meaning of MOSFET "linear region" in the context of switching losses

Question

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?

Answer 1

"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.

Answer 2

"Linear region" is unfortunately the most inconsistently used term when it comes to MOSFETs. It can mean exactly the opposite depending on the author. Compare:

Image from this appnote.

From this textbook, which calls the left region "linear region".

Also note that JEDEC has chosen "ohmic region" and respectively "saturation region" as their choice of standard terminology for MOSFETs (as in the 1st figure above). This is given in JESD77b on page "4-31". They avoided calling any region "linear".

Answer 3

Linear region in this context means thee region where you don't want to operate in because the product Id·Vds is big therefore you have a lot of losses. You want to minimize the losses in transistor by having the transistor either fully on or off.

Switching between the two states should be as fast as possible because being there generates losses.

The area under the blue curve is the energy dissipated in the device. Switching slower makes the area bigger.

If you take a look at typical hard switching turn-on

or turn-off

You can see that for some time there is high voltage and high current present on the device at the same time. Switching faster minimizes the time spent in that area.

There are ways to minimize the switching losses by using a zero-voltage or zero-current switching. You have to design your converter in such a way that it will switch only when either voltage or current on the transistor is close to zero. This way the power product of Id·Vds is also close to zero.

Answer 4

The graph which shows energy appears to have time as its axis. It may be helpful to graph power versus voltage drop, assuming a resistive load (e.g. figure 10 volt supply and a one-ohm load). When the device is fully off, zero current hence zero power. When fully on, very low voltage drop (e.g. 0.2 volts) and thus low power (9.8 amps, so 1.96 watts). When "half" on, significant voltage drop (5 volts) and significant current (5 amps), so big power (25 watts).

Answer 5

There is a bit of confusion as to which side of the graph is labeled as the "linear" region. If you are using a MOSFET for PWM switching, you should always try to stay within the left region of the graph.

Remember that a MOSFET is a voltage controlled, current limiting device. When enough voltage exists between the gate and source pins (\$V_{GS} > V_{th}\$), the MOSFET will allow current to flow, up to a limit. The current limit is determined by \$V_{GS}\$ and can vary depending on the specific part (see the graph in your datasheet).

If you attempt to draw more current that this limit, you are entering the right region of the graph. This is where the MOSFET will act as the amount of resistance necessary to maintain that limited current. Like any resistance with high current, it gets very hot. Because it is acting like a resistor, there is now a significant voltage across the drain and source pins (\$V_{DS} ≥ (V_{GS}–V_{th}\$)).

When using the MOSFET for PWM switching, make sure you are applying enough \$V_{GS}\$ so that the MOSFET's current limit is higher than the amount of current your fan/motor/etc. is going to draw. With Power MOSFETs, I recommend using the same voltage for \$V_{GS}\$ that you are using to power the fan/motor/etc. itself; this will ensure the fastest switching times, reducing the time you spend in the right region caused by charging/discharging the small capacitance of the MOSFET. Here is an example using an Op Amp to boost the PWM voltage:

UPDATE: Here is another example using a totem pole to drive the MOSFET gate. This has an advantage of driving the gate with a high current.

Note: due to the second N-ch MOSFET, the PWM signal gets inverted, I changed the schmitt trigger to the inverting type to rectify this.