Re: THOR: SMPS warmup behaviour followup

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "k= by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Steve-at-g8cyerichmond.freeserve.co.uk>

My only experience of ferrite grinding was when I wanted to make the legs on
an 'E' core equal (often the central leg is shorter than the other two, to
make an air-gap). The method I used was similar to that used for grinding
thin sections of minerals. Basically I used a thick glass plate, and a
slurry of carborundum powder and a mixture of 50/50 water and glycerine.
With the core held vertically I rubbed it face down in a figure of eight
motion, rotating the core by 90degrees every so often. It was a slow
process, I used the finest carborundum powder I could (from a local rock
shop). Using coarser powder meant that I ground too fast and the results
were very poor. It takes time and care, ferrite is very hard!!
----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, December 18, 2000 3:00 PM
Subject: THOR: SMPS warmup behaviour followup


> Original poster: "Marco Denicolai by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <Marco.Denicolai-at-tellabs.fi>
>
> Hello all.
>
> Maybe some of you recall that over a month ago I had a problem with Thor
> and its
> SMPS. Namely, after 15-20 seconds of running the performance got worse. It
was
> because the SMPS primary capacitor charging speed got slower.
>
> The supposed cause was long reverse recovery time of the BYM36G diodes
used to
> rectify the charging pulses.
>
> I did switch to using the UF5004 (faster, 75 ns vs 250 ns). That involved
> de-soldering 80 diodes and replacing them with 112 new ones.
>
> Here are the preliminary results:
>
> 1. Now the diodes run cool also after 1 minute: the old diodes got "can't
> touch"
> hot within 15 seconds. Good.
> 2. The performance is still getting lower after some 15-20 seconds!
>
> I have now identified what I believe is the actual cause: there is some
corona
> between the SMPS transformer secondaries and the ferrite cores. As the
corona
> grows together with charging voltage it is a good explanation for the
charging
> profile bending. I can also "ear" the corona. A week ago I had also a
discharge
> through the secondary (SMPS, not TC) coil former and the core, resulting
in one
> of the driver getting damaged.
>
> I am now going to try two remedies:
>
> 1. ground the ferrite core: at the moment it is floating. This should
> prevent it
> from collecting charge (if it does).
> 2. as the core is double-U shaped, smooth the ferrite sharp corners below
the
> secondary coil former. This should lower the electric field density in
that
> area.
>
> Can anybody give some hints about ferrite grinding?
>
> Regards
>
>
>
>
>
>