therefore becomes mandatory to design for sufficient switch drive during peak load conditions and also incorporate current-limiting or rapid overload protection systems.
Commercial wide-range power supplies invariably have output current limiting, but this does not limit the preregulator currents except during steady-state load conditions (including short circuits). Consider, for example, a power supply operating at short circuit and the short being removed suddenly. Referring to Fig. 8, the output would rise rapidly, reduce the passing stage voltage, and close the switching transistor. The resulting transient extends over many cycles (switching rate) so that the inductance of the preregulator filter becomes totally inadequate to limit current flow. Therefore, the current will rise until steady state is resumed, circuit resistance causes limiting, or insufficient drive causes the switch to come out of saturation. The latter condition leads to switch failure.
Other operating conditions that would produce similar transients include output voltage programming and initial turn-on of the supply. Momentary interruption of input power should also be a prime consideration.
One solution to the problem is to limit the rate of change of voltage that can appear across the passing stage to a value that the preregulator can follow. This can be done conveniently by the addition of sufficient output capacitance. This capacitance in conjunction with the current limiting characteristic would produce a maximum rate of change of
where
C0 = output capacity.
Assuming that the preregulator follows this change and has a filter capacitor Cl, then the switch current is
During power on, the preregulator reference voltage rise must also be limited.
Taking this into account,
where
ER = passing stage voltage
Tl = time constant of reference supply.
The use of SCR's to replace the transistors would be a marked improvement due to higher surge current ratings, but turning them off requires large energy sources. While the gate turn-off SCR seems to offer a good compromise to the overall problem, the severe limitations in current ratings presently restrict their use.
REFERENCES
[1] J. G. Truxal, Control Engineer's Handbook. New York: McGraw Hill, 1958, pp. 11-19.
[2] Motorola Zener Diode/Rectifier Handbook, 2nd ed. 1961. [3] W. Steiger, \tion to differential amplifiers,\vol. 1-8, pp. 82-91, December 1959.
[4] L. P. Hunter, Handbook of Semi-Conductor Electronics. New York: McGraw Hill, 1956, p. 13-3.
[5] \supplies,\Semi-Conductor Power Converter Section, NEMA.
[6] P. Muchnick, \plies,\
[7] R. D. Loucks, \regulators,\
[8] D. Hancock and B. Kurger, \supply utilizing high speed switching,\Winter General Meeting, New York, N. Y., January 27- February 1, 1963.
[9] R. D. Middlebrook, Differential Amplifiers. New York: Wiley, 1963.
[10] Sorensen Controlled Power Catalog and Handbook. Sorensen, Unit of Raytheon Company, South Norwalk, Conn.
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