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[Tom Rauch]
> It is MUCH better to catch the fault on the rise of the envelop, long
> before peak power is reached, than to have the protection
> "stupidly" sit back waiting for peak power to exceed a set value
> before reacting. In particular this is true with SSB and CW
> transmissions.
>
[Dave D'Epagnier]
Tom, I think you've been fooled all this time about the most common
failure mode of solid state transistor amplifiers. Again, the main cause of
failure is excess power dissipation in the transistor due to load mismatch,
not voltage breakdown. If voltage breakdown were the issue, you'd see
voltage peak detection circuits in amp finals. I haven't seen any of those,
although installation of transorbs wouldn't be a bad idea.
I ran across an interesting Motorola application report last night
written by one of the pioneers and masters of work in this area; the late
Helge O. Granberg. It echos the thermal issue. This article is a reprint
from a february 1991 RF Design article and is also in the Motorola HB215
Application Report Handbook. HB215 is a goldmine of information in this area
and is free for the asking from Motorola.
http://mot-sps.com/lit/html/ar510.html
AR510 addresses this very issue in detail an describes how Helge
would protect his amplifiers. He simply detected the output voltage and
current phase relationship (reflectometer) to give an indication of load
mismatch and reflected power. He then fed that back into the amp ALC input.
He states that as long as this is done in less time than the thermal time
constant of the transistor die, adequate protection is attained. The bottom
line is that there IS time to protect the transistor and it is MUCH longer
than 1us. This operation can indeed be done with a single chanel A/D
converter and uP, or better yet as some one suggested a few days ago,
without the A/D and uP at all. Just use fast analog circuitry and feed back
to the ALC input.
--Dave
K0QE
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5.5.2650.12">
<TITLE>RE: [AMPS] SWR Protection</TITLE>
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<BODY>
<BR>
<UL>
<P><B><I><FONT COLOR=3D"#0000FF" SIZE=3D2 FACE=3D"Arial">[Tom =
Rauch]</FONT></I></B><I></I><FONT SIZE=3D2 FACE=3D"Arial"></FONT>=20
<BR><FONT SIZE=3D2 FACE=3D"Arial">It is MUCH better to catch the fault =
on the rise of the envelop, long </FONT>
<BR><FONT SIZE=3D2 FACE=3D"Arial">before peak power is reached, than to =
have the protection </FONT>
<BR><FONT SIZE=3D2 FACE=3D"Arial">"stupidly" sit back waiting =
for peak power to exceed a set value </FONT>
<BR><FONT SIZE=3D2 FACE=3D"Arial">before reacting. In particular this =
is true with SSB and CW </FONT>
<BR><FONT SIZE=3D2 FACE=3D"Arial">transmissions.</FONT>
<BR><FONT SIZE=3D2 FACE=3D"Arial"> </FONT>
<BR><B><I><FONT COLOR=3D"#0000FF" SIZE=3D2 FACE=3D"Arial">[Dave =
D'Epagnier]</FONT></I></B><I></I> <FONT COLOR=3D"#0000FF" SIZE=3D2 =
FACE=3D"Arial"> </FONT>
<BR><FONT COLOR=3D"#0000FF" SIZE=3D2 FACE=3D"Arial">Tom, I think you've =
been fooled all this time about the most common failure mode of solid =
state transistor amplifiers. Again, the main cause of failure is excess =
power dissipation in the transistor due to load mismatch, not voltage =
breakdown. If voltage breakdown were the issue, you'd see voltage peak =
detection circuits in amp finals. I haven't seen any of those, although =
installation of transorbs wouldn't be a bad idea.</FONT></P>
<P><FONT COLOR=3D"#0000FF" SIZE=3D2 FACE=3D"Arial">I ran across an =
interesting Motorola application report last night written by one of =
the pioneers and masters of work in this area; the late Helge O. =
Granberg. It echos the thermal issue. This article is a reprint from a =
february 1991 RF Design article and is also in the Motorola HB215 =
Application Report Handbook. HB215 is a goldmine of information in this =
area and is free for the asking from Motorola.</FONT></P>
<P><FONT COLOR=3D"#0000FF" SIZE=3D2 FACE=3D"Arial"><A =
HREF=3D"http://mot-sps.com/lit/html/ar510.html"; =
TARGET=3D"_blank">http://mot-sps.com/lit/html/ar510.html</A></FONT>
</P>
<P><FONT COLOR=3D"#0000FF" SIZE=3D2 FACE=3D"Arial">AR510 addresses this =
very issue in detail an describes how Helge would protect his =
amplifiers. He simply detected the output voltage and current phase =
relationship (reflectometer) to give an indication of load mismatch and =
reflected power. He then fed that back into the amp ALC input. He =
states that as long as this is done in less time than the thermal time =
constant of the transistor die, adequate protection is attained. The =
bottom line is that there IS time to protect the transistor and it is =
MUCH longer than 1us. This operation can indeed be done with a single =
chanel A/D converter and uP, or better yet as some one suggested a few =
days ago, without the A/D and uP at all. Just use fast analog =
circuitry and feed back to the ALC input.</FONT></P>
<P><FONT COLOR=3D"#0000FF" SIZE=3D2 FACE=3D"Arial">--Dave</FONT>
<BR><FONT COLOR=3D"#0000FF" SIZE=3D2 FACE=3D"Arial">K0QE</FONT>
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