Abstract:
A high-power modulation system includes drive circuitry that receives input signals from the signal source via a series of transformers. The drive circuitry amplifies the input signals and provides the resulting amplified signals to the high-power switch. A storage capacitor within the drive circuitry stores energy derived from the input signals, and the stored energy is used to power the drive circuitry. One embodiment takes advantage of inductive ringing to more rapidly turn off the high-power switch. A diode connected in series between two drive transistors rectifies the ringing signals, pulling a control signal to the high-power switch negative.

Description:
FIELD OF THE INVENTION 
     This invention relates to high-power, high-voltage modulators. 
     BACKGROUND 
     A broad range of applications require high-voltage, high-power, variable-voltage sources with pulse-switching capabilities. Such applications include radar transmitters, X-ray machines, and semiconductor wafer manufacturing equipment. These machines and equipment employ such high-power amplifiers as cross-field amplifiers, traveling-wave tubes, magnetrons, and klystron amplifiers (collectively referred to as vacuum-electron devices). A number of high-power modulators are adapted to deliver pulsed power to these types of high-power amplifiers. 
     Conventional high-power modulators can be implemented using high-power vacuum tubes, but the technology is moving toward solid-state high-power switches, which are generally smaller and more robust. Insulated-gate bipolar transistors (IGBTs) are a common solid-state switch used in high-voltage applications. For a more detailed discussion of one type of conventional high-power modulator that employs IGBTs, see U.S. Pat. Nos. 5,444,610 and 5,646,833, both to Gaudreau et al., issued Aug. 22, 1995, and Jul. 8, 1997, respectively. Both of these documents are incorporated herein by reference. 
     IGBT-based high-power modulators provide excellent high-power, high-speed switching performance. There is always room for improvement, however, as competitive technology markets are ever watchful for cost-competitive systems that offer improved efficiency, reliability, speed performance, or a combination of these. 
     SUMMARY 
     The present invention is directed to an improved high-power modulation system. The system includes novel drive circuitry connected between a signal source and a conventional high-power switch. The drive circuitry receives input signals from the signal source via a series of transformers. The drive circuitry then amplifies the input signals and provides the resulting amplified signals to the high-power switch. A storage capacitor within the drive circuitry stores energy derived from the input signals, and the stored energy is used to power the drive circuitry. 
     Using energy derived from the input signal to power the driver circuitry eliminates the need to connect the driver circuitry to a separate power supply. This simplification allows the driver circuitry to be manufactured using fewer components, advantageously reducing size, cost, and power consumption. Also advantageous, reducing the number of components increases the mean time between failures. 
     In one embodiment, the high-power switch includes a series of power-switching devices that together switch current. The drive circuitry includes a number of drivers, one for each power-switching device. Each driver, in turn, includes a pair of series-connected drive transistors that alternately turn on and off the corresponding power-switching device via a pair of driver output lines. One of the driver output lines connects to a control terminal of the power-switching device; the other driver output line connects to a current-handling terminal of the power-switching device. In an embodiment in which the power-switching device is an IGBT, the driver output lines connect to the gate and emitter of the IGBT. 
     In one embodiment, the driver takes advantage of inductive ringing present on the output lines to more rapidly turn off the power-switching device. A diode connected in series between two drive transistors rectifies the ringing signals, pulling the control signal to the control terminal of the corresponding power-switching device negative relative to the respective current-handling terminal. Pulling the control signal to the power-switching device negative advantageously shuts the device off quickly, improving speed performance. 
     This summary does not limit the invention, which is instead defined by the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 depicts a high-power modulation system  100  in accordance with an embodiment of the present invention. 
     FIG. 2 depicts a portion of system  100  of FIG. 1, like numbered elements being the same. 
     FIG. 3A is a graphical depiction of transformer  113 , omitting primary winding  200  and secondary winding  210  for simplicity. 
     FIG. 3B is another graphical depiction of transformer  113 , this time omitting secondary windings  205  and  210  for simplicity. 
     FIG. 4 depicts a portion of high-power modulation system  100  of FIGS. 1 and 2, detailing driver  107  and high-voltage switch  130 . 
     FIG. 5 is a schematic diagram of a low-noise primary modulator  500  that can be used in place of modulator  105  of FIGS. 1 and 2. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 depicts a high-power modulation system  100  in accordance with an embodiment of the invention. System  100  includes a primary modulator  105  connected to a sequence of driver circuits  107 ,  109 , and  111  via a respective series of transformers  113 ,  115 , and  117 . Each of driver circuits  107 ,  109 , and  111  has a pair of output terminals A and B that connect to a high-voltage switch  130 . High-voltage switch  130  includes a sequence of power switching devices  132 ,  134 , and  136 , there being a power switching device for each of drivers  107 ,  109 , and  111 . 
     Each power switching device has a control terminal connected to the A output of the corresponding driver and a pair of current-handling terminals connected in series with a high-voltage source  140  and a load  150 . For example, power switching device  132  has a control terminal connected to output A of driver  107 , a first current-handling terminal connected to high-voltage source  140  via a conductor  141 , and a second current-handling terminal connected to output B of driver  107  and to switch  134 . High-voltage source  140  typically includes an energy storage capacitor that enables high-voltage source  140  to deliver high-energy pulses to load  150 . 
     Primary modulator  105  periodically completes the circuit defined in part by conductor  170  to introduce periodic signals into transformers  113 ,  115 , and  117 . The resulting output signals from transformers  113 ,  115 , and  117  are then applied to drivers  107 ,  109 , and  111  via respective buses  142 ,  144 , and  146 . Drivers  107 ,  109 , and  111 , in turn, trigger each of power switching devices  132 ,  134 , and  136  within high-voltage switch  130 . When high-voltage switch  130  turns on, high-voltage source  140  delivers a desired power pulse through load  150 . In one embodiment, the return side of load  150  and high-voltage source  140  is tied to a reference level (e.g., ground potential) for safety. 
     System  100  includes three transformers having series-connected primary windings (See FIG.  2 ). The actual number of transformers depends upon the number of power switching devices required to hold off the voltage applied across switch  130 . Assume, for example, that each of power switching devices  132 ,  134 , and  136  is rated to handle 1,200 volts, and that high-voltage source  140  produces 15 kV. Allowing a safety margin of 400 volts for each power-switching device, in that case high-voltage switch  130  would include nineteen (15 kV divided by 800V) power-switching devices triggered by nineteen corresponding transformers. Conductor  170  might be rated for about 50 kV of isolation. 
     FIG. 2 depicts a portion of system  100  of FIG. 1, like numbered elements being the same. The depicted portion details transformer  113 ; the remaining transformers  115  and  117  are identical to transformer  113 , and are therefore omitted for brevity. 
     Transformer  113  includes a primary winding  200  separated from first and second secondary windings  205  and  210  via a core  211 . Secondary winding  205  connects to driver  107  via a first low-voltage conductor LV 1  and a second low-voltage conductor LV 2 , and secondary winding  210  connects to driver  107  via a third low-voltage conductor LV 3  and a fourth low-voltage conductor LV 4 . 
     FIG. 3A is a graphical depiction of transformer  113 , mitting primary winding  200  and secondary winding  210  for simplicity. Secondary windings  205  and  210  are identical. Transformer  113  includes a ferrite core  211 , typically a toroid, wound with multiple turns of magnet wire. A single length of magnet wire wound about core  211  forms lines LV 1  and LV 2  and secondary winding  205 . FIG. 3B is another graphical depiction of transformer  113 , this time showing conductor  170  (the primary winding) and omitting secondary windings  205  and  210  for simplicity. Conductor  170  is a conventional high-voltage, insulated wire in which an electrical conductor  310  forms a single-turn primary through core  211 . In one embodiment, conductor  170  is a silicon-insulated, sixteen-gauge wire. As will be understood by those of skill in the art, the selection of insulation and the inside diameter of core  211  depend upon the desired voltage isolation between conductor  170  and secondary windings  205  and  210 . 
     Transformer  113  includes sufficient cross-sectional core material to support the desired voltage and pulse period for a single-turn primary. For example, let the desired secondary voltage be fifteen volts for five secondary turns. The single-turn primary voltage would have to be about three volts. For fifteen transformers, the required voltage provided by primary modulator  105  (FIG. 2) would be forty-five (three times fifteen) volts. 
     FIG. 4 depicts a portion of high-power modulation system  100  of FIGS. 1 and 2, detailing driver  107  and high-voltage switch  130 . A positive pulse on conductor  170  produces a positive voltage on low-voltage conductor LV 1  with respect to low-voltage conductor LV 2 . As a result, current flows through a resistor  400 , a capacitor  402 , and a diode  404 . This current flow charges capacitor  402  and reates a voltage drop across a resistor  406 . The voltage across resistor  406  turns on a transistor  408 , a conventional P-channel MOSFET in the depicted example. 
     Turning on transistor  408  causes current to flow between the top and bottom plates of capacitor  402  via transistor  408  and a pair of resistors  410  and  412 . The resulting voltage drop across resistor  412  turns on power-switching device  132 . As shown in FIG. 1, closing switching devices  132 ,  134 , and  136  allows current to flow from source  140  through load  150 . While switching device  132  is an insulated-gate bipolar transistor (IGBT) in the depicted example, other types of switches may also be used. Examples include field-effect transistors, triacs, silicon controlled rectifiers, thyristors, and power Darlingtons. IGBTs for use in the present invention include the BSM200GA120DN IGBT available from Eupec, Inc. The selection of the values for resistors  410  and  412  depends, in part, on the input capacitance of power-switching device  132 . 
     When a positive pulse on conductor  170  produces a positive voltage on low-voltage conductor LV 1  with respect to low-voltage conductor LV 2  to turn on transistor  408 , the same positive pulse on conductor  170  produces a negative voltage on low-voltage conductor LV 3  with respect to low-voltage conductor LV 4 . This negative voltage is applied to the control terminal of a transistor  414  via a capacitor  416  and a pair of resistors  418  and  420 . This negative voltage ensures that transistor  414  remains off while transistor  408  is on. A diode  422  limits the amount of current that flows from line LV 4  to line LV 3 , reserving most of the energy in core  211  for turning on transistor  408  and power-switching device  132 . 
     At the end of the input pulse on conductor  170 , the energy stored in core  211  reverses the voltages across windings  205  and  210 . Conductor LV 1  therefore goes negative with respect to conductor LV 2 , turning off transistor  408 . Capacitor  402  retains a level of charge between pulses, allowing transistor  408 —and consequently switch  132 —to trigger more rapidly upon receipt of the next input pulse. 
     The reversed voltage through winding  210  causes conductor LV 3  to go positive with respect to conductor LV 4 . Current therefore flows through resistor  418 , diode  422 , and resistor  420 . The resulting voltage drop across resistor  420  turns on transistor  414 , discharging the control voltage on switching device  132  via resistor  410 , a diode  430 , and transistor  414 . Transistors  408  and  414  thus provide low-impedance paths that rapidly turn switching device  132  on and off, respectively. 
     Signal lines A and B from driver  107  exhibit some series inductance, modeled as an inductor  450 , and have some capacitance between them, modeled as a capacitor  455 . When transistor  414  turns on, inductor  450  and capacitor  455  induce ringing on lines A and B. Diode  430  rectifies this ringing, limiting the positive swing on line A to quickly turn off switching device  132 . In some embodiments, the values of inductor  450  and  455  are such that the voltage between the gate and emitter of the IGBT is pulled below zero for an instant, helping switching device  132  turn off quickly. 
     Two transient-voltage suppressors  440  and  442  protect respective transistors  408  and  414  from excessive voltage, while a third transient-voltage suppressor  444  similarly protects power-switching device  132 . 
     A network  480  of transient voltage suppressors and a diode  490  limit the collector to emitter voltage of power-switching device  132  to a level below the safe operating voltage of power-switching device  132 . By adding or subtracting from the number of transient voltage suppressors in network  480 , the clamping voltage between the collector and emitter of power-switching device  132  can be adjusted to accommodate devices with different collector-to-emitter breakdown-voltage ratings. 
     When the voltage across network  480  increases above the selected breakdown voltage, the transient voltage suppressors conduct current through the gate/emitter junction of power-switching device  132  to keep device  132  out of the cutoff mode. Keeping device  132  out of the cutoff mode allows for current shunting from collector to emitter, thereby lowering the dynamic impedance of device  132 , and hence lowering the voltage across device  132 . This action protects device  132  from an over-voltage condition during turnoff and power-supply transients. 
     Diode  490  prevents gate/emitter junction charging and discharging currents transferred by high-voltage switch  130  from charging and discharging series capacitances related to network  480  and power-switching device  132 . Diode  490  ensures that a majority of the current through high-voltage switch  130  is delivered to the gate/emitter junction of power-switching device  132 , maintaining a fast turn-on and turn-off. 
     A resistor  492  and capacitor  494  connected across power-switching device  132  act as a voltage-transient dampening pair. When device  132  turns off, the initial transient energy caused by the series output inductance of device  132  is shunted through resistor  492  and capacitor  494 . The current passing through this capacitor/resistor pair slows the voltage slew rate to a time constant that is equal to or greater than the reaction time of the network  480 . The combination of network  480  and the capacitor/resistor pair protects device  132  from an over-voltage condition caused by high slew rate transients. These transients can also be caused by high-voltage power supply fluctuations. In general, this capacitor/resistor configuration slows the voltage slew rate across the collector-to-emitter junction of device  132  enough for network  480  to react and limit the collector-to-emitter voltage of device  132  to a value below the maximum operating voltage of device  132 . 
     A collection of balancing resistors  496  divides the voltage evenly across the collector-to-emitter junction of each power-switching device  132  within high-voltage switch  130 . Balancing resistors  496  ensure that each power-switching device  132  has the same collector-to-emitter voltage before high-voltage switch  130  turns on and after high-voltage switch  130  turns off (i.e., in the cutoff mode). 
     Capacitor  402  stores charge derived from input pulses on conductor  170  and uses this charge to supply current to transistors  408  and  414 . This configuration reduces or eliminates the need to connect drivers  107 ,  109 , and  111  to a separate power supply. This simplification allows drivers  107 ,  109 , and  111  to be manufactured using fewer components, advantageously reducing size, cost, and power consumption, while at the same time increasing the mean time between failures. 
     FIG. 5 is a schematic diagram of a low-noise primary modulator  500  that can be used in place of modulator  105  of FIGS. 1 and 2. Primary modulator  500  receives a periodic input signal on line IN and produces a corresponding sequence of periodic pulses to primary  200 . In one embodiment, the periodic pulses range in frequency from a few pulses per second to several thousand pulses per second. The maximum frequency depends in part on pulse width. 
     The primary modulator  500  receives power from a conventional 120-volt AC power source  502  through a transformer  504 . Transformer  504  has a dual-secondary output that can be configured in series or parallel to change the output voltage, as desired. In the depicted example, the secondary winding is configured in series. 
     The outputs of transformer  504  connect to a conventional rectifier  506 , the positive and negative output terminals of which connect across a filter cap  508  and a high-voltage linear regulator  510 . In one embodiment, the output of regulator  510  can be adjusted between about ten volts and 400 volts using a potentiometer  512  connected in parallel with a filter cap  514 . 
     The output VO of regulator  510  connects to the gate of a transistor  516  via a resistor  518 . Transistor  516 , typically a FET or IGBT, is used in a linear mode as a voltage follower to maintain the voltage across a capacitor  520 , which stores the energy used to pulse primary winding  200 . Transistor  516  ensures that the voltage across capacitor  520  returns to a desired level between pulses delivered to primary  200  via conductor  170 . 
     Referring to the lower half a primary modulator  500 , the input signal on line IN passes through a conventional driver  522  and a resistor  524  to the control terminal of a transistor  526 . Driver  522  converts a zero-to-five volt input signal on line IN into a zero-to-fifteen volt signal to resistor  524 . 
     Transistor  526  responds to signals from driver  522  by drawing current from capacitor  520  through a resistor network  528  and primary winding  200 . A pair of series-connected diodes  529  and  530  protects modulator  500  from noise spikes from primary  200 . 
     Driver  522  derives power from regulator  510  on a power-supply line that is filtered using a capacitor  532  and voltage-limited at the power-supply input to driver  522  by a zener diode  535 . A light-emitting diode  540  indicates whether modulator  105  is receiving an input signal. 
     While the present invention has been described in connection with specific embodiments, variations of these embodiments will be obvious to those of ordinary skill in the art. For example, the number of primary and secondary turns, the core size, and the primary voltage of the transformers may change, depending upon driver requirements. Moreover, some components are shown directly connected to one another while others are shown connected via intermediate components. In each instance, the method of interconnection establishes some desired electrical communication between two or more circuit nodes, or terminals. Such communication may often be accomplished using a number of circuit configurations, as will be understood by those of skill in the art. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description.