# Diode reverse recovery time and the speed (snap back) in which recovery takes place

I'm looking at generating a circa 1 nano second current pulse by utilizing the snap-back time (usually measured in the tens of pico seconds) when a diode is "forcibly" reverse polarized after being forward biased for a short time. This answer from Neil gives me some confidence in what I believed to be true i.e. a common or garden sloppy old diode like the 1N400x series (or the 1N540x as per Neil's suggestion in the link above) can achieve this.

For instance, in simulation, I apply a 1 MHz sinewave biased up at 10 volts (green waveform) to a diode whose anode is at 3 volts. The sinewave voltage is fed via a few ohms for current limiting: -

The blue waveform is the diode voltage and, you can see (highlighted in orange), it takes a while to recover but, when it does recover, it "snaps back" pretty darned quickly. And if I used a pulse (with moderate rise and fall times instead of a sinewave) I can get improvements in the current delivered into a 1 Ω load via a 100 pF capacitor: -

The above simulation uses ideal wires to connect things hence, I'm only showing its "results" as a way of demonstrating the potential fast snap-back time of sloppy old diodes.

Here I can get a 1 amp pulse of about 1.5 ns duration and about 100 ps rise time. But, do I really believe it? There isn't anything in the data sheets for these types of diode that indicate what the snap-back time is.

So, how can I calculate it for say the 1N540x or 1N400x series of diodes (question)?

Maybe I can't calculate it; maybe it's just a bit of pot-luck and trial and error. Maybe there is a formula somewhere that can reveal what I need to know? As you might expect I have searched for this on google and I'm aware that this sort of technique is used but, mainly with step-recovery diodes (hard to get and actually not what I think I want). Step recovery diodes have a very short recovery time and therefore operate differently to when using a long recovery time diode.

A few words about why I choose a diode with long reverse recovery time; the longer the reverse recovery time, the more time I have to reverse the applied pulse to a large reverse bias voltage and therefore the easier life is to produce a reverse current that is larger. A larger reverse current (prior to snap-back) means a higher value snap-back pulse.

• Comments are not for extended discussion; this conversation has been moved to chat. Commented Jun 13, 2022 at 15:32

My experience with my own recovery test jig, is that regular rectifier diodes exhibit specified behavior ($$\t_{rr}\$$ and softness) for $$\t_{on} \gg t_{rr}\$$. This is a hidden assumption that they don't specify. They can be pushed into drift step recovery* by using short pulses. This has to do with injecting a layer of charge with a brief forward bias, then reversing and waiting for it to snap. Consider the function of $$\t_b\$$ vs. $$\t_{on}\$$: at most pulse lengths (over $$\t_{rr}\$$ or so) it's as specified and fairly constant, but as you reduce $$\t_{on}\$$, it gets suddenly MUCH shorter; it's pretty cool to watch.

*I'm not real clear on the precise definitions of SRDs and DSRDs, so, something like this, but check the literature to be sure (Grehkov, etc.).

Note that $$\V_f\$$ may be quite large during such a pulse -- some types have worse forward recovery than others, but the key is that you're still injecting charge and the junction isn't in quasi-equilibrium yet. So things can get quite strange looking, like putting 40V across a diode that you're certain isn't entirely due to lead inductance.

• Can you explain what "chooch" means please? When you say $t_{on}$ >> $t_{rr}$ you mean it is forward biased for considerably longer than what the reverse recovery time is? But then you say "a brief forward bias" and this implies the opposite to me. Is $t_b$ what I have referred to as the snap-back time? Commented Jun 13, 2022 at 9:23
• @Andyaka That is, you get normal specified behavior for t_on >> t_rr, where slowness and softness applies, so we need to reverse that by going comparable or lesser than, and then we see interesting behavior. Yes, t_b as others have been using it. "Chooch" in the AvE sense, it does the thing; snap. Hm, I should just edit this, huh... Commented Jun 13, 2022 at 18:05
• "Chooch" in the AvE sense - no, I'm still lost as to what you mean!! Commented Jun 13, 2022 at 18:10
• @Andyaka Edited -- let me know if that clarifies things. Commented Jun 13, 2022 at 18:25
• Correct. And your example probably illustrates this, with the low duty cycle of conduction at 1MHz, so you're on the right path. Commented Jun 13, 2022 at 18:32

I believe--but am not 100% certain--that this "snap-back time" is what we in the semiconductor industry call $$\t_b\$$ (with $$\t_a\$$ being the rest of the recovery time--$$\t_{rr}=t_a+t_b\$$). Generally diodes are designed to have a slow snap-back to reduce high-frequency noise. The ratio $$\\frac{t_b}{t_a}\$$, called the "softness", can be found on datasheets for FREDs, and may be helpful for your purposes.

As I recall from using them to test some recovery measurement equipment I built, Vishay's E4PH and EPU diodes have particularly poor (low) softness, which may be what you're looking for here.

The topology of this circuit your are testing is unusual for a SRD, as it involves such low impedances, and ignores some key parasitics.

Usually, when we come to build a circuit physically, the L/C ratio of the unavoidable parasitics will tend to push us up into the 10s to 100 ohm range, and 1 ohm loads will be out of the question, especially in the face of a few nH series L in the switching diode.

This is the sort of basic circuit that is more typically used and analysed.

simulate this circuit – Schematic created using CircuitLab

During the charge stage, current flows from the source through L1, D1, L2, storing charge in D1.

During the discharge phase, current flows back out of the diode, through L1 and L2, storing energy in L2.

When D1 snaps off, the interrupted current in L2 generates a voltage that goes through C2 to the load. The now low self capacitance of D1 means that L3's energy does not couple significantly to the output. Additional biassing may be used to time the snap off to occur at the peak discharge current.

What to look for in diode data sheets?

Most rectifier diodes, if they mention it at all, will boast of soft recovery, which is not what you want. A snappy diode will not be advertised as such, at least not by the manufacturer.

One of the things to look for is evidence of high doping adjacent to the junction. PIN diodes for instance are exactly the wrong thing. We have had best performance in the 130 MHz in 1.5 GHz out ballpark in commercial equipment using BA482/682 switching diodes. These are heavily doped for low forward on resistance at modest currents.

• @Andyaka STRD - STep Recovery Diode, SRD - Step Recovery Diode. The first is easier to pronounce, as strud. Commented Jun 13, 2022 at 16:57
• Hmm a typo maybe or are they both different abbreviations for a step recovery diode? Commented Jun 13, 2022 at 17:17
• @Andyaka You see the three paragraphs under my circuit giving a blow by blow description of how it does what it does. It might be useful to you if you produce the same sort of narrative explanation for your circuit's operation. Commented Jun 14, 2022 at 7:51
• I'm still confused about what you mean by SRD or STRD. In your comment you call them both step recovery diodes and I'm sure you don't mean that i.e. you must have made a typo hence my earlier comments seeking clarification. Commented Jun 14, 2022 at 8:07
• @Andyaka no typo, just inattention to detail. The wikipedia article calls them SRD. However, try saying that, like you can say MOSFET, or PIN. In my lab we always said 'strud', putting the T in there from the word sTep, as I indicated in my first comment. Whenever I think step recovery diode, I hear strud, and type STRD. I'm sorry it's caused you so much bother. I've edited my answer to be consistent with practice. How's your narrative on how your simulated circuit produces its output pulse coming along? Commented Jun 14, 2022 at 19:50