Manufacturers are continually increasing adoption of electronically controlled motor drives in appliance applications such as compressors for refrigerators and air-conditioning to meet new energy efficiency regulations. The power semiconductor of choice in these systems is the IGBT, either in discrete form or in an integrated module. In these applications the switching frequencies are usually kept below 10kHz to increase efficiency, moreover trapezoidal modulation is often used to further reduce switching losses. This paper describes a new generation of IGBTs that uses an alternative approach to deliver benchmark VCE(ON) with zero temperature coefficient for lowest conduction losses possible, together with soft switching transients to reduce EMI.
Evolution of IGBT structures
The basic structure of an IGBT is composed of 2 elements:
• The top structure of the emitter where the gate is formed is very similar to a MOSFET, and has followed similar trends in increased densities, including the transition to trench gate structure in the last few years.
• The bottom structure of the collector is where the minority carriers are injected which allow the IGBT to exhibit very low VCE(ON) for its current and voltage rating. The main structures are punch through (PT), non-punch through (NPT) and Field-Stop
IGBTs are complex structures where elements interact and lead to radically different behaviors. To facilitate motor drive performance, International Rectifier has developed a new generation of IGBT devices, gen 7F, optimized to work in motor drive applications below 10kHz addressing the need for delivering lowest VCE(ON) possible, while limiting the switching speed for a balanced operation.
Best in-class performance at 5kHz
For motor drives running at 5kHz, the overall inverter losses are dominated by the conduction losses, so reducing VCE(ON) reduces the overall losses even if switching losses increase. Figure 2 compares the split between conduction and switching losses in a 5kHz motor drive inverter between trench gen 6 IGBTs and the new generation of optimized devices. This comparison is undertaken for devices of the same size but driven with different gate resistors as to achieve a maximum dv/dt at both turn-ON and turn-OFF of 5kV/µs.
The new generation of IGBTs not only achieves best-in-class VCE(ON), this low VCE(ON) has a zero temperature coefficient, which guarantees efficient performance throughout the temperature range. Table 1 compares the datasheet values of VCE(ON) between a gen 6 trench IGBT and the new generation IGBTs with the same die size. At high temperature, the zero temperature coefficient of the gen 7F generation IGBTs means the new device VCE(ON) is almost 400mV lower than the previous generation. Improved process control in the manufacturing process also reduced the maximum versus typical spread, resulting in lower maximum value which is very useful for worst case calculations.
As seen in the previous section, in applications for which this platform is optimized, conduction losses have a dominant effect when compared to the switching losses. This allows the device speed to be slowed and achieve smooth transitions during hard switching by reducing dV/dt and dI/dt and utilizing copack diodes with optimized reverse recovery. This not only improves EMI performance but can also help in reducing ground currents through the compressor which is a common issue in air-conditioning applications. The various switching transients are compared in Table 2 where it can be seen that the new generation of devices offer softer switching transients.
The new generation of IGBT also contributes to the overall system ruggedness in 3 major areas. Firstly the high gain of this generation allows these devices to be able to sustain significant over-currents which is particularly useful when starting up some types of compressors which require high starting torque. At the same time, this high gain means the saturation current reached during a short circuit event is high enough to allow designers a convenient way to set the protection thresholds for simple but reliable shunt resistor based protection circuits, even when taking threshold and power supply voltage variations into consideration. Finally, even with high saturation currents, these devices also offer short circuit rating which allows enough time for the system to safely turn off before any damage occurs to the IGBT.
This new generation of IGBTs is optimized for motor drives operating below 10 kHz such as compressors for refrigerators and air-conditioning. These devices show very low VCE(ON) with zero temperature coefficient for lowest conduction losses possible, together with soft switching transients to reduce EMI. They are also short-circuit rated for motor drive applications.