> Message: 5
> Date: Thu, 03 Feb 2011 00:14:09 +0000
> From: Manfred Mornhinweg <manfred@ludens.cl>
...
> 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.
While this is the normal practice with thermionic tubes, it is not a
recommended practice with silicon.
Bond wires in big devices are fatigued with pulsing high current. Thermal
performance is nothing like a big anode on a tube.
So when transistors are rated at a CW power, usually the pulsed performance is
not much more, for a reliable design.
I haven't seen an exact figure, but its nothing like tubes where 10 to 100x the
CW performance may be satisfactory.
> 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
> 50V.
LDMOS Gen 6 devices have made great strides in performance and a solid 50 volt
bias gets them there (to the kW level). I am watching SiC and GaN devices for
>100 volt VHF performance
in the workplace, due to the wide bandgap these materials have. Microsemi's
static junction FET can produce ~2 kW pulsed with only a single device (not
push pull).
> 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.
Certainly a good idea, for < 30 MHz operation. Operating with nonlinear classes
(like C, E)
and making a linear amplifier (like EER) will work with plastic switching
transistors too.
73
John
K5PRO
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