--- title: "Power diode V-I and reverse recovery" subject: "Power Electronics" chapter: 1 key: "semester_7/power_electronics:power-diode-vi-reverse-recovery" source: https://www.bctengineer.com/q/semester_7/power_electronics/power-diode-vi-reverse-recovery site: BCT Engineering --- # Power diode V-I and reverse recovery Power Electronics · Chapter 1: Characteristics and specification of power electronic devices - Asked in: 2080 Bhadra, 8 marks - Video: [Power Diode (Basics, Structure, Characteristics, Working, Applications, Biasing & Types) Explained](https://www.youtube.com/watch?v=3k9_YzcfGJo) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/power-diode-vi-reverse-recovery - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1 ## Question Explain the working of power diode with the help of V-I curve. What do you mean by reverse recovery characteristic of a power diode? ## Short answer A power diode is a P-i-N device with low forward drop and reverse blocking up to the breakdown voltage. Reverse recovery occurs at turn-off: stored minority carriers cause a reverse current peak before blocking resumes, characterized by recovery time trr and stored charge Qrr. ## Answer A **power diode** is a two-terminal P-N junction (with a lightly doped $n^-$ drift layer, the P-i-N structure) that conducts when its anode is positive with respect to the cathode and blocks when reverse biased. It is designed for large current and high reverse voltage. #### Structure ```text Anode | [ p+ ] heavily doped [ n- ] drift region (sets [ ] reverse voltage rating) [ n+ ] substrate | Cathode ``` #### V-I characteristic ```text iD ^ / forward | / conduction | / | / -VRRM | _/ slope 1/r ----+--------+---'--------------> vD | reverse Vcut-in | leakage (0.7-1 V) | (very small) | v reverse breakdown (avalanche) ``` - **Forward bias:** after the cut-in voltage (about 0.7 to 1 V), the current rises rapidly. The forward drop at rated current is about 1 to 2 V because of the drift-region resistance (on-state resistance $r$). - **Reverse bias:** only a small leakage current (µA to mA) flows until the reverse breakdown voltage $V_{BR}$. The diode is rated for a repetitive peak reverse voltage $V_{RRM}$ below this. Beyond breakdown, avalanche current flows and the diode may be damaged. #### Reverse recovery characteristic **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: $$ Q_{RR} = \tfrac{1}{2} I_{RR}\, t_{rr} $$ Depending on $S$, diodes are **soft recovery** ($S \approx 1$, less oscillation) or **fast/abrupt recovery** (small $S$). Reverse recovery causes extra switching loss and voltage spikes, so fast-recovery diodes are used in choppers and inverters. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the forward drop of a power diode at rated current about 1 to 2 V, higher than the cut-in voltage? A. Because of the leakage current B. Because of the drift-region (on-state) resistance C. Because of reverse breakdown D. Because of stored charge Answer: B. After the cut-in (0.7 to 1 V) the extra drop is I x r, from the resistance of the lightly doped n- drift region. (Key point: Forward conduction) 2. What is reverse recovery in a power diode? A. The current stops exactly at zero when reverse biased B. The current flows in reverse for a short time while stored minority carriers are removed, then decays to zero C. The diode breaks down in avalanche D. The diode conducts more in forward direction Answer: B. Stored minority carriers must be removed first, so the current passes through zero, reverses briefly, then decays. (Key point: Reverse recovery meaning) 3. How are trr and the softness factor defined? A. trr = ta - tb; S = ta/tb B. trr = ta + tb; S = tb/ta C. trr = ta x tb; S = ta + tb D. trr = tb; S = IRR/trr Answer: B. ta runs from current zero to peak IRR and tb from IRR to about 0.25 IRR; trr = ta + tb and S = tb/ta. (Key point: Definitions of ta, tb, S) 4. Which formulas give peak reverse current and recovery charge? A. IRR = ta (di/dt); QRR = 1/2 IRR trr B. IRR = trr/(di/dt); QRR = IRR x trr C. IRR = tb (di/dt); QRR = IRR/trr D. IRR = S x ta; QRR = 1/2 IRR ta^2 Answer: A. IRR = ta x di/dt, and QRR is the triangular area under the reverse current, 1/2 IRR trr. Soft-recovery diodes (S near 1) cause less oscillation. 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