A study of the Morgan D-C — D-C step-down circuit

In its simplest form, the Morgan circuit consists of four components: a silicon-controlled rectifier (SCR), a commutating capacitor, a saturable autotransformer, and a load resistor. When the gate supply puts out equally spaced trigger pulses to the SCR, the Morgan circuit provides equally spaced power pulses across load resistor R. The over-all technique is known as time ratio control (TRC), where a power pulse of given amplitude is turned on for Ton seconds of a tp-second period. When the pulse is rectangular, this ratio is TRC = Ton/tp = Ton/Ton + Toff = Vo/Vin. When this Morgan circuit is used as a d-c — d-c step-down converter which supplies a wide range of voltage and load requirements, it is advantageous to determine those circuit parameter values which produce the maximum possible TRC (highest output voltage) over the required range of load requirements, by using the laboratory design method presented in this paper. The high-current capabilities were found to be limited by capacitor size and the low currents usually by the maximum pulse-width capability of the trigger circuit. At intermediate currents and high TRC values, alternate pulse degeneration and its outgrowth, bistable operation, are encountered; at low TRC values, and over various currents, capacitor leakage results in pulse distortion. The causes of bistable operation and capacitor leakage are discussed in terms of two waveshapes, and proven remedies are presented. They result in a stable and regulated d-c-d-c step-down converter employing a Morgan circuit, filtering, and feedback capable of supplying a broad range of voltage and current requirements.