# [MOSFET] What is Power Dissipation PD?

Regarding the "Power Dissipation $$P_D$$ of MOSFET", this article will explain the information below.

• Power Dissipation $$P_D$$ of MOSFET
• $$P_D$$-$$T_c$$ Characteristics of MOSFET

## Power Dissipation of MOSFET

The power dissipation $$P_D$$ of a MOSFET is the power dissipated when the channel temperature $$T_{ch}$$ reaches the absolute maximum rating.

The symbol for power dissipation depends on the datasheet, but it is often expressed as $$P_D$$, $$P_d$$, $$P_T$$, $$P_{tot}$$, or $$P$$.

In the case of "packages to which a heat sink can be attached such as TO-220" or "packages that allow backside heat dissipation such as TO-252", the power dissipation $$P_D$$ listed in the datasheet is based on "case temperature $$T_c=25\mathrm{^{\circ}C}$$.

Therefore, the power consumption when the case temperature $$T_c$$ reaches the absolute maximum channel temperature rating $$T_{ch(MAX)}$$ from 25°C is the power dissipation $$P_D$$ listed on the datasheet.

The power dissipation $$P_D$$ is expressed by the following equation using the thermal resistance $$R_{th(ch-c)}$$.

Power Dissipation Formula

\begin{eqnarray}
P_{D}=\frac{T_{ch(MAX)}-T_c}{R_{th(ch-c)}}=\frac{T_{ch(MAX)}-25{\mathrm{[{^{\circ}C}]}}}{R_{th(ch-c)}}\tag{1}
\end{eqnarray}

The figure above shows the absolute maximum ratings and thermal resistance characteristics of a Toshiba N-channel MOSFET (2SK4017).

The absolute maximum channel temperature rating $$T_{ch(MAX)}$$ and channel-to-case thermal resistance $$R_{th(ch-c)}$$ listed in the data sheet can be confirmed to be the following values.

• Absolute maximum channel temperature rating $$T_{ch(MAX)}=150{\mathrm{[{^{\circ}C}]}}$$
• Channel-to-case thermal resistance $$R_{th(ch-c)}=6.25{\mathrm{[{^{\circ}C}/W]}}$$

Substituting the above values into equation (1), we obtain the following value for the power dissipation $$P_D$$, which is consistent with the power dissipation $$P_D$$ value listed on the data sheet.

\begin{eqnarray}
P_{D}&=&\frac{T_{ch(MAX)}-25{\mathrm{[{^{\circ}C}]}}}{R_{th(ch-c)}}\\
\\
&=&\frac{150{\mathrm{[{^{\circ}C}]}}-25{\mathrm{[{^{\circ}C}]}}}{6.25{\mathrm{[{^{\circ}C}/W]}}}\\
\\
&=&20{\mathrm{[W]}}\tag{2}
\end{eqnarray}

The data sheet also shows the thermal resistance $$R_{th(ch-a)}$$ between the channel and the ambient. The thermal resistance $$R_{th(ch-a)}$$ between the channel and the ambient shown in the data sheet can be confirmed to be the following value.

• Thermal resistance between the channel and the ambient $$R_{th(ch-a)}=125{\mathrm{[{^{\circ}C}/W]}}$$

Substituting the above values into equation (1), we can obtain the power dissipation $$P_D$$ with no heat sink attached.

\begin{eqnarray}
P_{D}&=&\frac{T_{ch(MAX)}-25{\mathrm{[{^{\circ}C}]}}}{R_{th(ch-a)}}\\
\\
&=&\frac{150{\mathrm{[{^{\circ}C}]}}-25{\mathrm{[{^{\circ}C}]}}}{125{\mathrm{[{^{\circ}C}/W]}}}\\
\\
&=&1{\mathrm{[W]}}\tag{3}
\end{eqnarray}

If no heat sink is installed, this means that the channel temperature will reach 150°C with only 1[W] of power consumption.

Supplement

• Note that in the case of chip components, the power dissipation varies depending on the mounting board.

## PD-Tc Characteristics of MOSFET

The power dissipation $$P_D$$ listed on the datasheet is based on the case temperature $$T_c=25\mathrm{^{\circ}C}$$. Therefore, if the temperature is higher than $$T_c=25\mathrm{^{\circ}C}$$, the power dissipation $$P_D$$ will decrease. The $$P_D$$-$$T_c$$ characteristics (power derating curve) showing the relationship between power dissipation $$P_D$$ and case temperature $$T_c$$ is shown on the datasheet.

The above figure shows the "$$P_D$$-$$T_c$$ characteristics (power derating curve)" of a Toshiba N-channel MOSFET (2SK4017).

When the case temperature $$T_c$$ is 25°C, the allowable power dissipation $$P_D$$ is 20 W. However, when the case temperature $$T_c$$ is higher than 25°C, the allowable power dissipation $$P_D$$ decreases. For example, when the case temperature $$T_c$$ is 120°C, the allowable loss $$P_D$$ is about 5W.

Therefore, the power dissipation $$P_D$$ must be reduced to match the case temperature $$T_c$$.

It is also possible to obtain the power dissipation $$P_D$$ for each case temperature using equation (1). Substituting the case temperature $$T_c=25\mathrm{^{\circ}C}$$ into equation (1), the following value is obtained, which can be confirmed to be the value in the "$$P_D$$-$$T_c$$ characteristics (power derating curve)".

\begin{eqnarray}
P_{D}=\frac{T_{ch(MAX)}-T_c}{R_{th(ch-c)}}=\frac{150{\mathrm{[{^{\circ}C}]}}-120{\mathrm{[{^{\circ}C}]}}}{6.25{\mathrm{[{^{\circ}C}/W]}}}=4.8{\mathrm{[W]}}{\;}{\approx}{\;}5{\mathrm{[W]}}\tag{4}
\end{eqnarray}

#### Summary

In this article, the following information on the "Power Dissipation $$P_D$$ of MOSFET" was explained.

• Power Dissipation $$P_D$$ of MOSFET
• $$P_D$$-$$T_c$$ Characteristics of MOSFET