- Necessity of IGBT Cooling
- Principle of IGBT Heat Generation
The fundamental reason for heat generation in IGBT (Insulated Gate Bipolar Transistor) devices is power loss during operation. Almost all of this power loss is converted into heat, causing the chip junction temperature to rise. Power losses are mainly divided into two categories: conduction loss and switching loss.
1.1 Conduction Loss
Conduction loss refers to the loss generated when the IGBT is fully turned on (saturated conduction) due to the voltage drop (Vce(sat)) between the collector and emitter.
Principle: When the IGBT is on, it behaves like a resistor, and current flowing through it generates a voltage drop Vce(sat), producing heat. Vce(sat) is not constant; it varies with collector current (Ic) and junction temperature (Tj) (usually provided in datasheets).
Formula: For a square-wave current with conduction time ton, the average conduction power loss Pcond is:
Pcond=Vce(sat) ×Ic×D
where D is the duty cycle, D = ton/T (T is the switching period).
1.2 Switching Loss
Switching loss refers to the loss generated during the transient process of turn-on and turn-off when the IGBT simultaneously withstands high voltage and large current.
Principle: During turn-on, current rises from 0 to Ic, and voltage drops from VDC bus voltage Vdc to Vce(sat); during turn-off, the opposite occurs. In the overlap period of voltage and current, significant instantaneous power (P=V×I) is generated. Integrating this power gives the switching energy loss.
Formula: Switching loss depends on the energy per switching event and the switching frequency.
Turn-on energy loss is Eon, turn-off energy loss is Eoff. The total average switching power loss Psw is:
Psw=(Eon+Eoff)×fsw
where fsw is the switching frequency. Eon and Eoff values can be found in datasheets under specified conditions of bus voltage, collector current, and junction temperature; adjustments are needed for real applications.
1.3 Total Power Loss
The total power loss Ptotal of an IGBT is the sum of conduction loss and switching loss:
Ptotal =Pcond+Psw
This Ptotal is the thermal power generated within the IGBT chip.
1.4 Example of Thermal Calculation
Let us calculate the junction temperature of an IGBT in a specific case.
Assumptions:
IGBT type: Infineon IGBT module, assumed model FF450R07ME4 (450 A, 1200 V), considering a single IGBT.
Operating conditions: DC bus voltage Vdc = 600 V, collector current Ic = 200 A, switching frequency fsw = 5 kHz, duty cycle D = 0.5, gate drive 15 V, typical switching performance assumed.
From datasheet (example values, actual values should be verified): At Tj = 125 °C, Ic = 200 A, Vce(sat) = 2.1 V. At Vdc = 600 V, Ic = 200 A, Tj = 125 °C, the turn‑on energy Eon = 15 mJ, the turn‑off energy Eoff = 10 mJ.
Calculation:
Step 1: Total power loss Ptotal
Conduction loss: Pcond = Vce(sat) × Ic × D = 2.1 V × 200 A × 0.5 = 210 W
Switching loss: Psw = (Eon + Eoff) × fsw = (15 mJ + 10 mJ) × 5000 Hz = 0.025 J × 5000 = 125 W
Total loss: Ptotal = Pcond + Psw = 210 W + 125 W = 335 W
This shows that IGBTs generate considerable heat during operation.
- Hazards of Insufficient Cooling
- Thermal breakdown: Reverse current of the PN junction increases exponentially with temperature, leading to possible thermal runaway.
• Parameter drift: Vce(sat) and switching time degrade as temperature rises, reducing efficiency.
• Solder joint failure: Thermal cycling causes cracks in the solder layer between IGBT die and substrate.
• System‑level risks: In EV inverters, overheating may cause power loss or failure; in PV inverters, overheating may lead to shutdown.
Based on the Arrhenius lifetime model, the lifetime of the same IGBT decreases as junction temperature increases (assuming 10,000 h at 100 °C). A common rule is that each 10 °C rise halves device lifetime.
Most industrial IGBTs specify maximum allowable Tj ≤ 175 °C. Therefore, heat sinks must ensure Tj < 150 °C, preferably ≤ 125 °C.
- Common Types of IGBT Heat Sinks
The following are commonly used, time‑proven heat sinks for IGBTs.
(1)Extrusion Heat Sink
Structure: Made from 6063 or 6061 aluminum alloy by extrusion, with continuous parallel fins. Surface anodizing enhances radiation and corrosion resistance.
Advantages: Lowest cost, mature process, lightweight, reliable, easy to install.
Disadvantages: Limited fin aspect ratio, thicker and fewer fins, lower heat dissipation efficiency (thermal conductivity ~180–200 W/m·K), performance affected by ambient temperature.
Applications: Low‑to‑medium power (<300 W per IGBT), such as inverters, UPS, welders, and PV inverters.
Example: Infineon FF300R06KE3 IGBT module (300 A, 600 V) often uses an extruded 6063‑T5 aluminum heat sink (250×150×50 mm). Under forced air cooling (5 m/s), it dissipates 200 W, maintaining Tj < 110 °C.
(2) Skived Fin Heat Sink
Structure: Thin fins are skived directly from a solid aluminum or copper base, forming a monolithic structure with no contact resistance.
Advantages: High aspect ratio, thin fins (<0.3 mm), dense fins, large effective area, excellent performance.
Disadvantages: Higher cost than extrusion, quality depends on process.
Applications: Medium‑to‑high power, compact designs, e.g., servo drives, telecom power, compact PV inverters, automotive drives.
Example: Mitsubishi CM300DU‑24NF (300 A, 1200 V) often uses a skived aluminum heat sink (120×80×30 mm). With a fan (0.5 m/s), it dissipates 150 W.
(3) Heat Pipe Heat Sink
Structure: Copper heat pipes embedded in baseplate transfer heat via phase‑change (evaporation/condensation).
Advantages: Very high effective conductivity (thousands of W/m·K), rapid thermal response, excellent uniformity, supports complex structures.
Disadvantages: Higher cost, possible long‑term degradation, orientation sensitive.
Applications: Compact, high‑power designs such as wind power converters, rail traction converters, servers.
Example: Huawei string PV inverters use 4×Φ6 mm heat pipes to transfer >500 W from IGBTs to fin array, managed with natural and forced convection.
(4) Liquid Cold Plate Heat Sink
Structure: IGBT module mounted on a plate with internal channels for coolant (water‑glycol mix), circulated by pump.
Advantages: Highest efficiency, very low thermal resistance, precise control, compact size.
Disadvantages: Most complex and costly, requires pumps, piping, coolant, maintenance.
Applications: Very high‑power systems such as EV traction inverters, HVAC compressors, rail converters, HVDC systems.
Example: In an EV inverter, a double‑sided liquid cold plate cools both surfaces of IGBT module, handling >750 A and several kW of heat.
III. Selection Guidelines for IGBT Heat Sinks
Recommendations based on power, cost, and space:
| IGBT Power Range (Single Module) | Recommended Heat Sink Type | Core Features & Advantages | Typical Application Scenarios |
| < 200W | Extrusion Heat Sink | Extremely low cost, mature manufacturing process, high reliability; compatible with natural convection or forced air cooling | Low-power inverters, UPS (Uninterruptible Power Supply), welding machines, servo drives |
| 200W – 600W | Skived Fin Heat Sink | Cost-effective option; provides larger heat dissipation area in compact space, with better performance than extrusion-processed heat sinks | Communication power supplies, photovoltaic inverters, high-end servo drives, auxiliary cooling for automotive electric drives |
| High-performance Extrusion Heat Sink (with forced air cooling) | Meets heat dissipation requirements by optimizing fin design and increasing air velocity; still maintains cost advantage | Industrial inverters, medium-power UPS | |
| 500W – 2000W
|
Heat Pipe Heat Sink | High-performance option; excellent temperature uniformity, fast thermal response, capable of long-distance heat transfer, and solves local hot spot issues | Wind power converters, high-speed rail auxiliary converters, high-end server power supplies |
| Skived Fin Heat Sink (with forced air cooling) | A compromise between cost and performance for applications with relatively loose space constraints | High-power photovoltaic inverters, industrial high-power power supplies | |
| > 1500W | Liquid Cold Plate Heat Sink | Optimal solution; extremely high heat dissipation efficiency, compact size, low noise, and precise temperature control | Main drives of new energy vehicles, traction converters for high-speed rail/rail transit, HVDC (High-Voltage Direct Current) transmission, ultra-high-power industrial converters |