Re: IRF510 amplifier failures

Arv Evans

Marco - KG5PRT

That is very interesting indeed.  Implementing this current based bias control
should be easy and could be compact. 

Inline image 1

If I understand this correctly...
  • Voltage drop across R1 forward biases T2 to ON state
    This lights the LED.
  • Voltage on the collector of T2 turns T1 OFF, removing supply voltage
    to the 5V bias regulator in the RF PA circuit.

I was looking toward a similar control based on reflected power but the current
based approach is easier, as long as it works.  This adds another project to my
to-do list.

Thanks for the URL.

Arv  K7HKL

On Thu, Jan 25, 2018 at 10:11 PM, M Garza <mgarza896@...> wrote:
Here is another:

In the middle of the page, is this:

"I destroyed many IRF510 FETs during testing. In fact I blew a small hole in one and another into several pieces. It was quite a shock when the first one was destroyed because it made a loud noise like a rifle being fired.

Once I got tired of replacing the FETs, I built a current sense circuit, which shuts off the bias once the amplifier draws more than about 3 amps from the PSU. I think this circuit is essential. You can build it into the Power Supply or into the Amplifier. I built it into the Amplifier because the power supply, which is also homemade, does not limit until 7 amps. With the current limit circuit the amplifier now survives transmitting into any SWR from an open circuit to a short."

There is a schematic for the current limiting circuit that is used.  This might be something to incorporate into the design.  It is only 7 more parts.

Marco - KG5PRT

On Thu, Jan 25, 2018 at 10:48 PM, Arv Evans <arvid.evans@...> wrote:

The problem with IRF510 RF amplifiers failing seems to be a recurring one for those who
are not quite careful with antenna matching, bias level, and drive level.  As a way to start
looking into this situation I have performed some on-line searches to see how others are
And there is much more out there to be Google searched and reviewed.

There are a number of potentially useful ideas contained in those articles and discussions,
but nothing that obviously applies directly to the problem of blowing IRF510 devices at only
a few watts of power if the antenna is mis-matched.  Mention of using small resistance values
in series with gate drive is interesting, as is use of pi-net attenuators between exciter and
RF PA help control impedance?  While we look upon the MOSFET internal capacitance
as being a problem, it is interesting that some designs add a capacitor on the drain side of
things, apparently to limit the upper frequency capability and reduce 'spikes'.  The discussion
on single-ended versus push-pull is interesting from a technical view, but did not introduce
anything obvious that could help.

I suppose we have to first determine just what the exact cause of IRF510 failure might be,
then use that as the basis for designing a suitable solution.

Arv  K7HKL

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