Protecting diodes in a rectifier with a capacitor-input filter: Which component is typically inserted to limit the inrush and peak currents and thereby protect the rectifier diodes?

Introduction / Context:
Capacitor-input (C-filter) rectifiers draw very high current pulses when the input sine peaks exceed the capacitor voltage. These brief but large surge currents stress rectifier diodes and transformer windings. Designers add impedance to limit surge and extend diode life.

Given Data / Assumptions:

• Single-phase rectifier feeding a large reservoir capacitor.
• Goal: reduce peak repetitive current and initial inrush at power-up.

Concept / Approach:

A small series resistance in the AC path limits di/dt and peak current during the short conduction angle. This “surge resistor” (sometimes NTC inrush limiter) is the simplest, most common, and predictable solution. Although a choke (inductor) can also limit current, a true choke-input filter changes the operating mode and requires a sufficiently large inductance to maintain continuous current; it is not what is “necessary” for a capacitor-input filter dedicated only to diode protection.

Step-by-Step Solution:

Verification / Alternative check:

Simulate with and without a series resistor: peak current waveform amplitude reduces substantially, diode junction temperature and transformer copper loss also reduce.

Why Other Options Are Wrong:

• Series inductor: can work, but not typically required for a capacitor-input design devoted solely to diode protection; it transforms the filter style.
• Surge capacitor across secondary: increases current spikes; not protective.
• Both (a) and (b): overstates what is necessary; a resistor alone suffices in common designs.

Common Pitfalls:

• Placing a capacitor across the transformer in hopes of “smoothing” actually raises reactive currents.

Final Answer:

Surge (series) resistor.