Looking for pulse-rated zener.

[colin]

"Winfield Hill" <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote in
message news:d9cgmt0100a@drn.newsguy.com...

colin wrote...

Winfield Hill wrote ...

This active-zener method works well with low-voltage power MOSFETs,
such as under 100V, but it's dangerous with high-voltage FETs, 200V
and up, because they have a bad tendency to go into RF oscillation.
This is a high-power RF oscillation at frequencies of 15 to 40MHz,
which is very difficult to damp with external parts such a ferrite
beads, gate resistors, etc. That's because the RF oscillation is
internal to the FET, employing its inductance and self capacitance.
The required linear properties occur whenever a high current flows
while the drain-source voltage is higher than 10 to 20V. The latter
condition causes the FET capacitances to drop to the levels where RF
amplification is efficient.

If the RF feedback path is wholy internal how would this affect the
method of using a higher gate drive resistance to slow the current
fall to limit the voltage to less than the breakdown voltage?

Two different effects... The slowing of the turnoff means the
coil can flyback and dI/dt discharge as it's doing so, without
reaching the avalanche voltage, if carefully done.

or does the zener just add more parasitics to make the difference?

You're asking if oscillation doesn't happen in the event of a
slowed transition, as in the zener case? It certainly can with
high-voltage MOSFETs, although the dV/dt slewing output helps to
hide it, on the one hand, and perhaps to dampen it, on the other.

Yes thanks thats what i was asking, as both cases have the vds>20v @ high
curent. Trying to think of a way of avoiding it yet still using a more
deterministic way of setting the peak voltage.

Actualy i was wondering if a cascode mosfet arangement would behave any
better, again it might make it less noticable as the bottom device would
stay more in control of the current, although i would be worried about this
as long ago I had some nasty oscilations when i was trying to make a high
voltage power supply with several series mosfets (600v mosfets were very
limited at the time), but unfortunatly i never had the time (or the
experience back then) to get to the bottom of all the diferent modes of
oscilations.

Colin =^.^=

 

[Robert Baer]

Mike Monett wrote:

Winfield Hill wrote:

[...]


One question, Mike. Did the house have a good gutter system, with
drainpipes, to keep roof drainage away from the basement walls?

Thanks,
- Win


Very astute question, Win. The drainage was very poor. The gutters were
full of leaves and warped so they wouldn't drain properly, and water
just spilled over in the middle. But you don't notice these things until
the first storm, after you have signed the lease and moved in.

The downspouts were normally connected to drainage pipes to take the
water away from the house, but these had been removed to make cutting
the grass easier. So the water stayed along the exterior walls.

The basement wasn't sealed - the concrete was bare. After the first
storm, there was several inches of water in the basement. A great deal
of wood that was stored lying flat on the floor turned out to have mold
underneath.

The landlord promised to fix everything but it never happened. Then
winter set in and it became impossible to do anything. Ottawa had an
extremely low vacancy rate at the time, and the landlords could get away
with anything. You counted yourself lucky if there was a roof over your
head. Some of the places I looked at were just incredible.

All these things were against city code. But lack of enforcement meant
doing the best to try to fix things myself. This turned out to be
impossible. An amateur with a paintbrush cannot seal a concrete
basement. It takes professionals with the proper equipment, and it
should have been done when the house was built.

I tried to leave as soon as the first symptoms started, but a vacancy
rate of 0.3% in the middle of winter meant there was nowhere else to go.

My big mistake was underestimating what mold can do to your health. The
worst symptoms didn't really show up until after I moved. By then it was
too late.

Since then I have found many basements that are sealed properly but may
have accumulated cardboard boxes full of paper or clothes. Even if the
basement doesn't leak, the cardboard accumulates moisture and provides
an excellent place for mold to grow.

So if you can smell a musty odor after returning from shopping, mold is
growing somewhere and it should be corrected.

Mike Monett
Check the rental laws.

I understand that in most states (US) that a lease of one year or
more, even tho it *states* one year (or more) in reality means
"indefinite" which translates to *any* amount of time the renter
desires. Some states may require the lease to state one year and one day
at minimum for that interpretation.
A lease less than one year runs for the specified period.
Something to consider and use.

 

[Robert Baer]

Watson A.Name - "Watt Sun, the Dark Remover" wrote:

"Winfield Hill" <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote in
message news:d9cfvj0su1@drn.newsguy.com...

Jim Thompson wrote...

archive storage (I rent an off-site, air-conditioned, 10'x10'
storage "cell" that I have filled with file drawers) ...

I never could understand why Western houses eschew basements.
A full basement is a VERY useful thing to have. Get double
or half-again the floor area of the house in one quick step.


Well, some areas of the coutnry have to have basements because of the
tornadoes. But here in So. Calif, we seldom have basements, I'm
guessing the reasons are:

Because the dwelling is on a sandy soil that has a shallow water table,
so the basement would be easily flooded.

Because most of the housing built after WW2 used a poured cement slab
for the floor.

Because in a bad earthquake, the rest of the house would fall into the
basement.

Because it's cheaper to build a second story out of wood than dig a hole
and pour concrete walls, needed for earthquake reasons. Also since it
doesn't get cold here, the insulation doesn't have to be that great, so
the second story is even cheaper.

Probably a combination of the above.

I think that this place has a cool basement.
http://www.earth-house.com/Homes_4_Sale/Atlas_E_Silo/atlas_e_silo.html


--
Thanks,
- Win



I guess i better use some of my "pocket lint" and get that shack...

 

[Ian Stirling]

In sci.electronics.design Winfield Hill <hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Winfield Hill wrote...

Lasse Langwadt Christensen wrote...
snip

I've seen an app-note, onsemi I think, that stated that the
fets could handle more power with the active clamping
snip
The predictability of an added gate-zener function may be desirable.

Actually, there is another possible reason. Higher inductor flyback
voltages mean shorter inductor discharge times. Dissipating a fixed
amount of energy in a shorter time means higher junction temperatures
because the heat can't diffuse as far through metal frames, and into
the heatsink, etc. So that's an argument both for lower FET clamping
voltages and for more predictable voltages.

Not in a microsecond or shorter (the time in question).
Then it's stuck in the few microns outside junction.

 

[Spehro Pefhany]

On 22 Jun 2005 12:59:47 -0700, the renowned Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Jim Thompson wrote...

archive storage (I rent an off-site, air-conditioned, 10'x10'
storage "cell" that I have filled with file drawers) ...

I never could understand why Western houses eschew basements.
A full basement is a VERY useful thing to have. Get double
or half-again the floor area of the house in one quick step.

For sure! Half-again on a 2-storey and modern basements are very
useful (unlike the dungeons of old).


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com