So basically I want to be able to determine if a MOSFET is suitable for a certain circuit or not.
I will be driving the MOSFET like this:
The gate signal will be a PWM signal from an MCU.
MOSFET driver is not an option because they cost 4 times the cost of a MOSFET in here and I don't think I'll ever use one in my hobby projects.
My question is, how do I determine if a MOSFET can handle a certain current at a certain frequency?
Lets say we're using the IRF530
As far as I know what I need to calculate is the switching speed and power dissipation. If those are within the MOSFETs specifications then I'm good to go.
Qeustion #1: Is there anything else other than switching speed and power dissipation I need to consider in order to choose a MOSFET? (Other than the obvious voltage and current capability)
For calculating switching speed my research tells me the best way is to use the gate charge from the datasheet as opposed to rise/fall/on/off times. Those timings seem to be only specific to the datasheet test circuit and don't seem to be meaningful for an arbitary circuit.
https://e2e.ti.com/blogs_/b/powerhouse/archive/2015/07/16/understanding-mosfet-data-sheets-part-5-switching-parameters (last paragraph)
https://www.infineon.com/dgdl/mosfet.pdf?fileId=5546d462533600a4015357444e913f4f (gate charge section)
So I'm using the equation Q(total) = I(gate)t to calculate the time it takes to turn on the MOSFET. As opposed to using the turn-on or rising times from datasheet, or using the gate capacitance and RC time constant.
So with the equation Q(total) = I(gate)t we can determine how long it takes to turn on the MOSFET.
Question #2: How do I calculate gate current? Is it enough to imagine the gate as ground and solve for the value of I based on +12v and 10k (in above circuit)?
Qeustion #3: When I calculated t, should my PWM wave period be t2 (on+off time). Or should it be (t2*100)? Because the fastest the MOSFET will have to switch on and off will be at 1% duty cycle, so my period should be 100 times that.
In the above circuit assuming gate acts like ground our gate current will be ~1mA, then the time needed to turn on the MOSFET would be ~50us, so my PWM would need to be 100us which is a maximum of 10kHz. Or 10000us which will be 100Hz.
So we now know the maximum frequency that the MOSFET can switch with this setup.
Next we should calculate power dissipation. Assuming a 12v gate voltage the resistive power dissipation is easy to calculate
Now comes the tricky part that I find difficult to find a formula for. Power dissipated due to switching.
One source claims you can use the following formula to calculate switching power dissipation:
PD(sw) = [C(rss) × VIN² × f(sw) × I(load)]/I(gate)
Now all these are available except for I(gate) which needs to be calculated. I haven't seen this formula elsewhere and it doesn't seem right to me because it doesn't use neither on/off times nor gate charge.
A more promising formula is this:
PD(sw) = 1/2 * VIN * I(load) * [t(rise) + t(fall)] * f(sw)
Now this formula is using rise and fall times. Which as far as my research and my common sense tell me are useless values. (discussed above).
I was wondering if I could use the on/off time I calculated using the total gate charge instead of rise and fall time and put it in this formula and use it as my guide to determine how much power will be dissipated by a MOSFET at a certain frequency and a certain load.