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Reply to Thomas W. Day regarding 12V Power Amplifier Desig

TW>From: thomasd@tps.COM (Thomas W. Day)
TW>Organization: Telectronics Pacing Systems
TW>
TW>Can anyone give me some advise on power amplifier design for 12VDC
pow
TW>supplies (automotive applications)?  Specifically, what kinds of
power
TW>supplies are typically used in high power (50W+) designs?  I am
hoping
TW>find a practical way to get a high voltage (>+/-25VDC) supply for a
bi
TW>amp.
TW>
You probably have gobs of replies on this one.

So long as you remember that powerout=Vrms^2/Rload, the choices
become
apparent.  Boost the Vrms or reduce the Rload.

With a Peak-to-peak swing of, say, 12V, the Vrms (AC coupled) is
6/sqrt(2).  If, however, you use *two* amplifiers, with one being the
inversion of the other, and the load between the output of the two
amps (a
bridge circuit) then you can get Vrms of 12/sqrt(2).  But, the supply
voltage in an auto with the alternator running is closer to 14V.  If
you
use a saturating bridge output stage (output comes from the collectors
of
two bipolar transistors ... or use a capacitive-bootstrap drive to
large
area (very low saturation) FETs, then 14*.707 or 10Vrms is about the
best
you could do.  10^2/4ohms=average power(sine wave) of 25 watts.  Use 2
ohm
speakers, and you have 50W.  1 ohm -> 100W.  At least one commercial
system that I know of used 0.7 ohm loudspeakers.  (The transmission
losses
in reasonable wire gauges are excessive.)

Or -- you *could* use an output transformer.

Or -- you *could* boost the car's battery voltage.  (Back in the
*old*
days [gasp -- I remember those old days] the approach was to use a
"vibrator" (a mechanical chopper, something like a doorbell) to
convert
the auto battery 12V to AC, followed by a transformer to "step up"
the
voltage to the 100V or so required by the thermionic emission valves
(vacuum tubes) in the radio.  Today, of course, you would use a
circuit
with a topology essentially identical to the switching power supply
in
your computer.  (A switching power supply starts by rectifying the
A.C.
line voltage, filtering it with capacitors to make it D.C..  In a
computer
switching supply, you end up with about 180 or 360 VDC, which is then
chopped at a frequency between 20kHz and perhaps 200kHz, then supplied
to
a transformer followed by rectifier diodes and capacitor filters.  In
an
auto, the initial step of AC to DC conversion is unnecessary ... the
rest
of the circuit is essentially unchanged except for the transformer
winding
ratios.

Now, as for designing a DC to DC converter.  You'll probably get loads
of
references for construction or theoretical articles.  You *may* even
get
the suggestion that it's easy.  It ain't.  At least, not the first
time.
It *might* be easy, if you go with a cookbook design for which
components
actually exist -- but *NOT* if you need a commercially viable design
that
is reliable and marketable (i.e. not $20 when your competitor does it
for
$3).  Assuming you intend to go for a full design, read the
following:

Recommendation #1:  Understand the differences between the switching
circuit types.  Flyback is generally cheap overall, push-pull has a
better
transformer utilization.  Read all you can, *particularly* the
appnotes
for integrated circuits designed for the job.  But, don't become too
easily convinced because *this* appnote is charming and reassuring
while
*that* appnote is obscure and overly technical.

Recommendation #2, #3, #4, #5, #6, and #7.  Do *NOT* depend on a
custom
transformer design from a transformer manufacturer.  *UNDERSTAND*
what
core saturation is all about, how to calculate the transformer air
gap,
the interrelationship between operating frequency and inductance
requirements and saturation and parasitic (generally winding
capacitance)
components.  FYI, I spent several months with a fire extinguisher
because
I believed that the transformer manufacturer *met* my 2A saturation
specification.  (The transformer actually saturated at 1.1A, but when
things burst into flame in a few microseconds, it's tough to get a
'scope
probe on it!)

Recommendation #8:  Have a current probe for your 'scope.  Switching
supplies operate in a way that is grossly misleading to anyone who
(deep
down inside) feels that current is just one of those nasty things
that
happens, but voltage it the thing that matters.

Good luck.  If *I* were designing a system, I would do everything I
could
to work with the intrinsic battery voltage of the auto.  If it were
for a
"one-off" system, then no contest:  buy, don't build.  If it were for
a
commercial application, then I *might* consider a switching supply. 
But,
a purchased unit is unlikely to do exactly what you want.  The
learning
experience is tractable (and, if you are, like me, a masochist,
rewarding)
but, the first time, it ain't easy.  No how.

-Dick
rplourde@leotech.mv.com

 ~ WinQwk 2.0 a#746 ~ A loaf of bread, a jug of wine and <occupant>.


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