Optimized Design of Transformers for Application in Silicon Controlled Rectifier Extinction and Ignition Circuits

In recent years the evolution of more sophisticated military avionic systems has required a corresponding reduction in component size and an increase in component life, precision, and reliability. To satisfy these more difficult design requirements, silicon solid-state devices are being used more extensively in power switching applications. Silicon controlled rectifiers (SCR's) can be used in place of relays or to drive relays with the proper ignition and extinction control circuits. The most general purpose external control circuits are the use of a transformer winding in series or in parallel with the SCR for quenching and a transformer winding connected between the trigger and the cathode for ignition. The use of separate transformers for exact timing and control of SCR's in dc circuits is an optimum control method since transformers have no moving parts and are usually more reliable than relays or pure transistor networks. In ac circuits, where waveform zeros are not used for switching, transformers can provide a convenient, precise method of SCR control. The purpose of this paper is to provide an optimum procedure for the designing of single primary and secondary winding ignition and extinction transformers (as shown in Fig. 1) for controlling SCR's. In addition, this paper provides a transformer core and winding specification synthesis that minimizes the over-all transformer size. A transformer secondary can provide a voltage zero in series with the SCR which allows the center p-n region to naturally recombine its excess charge carriers. It can also reset an SCR by providing both an external short across the SCR and the required SCR reverse current to deplete the excess charge carriers in the region of the center junction. This paper presents six circuits and their design parameters for setting and resetting SCR's. There are two ignition circuits and four quench circuits. For each of the six transformer circuits a relizability theorem, using the external circuit parameters, is provided so that a nonrealizable synthesis can be avoided. Ignition and extinction of SCR's require either a current impulse or a voltage impulse. Consequently the transformer design procedure is based upon certain essential elements of the full equivalent circuit, which is shown in Fig. 2. These transformers are intermittent in operation and do not require phase and amplitude linearity; hence the significant circuit design parameters are the turns ratio N, coil resistances R
ss
and R
sp
, and primary shunt inductance L
p
. The procedure developed in this paper is based upon these fundamental transformer parameters with derived criteria for maximum values of leakage inductance L
s
and equialent shunt capacitance C
w
(both referred to the primary side). A simple synthesis procedure is also presented in this paper for the determination of actual transformer core and winding dimensions based upon the equivalent circuit parameters of Fig. 2 and the known external circuit environment.