#201: Basics of Reverse Recovery Time in a Diode

Now, in my previous video, I built this transmit/receive switch based on pin

diode

s. Now I use some pin

diode

s that I picked up at a parts collection at a ham party and they just weren't labeled, like pin diodes, no markings, no markings on the components. so I wanted to verify that they were indeed pin diodes and the way to do that is to look at the capacitance of the device and the

reverse

recovery

time

and while I was doing that I thought maybe a video on diode

reverse

recovery

time

would be helpful. will be good to do, that's what we are going to talk about today is the reverse recovery characteristics of diodes, so what is the reverse recovery time of diode?

From what you change a diode from forward to reverse bias, a reverse current actually flows for a short time. period of time one way to illustrate this let's say we had the throat of the switch in this position here we're sending a positive voltage, you know, across this current limiting resistor through the diode, so the diode current is shown so when the switch is operated in the opposite direction. You know that essentially a negative voltage is applied to the diode. You say it will act like a diode and block all current flow. That's true, but not instantly.

What happens is that there is an investment. The current flowing can sometimes go down and be limited by voltage and resistance, sometimes they can just go down and then recover, so you actually get this reverse current flowing where the diode is not blocked like a diode by a short time. period of time, this is essentially due to the charge stored in the PN junction, when the junction is forward biased there is a space charge that builds up around the junction which basically allows the junction to conduct when the polarization is reversed, that charge it must be removed and then essentially the carriers have to recombine to reform the depletion region which causes the diode to latch and this does not happen instantaneously.

The reverse recovery time can vary from a few nanoseconds or less for high-speed switching diodes to a few microseconds or more for already known high-power rectifiers or even pin diodes, so the reverse recovery time will largely depend on of the particular device and what its application was designed for, as well as the current levels involved for a given device. If we operate it at a higher forward current, the reverse recovery time will be longer because there is more space charge that must be removed, for example. which we will see that the reverse recovery time is proportional to the forward current and it is also proportional to the amount of voltage applied in the reverse direction, so how fast can we remove that charge, so test the reverse recovery time It will largely depend on the test setup.

Now, many of the diode manufacturers, when they specify a reverse recovery time, will also include their test configuration. So the values ​​that are in the spec table will actually only apply under those specific test conditions, so the little test method I show here might not match the results of a line item in the datasheet because of this test setup I use. This may not match what the manufacturer used when they actually did that characterization, so why would we care about reverse recovery if you had a diode that had a long reverse recovery time which could be a major source of losses like a switching regulator? where we want to cut the current very quickly, it could also be a source of inaccuracy from a spike detector or if we are trying to charge a capacitor to measure the maximum voltage level, if there is any reverse current then that would bleed the charge. of that capacitor and cause an inaccuracy in that measurement, so there may be a few reasons why you would want to think about what the reverse recovery is in your application.

There may also be some benefits. It may be beneficial in some signal switching applications. and again, if we take a look at my video on pin diodes and RF switching, we'll see that that stored charge allows us to use the diode to change our currents even when those are currents and voltages that would normally reverse the bias of the diode we're taking. an advantage of the fact that the diode still conducts even when the voltage across it is reverse biased to be an effective switch so there are advantages and disadvantages so you know the effect we have here now as I mentioned different manufacturers They will have different tests. settings to measure reverse recovery time.

I'm using a very, very simple setup here just to show the effect. I have a pulse generator set up so that, you know, it outputs a pulse going from a negative voltage starting at let's say minus 1. going to plus 1 and then going back to minus 1, I set it to just a few microseconds wide, the The pulse generator itself, like most function generators, essentially has a 50 ohm output impedance, so it will tend to limit the current that basically we'll send that directly through our device under test, our diode that we are testing and will go into a 50 ohm termination.

In my case, I am using the 50 ohm termination that is built into the oscilloscope, but if your oscilloscope does not. If you don't have a 50 ohm termination that you can turn on, you can use a real 50 ohm only termination, a 50 ohm resistor and test it with, for example, a 10x probe with your scope or use a direct 50 ohm termination of 3 directly into the input of the scope, so this is the test device I use to test devices like this, it's just a pair of SMA connectors soldered to a piece of scrap circuit board. I used an exact knife to hoard, you know, essentially a conductive strip between the two connectors and I hoarded the center a little bit to create two small insulated strips and soldered on a couple of pin sockets of a couple of different sizes that make it easier for me to grab a device , you know, and stick it, you know, into the socket to make measurements now with a piece of wire inserted into my test device, we can easily see the applied pulse that I'm going to essentially pass through the diode, so in this case starts with minus 1 volt and goes to plus 1 volt. and that's looking at 50 ohms, of course, when we go through the diode, the diode drops and things like that will cause these voltages to change, but okay, we're just going to use this as a quick way to visualize the reverse recovery effect, right?

OK? If I move my short circuit wire here and we replace it with the first device we're going to test, it's this little power rectifier diode that we stick in here, now we can see the effect of that reverse recovery, the horizontal scale here is about four hundred nanoseconds per division, so it probably took about a microsecond or so for this to get pretty close to turning off now, of course it'll all depend on the bias conditions that we're looking at, about 600 millivolts or so in the forward direction here, that's . 600 millivolts across the 50mm termination, so you know 10 to 12 milliamps forward bias and then when we turn off, we can see this drops to about 1.3 volts, roughly, so we're at about 25 milliamps. in the reverse direction for a while and then we can see a tail returning to zero, so if I increase the forward bias, it will increase the amount of charge being stored at the junction and generate a correspondingly longer reverse recovery time, so let me increase. that we can actually see that effect, the forward bias is increasing, that reverse recovery time is also increasing, so you can see how that effect works now.

Also, if I increase the negative bias, make it more negative when we turn off the device, we can eliminate it. I charge faster, so if I take the low level and turn it down, we can see that the peak current increases, but the reverse recovery becomes a little bit shorter and if I reduce that reverse bias, we can see how it can take the reverse recovery. very long, so increases in reverse bias may tend to shorten the reverse recovery time, of course this all depends on the particular devices used, so let's get this rectifier out, if I put this guy in as a 4001 end, I would we put in the plug here. you can see it has a slightly different characteristic, a bit longer reverse recovery time, it's not as deep in terms of maximum reverse current.

I have another even higher power diode here, this one is similar to a 1n 4004 I think and We can see that again a little bit more, you know, on the reverse recovery pedestal, but then the recombination time is a little bit faster in terms of that exponential. We have a DK DK here, so all of these devices behave a little differently. Here we have an unmarked glass switching diode and if we place it in the socket we can see that its recurring reverse recovery time is actually much shorter; actually we can zoom in and get a better idea so it's about 40 nanoseconds of division now so let's say 40 80 you know you know 1.2 or you know 120 nanoseconds that was basically pretty off so it's probably a order of magnitude faster than switching diodes.

Irasema, the rectifier diode that I was showing you before, now high speed switching. diodes that are designed specifically for things like 1 in 914. I have a very fast reverse recovery time, so if we put that diode here we can see that I can barely see what's going on here, but if we zoom in, it's fine. We're not going to get even closer to that, let's put the Earth more or less and bring that closer, okay, now I'm at 4 nanoseconds of division and I can see that you know I'm recovered, you know, from going through zero and we cover again in about 4 nanoseconds, very, very fast in terms of its reverse recovery characteristic and that is typical of 1 in 9 14 or 1 in 40 148 that are designed for high speed switching applications now Schottky On the other hand, diodes, since they don't have a PN junction per se, they don't have the same stored charge.

If I put this 57:11 Schottky diode on one end, we see just a slight undershoot here and that's probably more. Just because you know reflections and whatnot on my device rather than any actual reverse recovery, now let's go back and take a look at the original reason I started playing with RAM with this device was to take a look at recovery reverse. of these pin diodes that I used in my transmit and receive switch so I can put it in the device here and be able to see here now we expand this here that the reverse recovery of this pin diode is in the order of see There are about 40 nanoseconds of division, already you know, 40 80, you know, those same hundred and 20 nanoseconds, but I can see that the recombination time is actually quite fast on this, so this is really such a long duration here where we are. only the current limited by the source and the load is really what is advantageous for us in the RF switching application as I use the pin diodes for that and the low capacitance even when the device is off so I hope you have learned something about what reverse recovery is. in a diode, a quick way to measure it and know some of the conditions that affect the behavior of that reverse recovery.

Thanks again for watching the common release, you're welcome and we'll see you next time.