Rgate of a power MOS using LTSpice

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Alejofnb

Joined Dec 24, 2019
20
I am doing a project with a BLDC motor and I am using a half-bridge configuration with power MOS, but I have to feed them in their Gate with a driver. In order to determine the load value that the MOS need to switch, I have my doubts on how to proceed, because in the datasheet of the MOS there are values of Ciss, Coss, Crss and even Qg, Qgs and Qgd, but to know the load, which one do I choose?
On the other hand, I have a doubt about how to determine the value of the gate resistance of the power MOS that allows me to provide the sufficient current that the transistor needs instantaneously, that is to say, that the Rgate is not too big in value, since it would make the MOS take too long to switch, but that it is enough in value so that the SOA graph on the page 4 of the MOS is fulfilled. For this I have made the following simulation, where I understand that I would have to vary the Ton parameter in 10ms, 1ms, 100us and 10us, as it appears in the SOA graph of the datasheet. Maybe I am wrong and I am misunderstanding it. If so, if someone could explain it to me. Thanks!
 

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crutschow

Joined Mar 14, 2008
27,759
that it is enough in value so that the SOA graph on the page 4 of the MOS is fulfilled.
You don't "fulfill" that value or slow down the switching speed.
You want the MOSFET to switch as fast as possible to be in the low Ton region region of the SOA.
Notice how, the faster the switching, the better the SOA value?

For switching you usually use the Qg Total Gate Charge value to determine the drive you need to get fast switching.

A series gate resistor is usually no more than 10 ohms to minimize parasitic oscillations.
 
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Papabravo

Joined Feb 24, 2006
16,832
You don't "fulfill" that value or slow down the switching speed.
You want the MOSFET to switch as fast as possible to be in the low Ton region region of the SOA.
Notice how, the faster the switching, the better the SOA value?

For switching you usually use the Qg Total Gate Charge value to determine the drive you need to get fast switching.

A series gate resistor is usually no more than 10 ohms to minimize parasitic oscillations.
You can try a class-B stage made from BJTs, an NPN and a PNP. If that doesn't work for you then a dedicated gate driver with a boost supply for the gate voltage might be an option
 
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