Table of Contents
Why Use a Freewheeling Diode?
The primary purpose of the FD is to provide a path for the inductive current to circulate once the source voltage goes negative. This modifies the circuit behavior in two major ways:
- Voltage Clamping: It prevents the output voltage ($V_o$) from becoming negative.
- SCR Protection: It allows the Thyristor (SCR) to turn off (commutate) exactly at $\pi$, reducing the stress on the switch.
Principle of Operation
Mode 1: Conduction Period ($\alpha \le \omega t \le \pi$)
The SCR is triggered at the firing angle $\alpha$. During this interval, the source supplies power to both the resistor and the inductor. The load voltage $V_o$ follows the source voltage $v_s$, and the inductor stores energy.
Mode 2: Freewheeling Period ($\pi \le \omega t \le 2\pi$)
As soon as $v_s$ crosses zero and enters the negative half-cycle at $\omega t = \pi$, the Freewheeling Diode becomes forward-biased.
The inductor now acts as a source, discharging its stored energy through the resistor and the diode. The output voltage is effectively clamped to zero.
Mathematical Analysis
Because the negative “tail” of the voltage waveform is eliminated, the average DC output increases.
Average Output Voltage ($V_{dc}$):
The integration is now only valid from $\alpha$ to $\pi$ (similar to a purely resistive load):
$$V_{dc} = \frac{1}{2\pi} \int_{\alpha}^{\pi} V_m \sin(\omega t) \, d(\omega t) = \frac{V_m}{2\pi} (1 + \cos \alpha)$$
Comparison: Without the FD, the formula was $\frac{V_m}{2\pi} (\cos \alpha – \cos \beta)$. Since $1 > -\cos \beta$, the FD version provides a higher average voltage.
Comparison Summary
| Feature | Without Freewheeling Diode | With Freewheeling Diode |
|---|---|---|
| Output Voltage ($V_o$) | Contains negative spikes (up to $\beta$) | Clamped to zero at $\omega t = \pi$ |
| Average Voltage ($V_{dc}$) | Lower (due to negative area) | Higher (negative area eliminated) |
| SCR Turn-off | Delayed until extinction angle $\beta$ | Occurs exactly at $\pi$ |
| Power Factor | Lower | Improved |
Key Benefits
- Smoother Current: The load current becomes more continuous and has reduced ripple.
- Improved Efficiency: Energy is dissipated in the load rather than being returned to the source.
- Better Power Factor: Reducing the reactive power return improves the overall input power factor of the system.