Freewheeling Diode

A freewheeling diode is placed across an inductive load to provide a path for the inductor current when the main switch turns off. It is vital in buck converters and relay coil drivers.

Core Concept

Inductors oppose sudden changes in current. If a switch interrupts an inductive circuit the inductor generates a massive negative voltage spike L di/dt to keep current flowing.

A freewheeling diode becomes forward biased by this voltage spike clamping the voltage and allowing the stored magnetic energy to circulate safely through the diode and load.

In phase controlled rectifiers freewheeling diodes prevent the output voltage from going negative improving the power factor and smoothing the load current.

Key Formulas

Voltage spike without FWD V equals L multiplied by di/dt. The decay of current through the FWD follows an exponential curve based on the L/R time constant.

Given: I = 10 A, L = 50 mH, R = 5 ohms
Formula: Decay Time Constant Tau = L / R
Steps: Tau = 50e-3 / 5
Final Answer: 10 milliseconds

Exam Tip: In a single phase half wave controlled rectifier with a highly inductive load a freewheeling diode makes the extinction angle beta equal to pi radians.

Performance Parameters

• Prevents switch destruction from voltage spikes
• Improves load current waveform
• Increases average output voltage in rectifiers
• Improves system input power factor
• Must have fast reverse recovery times

Quick Revision

• Protects switches from inductive kickback
• Provides path for continuous conduction
• Clamps negative voltage excursions
• Enhances power factor
• Essential in step down choppers
• Exam trap: Thinking the FWD conducts when the main switch is ON