can someone explain this to me?

Discussion in 'Wireless & RF Design' started by saintcostas, Feb 15, 2018.

  1. saintcostas

    Thread Starter New Member

    Feb 15, 2018
    7
    0
    hello

    i am testing an hf amp with low cost mosfets in AB class push pull.
    mosfets are IRFP150.
    i have a power consumption of 11 amperes at 12 volts.
    my power bridge shows 200 watts output power at a 50Ω dummy load.
    frequency is 1.8 MHz.input power 5 watts with AM or FM modulation.

    How is that possible?
     
  2. loulou31

    New Member

    Feb 23, 2018
    15
    1
    hello,

    That is not possible!
    Please check your power bridge calibration, first with the input power if your are confident on this power level.
    You can also verify the temperature on the dummy load : e.g. compare temperature on dummy load and when you inject 2 A DC current and when you measure the unknown RF power... It shall be similar....

    Jean-Louis
     
  3. saintcostas

    Thread Starter New Member

    Feb 15, 2018
    7
    0
    my bridge is diamond sx-200.my input power is shows right.
    exactly as it must be.i take such measurements when i work the amp
    at 12 volts.when i work it at 24 volts the measurements are reasonable,
    about 60% efficiency.any ideas?
    and something else,i take 160 watt output at 12 volt ,11 amperes ,5 watt input,
    and also 160 watt output 24 volt 11 amperes 5 watt input.
    for 24 volts i change output transformer.
     
    Last edited: Feb 23, 2018
  4. saintcostas

    Thread Starter New Member

    Feb 15, 2018
    7
    0
    my dummy load is small and it gets hot but i take these measurements from the beginning
     
  5. loulou31

    New Member

    Feb 23, 2018
    15
    1
    Hi,

    Please could you check your dummy load. What is the resistance when cold. What the power capability of this dummy load?


    Jean-Louis
     
  6. bertus

    Administrator

    Apr 5, 2008
    19,507
    3,972
    Hello,

    The gain you describe could well be possible.
    I have seen higher frequency RF mosfets that can put-out 200 Watts with 5 Watts input @ 225 Mhz.
    See the attached datasheet of the MRF275G, wich I have worked with.

    If the dummyload is getting hot from the beginning, it is not capable to manage the power.

    Post the schematic you are working with.
    Is there a harmonics filter at the output?
    If not, the powermeter can show more power than actual wanted.

    Bertus
     
  7. saintcostas

    Thread Starter New Member

    Feb 15, 2018
    7
    0
    my dummy load resistance is ok.50 Ω cold , 50 Ω hot.
    i stop it before it gets too hot.
    i do not have a filter yet.the weird is that i have two situations here.
    the first is how i take more watts than i give and why this happens
    only at 12 volts.and of cource this cannot be happened but what is the difference
    at 24 volts?
    thanks
     
  8. bertus

    Administrator

    Apr 5, 2008
    19,507
    3,972
    Hello,

    Did you have a look at the PDF I posted?
    In figure 5, you will see the effect of the supply voltage on the power.

    Bertus
     
  9. saintcostas

    Thread Starter New Member

    Feb 15, 2018
    7
    0
    bertus thank you for your interest.

    i saw your pdf.i see the output differencies but my question was
    about efficiency.at 12 volt i get more power that i give from
    the power supply consumption side.
    anyway i understand that something confusing my bridge and
    shows impossible indications or i have made the magic transformer.
    by the way i see you have worked with mosfets i am looking a
    way to work cheap mosfets directly at 300 vdc.i do not want
    them with a Vds 900 volts because they are still quite expensive.
    do you think mosfets at 500V V ds will be ok?

    thanks again
     
  10. loulou31

    New Member

    Feb 23, 2018
    15
    1
    Yes the problem is that you read more HF power than power supply is providing.
    Did you test your bridge and dummy load with the input power of 5 W with same range for verification?

    Jean-Louis
     
  11. saintcostas

    Thread Starter New Member

    Feb 15, 2018
    7
    0
    sorry what do you mean by "with same range for verification''
     
  12. bertus

    Administrator

    Apr 5, 2008
    19,507
    3,972
    Hello,

    It looks like that the power is not measured correctly.
    How is the SWR in both cases?

    Also the mosfet is designed for switching and not for RF amplification.
    There are special RF mosfets around, as I already showed you one.

    Bertus
     
  13. loulou31

    New Member

    Feb 23, 2018
    15
    1
    I suppose you have some different ranges of measurement on your bridge. Measure 5W power on same range that the you amplifier output. The accuracy will be not good but you want to know to have a rough of magnitude.

    Jean-Louis
     
  14. saintcostas

    Thread Starter New Member

    Feb 15, 2018
    7
    0
    my swr is 1:1.i know about rf mosfets.they are quite expensive.
    that is a reason i am testing power mosfets.they work
    very good.and i make efforts to use them directly at 300 Vdc
    so i will not need a special power supply.

    loulou31 i tested my bridge ranges,they all show the same output.
     
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