With growing emphasis on motor efficiency across various applications, the demand for high-efficiency drives is increasingly important. These motor applications—such as those used in electric vehicles, pumps, and fans—are driving the adoption of motor drives to reduce overall cost and energy consumption. Therefore, selecting efficient devices for both the motor drive, and ensuring each component meets the specific application requirements, has become even more critical.
Insulated-gate bipolar transistors (IGBTs) are a proven switching device solution that meets the higher voltage, higher current, and low-frequency requirements of motor drive applications. Since these motors need to maximize efficiency and often require a robust safe operating area (SOA) and short-circuit ratings, IGBTs with co-packaged fast recovery diodes (FRDs) are recommended.
IGBTs in Motor Drive Design
Motor drive designs are typically powered by an AC mains supply and controlled based on user input (Figure 1). Power factor correction (PFC) rectifiers are implemented using IGBTs, similar to those used in uninterruptible power supplies (UPS). The motor braking circuit consists of IGBTs that dissipate motor power or transfer excess energy back to the AC input during regenerative braking when the motor is stopped. The motor drive inverter converts the DC energy stored in the capacitors into an AC waveform at a specified voltage and frequency to control the motor at the desired speed and torque.
Figure 1: Illustration of a typical motor drive design with a power factor correction (PFC) input rectifier.
To keep IGBTs within their SOA ratings in various parts of a motor drive design, heat must be effectively removed from the transistor package. For this reason, designers should evaluate IGBTs in smaller packages with enhanced thermal dissipation capabilities. For example, IGBTs are available in the thermally efficient TO-247 package, which provides effective heat dissipation for power losses caused by switching transients and forward conduction in the IGBT and FRD.
In motor control applications, designers need to consider the impact of power dissipation on system performance under high ambient temperatures and low or no airflow. Furthermore, using IGBTs optimized for high efficiency means they generate less heat, reducing the cooling requirement. Additional benefits of smaller IGBTs include lower costs and simplified thermal management designs.
Switching and Conduction Performance
The IGBT's device structure determines its efficiency and performance. Advanced IGBTs with an asymmetric structure help improve conduction losses and switching speeds in motor control applications. A key feature of this structure is the field-stop layer, formed by an n+ buffer layer inserted below the n-drift region and above the lower p-doped layer. This buffer layer supports the electric field and allows for a thinner n-drift region, which significantly reduces conduction losses.
The trade-off between switching losses (Eoff) and conduction losses (Vce(sat)) is shown in Figure 2. This illustrates why understanding both low-frequency and high-frequency system requirements is essential for selecting the appropriate IGBT. In many new applications, IGBTs using advanced trench gate field-stop (TGFS) technology offer higher cell density, enabling improved Vce(sat)/Eoff performance.
IGBTs with advanced TGFS technology, such as those from Bourns, may experience high stress, leading to transient short-circuit conditions. These conditions can cause a short-circuit path from the DC bus to ground (as a shoot-through current) or between motor phases or to ground. Therefore, the selected IGBT must be able to withstand these faults within the time required for fault detection in the application.
Electric motors can draw very high current levels for relatively long periods (milliseconds to seconds); however, IGBTs typically used in motor drive inverters have very short (microsecond-range) short-circuit withstand times. To address this, suppliers such as Bourns offer specific IGBT models with 10µs short-circuit withstand capability.
IGBTs generally provide robust short-circuit withstand capability in the range of 5µs. The benefit of high short-circuit current tolerance and shorter withstand times is reduced conduction losses, which also helps lower overall BOM costs. Advances in IGBT technology can compensate for some of the limitations in short-circuit performance. For example, some of the latest IGBTs offer higher transconductance and lower thermal resistance, resulting in lower conduction losses and higher efficiency—even with reduced short-circuit withstand times.
Evaluating IGBT Trade-offs
Selecting IGBTs with high switching frequencies to reduce switching losses typically results in higher conduction losses. Furthermore, higher conduction losses lead to increased power dissipation, necessitating larger heat sinks. This raises system cost and occupies more space. Conversely, IGBTs with lower conduction losses are more expensive. Devices with reduced losses operate efficiently at lower frequencies, but often have diminished short-circuit withstand capability. This trade-off is illustrated in Figure 3.
Figure 3: Trade-offs between conduction losses, switching losses, and short-circuit withstand capability in motor control designs, with reference to the associated safe operating area.
Safe Operating Area (SOA) Considerations
Selecting an IGBT that operates near its maximum current and voltage ratings requires careful attention to maintaining these parameters within datasheet specifications. The primary goal is to keep the collector current below the maximum rating while ensuring the collector-to-emitter voltage remains within the specified limit.
When operating within the forward-biased safe operating area (FBSOA), the maximum pulsed collector current can be achieved depending on pulse width and thermal impedance. The FBSOA defines the maximum saturation collector-emitter voltage. Under reverse bias conditions within the reverse-biased safe operating area (RBSOA), the maximum collector current—typically applicable to inductive loads—is limited by the peak collector-emitter voltage during turn-off. To protect the fast recovery diode (FRD) at the maximum junction temperature, these limits must be strictly adhered to.
Conclusion
Using IGBTs in motor control inverters helps designers achieve lower system costs due to their smaller chip size and higher current density. Designers should also seek advanced IGBT discretes that support high-temperature operation and provide enhanced thermal performance. The result is a highly efficient switching solution for motor control applications.
As mentioned, optimizing efficiency requires balancing the IGBT’s conduction and switching losses. It is also essential to tailor the design to the specific motor type and application requirements. For most motor control applications, 600 V/650 V IGBTs using trench gate field-stop (TGFS) technology with co-packaged FRDs in a compact TO-247 package are considered an ideal solution. The star product of SHYSEMI can fully meet the above requirements. Compared with the previous generation of planar IGBTs, the IGBT devices of SHYSEMI offer higher thermal performance, lower VCE(sat), and higher efficiency due to lower total power consumption, while also providing high reliability.