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Transistor Failure Under High VSWR Load - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


mthiele
Post subject: Transistor Failure Under High VSWR Load Posted: Thu Feb 08, 2007 12:13 am

Lieutenant

Joined: Wed Feb 07, 2007 11:51 pm
Posts: 4
Location: Canada
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 the cause.

Thoughts and/or links to material on this subject?
Thanks.


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IR
Post subject: Posted: Thu Feb 08, 2007 3:04 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
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 failure.


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mthiele
Post subject: Posted: Thu Feb 08, 2007 4:10 pm

Lieutenant

Joined: Wed Feb 07, 2007 11:51 pm
Posts: 4
Location: Canada
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 info.


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IR
Post subject: Posted: Thu Feb 08, 2007 4:13 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello again,

regarding the diode, I just recall that there was a discussion about this issue in the past. Here is a link to the posts:

http://www.rfcafe.com/phpbb2/viewtopic. ... ght=mrf372

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mthiele
Post subject: Posted: Thu Feb 08, 2007 4:30 pm

Lieutenant

Joined: Wed Feb 07, 2007 11:51 pm
Posts: 4
Location: Canada
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?


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IR
Post subject: Posted: Thu Feb 08, 2007 4:38 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
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.


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mthiele
Post subject: Posted: Fri Feb 09, 2007 10:17 am

Lieutenant

Joined: Wed Feb 07, 2007 11:51 pm
Posts: 4
Location: Canada
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.







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