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[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

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

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.

S