Linear Power Supplies, Rectification & Regulation

A linear supply separates two jobs. The rectifier and reservoir make an unregulated DC rail whose instantaneous value carries mains, load, and source-impedance variation. A feedback regulator then controls a series or shunt element so the load sees a cleaner voltage. Regulation cannot create missing headroom, and its rejected energy becomes heat.

Energy and control path #

flowchart TB
  SRC["source / transformer"] --> BR["rectifier"]
  BR --> RES["reservoir capacitor"]
  RES --> PASS["pass element"]
  PASS --> LOAD["load"]
  LOAD --> SENSE["sense divider"]
  REF["reference"] --> ERR["error amplifier"]
  SENSE --> ERR
  ERR --> PASS

The reservoir ripple estimate $\Delta V\approx I_L/(f_rC)$ uses $f_r=f_{line}$ for half-wave and $2f_{line}$ for full-wave rectification. The minimum unregulated voltage, including ripple valley, low line, transformer regulation, diode drop, and wiring drop, must remain above $V_{OUT}+V_{dropout}$ during the transient of interest.

Linear regulation #

A series regulator compares a fraction of output with a reference and varies pass-device conduction. Its approximate dissipation is

$$P_{pass}\approx(V_{IN}-V_{OUT})I_{OUT}.$$

Efficiency is therefore bounded roughly by $V_{OUT}/V_{IN}$, excluding control current. A shunt regulator diverts current away from the load and can be simple and quiet at low power, but wastes current continuously.

An LDO uses a pass device that can regulate with small input–output difference. Dropout is specified at a particular current and temperature; it is not a fixed threshold independent of load. Ground current, reverse current, startup, current limit, thermal shutdown, and foldback influence system behavior.

Capacitors close the loop #

Input and output capacitors supply transient current and shape regulator loop gain. Some regulators require an ESR range; others are designed for low-ESR ceramics but still require minimum capacitance and placement. A capacitor value printed in a schematic is incomplete without bias derating, tolerance, ESR, ESL, ripple current, temperature, and distance to the regulator.

Protection and reverse paths #

Large output capacitors or multiple rails can drive current backward through a regulator when input collapses. Add the manufacturer-recommended discharge path or protection diode where necessary. Include transformer/input fuse behavior, inrush limiting, rectifier surge, capacitor discharge, overvoltage, reverse polarity, and earth/isolation requirements.

Verification #

Measure low/high line, no/full/dynamic load, cold/hot startup, dropout, ripple, load-step deviation and settling, output noise, loop stability with actual capacitors, pass-device temperature, and fault recovery. A steady DMM reading validates none of the transient or thermal contract.