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Linear RF power amplifier

Discussion in 'Radio and Wireless' started by davelectronic, Nov 17, 2011.

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  1. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    I am not sure on a little issue, i have a small 25 watt linear RF power amp running on TX only, temperature concerns, or are my worry's needless.
    Operation at full rated power the heat sink body is reaching about 80 degrees C.What i am wondering is the temp fine, oh i cant find the spec on the units safe temperature limits, anyway the SWR of my set up is less than 1.2 on the SWR meter, so reflected RF energy is being kept with in the units operating parameters there, but the unit is only small, do i need to add extra heat sinking, i dont know.

    The rated supply voltage is 13.80 volts, i am running it at 12.00 volts, so very slightly under powered.

    A lot of power transistors can operate above this temperature, that temperature observation is the case and not the junction temperature, i cant operate the unit and get a junction temperature, enclosure etc.

    Can anyone advise on the safe area temperature of this 25 watt linear RF amplifier. are the 80 degrees C typical and normal, i am expecting heat, as it is a power amplifier, and the case is extruded aluminum. So in summary will it serve a useful life at this operating temperature, i am not looking for any 100% guarantees, more a sensible operational life, the unit came with spec on input output power, but no in depth data, i will post a page link of the unit, in the hope some one has some ideas.
    Dave.D :)

    http://www.moonraker.eu/Citizens-Band-Radio/Linear-CB-Amplifiers/MOONRAKER-L30-LINEAR-AMPLIFIER
     
  2. OLIVE2222

    OLIVE2222

    690
    25
    Oct 2, 2011
    80° seems acceptable, but it' s not optimum for the amplifier life.

    In the document here under they claim that package with a 25% improvement in thermal resistance which corresponds to a operating life improvement of approximately 400%.

    http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXyxusv.pdf

    In the high-rel product we sometimes work with I find out that everything is over sized to keep component deadly cold. So it' s probably worth too cool down your amplifier.

    Olivier
     
  3. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Hi Olivier. Thanks for your reply, i did think adding some extra heat sink ability would extend the useful life of the unit, so although 80 degrees C is in safe limits, say 50 degrees C would extend life for possibly longer, as evey thing is running cooler, a little disappointing as this will increase the units size over all. But i cant have my cake and eat it, an old british saying.
    Dave.D
     
  4. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Ive found some PC Ram heat sinks with a thermal self addesive backing, if i strategically place one over the power transitors mount position on the cases aluminium this should increase thermal dissipation of the power transistor, well thats the plan, and its still discreet so not ugly and over size look to the unit.
    Dave.D
     
  5. MattyMatt

    MattyMatt

    161
    0
    Mar 24, 2011
    Well Dave, your theory is sound... the PC RAM heat sinks tend to do pretty well. most of them have the heat sink sticky stuff (the adhesive itself), which doesn't generally do as well as it could as far as the heat transfer, but it does work, I did notice a serious stability improvement a few years back before heat sinks were commonplace on RAM.

    As far as how much improvement you will see... couldn't tell ya, but hopefully it will drop about 5 to 10 C off the top, which will defiantly help.

    Question though, are there any aluminum fins coming off the back where these power transistors are mounted? Because what I was thinking, is that if you could remove them, and replace them with a more efficient design or material... or maybe ones that have a fan that would match the size of your current ones... you may be a little better off.

    Also, I was thinking, depending on size, I may have some specialized heat sinks kicking around that I could send you, but I would need a couple of pictures and measurements before hand.
     
  6. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Hi Matt.
    Thanks for the offer, appreciate that, any way no the back has no finned heat sink on it, rather the whole unit is constructed of extruded aluminum, so yes some Ram heat sinks will help, i would open it as ive looked in all ready, the clearance between the back of the case inside and the pcb wont allow even a machine screw or bolt, even flush the gap is about 1.5mm or less, and the pcb has the SO239 sockets soldered to the pcb as well, big solder joints done with a powerful gun i expect.

    I have an AMD CPU heat sink, that would have been cut in half very neatly and fitted a treat, but the clearance in the back wont allow even the lowest of profile fasteners, i would be lucky even to get away with rivets.

    So the Ram sinks will do for now i hope. :)
     
  7. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Well i tried the Ram heat sinks, strange things happened, it seemed to get hotter and during a TX over report of buzzing, so i removed them and used the radio with the amp this morning and its fine. I dont think the HS was substantial enough and heat lingered on the case back, so i will try and get that AMD HS cut in half as that really is much bigger, as i cant bolt up, ive got some titanium epoxy, but no data on heat transfer, only it can handle 200 degrees C but not sure as a thermal interface.
     
    Last edited: Nov 18, 2011
  8. MattyMatt

    MattyMatt

    161
    0
    Mar 24, 2011
    Can you post a picture of where those amps are? I'm rather curious considering the space constraints.... maybe we can think outside the box :)

    Matt-
     
  9. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Hi Matt. Not sure what you mean where they are. Its a unit conected to my radio via input output 50 ohm RG58 cable via PL259 plugs into SO239 sockets on the unit, i can open one up and post pic's later on the pc, plus a pic of the unit in situe connected to the radios RF output. I will get the pysical dimmensions of the unit as well. maybe these models are meant to run hot, ive tried to see the transiators model number type but its obscured bit of a pain as that data would have helped determine temp.
     
    Last edited: Nov 18, 2011
  10. MattyMatt

    MattyMatt

    161
    0
    Mar 24, 2011
    Yes, sorry, I was referring to the internals of the unit in question, just to see what kind of constraints you were working with.
     
  11. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Hi Matt.
    Not sure if my images are going to be brilliant but its all ive got at the minute, other than 35mm SLR's etc not digital.
    Any way it worth a try on posting the images. :)


    [​IMG]


    [​IMG]


    [​IMG]


    [​IMG]



    [​IMG]


    [​IMG]


    [​IMG]


    Might give some insite to better heat sink ideas. Not very clear but there are the two SO239 solder socket joints, melting these for removal might damage some of the SMD components from heat, there are some close to both joints. I have two units, the plan was me Mum was going to use the other at her place, but ive got to get round her using a small antenna and running power in a flat / apartment, ive tried and blotted out her TV on TX oop's, so not one at hers for now, i can talk to her on the allowable 4 watts. The output power transistor is clearest in the last picture in the shielded box.
     
    Last edited: Nov 18, 2011
  12. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    To try and get enough clearance for bolting a heat sink on presents a problem or two, the machine screw heads that is, i am contemplating using an AMD CPU heat sink as the device is getting to hot.

    Ok ive found a product, but in the questions link on this page it says would be suitable for a CPU's use as an interface material, but its thermal dissipation is rated in Degrees C mW/ C per watt, well milliwatt, some advice, does anyone think it a suitable alternative to a hard fix, appreciate any feed back. :)
    PS, Link to the product below.

    http://www.maplin.co.uk/thermal-adhesive-tape-221326
     
  13. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Hi Dave,
    To get the temperature down, you will need to substantially increase the surface area, I woild look to double it. A big heat sink fitted to each side with self tapper screws would do.

    The alternative would be to fit an external fan. Only a small increase in air movement can have a large effect on the heat transfer. Make sure your mum doesn't put her knitting on it!

    Duke
     
  14. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Hi Duke thanks for looking in.
    I was going or at least thinking on the fan idea, ive a coupe of 40mm axial fans put away for other uses, but i could pinch one of them for now, yes i know that would improve the heating problem, as i say my SWR is low less than 1.2 on an 18 " glass fiber virticle an antenna known as an A99 Antron 99 but with the amp i think in the production stage they try to keep metal down to a minimum to save money, to hazard a guess i would say the metal work, aluminum only just covers the spec of the power transistors power heat transfer for heat sink size.

    Yes will tell me Mum not to put anything flammable or items that might send the temp up lol.
    These types of amps have the nickname ( burner ) but i think you probably know this anyway.
    Thanks again Duke.
    Dave. :)
     
  15. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Hi to all.
    Well i opted for passive over the forced air cooling of an axial fan, as its right next to my chair in the living room, noise etc.

    It was a tight squeeze with the fastenings, but rivet heads side in offered the lowest safest profile, i wanted the heat sink in the other position, but 2 x SO239 sockets would not allow room, about half the amplifier is covered by the heat sink, the important part the power transistors fixing area part.

    Bit of an ordeal unbolting the SO239 sockets, and unsolder the power transistor, allowing room on the hole drilling and metal work, so not the most attractive of thermal solutions, but it should do the job, and i keep telling myself its practical, not suppose to win a beauty contest, hope it works, tested short over only made sure ive not killed the amp, longer overs later. :)

    [​IMG]

    [​IMG]
     
  16. robert1942

    robert1942

    1
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    Nov 25, 2011
    FAN

    Why not install a small fan on the unit you can get nice small 12 volt fans at a computer store or just take one out of an old junk computer power supply I have fans on a lot of my equipment just to keep it cool . Robert1942
     
  17. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Hi Robert welcome to the forum. Yes i was contemplating using the addition of a 40 mm chip set heat sink and its fan behind the power transistor, but noise in my ear put me of that idea, i could have incorporated a thermal switch so its not running all the time. But i dont even think that could have coped with the heat on TX. The AMD heat sink has got the temp down to 45 degrees C even with the heaviest of use, its added bulk to the unit, but runs cool now. These units are known for running hot anyway.
     
  18. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Using my old phone this morning, i only get 512 characters on it LOL, anyway hope you enjoy the forum, and thanks for your input on the axial fan idea, i am thinking on modding a second unit i have with some kind of extra heat sink and fan combination. The linear amps are cheap enough £ 25 quid each, but the down side is they lack the surface area for there power and heating levels. The current one in use is running really cool now, the AMD heat sink is cool for 65 watts or so, the standard unit is 25 watt.
     
  19. MattyMatt

    MattyMatt

    161
    0
    Mar 24, 2011
    Well Dave,

    I gotta say, it looks good to me! but then again, I am less concerned about some of aesthetics of things. That is something in the line of what I was thinking of... but I just wasn't sure it would be very effective. I'm glad to hear its running a bit cooler.

    Matt-
     
  20. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Hi Matt. Yes no problems with it now, been using it for the best part of a week, three times a day keep in contact with me Mum, first laid up after a knee operation, she just got walking again, then 2 weeks ago was attacked in the street by a mugger trying to get her bag, pulled her to the floor smashing her hip, another operation, plate screws etc, laid up again, the radio is a life line in terms of contact, anyway wont dwell on the negatives, but yes amplifier runnig fine.
     
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