LDMOS amplifiers

[w4bfs]

Does anyone have direct experience with these high power amps.   Looked on utoob at a 3kw out using 2 big devices (LX188 etc) and it was impressive ... how reliable are these things or are they Chinese firecrackers ?

[WBear2GCR]

They're used all the time in commercial FM & TV transmitters.

Reliable when used appropriately.

Chinesium copies, can't vouch for.

Real ones are usually moderately expensive.

                        _-_-

[KD6VXI]

There is a thread on EHAM about the Mercury III now.


I inquired about power draw based upon a 4 to 1 and 5 to 1 carrier to pep ratio.

Flatly, they suck.  As power goes down the efficiency drops like a rock.

For ssb and cw they are alright, but the general consensus was they are NOT OK for AM use.

To quote,  "At approx 250 watts of carrier the Drain voltage is 53.1 volts and the drain current is 31.3 amperes.  so as you can see the amp is very ineffecient at this low power level, converting most of that energy into heating the heatsink.

Now with modulation applied at the 250 watt carrier level and with SWR starting (actual antenna) at 1.3:1 the the drain current was 33.1 amps.  As you can see with about 800 watts PEP out the amp is much more efficient, in that more of the current went out the antenna due to the increase in drive from the modulating sidebands. BUT still pretty ineffecient overall.  Switching to SSB for 1150 watts out (PEP) the current was 36.1 amps

As far as heating goes, during the AM carrier test, the internal sensor rose from 24 degrees C (room temp) at the start of the test to 38 deg. C pretty quickly (about 2 minutes of testing). It seemed to rise at about the rate that a 600 watt FT8 QSO would make it rise. The internal fans kick into high gear at 51 degrees and the amp goes offline at 65 degrees C

Conclusion, I don't think you will like the duty cycle and heat rise associated with long winded AM transmission at this power level of 250 watts carrier and 800 watts PEP.  "

--Shane
KD6VXI

[w4bfs]

thanks all .... Shane sounds like that amp may not be biased right .... perhaps closer to class A to make distortion spec .... ??

[KD6VXI]

I have no idea the biasing, etc.  I've not run that amp before.

I do know the CBers have them running, but I also know they have some reliability issues.....  But, CB...

I got a Pm from Frank with his current figures.  I tried to PM him back but it's blocked.  I'm not sure what voltage or topology he is using.

--Shane
KD6VXI

[M0VRF]

They're designed for SSB linears and hence a relatively low duty cycle.

Pointless using them for anything else.

JB.

[W3GMS]

I have looked into most of near legal limit and legal limit solid state amps and the twin pair of LDMOS seems to be the choice for most.  When I crunched the numbers, I was not happy with how much heat each device generated.  Then getting rid of the heat from attaching each of the two devices to the heatsink was scary for legal limit AM, especially when adding up all the thermal resistances.  In the end, fan roar in an attempt to keep things within the safe operating area of the devices.  It would be interesting to see an MTBF analysis based on actual junction temperatures.

There was one amplifier manufacture which appeared to come up with a better solution.  Ameritron uses eight rugged MRF-150 power FETs in their 1200W amplifier. The devices are  properly arranged to spread out the heat over a large surface.  That gives them a very big advantage over the other amplifier manufactures.  The junction temperatures should be considerable lower since it gets divided by 8 or 4 if you comparing it to the two LDMOS topology.         

[WBear2GCR]

Joe, your concerns make sense. I do not know the answers. But, the manufacturers of the LDMOS devices
will have the thermal considerations on their site or data sheets. They may be counting on a high efficiency
application, Class E or D for example, to keep the heat low.

Otoh, the usual technique in these cases is to
use a very low thermal resistance material as a "spreader" before going to an aluminum heatsink.
that is usually copper. That and low thermal resistance compound, as needed. Flatness counts
too. So getting mating surfaces to be truly flat helps considerably.

I used to build a Class A audio amp that dissipated ~125watts of thermal power - for ~30watts of audio! -
and the heatsink required consisted of a thick aluminum plate and very long aluminum fins, with extra wide
spacing to permit natural convection. The good part is that a rather modest air flow (low noise) will boost
the efficiency and power handling of a heatsink by rather a lot.

Turns out that tubes can be somewhat easier to deal with WRT thermal design!

The distributed thermal sources via multiple devices has more merit as the total thermal load increases.
So, IF the amp is ~50% efficient, then 500watts of thermal dissipation? THAT is going to be a big problem!
As much time and effort needs to go into the thermal design as does the circuit design to make that
practical, imo.

                         _-_-bear

[KD6VXI]

Bear,

The better LDMOS Amplifiers don't use any type of heat sink compound, lapping, etc.  They directly solder the device to the spreader.

I got another PM from Frank, GFZ.  He's seeing 25 pct efficiency on his moafef amplifier at carrier.

The Ameritron stuff isn't all its cracked up to be.  Plenty of them have blown up.

There is a guy in CT building around the VMOS devices now.  From what I understand the amp is in testing.  It has 8 way protection for the devices so it should be fairly reliable.

Since the VMOS can handle 700 volts, it's a no transformer design.....  I'm don't feel very good about that, it would need to be double insulated.  I believe it's just power line rectification and the boards float above ground then use a magnetic coupling to get RF out the antenna socket....  So far it's pretty hush hush and details are not available for the most part.

Anywho.....   Frank, unblock me 😎. I'll send you a cookie 😎

--Shane
KD6VXI

[WBear2GCR]

Soldering directly to a copper spreader of any size is going to require a lot of
heat. Over heating the device would be a concern. Guess I'd like to see the actual
package and how that is done.

[W3GMS]

Bear,

The better LDMOS Amplifiers don't use any type of heat sink compound, lapping, etc.  They directly solder the device to the spreader.

I got another PM from Frank, GFZ.  He's seeing 25 pct efficiency on his moafef amplifier at carrier.

The Ameritron stuff isn't all its cracked up to be.  Plenty of them have blown up.

There is a guy in CT building around the VMOS devices now.  From what I understand the amp is in testing.  It has 8 way protection for the devices so it should be fairly reliable.

Since the VMOS can handle 700 volts, it's a no transformer design.....  I'm don't feel very good about that, it would need to be double insulated.  I believe it's just power line rectification and the boards float above ground then use a magnetic coupling to get RF out the antenna socket....  So far it's pretty hush hush and details are not available for the most part.

Anywho.....   Frank, unblock me 😎. I'll send you a cookie 😎

--Shane
KD6VXI

Shane, I have not heard about any problem with the ALS-1300 amps from folks I know that have them.  The 8 FETS sure make the thermal management easier and the junction temperatures will be far lower then the 2 device LDMOS amps.   Protection circuits are one thing, but the goal is, not to have the protection circuit come into play due to the thermal and electrical design margin of the amplifier. 

Joe     

[KD6VXI]

Joe,

There is plenty of anecdotal evidence the ALS600 and the 1300 have problems.  Google searches of the zed and EHAM will return quite a few stories.  In addition, I've seen quite a few people selling theirs after it being returned from the factory with a fresh set of finals.

It may be 100 pct the priblem of the guy in front of the amp.  I've also heard it discussed more on the 600,but that could be due to sales numbers being a lot more in the 600.

I completely agree with you about having the spreader with multiple devices.  I've probably built a few hundred 12v  solid state amps in and right after high school (don't ask lol) a copper spreader went miles towards cooling of the individual transistors on the 2 to 8 transistor amps (used 454s, 2290s and 2879s).

LDMOS must work or it wouldn't be being used in MRI, Plasma generation, etc.  Possibly the difference is in lower duty cycles and class of operation.  I've often thought of running one of the 1800 watt devices in class e.  Price and the lack of need keeps me from doing so.

Solid state is great, when it works.  It's not as rugged as tubes and not nearly as forgiving as tubes.  But as Steve and others have shown, it does work.  I've got a 16 x 2879 amp I built in the late 80s that still works fine business and I used it for legal limit mobile operation for a decade driving truck.

--Shane
KD6VXI