POWER DIODE TYPES - EEE Made Easy

Contents

Types of power Diodes

Power diodes can be classified as

General purpose diodes.
High speed (fast recovery) diodes.
Schottky diode.

Books for Power Electronics

Power Electronics: Devices, Circuits and Applications, by Muhammad H. Rashid
Power Electronics, by P.S. Bimbhra
Power Electronics, by M Singh & K Khanchandani

General Purpose Diodes

The diodes have high reverse recovery time of about 25 microsecs ( sec).

They are used in low speed (frequency) applications. e.g., line commutated converters, diode rectifiers and converters for a low input frequency upto 1 KHz.

Diode ratings cover a very wide range with current ratings less than 1 A to several thousand amps (2000 A) and with voltage ratings from 50 V to 5 KV.

These diodes are generally manufactured by diffusion process.

Alloyed type rectifier diodes are used in welding power supplies.

They are most cost-effective and rugged and their ratings can go upto 300A and 1KV.

Fast Recovery Diodes

The diodes have low recovery time, generally less than 5 s. The major field of applications is in electrical power conversion i.e., in free-wheeling ac-dc and dc-ac converter circuits.

Their current ratings is from less than 1 A to hundreds of amperes with voltage ratings from 50 V to about 3 KV.

The use of fast recovery diodes are preferable for free-wheeling in SCR circuits because of low recovery loss, lower junction temperature and reduced di /dt .

For high voltage ratings greater than 400 V they are manufactured by diffusion process and
the recovery time is controlled by platinum or gold diffusion.

For less than 400 V rating epitaxial diodes provide faster switching speeds than diffused diodes. Epitaxial diodes have a very narrow base width resulting in a fast recovery time of about 50 ns.

Schottky Diodes

A Schottky diode has a metal (aluminum) and semi-conductor junction.

A layer of metal is deposited on a thin epitaxial layer of the n-type silicon.

In the Schottky diode, there is a larger barrier for electron flow from metal to semi-conductor.

The figure shows the Schottky diode.

When the Schottky diode is forward biased free electrons on the n-side gain enough energy to flow into the metal causing forward current.

Since the metal does not have any holes there is no charge storage, decreasing the recovery time. Therefore a Schottky diode can switch off faster than an ordinary p-n junction diode.

A Schottky diode has relatively low forward voltage drop and reverse recovery losses.

The leakage current is higher than a p-n junction diode.

The maximum allowable voltage is about 100 V. Current ratings vary from about 1 to 300 A.

They are mostly used in low voltage and high current dc power supplies.

The operating frequency may be as high as 100-300 kHz as the device is suitable for high-frequency
application.

Comparison Between Different Types Of Diodes

General Purpose Diodes	Fast Recovery Diodes	Schottky Diodes
Upto 5000V & 3500A	Upto 3000V and 1000A	Upto 100V and 300A
Reverse recovery time – High	Reverse recovery time – Low	Reverse recovery time – Extremely low.
t rr = 25 Micro Sec	t rr = 0.1- 5 Micro Sec	t rr = a few nano seconds
Turn off time – High	Turn off time – Low	Turn off time – Extremely low
Switching frequency – Low	Switching frequency – High	Switching frequency – Very high.
V = 0.7V to 1.2V	V = 0.8V to 1.5V	V 0.4V to 0.6V