In the context of MOSFET switching circuits (PWM, motor control, etc) I've read the "linear region" of operation is where you don't want to be for long, because here is where there is large power in the MOSFET. For example, this answer:
you are driving the MOSFET into its linear (power dissipating) region
Or this application note from International Rectifier:
If the device is operated as a switch, a large transient current
capability of the drive circuit reduces the time spent in the linear region, thereby reducing the switching losses.
Yet, Wikipedia offers these definitions:
- linear region: \$V_{GS} > V_{th}\$ and \$V_{DS} < ( V_{GS} – V_{th} )\$
- active mode: \$V_{GS} > V_{th}\$ and \$V_{DS} ≥ ( V_{GS} – V_{th} )\$
That is, \$V_{DS}\$, and thus the power in the MOSFET, is less in the linear region than in active mode. Therefore, I would think it's time in active mode that one would want to avoid. As one switches from off to on, one starts in cutoff, moves through active mode as quickly as possible to minimize losses, then ends in the linear region.
But, I can't reconcile this with the examples above, which discuss minimizing time in the linear region. Where is the inconsistency?
Best Answer
"Linear region" in the answers you quote is used somewhat loosely. Often we say "linear region" or "linear operation" in electronics when we mean in-between operation where a voltage is kept somewhere between the power supply rails (as apposed to clamped to near one of them) or a device like a transistor is kept in the middle region where it is not fully on or fully off. Often devices aren't all that linear in this "linear region", but it's a name that stuck from long ago where linear region was as opposed to in switching operation or the clipped region.
It is this middle "linear" region where the device will dissipate significant power. If the device is a ideal switch, then it can't dissipate power when open since the current is zero, or when closed since the voltage is zero.
This is different from "linear region" when talking about the device physics or details characteristics of a MOSFET. There "linear" can mean "roughly linear current with applied voltage", which also means the MOSFET is acting like a resistor as apposed to more like a current source. That's different from "linear region" from the overall circuit perspective.
Yes, it's context-dependent and can be confusing. If you need to be precise, use real numbers.