Question

A power diode is open circuited, and a reverse dc voltage is applied to get a faster turn-off time. During reverse recovery time
(a) When current is negative peak, voltage is zero
(b) When voltage is negative peak, current is zero
(c) When current is negative peak, voltage is not zero
(d) When voltage is negative peak, current is not zero
Choose the correct answer

• A. \( \text{(a) true, (c) true and (d) true} \)
• B. \( \text{(b) true, (c) true and (d) false} \)
• C. \( \text{(b) false, (c) false and (d) true} \)
• D. \( \text{(a) true, (c) false and (d) true} \)

Hint:

Remember the physical sequence during reverse recovery: 1. Current hits its negative peak first, while the voltage is still zero because the junction is full of carriers. 2. Voltage hits its negative peak later, while current is still decaying back toward zero. This phase shift between the current and voltage peaks prevents them from happening at the same time.

Answer 1

The Correct Option is D

Concept: When a forward-conducting power diode is rapidly switched off by applying a reverse bias voltage, it cannot stop conducting instantly. This delay occurs because stored charge carriers (minority carriers) near the p-n junction must be removed before the device can block reverse voltage. The time required for this to happen is called the Reverse Recovery Time ($t_{rr}$). Let us analyze the standard reverse recovery waveform phases:
• Initially, the current decreases from its forward value, crosses zero, and continues downward in the negative direction as a reverse current to sweep out stored charges.
• The reverse current reaches a maximum negative value, denoted as $I_{RM}$ (negative peak current). At this specific instant, the junction has not yet developed a charge depletion layer, so the voltage across the diode remains zero.
• After this peak, the stored charge drops significantly, a depletion region begins to form, and the reverse current decays back toward zero. As the current decays, the diode begins blocking voltage, causing the reverse voltage to rise toward its peak negative value ($V_{RM}$). At this point, current is still flowing, so the current is not zero.

Step 1: Analyzing Statement (a) and Statement (c).
As the minority carriers are swept out, the reverse recovery current reaches its maximum negative value ($I_{RM}$). At this exact moment, the p-n junction is still flooded with excess carriers and cannot block any voltage. Thus, the voltage across the diode remains zero:
• Statement (a) ("When current is negative peak, voltage is zero") is True.
• Statement (c) ("When current is negative peak, voltage is not zero") is logically False.

Step 2: Analyzing Statement (b) and Statement (d).
Once the current starts decaying from its negative peak, the depletion layer begins to widen rapidly, allowing the diode to support a reverse voltage. The reverse voltage reaches its maximum negative peak ($V_{RM}$) while the current is in the middle of decaying back to zero. Because the current has not finished decaying completely, it is not zero at the voltage peak:
• Statement (b) ("When voltage is negative peak, current is zero") is False.
• Statement (d) ("When voltage is negative peak, current is not zero") is True.

Step 3: Combining the true statements.
Reviewing our findings:
• (a) is True
• (b) is False
• (c) is False
• (d) is True This combination matches option (4): "(a) true, (c) false and (d) true". Hence, the correct choice is option (4).