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Are logic level gate mosfet slower than regular mosfet?
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T3sl4co1l:
doc format?  But... that's almost as bad, what's wrong with plain text? |O

Here's plain text, cleaned up a bit; view with tab size 4.  I'm not sure if that imports into, say, Excel cleanly.

Tim
exe:

--- Quote from: spec on November 01, 2018, 05:45:19 am ---By the way, it is generally a good idea to connect a 100 Ohm resistor directly to the gate of a MOSFET to act as a gate stopper.

--- End quote ---

Do you mean in series with the gate? To me value seems to be too high, I saw much lower values (used to slow down switching to reduce noise). BTW, what is "gate stopper"?
spec:

--- Quote from: exe on November 01, 2018, 09:13:58 am ---
--- Quote from: spec on November 01, 2018, 05:45:19 am ---By the way, it is generally a good idea to connect a 100 Ohm resistor directly to the gate of a MOSFET to act as a gate stopper.

--- End quote ---

Do you mean in series with the gate? To me value seems to be too high, I saw much lower values (used to slow down switching to reduce noise). BTW, what is "gate stopper"?

--- End quote ---

Yes, in series with the gate and mounted directly to the gate terminal. In fact, a resistor on the gate serves quite a few functions, one of which is to act as a gate stopper which is akin to the grid stopper used on thermionic valves.

MOSFETs have a very high frequency response, but unfortunately they also have amazingly high parasitic capacitances. The capacitance from the drain to the gate Cdg often being the most troublesome. The upshot of all this is that MOSFETs have a tendency to ring and even oscillate. A gate stopper reduces the gain of the MOSFET at high frequencies and thus makes them more stable. Of course, good grounding and short wires and decoupling also help.

But the gate resistor has another function too- it is used to shape the gate drive waveform. There is a lot going on when the gate of a MOSFET is being driven by a reasonably fast waveform. For example, the already large Cdg appears at the gate as a virtual capacitor of ACdg (A= MOSFET voltage gain) due to the Miller effect. And if that were not enough, the capacitance values vary throughout the voltage excursion of the input waveform and output waveform.

As to the value of the gate resistor, it takes a bit of working out, but as a rule of thumb 50 Oms to 100 Ohms seems to be a good compromise. But you will see much lower values in high speed, high power applications.

Gate pulse shaping is not really that important if you are just turning a LED on and off now and then. But, in more critical applications the gate resistor can help protect the MOSFET from damage. Imagine MOSFETs being used in a welder or an inverter.

While on the subject, just a word about gate drive voltage and current. In general, the higher the gate drive voltage the lower the drain source resistance (RDS), which means that your circuit is more efficient, but it also means that the MOSFET is dissipating less power and thus is under less stress.

On to the gate drive current. It seems strange that a device with an input resistance of megaohms should require any base current, and at DC that is true. But as the frequency increases, more and more the effects of the MOSFET's parasitic capacitances come into play and the more input/output current is required. Many people are amazed to learn that gate drivers can source and sink from 100ma to 7A.

I have been rabbiting more than intended, so i will stop now, But just to say that this is a complex subject and I have only skimmed the surface. :)
exe:
BTW, while higher gate voltage means lower Rds(on), I found there is a diminishing return. E.g., a 10V drive was barely better than 7V drive. So, it may not make much sense for charge pumps, etc for a logic-level fet. I suggest do some measurements (but leave some voltage headroom for process variation, temperature effects, etc).

Also datasheets often provide guaranteed Rds(on) at specific voltages, the actual resistance is even smaller (but do not rely on this in mass production).
spec:

--- Quote from: Ian.M on November 01, 2018, 08:48:44 am ---Interesting chip, but its strictly for low speed switching.   Even 1KHz PWM would be too fast for it.

--- End quote ---

Yes, quite true- I just mentioned it as an example. There are no doubt better standard MOSFET drivers.
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