6. I actually intend to change this thing to FQP13N10's after I get my Mouser order made in order to take advantage of suggestions in this website: http://m0rzf.co.uk/20W_Amplifier/index.html. I see the linked website has diodes as temp compensators.
Some quick excerpts to hopefully trigger your skepticism:
TO-220 packages have inductive wire bonds
Maybe they do, maybe they don't. The leads are ten times longer and therefore higher in equivalent inductance -- you might want to worry about those first!
Switching FETs hotspot badly and the bias point is unstable… OnSemi agree, Infineon agree, Microsemi agree…
A false generalization -- the bias point is only unstable in the region where it is unstable. This region is smaller for some types than others.
It is always present. Some types just burn up (wholesale) before reaching it.
All you must do is simply choose a device which has an SOA which includes all available bias conditions*.
*For any angle of the trimpot, too, mind. It would be a crummy circuit indeed that cannot save itself from the errant turn of the technician's screwdriver, especially an untrained amateur! I hate suicidal circuits, and call them out every time I see them!
I found these are most suitable in terms of medium gate capacitance, low-ish transconductance and good thermal conductivity:
The choices listed are all newer types -- calling them "low-ish" is peculiar, because newer types have more Gm/Id than ever before. This can make them even more prone to runaway! We must depend on other design characteristics of these parts, to obtain good linear operation.
Curiously, the single most powerful development, SuperJunction technology (applicable for high voltage MOSFETs, over ~300V I think), is apparently free from breakdown. If not by nature, then by additional design steps, that seemingly all manufacturers have followed. It sure is nice, however it's happened -- high voltage transistors are higher performance, and more affordable, than ever!
The article is hardly bald-faced-wrong, no -- but it is also nowhere near something to believe unconditionally. Likewise the articles referred to, from the manufacturer's mouth even, are far from infallible (the On Semi article in particular lacks critical context in an early paragraph: when talking about PTC, they're referring specifically to the resistive saturation (triode) region only, not the linear (FET "current saturation") region; this is clearly distinguished in the Infineon article, however).
8. Any advise on VHF oscillation? Should I be able to pick it up with my DS1104Z?
Yes*.
*For the IRF510, most likely. For the STW13N60M2, maybe not. Those types tend to sing at 200-400MHz, with the cause usually being a G-S capacitance that instead forms a tuned-gate resonant circuit with the lead inductance. Again, TO-220 works against you, severely.
The usual fix for such oscillations is a ferrite bead on the gate lead, but that's only practical at low frequencies, or repeat rates (SMPSs are, in a sense, pulsed RF amps, with a typical repeat rate in the 100s of kHz to low MHz).
Well, if we're only talking a few MHz here anyway, that's not a problem. Maybe an 11m amp would cook the ferrite bead (from gate drive power) though. (Note: ferrite beads don't "cook off", they just thermoregulate around Tc, as that causes their added impedance to drop to nil. So, such an amp might be okay at key-down, but a couple seconds later, you get UHF whistlers!)
It's quite practical to build an amp with SMTs -- a two layer PCB with 2oz copper and lots of vias can draw 5 or 10W out of a PDSO-8 sized part, and some quite beefy transistors come in such packages. (Although the opposite case is usually the problem: that you're looking for smaller transistors, so that capacitance is small; but not so small that the power dissipation is pitiful.) Packages like these (DFNs and such) have essentially no internal lead length, beyond what you can see, giving you maximal control over parasitics.
Not that DFNs are the easiest thing for an amateur to build, but leaded SMTs (like DPAKs and SOICs) are available in all sorts, at modest expense to parasitics or power dissipation. People need to realize that SMTs are actually easier; heck, it almost seems like coddling to keep putting THTs out there where they're not necessary!
My goal essentially is to generate sine waves at around 20-50 watts and 1.5 to 3 MHz.
Hardly a challenge -- these can be synthesized digitally at high efficiency, with a switching converter. Assuming you didn't need modulation on that, that is (but then, that wouldn't be a sine wave?).
Tim