Rectifiers and Smoothing

Mains supplies alternating voltage and almost everything downstream wants a steady one. The negative half is not merely useless — it reverse-biases the circuit it feeds. The diode's one-way behaviour is exactly the tool for removing it.

Doubling the ripple frequency halves the smoothing capacitance needed for the same ripple, so full-wave wins twice over — a higher average and an easier filter.

A reservoir capacitor charges to the peak and then discharges into the load until the next peak arrives. The ripple is approximately V_r ≈ I_load/(f·C) — worse with more load current, better with more capacitance, better with a higher ripple frequency.

Design to the ripple trough, not the average. If the trough falls below a downstream regulator's dropout voltage, the output dips every cycle and the result is audible hum.

The diode conducts only while the input exceeds the capacitor's voltage — a narrow window near each peak. All the charge has to be delivered in that window, so the peak current is many times the average.

• It sizes the diodes for a surge far above the DC output current.
• It heats the transformer, because RMS current is much higher than the average.
• It injects harmonics into the mains, which is why power-factor correction is mandatory above modest power levels.
• Inrush at switch-on is worse still, since the capacitor starts empty — hence NTC thermistors and soft-start circuits.

A bridge puts two forward drops in the current path. At 12 V that is a 12% loss; at 5 V it is 28%. Schottky diodes roughly halve it, and synchronous rectification — MOSFETs turned on in place of diodes — nearly eliminates it, which is standard in modern low-voltage supplies.

A switch-mode supply avoids most of this by rectifying the mains first and then switching at 100 kHz, where the transformer and capacitors shrink by orders of magnitude.