This paper describes in detail the principle of operation of the MC34063 and ìA78S40 switching regulator subsystems. Severalconverter design examples and numerous applications circuits with test data are included.

 

The SE34063 is a monolithic switching regulator subsystems intended for use as dc to dc converters. The devices represent a significant advancement in the ease of implementing highly efficient and yet simple switching power supplies. The use of switching regulators is becoming more pronounced over that of linear regulators because the size reductions in new equipment designs require greater conversion efficiency. Another major advantage of the switching regulator is that it has increased application flexibility of output voltage. The output can be less than, greater than, or of opposite polarity to that of the input voltage.

 

 

In order to understand the difference in operation between linear and switching regulators we must compare the block diagrams of the two step-down regulators shown in Figure1. The linear regulator consists of a stable reference, a high gain error amplifier, and a variable resistance series-pass element. The error amplifier monitors the output voltage level, compares it to the reference and generates a linear control signal that varies between two extremes, saturation and cutoff. This signal is used to vary the resistance of the series-pass element in a corrective fashion in order to maintain a constant output voltage under varying input voltage and output load conditions.

The switching regulator consists of a stable reference and a high gain error amplifier identical to that of the linear regulator. This system differs in that a free running oscillator and a gated latch have been added. The error amplifier again monitors the output voltage, compares it to the reference level and generates a control signal. If the output voltage is below nominal, the control signal will go to a high state and turn on the gate, thus allowing the oscillator clock pulses to drive the series-pass element alternately from cutoff to saturation. This will continue until the output voltage is pumped up slightly above its nominal ＆#118alue. At this time, the control signal will go low and turn off the gate, terminating any further switching of the series-pass element. The output voltage will eventually decrease to below nominal due to the presence of an external load, and will initiate the switching process again. The increase in conversion efficiency is primarily due to the operation of the series-pass element only in the saturated or cutoff state. The voltage drop across the element, when saturated, is small as is the dissipation. When in cutoff, the current through the element and likewise the power dissipation are also small. There are other variations of switching control. The most common are the fixed frequency pulse width modulator and the fixed on-time variable off-time types, where the on-off switching is uninterrupted and regulation is achieved by duty cycle control. Generally speaking, the example given in Figure 1b does apply to SE34063.