[uCsimm] Re: [gnucsimm] Power supply blues

From: Tom Walsh (tom@cyberiansoftware.com)
Date: Thu Mar 30 2000 - 10:48:09 EST


Micah Dowty wrote:

> I have been having trouble with the Maxim MAX717 and MAX722 power supply
> chips. They're working ok for the most part, but the efficiency is
> terrible. Does anybody have experience with these parts? (or switching
> power supplies in general?)
> I have schematics and info in the hardware and status pages at
> homesoftware.com
>

Two common switching supply "killers" are inferior grade components in the
switcher, and component layout. Common mistakes are substituding a Silicon
Diode for when a Schottky Diode is called out.

Also, the Capacitor used within the switched element itself MUST be a
Low-ESR (for example, C3 & C9) device (Low Equivilant Series Resistance
somewhere on the order of 0.2 - 0.5 ohms @ 1kHz), these caps are a bit
pricey, when compared with a a garbage-variety electrolytic, but absolutely
necessary!

The inductors used within the switch elecment are generally torroidal (or
pot) cores with a nice cross sectional area so as to store the needed power
required by the switcher load. A cheap-o J.W. Miller choke is not going to
work here. Wire gauge should be about 24ga.. Look very closely at
'http://www.openhardware.net/Users/TomW/drawings/Large.Motherboard.jpg' and
you will see the physical sizes of the inductors I used in my three
switching supplies, the first two supplies (near the heatsink) are buck
supplies and will deliver about 3 Amps @ 3.3 & 5 volts. The supply to the
left is a boost supply and will deliver up to 700ma @ 38 volts. By the
way, if your inductor gets REAL hot(!), it is too small --- you have
magnetically saturated the device and it WILL overheat, ..., real fast!

What you cannot see from the picture is perhaps the most critical part of
designing with switchers, and that is the component layout. To recall a
story: the "guru" of drafting (where I had worked once upon a time) turned
the layout AutoRouter loose on a switching supply I'd designed, simply
amazing things happened to that supply! He had some nasty current loops in
the PCB layout that were so bad that the power supply actually broke into
oscillation! Amaziing!!! But, I learned my lesson and personally oversee
any board someone does the layout for me.

Probably your best source of information about switching supplies will come
from the app notes up at the Linear Technologies website. These people
have a rich design history with supply circuits and the app notes are
somewhat technical, but not all are that way, these notes may help shed
some light...

Whenever you can, try to look at the suggested PCB layout "pattern" that a
manufacturer will sometimes give you. Note that the switch element
components (diode, inductor, FeedBack sense) are on very short, very thick
foils traces? This is keep stray inductances down, and provide a low-ohmic
path between the components. Grounding between the devices of the switcher
is critical, you have to keep the paths between components short & sweet
(thick).

 Don't mess with quality, you will suffer for it.

Regards,

TomW

--
Tom Walsh - WN3L - Embedded Systems Consultant
'www.openhardware.net', 'www.cyberiansoftware.com'
"Windows? No thanks, I have work to do..."

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