Transistor Failure Under High VSWR Load - RF Cafe Forums
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Post subject: Transistor Failure Under High VSWR Load Posted:
Thu Feb 08, 2007 12:13 am
Joined: Wed Feb
07, 2007 11:51 pm
I am wondering
what the failure mode of an LDMOS type transistor is under high VSWR
conditions (say VSWR of 3:1). Is it simply voltage breakdown where the
forward and reflected voltages add to the supply voltage and destroy
the device? The output impedance of matched transistors is still rather
low (eg. 5-8 ohms for MRF372) and it seems that over voltage may be
Thoughts and/or links to material on this subject?
Post subject: Posted: Thu Feb
08, 2007 3:04 pm
Joined: Mon Jun 27, 2005
The failure mechanism
is as you indicated a reflected wave which is added to the supply voltage
and causes to voltage breakdown.
Usually LDMOS transistors can
withstand higher mismtach values
(Up to 10:1).
Just FYI the
MRF372 has a known failure with a protection diode at the input which
causes to short-circuit of the device. It is better to use equivalent
devices from Philips, but maybe Freescale has already solved this bug.
I don't know about any article discussing about this kind of
Post subject: Posted: Thu
Feb 08, 2007 4:10 pm
Joined: Wed Feb 07, 2007
Thank you for your input.
I was not aware of this input diode problem on the MFR372 that you mentioned.
Is the failure of this diode random, does it occur under high VSWR conditions,
or some other way? I will see if a search on the web turns up any other
Post subject: Posted: Thu Feb 08,
2007 4:13 pm
Joined: Mon Jun 27, 2005
regarding the diode, I just recall that there was a discussion about
this issue in the past. Here is a link to the posts:
Post subject: Posted:
Thu Feb 08, 2007 4:30 pm
Joined: Wed Feb 07,
2007 11:51 pm
I ran my web search
and found two previous discussions here in the RF Cafe (which I read
with interest). Forgive my ignorance, but how would one determine whether
the gate diode had failed versus an over-voltage condition on the drain?
Post subject: Posted: Thu Feb 08, 2007 4:38
Joined: Mon Jun 27, 2005 2:02 pm
What happened in my case was that
I had to build a PA with a given output power for 470-806MHz and there
was no available Freescale LDMOS device for the middle stage, meaning
with a proper output power. Therefore, I had to use the MRF372 for both
the middle and final stages, which is wrong to do!
When I used
the MRF372 as driver to another MRF372 the input power, and hence the
voltage swing to the final stage was exceeding the maximal required
value and the input diode of the final stage clamped this extra voltage.
Post subject: Posted: Fri Feb 09, 2007
Joined: Wed Feb 07, 2007 11:51 pm
I've been trying to find an original
data sheet for the MRF372 (ie. Rev 0) to see what the maximum allowable
Vgs is. If the gate diode was intended to clamp negative voltages, then
it may have been removed at later revisions. Current spec sheets for
the LDMOS parts MRF372 and MRF374A give a maximum allowable range for
Vgs of -0.5V to +15V. I have an older (1999) Motorola catalogue showing
the MRF374 and it gives a maximum Vgs range of +/-20V. I suspect the
same may be true for the original MRF372 parts and so I am trying to
track down an old data sheet (we must have one in a file somewhere!).
So, it looks like Motorola / Freescale may have removed this diode and
lowered the allowable negative Vgs spec.
Thanks for all of you help.