[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 $R_{th(j-c)}$.

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.

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

P_d-T_a 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.