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.

## 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.

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