Optimizing the FET The requirements of the FET (voltage input, voltage output(s), the current draw, and effective power dissi...
Optimizing the FET
The requirements of the FET (voltage input, voltage output(s), the current draw, and effective power dissipation) will change for every design. There isn’t a perfect FET that will work for every application, but there are a few key criteria useful for maximizing efficiency.
Reduce the resistance between the Drain and Source (RDS) of the FET. RDS should be as low as possible, a 1Ω resistance at 2A becomes a 4W power loss, I aim for below 50mΩ as a rule of thumb for most applications.
After you’ve done the math to determine the power loss, confirm that the FET or integrated circuit (IC) can effectively dissipate that power as heat.
It is important to remember that as semiconductors heat up, the resistance will decrease.
With the temperature leading to self heating, this could create a feedback loop and burn out the circuit.
Minimizing the gate charge (QG) will decrease switching losses in the FET by shortening the duration of the ID x VDS peaks that occur while the FET is switching.
Even at heavy load where it’s not the dominant source of loss, it may still be worth reducing the gate switching loss. Remember, the voltage required for the FET to fully conduct will be higher than the advertised minimum turn on voltage.
Consult the graphs in the FET datasheet to confirm the correct voltage for maximum conductivity. A lot of converter ICs have an integrated FET, and there may be no useful information on the VGS or CGSS.
NOISE!!
Faster switching usually results in worse Electro Magnetic Compatibility (EMC), so be sure to employ strategies that reduce EMI (Electro Magnetic Interference)
In high power situations the EMI could be much worse, it may be worth increasing R with a resistor to slow the switching even though it will result in lower efficiency.
Consult the Datasheets
Many datasheets now have efficiency graphs and recommended component values.
Sometimes they even include recommended part numbers.
The manufacturer has also worked to get the most out of their DC-DC converter.
While it’s unlikely you will see the same results they achieved in their near ideal test environment, it is a good idea to use them as a starting point to check your numbers.
