Hi Bill, Baruch, Alex, and all,
Thanks for all the info. I was hinted about this new transistor a few
days ago, and tried to make up my mind whether to try it. And now I'm
still trying to make up my mind! ;-)
There are several fishy things in the datasheet. For example, how can
this transistor be rated for the same peak pulse power, and CW power?
That defies logic. Normally pulse power is dictated by electrical
limitations, and CW power by dissipation capability, with pulse power
being higher than CW.
They rate the power in class AB, not C, as indicated by the idling
current of 100mA. It seems that this idle current is too small for good
linearity, but no good data about this is given. And despite the power
being rated in class AB, I suspect it is saturated power output (in the
specific test circuit), not linear. After all, nothing prevents a class
Ab amplifier from saturating, and it will have an efficiency approaching
that of class C when saturated deep enough!
So there are quite a few questionmarks as to what this transistor can
really do. It looks very attractive because it packs a lot of power and
very little capacitance. But I wouldn't really like to work with 50
volts nowadays, and pretty tight absolute maximum ratings, having been
spoiled by transistors that have an absolute maximum rating of five
times the operating voltage, and that operating voltage being well above
The thermal specs are pretty good. If I was to try this transistor, I
think I would _solder_ it to the heat spreader (or watercooled block),
rather than bolting it on. Since solder is a much better heat conductor
than heat transfer grease is, soldering the transistor on would likely
remove the need for an accurate surface finish of the spreader, and
produce better heat transfer.
But spending well over USD 200 on a transistor, to experiment with it
and possibly blow it up, when I can't even get a basic transfer curve
showing how temperature affects bias, for example, is beyond my level of
fanatism at this moment!
The legal power limit for ham radio in my country is 1200 watts. So when
this transistor rated at 1250 watts showed up, that was sort of love at
first sight. But on closer examination, it seems to me that in actual
real world operation at 1200 watts it might be a bit overstressed,
unless heatsinking is fabulously good.
The 65:1 SWR survival spec is also impressive - but is valid only in
pulsed service, as I understand it!
So, after drooling for a few days about this transistor, I'm back to
something much more ham-like: Thinking about the details of an amp using
a large number of cheap TO-220 switching MOSFETs, paralleled using
separate gate and source resistors for each transistor, to make sort of
a home-built ballasted super-MOSFET. This technique was well documented
by DL9AH, and it looks like quite a few German hams built their own
versions of this amp. My version of it would have some (hopefully)
clever innovations in the power supply. The original DL9AH amp has a
marvelously simple and clever power supply, but its input power factor
is dismally poor, which would rule it out for me. I power my home from a
little turbine in a creek, and if I load that thing with an amplifier
taking 2.5 kilowatts at a power factor of 0.3 or even less, nothing good
Today I have been looking at which MOSFETs are available that would be
best suited to that project. DL9AH used the IRF710, but that was about
15 years ago. Other builders used the IRF820, but that wasn't much more
recent. I was looking at some newer MOSFETs. Most of them pack too much
power (and capacitance), so that the source lead becomes the bottleneck,
due to its unavoidable inductance. But there are a few which seem
promising, such as the NDP02N60ZG, or the IXTP1R4N60P. I would probably
use about 40 to 50 MOSFETs, powered from 130 to 150Vdc, with a 1:4
impedance ratio in the output transformer.
Any ideas, comments, bullets or missiles?
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