Abstract:
A method and apparatus for generating low-jitter, high-voltage and high-current pulses for driving low impedance loads such as detonator fuses uses a MOSFET driver which, when triggered, discharges a high-voltage pre-charged capacitor into the primary of a toroidal current-multiplying transformer with multiple isolated secondary windings. The secondary outputs are suitable for driving an array of thyristors that discharge a precharged high-voltage capacitor and thus generating the required high-voltage and high-current pulse.

Description:
The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for Management of the Lawrence Livermore National Laboratory. 
    
    
     FIELD OF INVENTION 
     The invention relates to the field of high-current and high-voltage (i.e. high power) thyristor pulse drivers and generators such as used for explosive detonating firesets, cable and printed circuit board fault clearing, and for driving low impedance transducers and loads. 
     BACKGROUND OF THE INVENTION 
     The generation of high power electrical pulses is often required in various applications such as in the firesets used for detonating explosives. These firesets typically use one or more MOS (metal-oxide-semiconductor) controlled thyristors (MCTs) to drive an array of serial and/or parallel connected explosives. Controlled precision timing of detonation is desired in order to control the effect of the detonation of an array of explosives as well as for studying the details of how such explosive events unfold. 
     Some prior-art devices use sparkgap-based driver devices for discharging an energy storage capacitor for generating high power pulses. Others use CMOS or TTL transistors to directly drive one or more MCTs. These devices exhibit time-jitter and/or deteriorate rapidly when subjected to repeated use. The present invention corrects these prior-art problems. 
     BRIEF SUMMARY OF THE INVENTION 
     A low-jitter pulse-driver for driving a thyristor array and for producing a high-voltage and high-current output pulse from the thyristor array with fast rise-time and low onset jitter includes: 
     a) a trigger controlled low-voltage driver for driving a fast high-voltage switch; 
     b) a fast high-voltage switch for discharging a precharged capacitor into the primary of a current-multiplying transformer; and 
     c) a current-multiplying transformer with its primary winding connected in series with the fast high-voltage switch and a precharged capacitor, the transformer having at least one electrically isolated secondary winding for outputting a fast rise-time electrical pulse suitable for driving one or more prescribed thyristors. 
     The low-jitter driver is suitable for many different applications requiring a high power input pulse with low time-jitter. A precharged capacitor is selected so that the energy stored in the capacitor produces the desired current pulse when discharged into the primary of the current-multiplier transformer and produces the required electrical pulse at the secondary output to drive a prescribed load, such as a thyristor. 
     One embodiment includes a high-voltage charging circuit for pre-charging the capacitor for providing the energy to drive the current-multiplying transformer. 
     Another embodiment includes a thyristor array for generating a high-voltage and high-current pulse with low-jitter, the thyristor array driven by multiple electrically isolated secondary windings of the current-multiplying transformer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a prior-art thyristor driver circuit. 
     FIG. 2 is a block diagram of a low-jitter, high current, and high voltage, thyristor-array pulse driver. 
     FIG. 3 is a circuit diagram of low-jitter, high current, and high voltage, thyristor pulse system including a driver circuit and thyristor array. 
     FIG. 4 shows a set of waveforms that demonstrate the operation of the circuit of FIG.  3 . 
     FIG. 5 is a typical current-waveform pulse produced by the thyristor-array driver circuit outputting into a short-circuit load. 
     FIG. 6 shows a superposition of five output current-waveform pulses into a short circuit load with less than 1 ns of jitter. 
     FIG. 7 shows a typical thyristor output voltage-pulse and current-pulse waveforms into a detonator load. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a block diagram of a prior-art thyristor array high-power pulse generator system  100 . Driver circuit  101  is typically a set of one or more CMOS (complementary metal-oxide-semiconductor) or TTL (transistor-transistor-logic) power transistor circuits for driving one or more thyristors in thyristor array  102 , the thyristor array providing a high current and voltage pulse to a fireset detonator load. A low-voltage input pulse at input terminal  103  initiates the circuit action. Alternatively, a suitable high-voltage driver circuit call be used to drive a triggered spark-gap switch. The spark-gap switch is used to discharge a precharged high-voltage capacitor for producing a high-voltage and high-current output pulse. However, spark-gap switches have proven to have low reliability and high jitter in the generated output pulse waveforms. 
     FIG. 2 shows a block diagram of a low-jitter, high-current and high-voltage thyristor pulse generator system  200  which overcomes the shortcomings of the prior-art. Thyristor driver  201  accepts a low-voltage input trigger on input line  206  and produces a low-voltage output pulse to high-voltage power switch  202  on line  207  for switching-on power switch  202 . When on, switch  202  is connected in series with precharged high-voltage capacitor  203  by line  208  and in series with the primary winding of current multiplying transformer  204  by means of line  209 . This provides a conductive path for discharging precharged capacitor  203  through the primary winding of transformer  204  and produces a current-amplified pulse on each of the secondary output lines  210 ( 1 ) . . .  210 (n) for driving selected thyristor sub-arrays  205 ( 1 ) . . .  205 (n) in thyristor array  205 . Each thyristor sub-array  205 (k) may consist of one or more parallel thyristors for providing higher output current to their loads. Stacking sub-arrays in series generates a higher voltage output pulse. 
     FIG. 3 is a circuit diagram of a low-jitter MOSFET-driven thyristor pulse generator system  300  that includes a preferred embodiment of a low-jitter pulse driver circuit  310  and a thyristor-array  350 . A positive trigger, on input line  301 , is applied to input terminal  2  of MOSFET driver unit U 1 . A suitable choice for driver unit U 1  is the model MIC4452 MOSFET driver (manufactured by Micrel Semiconductor, 1849 Fortune Drive, San Jose, Calif. 951331) which is a fast non-inverting driver and accepts an input trigger level from 2.4 volts minimum to a maximum of V 5 =15 volts (the supply voltage applied to terminals  1  and  5 ). The output level at pins  6  and  7  is within 25 mV of ground and V 5  for low and high logic levels, respectively. When an appropriate positive trigger level is applied to input line  301 , driver U 1  turns on n-channel MOSFET high-voltage switch SW 1  by applying the output of terminals  6  and  7  to the control gate of SW 1  at terminal  1 . The drain terminal is connected to drain voltage supply V dd  (+165 volts) through resistor R 2 , The preferred embodiment uses a model STW5NB100 high current and high speed switch manufactured by SGS-Thomson (STMicroelectronics, 1060 E. Brokaw Road, San Jose, Calif. 95131) which is capable of switching drain-to-source voltages up to 1 kV. Capacitor C 2  of 0.01 uF has one side connected to the drain terminal  2  of SW 1  and the other side to terminal  1  of the primary of current multiplying transformer T 1 . The source terminal  3  of SW 1  and terminal  2  of the primary of T 1  are both connected to common ground. The secondary of transformer T 1  can have multiple independent (i.e. electrically isolated) secondary windings. The number of independent windings depends on the type of devices that are to be driven by the output of transformer T 1  The current multiplying factor between the primary current and a secondary current is determined by the ratio of primary winding turns to secondary winding turns, For the preferred embodiment of FIG. 3, T 1  is a toroidal transformer with a  10  ram primary and two isolated one-turn secondary windings for matching the requirements of the load represented by thyristor array  350 . 
     Thus, when SW 1  is not conducting from drain to source, capacitor C 2  is precharged to the +165 volt supply voltage through resistor R 2 . But when SW 1  is triggered into its conducting state by the positive trigger output of U 1 , the charge on C 2  is discharged as a current pulse through SW 1  and the primary of transformer T 1 . 
     Thyristor array  350  has two series-stacked sub-arrays of parallel-paired MOS-controlled thyristors, (MCT 1 , MCT 2 ) and (MCT 3 , MCT 4 ), for doubling the current and voltage capacity of the array output. The load that the thyristor-array  350  must drive determines the array size and configuration. In a preferred embodiment, MCT 1  . . . MCT 4  are model SMCT2TA65N14A10MCTs (manufactured by Silicon Power Corporation, Commercial Power Division, 3 Northway Lane North, Suite 1, Latham, N.Y. 12110-2204). 
     Referring back to FIG. 3, transformer T 1  secondary output terminal pairs ( 3 , 4 ) and ( 5 , 6 ) of MOSFET driver  310  are respectively connected to the gate G and anode A of parallel thyristor pairs (MCT 1 , MCT 3 ) and (MCT 2 , MCT 4 ) for providing the required gating current to trigger the thyristor pairs into conduction. Resistors R 4  and R 5 , connected across the secondary terminals ( 3 , 4 ) and ( 5 , 6 ) are dampening resistors for controlling overshoot of the current pulse leading edge, while resistors R 9  and R 10  ensure that the 2 kV charge across capacitor C 3  is evenly split between the two series thyristor pairs (MCT 1 , MCT 2 ) and (MCT 3 , MCT 4 ). The thyristor array  350  high-voltage supply of 2 kV is applied to input terminal  302  for precharging capacitor C 3  through resistor R 8 . Precharged capacitor C 3  becomes the current source for the thyristor array when it is conducting. (N.B.: each application, with its own peculiar high-voltage and current requirement, will determine the thyristor array configuration.) 
     When MOSFET driver  310  is triggered, the two secondary windings produce a current pulse that drives thyristor array  350  into conduction and causes the precharged capacitor C 3  to discharge through thyristors MCT 1  . . . MCT 4  and through output line  303  into the detonator load R L . Conduction stops when the capacitor C 3  is discharged because the current limited by resistor R 8  is too small for sustained conduction of the thyristors. 
     Waveforms (a) . . . (e), shown in FIG. 4, summarize the operation of the circuit in FIG. 3 when driving a resistive load. Waveforms (a) and (b), from t=0 to t=t 0 , show the charging voltage across capacitors C 2  and C 3 , respectively, when the power supplies are turned on at t=0. At t=t 0 , a positive trigger, V T , is applied on trigger input line  301 , and remains on for at least as much time as capacitor C 2  requires to discharge as shown by waveform (c). Waveforms (d) and (e) respectively show the resulting discharge currents from capacitor C 2  through transformer T 1 and switch SW 1 , and from capacitor C 3  through the thyristor array and output load R L . After the trigger input is removed at time t 1  (waveform (c)), SW 1  is turned-off and capacitor C 2  recharges to 165 volts (waveform (a)). When the thyristor array stops conducting at t=t 2  in waveform (e), capacitor C 3  begins to recharge through R 8  to 2 KV, and the system is ready to be used again. 
     A convenient way to test the MOSFET driven thyristor system of FIG. 3 is by short-circuiting output line  303  and observing the short-circuit output current when the input line  301  is triggered. The resulting short-circuit current waveform is shown in FIG.  5 . This test, conducted with C 3  having 0.5 uF capacitance, results in an output current-pulse having a damped sinusoidal-waveform due to the C 3  discharge-path inductance and resistance. The approximately 1 us spacing between zero-crossings indicates a natural resonant frequency of about 1 MHz while the peak current exceeds 6000 amperes. 
     The low-jitter characteristic of the previously described low-jitter MOSFET-driven thyristor system is demonstrated in FIG. 6 where the early onset portion of the short-circuit current waveform is shown expanded and the superposed current waveforms from five successive firings of the low-jitter MOSFET-driven thyristor system are shown. Examination of the spread between traces indicates that the jitter of the output short-circuit current waveforms is less than 1 nS. 
     FIG. 7 shows another example of the low-jitter MOSFET-driven thyristor system output load-current, I L , and load-voltage, V L , waveforms for the case when capacitor C 3  of FIG. 3 has a value of 0.82 uF and the load is a typical exploding bridgewire (EBW) or slapper detonator. Load current, I L , rises to a peak-plateau of almost 5000 amperes and eventually drops-off after about 280 nS. The load voltage peaks and drops-off more sharply after exceeding 1500 volts. 
     The present invention has been described in terms of specific embodiments, which are illustrative of the invention, and are not to be construed as limiting.