Date: Mon, 20 Dec 1999 12:50:54 -0800 (PST) From: Matthew Dillon <dillon@apollo.backplane.com> To: Warner Losh <imp@village.org> Cc: Wes Peters <wes@softweyr.com>, freebsd-hackers@FreeBSD.ORG Subject: Re: Cool little 100BaseTX switch - they're coming down in price Message-ID: <199912202050.MAA17151@apollo.backplane.com> References: <199912202024.MAA17008@apollo.backplane.com> <199912190410.UAA01049@apollo.backplane.com> <385C60FC.7613CB55@bellatlantic.net> <19991218225758.A23729@futuresouth.com> <199912190556.AAA08484@whizzo.transsys.com> <199912191943.LAA06826@apollo.backplane.com> <385D47D3.FCEE9EAB@softweyr.com> <199912192127.NAA09156@apollo.backplane.com> <385DDE7A.1A0ED466@softweyr.com> <199912201949.LAA16719@apollo.backplane.com> <199912202034.NAA18342@harmony.village.org>
next in thread | previous in thread | raw e-mail | index | archive | help
:Yes. The nice thing about modern swithcing power supplies are that if
:you DO hear any vibration, you know you have big problems and are
:courting disaster. At least that's my experience in homebrewing a 12V
:-> 16V converter for my Sony VAIO 505TS. The original magnetics I
:chose easily overloaded, producing a noticable hum. I didn't think
:much of this at the time, but about 3 hours into my test the coil
:started smoking a little, and then the cascade failure from that
:caused an overload on the regulator ic I was using and after the pop
:no more hum, but no more +16V either :-( And I was only running at
:20 kHz....
:
:Warner
Also, step-up switching supplies are a lot more finicky then
step-down supplies. We usually use a flyback configuration for step-up
but the input voltage range tends to be more constrained and the
efficiency is lower so I try to avoid step-up whenever possible. I
used a step-up (flyback) in one of my telemetry boards - incredible
sucker, it took 12V in and produced 24V at 3A. I made one mistake in
the design and that was to use too low a wattage on the 0.05 ohm current
limit resistors (the last thing you'd expect to overheat).
My personal preference is to use an *unregulated* DC wall plug (which
is essentially just an AC transformer, rectifier, and big caps) that
produces somewhere on the order of 15-25V and then put a step-down
switching supply on the motherboard. This way we don't really care what
the DC power going into the device is, or how noisy it is, just as long
as it's at least 1.5V more then our regulator output. It lets us use
virtually any wall plug, or a solar panel, battery, or DC power without
making a single change to the design.
The standard national switching supply chips can handle DC inputs up to,
what, 40V? I think they have an industrial (read: for a car) version
that can handle even more. They are efficient enough that even worse
case inputs have low enough heat dissipation. I usually use the 1A parts
though one recent design uses their 5A part. If we needed more power we
used to use TO-220 packaged PFETs for the switching circuit. Now days
we use SMT duel-PFETs which are actually rated for higher current in
plastic then the old PFETs were in ceramic. But for low-current (< 3A)
applications the national parts can handle the switching natively, which
I really really like.
-Matt
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-hackers" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?199912202050.MAA17151>