Dr. Middlebrook’s Impedance Conditions
Power Supply Basics
The idealized circuit above represents the simplest power supply imaginable, the trusty voltage divider! Vi represents the input voltage, while Io is the output (load) current.In a switcher, a somewhat more complex circuit, the arrangement is remarkably similar. R1 represents the series losses in the converter, while R2 represents the parallel losses.Series losses consist of wire resistance in magnetics and traces, esr’s of power capacitors, and forward voltage drops across the switch (ON voltage).
Parallel losses are less important in switchers, and consist of AC losses in the magnetics (core loss), leakage losses in capacitors, and leakage current in the switch (OFF current). It will also help to include power consumed by the housekeeping and drive circuitry at first, though these will not enter into our final discussion.
Our power supply will, of course, have to meet certain specs. It will have to regulate. That is, it must maintain the output voltage Vo within a defined band of regulation as the load current increases from 0 to maximum load.Further, when the load current exceeds a given safe limit, the p.s. must go into current limit, with a max short circuit current.
Examination of the Vo vs. Io plot above reveals that our circuit performs admmirably! The output stays in reg from Io=0, until Vo has sagged to Vreg, our minimum spec voltage. At this time, the converter begins a smooth transition into current limit. Beyond Icl, we are in current limit, with a maximum current Imax drawn at dead short!