--- title: "Diode reverse recovery characteristics" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:diode-reverse-recovery" source: https://www.bctengineer.com/q/semester_7/power_electronics/diode-reverse-recovery site: BCT Engineering --- # Diode reverse recovery characteristics Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2076 Asoj, 8 marks - Video: [Power Diode Reverse Recovery Characteristics | Basic Concepts | Power Electronics](https://www.youtube.com/watch?v=jAkdiEZgOd0) — ENGINEERING TUTORIAL - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/diode-reverse-recovery - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Explain the reverse recovery characteristics of diode. ## Short answer Reverse recovery occurs when a conducting diode is switched to reverse bias: stored minority carriers cause a reverse current flow for a time trr, with peak Irr, before the diode blocks. This stored charge leads to switching losses and voltage spikes, requiring fast-recovery or Schottky diodes to minimize effects. ## Answer **Reverse recovery:** when a conducting diode is switched to reverse bias, its current does not stop at zero. The minority carriers stored in the junction must first be removed, so the current falls through zero, flows in the reverse direction for a short time and then decays to zero. ```text iD ^ IF |-------. | \ slope = di/dt | \ --+----------\--------------------> t | \ .-------- | \ .' | - IRR ....\.' tb | ta |<->| | |<-->| | |<---trr--->| ``` - $t_a$: time from current zero to the peak reverse current $I_{RR}$, while charge stored in the depletion region is removed. - $t_b$: time for the reverse current to decay from $I_{RR}$ to about $0.25 I_{RR}$, while charge in the bulk is removed. - **Reverse recovery time** $t_{rr} = t_a + t_b$; **softness factor** $S = t_b / t_a$. - **Peak reverse current:** $I_{RR} = t_a \dfrac{di}{dt}$. - **Reverse recovery (stored) charge** $Q_{RR}$ is the area under the reverse current, approximately a triangle: $$ Q_{RR} = \tfrac{1}{2} I_{RR}\, t_{rr} $$ #### Soft and abrupt recovery | Point | Soft recovery | Abrupt (fast) recovery | |---|---|---| | Softness $S = t_b/t_a$ | About 1 | Much less than 1 | | Reverse current decay | Gradual | Sharp (snap-off) | | Voltage spikes and EMI | Small | Large, with oscillation | | Uses | General purpose, inductive circuits | High-frequency choppers and inverters | #### Effects of reverse recovery - Extra switching loss $\approx V_R Q_{RR} f$. - Over-voltage spikes $L\,di/dt$ in stray inductances. - In bridge circuits, the recovering diode draws a current spike through the incoming switch. Hence power circuits use **fast recovery** or **Schottky** diodes, which have small $t_{rr}$ (a few µs down to ns). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the reverse recovery waveform, what does ta represent? A. Time for reverse current to decay from IRR to 0.25 IRR B. Time from current zero to peak reverse current IRR, while depletion region charge is removed C. Time for forward current to build up D. Total storage time of the load Answer: B. ta is from zero crossing to IRR; tb is the decay from IRR to about 0.25 IRR. (Key point: ta and tb) 2. What is the softness factor and what does a value much less than 1 mean? A. S = tb/ta; abrupt recovery with sharp snap-off and large voltage spikes B. S = ta/tb; soft recovery C. S = IRR/IF; no recovery D. S = trr/ta; slow diode Answer: A. S = tb/ta. A value near 1 is soft; much less than 1 is abrupt, giving snap-off, spikes and EMI. (Key point: Softness factor) 3. Which is an effect of reverse recovery in power circuits? A. Reduced switching loss B. Over-voltage spikes L di/dt and extra switching loss about VR*QRR*f C. Lower EMI D. Higher forward voltage Answer: B. Reverse recovery causes extra loss VR QRR f and L di/dt voltage spikes in stray inductance. (Key point: Effects of recovery) 4. Which diodes are used in fast switching power circuits to limit reverse recovery problems? A. General purpose rectifier diodes B. Fast recovery or Schottky diodes with small trr C. Zener diodes D. Point contact diodes with large trr Answer: B. Fast recovery and Schottky diodes have small trr (a few us down to ns). 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