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[Amps] MOSFET ratings

Subject: [Amps] MOSFET ratings
From: Manfred Mornhinweg <>
Date: Sun, 17 Jul 2011 18:36:51 +0000
List-post: <">>
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?


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