Power MOSFETs are widely used in switching high voltage and/or high current loads in industrial, household, telecommunication, and automotive applications. The power loss is a primary concern facing component selection and conducting optimal design. The power loss of MOSFETs varies with different applications, components and working conditions. We can categorize all types of the power loss of MOSFETs into following types:

**Conduction loss**caused by the on resistance of the Drain-source channel,*R*;_{DS}(on)**Switching loss**caused by MOSFET’s parasitic parameters;**Dead time loss**caused by body diode dissipation during the dead time;**Gate charge loss**caused by the gate electric charges;**Operating loss**caused by IC control circuit;

Now we focus on understanding the MOSFET loss caused by switching. The loss of switching occurs in two critical periods, turn-on and transition between turn-on and turn-off. The switching loss is becoming increasingly significant for low power applications.

The *Coss* includes the capacitance between the drain and source structure, *Cds*, and the capacitance between the gate and drain, *Cgd, Coss = Cds + Cgd*

In the switching operation, the power source charges *Coss* to store the energy during the turn-on phase. When the MOSFET is turned off, the stored energy in *Coss* discharges via the body diode and causes the turn-on loss.

Theoretically, the Coss loss can be modeled by:

The formula shows the power loss caused by *Coss* is proportional to *Coss* value, the switching frequency, *f _{switch}* and squared drain-source voltage,

*Vds*. The energy dissipated by

*Coss*is denoted as

*Eoss*in the datasheet.

Or:

The power loss due to *Coss* is different for MOSFETs with different structures as shown below.

The turn-on loss is calculated by the following formula:

Where:

*Vds* = Drain-Source voltage

*C _{L}* = Load capacitance and wiring parasitic capacitance

*f _{switch}* = Switching frequency of the MOSFET

*Coss* = Drain-source parasitic capacitance

In many low voltage applications, as those in laptop and portable devices, the input voltage of the main power source is normally less than 20V and the voltages at various points of load are 12V. Due to the low working voltage, the power loss caused by *Coss* is small compared to the crossover loss during transition between the ON and OFF states. For other low power applications, as the switching frequency increases significantly, the power loss caused by *Coss* cannot be ignored.

The capacitance of MOSFETs is normally nonlinear. *Coss* capacitance changes nonlinearly as *Vds* changes.

We can see from the above figure, the value of *Coss* is not constant. The *Coss* value listed in the datasheet is the value under a certain conditions, such as *Coss* = 59pF @ *Vds* = 100V, *V _{GS}* = 0V and

*f*= 1MHz.

_{switch}Now, how to obtain the value of *Coss* becomes the main task for determining the power loss by *Coss*. Mathematically, we can use integral method to determine the charge in *Coss* by calculating the area under the *Coss* curve.

As shown above, we want to determine the *Qoss* at *Vds* = 30V. The area encompassed by the *Coss* curve, X-axis, *Vds* = 30V and the Y-axis is the electric charge stored in *Coss*.

The output capacitance of a MOSFET depends on the drain-source voltage applied; therefore a single point measurement cannot exactly stands for the capacitive characteristics of the MOSFET. We can use a method called curve fitting to find the equation of the fitting curve for the output capacitance *Coss* from every single point we can specify. The following equation is an example of the *Coss* based on the output capacitance at 100V for Vishay SiHF10N40D.

Then, we plug *Coss* into the *Qoss* integral equation:

From the datasheet of SiHF10N40D, we have *Coss(100V)* = 59pF.

From *Qoss*, we can calculate *Eoss* by integral of the *Qoss* – *Vds* curve.

Because,

We have

Considering that *Coss* changes as *Vds* changes and other nonlinear factors, we cannot use the above formulas to directly calculate *Eoss*. Instead, we can use some techniques to calculate the *Eoss* from *Qoss*. Assume the capacitance is constant for the applied voltage with small changes. So, we increase *Vds* from 0V discontinuously at small interval of 10V to 10V, 20V, 30V, 40V, 50V, …400V, we calculate the *Coss* value for each interval using the average voltage within that range to represent the interval.

When *Vds* changes from *Vds(n)* to *Vds(n+1)*, e.g., from 1.0V to 1.5V, the increased charge *ΔQoss* can be calculated:

The corresponding energy change is calculated by:

And, the energy corresponding to Vds(n+1) is:

Using this method, we can obtain the *Eoss *vs. *Vds* curve as:

The calculation is not perfect, but it will be very close if we use smaller interval than 10V.