Classification of Power Diodes
Fig: Operating zones for operating a Bipolar Junction Transistor as a linear and a switching amplifier
- Silicon Power diodes are the successors of Selenium rectifiers having significantly improved forward characteristics and voltage ratings. They are classified mainly by their turn-off (dynamic) characteristics.
- The minority carriers in the diodes require finite time - trr (reverse recovery time) to recombine with opposite charges and neutralise.
- Large values of Qrr (= Q1 Q2) - the charge to be dissipated as a negative current when the diode turns off and trr (= t2 - t0) - the time it takes to regain its blocking features, impose strong current stresses on the controlled device in series.
Types of Power Diodes:
There are broadly three types of diodes used in Power electronic applications:
- Line-frequency diodes: These PIN diodes with general-purpose rectifier type applications are available at the highest voltage (~5kV) and current ratings (~5kA) and have excellent over-current (surge rating about six times average current rating) and surge-voltage withstand capability. They have relatively large Qrr and trr specifications.
- Fast recovery diodes: Fast recovery diffused diodes and fast recovery epitaxial diodes, FRED's, have significantly lower Q rrand trr (~ 1.0 usec). They are available at high powers and are mainly used in association with fast controlled-devices as free-wheeling or DC-DC choppers and rectifier applications. Fast recovery diodes also find application in induction heating, UPS and traction.
- Schottky rectifiers: These are the fastest rectifiers being majority carrier devices without any Qrr.. However, they are available with voltage ratings up to a hundred volts only though current ratings may be high. Their conduction voltages specifications are excellent (~0.2V). The freedom from minority carrier recovery permits reduced snubber requirements. Schottky diodes face no competition in low voltage SPMS applications and in instrumentation.