Power Tips #154: Finding the thermal and current limits of high-power GaN devices through simulation

High power density power-supply modules based on gallium nitride (GaN) devices are core components in the automotive, industrial and data-center sectors. As their integration level and power density continue to rise, the issue of dissipation of concentrated internal heat becomes increasingly prominent. Device overheating leads to thermal failure and degrades system reliability; therefore, sound thermal design is of paramount importance.

Thermal resistance analysis and power-loss calculations form the theoretical foundation of thermal design. The thermal resistance (R_QJA) of a complex power system represents a coupling of the thermal resistances of numerous components, however, making it difficult to calculate precisely using theoretical formulas alone. Thermal simulation software can directly yield the coupled R_QJA of the system and rapidly identify a significant operating condition – the maximum power dissipation sustainable at a given ambient temperature – thereby providing precise data guidance for thermal designs. Figure 1 shows the simulation of temperature distribution of a GaN-based power-supply system using Ansys Electronics Desktop (AEDT) Icepak software. By referring to the temperature color scale on the left, we can intuitively observe the temperature distribution and heat dissipation status of different areas based on their colors.

Figure 1 This simulation of the temperature distribution of a GaN-based power-supply system uses Ansys’ Electronics Desktop Icepak software. Source: Texas Instruments

Understanding conduction, convection and radiation R_Q_JA

Heat transfer in a power supply occurs in three forms: conduction, convection and radiation. Thermal resistance is the core parameter for characterizing the ease or difficulty of heat transfer. Within a power supply, heat generated by semiconductor chips is transferred layer by layer through the package, solder joints, printed circuit board (PCB) copper traces, thermal interface materials and heat sinks – a process that constitutes classic thermal conduction but requires direct physical contact between materials. Equation 1 gives the thermal conduction resistance as:

$R_{cond} = \frac{L}{kA}\text{ (1)}$

where L is the length of the conduction path, k is the thermal conductivity, and A is the cross-sectional area of heat transfer.

Once heat reaches a material surface, it is transferred to the surrounding air. Taking a heat sink as an example, its fins transfer heat to the adjacent air, which rises upon heating to form natural convection, or is driven by a fan for forced convection cooling. Convective thermal resistance represents the resistance to heat exchange between a solid surface and a cooling medium, expressed in Equation 2 as:

$R_{cond} = \frac{1}{hA}\text{ (2)}$

where h is the convective heat-transfer coefficient and A is the convective heat-transfer surface area.

In addition, heat radiating from the enclosure and heat sink rises toward cooler surrounding walls or the ambient environment. Particularly under natural cooling conditions with a high temperature rise, radiated heat can account for 20% to 30% or even more of the total heat dissipation and therefore requires attention. Equation 3 gives the radiative heat flux as:

$\Phi = \varepsilon\sigma A(T^{4}_{s} - T^{4}_{surf})\text{ (3)}$

where ε is the surface emissivity, σ is the Stefan-Boltzmann constant, A is the radiating surface area, T_s is the absolute temperature of the solid surface, and T_surf is the absolute temperature of the surrounding ambient walls.

In a practical power-supply thermal design, the three conduction, convection and radiation modes of heat transfer occur simultaneously and are mutually coupled: heat from the chip first reaches the heat sink by conduction and then flows into the environment from the heat sink’s surface by convection and radiation. Thoroughly understanding their physical meanings and governing equations is the foundation for performing thermal design and estimating R_Q_JA.

Calculating MOSFET and magnetic component power losses as heat sources

The heat sources in a power supply originate from the power losses of its core devices, which constitute the fundamental input to thermal design.

The losses of a GaN metal-oxide semiconductor field-effect transistor (MOSFET) consist primarily of conduction losses and switching losses. Conduction loss (P_cond) is the loss produced by the root-mean-square (RMS) drain current flowing through the on-state resistance during conduction, calculated using Equation 4:

$P_{cond} = I^{2}_{D (RMS)} \times R_{DS (on)}\text{ (4)}$

$I_{D (RMS)} = I_{D (on)} \times \sqrt{D}$

Equation 5 and Equation 6 calculate the turnon (P_on) and turnoff (P_off) losses of the MOSFET, respectively:

$P_{on} = \frac{1}{2} \times I_{D (on)} \times V_{DS} \times (t_{fv} + t_{ri}) \times f_{sw}\text{ (5)}$

$P_{off} = \frac{1}{2} \times I_{D (on)} \times V_{DS} \times (t_{rv} + t_{fi}) \times f_{sw}\text{ (6)}$

where V_DS is the drain-to-source voltage before turnon or after turnoff; t_fv and t_ri are the drain-to-source voltage fall time and current rise time during turnon; f_sw is the switching frequency; and t_rv and t_fi are the drain-to-source voltage rise time and current fall time during turnoff. Equation 7 expresses the total losses of each MOSFET as:

$P_{MOS} = P_{cond} + P_{on} + P_{off}\text{ (7)}$

The losses of magnetic components such as transformers and inductors are the sum of core losses and winding losses. Core losses (P_core) comprise hysteresis losses and eddy current losses, while winding losses (P_coil) comprise DC losses and AC losses, making it one of the primary heat sources in high-frequency power supplies. Equation 8 and Equation 9 are the relevant expressions:

$P_{core} = P_{CV} \times V_{e}\text{ (8)}$

$P_{coil} = R_{DC} \times I^{2}_{coil (RMS)} + R_{AC} \times I^{2}_{coil (RMS)}\text{ (9)}$

where P_CV is the volumetric core loss, V_e is the effective core volume, R_DC is the DC winding resistance, I_coil(RMS) is the RMS winding current, and R_AC is the AC winding resistance.

Using Icepak simulation to extract R_Θ_JA and determine the maximum current and temperature limits

The R_Θ_JA of a complex power system is difficult to solve precisely through analytical methods. Icepak, a thermal simulation software package for electronic equipment from Ansys, enables system-level modeling and thermal field solving, allowing you to directly obtain the total R_Θ_JA from simulation results, thereby compensating for the limitations of theoretical calculations.

The Icepak thermal simulation workflow comprises three broad steps:

Model construction, which retains the heat-generating components and thermal-management structures (including chips, PCBs, magnetic components, heat sinks and enclosures); assigns the corresponding material thermal parameters; and generates the mesh.
A boundary condition setup that applies theoretically calculated device losses as heat sources and specifies the ambient temperature, air-domain boundaries and convection mode.

Solution and output: after iterative solving, the Icepak tool obtains the temperature field distribution and junction temperatures of important devices, which it then uses to compute the total R_ΘJA along with the heat dissipation path from chip to ambient. Equation 10 is the formula for R_ΘJA:

$R_{\Theta JA} = \frac{T_j - T_a}{P_{loss}}\text{ (10)}$

where T_j is the chip’s junction temperature, T_a is the ambient temperature, and P_loss is the chip’s power dissipation.

After obtaining R_ΘJA through simulation, Equation 11 is the fundamental heat-transfer equation:

$T_j = T_a + P_{loss} \times R_{\Theta JA}\text{ (11)}$

Applying this formula determines the most significant operating conditions of the power supply, yielding both the maximum permissible power dissipation at different ambient temperatures and the maximum safe ambient temperature at the rated power dissipation.

Here is an example. As shown in Figure 2, under a certain working condition, the GaN device’s power consumption is 1.16W, the ambient temperature is 70°C, and the simulation results show that the chip’s temperature is about 95°C. According to the thermal resistance formula, R_ΘJA is 21.55°C/W.

Figure 2 The temperature distribution of chips and the PCB they’re mounted on commonly varies. Source: Texas Instruments

After obtaining the R_Θ_JA parameter, it is possible to calculate the chip’s power dissipation based on the specified junction temperature and ambient temperature. The chip’s power dissipation formula then determines the maximum current that can pass under different operating conditions (for calculation convenience, assume that the chip’s power dissipation equals the conduction losses, although in reality they are different). Table 1 shows the maximum allowable load current under various ambient and junction temperatures.

T_j (°C)	Ambient temperature (°C)	Power losses per GaN (W)	R_DS(on) (Ω)	I_max (A)
150	25	5.80	0.00210	74.33
110	70	1.86	0.00188	44.4
125	25	4.64	0.00196	68.8

Table 1 This table documents the maximum allowable load current under various ambient and junction temperatures.

Conclusion

Thermal design is one of the most important steps to help ensure the reliability of high-power-density power supplies. Theoretical calculations of power losses and thermal resistance can clarify the heat generation and heat-transfer behavior of individual components, but cannot accurately characterize the coupled thermal resistance of complex systems.

Thermal simulation software enables efficient extraction of the total system R_Q_JA, rapidly determining operating boundaries under varying power dissipation and ambient temperature conditions, and achieving quantitative analysis and precise optimization of thermal design.

The combination of theoretical calculation and simulation represents an efficient methodology for modern power-supply thermal design and can significantly enhance heat dissipation capability and system reliability, particularly for high-power-density GaN-based designs.

Bert Zhang (Haobo Zhang) currently works as a systems engineer in Texas Instruments’ Power Design Services team to develop power solutions using thermal simulation, magnetic simulation, and power design techniques. He earned a Master’s degree from Nankai University.

Source link

Understanding conduction, convection and radiation RQJA

Calculating MOSFET and magnetic component power losses as heat sources

Using Icepak simulation to extract RΘJA and determine the maximum current and temperature limits

Conclusion

Understanding conduction, convection and radiation R_Q_JA

Using Icepak simulation to extract R_Θ_JA and determine the maximum current and temperature limits