Thermal Design Fundamentals of Power Devices (V) - Thermal Capacity of Power Semiconductors

Latest update time：2024-11-18

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/ Preface /

Power semiconductor thermal design is the basis for achieving high power density of IGBT and silicon carbide SiC. Only by mastering the basic knowledge of thermal design of power semiconductors can we complete accurate thermal design, improve the utilization rate of power devices, reduce system costs, and ensure system reliability.

The series of articles on the basics of thermal design of power devices will systematically explain the basics of thermal design, relevant standards and engineering measurement methods.

Heat capacity

Thermal capacitance _Cth is an important physical quantity like thermal resistance _Rth , and they have similar dimensional structures. Thermal capacitance and capacitance are both physical quantities that describe storage capacity. The comparative relationship between capacitance and thermal capacitance of a flat plate capacitor is shown in the figure.

The corresponding relationship between capacitance and thermal capacity of flat plate capacitor

Heat capacity of a plate

Capacitance C _el (in A·s/V ) represents the relationship between charge Q and voltage U.

Heat capacity Cth (in J/K ) is the relationship between heat Qth and temperature difference ΔT, as shown in Equation 1. In other words, heat capacity can be described as the ratio of heat change to temperature difference , _that is _:

_The heat Qth can be obtained from the specific heat capacity _cth , mass m and temperature difference ΔT , that is:

_{The specific heat capacity cth} of a certain material is a constant, with the unit of J/(kg·K) (see the table _below ). If equation (2) is used to replace ΔQth in equation (1) , the relationship of heat capacity becomes:

Specific heat capacity of the material _cth

Since mass m = ρ·d·A ( d is thickness, A is area, and ρ is density ), the heat capacity of power electronic devices can be calculated using the material's specific heat capacity c _th , relative density ρ , and volume.

Thermal impedance

Using thermal resistance R _th and thermal capacitance C _th , a thermal model similar to an RC low-pass circuit can be constructed. This model can be represented by transient thermal resistance or thermal impedance Z _th , and every real object has thermal resistance and thermal capacitance.

Transient thermal impedance Z _{th
, including the thermal resistance R}_th and thermal capacitance C _th of the plate

The figure above shows the transient thermal impedance Z _th , including the thermal resistance R _th and heat capacitance C _th of the plate. The thermal impedance Z _th can be described in the time domain , that is, due to the heat capacitance, the temperature difference ΔT changes with time, and we have:

Similar to the definition of time constants in electrical engineering, the time constant τ for thermal capacitance filling is:

The time of the transition process is 0~5τ , which represents the time to reach 0~99.3% of the final value. The time after 5τ or 99.3% is regarded as steady state (i.e. thermal equilibrium). At this time, it is assumed that ΔT _max no longer changes and the heat capacity no longer has any effect on the thermal impedance, so the thermal impedance _{Z th} can _be regarded as the same as the thermal resistance R _{th
.}

The figure below shows the change of thermal impedance _Zth over time. The thermal impedance can be calculated by ΔT(t) and Pth _,C , that is:

Relationship between thermal impedance Zth _{and
time}

In the actual device data sheet, the thermal impedance _Zth graph has the X-axis as time.

Thermal impedance of actual device

The transient thermal impedance Z _thjc of the power semiconductor junction to the case will be given in the data sheet. Common packages for power semiconductors are power modules with copper substrates, DCB modules without copper substrates, and single tubes based on copper frame structures. Due to the different materials of the heat transfer path and the different weights and volumes of the materials, the transient thermal impedance Z _thjc is different.

Copper base module

DCB Module

Single tube

Copper base module

Copper-based modules are heavy, mainly because of the copper base plate. The copper base plate thickness of EconoDUAL™ 3 is 3 mm, which plays an important role in the transient thermal impedance Z _thjc . The heat will spread longitudinally and transversely in the copper layers on both sides of the DCB and the copper base plate. The 5 τ value is greater than 2 seconds (diagram taken from FF900R12ME7_B11 900A 1200V half-bridge module).

DCB module:

The DCB module without copper substrate is much lighter. The copper thickness of DCB is 0.25-0.30mm, and the heat capacity is much smaller than that of the module with copper substrate. The heat will only diffuse in the longitudinal and lateral directions of the copper layer on both sides of the DCB. The τ value is about 0.4 seconds (the chart is taken from FS200R12W3T7_B11 200A 1200V three-phase bridge module).

Single tube:

The single tube does not have a DCB board, and the chip is directly soldered to the copper frame. The heat of the chip is directly added to the copper frame, and the heat can be well diffused on the copper frame. The 5 τ value is about 0.02 seconds (the chart is taken from IKY140N120CH7 140A 1200V IGBT single tube).

summary

This article introduces the concept of thermal capacitance, proposes transient thermal characteristics, and compares the transient thermal resistance of different packages. The next article will introduce transient thermal measurement in detail.