Power Diode

Power diodes are unipolar devices used for uncontrolled rectification in power electronics. They are widely used in freewheeling paths and front end rectifiers for DC motor drives.

Core Concept

A power diode conducts when the anode is positive with respect to the cathode. It blocks current when reverse biased. High power devices like the 1N5408 have a larger drift region to withstand high reverse voltages.

During turn off a reverse recovery current flows due to stored charge in the drift region. The reverse recovery time trr determines the maximum switching frequency. Fast recovery diodes operate efficiently at 20 kHz.

The drift layer dictates the Peak Inverse Voltage rating. A thicker drift layer increases PIV but also increases the forward voltage drop due to higher resistance.

Key Formulas

Reverse recovery charge Qrr equals 0.5 multiplied by reverse recovery time trr multiplied by peak reverse current Irr. Softness factor defines the ratio of fall time to storage time.

Given: trr = 4 microseconds, Irr = 20 A
Formula: Qrr = 0.5 * trr * Irr
Steps: Qrr = 0.5 * 4e-6 * 20
Final Answer: 40 microcoulombs

Exam Tip: The drift region in a power diode is lightly doped to handle high voltages. Standard signal diodes lack this region which is why they cannot block high PIV.

Performance Parameters

• Forward voltage drop: 1.0 V to 1.5 V
• Peak Inverse Voltage: up to 5000 V
• Reverse recovery time: 100 ns to 5 microseconds
• Maximum forward current: 1 A to 1000 A
• Junction temperature: typically 150 C max

Quick Revision

• Uncontrolled unidirectional switch
• Drift region increases blocking voltage
• Reverse recovery causes switching loss
• Schottky diodes have zero reverse recovery
• Used as freewheeling diodes in buck converters
• Exam trap: Assuming power diodes switch instantly