Re: Rectification

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.
--
Bill Sloman, Sydney