MarkT:
Everyone uses gate driver chips which are MOSFETs internally (faster), one cheap chip, does everything, no need to
build a discrete gate driver circuit. My favorite low-side MOSFET driver is MIC4422, plenty of current
ability, you can even use it as a class-D audio output stage by itself! Favorite because it will work from 5V to
18V and is DIP or surface mount and powerful enough for any MOSFET of IGBT you can imagine.
Also gate driver chips usually have very useful protection features like under-voltage shutdown which
can save you from an expensive power-excursion event.
I can get MIC4422 for 2.35EUR each... not cheap at all IMOHO. And then it needs some decoupling caps?...
There are literally thousands of gate driver chips out there, you'll find a lot cheaper. Yes, good decoupling, I use 10uF ceramic due to the high currents, but 1uF ceramic plus 10+uF electrolytic will probably do.
I plan to get logic level gate MOSFETs (chosen by price, availability and Rds between the logic gate)
PSMN2R7-30PL rated 30volts so I'll use it in 12volts applications a a
IRLZ44NPbF rated 55volts so I'll use it in 24volts applications
if used for switching applications or low frequencies PWM (hard to tell the edge of "low" but assuming the <1kHz native PWM freq. of the arduino is "low") I'll connect the gate directly to the arduino's pin (220Ohm limiting and 10kOhm pull-down)
if I decide to use it on a high frequency PWM control, I'll put a gate driver between them. on the same criteria of price, availability, I got the MCP1407
datasheets:
logic level MOSFET
logic level MOSFET
MCP1407 MOSFET driver
and here some "final" questions:
any thoughts on previous statements ?? do I seem to be in the right way?
MCP1407 datasheet states
To operate the MOSFET driver over a wide frequency
range with low supply impedance, a ceramic and a
low ESR film capacitor are recommended to be placed
in parallel between the driver VDD and the GND. A
1.0 µF low ESR film capacitor and a 0.1 µF
ceramic capacitor placed between pins 1, 8 and 4, 5
should be used.
and on their example circuit the add a 2500pF on the output, and they say "For example, 2.25A are
needed to charge a 2500 pF load with 18V in 20 ns."
so:
2.1) how do I dimension the output cap? knowing the capacitance of my gate, the Rising time of the MOSFET and the PSU voltage?? I guess this is tricky cuz if I under-dimension it, the gate will take longer to saturate, and if I over-dimension it, it will waste a lot of energy trough the driver...
2.2) do I need current limiting resistor and pull-down resistors on the gate? or the driver would take care of it?
2.3) what kind of cap is a low Equivalent Series Resistor Cap ? just a "standard" film cap would do? When I look for low ESR cap, all I get is electrolytic caps and the smallest one with the mention "low ESR" is 47µF
jorgemarmo:
and on their example circuit the add a 2500pF on the output, and they say "For example, 2.25A are
needed to charge a 2500 pF load with 18V in 20 ns."
This has been misunderstood before.
You DON't use an output cap.
In those diagrams they use a capacitor, to simulate a mosfet gate, for testing purposes.
Leo..
Why limit the current to the gate, if you want a lower current and slower switching (to reduce EMI for example)
then use a gate driver with lower output current! Remember the output stage of a gate driver is just a smaller
MOSFET with its own on-resistance of a few ohms (typically).
However if you drive MOSFETs in parallel from one driver it is advisable to use gate resistors per device to both
share current equally and prevent any differential oscillation modes between devices.