Abstract:
A two-phase control system for sequential switching of a.c. power. An a.c.nput is split into two phases. The two phases are switched simultaneously or independently by a plurality of switches in sequence. The output from the switches is recombined to provide the original a.c. signal at a desired voltage level.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to power control systems, and more particularly to a two-phase control system for sequential switching of a.c. power. 
     2. Description of the Prior Art 
     In aerospace applications it is desirable to convert medium voltage signals to high voltage signals upon command to activate various pyrotechnic events in a predetermined sequence. As shown in FIG. 1 prior art techniques used separate transformers T1&#39;, T2&#39;, T3&#39; between the input, output, and each switching function SW1&#39;, SW2&#39;. The switching function was performed between the center-tapped secondary of one transformer and the center-tapped primary of the next transformer, and at an intermediate voltage by relays, high voltage transistors or the like, which components are less reliable in the severe aerospace environments encountered than low voltage d.c. switching devices. 
     Therefore, it is an object of the present invention to provide for sequential switching of a.c. power at low voltages. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a two-phase control system for sequential switching of a.c. power. The a.c. power input is split into two phases at a low voltage. Each phase is sequentially switched simultaneously by d.c. switches. After the final switching function the two phases are recombined and output to provide a high voltage a.c. power output. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a prior art a.c. sequential switching control system. 
     FIG. 2 is a block diagram of the two-phase control system of the present invention. 
     FIG. 3 is a schematic diagram of one embodiment of the two-phase control system. 
     FIG. 4 is a schematic diagram of a typical circuit controlled by the two-phase control system. 
     FIG. 5 is a schematic diagram of a typical d.c. switching function for the two-phase control system. 
     FIG. 6 is a timing diagram for the two-phase control system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 2 a.c. voltage, typically on the order of 100-200 volts at 3-6 amps, is input to the primary of transformer T1. The secondary of transformer T1 has a grounded center tap and diodes D1, D2 to provide two phases φ 1 , φ 2  180° out of phase with each other. Switching functions SW1, SW2, SW3 in each phase path operate in response to d.c. control signals to either enable or block each phase path by closing or opening the paths for both phases simultaneously. After all the switching functions have been applied closing the switching functions, transformer T2 with a grounded primary center tap recombines the two phases φ 1 , φ 2  to produce an a.c. voltage output at the secondary. Since each phase φ 1 , φ 2  is merely a unipolar signal referenced to ground and shifted 180° from the other (see FIG. 6), the switching functions SW1, SW2, SW3 may be d.c. switches implemented by solid state devices operating at a low voltage, typically 30 volts peak for each phase. The a.c. voltage output is typically on the order of greater than 1,000 volts to provide the high power impulse required by pyrotechnic ignition circuits. 
     A typical application of a two-phase control system is illustrated in FIG. 3. A battery B, such as a missile flight battery, provides power to an inverter/converter 10. The inverter/converter 10 converts the power from the battery B into necessary system d.c. voltages and an a.c. voltage, typically 100-200 volts at 400 Hz. A transformer T, having a center tapped secondary connected to the return line 12, together with diodes D splits the inverter a.c. output voltage into two phases φ 1 , φ 2  with peak values of typically 30 volts. The two phases φ 1 , φ 2  are applied to the collectors of transistors Q 11 , Q 21 , respectively. Application of a control signal DC1 to the base of transistors Q 11 , Q 21  simultaneously allows the two phases to appear as an output at the respective emitters. The output of transistors Q 11 , Q 21  is then applied as an input to an output circuit FU1. A d.c. voltage on line 14 from the inverter/converter 10 can likewise be switched by the control signal DC1 through transistor Q 10  to provide a d.c. voltage to the output circuit FU1. The two phases φ 1 , φ 2  are also applied to the collectors of transistors Q 12 , Q 22  in parallel with transistors Q 11 , Q 12 . When a second control signal DC2 is applied to the bases of transistors Q 12 , Q 22 , the transistors conduct and provide the two phases φ 1 , φ 2  as an output to a second output circuit FU2 and to transistors Q 13 , Q 23  in series with transistors Q 12 , Q 22 . The d.c. voltage for the second output circuit FU2 may be switched on by a third control signal DC3 applied to transistor Q 20 . Likewise a fourth control signal DC4 applied to transistors Q 30 , Q 13 , Q 23  switches the two phases φ 1 , φ 2  and the d.c. voltage to a third output circuit FU3 as well as passing these signals on to the next switches in sequence. The final switching stage would switch the two phases φ 1 , φ 2  and the d.c. voltage to the final output circuit FUη. The d.c. voltage signal line 14 could be eliminated for those output circuits which do not need a d.c. voltage signal. 
     Using integrated circuits and micro-electronics technology redundancy can be readily achieved by providing more than one series diode D in each phase path to protect against a short circuit failure of one diode, and by providing parallel redundant transistors for each switching function to protect against an open circuit failure of one transistor. 
     One type of output circuit is shown in FIG. 4. The two phases φ 1 , φ 2  are applied to each side of the center tapped primary of an output transformer T o . The d.c. voltage signal is applied to the base of a transistor Q o  situated between the center tap of the primary of transformer T o  and the return line so that the d.c. voltage signal enables the return line for the transformer T o . A full wave bridge rectifier CR is connected across the secondary of transformer T o  to provide a high voltage d.c. output to a storage capacitor C. The storage capacitor C provides the high voltage energy necessary to detonate a pyrotechnic device, for example. 
     A control circuit for transistor Q 11  is shown in FIG. 5. A voltage source V dc  having a value greater than the peak value of phase φ 1 , such as 40 volts as opposed to 30 volts for phase φ 1 , is applied to the emitter of transistor Q A . The collector of transistor Q A  is connected to the base of transistor Q 11 . The control signal (DC1) is applied to the base of transistor Q B  which is in a common emitter configuration with the collector connected to the base of transistor Q A . Transistors Q A  and Q B  are of opposite type, Q A  being PNP and Q B  being NPN. A relatively small value control signal (DC1) switches on transistor Q B  which in turn switches on transistor Q A  to apply V dc  to the base of transistor Q 11 , thus switching on transistor Q 11 . 
     Transformers used in the two-phase control system are square loop transformers which reproduce any input a.c. wave shape such as that shown in FIG. 6 so long as the flux, φ, does not exceed saturation, i.e., the flux density, B, remains within the hysteresis loop. This characteristic of the square loop transformer places a lower frequency limit on the a.c. waveform which can be faithfully reproduced, i.e., the saturation time must be greater than one-half the a.c. period. Such transformers are readily available. 
     Thus, the present invention eliminates the multiple transformers of the prior art by switching each phase of the a.c. power signal separately and recombining the two phases only after all control switching has been completed.