Electronics > Beginners
When to consider MOSFET capacitance?
doublec4:
Hi all,
So I have some experience using MOSFETs to control loads on/off with a microcontroller. I understand them enough to see them as a voltage controlled switching device in my layman's terms.
On the datasheet, and when searching some topics related to MOSFETs I occasionally come across someone referencing the "input capacitance" or other capacitance related to the MOSFET itself. For example, a quote from some other forum post, someone states: "You can omit the series resistor between the Arduino and the gate (because it is a small mosfet with low input capacitance)"
So why is this the case?
I tried to look into when you would consider such things, but I can't seem to find a clear answer. I've found videos explaining the construction of a MOSFET and why the capacitance exists in different regions, but not when/why/how you would need to consider it in a design.
I imagine it would have an effect on the switching speed for high speed applications? For simple applications not requiring "high speed" (kind of subjective term I guess) is there any point where any of the capacitance properties come into play or would be considered too high / not desirable?
Thanks!
SparkyFX:
In applications in which you need to switch as fast as possible (or the power dissipation during switching will overheat the MOSFET), you want to drive the gate with enough energy in a short amount of time. That´s when the gate capacitance becomes important, which you need to overcome then.
TheHolyHorse:
--- Quote ---On the datasheet, and when searching some topics related to MOSFETs I occasionally come across someone referencing the "input capacitance" or other capacitance related to the MOSFET itself. For example, a quote from some other forum post, someone states: "You can omit the series resistor between the Arduino and the gate (because it is a small mosfet with low input capacitance)"
So why is this the case?
--- End quote ---
A microcontrollers pins can't withstand everything, so you might need a resistor in series to limit the high inrush currents when driving something with high capacitance. This information will be found in the datasheet.
MagicSmoker:
--- Quote from: doublec4 on July 12, 2019, 11:59:34 am ---...
On the datasheet, and when searching some topics related to MOSFETs I occasionally come across someone referencing the "input capacitance" or other capacitance related to the MOSFET itself. For example, a quote from some other forum post, someone states: "You can omit the series resistor between the Arduino and the gate (because it is a small mosfet with low input capacitance)"
So why is this the case?
--- End quote ---
"Someone" is relying on the Rds[on] of the internal CMOS output drivers to provide the necessary damping and current-limiting required when directly driving a capacitive load like a MOSFET. While this works fine with small MOSFETs 99% of the time, it's not a habit you should get into, and it still can cause ground-bounce and similar problems because turning the MOSFET on/off requires a brief spike of current (into or out of the gate, respectively). So, always use a resistor between the MCU pin and the MOSFET (typically 10 to 100 Ohms) and for larger MOSFETs (basically, anything in a package bigger than SOT-23) put a buffer or gate driver in between the MCU and MOSFET.
T3sl4co1l:
Always!
Not that that's a terrifically useful opinion to a beginner, but the fact is, you can't simply ignore many things -- maybe you'll get away with it, but they tend to have unpleasant ways of reminding you. :P
But you can also mitigate them. In the case of MOSFET gate capacitance, it can oscillate with connecting wires (the MOSFET itself is an amplifier with quite reasonable gain under certain conditions), and this can usually be avoided by placing a small resistor directly in series with the gate pin. Typical value is dictated by the driving source, which might be 33 to 1k ohms for a micro pin, or down to 1 ohm or even less for a power switching circuit. (Sometimes a direct connection can be used, or other components like ferrite beads or diodes; you probably won't need to know how to choose these cases, just mentioning them for information.)
If you're just switching slow, boring stuff, you should ensure that you are, in fact, switching it as slowly as you should be! Only use as much bandwidth as you need. MOSFETs perform quite well these days: even with a 1k gate resistor to really slow it down, you can still expect sub-1µs rise times on the drain. Into a capacitive load, this can draw large surge currents; into an inductive load, this can develop large (flyback) voltages. And it doesn't take much of either to kill a transistor, and it only takes one pulse to do it.
So to cover these cases, you may think about simple, crude ways to control the environment around the transistor -- you can limit surge current by placing series resistance around it (with the load, or transistor drain or source), or using a current limiting circuit; you can limit peak voltage by placing a TVS diode from source to drain. (Pick the diode for the maximum nominal voltage applied to the terminal, then pick the transistor for the maximum peak voltage across the diode, typically 1.3-1.5 times its rating. So, a 12V supply might pick a 15 or 18V TVS, and a 30V transistor. Anything higher is fine, of course!)
This covers slow stuff; if you need faster stuff, then you should ensure that it's fast enough, but not too fast -- that's an invitation for generating radio interference. Consider applying a filter -- sometimes this can be as simple as a single capacitor or ferrite bead, or a combination with resistance (lone C's can resonate with unlucky strays or wiring; an R+C can prevent that). Typical example, digital logic signals. There are good reasons why standards exist for these sorts of things -- for example, RS232 is a slow, fairly wide voltage, single-ended standard, while RS485 is modest voltage, good current, fairly fast, and differential. Both are easy to use and well behaved. :) Just wiring logic gates or MCU pins to wires -- not so well behaved, use with caution!
Tim
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