On 5/1/2012 3:35 PM, Manfred Mornhinweg wrote:
> Dear all,
>
> I have been following this thread, and at some moments shuddered at what
> some of you opine, and assented at what others write, and several times
> I would almost have replied to something, to set it straight, but always
> somebody did it before I got a chance.
>
> Interesting thread, but I can't help the impression that most of the
> people who give their opinions have never in their life actually
> designed some piece of equipment, let alone a high power solid state
> circuit of any sort. And many are also a bit weak in the field of vacuum
> state power electronics.
>
> I would like to tell you that I have been working over the last months,
> as a background hobby work, on a project of a legal limit HF solid state
> power amplifier. At this time I'm getting a pretty clean 1200 watts
> output, and saturated 1700 watts, from 160 to 15 meters, with slightly
> less power on 10 meters, from a set of MOSFETs costing a total of 75
> dollars. No, the number is NOT missing a zero.
IF and I have to emphasize the IF these transistors were rugged enough
for prime time and I'm quite willing to take your word on the power you
are getting out, the manufacturers would be jumping on them like flys
on...er... honey because they could run them PP/parallel for the legal
limit out plus comfortable orverhead and couldn't build them fast enough
to meet the demand. There has to be a reason they are not doing so.
As to home brew, there are very few of us who actually do that be it SS
or tubes. Like the general public most hams are clueless when it comes
to circuit design and engineering. I'd hope we at least have a better
percentage than the general public, but I'd bet it's still a pretty
small number. When I was a tech they had me teaching the skilled trades
every thing from introductory electricity to solid state circuit design
for my last 5 years. Most of the circuits were simple was back then it
was simply bipolar along with a brief mention of FETs. Of course they
still expected me to keep the chronographs running while doing this.
The techs did pretty well on Instrumentation, but not very well on the
SS circuit design
>
> I have tried several slightly different but generally similiar MOSFETs
> from different manufacturers, so that if one device is pulled from the
> market, I have a choice of others to fall back upon. And while I have
> been working on this circuit, newer, better and even cheaper devices
> have been showing up.
I'd be interested in the device, and circuit design. Most designs are
pretty straight forward, running PP through transformers and typically a
pair of PP amps in parallel. Roughly 32 to 34 years ago I had a solid
state 2-meter amp that was almost as large as my Emtron DX-2sp, but not
quite as tall and considerably lighter although it was still beefy. It
had 4 PP amps combined and ran 600W out. i wish I had it back. The whole
top was a heatsink that was hinged with the large circuit board on the
underside.
At any rate all commercial amps and projects I've seen had very
elaborate protective circuitry which I said before was pretty simple
when looking at each section, but kinda intimidating when looked at
overall. Course most people make the mistake of looking at the whole
thing instead of one step at a time.
Of course when you look at my legal limit Tetrode amp the bias and
protective circuitry looks pretty intimidating too. <:-)) OTOH it's no
more so than the circuitry for a SS amp, but both are beyond the typical
homebrewer. OTOH I don't consider a lot of the hams on the reflectors I
follow to be typical.
> You might also be interested in learning that ALL of the parts for this
> amplifier, including the power supply, heat management, protection, low
> pass filtering, and automatic bandswitching, will likely end up costing
> below 1000 dollars total. That's using all new parts. An antenna tuner
> is NOT included in this amplifier, so if you want to use antennas having
> an SWR higher than 1.5 or so, you would need an external tuner.
>
> Working on my little pet project, and reading how people write that
> solid state amps are not for the ham, or not for the home builder, I
> tend to smile, and sometimes I tend to become slightly angry too. People
> who say it can't be done shouldn't be discouraging people who are doing it.
If people are doing it, they aren't going to be discouraged from
experimenting. OTOH it might save the uninformed a lot of frustration.
I still maintain SS is not ready for prime time, but for those with the
knowledge and desire to experiment it's a great field.
>
> Some of the last messages regarding water and vapor cooling have shown
> that some people here don't understand the thermal specifications of
> semiconductors. So let me explain: The maximum power dissipation, given
> in the datasheets, in most cases refers to the maximum power a device
> can safely dissipate AS LONG AS _YOU_, the designer/builder, make sure
> that the case of the device will never exceed 25 degrees Celsius.
It's also, often at pulse with enough off time to allow the heat to
migrate to at least the spreader
> In
> practice that's almost always imnpossible, and for that reason the
> actual power that a device can safely dissipate is always lower than the
> rated power. This is not any sort of de-rating for safety or improved
> life - it is instead an essential part of designing the circuit, to keep
> the device operating AT (not below) its maximum safe internal
> temperature, which should result in a reasonable lifetime. If you truly
> derate it from this point, so that the silicon runs at significantly
> less than 150 degrees Celsius (that's the typical silicon temperature
> allowed for plastic-cased devices), additional (speak: essentially
> unlimited) lifetime will result.
Depending on frequency we set the internal limit at 100C and max
internal operating temp at somewhere between 70 and 90C.
dopant migration was starting to become noticeable at 100C so when I was
in the industry.
> Vapor cooling isn't practical with solid state devices, because between
> water boiling temperature and the device's internal absolutely maximum
> temperature you have just 50 degrees Celsius difference, and this
> temperature difference must push all the heat from the silicon to the
> water. On its path, that heat will find the internal thermal resistance
> from the silicon to the case, more resistance from the case to the
> copper block, more resistance through the copper into its fins, more
> resistance from the fins to the water, and then some, as the water is
> stirred around only by the vapor bubbles. If you allow one half of this
> 50 degree gradient to the whole path from the device's case to the
> water, and the other half just to the path from the silicon to the case,
> then the maximum allowable power dissipation will be only ONE FIFTH the
> rated power! This is because the rated power is based on having 125
> degrees from silicon to case, and you are allowing only 25.
>
> It's a bad bargain to use transistors in such an inefficient way, so
> forget vapor cooling.
Agreed
>
> When using water cooling, you have basically the same thermal
> resistances, except that the resistance from the fins into the water is
> much lower,
You can also use *cold* or chilled water. Just draw the water out of an
open PVC tank and return to the same tank. 20 gallons will run a KW amp
for quite a while. Throw in a few chunks of ice and have at it. We use
the water in plastic bags for our picnic coolers , so you just grab a
couple and drop in the tank
> if you pump the water by fast enough to create a lot of
> turbulence. But the greatest benefit of water cooling over vapor is that
> your cooling medium might be just 30 degrees Celsius warm,
You can take it down as far as you like as long as you don't get
condensation on the circuit board, or ice up the pump.
I had a lot of maintenance and building in the 100 to 200 KW range and
even used electron beams on the order of 30 KW (give or take) That is
enough to
BTW there is a fungicide used in swimming pools that will do a good job
of keeping *stuff* from growing in your cooling system. I'm not
referring to Chlorine.
73
Roger (K8RI)
www.rogerhalstead.com
>
> I'm in between, so I think I understand both extremes! ;-)
>
> 73,
> Manfred
>
> ========================
> Visit my hobby homepage!
> http://ludens.cl
> ========================
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|