  in the schematic of FIG. 4, is typically used on the output of the PWM amplifier for various reasons. It may limit the power dissipation in the load at the MOSFET clock frequency and its harmonics. Second, it may limit the spectrum of the signal sent down the output cable  55 , hence controls EMI. Third, it may isolate the MOSFETs from capacitive loading, which may limit internal dissipation. 
     The inclusion of the L-C-L filter  56  in the amplifier load circuit  28 , however, may cause two additional concerns during design of the amplifier channel  28 . The first problem is that the inductors (L 1 ) in the L-C-L filter  56  may dissipate power within the amplifier channel  28  due to both copper and core losses in the inductors. Also, since the amplifier channel  28  is a current feedback system, the admittance of the filter  56  may significantly affect the feedback, particularly near the L-C-L series resonance frequency, where the admittance exhibits a complex pole pair and may cause the current feedback loop to approach instability. 
     Power dissipation in the filter  56  is diminished by the choice of a low-loss core material for the chokes, but this has the concomitant effect of reducing the damping of the series resonance, raising the Q of the L-C-L filter  56  and further exacerbating the current loop stability problem. The more lossless the core, the greater the potential stability problem. From the power dissipation standpoint, however, an infinite-Q resonance, if it could be obtained, would be very desirable. 
     According to embodiments of the present invention, the high-Q series resonance among the components of the L-C-L filter  56  may be damped electronically without damping losses by using a damping loop  50  nested within the current loop, as this may make the resonance essentially invisible to the loop transfer function of the current loop. 
     As illustrated in FIGS. 4 and 5, the damping loop feedback  57  (which includes resistor R 59  and capacitor C 42  and current feedback summer  58 ) closes the damping loop  50 . The main current feedback signal  59  is used for the main current loop  48 . Alternatively, the damping loop feedback could be derived from the voltage across the capacitor in the L-C-L circuit  56  (i.e., C 31  in the schematic) but this requires additional known circuitry not shown in the FIGS. as will be understood by those of skill in the art. 
     Frequency compensation of the damping loop  50  may be accomplished with differentiating input circuitry  60  (including R 59  and C 42  in the schematic) in the damping loop. Support for this result recognizes that the external actuator  30  is essentially a d-c path, in which case the input admittance, Y, to the L-C-L filter  56  with actuator load functions as a low pass second order filter with a high resonant peak at the band edge, i.e., Y has the form:          Y        (   s   )       =       ω   o   2         s   2     +     2                 ξ                   ω   o        s     +     ω   o   2                                
     where Y(s)=admittance, i.e., the ratio of the input current  61  to the L-C-L filter  56  to the applied voltage  62  across the L-C-L Filter  56 , ω o =the resonant frequency of the L-C-L filter  56 , ξ=the damping factor of the resonance of the filter  56 , and s=the Laplace complex frequency variable. 
     Closing the damping loop  50  around this function with differentiating compensation of the form, Ks, gives as the denominator of the damping loop closed loop transfer function the following: 
      Zeros{1 +KsY ( s )}= s   2 +(2 ξ+K )ω o   s+ω   o   2   
     in which, for example, if 
     
       
         K=2(1−ξ 
       
     
     then 
     
       
         Zeros{1 +KsY ( s )}= s   2 +2ω o   s+ω   o   2 =( s+ω   o )) 2   
       
     
     With this technique, resonance damping can be changed to critical (two real roots), or other acceptable level. The components C 42  and R 59  give a differentiating compensation of the form Ks at the frequency ω o . 
     The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although a few exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Therefore, it is to be understood that the foregoing is illustrative of the present invention and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.