# [Zener Diode] What is Power Dissipation Pd?

Regarding the "Power Dissipation $$P_d$$ of Zener Diode", this article will explain the information below.

• Power Dissipation $$P_d$$ of Zener Diode
• $$P_d$$-$$T_a$$ Characteristics of Zener Diode

## Power Dissipation of Zener Diode

The power dissipation $$P_d$$ of a zener diode is the power dissipated when the junction temperature $$T_j$$ reaches the absolute maximum rating.

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

The power dissipation $$P_d$$ listed on the datasheet is based on the ambient temperature $$T_a=25\mathrm{^{\circ}C}$$. Therefore, the power consumption when the junction temperature $$T_j$$ reaches the absolute maximum rating $$T_{j(MAX)}$$ 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 $$R_{th(j-c)}$$.

Power Dissipation Formula

\begin{eqnarray}
P_{d}=\frac{T_{j(MAX)}-T_a}{R_{th(j-c)}}=\frac{T_{j(MAX)}-25}{R_{th(j-c)}}\tag{1}
\end{eqnarray}

The datasheet shown above is the absolute maximum ratings of ROHM's UDZV series of zener diodes.

The data sheet shows that the power dissipation $$P_d$$ is 200mW (the symbol for power dissipation on the data sheet is '$$P$$').

Since the data sheet gives the power dissipation $$P_d$$, junction temperature $$T_j$$, and ambient temperature $$T_a$$, we can also calculate the thermal resistance $$R_{th(j-c)}$$ from equation (1). The thermal resistance $$R_{th(j-c)}$$ is the following value.

\begin{eqnarray}
R_{th(j-c)}=\frac{T_{j(MAX)}-T_a}{P_{d}}=\frac{150{\mathrm{[{^{\circ}C}]}}-25{\mathrm{[{^{\circ}C}]}}}{200{\mathrm{[mW]}}}=0.625{\mathrm{[{^{\circ}C}/mW]}}\tag{2}
\end{eqnarray}

Supplement

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

## Pd-Ta Characteristics of Zener Diode

The power dissipation $$P_d$$ listed on the datasheet is based on the ambient temperature $$T_a=25\mathrm{^{\circ}C}$$. Therefore, if the temperature is higher than $$T_a=25\mathrm{^{\circ}C}$$, the power dissipation $$P_d$$ will decrease. The data sheet shows the "$$P_d$$-$$T_a$$ characteristics (power derating curve)" that indicates this.

The above figure shows the "$$P_d$$-$$T_a$$ characteristics (power derating curve)" of ROHM's UDZV series of Zener diodes. When the ambient temperature $$T_a$$ is 25°C, the power dissipation $$P_d$$ is 200 mW, but as the ambient temperature $$T_a$$ becomes higher than 25°C, the power dissipation $$P_d$$ decreases.

For example, if the ambient temperature $$T_a$$ is 125°C, the power dissipation $$P_d$$ is 40 mW. Therefore, it is necessary to reduce the allowable power dissipation $$P_d$$ according to the ambient temperature $$T_a$$.

Note that it is also possible to determine the power dissipation $$P_d$$ using equation (1), depending on the ambient temperature.

Substituting the ambient temperature $$T_a=125\mathrm{^{\circ}C}$$ into equation (1), the following equation is obtained, which is consistent with the value in "$$P_d$$-$$T_a$$ characteristics (power derating curve)".

\begin{eqnarray}
P_{d}=\frac{T_{j(MAX)}-T_a}{R_{th(j-c)}}=\frac{150{\mathrm{[{^{\circ}C}]}}-125{\mathrm{[{^{\circ}C}]}}}{0.625{\mathrm{[{^{\circ}C}/mW]}}}=40{\mathrm{[mW]}}\tag{3}
\end{eqnarray}

#### Summary

In this article, the following information on the "Power Dissipation $$P_d$$ of Zener Diode" was explained.

• Power Dissipation $$P_d$$ of Zener Diode
• $$P_d$$-$$T_a$$ Characteristics of Zener Diode

Thank you for reading.

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