Abstract:
An apparatus and method for high frequency alternating power generation to control kilowatts of supplied power in microseconds. The present invention includes a means for energy storage, push-pull switching means, control electronics, transformer means, resonant circuitry and means for excess energy recovery, all in electrical communication. A push-pull circuit works synchronously with a force commutated free-wheel transistor to provide current pulses to a transformer. A change in the conduction angle of the push-pull circuit changes the amount of energy coupled into the transformer&#39;s secondary oscillating circuit, thereby altering the induced secondary resonating voltage. At the end of each pulse, the force commutated free-wheel transistor causes residual excess energy in the primary circuit to be transmitted back to the storage capacitor for later use.

Description:
This invention was made with Government support, Contract Number W-7405-ENG-36 awarded by the United States Department of Energy to The Regents of the University of California. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to an apparatus and method of power conversion to efficiently convert a DC voltage to a high frequency and high voltage AC waveform with high power capability for a variety of applications, including pollution control, environmental remediation equipment, and medical apparatus. Specifically, the present invention relates to an optically controlled power converter which incorporates push-pull technology and an efficient transformer (with a 0.999% coupling) to produce a high voltage and high frequency waveform for a load (such as, for example, a dielectric barrier plasma discharge) by conditioning the input power supplied to the load while also recovering and storing unused converted energy, for subsequent use by the load. 
     BACKGROUND OF THE INVENTION 
     High frequency and high voltage waveforms with high power capability are used in various industrial, commercial and military applications, such as in environmental remediation equipment and processes. 
     In the past, equipment for the creation of such waveforms required the use of series resonant converters, which are electrically inefficient because of the circuit topology employed. A series resonant converter uses a step-up transformer with a high leakage inductance to resonate with the load capacitance. Additional resonant capacitors are located on the transformer primary. The transformer conducts electricity for the full 360 degree cycle. In such systems, the winding resistance of the primary and secondary transformer windings, along with the losses of the oscillating circuitry, contribute to an inordinately high total energy loss. Thus, for example, a typical prior art system designed to deliver 15 kilowatts (kW) is likely only to deliver 3 kW of power to the load. Another disadvantage of series resonant converters is that they must employ switching transistors to switch for a full 180 degree conduction angle and operate almost exactly at the resonate frequency to avoid large destructive transient voltages. Finally, these types of systems are bulky, heavy, require large amplifiers to drive loads through variable frequencies, are limited in output power, and are quite expensive to operate. Thus, a highly efficient, high frequency power converter is needed for applications which require a large amount of power. 
     Accordingly, it is an object of the present invention to provide high frequency, high voltage AC power that includes operation at up to 25 kHz, 25 kW with high efficiencies. 
     It is another object of the present invention to provide for efficiencies up to 85%. 
     It is another object of the present invention to provide variable power (to 25 kW) by changing the pulse duration of pulse-pairs, by regulating the applied voltage. 
     It is still another object of the present invention to match the timing of the switching transistors to the timing of the load, to optimize power transfer. 
     It is still another object of the invention to vary the voltage by changing the individual pulse width of the power switching transistors. 
     It is yet another object of the present invention to control power switching with the use of insulated gate bipolar transistors (IGBTs). 
     It is yet still another object of the present invention to provide a pulse width modulated, push-pull driven parallel resonant converter which has an energy recovery system. 
     It is yet still another object of the present invention to provide a pulse width modulated, push-pull driven parallel resonant converter with an active free-wheel. 
     It is an additional object to provide a transformer: (1) which is highly efficient; (2) has a better than 0.999 coupling coefficient; (3) in which both sides of the transformer receive power and transmit power on both sides of the positive and negative sine wave; and (4) which has multiple windings arranged such that the high voltage output is shielded by components near ground potential which, in turn, permits higher voltage with improved efficiency. 
     It is an additional object of the present invention to provide a step up power converter using transistors configured in a push-pull circuit with a multiple winding transformer to couple power efficiently to a resonant load. 
     It is a further object of the present invention to provide a step up power converter which drives any load by conditioning the input power supplied to the load while also recovering and storing unused energy for subsequent use by the load. 
     Additional design advantages and novel features of the invention will be evident from the drawings and the description set forth herein. 
     SUMMARY OF THE INVENTION 
     The present invention is a highly efficient, high frequency power generator for applications which require a large amount of power (over 5 kW) in either a pulse or continuous mode. The present invention utilizes an optically controlled power converter to control kilowatts (or more) of supplied power in microseconds by employing an input rectifier, an energy storage network, a resonant power switching mechanism, control electronics optically coupled to the resonant power switching mechanism, a coupling transformer, a secondary oscillating circuit having the load, and electronics for energy recovery. The power converter of the present invention employs pulse width modulation techniques to deliver short pulses of energy to a secondary oscillating circuit (which is coupled to the load to be driven) to provide regulation and control. The electrically controllable conduction angle of the secondary oscillating circuit determines the amount of energy delivered to the load and, thus, can be used to regulate the output voltage. The resonant power switching mechanism employs a free-wheel transistor which allows the remaining energy in the current limiting inductor included in the transformer primary circuit to be returned to an energy storage capacitor via the energy recovery network, maintaining a high system efficiency in addition to avoiding the large transients from off-frequency operation. 
     The transformer of the present invention has a better than 0.999 coupling coefficient. It has multiple windings arranged such that the high voltage output is shielded by components near ground potential which, in turn, permits higher voltage with improved efficiency. Further, both sides of the transformer receive power and transmit power on both sides of the positive and negative sine wave. 
     The novel features of the present invention will become apparent to those of skill in the art upon examination of the detailed description or can be learned by practice of the invention. It should be understood, however, that the detailed description is provided for illustration purposes only. Various changes and modifications within the scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the preferred embodiment of the present invention, illustrating the basic functional assemblies of the resonant converter; 
     FIG. 2 is an electrical schematic illustrating the preferred embodiment of the present invention; 
     FIG. 3A is a partial cross-section of the transformer core illustrating the relationship between the primary and secondary windings; 
     FIG. 3B is a plain view schematic of the transformer of the present invention showing the primary and secondary windings pulled apart for purposes of illustration (instead of in their actual, overlapping configuration); and 
     FIGS. 4A and 4B are schematics how the variation in pulse width affects the output power and voltage. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As seen in FIG. 1, optically controlled power converter  10  includes an input power line rectifier  11 , energy storage circuitry  21 , resonant power switch  31 , network control electronics  41 , transformer  51 , secondary oscillating circuit  61 , and an energy recovery network  71 . 
     In the high power applications, for which the present invention is suited, three phase input power is typically required (e.g., 480 volts in industrial applications). This AC power is converted to direct current by a conventional rectifier  11 . Because the whole system, including rectifier  11 , floats with the utility input power grid, isolation transformers are unnecessary. Further, because the present invention operates at the input power source&#39;s voltage level (e.g., 480 volts), power line rectifier  11  further increases overall system efficiency and minimizes high power components and related physical size. If DC input power is used, rectifier  11  would be unnecessary. 
     With reference to FIG. 2, energy storage circuitry  21  includes capacitor C STORE , which stores electrical energy originating from the input power source and that also recycled from C REC  of energy recovery electronics  71 . Diodes D 1  and D 2  free-wheel the leftover energy from each pulse and transmit that energy to C REC  via the forced commutation of transistor Q 2 . C STORE  is then recharged through L REC  and diode D 3 . 
     The resonant power switching network  31  includes optional transient network varistors VR 1   TRANS  and VR 2   TRANS , current limiting inductor L LIM , and transistors Q 1 , Q 2  and Q 3 . All three transistors all controlled and driven by control electronics  41 , via optic coupling  33 . Transistors Q 1 , Q 2  and Q 3  are arranged in a push-pull circuitry arrangement operating at a predetermined conduction angle, preset by conventional control electronics  41 , that generate the ultimately desired voltage AC signal across the secondary oscillating circuit L OSC  of secondary oscillating circuitry  61 . Transistor Q 2  is an active free-wheel transistor and operates synchronously with transistors Q 1  and Q 3 . By modifying the conduction angle of the push-pull circuitry (and, thus, modifying the amount of energy through primary winding W P  of transformer  51 ), the amount of energy applied to the secondary winding W S  of transformer  51  can be changed, which alters or regulates the induced secondary resonating voltage applied to the load (identified as C OSC ). 
     With the circuitry set forth in FIG. 2, at the end of each pulse period generated by the control electronics  41  to transistors Q 1 , Q 2  and/or Q 3 , any residual energy stored in current limiting inductor L LIM  is forced to ring back into storage capacitor C REC  through free-wheeling diodes D 1  and D 2  by the forced commutation of active free-wheel transistor Q 2 . Preferably, transistors Q 1 , Q 2  and Q 3  are equivalent devices and, more preferably, insulated gate bipolar transistor (IGBT) switch modules having the desired power ratings. Those of skill in the art will realize that other similar devices (e.g., metal oxide semiconductor field effect transistors (MOSFETs) and isolated gate control thyristors (IGCTs)) could be used depending on system requirements. Further, current limiting inductor L LIM  can be optimized for various powers or frequencies. While transient varistors VR 1   TRANS  and VR 2   TRANS  are optional, their inclusion is desirable to assist in the prevention of unexpected voltage surges, primarily from transformer leakage inductance. Preferably, transient varistors VR 1   TRANS  and VR 2   TRANS  are rated within a range of 700-1000 volts. In the preferred embodiment the design of resonant power switch  31  provides a high frequency bypass assembly, transient circuit protection, transistor switching networks, and a current limiting inductor. 
     Control electronics  41  is of conventional design that controls the timing and pulse width of the transistors Q 1 , Q 2  and Q 3  by conventional methods known in the art. However, because transistors Q 1 , Q 2  and Q 3  float at the potential of the utility power grid, control electronics  41  must communicate with the resonant power switch  31  by optic coupling  33  which is, preferably, a fiber optic communications cable as illustrated in FIGS. 1 and 2. Other isolation techniques would work (e.g., isolation transformers). 
     As seen in FIG. 2, R EQ  is an equalizing resistor which may be set to assist in establishing the initial operating conditions of the present invention. In particular, R EQ  is used to start and maintain voltage equilibrium between C STORE  and C REC , when the system is idle, and sets the initial conditions at turn-on. 
     Transformer  51 , illustrated in FIGS. 2,  3 A and  3 B, is a high-frequency transformer that has a high electrical efficiency at the frequency of operation. This means that it has a low winding power loss, high self-resonant frequency, and low leakage inductance to transfer power efficiently. Low primary winding power loss is obtained with the utilization of two parallel wires in  81  and  83  in a single winding layer of flat strap conductor, litz or braid, closely coupled to the core  85 . See FIG.  3 A. This technique provides a high current rating and low resistive and skin depth losses. Additionally, for each transformer leg (or post), is then wound with two mono-planar secondary windings  87  and  89 , each insulated from primary windings  81  and  83  by insulation  91 . Again, see FIG.  3 A. Finally, the primary windings from each leg of the transformer are parallel connected to the windings on the opposite transformer leg (or post). This winding configuration is interconnected to the power switching transistors Q 1 , Q 2 , and Q 3 , as illustrated in FIG. 2, such that symmetrical drive flux to both transformer core legs is derived for each pulse, whether generating the positive or negative half wave. 
     To couple transformer magnetic flux efficiently between the primary and secondary, all winding sets are wound with a single layer. Again, see FIG.  3 A. To further maximize the magnetic flux coupling, the winding space between the primary and secondary is minimized. With this minimization of distance, the high secondary voltage and related high frequency present a problem of arcing and short circuiting. To ameliorate the affects of the secondary high voltage and high frequency, a special winding technique for the secondary is also used. Specifically, a split mirror winding is used for each of the secondary windings on each leg of the transformer. See FIG.  3 B. The winding initially starts at H, winds to the middle I of the transformer post assembly, and then reverses pitch back down to the other end J. The start and end are then connected to each other and grounded. This results with the high-voltage being in the middle of the assembly. The secondary winding assemblies for both legs of the transformer are then parallel connected to one another. This results with four parallel single layer secondary windings, with the two high voltage nodes I, I in the center. The center connection of the high-voltage provides field gradients around the transformer that are constant potentials to other structures. This design provides the highest electrical safety margin for the transformer along with the desired parameters of low loss, highest power transfer efficiency, and high resonant frequency. To control the electric field at the high voltage output, a corona ring  93  is provided as schematically illustrated in FIGS. 3A and 3B. 
     Transformer  51  is designed to couple energy, preferably short pulses of energy, to the secondary oscillating circuit  61  and, thus, provide controllable, highly efficient, high frequency, high voltage power to the load C OSC . 
     Secondary oscillating circuit  61  includes oscillating inductor L OSC  and capacitor (or, load) C OSC . The ratio of L OSC  and C OSC  determine the operating resonant frequency for the load to be driven. RG 220 , in FIG. 2, is a high voltage transmission cable to provide interconnection. 
     Energy recovery electronics  71  includes isolating diodes D 1 , D 2  and D 3 , capacitor C REC  and recovery inductor L REC . Preferably, C REC  is a 1 μF capacitor, while recovery inductor L REC  is a 110 μH inductor. Because active free-wheel transistor Q 2  operates synchronously with each of transistors Q 1  and Q 3 , at the end of each pulse cycle generated by control electronics  41 , any residual current in the current limiting inductor L LIM  is free-wheeled (using free-wheeling diodes D 1  and D 2 ) with Q 2  turn off, forcing current back to storage capacitor C REC . The use of diode D 3  and recovery inductor L REC  as illustrated in FIG. 2 allows C REC  and C STORE  to operate at different frequencies. C STORE  operates at 60 Hz. C REC  operates at a faster time scale, depending on the frequency of the power output of the secondary winding Ws. 
     In operation, control electronics  41  controls and drives the resonant power switch  31 , controlling transistors Q 1 , Q 2  and Q 3 . In particular, control electronics  41  is used to generate the desired pulse width and timing signals to each of transistors Q 1 , Q 2  and Q 3 . By controlling the pulse widths to each of the transistors the output power (magnitude) to the load oscillator circuit can be regulated. The wider the pulse width the greater the power and voltage. Similarly, by adjusting the repetition rate of the pulses generated to each of the transistors Q 1 , Q 2  and Q 3 , the output frequency to the load can be optimized. These operational modes are illustrated in FIGS. 4A and 4B. As seen in FIG. 4A, with a larger pulse width, the power to the load increases as transistors Q 1  and Q 2  are pulsed to begin a positive polarity phase. When transistors Q 2  and Q 3  are pulsed, the power to the load alternates to the negative polarity phase. Similarly, as seen in FIG. 4B, with a narrow pulse width the power to the load decreases. 
     The present invention is useful in those applications, for example, which require generation and control of a large amount of high frequency power to a load in a controlled manner. One such application, for example, is for use with a non-linear silent discharge plasma (SDP) cells. The present invention, when used with SDP cells, can be applied to large commercial installations, such as semiconductor gas processing, chemical processing, flue-gas process remediation for cleaning solvents, adhesive vapor remediation, as well as pyrolysis of off-gases. The present invention is also useful for those defense programs which are adopting or have adopted a zero emissions policy from manufacturing or processing facilities. 
     Other variations and modifications of the present invention will be apparent to those skilled in the art, and it is the intent of the appended claims that such variations and modifications be covered. The particular values and configurations discussed above can be varied, are cited to illustrate the preferred embodiment of the present invention, and are not intended to limit the scope of the invention.