I've been studying MOSFET switching applications recently, and have been playing with the Circuit Simulator Applet here. In my diagram below, I have Vds usually set to either 12 or 14, as I am trying to design a decent buck converter for automotive applications, in which a constant DC output that has minimal oscillations is necessary.
I've noticed that using a Vgs of 12V, while also have Vds being 12V, results in a lot of heat being dissipated over my MOSFET. I've attempted to minimize switching losses by playing with the timing and inductance values, but have hit a wall.
However, I noticed that upon increasing Vgs higher than Vds, I minimize my switching losses almost entirely, if Vgs is set high enough. In this case, I have it set to 18V, which I cannot really hope to achieve in my application without some kind of voltage doubler/charge pump, of which I have not yet played with.
I'm wanting to make sure the behavior I am seeing here is consistent with reality and not just something happening in the simulator. Is it better to have Vgs higher than Vds? If so, am I wrong in my understanding that MOSFETs are designed to be low-voltage triggered switches, designed to control high voltage, high current loads?
My circuit can be found here that I have set up. Along with some of the sliders I was playing with to try to make it efficient. To anyone that knows whether this is true or not, please explain as much as you can! Very interested in this subject.
Best Answer
The configuration you have is a high side switch. If MOSFET is fully on then VS=VD (assuming very low RDSon and thus low voltage drop). But for this to happen, VG needs to be sufficiently high enough, for example 10 to 12V higher than VS. This is why VG =18 V works better than VG = 12 or even VG=14V. Since for these lower voltages the VGS is only 0V and 2V. Not enough to properly turn on, and thus causing power dissipation. In this configuration a charge pump is required.
AS a side note, if you use N Channel MOSFET in low side configuration then VDS can be more than VGS with no problem since it’s VGS that determines how much MOSFET is on. For example VDS of 20V with a VGS of 10V.