Hi all,
Over the last days I have been experimenting with low cost switching
MOSFETs pressed into RF power amp service. The results are quite
encouraging, such as 100 watts of pretty clean output (and nearly 200
watts in saturated mode) from a pair of MOSFETs which cost one dollar each.
That's on the lower bands. On the higher bands, these particular MOSFETs
tend to be rather lazy. On 10 meters they just don't work. But faster
ones do exist.
I have been trying to make sense of the data sheets, hoping to find a
way to tell from a data sheet (aimed at switching applications) how a
MOSFET will perform in linear service at RF. And there are some things
that look odd, and I hope that somebody here can explain them.
For example, take the data sheet for the IXFP3N80. This transistor is
rated for a rise time of 14 ns, to slew from zero to 400V, under certain
conditions of base drive and drain current. Assuming that the voltage
ramp is linear, that would be a dv/dt of 28 V/ns. If the ramp is
nonlinear, the peak dv/dt would be even larger. But this same
transistor, in the same data sheet, is rated for an absolute maximum
dv/dt of only 5 V/ns!
The same discrepancy happens in the data sheets of many other MOSFETs. I
don't know what I'm misunderstanding here, if perhaps that low absolute
maximum dv/dt rating is valid under totally different conditions, or if
these data sheets are simply wrong! I notice that many manufacturer
specify either 5 or 10 V/ns of absolute maximum dv/dt, for transistors
that actually seem to have very different rise times. Could it be that
these dv/dt ratings are simply copied from one data sheet to the next,
and do NOT reflect the true capability of each transistor?
Can anybody explain this?
Thanks,
Manfred.
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