# Turn off MOSFET with diode - PWM controller Maximal output current

Here is my circuit (and I know that this circuit does not cause any problems at least for the moment):

For turning on, the maximal current from the pin "OUT" is limited by the resistance Rgate and the voltage VDRV as a worst case. Nevertheless for turning off, the maximal current is only limited by the impedance of the diode and the internal resistance of the PWM controller. For the PWM controller that I am currently using the maximal output current is 1A. So the impedance of the diode plus the internal resistance should be at least equal to 13V5/1A, ie 13.5 Ohm. I would be surprise to have a diode with an impedance equal to 13.5 ohm. The diode used is the 1N4148 (CMS).

1N4148 datasheet : https://www.vishay.com/docs/85748/1n4148w.pdf

What do you think about it ? Do you think that it may degrades the components over time ? Or do you think as the current exceed 1A during a very short time, it doesn't cause a problem as it probably depends on the junction temperature of the output stage of the output pin which probably does not have the time to exceed the maximal temperature ?

Thank you very much !

• 1N4148 could have large capacitance, usually schottky diodes are used, they also exhibit larger resistance. Commented Jul 1, 2020 at 16:37
• Thank you for your comment. It is interesting ! According to you it is not safe to put a diode unless this diode has a sufficiently high resistance... So for the moment I will remove the diode even if it is a Schottky, unless you know a mean to know what is the resistance of a schotkky diode when his forward current is equal to 1A during a very short pulse. (I do not want to use a large package).
– Jess
Commented Jul 2, 2020 at 6:24
• @MarkoBuršič This is a very vague comment. It is not clear which of these two "exhibit larger resistance", is it good or bad, and why. Commented Dec 3, 2020 at 14:16

No need to worry about the driver, it will not sink more current than its own abs max regardless of how little impedance you have to the gate.

The part is designed to drive FET gates without the user having to worry about the peak current due to the gate capacitance.

It's the internal driver circuitry that limits the current, not the external components connected to the gate.

Update with examples of designs where no gate drive resistors are used (albeit with different controllers from TI) See here. And here and here.

And finally from the datasheet:

OUTPUT is the gate drive for the external MOSFET. OUTPUT is the output of the on-chip driver stage intended to directly drive a MOSFET. Peak currents of up to 1 A are sourced and sunk by this pin.

The only discussion in the datasheet of a gate drive resistor is the trade off between switching losses and EMI. There's no reference to a need to limit the driver current.

• This of course is not true. If the gate capacitance is large, then the current is also very large. Theoretically it would be infinite without a resistance, so there must be a gate resistor, why do you think they exist - it would be simpler to connect without them. Commented Jul 1, 2020 at 16:40
• @MarkoBuršič So you are assuming the output stage of the driver can sink infinite current? There's no internal limiting in the IC? Driver ICs often connect to FET gates with no limiting resistor in series. Gate drive resistors damp oscillations and with a diode can control rise and fall times and EMI, but the driver generally has internal current limiting to prevent damage. Commented Jul 1, 2020 at 20:10
• That's true for this specific device, but generally gate drivers do need external resistors to limit the peak current, besides the EMI,... Commented Jul 2, 2020 at 8:19
• @MarkoBuršič We can agree to disagree on that. I don't know of any gate drivers that don't internally limit the current to a safe value for the output stage of the driver. If you push the switching frequency very high you might run into thermal problems, but a resistor to limit the peak current during a single switching cycle isn't necessary. Witness all the fast turn-off diodes in parallel with gate resistors. As the OP said, most of them will have a small dynamic impedance. Commented Jul 2, 2020 at 15:07