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Re: [softrock40] Re: How high can you safely push the voltage to on a Softrock RXTX?

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  • Jack Smith
    Thermal runaway is less of a problem with MOSFET than bipolar because the majority carrier mobility decreases with increasing temperature, unlike a BJT, where
    Message 1 of 13 , Jun 18, 2013
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      Thermal runaway is less of a problem with MOSFET than bipolar because the majority carrier mobility decreases with increasing temperature, unlike a BJT, where the Vbe decreases as temperature increases and hence thermal runaway can occur for fixed bias voltage operation.

      You might also consider what happens to the drain voltage if operating into an open circuit at maximum power and maximum voltage. The higher the power supply voltage the greater the chance of exceeding the drain/source voltage rating of the device.

      If always operated into a 50 ohm load, then no problem. But part of a check of good design practice is to operate into a short circuit and an open circuit at maximum drive and on the worst case frequency. Even better to verify proper operation with a range of load Z.



      Jack K8ZOA

      On 6/18/2013 8:01 PM, warrenallgyer wrote:
       

      (fully bandaged following Alan's dig)

      I actually attempted destructive testing with the RXTX. I was willing to sacrifice the finals and surrounding components to see if I could make the radio go into thermal runaway. Bottom line is I could not.

      I ran my 15/12/10 at 16 volts, full power test tone with 3.3 V P_P audio drive, for one hour. I logged total current consumption and power output at one minute intervals initially, and then when that got too boring, at 5 minute intervals.

      The final heat sink assembly got too hot to touch, the current rose to about 110% of startup and then stabilized. The power output was quite high because of the 16V and drifted down maybe 10% then stabilized.

      I let it go in that condition another 20 minutes beyond the hour, then shut it down. There was no detectable damage and I continue to use the rig today. It gave me a new degree of confidence in the reliability and respect for the design.

      Regarding Alan's concern about me leading a newbie into a dark dungeon of destruction I offer this disclaimer:

      If you operate the Softrocks within the ratings for which they are designed you will have a fun, reliable, compliant radio that will last a long time. If you push the limits you need to have instruments to assure you remain in compliance with the rules and a well-stocked spares drawer, just in case.

      Warren Allgyer - 9V1TD

      --- In softrock40@yahoogroups.com, "Alan" <alan4alan@...> wrote:
      >
      >
      > ----- Original Message -----
      > Subject: Re: [softrock40] Re: How high can you safely push the voltage to on a Softrock RXTX?
      >
      >
      > >
      > > With me it's more about playing with my new test gear and seeing how
      > > changes affect measurements, and indeed, seeing how to DO the
      > > measurements in the first place. Rather than trying to pull the
      > > ultimate out of a Softrock RXTX.
      >
      > Chris,
      >
      > I know you are not pushing to the limit.
      > I was having a playful dig at Warren, who is.
      > I'd guess that higher voltage with no increase in drive should improve linearity. But maybe it's not that simple.
      >
      > 73 Alan G4ZFQ
      >


    • warrenallgyer
      Jack I cannot explain the phenomenon but I have observed very quick and devastating thermal runaway in the RXTX by breaking the thermal coupling between the
      Message 2 of 13 , Jun 18, 2013
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        Jack

        I cannot explain the phenomenon but I have observed very quick and devastating thermal runaway in the RXTX by breaking the thermal coupling between the two finals and the bias regulator.....The third BS170 in the heat sink assembly. Without negative bias feedback from increased temperatures these finals will destroy themselves in a few minutes. I am particularly careful to maximize thermal coupling in among all three of these devices.

        I have not tried operating the RXTX with no load at 16V but I have inadvertently done it for several minutes at 13.8 V with no ill effects.

        --- In softrock40@yahoogroups.com, Jack Smith <Jack.Smith@...> wrote:
        >
        > Thermal runaway is less of a problem with MOSFET than bipolar because
        > the majority carrier mobility decreases with increasing temperature,
        > unlike a BJT, where the Vbe decreases as temperature increases and hence
        > thermal runaway can occur for fixed bias voltage operation.
        >
        > You might also consider what happens to the drain voltage if operating
        > into an open circuit at maximum power and maximum voltage. The higher
        > the power supply voltage the greater the chance of exceeding the
        > drain/source voltage rating of the device.
        >
        > If always operated into a 50 ohm load, then no problem. But part of a
        > check of good design practice is to operate into a short circuit and an
        > open circuit at maximum drive and on the worst case frequency. Even
        > better to verify proper operation with a range of load Z.
        >
        >
        >
        > Jack K8ZOA
        >
        > On 6/18/2013 8:01 PM, warrenallgyer wrote:
        > >
        > > (fully bandaged following Alan's dig)
        > >
        > > I actually attempted destructive testing with the RXTX. I was willing
        > > to sacrifice the finals and surrounding components to see if I could
        > > make the radio go into thermal runaway. Bottom line is I could not.
        > >
        > > I ran my 15/12/10 at 16 volts, full power test tone with 3.3 V P_P
        > > audio drive, for one hour. I logged total current consumption and
        > > power output at one minute intervals initially, and then when that got
        > > too boring, at 5 minute intervals.
        > >
        > > The final heat sink assembly got too hot to touch, the current rose to
        > > about 110% of startup and then stabilized. The power output was quite
        > > high because of the 16V and drifted down maybe 10% then stabilized.
        > >
        > > I let it go in that condition another 20 minutes beyond the hour, then
        > > shut it down. There was no detectable damage and I continue to use the
        > > rig today. It gave me a new degree of confidence in the reliability
        > > and respect for the design.
        > >
        > > Regarding Alan's concern about me leading a newbie into a dark dungeon
        > > of destruction I offer this disclaimer:
        > >
        > > If you operate the Softrocks within the ratings for which they are
        > > designed you will have a fun, reliable, compliant radio that will last
        > > a long time. If you push the limits you need to have instruments to
        > > assure you remain in compliance with the rules and a well-stocked
        > > spares drawer, just in case.
        > >
        > > Warren Allgyer - 9V1TD
        > >
        > > --- In softrock40@yahoogroups.com
        > > <mailto:softrock40%40yahoogroups.com>, "Alan" <alan4alan@> wrote:
        > > >
        > > >
        > > > ----- Original Message -----
        > > > Subject: Re: [softrock40] Re: How high can you safely push the
        > > voltage to on a Softrock RXTX?
        > > >
        > > >
        > > > >
        > > > > With me it's more about playing with my new test gear and seeing how
        > > > > changes affect measurements, and indeed, seeing how to DO the
        > > > > measurements in the first place. Rather than trying to pull the
        > > > > ultimate out of a Softrock RXTX.
        > > >
        > > > Chris,
        > > >
        > > > I know you are not pushing to the limit.
        > > > I was having a playful dig at Warren, who is.
        > > > I'd guess that higher voltage with no increase in drive should
        > > improve linearity. But maybe it's not that simple.
        > > >
        > > > 73 Alan G4ZFQ
        > > >
        > >
        > >
        >
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