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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Rectification
Date: Sun, 3 Nov 2024 21:07:17 +1100
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On 3/11/2024 7:50 pm, Cursitor Doom wrote:
> On Sun, 3 Nov 2024 13:53:49 +1100, Bill Sloman <bill.sloman@ieee.org>
> wrote:
> 
>> On 3/11/2024 6:03 am, Cursitor Doom wrote:
>>> On Sat, 02 Nov 2024 08:15:17 -0700, john larkin <JL@gct.com> wrote:
>>>
>>>> On Sat, 02 Nov 2024 12:34:44 +0000, Cursitor Doom <cd@notformail.com>
>>>> wrote:
>>>>
>>>>> Greetings mesdammes et messureses,
>>>>>
>>>>> Say I'm using a regular jelly bean diode to rectify an AC waveform to
>>>>> a light load. Everything's hunky dory at 50hz and the negative
>>>>> portions of the wave are neatly removed. I up the frequency to say
>>>>> 1khz and all is still well.... and repeat. Eventually I will notice
>>>>> that there's insufficient recovery time for the diode to function as
>>>>> it formerly was. At still higher frequencies, the inherent capacitance
>>>>> of the diode is leaving just a flat DC voltage with no longer any
>>>>> peaks visible. If I keep going up and up in frequency, will this
>>>>> situation continue indefinitely or will I eventually run into some
>>>>> weird unexpected effects like negative resistance/parametric
>>>>> amplification etc etc?
>>>>>
>>>>> CD.
>>>>>
>>>>> PS: Please don't suggest using a fast recovery diode as that's not
>>>>> what the question is getting at. I'm not after a solution to a
>>>>> problem, just an answer to this entirely theoretical quesition.
>>>>
>>>> Eventually the ESL of the diode will series resonate with its
>>>> capacitance, and beyond that it's an inductor.
>>>>
>>>> By that time, it's not much of a diode. In a PN silicon diode the
>>>> carriers can't move very fast so it starts to look ohmic at high
>>>> frequencies. PN diodes have both reverse and "forward recovery" time
>>>> delays.
>>>>
>>>> I've used SiC diodes for fast high-voltage things and they are much
>>>> better than silicon.
>>>>
>>>> It is an interesting problem, how to make a power rectifier at very
>>>> high frequencies. It's been proposed to put solar arrays in orbit and
>>>> beam the power down as microwaves, into "rectennas" on the ground.
>>>> They must have some sorts of diodes in mind.
>>>>
>>>> The step recovery effect is cool too. Look up "drift step recovery
>>>> diode" aka Grekhov diode for some other interesting effects. I made
>>>> one thing that forward biases a power diode at +50 volts for a while
>>>> and then reverses it at a couple hundred amps. I didn't sell many but
>>>> it was fun.
>>>>
>>>> https://www.dropbox.com/scl/fi/7r128d5fny7kj403ozk5q/Neon_5.JPG?rlkey=6gz93k2xr1bvsxljaxomd8swg&raw=1
>>>
>>> Cool. Like everyone else here I suppose, I have thousands of diodes of
>>> all kinds, some more easy to identify than others. I have the
>>> capability to characterize them for their different high speed
>>> attributes, but doing so *properly* would take up huge amounts of time
>>> and sadly I just can't spare it. Life gets shorter every year it
>>> seems.
>>
>> It would take even longer for you to learn enough to be able to classify
>> them properly.
>>
>> Step recovery diodes are fun, as John Larkin points out, but even if you
>> spend the money to buy one that is properly characterised for the task
>> you have to wait a while for the snap-off edge that you want and the
>> stored charged that works the magic isn't all the well-defined.
> 
> Pretty good for generating a comb, though. I would guess there are
> more varieties of diode than any other discrete component there is.
> Quite a remarkable little device and - usually - trivially cheap.

"Generating a comb" relies on the fact that a Dirac impulse - of finite 
area and zero width - would generate all the harmonics of its repetition 
rate up to infinity. Step recovery diodes generate narrow pulses and the 
upper limit to the harmonics they generate is set by the width of the spike.

-- 
Bill Sloman, Sydney