Rectifier Diodes

A rectifier diode is a diode capable of converting AC into DC. It can conduct 1 A or more or dissipate 1 W or more of power. Most rectifier diodes are now made from silicon. The dies have large PN junctions to eliminate or minimize damage from heat produced by power dissipation. Typically packaged as discrete devices, the rectifiers can be paralleled to increase their power-handling ability. Rectifiers rated for less than 6 A are usually packaged in axial-leaded glass or plastic cases. However, those with 8- to 20-A ratings are usually packaged in flat plastic cases with copper tabs that can act as heat sinks or metal-to-metal interfaces with larger heat-dissipating busbars. Rectifiers rated from about 12 to 75 A are usually packaged in metal cases. Some have threaded base studs for fastening the case directly to a larger heat-dissipating surface.

The most important electrical ratings for rectifier diodes are:

• Peak repetitive reverse voltage VRRM
• Average rectified forward current IO
• Peak repetitive forward surge current IFSM

Standard PN junction rectifiers are specified for linear power supplies operating at input frequencies up to 300 Hz, but they are inefficient in switching power supplies that switch at frequencies of 10 kHz or higher because of their slow recovery time. This is the finite amount of time required for the minority and majority carriers—electrons and holes—to recombine after a polarity change of the input signal. The minority carriers must be removed before full blocking voltage is obtained.

Despite their slow recovery time, standard PN junction rectifiers have lower reverse currents, can operate at higher junction temperatures, and can withstand higher inverse voltages than faster rectifiers designed to overcome this speed limitation.

Three types of fast silicon rectifiers perform more efficiently at the higher-frequency switching rates:

1. Fast-recovery rectifiers.
2. Ultrafast- or superfast-recovery rectifiers.
3. Schottky rectifiers.

FAST-RECOVERY RECTIFIERS

A fast-recovery rectifier is a PN junction rectifier made by diffusing gold atoms into a silicon substrate. The gold atoms accelerate the recombination of minority carriers to reduce reverse recovery time. These rectifiers can be switched in 200 to 750 ns. They have current ratings of 1 to 50 A and voltage ratings to 1200 V. Forward voltage drop is typically 1.4 V, higher than the 1.1 to 1.3 V of the standard PN junction. The maximum allowable junction temperature is about 25°C. This value is lower than that for a standard PN junction. The maximum reverse voltage for a fast-recovery rectifier is about 600 V.

ULTRAFAST- OR SUPERFAST-RECOVERY RECTIFIERS

An ultrafast- or superfast-recovery diode is a PN junction rectifier whose reverse recovery time is between 25 and 100 ns. Gold or platinum is also diffused into the silicon wafers from which the rectifier is made to speed up minority carrier recombination. These rectifiers are specified for power supplies with output voltages of 12, 24, and 48 V.

SCHOTTKY RECTIFIERS

A Schottky rectifier has a metal-to-semiconductor junction rather than a PN junction, so it does not have minority charge carriers. The die is in direct contact with one metal electrode, so recovery time, although not specified, is typically less than 10 ns. Recovery current is principally caused by junction capacitance. Schottky rectifiers provide lower forward voltages (VF) than the PN rectifiers (0.4 to 0.8 V vs. 1.1 to 1.3 V). Hence power dissipation is lower and efficiency is higher. One drawback of the Schottky rectifier is its low blocking voltage, typically 35 to 50 V. However, Schottky rectifiers with maximum blocking voltages of 200 V are available. These rectifiers require transient protection, and they have inherently higher leakage current (IRRM) than PN junction rectifiers. This makes them more susceptible to destruction by overheating (thermal runaway). Schottky rectifiers can be paralleled in the output stages of switching power supplies, where they are usually used with output terminals rated for 5 V or less.