In this study, we focus on the thermal management of a single-phase 6 kW low-power grid-connected inverter operating under natural convection cooling. The inverter is one of the most common types of solar inverter used in residential photovoltaic systems. To enhance reliability and lifespan, we must ensure that all critical components operate within their safe temperature limits, especially under harsh ambient conditions. We employ the electronic system thermal simulation software FLOTHERM to construct a full three-dimensional numerical model of the inverter. Through systematic parameter assignment, iterative model calibration, and experimental verification, we establish a highly accurate simulation platform. This platform enables us to evaluate the effect of adding thermally conductive silicone pads on component temperatures. Our results demonstrate that a 1.5 W/(m·K) thermal pad significantly reduces the temperature of sensitive components such as electrolytic capacitors and relays, thereby improving the overall natural heat dissipation efficiency without increasing the size or cost of the heatsink. This work provides practical guidance for the thermal design of various types of solar inverter in the industry.
Modern photovoltaic systems rely on inverters to convert direct current (DC) from solar panels into alternating current (AC) for grid connection. Among the many types of solar inverter, single-phase grid-connected inverters are widely adopted in residential and small commercial installations due to their simplicity and cost-effectiveness. However, the trend toward higher power density and compact enclosure design poses significant thermal challenges. Heat generated by power semiconductors, magnetic components, and passive elements must be efficiently dissipated to prevent premature failure. Natural convection cooling is often preferred for low-power inverters (typically below 10 kW) because it eliminates the need for fans, reducing maintenance and improving reliability. Nevertheless, the absence of forced airflow makes thermal design more demanding, as the heat transfer coefficients are low and the internal temperature gradients can be large.
Our research targets a specific 6 kW single-phase grid-connected inverter, representing a typical product among types of solar inverter. The inverter consists of an aluminum heatsink, a sealed enclosure with an IP66 rating, a power PCB with IGBTs, MOSFETs, inductors, capacitors, and relays. The heatsink and top cover form a closed cavity without any internal fan. Heat is transferred from the power devices to the heatsink base via conduction through thermal interface materials, and then dissipated to the ambient air by natural convection and radiation. The challenge is to keep the temperature of electrolytic capacitors and relays below their rated limits (typically 85°C) while also managing the temperature of IGBTs and inductors that can reach up to 130–150°C. We need a reliable simulation model to explore cost-effective improvements, such as adding thermally conductive pads to critical hot spots.
Our approach begins with building a detailed geometry model in FLOTHERM. The inverter dimensions are 362 mm × 392 mm × 82 mm for the top cover, 335 mm × 345 mm × 62 mm for the heatsink, 47 mm × 342 mm × 65 mm for the inductor assembly, and 326 mm × 385 mm × 66 mm for the power PCB. The top cover is modeled as two cuboids (box and lid), the heatsink uses the specialized ‘Heat Sink’ smart part with defined base and fin geometries, and the PCB is represented as a ‘PCB’ smart part with copper layers. IGBTs are modeled as assemblies: a heat generation layer, a copper layer, a DBC layer, another copper layer, and a baseplate layer. Electrolytic capacitors are modeled as two coaxial cylinders (electrolyte and case), and relays are composed of internal copper block, FR4 block, and enclosure cuboid. Inductors, which have complex winding structures, are simplified as uniform cylinders with equivalent thermal properties. The inductor assembly includes a plastic housing, four separate inductors (two boost and two inverter inductors), and potting compound that fills the housing to half the inductor height.
The heat conduction equation under steady-state conditions is given by Equation (1):
$$
\frac{\partial^2 t}{\partial x^2} + \frac{\partial^2 t}{\partial y^2} + \frac{\partial^2 t}{\partial z^2} + \frac{q_v}{\lambda} = 0
$$
where \(t\) is temperature, \(q_v\) is the volumetric heat generation rate, and \(\lambda\) is the thermal conductivity. Fourier’s law relates heat flux \(q\) to temperature gradient:
$$
q = -\lambda \left( i \frac{\partial t}{\partial x} + j \frac{\partial t}{\partial y} + k \frac{\partial t}{\partial z} \right)
$$
For numerical solution, we discretize the domain using a finite volume method. A two-dimensional steady-state heat conduction problem in a rectangular region (Figure 3 in the original paper) serves as an analogy. For a node \((i,j)\), the second-order central difference approximation yields:
$$
\frac{t_{i+1,j} – 2 t_{i,j} + t_{i-1,j}}{\Delta x^2} + \frac{t_{i,j+1} – 2 t_{i,j} + t_{i,j-1}}{\Delta y^2} = 0
$$
Solving for \(t_{i,j}\):
$$
t_{i,j} = \frac{\Delta y^2}{2(\Delta x^2 + \Delta y^2)} (t_{i+1,j} + t_{i-1,j}) + \frac{\Delta x^2}{2(\Delta x^2 + \Delta y^2)} (t_{i,j+1} + t_{i,j-1})
$$
We iterate until the maximum temperature difference between successive iterations is less than 0.05°C.
The simulation environment is set to an ambient temperature of 45°C, consistent with the test chamber conditions required by the standard GB/T 37408-2019. We apply a natural convection coefficient of 5 W/(m²·K) on external surfaces. The top cover, coated with paint, has a surface emissivity of 0.38. The heatsink, with an anodized surface, has an emissivity of 0.9. Gravity is directed downward along the vertical mounting orientation, and a 30 mm thick collapsed cuboid wall is placed behind the inverter to represent the installation wall.
The key component parameters, including power losses, material properties, and contact thermal resistances, are listed in the following tables. Table 1 summarizes the losses of major components, with reference to the component numbers shown in Figure 2 of the original paper (we do not reference the figure number in text but provide the table).
| Component Number | Component Type/Model | Power Loss (W) |
|---|---|---|
| 1–2 | TGAN40N65F2DS | 16.7 |
| 3–4 | G3S06530PM | 11.5 |
| 5 | Common mode choke | 1.65 |
| 6 | PV Film CAP | 1.0 |
| 7 | BUS capacitor | 0.2 |
| 8–11 | TGAN40N65F2DS | 19.4 |
| 12–13 | TGAN40N65F2DS | 30.5 |
| 14–17 | INV relay | 3.2 |
| 18–19 | Boost power inductor | 10.5 |
| 20–21 | Inverter power inductor | 14.0 |
| 22 | AC output common mode choke | 4.0 |
| Component | Density (kg/m³) | Specific Heat (J/(kg·K)) | Thermal Conductivity (W/(m·K)) |
|---|---|---|---|
| Heatsink | 2700 | 900 | 201 |
| Enclosure (chassis) | 2720 | 890 | 138 |
| Top cover | 2650 | 880 | 96 |
| Electrolytic capacitor (X/Z/Y) | 2702 | 903 | 5 / 237 / 5 |
| Ceramic thermal pad | 3960 | 850 | 30 |
| Thermal grease | 1300 | 1200 | 2.7 |
| IGBT DBC layer (X/Z/Y) | 3900 | 1150 | 22 / 5000 / 22 |
| IGBT Cu baseplate (X/Z/Y) | 8940 | 386 | 398 / 5000 / 398 |
| Thermal pad (silicone) | 1200 | 1660 | 1.5 |
| Inductor housing | 2720 | 890 | 138 |
| Contact Interface | Thermal Resistance (m²·K/W) |
|---|---|
| Inductor – Enclosure | 5×10⁻⁴ |
| Cu baseplate – Ceramic pad | 4×10⁻⁵ |
| Ceramic pad – Heatsink | 4×10⁻⁵ |
| Potting compound – Inductor housing | 2×10⁻⁴ |
After setting up the model, we perform a steady-state simulation. The computed temperature distribution is shown in Figure 4 (original paper). High temperatures are concentrated on the right-side IGBTs (components 8–10) and the inverter inductors (20–21), corresponding to their higher power losses. However, all temperatures remain within the rated safe limits of the devices (130–150°C). The electrolytic capacitor (component 7) and relays (14–17) exhibit temperatures below 85°C in the baseline simulation, but they are uncomfortably close to the critical limit. In the actual test, they exceeded the 85°C threshold, indicating a need for improvement.
We therefore investigate the addition of thermally conductive silicone pads. Three scenarios are simulated: no pad, a 1.0 W/(m·K) pad, a 1.5 W/(m·K) pad, and a 2.0 W/(m·K) pad. The temperature changes of critical components are compared. Table 4 and Figure 5 (original) illustrate the results. The 1.5 W/(m·K) pad provides a good balance: it reduces the relay temperature by 28°C and the electrolytic capacitor average temperature by 13°C, while all other components stay optimal. The 1.0 W/(m·K) pad is insufficient, and the 2.0 W/(m·K) pad yields only marginal further improvement at a higher cost.
| Component | Without Pad (°C) | 1.0 W/(m·K) Pad (°C) | 1.5 W/(m·K) Pad (°C) | 2.0 W/(m·K) Pad (°C) |
|---|---|---|---|---|
| Relay (No. 14) | 92 | 72 | 64 | 61 |
| Electrolytic Capacitor (No. 7) | 84 | 76 | 71 | 69 |
| IGBT (No.10) | 118 | 116 | 115 | 114 |
| Inverter Inductor (No.20) | 105 | 104 | 103 | 103 |
We conduct experimental validation in a temperature chamber set to 45°C. The inverter operates at 230 V AC output, and we measure steady-state temperatures using thermocouples. The test results match the simulation well: for power semiconductors, the error is within ±3°C, and for other components, within ±6°C. The temperature rise curve (Figure 6 in original) shows that the inverter reaches thermal equilibrium within one hour, with less than 1°C variation thereafter. This confirms the high fidelity of our FLOTHERM model.
In conclusion, we have developed a reliable FLOTHERM-based simulation model for natural heat dissipation in single-phase grid-connected inverters, which is one of the most popular types of solar inverter. The model accurately predicts component temperatures within an acceptable error range. By adding a 1.5 W/(m·K) thermally conductive silicone pad to the surface of temperature-sensitive components, we effectively lower the hotspot temperatures without increasing the heatsink size or overall cost. This improvement enhances the reliability and lifespan of the inverter. The methodology can be extended to other types of solar inverter such as three-phase inverters, off-grid inverters, or battery-integrated inverters, providing a general framework for thermal optimization under natural convection. Future work may include transient thermal analysis and the consideration of different ambient conditions to further validate the robustness of the design.
Our findings emphasize the importance of careful thermal interface material selection in the design of modern types of solar inverter. The combination of simulation and experiment reduces development cycles and ensures that products meet stringent thermal requirements. As the market demands higher power densities and longer warranties, such thermal management strategies become increasingly critical. We believe that the approach presented here will aid engineers in designing more efficient and durable inverters for the growing photovoltaic industry.