POWER ELECTRONICS – PAGE 15
🔁 Freewheeling Diode in Controlled Rectifiers
1️⃣ Why Freewheeling Diode is Needed?
In R-L load rectifier, when supply voltage becomes negative, the inductor tries to maintain current.
This causes negative output voltage.
To avoid this, we use a Freewheeling Diode (FD).
2️⃣ Working Principle
- When SCR turns OFF
- Inductor current cannot become zero instantly
- Current flows through Freewheeling Diode
Load current circulates within load loop.
3️⃣ What Happens Without FD?
- Output voltage becomes negative
- Higher ripple
- Lower power factor
- More stress on SCR
4️⃣ What Happens With FD?
- Negative portion eliminated
- Output voltage improves
- Ripple reduces
- Power factor improves
- SCR stress reduces
Freewheeling diode improves overall performance.
5️⃣ Average Output Voltage (With FD)
For full wave controlled rectifier:
Vavg = (Vm / π) (1 + cos α)
Note: Negative region removed.
6️⃣ Waveform Behaviour
- Voltage becomes zero during freewheeling
- Current becomes smoother
- Continuous conduction easier
🎯 GATE / PSU Important Points
- Difference between with & without FD
- Waveform identification question
- Average voltage comparison
- Power factor improvement concept
Very Important: Freewheeling diode provides alternate path for inductive current.
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