The heat dissipation design of solar inverters is the core link to reduce power loss, improve operational efficiency and reliability. When the inverter is working, the losses of power devices (such as IGBT, MOSFET) and passive components (transformers, inductors, etc.) will be converted into heat. If the heat accumulates and causes the temperature to be too high, it will not only increase the conduction/switching losses of the device (such as the on resistance of semiconductor devices increasing with temperature), but also may cause device failure. The heat dissipation design needs to start from the full path of "reducing heat generation → optimizing heat conduction → enhancing heat dissipation", and optimize the system based on device characteristics, structural layout, and environmental conditions.
1、 Reduce heat from the source: reduce inherent losses of devices
The underlying logic of heat dissipation design is "less heat generation" - through device selection and parameter optimization, heat generation is reduced from the source, directly reducing heat dissipation pressure.
1. Power devices: prioritize low loss types
The core heat source of an inverter is power semiconductor devices (IGBT, MOSFET, diode, etc.), and their losses (conduction loss+switching loss) account for more than 60% of the total losses. Choosing low loss devices can significantly reduce heat generation:
SiC/GaN wide bandgap devices replace traditional Si devices: Compared with silicon-based IGBTs, the on resistance (R {ds (on)}) of SiC MOSFETs changes more smoothly with temperature, and the switching loss (on/off loss) can be reduced by more than 50% (such as 650V SiC MOSFET switching loss is only 1/3 of the same specification Si MOSFET), and the high temperature resistance is stronger (the upper limit of junction temperature can reach over 200 ℃, while Si devices are usually 150 ℃), fundamentally reducing heat generation;
Optimize device parameter matching: Select devices with appropriate rated current and withstand voltage according to the inverter power level to avoid "overusing" (devices that exceed specifications may increase parasitic parameters and potentially increase losses) or "small horses pulling big cars" (devices that operate at full load for a long time leading to a surge in losses).
2. Passive components: low loss materials and structural design
The iron loss (core loss) and copper loss (winding loss) of passive components such as transformers and inductors account for about 30% of the total loss, and it is necessary to reduce heat generation through material and structural optimization:
Transformer: using low loss magnetic core materials (such as nanocrystalline alloys, amorphous alloys, with iron loss only 1/5-1/10 of traditional silicon steel sheets); Replace single wire with multi stranded enameled wire (Litz wire) in the winding to reduce copper loss caused by skin effect and proximity effect; Optimize the air gap of the magnetic core and the ratio of winding turns to reduce leakage inductance and eddy current losses;
Inductance: Choose high magnetic permeability and low loss magnetic powder cores (such as iron silicon aluminum magnetic powder cores) to reduce hysteresis losses; The winding adopts flat wire or multi stranded wire to increase the conductive cross-sectional area and reduce DC resistance loss;
Capacitor: Choose thin film capacitors or ceramic capacitors with low ESR (equivalent series resistance) to reduce losses at high frequencies (ESR losses are proportional to the square of the frequency).
2、 Optimize heat conduction: shorten the path from heat source to radiator
The heat generated by the device needs to be conducted to the heat sink, and the key is the "short, wide, and low resistance" path - which requires layout design, selection of thermal conductive materials, and contact optimization to reduce thermal resistance.
1. Layout design: make the heat source "close" to the radiator
The layout of PCB and components directly determines the length of the thermal conduction path, and the core principle is to "keep the heat source close to the heat sink and avoid the concentration of hot spots":
Centralized arrangement of power devices: IGBT, diode and other main heating devices need to be directly installed on the heat sink substrate (such as fixed by copper pillars or screws), reducing the indirect path of "device → PCB → heat sink" (PCB copper skin has high thermal resistance, direct contact can reduce thermal resistance by more than 50%);
Avoid local hotspots: Heating devices (such as transformers and inductors) and power devices should be arranged in a dispersed manner (with a spacing of ≥ 2cm) to avoid the accumulation of heat and the formation of local high temperature zones (such as the surface temperature of transformers often reaching 80 ℃ or above, and if they are adjacent to IGBTs, it may cause the junction temperature of IGBTs to rise by 10-15 ℃);
PCB auxiliary heat dissipation: The PCB copper skin of the power circuit adopts a "thick copper design" (copper thickness ≥ 2oz, i.e. 70 μ m) to increase the thermal conductivity cross-sectional area; Set up "heat dissipation vias" at the device pads to conduct heat from the top layer to the bottom layer through the vias (the bottom layer can be equipped with heat dissipation fins or use the metal shell of the chassis for heat dissipation), and use the overall area of the PCB to diffuse heat.
2. Thermal conductive material: Fill gaps to reduce contact thermal resistance
There is a small gap between the device and the heat sink (with an air thermal conductivity of only 0.026W/(m · K), which is the main source of thermal resistance), which needs to be filled with thermal conductive materials and insulated (in some scenarios):
Insulation and thermal conductive materials (when electrical isolation is required):
Ceramic chips (Al ₂ O Ⅲ, AlN): AlN ceramics have a thermal conductivity of 150-200W/(m · K) (Al ₂ O Ⅲ is about 30-40W/(m · K)), high insulation strength (≥ 20kV/mm), and are suitable for high-power scenarios (such as between IGBT and radiator in inverters above 10kW), but the cost is relatively high;
Thermal conductive silicone sheet: thermal conductivity of 1-5W/(m · K), good flexibility (can adapt to uneven device surfaces), low cost, suitable for low to medium power scenarios (such as between low-power MOSFETs and heat sinks), pay attention to selecting models with a thickness of ≤ 0.5mm (increasing thickness will cause linear increase in thermal resistance);
Non insulating thermal conductive material (when no isolation is required):
Thermal paste (silicone grease): with a thermal conductivity of 2-8W/(m · K), used to fill the micro gaps between devices and heat sinks (such as the contact between CPU and heat sink). The application should be uniform (thickness ≤ 0.1mm), and excessive use may affect thermal conductivity due to the thermal resistance of the silicone grease itself;
Thermal conductive gel: it is easier to apply than silicone grease, and it does not lose and solidify, so it is suitable for long-term operation scenarios (such as outdoor inverter).
3. Contact optimization: Ensure a "tight fit"
The effectiveness of thermal conductive materials depends on good contact between the device and the heat sink, and needs to be optimized through mechanical structure:
Control contact pressure: When tightening the device and heat sink with screws, apply pressure according to the recommended torque in the device manual (such as 2-3N · m for IGBT modules). Insufficient pressure can cause a large contact gap, while excessive pressure can damage the device or heat sink;
Surface treatment of heat sink: The surface of the heat sink in contact with the device needs to undergo "precision milling" treatment (roughness Ra ≤ 1.6 μ m) to reduce surface roughness (excessive roughness can cause the thermal conductive material to be unable to completely fill the gaps);
Avoid intermediate layer redundancy: reduce the multi-layer structure of "device → thermal conductive material → PCB → thermal conductive material → heat sink", and prioritize the direct path of "device → thermal conductive material → heat sink" (such as directly attaching the IGBT module substrate to the heat sink, eliminating PCB transition).
3、 Strengthening heat dissipation: designing efficient heat dissipation terminals
After heat is conducted to the radiator, it needs to be dissipated into the environment through convection (air/liquid flow), radiation (thermal radiation), and conduction (contact with the environment). Suitable cooling methods (natural cooling, forced air cooling, liquid cooling) should be selected based on the power level and application scenario of the inverter.
1. Natural cooling: Low power scenarios (≤ 5kW)
Natural cooling is achieved through the natural convection of radiator fins (air flows due to temperature differences) and thermal radiation, without the need for additional equipment. It is suitable for household low-power inverters (1-5kW). The core of the design is to increase the heat dissipation area and optimize the fin structure
Heat sink selection: Aluminum profile heat sink (aluminum thermal conductivity of 200W/(m · K), low cost) is used, and the fin design needs to balance "area" and "convection efficiency":
Fin height: usually 50-100mm (too high can cause a large temperature difference between the top and bottom of the fins, making it difficult to transfer heat to the top);
Fin spacing: 8-15mm (too small spacing will hinder air flow, too large spacing will reduce the number of fins per unit volume and decrease the area);
Fin thickness: 1.5-2mm (being too thin can cause deformation, while being too thick can increase the weight of the radiator and limit thermal conductivity gain);
Enhance radiative heat dissipation: The surface of the radiator is treated with "anodizing" (black oxide layer) to increase the emissivity (ε) from 0.1 (untreated aluminum) to 0.8-0.9, and the proportion of radiative heat dissipation can be increased from 10% to over 30% (radiative heat dissipation power is proportional to ε and temperature to the fourth power);
Chassis coordination: Ventilation holes should be left at the bottom/side of the inverter chassis (dust-proof and waterproof, such as installing dust-proof nets and waterproof eaves), forming a natural air duct with "cold air entering from the bottom and hot air exiting from the top" (the density of hot air is low and will naturally rise and be discharged).
2. Forced air cooling: medium power scenario (5-50kW)
When natural cooling cannot meet the heat dissipation requirements (such as device junction temperature exceeding 80 ℃), it is necessary to use a fan to force air flow to improve convective heat dissipation efficiency (forced convective heat dissipation coefficient is 5-10 times that of natural convection). The design core is "matching air volume and pressure, optimizing air ducts":
Fan selection: Select the fan based on the resistance of the radiator and the required air volume:
Airflow: Estimated based on "1-2m ³/h airflow required for every 100W loss" (e.g. 10kW inverter loss is about 300W, requiring 3-6m ³/h airflow);
Wind pressure: It is necessary to overcome the resistance of radiator fins (the denser the fins and the longer the path, the greater the resistance, usually requiring 5-15Pa wind pressure);
Type: Axial fan (with high air volume and low air pressure, suitable for short air ducts) or centrifugal fan (with low air pressure and high air volume, suitable for complex air ducts), with priority given to variable frequency fans (which can adjust speed according to temperature to reduce noise and power consumption);
Air duct design: Avoid "hot air backflow" and ensure that the airflow flows "efficiently through the radiator in one direction":
Air duct path: Cold air is drawn in from one side of the chassis (low temperature area), pressurized by a fan, and flows through the radiator fins. Hot air is discharged from the other side (away from the heat source area);
Isolation design: Use deflectors to isolate the "cold air zone" from the "hot air zone" (such as adding baffles between the radiator and other components), to prevent hot air from being sucked back in;
Location of radiator: Arrange the radiator fins along the airflow direction (with fin gaps parallel to the airflow direction) to reduce airflow impact losses.
3. Liquid cooling heat dissipation: high-power scenarios (≥ 50kW)
High power inverters (such as centralized inverters, above 100kW) generate a large amount of heat (single unit loss can reach several kW), and air cooling is limited by the thermal conductivity of air (air thermal conductivity coefficient of 0.026W/(m · K)), requiring liquid cooling (coolant thermal conductivity coefficient of 0.5-0.8W/(m · K), high specific heat capacity). Common types include "cold plate liquid cooling" and "immersion liquid cooling":
Cold plate liquid cooling: The principle is to tightly attach a "metal cold plate with internal coolant flow channels" to heating devices (such as IGBT modules, transformers), and the coolant (water+ethylene glycol, antifreeze and anti-corrosion) flows through the cold plate to take away heat. Design focus:
Cold plate channel: using "serpentine channel" or "microchannel" (channel width 1-3mm) to ensure sufficient contact between the coolant and the cold plate (flow rate 1-2m/s, avoiding uneven heat dissipation caused by low local flow rate);
Flow control: Designed according to the principle of "0.5-1L/min flow rate required per kW loss" (for example, a 100kW inverter with 5kW loss requires 2.5-5L/min flow rate), the flow rate is controlled by a water pump (brushless water pump with long service life);
Cooling terminal: After the coolant absorbs heat, it needs to be dissipated into the environment through a "heat exchanger" (such as a finned heat exchanger+fan) (closed-loop system);
Immersion liquid cooling: The core components of the inverter (power module, transformer) are immersed in a "non-conductive dielectric coolant" (such as mineral oil, fluorinated liquid), and heat is dissipated through convection and phase change of the coolant (some of the coolant evaporates and absorbs heat), with an efficiency 2-3 times that of a cold plate. Design focus:
Coolant selection: Choose a model with high boiling point (≥ 100 ℃), low viscosity (to avoid high flow resistance), and non corrosive properties (such as 3M fluorine solution);
Chassis sealing: It needs to be completely sealed (to prevent coolant leakage) and equipped with a condensation recovery device (to condense and reflux evaporated coolant).
4、 Simulation and verification: Ensure effective heat dissipation design
The heat dissipation design needs to be optimized through simulation and verified through actual testing to avoid "empty talk":
Thermal simulation: Use ANSYS Icepak, Flotherm and other software to establish a model, input device losses (such as IGBT conduction loss of 0.5W, switch loss of 0.3W), thermal material parameters (thermal conductivity, thickness), heat sink structure, ambient temperature (such as outdoor 40 ℃), simulate temperature distribution:
Pay close attention to the "hot spot temperature" (such as IGBT junction temperature, transformer winding temperature), and ensure that the junction temperature is ≤ the rated junction temperature of the device (such as SiC devices ≤ 175 ℃);
Optimization direction: If the temperature in a certain area is too high, the number of radiator fins can be increased, device layout can be adjusted, and materials with higher thermal conductivity can be replaced;
Experimental verification: After making the prototype, use an infrared thermal imager (such as FLIR) to capture the surface temperature, and use a thermocouple (attached to the device surface) to measure the actual temperature. Compare the simulation results:
If the measured temperature is higher than the simulation by more than 5 ℃, it is necessary to check whether the thermal conductive material is installed in place (such as whether it is not compacted) and whether there are processing errors in the radiator (such as small fin spacing).
summary
The heat dissipation design of solar inverters requires "multi link collaboration": from reducing heat generation through device selection, optimizing conduction paths through layout and thermal conductive materials, enhancing heat dissipation through radiators and heat dissipation methods, and finally verifying through simulation and actual testing. The core goal is to control the device temperature within a reasonable range (such as Si devices ≤ 125 ℃, SiC devices ≤ 175 ℃), which can not only reduce the additional losses caused by temperature rise (such as semiconductor losses increasing by 5% -10% for every 10 ℃ temperature rise), but also avoid device overheating and failure, thereby improving the efficiency and lifespan of the inverter.
