The Normalized Effective Transient Thermal Impedance in Semiconductor Devices

September 16, 2021 Cadence System Analysis

Key Takeaways

  • Use transient thermal impedance rather than steady-state thermal resistance for thermal characterization of semiconductor devices under pulse current operation.

  • Normalized effective transient thermal impedance = (D+(1-D))*(transient thermal impedance for single pulse duration, t).

  • The repetitive peak junction-to-case temperature rise during the conduction time (Ton) of a semiconductor device can be determined as transient thermal impedance multiplied by the power dissipation during the same conduction time, Ton.

 Semiconductor device

High power and current density demands at the chip level challenge the reliability of semiconductor devices and depend heavily on die size, power dissipation, maximum junction temperature, layout, packaging, cooling, and thermal characteristics.

Among thermal parameters, normalized effective transient thermal impedance is the thermal metric most suitable to measure the thermal response of semiconductors at pulsed current mode. This metric can determine the junction temperature, thereby preventing the overheated operation of a semiconductor device.

Electrical and Thermal Characteristics of Semiconductor Devices

The electrical performance of a semiconductor device is temperature-dependent. The electrical characteristics provided in the datasheet of a semiconductor device are at a specified temperature; when the device is operated above this temperature, performance derating occurs.

In semiconductor datasheets, the derating curve for output current and power dissipation is usually provided. Additionally, a transient thermal impedance graph is provided to help designers understand the temperature difference between the junction and case of a semiconductor device. 

Transient Thermal Impedance

Most semiconductor devices operate in pulsed current mode rather than in DC mode or continuous current mode, which is why it is appropriate to use transient thermal impedance rather than steady-state thermal resistance for thermal characterization.

When semiconductor devices operate in pulse current mode, the power dissipation changes with the pulse duration. Under pulse currents, the junction temperature increases exponentially. This rise can be predicted from the thermal response curve—the curve plotted with surge or pulse duration in the x-axis and transient thermal impedance, ZΘ(JC)(℃/W) in the y-axis. From the transient thermal impedance graph, the peak junction temperature (TJ) of a semiconductor device can be calculated as follows, where PDM is the maximum power dissipation in the semiconductor device in watts and TC is the case temperature in ℃.

TJ=PDM*Z(JC)+TC

Normalized Effective Transient Thermal Impedance

The frequency of the pulse is a significant factor affecting the junction temperature of a semiconductor device. Pulses above a few kHz and duty cycle (D) increase the peak junction temperature, which can be considered equivalent to the average power dissipation multiplied by DC junction-to-case thermal resistance.

Peak junction temperature caused by low-frequency pulses is greater than high-frequency pulses. The thermal response curve is useful in determining the peak junction temperature arising from pulses of a low repetition rate. Usually, the thermal response of a semiconductor device shows the single pulse curve along with the curves corresponding to the effective transient thermal impedance at different duty cycles. The normalized effective transient thermal impedance of a semiconductor device can be defined as:

Normalized effective transient thermal impedance = (D+(1-D))*(transient thermal impedance for single pulse duration, t)  Where D=TonTs and 1-D=ToffTs   Ts=Ton+Toff  Ton is the pulse duration for which the semiconductor device is on or conducting and Toffis the pulse duration where the semiconductor device is off.

Calculating the Junction Temperature of a Semiconductor Device for Pulsed Currents

The repetitive peak junction-to-case temperature rise during the conduction time (Ton) of a semiconductor device can be determined as transient thermal impedance multiplied by the power dissipation during the same conduction time, Ton. The power dissipation can be calculated from the applied voltage and current through the semiconductor device during on-time, Ton.

The junction temperature of a semiconductor device is significant in maintaining its operational reliability. Understanding a device’s thermal response helps the user utilize the semiconductor device to the maximum ratings without derating the electrical performance. 

Cadence software offers the Celsius Thermal Solver (featured below) to analyze the thermal status of solid-state electronics circuits. 

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