[AMPS] SWR Protection

Mon, 4 Dec 2000 12:48:43 -0700

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> However, transistors can fail very rapidly from excessive voltage. 
> You have zero-time to prevent that failure, since it is a punch-
> through failure. Temperature really isn't the major concern. Excess 
> voltage is, and exciters with keying transients (common today) can 
> really hammer the transistors with collector or drain voltage spikes. 
> That's another reason why the optimum load impedance is 
> generally set to be higher than the working impedance.   
> 
	[Dave D'Epagnier]  Tom, I know of at least one prominent RF
transistor manufacturer that would take issue with your statements about
temperature above. My experience as well shows that high temperature is
almost always the cause of failure in a properly designed solid state amp.
This high temp is usually cause by load mismatch. If voltage breakdown is a
problem then improper devices are being used and it's a poor design to begin
with.

>  > a transistor you will see that the die has a typical response of 10ms
> or
> > so. So if you can react quicker than this (say in about 1ms) then you
> can
> > adequately protect the transistor. They do not instantaneously vaporize
> > under adverse load conditions. 
> 
> They most certainly do. Thermal concerns from excessive 
> dissipation are almost never the failure mode. Voltage breakdown 
> is immediately catastrophic.
	[Dave D'Epagnier]  
	Again, if voltage breakdown is the problem, then all bets are off.
The wrong device is being used and there isn't a whole lot that can be done
for protection. If voltage breakdown were that big of an issue in amateur
gear almost everyone with an exciter would have blown it up by now due to
load mismatches or accidentally transmitting with no load connected at all
(improper antenna selected etc). 

> Therefore simultaneous detection of forward
> > and reflected power is unnecessary, and a single A/D converter is
> > typically used. 
> 
> You MUST use a directional coupler, and you should always detect 
> reflected power as well as forward power. Forward power sets the 
> drive limits, and reflected power sets the SWR limit. 
	[Dave D'Epagnier]  Yes, you need a directional coupler, but
simultaneous (instantaneous) sampling of forward and reflected power is not
necessary.

> > time of the amplifier is this quick if that's the method used for
> shutdown
> > under high SWR conditions. Severe load mismatches can cause the
> > transistors to see higher voltages, but typically not as high as the
> > breakdown rating of the device. 
> 
> Not so. They can easily produce such voltages unless the voltage 
> breakdown of the device is several times the operating voltage. 
> While commercial equipment often has enough headroom, amateur 
> gear doesn't.
	[Dave D'Epagnier]  In that context, I guess you could say I design
commercial grade equipment at home. Take a look at Advanced Power
Technologies RF transistors. Every one of their RF power transistors has a
breakdown rating of 3x the recommended operating line voltage.

> One of the cures for the common PA failures in Drake TR5 
> transceivers was to use a higher voltage output device.
> 
> Have a look at Motorola's data, and you will see transistors are 
> typically recommended to be used with supply voltages just less 
> than half of the working voltage. In normal operation, that device 
> would be coming close to peak voltage rating on every cycle at 
> maximum output. If the collector impedance increases, the voltage 
> can easily exceed twice the supply rail.
	[Dave D'Epagnier]  Transistor output impedances are generally much
less than 50 ohms. The output matching network steps this impedance up to 50
ohms. As a result the  [Dave D'Epagnier]  voltage on the collector (or
drain) of the transistor is much less than voltage at the 50 ohm load during
normal operation. The transistor should not even be close to breakdown under
this condition.

> I've measured voltages of 100 volts on the drains of FET's operated 
> at 12 volts, under some conditions of load mismatch. If the device 
> have a breakdown voltage of less than that, it will instantly fail. Just 
> one RF cycle over the peak voltage breakdown will do it.
> 
> 
> 
> 73, Tom W8JI
> w8ji@contesting.com

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<TITLE>RE: [AMPS] SWR Protection</TITLE>
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<UL>
However, transistors can fail very =
rapidly from excessive voltage. 
 You have zero-time to prevent that =
failure, since it is a punch-
 through failure. Temperature really =
isn't the major concern. Excess 
 voltage is, and exciters with keying =
transients (common today) can 
 really hammer the transistors with =
collector or drain voltage spikes. 
 That's another reason why the optimum =
load impedance is 
 generally set to be higher than the =
working impedance.&nbsp;&nbsp;

[Dave =
D'Epagnier]&nbsp; Tom, I know of at least one prominent RF transistor =
manufacturer that would take issue with your statements about =
temperature above. My experience as well shows that high temperature is =
almost always the cause of failure in a properly designed solid state =
amp. This high temp is usually cause by load mismatch. If voltage =
breakdown is a problem then improper devices are being used and it's a =
poor design to begin with.

&nbsp;&gt; a transistor you will see =
that the die has a typical response of 10ms or
 &gt; so. So if you can react quicker =
than this (say in about 1ms) then you can
 &gt; adequately protect the =
transistor. They do not instantaneously vaporize
 &gt; under adverse load conditions. =

They most certainly do. Thermal =
concerns from excessive 
 dissipation are almost never the =
failure mode. Voltage breakdown 
 is immediately catastrophic.
 [Dave =
D'Epagnier]&nbsp; 
 Again, if voltage =
breakdown is the problem, then all bets are off. The wrong device is =
being used and there isn't a whole lot that can be done for protection. =
If voltage breakdown were that big of an issue in amateur gear almost =
everyone with an exciter would have blown it up by now due to load =
mismatches or accidentally transmitting with no load connected at all =
(improper antenna selected etc).

Therefore simultaneous detection of =
forward
 &gt; and reflected power is =
unnecessary, and a single A/D converter is
 &gt; typically used.

You MUST use a directional coupler, =
and you should always detect 
 reflected power as well as forward =
power. Forward power sets the 
 drive limits, and reflected power =
sets the SWR limit. 
 [Dave =
D'Epagnier]&nbsp; Yes, you need a directional coupler, but simultaneous =
(instantaneous) sampling of forward and reflected power is not =
necessary.

&gt; time of the amplifier is this =
quick if that's the method used for shutdown
 &gt; under high SWR conditions. =
Severe load mismatches can cause the
 &gt; transistors to see higher =
voltages, but typically not as high as the
 &gt; breakdown rating of the device. =

Not so. They can easily produce such =
voltages unless the voltage 
 breakdown of the device is several =
times the operating voltage. 
 While commercial equipment often has =
enough headroom, amateur 
 gear doesn't.
 [Dave =
D'Epagnier]&nbsp; In that context, I guess you could say I design =
commercial grade equipment at home. Take a look at Advanced Power =
Technologies RF transistors. Every one of their RF power transistors =
has a breakdown rating of 3x the recommended operating line =
voltage.

<P><FONT SIZE=3D2 FACE=3D"Arial">One of the cures for the common PA =
failures in Drake TR5 </FONT>
<BR><FONT SIZE=3D2 FACE=3D"Arial">transceivers was to use a higher =
voltage output device.</FONT>
</P>

Have a look at Motorola's data, and =
you will see transistors are 
 typically recommended to be used with =
supply voltages just less 
 than half of the working voltage. In =
normal operation, that device 
 would be coming close to peak voltage =
rating on every cycle at 
 maximum output. If the collector =
impedance increases, the voltage 
 can easily exceed twice the supply =
rail.
 [Dave =
D'Epagnier]&nbsp; Transistor output impedances are generally much less =
than 50 ohms. The output matching network steps this impedance up to 50 =
ohms. As a result the&nbsp; [Dave =
D'Epagnier]&nbsp; voltage on the collector (or drain) of the transistor =
is much less than voltage at the 50 ohm load during normal operation. =
The transistor should not even be close to breakdown under this =
condition.

&nbsp;
 I've measured voltages of 100 volts =
on the drains of FET's operated 
 at 12 volts, under some conditions of =
load mismatch. If the device 
 have a breakdown voltage of less than =
that, it will instantly fail. Just 
 one RF cycle over the peak voltage =
breakdown will do it.

<P><FONT SIZE=3D2 FACE=3D"Arial">73, Tom W8JI</FONT>
<BR><FONT SIZE=3D2 FACE=3D"Arial">w8ji@contesting.com</FONT>
</P>
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