Author Topic: When to consider MOSFET capacitance?  (Read 408 times)

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Offline doublec4

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When to consider MOSFET capacitance?
« on: July 12, 2019, 09:59:34 pm »
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!
 

Offline SparkyFX

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Re: When to consider MOSFET capacitance?
« Reply #1 on: July 12, 2019, 10:47:02 pm »
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.
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Offline TheHolyHorse

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Re: When to consider MOSFET capacitance?
« Reply #2 on: July 12, 2019, 11:55:46 pm »
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? 

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.
 

Online MagicSmoker

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Re: When to consider MOSFET capacitance?
« Reply #3 on: July 13, 2019, 01:19:48 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?

"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.
 

Offline T3sl4co1l

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Re: When to consider MOSFET capacitance?
« Reply #4 on: July 13, 2019, 01:33:27 am »
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!

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Offline capt bullshot

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Re: When to consider MOSFET capacitance?
« Reply #5 on: July 13, 2019, 01:51:43 am »
Always!

I had the same thought while reading the topic. Thanks. ;)

So somewhere on the internet is a story about what gate (MOSFET)/ base (BJT) resistor to use -> the greybeard just says 100 Ohms, the novice asks for a more detailed explanation and gets one, resulting in a 100 Ohm gate resistor after doing all the math (AFAIR).

So for part of the original question -> just use a 100 Ohm series resistor for the MOSFET connected to the Arduino. Works nice as a rule of thumb if you know your stuff.
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Offline David Hess

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Re: When to consider MOSFET capacitance?
« Reply #6 on: July 13, 2019, 01:31:08 pm »
The input capacitance and reverse transfer capacitance become an issue when combined with the source impedance, they limit switching speed to slower than desired.  This is more common with MOSFETs but it applies to bipolar transistors as well.

So driving a small MOSFET like a 2N7000 from a logic output will produce fast switching but the same logic output will produce slow switching when driving a big TO-220 part because of excessive input and reverse transfer capacitance.
 

Offline David Hess

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Re: When to consider MOSFET capacitance?
« Reply #7 on: July 13, 2019, 01:46:54 pm »
So somewhere on the internet is a story about what gate (MOSFET)/ base (BJT) resistor to use -> the greybeard just says 100 Ohms, the novice asks for a more detailed explanation and gets one, resulting in a 100 Ohm gate resistor after doing all the math (AFAIR).

So for part of the original question -> just use a 100 Ohm series resistor for the MOSFET connected to the Arduino. Works nice as a rule of thumb if you know your stuff.

The rule is obviously more complicated; maintaining the same switching speed with larger MOSFETs would require a lower gate series resistance simply because the combined RC time constant from the series resistance and total equivalent input capacitance would limit switching speed.

The problem comes about because a low impedance load, including parasitic capacitance, reflects thought the transistor to produce a negative resistance at its input.  If the resistive part of the source impedance driving the MOSFET is less than this, then the result is a negative resistance oscillator.

A series gate resistor is not the only way to prevent this problem; a lossy element in series with any of the three leads will work but usually a gate series resistor is the easiest.  In higher performance applications, a lossy ferrite bead might be used in series with the gate or source.  Sometimes this is also done with bipolar transistors for the same reason.

In contrast to linear operation, a saturated switching application likely requires no protection at all because so little time is spent in the linear part of the transfer curve.  Then the gate series resistor is used only to limit the current from the gate driver or to slightly soften the hard switching of the transistor.

Even in switching applications, I have gotten into the habit of including a gate series resistor because it makes layout easier and makes for a convenient spot to measure the gate current and voltage if necessary.
« Last Edit: July 13, 2019, 01:50:06 pm by David Hess »
 

Online Psi

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Re: When to consider MOSFET capacitance?
« Reply #8 on: July 13, 2019, 03:10:28 pm »
Simply put...

If you have a mosfet wired to power a 1R load from 10V  (so 10A / 100W)
And it's RdsOn is 0.016 ohms then the power wasted in the mosfet normally is  R * I^2  so  0.016 * (10*10) = 1.6W   (~1.6% of your 100W lost as head in the fet)

But, when you feed voltage to the gate and change its state the resistance must go from infinite and move down to 0.016 ohms, or vise versa.

Consider that, at some point, the mosfet resistance is the same resistance as the load resistance.
You have a 1ohm mosfet and a 1ohm load in series, so you have effectively 2 ohms from VCC to GND.
So 10v / 2 ohms = 5A
and 5A * 10V = 50W total
25W in the mosfet and 25W in the load.

25W is a massive amount more heat in the mosfet than the normal 1.6W.

Therefore, you want the time in this transition state as small as possible compared to time in the normal on state.

To do this you:
- Do less changes per second, eg a 100hz pwm wastes less heat than a 1000hz pwm
- Use a mosfet with low gate capacitance (so it charges quickly)
- Use a high current mosfet driver IC to feed lots of energy into the gate to charge it fast. (MCU IO can only push ~20mA so takes a while to charge)
« Last Edit: July 13, 2019, 03:15:18 pm by Psi »
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Offline capt bullshot

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Re: When to consider MOSFET capacitance?
« Reply #9 on: July 13, 2019, 05:53:51 pm »
So for part of the original question -> just use a 100 Ohm series resistor for the MOSFET connected to the Arduino. Works nice as a rule of thumb if you know your stuff.

The rule is obviously more complicated; ...

Yes, it is. That why I added "if you know your stuff" and "always" is the correct answer for the orignal question here. I never answer "100 Ohm is just fine" without knowing the whole context of the circuit. So usually, if someone asks something like this thread, I ask for the schematics and/or other context before answering.

OP told about "switching loads on and off with arduino and MOSFET" -> this lead me to omit further questions, assuming switching frequency is low. This assumption might be wrong here, have to admit this.
« Last Edit: July 13, 2019, 05:56:59 pm by capt bullshot »
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Online magic

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Re: When to consider MOSFET capacitance?
« Reply #10 on: July 13, 2019, 07:34:47 pm »
In contrast to linear operation, a saturated switching application likely requires no protection at all because so little time is spent in the linear part of the transfer curve.  Then the gate series resistor is used only to limit the current from the gate driver or to slightly soften the hard switching of the transistor.

Even in switching applications, I have gotten into the habit of including a gate series resistor because it makes layout easier and makes for a convenient spot to measure the gate current and voltage if necessary.
Funnily enough, I once replaced a MOSFET which simply switched some device's voltage rail ON/OFF with a different type and it oscillated during the transition through linear region, driving the control circuit bonkers and turning the rail OFF again. There was no resistor directly at the gate and the PCB trace was long and meandrous. And the controller was open drain with pullup, which probably was why the MOSFET had enough time to develop oscillation.
 

Offline T3sl4co1l

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Re: When to consider MOSFET capacitance?
« Reply #11 on: July 13, 2019, 11:47:15 pm »
Doesn't take much time to develop oscillations -- high voltage SuperJunction FETs will happily oscillate at ~300MHz, in a burst during the tens of nanoseconds spent switching, whether under 100kHz or over 2MHz. :)  Modern, lower voltage FETs offer comparable performance, too.

Tim
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