Hi there,
about the question regarding "
which MOSFET to use". This is fairly technical now but may offer some insight into the technical background.
Basicly, almost every common (Power-) MOSFET can be used in the HF Bands up to 50Mhz. _
When_ the features are suitable and it's driven properly, they all will perform about the same. The issue here is "
driving properly".
There are several reason why the IRF510 is used so often:
- It's widely available (from misc manufacturers).
- It's cheap (we all managed to blow up a dozen FETs on a single day full of tinkering, eh? ;p).
- With a Drain-Souce-Voltage of about 100V it's suitable for most PAs up to 30W, which is enough for most people.
- It has a relative low RDS(on) resistance which qualifies it for pretty low PA output impedances (and therefore high power).
- And most important: the Gate-Source Capacitance is relative low. In fact. that low that not much attention needs to be paid when designing a driver stage.
- (Another, but not too important reson is the relative low Gate-Source Threshold Voltage.)
Why are those values important? Well, the perfect MOSFET for a HF PA should have:- A low Drain-Source(on) Resistance Value (RDS(on)). The lower the better. It defines how how much power will be converted into heat (during the transition phase but mostly the full-on state). This value isn't really critical. It affects the PA output impedance but does not much harm. It limits the maximal obtainable output power. But a bigger heatsink solves the problem in case the chosen FET is able to deliver the desired output impedance and power.
- A high Drain-Source Breakdown Voltage (UDS). It should always be twice the expected voltage on the output. More is better because a bad SWR will reflect power back into the PA which caused voltage peaks. Voltage peaks above the breakdown voltage will, in fact, just cause the inner structure of the FET break down. BAM!
- A low Gate-Source Capcitance (Cds). This is most critical. A MOSFET behaves almost like a perfect capaciter when driven with AC (especially HF). Which means that the driver circuit basically sees a capaciter on the output and must be able to deal with reactive loads. On lower frequencies (say... below 20Mhz) that isn't much of a problem when the GS-Capacitance is relative low. But the bigger this value becomes (or the frequeny) the more attention needs to be paid. Wrong coupling can cause oscillation (harmonics, which cause excessive heat in the FET and Filter) or a not fully driven FET (which will likely cause the FET to become terribly hot). So, when using High Power MOSFETS, one needs to pay attention to the driver stage. It must be able to deliver enough power and (depending on the driver) an inductive compensation must be added to take care of the capactive behaviour of the FET. But this may cause more problems.
Like most times, there is no such think like a "perfect MOSFET for RF use". Here goes the rule:
pick two out of three.
- A low RDS(on) means: a low DS breakdown voltage.
- A Low RDS(on) and high DS breakdown voltage means: a big GS Capacitance.
- ... and so on.
And yeah, that also not the full truth. There's wayyy more to add.
Greetings,
~Zazzle
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