Abstract:
The primary of a transformer is driven at low voltages to provide high-voltage dynamic drive from the secondary to a load. A high-current source is placed in series with both the transformer secondary and load. At least secondary inductance of the transformer, hence impedance, is controlled through core saturation to transition secondary output to the load between high-voltage dynamic drive inductively coupled from the primary, and high-current drive serially connected through the secondary. Switching between high voltage and high current output is accomplished through the transformer; no additional switching devices need exist in the high-voltage path. Broad voltage and current capabilities of the configuration inexpensively improve transient drive of highly reactive loads.

Description:
REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. Provisional Patent Application Ser. No. 61/181,321, filed May 27, 2009, the entire content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to electronic power circuitry, and particularly to methods and apparatus to drive highly reactive loads. 
     BACKGROUND OF THE INVENTION 
     The majority of loads to be driven by electronic devices are designed so as to present impedances consistent with low-cost semiconductor devices. This implies operation at readily available voltage or current sources. Resistive and low-reactance loads therefore present no challenge to conventional drive techniques. 
     Highly reactive loads and loads with inconstant impedances, such as gas tubes or motors, sometimes however require voltages or currents for transient behavior which are totally inconsistent with those required for later static operation. Multiple energy sources with entirely disparate characteristics are therefore required for tightly-controlled transient behavior. 
     Controlled magnetic core saturation to form switches or amplifiers of inductive components has been in use for many years to inexpensively drive large and/or unusual electrical loads. Current examples of these approaches include U.S. Pat. No. 7,706,424—‘Gas discharge laser system electrodes and power supply for delivering electrical energy to same’, #7,675,761—‘Method and apparatus to control two regulated outputs of a flyback power supply’, and #7,675,242—‘Electronic ballast’. Prior art furnishes many examples of single-path control using magnetic components, but does not teach inexpensive control of multiple energy sources within a single device. 
     Highly inductive motors belong to a class of devices which initially require high winding voltage in order to quickly develop magnetic flux, but subsequently require high current at low voltage to perform work. Common practice of operating motors within the fixed voltage range of a power supply therefore forces a compromise between allowable winding inductance and transient response. Low inductance, however, exacerbates ohmic losses in high power applications where drive current must be increased to maintain output power requirements. A burgeoning application encountering these obstacles is found in electrically-powered transportation vehicles, the motors for which typically have compromised torque curves in order to meet system voltage constraints. 
     This category of loads therefore is much more expensive to drive quickly than more pedestrian loads, in that requisite drive circuitry often must be doubled to achieve dual voltage and current requirements. The use of semiconductors in the high-voltage path or multiple controlled reactors as well increases cost, in that high-voltage production processes are more expensive than processes for lower voltages. A need exists for a method and apparatus whereby loads of unusual or inconstant impedance may be inexpensively driven without degrading system transient performance. 
     SUMMARY OF THE INVENTION 
     This invention resides in the advantageous exploitation of controlled transformer core saturation to select one of multiple energy forces for application to a reactive or nonlinear load at a transformer output. A minimal configuration teaches selection between one force possessing high voltage capability and a second force possessing high current capability. 
     A method for inexpensively driving a reactive or nonlinear load with improved transient response comprising the steps of:
         1. Coupling current into at least one primary winding of a transformer during a first period of time, so as to invoke an output response in at least one secondary winding;   2. Coupling current through at least one secondary winding of said transformer to said load during a second period of time; and   3. Employing magnetic core saturation to reduce the impedance of said at least one secondary winding during said second period of time, so as to facilitate current flow in a desired direction through both secondary winding and load.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of a preferred embodiment of the present invention. 
         FIG. 2  shows the voltage and current waveforms of the embodiment of  FIG. 1  in normal operation. 
         FIG. 3  shows a block diagram of an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , incoming command Signal  101  is applied to both Delay  102  and Differentiator  103 . Although assumed to be a pulsed voltage signal herein, Signal  101  may as well effect dynamic control in other forms known to the art. Differentiator  103 , under control of Signal  101 , applies a voltage Signal spike  108  to the upper primary terminal of Transformer  104  at the incoming rising edge. The lower primary terminal of Transformer  104  is grounded, allowing Signal  108  to induce current in the primary of Transformer  104 . Although it is assumed for this example that Differentiator  103  operates only on rising events, alternative implementations are anticipated to require differentiator outputs, usually bipolar in nature, on both rising and falling events. In response to Signal  101 , Delay  102  outputs Signal  107 , a replication of Signal  101  which is presumably delayed slightly less than the width of differentiated Signal  108 . Termination of Signal  108  therefore occurs slightly after initiation of Signal  107 . Signal  107  is supplied as input to controlled Current Source  105 , the output of which is connected in series with the ground path of the lower secondary terminal of Transformer  104 . The upper secondary terminal of Transformer  104  is connected to one terminal of Load  106 , the second terminal of which is grounded. Under control of Signal  107 , Current Source  105  therefore induces current (if possible) in both the secondary of Transformer  104  and Load  106 . 
     From a quiescent state with no current flowing, Signal  101  initiates current in the primary of Transformer  104 , through the action of Signal  108 . This transformer current n this form, Signal  101  therefore causes  103  to apply controlled current pulses (within voltage and current constraints of the device) to the lower secondary terminal of Transformer  104 . Delay  107  equally retards rising and falling events of incoming Signal  101 , to become Signal  107 , applied as input to Differentiator  103 . 
     It is assumed that voltage constraints of Source  105  prevent achievement of core saturation in Transformer  104  without additional assistance. It is as well assumed that Transformer  104  is of high secondary-to-primary turns ratio. The secondary output of Transformer  104 , shown as Signal  109 , directly drives Load  106 , shown to be a gas discharge tube which exhibits low impedance only after receipt of a high breakdown voltage. Composite effect of these conditions is that Transformer  104  produces a very high secondary voltage at Signal  109  as a direct result of the output of Differentiator  103 . 
     The high secondary Signal  109  spike therefore ionizes the gas in Load  106 , immediately decreasing its impedance. As this impedance drops, the resultant current developed saturates the core of Transformer  104 , causing its secondary to become a low-impedance path for the current provided by Source  105 . The series connection of low impedances of Load  106  and Transformer  104  secondary lower the voltage required at Source  105  to be within its voltage constraints, now facilitating current control and presumably subsequent cessation by Source  105 . 
     Referring now to  FIG. 2 , Trace  201  shows the voltage Signal  101 , Trace  202  shows Signal  107 , Trace  203  shows Signal  108 , Trace  204  shows voltage of Signal  109 , and Trace  205  shows current of Signal  109 ; all of  FIG. 1 . 
     The incoming Signal  201  can be seen to be delayed at Signal  202 , and the resultant derivative spike from Differentiator  203  of  FIG. 1  can be seen in Trace  203  at the rising event shown at Time Markers  206 . In that Transformer  104  current at Time Markers  206  is inadequate to saturate the core, the resultant high effective turns ratio of Transformer  104  produces the high voltage spikes shown in Trace  204  at the rising event of Trace  203 . Current allowance through Source  105  is then initiated at Time Marker  207 , as shown in Trace  202 . These high voltage spike of Trace  204  at Time Marker  206 , when applied to Load  106  of  FIG. 1 , create a ionized path through Load  106  which lowers its impedance as breakdown is achieved. The increased current allowed by this reduced impedance then saturates the core of Transformer  104  of  FIG. 1 . In a saturated state, the secondary of Transformer  104  represents a low effective turns ratio, but most importantly, a low secondary impedance. The saturation of Transformer  104  therefore serves as a current switch to enable Load  106  current control by Source  105 , both of  FIG. 1 . The combination of secondary voltage spike and switched current source therefore causes the current, as shown in Trace  205 , to be a close replica of the incoming control voltage shown in Trace  201 . Current is commanded to zero by Signal  101 , shown in Traces  201 , at Time Marker  208 . Note that Load  106  ionization ceases of its own accord at the Time Marker  209 , as current Source  105  is disabled. Lacking secondary current, Transformer  104  recovers from saturation at Time Marker  209 . Time Marker  210  shows initiation of a second pulse cycle similar to that initiated at Marker  206 . 
     Referring now to  FIG. 3 , incoming Signal  301 , Delay  302 , Differentiator  303 , Signal  308 , Signal  309 , and current Source  305  are analogous to Signal  101 , Delay  102 , Differentiator  103 , Signal  108 , Signal  109 , and current Source  105 , respectively, of  FIG. 1 . Transformer  304  differs from Transformer  104  of  FIG. 1  by the addition of Control Winding  310 , used to control saturation of the Transformer  304  secondary core. Gas discharge Load  106  of  FIG. 1  has been replace by Motor Load  306 , which exhibits high inductance. 
     The primary significant difference between Load  106  of  FIG. 1  and Load  306  of  FIG. 3  is that inductance of Motor Load  306  will resist current cessation, whereas the Load  106  of  FIG. 1  exhibits no such behavior. Current interruption by Source  305  is therefore inadequate to terminate core saturation of Transformer  304  in a timely manner. 
     The circuit of  FIG. 3  is distinguished from that of  FIG. 1  by the extension of Signal  307  to additionally drive Control Winding  310 . It is assumed that the core of Transformer  304  is less susceptible to saturation than that of Transformer  104 , and that operational current through Load  306  is of itself inadequate to cause Transformer  304  core saturation. In that characteristics of Load  306  are antipathetic to secondary current cessation; Signal  307 , used as Signal  107  in  FIG. 1  to control Transformer  104  secondary current, additionally directly controls Control Winding  310 . Under control of Signal  307 , Winding  310  presumably adds or removes sufficient secondary core field strength to cause or disallow, respectively, secondary core saturation of Transformer  304 , thus deterministically controlling current in Load  306 . The addition of Winding  310  therefore extends use of the current invention to highly inductive loads. 
     Expansion of the simple control winding activation scheme used for exemplary purposes is anticipated to minimally include control of motor back-EMF as necessary. 
     Although exemplary specification of a single secondary core saturation is used above to select one of two possible energy sources (high voltage or high current), application of the current invention to magnetic topologies with multiple magnetic regions or which saturate in entirety will result in minor and anticipated departures from the embodiments shown. Resultantly, use of the invention with more than the two energy sources described is anticipated. 
     Although shown in examples of unipolar impedance change, those skilled in the art will readily apply the current invention in applications utilizing additional voltages, currents, polarities, and/or phase relationships. The relatively minor circuit and timing modifications to facilitate use of the current invention in controlling capacitive loads, in contrast to the exemplary inductive loads, is as well anticipated. 
     By the disclosure above, it can be seen that extremely fast and accurate current control may be effected in an unusual or highly reactive load, through use of controlled core saturation to select one of a plurality of energy sources in a transformer. The simplicity of the approach avoids the cost of commensurately unusual semiconductors or multiple magnetic devices.