How to size FETs?

[redgear]

TI's design resources say :

To select a FET, take four main parameters into account: the voltage, current, thermal performance, and the switching time of the FET.
The voltage rating of the FET must be higher than approximately 5-V DC per cell in series and 10-V transient per cell in series. In this design, 10 cells are used in series, which means that the FETs (Q13, Q14, Q15, and Q16) should be rated higher than 50-V DC and 100-V transient.

I will be using a 6S pack, so my DC voltage should be 30V and transient should be 60V.

The current requirements are 75-A continuous, 300 A for 400-μs, and 200 A for 40-ms transient.

How do I calculate the current requirements? My motor draws a max of 15A and I'm yet to calculate the current required for the display and other parts.

The design uses two FETs in parallel to increase thermal performance, do I need to use two FETs for the low current applications like mine?

[ratatax]

For the thermal side :
On the datasheet you should have a line telling you how many °C the part increase relative to ambient temperature for each Watt of power dissipiation. Calculate the power dissipation using the internal FET resistance, the voltage and current needed.
If the temp rise (°C) is too much, you need a heatsink or split the current with multiple FETs in parallel.

I think you can ignore the current needed by the display, it should be peanuts compared to the 15A needed by the motors

[Kleinstein]

Is the FET also switching the display ? Normally the display and control part would not be switched together with a motor.

For the choice of FET the frequency is an important factor. For just occasionally switching in an on/off way, the FETs can be relatively large (high current). For a higher frequency switching like fast PWM, high frequency SMPS the switching losses are important and just large enough it not good. Larger FETs will have more switching loss.

[redgear]

For the thermal side :
On the datasheet you should have a line telling you how many °C the part increase relative to ambient temperature for each Watt of power dissipiation. Calculate the power dissipation using the internal FET resistance, the voltage and current needed.
If the temp rise (°C) is too much, you need a heatsink or split the current with multiple FETs in parallel.

I think you can ignore the current needed by the display, it should be peanuts compared to the 15A needed by the motors

Ok, Thanks! How much is too much? >50 deg C?

Is the FET also switching the display ? Normally the display and control part would not be switched together with a motor.

No, FET does not switch the display!

For the choice of FET the frequency is an important factor. For just occasionally switching in an on/off way, the FETs can be relatively large (high current). For a higher frequency switching like fast PWM, high frequency SMPS the switching losses are important and just large enough it not good. Larger FETs will have more switching loss.

Yeah, I read that but idk how to what I can do to make it better.