I'm designing a circuit for an automotive application (chassis ground) that has a number of MOSFET outputs to drive large LED arrays and smaller motors (application tends to favour non inductive modes, but it's still an important consideration). One of the goals here is to be able to PWM switch a 300W LED light using a MOSFET or similarly appropriate transistor.
I'm trying to understand now how to understand requirements and limitations of MOSFETs in high frequency switching. I'm targeting a switching rate of 20khz such that it's outside of the audible range as some of the cheaper LED lights i'm trying to switch use inexpensive filter caps that tend to mechanically oscillate at the input frequency.
I've drawn up a very simple circuit in which a simple NPN BPJT (BC848) is used to invert the positive 3.3V output from a micro controller and drive the gate on a low Rds power FET like the IRF5305.
My understanding at this point is that the goal of the driver here is to saturate the FET as fast as possible such as to minimise the Rds as it's turning on; and to do this with some sort of capacitive driver. (I'm new to this and my knowledge is coming from unverifiable sources).
I've read into some of the merits of gate driver IC's which seem to tick many of the boxes (other than cost, but that's a sensible sacrifice to make here) though I'm having a hard time understanding how to indentify the associated rise and fall times required from an application (20khz @ 0-100% duty cycle, 40Khz??) and what that does to the gate at any specified current. If I were looking to drive a FET at:
- 20KHz PWM
- Logic voltage drive (3.3v)
- 0-100% duty cycle
- 20A load
How would I determine an appropriate FET and how would I determine the driving requirements?