Diode turn-off time PUBLIC

Diodes can take a surprisingly long time to turn off, as we show in this demo.

This circuit shows a voltage square wave put across a 1N4007 diode (datasheet PDF). This diode is meant for relatively large currents (1A continuous, 30A pulsed), but it's not a particularly fast switching diode. That can have serious consequences for power electronics circuits! For more about this topic, see this article about diode turn-on/turn-off time and inductive loads.

A few things to try in CircuitLab

Load the simulation by clicking "Open in editor" above, and then click "Simulate" at the bottom. Run a time domain simulation.

Switching time

Inspecting this circuit, what we'd "expect" to see is that when Vsrc is positive, the diode should be conducting. A substantial amount of current should flow through the diode. V(out) should be whatever the forward diode drop is for that current, somewhere in the 0.7 to 1.0 volt range. When Vsrc is negative, we'd "expect" that the diode would block almost all current, perhaps letting a few nanoamps trickle through in reverse, and so V(out) would be about the same as Vsrc.

However, when you run it for yourself, you see that there's a transient effect when Vsrc switches from positive to negative. For a few microseconds, the charge stored in the diode keeps V(out) positive. But if we look at the current through the diode, we can see it's actually conducting a lot of current in the negative direction! The diode stays on even when it's supposed to be off for about 1.5 microseconds.

Different diode model numbers

Double-click on D1. You can see we have it set to use the 1N4007 part number, but try changing it instead to the 1N4148, which is a small, fast signal diode. Run the simulation again.

The forward diode drop is now about 1.6 volts -- this diode simply can't handle as much current as we're trying to force through it -- but there's no longer any switching effect. The edges are clean and sharp.

Of course, you could start to experiment with the frequency and discover just how fast you can push the 1N4148, and you'll find that it too has a finite turn-off time -- it's just much faster than the 1N4007.