@hevans1944, can I ask you a bit more about the MOSFETS? When we talk about the gate-to-source threshold voltage, we're talking about the difference between potential across these two terminals, right? Since in this case we're using this chip to regulate a certain voltage with a smaller voltage, we need to hook the Source to ground, that way the difference will always be equal to whatever the Gate is receiving, correct? And since we're working with PWM, the signal itself is not analog, so there's no reason we need to worry about the
range of voltage being applied to Gate; the only certainty we require is that it will switch on when hit with the 3.3 or 5 V supplied by an Arduino pin during rapid on-off-on PWM, correct? The part I bought is the FQP30N06L. The spec in question is the Gate Threshold Voltage, which is spec'd as "min" at 1.0 and "max" at 2.5 V, according to
this document. (I ordered from Mouser because it was a bit cheaper than sparkfun and I like them).
If we were controlling a larger voltage with a smaller analog (fluctuating) voltage, we would have needed to find something that had upper and lower threshold limits that matched the limits in our controlling voltage, correct? Out of curiosity, what kind of application would have voltage going to the source? What else are these guys used for?
Look at
Figure 2. Transfer Characteristics for the document you reference (Fairchild FQP30N06L N-Channel QFET datasheet):
Note the drain current scale is logarithmic. At 25° C, for Vds=25 V, the drain current varies from about one ampere, near the Vgs threshold of about 2.3 V, to about 10 amperes for Vgs equal to about 2.6 V, increasing to 90+ amperes for Vgs equal to 10 V. This is highly non-linear behavior typical of a power MOSFET switch. You want to operate this device as a switch, driving the gate-to-source voltage to zero to turn it off, and to at least 5 V (preferably 10 V) to turn it fully on. This will minimize the power dissipation in the device but will not totally eliminate it because there are switching losses.
As conduction changes between full-on and full-off or from full-off to full-on, the drain current and the drain-to-source voltage (which is dependent on the load impedance and power supply voltage) both pass through a region of so-called "linear" conduction where drain current and drain-to-source voltage are sufficient to cause significant power dissipation in the MOSFET. This particular device is rated for 79 W dissipation. It is also rated for 60 V drain-to-source voltage and 32 A continuous DC current. Clearly both of these conditions cannot occur at the same time because that would result in 1920 W dissipation! It is necessary to traverse the region between full-on and full-off very quickly (nanoseconds) to minimize the power dissipation that occurs in this region, but the ability to add or remove charge from the gate-to-source capacitance (including the Miller capacitance) determines how fast this can occur. You need to drive the gate with a low impedance source to minimize the switching time.
If we were controlling a larger voltage with a smaller analog (fluctuating) voltage, we would have needed to find something that had upper and lower threshold limits that matched the limits in our controlling voltage, correct? Out of curiosity, what kind of application would have voltage going to the source? What else are these guys used for?
No. You adjust bias-point and feedback and load impedance to fit the device. Also, the "upper" and "lower" threshold "limits" represent a range of values that any particular part will fall between. For Vgs threshold, these values are measured at a particular Vds and Ids and devices that fall outside the specified Vgs range result in either part rejection or re-binning to some other spec. Be careful relying on "min", "typ", and "max" specifications unless the manufacturer defines (somewhere in their literature) what these terms mean. It is not unusual to see only a "typ" specification without a high or low limit. Who the hell knows what that means? What recourse do you have if the part you receive isn't "typical"?
These power MOSFETs are intended to be used only as switches, driven fully on with milli-ohm Rds at high current, or driven fully off with essentially no current flowing from drain to source. It is a whole different ball game to design a power MOSFET circuit for stable linear operation as a power amplifier or power oscillator.
Here is a link to get you started if you want to explore that fascinating application area.
(I ordered from Mouser because it was a bit cheaper than sparkfun and I like them).
Cost is certainly a factor in any engineering design, but to choose a vendor based on whether you "like" them or not is not good engineering practice. I choose a vendor based on many criteria such as cost, delivery, availability of parts, reliability of parts (freedom from counterfeits), and vendor reputation. I "like" Mouser... and many others such as Digi-Key, Jameco, Allied Electronics, Newark (Element 14), McMaster-Carr, Microchip, Texas Instruments... the list goes on and on, and all have "good" reputations, but cost, delivery, availability, and reliability are my usual determining factors in which vendor I choose.
I don't much "like" Asian vendors because it is difficult to know exactly what you are getting from them. I have to rely on experience with a particular vendor, or their eBay rating, before deciding whether to order from China... and then its a crap shoot. I have had some pretty good experience with Chinese carbide drills for PCB work (so far), but China makes a lot of counterfeit electronics parts that may or may not be equivalent to a world-class vendor's stock of a brand-name manufacturer's parts.
Caveat emptor (buyer beware) until you have thoroughly vetted a vendor.
