Created by
Created March 19, 2012
Last modified March 20, 2012
Tags diode   flyback   switching  


Diodes have a "reverse recovery time". When switched from on to off, the space charge within the semiconductor must return to its original location. The diode appears to conduct briefly in reverse.


Inspired by -- check that page out for some great explanations.

I wanted to provide a demonstration of what reverse recovery time looks like in diodes.

V1 provides a 10kHz square wave, alternating between +5V and -5V. Thus, the diodes are being switched from on to off to on again and again. But this demo shows that they don't switch from state to state instantaneously. Run the time domain simulation, and see how the diode current for D2, the "slow" diode, actually goes significantly negative for a few microseconds after the voltage switches from forward to reverse bias.

As CircuitLab has detailed diode simulation models which include these effects, the approximately 2us reverse recovery time of the simulated 1N4007 is roughly in accordance with the 1N4007 datasheet!

You may also want to try plotting V(A) and V(B).

My best explanation for "why" there is a reverse recovery time:

A space charge within a P-N junction needs to be established before forward current can flow. From zero, this space charge can be established quite quickly, because an externally applied forward bias voltage can route electrons externally around. Electrons diffuse from the n-type material into the edge of the p-type material, holes in the p-type material diffuse into the edge of the n-type material, and at the metal interfaces, new electrons are injected into the n-type end and holes are generated at the p-type end to produce free electrons that can flow in the external circuit. All of these flows are flows of majority carriers in their respective materials, so diffusion happens quickly driven by a much larger concentration gradient. A space charge develops rapidly because majority carriers are flowing to turn the diode on -- electrons in the n-type material, and holes in the p-type material.

However, if the external voltage is then reversed to be a reverse bias, the space charge is attracted to itself to recombine. However, this recombination only happens through the diffusion of minority carriers. This minority carrier diffusion has much smaller concentration gradients, and therefore diffuses orders of magnitude more slowly. An external circuit providing reverse bias can aid in speeding this recombination, as it can allow for faster neutralization of excess holes that migrated back to the p-type material, and removal of excess electrons that migrated back to the n-type material. That's why there can be huge reverse currents during the reverse recovery time.

(See also a PDF titled "Recombination Time in Semiconductor Diodes".)

Other things to try in CircuitLab

The DC simulation shows the DC values for the diodes currents and voltages.

The DC Sweep shows the diode characteristic curves (when in series with the 50 ohm resistors as shown).

The Frequency-domain simulation shows how a small AC voltage signal at the input would be reflected at the output V(A) or V(B), filtered by the combination of the small-signal incremental resistance of the diode under forward bias (in parallel with the 50 ohm resistor), and the incremental capacitance of the diode. The 1N4007 has much much larger stored charge and incremental capacitance, and thus there's a pole at much lower frequency!


i love it! looks like a 4007 is still fast enough for audio, though. (darn, i was hoping for artifacts).

by danielfiction
March 21, 2012

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