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J

Joerg

Jan 1, 1970
0
Hi SioL,
Hmm, this sounds interesting. How exactly did you do this toroid part?
Sometimes just by winding primary and secondary on top of each other,
with a ration of 1:2 or a bit higher. The best results require a
trifilar winding where you twist three wires, wind that bundle around a
core and wire it up so the secondary towards the FET has two in series.
But watch out that the FET gate can tolerate the resulting pulse
amplitude of four times the logic level with a healthy margin. You don't
want to pulse a FET with an abs max Vgs of 20V right smack at 20V. The
goal is to transit through the threshold region of the FET as fast as
possible but not run it 'into the red'. When it's all working as desired
measure carfully with a scope to make sure there are no spikes and other
adversities that could shorten the life of the FET or the driver.

Then there is the trick of running inverted and non-inverted into the
primary and get twice the logic level out of a secondary. This only
requires bifilar winding. You could take a CAT-5 pair, twist it a bit
more, about 2 twists every inch or so. But be careful, these drivers
must be in 100% transition time sync or they might blow out. That is
less easy than it seems to be.

Mostly I just used 43 material because I have lots in the lab. This is
also the material most popular for EMI remedies, the cores you slip over
offending cables. For pulsing a FET you only need a very small core 1/4"
or so. Nowadays I often need my glasses to wind these.

To try it out use a large ferrite bead that you find in the box, or one
of those six-holer 'VK200 style' cores. A really good source on how to
wind bifilar and trifilar is the ARRL Handbook. It's a good investment
anyway.

The bottomline is that 5V logic can't directly drive high speed pulser
FETs well enough, no matter what the marketing folks say about the FET.
With 3.3V logic there is not a chance. You need a higher gate drive.

Regards, Joerg
 
S

SioL

Jan 1, 1970
0
Sometimes just by winding primary and secondary on top of each other, with a
ration of 1:2 or a bit higher. The best results require a trifilar winding
where you twist three wires, wind that bundle around a core and wire it up so
the secondary towards the FET has two in series. But watch out that the FET
gate can tolerate the resulting pulse amplitude of four times the logic level
with a healthy margin. You don't want to pulse a FET with an abs max Vgs of
20V right smack at 20V. The goal is to transit through the threshold region of
the FET as fast as possible but not run it 'into the red'. When it's all
working as desired measure carfully with a scope to make sure there are no
spikes and other adversities that could shorten the life of the FET or the
driver.

Then there is the trick of running inverted and non-inverted into the primary
and get twice the logic level out of a secondary. This only requires bifilar
winding. You could take a CAT-5 pair, twist it a bit more, about 2 twists
every inch or so. But be careful, these drivers must be in 100% transition
time sync or they might blow out. That is less easy than it seems to be.

Mostly I just used 43 material because I have lots in the lab. This is also
the material most popular for EMI remedies, the cores you slip over offending
cables. For pulsing a FET you only need a very small core 1/4" or so. Nowadays
I often need my glasses to wind these.

To try it out use a large ferrite bead that you find in the box, or one of
those six-holer 'VK200 style' cores. A really good source on how to wind
bifilar and trifilar is the ARRL Handbook. It's a good investment anyway.

The bottomline is that 5V logic can't directly drive high speed pulser FETs
well enough, no matter what the marketing folks say about the FET. With 3.3V
logic there is not a chance. You need a higher gate drive.

Regards, Joerg

Hey, this is really cool. I had that problem with 5V logic, never occured to me
to use toroid at that place. Thanks for a good idea.
I ended up using BS170 and BS250 at the time.

SioL
 
J

Joerg

Jan 1, 1970
0
Hi SioL,
Hey, this is really cool. I had that problem with 5V logic, never occured to me
to use toroid at that place. Thanks for a good idea.
I ended up using BS170 and BS250 at the time.
You are welcome. Just one word of caution. Since toroids do not transfer
DC these schemes are meant for short pulses. They are not very suitable
for longer pulse durations and large duty cycles.

I did use toroids for duty cycles up to 50%, like it almost would be
necessary in Winfield's application. But then you end up with 'half the
amplitude above GND, the other half below'. In some cases that is fine
and can actually help turning the FET off faster. When pulses are
significantly longer than 1usec you have to select other ferrite
materials. Amidon has a huge selection and guidelines. The longest
pulses I did with toroids were around a millisecond but that, of course,
was not with 43 material anymore.

Regards, Joerg
 
P

Paul Burridge

Jan 1, 1970
0
Mostly I just used 43 material because I have lots in the lab. This is
also the material most popular for EMI remedies, the cores you slip over
offending cables. For pulsing a FET you only need a very small core 1/4"
or so. Nowadays I often need my glasses to wind these.

Don't you find that stuff far too lossy for transformers?
 
J

Joerg

Jan 1, 1970
0
Hi Paul,
Don't you find that stuff far too lossy for transformers?

Not really. Sure, it isn't ideal but very cheap and tons of sources. The
latter makes the purchasing folks happy. In the old days when
distributors brought chocolate boxes to the purchasing department around
Christmas that could place you high up the 'food chain'.

Some of the EMI material is more lossy but that is often not a clean 43
material. If the design needs tight specs I make sure that the ECO calls
for vendors where detailed data sheets are furnished, such as Amidon. In
Europe that would be larger companies such as Philips or smaller ones
like Vogt.

Regards, Joerg
 
T

Tim Wescott

Jan 1, 1970
0
Joerg said:
Hi Paul,


Not really. Sure, it isn't ideal but very cheap and tons of sources. The
latter makes the purchasing folks happy. In the old days when
distributors brought chocolate boxes to the purchasing department around
Christmas that could place you high up the 'food chain'.

Some of the EMI material is more lossy but that is often not a clean 43
material. If the design needs tight specs I make sure that the ECO calls
for vendors where detailed data sheets are furnished, such as Amidon. In
Europe that would be larger companies such as Philips or smaller ones
like Vogt.

Regards, Joerg

It's interesting that you should cite Amidon -- to my knowledge they
just resell Fair-Rite ferrites and Micro Metals iron powder cores.
 
P

Paul Burridge

Jan 1, 1970
0
It's interesting that you should cite Amidon -- to my knowledge they
just resell Fair-Rite ferrites and Micro Metals iron powder cores.

IIRC, MM's powder iron toroids are about the best thing out there to
wind RF transformers on. The lossier ferrites do enable high values of
inductance to be obtained from very few windings, though, if you can
live with that loss.
 
J

Joerg

Jan 1, 1970
0
Hi Tim,
It's interesting that you should cite Amidon -- to my knowledge they
just resell Fair-Rite ferrites and Micro Metals iron powder cores.


Yes, but Amidon is a great source of information for engineers who are
pretty new to ferrites and iron powder cores. It is also a place with a
tremendous variety of cores in one location. Their booklets were what
taught me the ropes a long time ago. When a client has EMI problems or I
design isolation amps for them I usually point their engineers to
Amidon. There they can buy a large variety of cores to try things out on
their own. Literature from the mainstream manufacturers is great as well
but for the uninitiated it is often too heavy on math and formulas they
were never used to.

In Europe it used to be even more difficult. Once I saw a whole crate of
cores at a company. Unfortunately we needed a different ferrite so I
asked them why they bought this many. It was the minimum quantity...

Regards, Joerg
 
T

Tim Wescott

Jan 1, 1970
0
Paul said:
IIRC, MM's powder iron toroids are about the best thing out there to
wind RF transformers on. The lossier ferrites do enable high values of
inductance to be obtained from very few windings, though, if you can
live with that loss.

You don't lose that much to ferrite if it's a wide-bandwidth
transformer, because the main job of the core is to reject flux changes,
which it does by forcing the windings into sync. Small flux change
means there isn't much loss, even with a lossy material*.

Resonant transformers are an entirely different matter.

* At least that's how it's been explained to me. On reflection it
sounds like total BS -- I'll have to do the math sometime and see what
falls out the bottom.
 
L

Lady Chatterly

Jan 1, 1970
0
From what I've seen, these power MOSFETs aren't much use for anything
beyond HF.
Check these out, though:
2SC1969
2SC1971
2SC1972

The Internet is still alive and well, though people are spend more
time talking because life is more centered on the community.
These are BJTs but some sort of arrangement of them should get you
near the mark.

Are you sure?
 
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