I have to believe that M2 knows what they are doing with these Amps. They
offer a 12 month limited warranty on them.
If it's such a poor design, they will be replacing a lot of them. but, I
doubt that.
73, Dick, W1KSZ
On Fri, Sep 13, 2013 at 12:14 PM, Manfred Mornhinweg <manfred@ludens.cl>wrote:
> Jim,
>
> ## What about these freescale devices ? That company in OH land manages
>> to get 800w to 1 kw out of just one device. I saw on U tube, 1 of em
>> being used in a 1250w CCS FM broadcast TX, 88-108 mhz
>> variety.
>>
>
> That's in saturated service. Dissipation should be no more than perhaps
> 400 watts. That can be handled.
>
> Also on U tube, was a 2.5 kw CCS eme 144 mhz amp... using 2 of em.
>>
>
> I would think that this one also operated in saturation, at high
> efficiency. So it was not linear. Surely it was used in modes that don't
> require linearity, like CW or some digital modes that use slowly single
> tones. Was it that way?
>
> The cooling used was unique on all 3 of them.
>>
>
> The biggest problem when one designs cooling systems for such devices is
> getting the heat away from the small device. There is just simply an
> awful lot of heat coming out of their small mounting surface. A copper
> spreader is the usual answer, but to get the heat into the bulk of a
> heatsink large enough, it might have to take a 10cm long path through
> copper, and the device end of this path is narrow and tight. The thermal
> resistance in this area is already so much that it severely limits the
> maximum safe total dissipation.
>
> I did a design where I got water flow as close as possible to the
> device. The trick was getting enough water/copper interface surface,
> with short enough in-copper path length.
>
> As soon as the amount of heat is less, or the device mounting surface is
> larger, or it's spread out among several devices, or the device can run
> hotter, things get easier.
>
> Then the 3 x 150 ohms are all in parallel to make 50 ohms. They are
>> rated at 800w CCS each. The air cooling was not up to snuff..and one
>> exploded. Plan B was to lower the fins face down into a tub of ice
>> water..problem solved.
>>
>
> But when it comes to power transistors instead of resistors, even that
> trick is often not good enough. Instead you need copper, not aluminium,
> and you need to move the water fast, to have enough turbulence to get a
> low thermal resistance between the metal and the water.
>
> And just a little further up the scale you near the point where the
> copper between the device and the water is the main limiting factor
> (other than the internal resistance of the device, of course). At that
> point, it's time to say "enough"!
>
> + 69dbm was being used at the time.
>>
>
> And may I ask what's the connection between +69dBm and ham radio?
>
>
> Dick,
>
> M2 markets both a 6 and 2 Meter Monoband Amp, 1250 watts out, uses a
>> single device. Granted there's a time limit on AM and JT modes, but
>> there are no limits on SSB.
>>
>
> I just had a look at 2m version's manual. I see that it switches between
> class AB and class C, depending on the drive envelope. In CW and such it
> runs 950-1000W in class C, while in SSB it runs 1250W in class AB. But
> drain matching doesn't change. I didn't check if perhaps they lower the
> supply voltage when in class C. That's likely - otherwise I don't see how
> they could get the 65-80% efficiency of teh rating!
>
> Anyway, with 1000W output and 60% efficiency in class C, the FET might be
> running barely in the green area, as long as the heat sink remains cool
> enough.
>
> But with 1250W in class AB, never! It has 1:4 matching, so it has a swing
> of +/-88V at 1250W output, which at 50V supply is OK for a push pull stage
> - but results in only slightly over 50% efficiency. That means about 1200W
> dissipation at the envelope peaks, and an average dissipation not very much
> lower, given that efficiency falls at lower amplitudes. During sustained
> speech in SSB, this might result in about 900-1000W of dissipation - if the
> SWR is perfect. The device is rated at 1333W dissipation, if it is kept
> under 25 degrees Celsius. M2 activates thermal protection at 90 degrees - I
> don't know if they measure this at the heat sink, the spreader, how close
> to the FET, or the case of teh FET proper - but certainly the mounting
> surface of the FET will be hotter! Even if the spreader had zero thermal
> resistance, so that the maximum possible temperature at the FETs mounting
> surface would be 90 degrees, that would still limit maximum dissipation to
> 900W - and that is for 225 degrees junction temperature, which is higher
> than most manufacturers would rate their devices to survive!
>
> All this means that unless I'm missing something, this amplifier stressed
> the MOSFET to its absolute limit when operating with a very light hand (no
> speech processor, short transmissions, lots of RX time, low ambient
> temperature). In any other case, the MOSFET gets stressed beyond its
> absolute maximum thermal ratings.
>
> I hope I am wrong, but I would expect those amplifiers to fail easily, due
> to MOSFET meltdown.
>
> > Look at the W6PQL site for info on cooling one of these LDMOS devices.
>
> He solders the MOSFETs to a thick but rather smallish copper heat
> spreader, and attaches that to a rather small heat sink.
>
> Soldering is good, far better than using screws and thermal grease. The
> spreader is thick, but too small. The edges of the heat sink will be a
> several degrees cooler than the middle, wasting capacity.
>
> Want to do some math? Let's do it, very simply and not very accurately,
> but enough to get an idea:
>
> The thermal resistance inside the device is 0.15 degrees per watt. This
> part is easy, it's in the data sheet.
>
> The thermal resistance of the soldered connection is probably low enough
> to ignore.
>
> The copper spreader seems to be about 1cm thick. The hard part is
> estimating the thickness of the heath path through the copper. It starts as
> small as the MOSFET is, about 3cm^2, and expands to about 50cm^2 at the
> spreader's edges. Heat leaves it all along that path. The average bit of
> heat might travel about 5cm through the copper - this is a coarse
> approximation! Actually we should be using integral calculus here, and
> that's beyond my range of most beloved hobbies. So let's take a 5cm long
> copper path, that starts with 3cm^2 cross sectional area, and ends with
> about 20cm^2, which is what it would have at the 5cm path length periphery.
>
> A cube of copper, 1cm on each side, has a thermal resistance across it of
> about 0.27 degrees per watt. The spreader starts with about 3 such cubes
> side by side under the MOSFET, so that gives about 0.09 degrees per watt.
> As we get farther away from the MOSFET, the path widens, and resistance per
> length drops, all the way to about .013 degrees per watt per cm at the
> periphery of our mean path length. The average will be around .03 degrees
> per watt per cm along the path, it's 5cm long, so we get about 0.15 degrees
> per watt total thermal resistance for the spreader. Remember that this is
> VERY CRUDE, and it would be good to make the exact math. If anyone knows
> how to do that, I would like to learn!
>
> So, we have 0.15 degrees per watt inside the device, and another 0.15 in
> the spreader! Now comes the interface to the heatsink, which will add very
> little, because it's a large surface. Then comes the thermal resistance of
> the heatsink, considering that we have to take a value slightly worse than
> its rated one, because the heat isn't spread out evenly over the full
> mounting surface!
>
> That heat sink used by W6PWL looks like about 0.2 degrees per watt, if it
> gets a good air flow from strong fans. Let's de-rate it to 0.25, because of
> the non-even heat distribution. Again this is pure guesswork, but based on
> some practical experience.
>
> So we would have a total thermal resistance of 0.15+0.15+0.25=0.55 degrees
> per watt.
>
> How warm will the air be? Let's suppose, at most 35 degrees Celsius. And
> the MOSFET is rated to be able to work at up to 225 degrees. So we have 190
> degrees difference. With 0.55 degrees thermal resistance, this allows
> continuously dissipating 345 watts. And nothing more!!!
>
> In ICAS one can go higher, because we can store some heat in the spreader
> and heatsink and release it later. But we cannot go very much higher,
> because some important part of the total thermal resistance is very close
> to the MOSFET, even inside it, where the thermal mass is very small! The
> heat storage within a few millimeters of the chip is great to inflate the
> pulse power capability, but not the dissipation over a one minute
> transmission! The spreader and heatsink can store quite some heat for one
> minute, though.
>
> So, this arrangement can dissipate about 345W continuously, perhaps 500W
> for one minute, and well over a kilowatt for a few milliseconds. This is
> enough for 1kW continuous output in high efficiency, well saturated class
> C, well over 1kW in class E or class F, but only about 800W or so PEP in
> brief, processor-less SSB when running class AB, and even less when being
> as long winded on the air as I am in this post! ;-)
>
> Don't forget that I did some gross estimations. Maybe I'm wrong. But then
> I would like someone to PROVE me wrong, either by doing the precise math,
> or by actually measuring the temperature of the MOSFET's mounting surface
> (not the top of its case!!!) during full power operation!
>
> The problem is that this kind of design leads to amplifiers that seem to
> work just fine, but then tend to fail very easily, or to fail consistently
> after just a few hundred hours of use, for the simple reason that the
> MOSFETs are running far too hot.
>
> Phew!
>
> Manfred
>
> ========================
> Visit my hobby homepage!
> http://ludens.cl
> ========================
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