[Amps] New solid state kid on the block

[John Lyles]

> Message: 5
> Date: Thu, 03 Feb 2011 00:14:09 +0000
> From: Manfred Mornhinweg <manfred at 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