FET selection and quantity for buck-boost converter design

Question

I am working on designing a buck-boost converter where the input voltage is 24 V regulated, and the output voltage ranges from 12 to 48 V with a current rating of 5 A. The design should also allow for easy adjustment when the output voltage is set to a fixed value.

Here's my approach to selecting the FET and determining the number needed:

1. For the high side and low side of the buck portion, I've chosen the CSD18543Q3A FET, which has VDS = 60 V and ID = 35 A. I selected this FET to ensure that it can handle at least twice the input voltage, which is 24 V.
2. Since the VDS of the selected FET is less than 2 x 48 V (the maximum output voltage), I decided not to use the same FET for the high side and low side of the boost. Instead, I chose the CSD19531Q5A, which has VDS = 100 V and IDS = 100 A. This FET has a higher voltage rating than needed, but it addresses the voltage requirement. However, it may result in size issues due to its high current rating.
3. Here's where I'm encountering uncertainty: Do I need more than one FET, for example, for the higher FET in the buck switch? The datasheet specifies a power dissipation (PD) of 66 W at 25°C. Considering that in the buck configuration, Vin = 24 V and I = 5 A, which gives a power dissipation of 120 W, would I need two FETs to handle this power?

Answer 1

Your FETs don't (i.e. shouldn't) dissipate 120 W -- when ON, the current may be 5 A, but the VDS will be low (100s mV), and when off, the VDS will be high, but the current nearly zero.

Properly designed, one FET is sufficient.