Power Electronics

Trench IGBTs Improve Appliance Motor Drives

The device physics of DS trench IGBTs translate into higher efficiency in appliance-motor driver applications.

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The latest trench IGBTs optimized for appliance-motor controls display lower VCEON and lower switching loss than previous punch-through (PT) and non-punch-through (NPT) planar IGBTs. In white-goods applications, the depletion-stop (DS) trench IGBTs developed by International Rectifier reduce losses and deliver up to 60% more root-mean-square (rms) current than the previous generation of devices. This enables designers to reduce heatsink size by about 50% and achieve a 25% reduction in the size of integrated IGBT modules.

Though the development of new IGBT devices concerns primarily IGBT designers, an understanding of device structures and their electrical characterizations can provide the OEM designer with useful insights into these power semiconductors' in-circuit behaviors and can form the basis for comparing multiple, contending devices for a given application. This knowledge also can provide the OEM designer with the context to compare competing technologies.

The IGBT has evolved to become the switching device of choice in the industrial-drives market. The next big opportunity is the increasing penetration of motion controls into domestic white-goods products. Appliance designers are seeking convenient and cost-effective methods of variable-speed control of permanent magnet-motor drivers. In the case of water-consuming appliances, greater energy efficiency and low water consumption are emerging as the new figures of merit in the domestic appliance marketplace. To help meet these goals, IGBTs must evolve even further to deliver a subtly different set of characteristics for consumer applications. These include improved high-temperature performance and enhanced current handling to enable significant device and heatsink size reductions for discrete IGBTs and IGBT modules. An even better tradeoff between conduction and switching losses is also desirable to further improve energy-efficiency ratings in the end product.

The goal for the DS trench IGBT development was to improve upon switch performance for appliance applications. These applications require lower-voltage (600-V) devices suitable for 110-Vac or 220-Vac mains, saving cost, with improved switching performance without trading off the inherently low conduction-loss properties of the IGBTs.

Switching losses in IGBTs result from the slow removal of holes in the drift region after the gate-emitter voltage falls below the threshold voltage to turn the device off. The holes either recombine or are swept out by the voltage gradient, but until this process completes, the IGBT exhibits a current tail. PT IGBTs have a buffer layer adjacent to the drift region to absorb the remaining holes quickly during turn off. However, this enhanced switching performance is at the expense of higher VCEON in PT IGBTs. In addition, PT IGBTs do not exhibit the short-circuit withstand capability most motor-control applications require.

Moving from the planar PT and NPT IGBTs to a trench IGBT structure allows the device designers to regain the conduction performance lost by the PT IGBT. The trench structure achieves this by enabling high channel density, enhancing accumulation-layer injection and eliminating the parasitic JFET resistance inherent in the planar IGBT.

The next evolution is the DS trench IGBT, which aims at delivering IGBTs suitable for operation from the domestic mains in white-goods applications as well as a wider range of industrial drives and HVAC applications. The DS layer allows a thinner n-base with a higher transistor gain and switching speed. In addition, the optimized device has an efficient anode, which controls minority-carrier injection and reduces the current tail at turn-off, resulting in lower turn-off losses.

By using 600-V thin-wafer DS trench IGBTs, appliance designers can achieve improved efficiency while maintaining the smooth turn-off characteristics and robust safe operating area (SOA) that hard-switching applications require. Co-packaged with ultrafast soft-recovery diodes, these IGBTs have lower collector-to-emitter saturation voltage (VCEON) and total switching energy (ETS) than planar PT- and NPT-type IGBTs. The combination of low collector-to-emitter saturation voltage and total switching energy of trench IGBTs results in reduced power dissipation and greater power density in motion-control applications over a wide range of switching frequencies.

Fig. 1 shows the typical cross section of the DS trench IGBT device. The emitter N+ regions form adjacent to the trench, which is filled with gate oxide and polysilicon. A p-base diffusion and heavy P+ implant form the channel and base contacts, respectively. The deep trench extends below the p-base junction to form a gate-bias-induced channel between the N+ emitter and N- drift regions. The high channel density combines with an efficient anode that the P+ region forms on the backside of the wafer, to give a high carrier density in the drift region and a low forward-voltage drop.

The wafer thickness of these trench-IGBT devices is only 70 µm, which allows for a lightly doped anode. This helps reduce the total stored charge, thereby improving the switching performance of the device, especially at higher temperatures.

Device modeling aided the optimization of the physical transistor design and fabrication process technology. Characterization of finished devices served to verify the models.

The optimized device exhibits lower VCEON and lower switching loss than the previous PT and NPT IGBT devices. In practical applications, DS trench IGBTs reduce losses and deliver up to 60% more rms current than the previous generation of devices. This also leads to about a 50% reduction in heatsink size. The technology is suitable both for discrete IGBTs and for emerging families of smart power modules combining gate-driver circuits with 600-V IGBTs to ease appliance motor-control design. DS trench IGBT technology has been shown to enable size reduction greater than 25% for such integrated modules.

Electrical Characterization

Fig. 2 shows the VCEON versus forward current characteristic curves of planar PT, planar NPT and DS trench IGBTs at a junction temperature of 150°C. All measurements are done using an 8.2 mm2 IGBT. This chart demonstrates that the DS trench IGBT has the lowest VCEON for any given current density.

Fig. 3 shows the typical switching waveforms for turn-on and turn-off of the DS trench IGBT. As the figure shows, the DS trench IGBT exhibits much smoother turn-off waveform and a smaller current tail at turn-off. In addition, the turn-off voltage spike is small as well. All of this results in reduced EMI and better device ruggedness, which in turn allows the designer to achieve a better tradeoff between device electrical performance and ruggedness.

Fig. 4 shows the ETS versus current and RG characteristics of planar PT, planar NPT and DS trench IGBTs at a 150°C junction temperature. This chart demonstrates that the DS trench IGBT has the lowest ETS at any given operating point. The DS trench IGBT has approximately twice the current density and yet exhibits lower switching energy. So, effectively, the designer can use a smaller IGBT and expect to reduce the total switching losses in a given application.

Increasing Current and Efficiency

The reduced VCEON and ETS mean lower losses in the inverter. The lower VCEON reduces the conduction losses and the lower ETS reduces the switching losses. The combination of the lower VCEON and lower ETS reduces the losses across a wide range of switching frequencies. Fig. 5 shows the breakdown of the estimated conduction and switching losses in a motor-drive application for 14.4-mm2 planar PT, planar NPT and DS trench IGBTs. From that chart, it is evident that the trench IGBT has lower total losses in the 4-kHz to 20-kHz range. This reduction in losses allows the drive designer to reduce the heatsink size as well as the system size and cost.

Fig. 6 shows the estimated rms current versus switching-frequency characteristic for 14.4-mm2 planar PT, planar NPT and DS trench IGBTs. This chart shows that the DS trench IGBT delivers higher rms current in the 4-kHz to 20-kHz switching-frequency range.

OEM designers do not need to make changes to their gate-drive circuitry because the threshold voltages for these devices are in the same range as PT and NPT devices; they too have a 20-V gate rating. The trench IGBT also has lower total gate charge and shorter switching times, as well as shorter TDON+TRISE and TDOFF+TFALL timings. Therefore, drive circuits need not modify dead time or minimum-pulse-width settings. Finally, higher CGE/CRES ratios make trench IGBTs immune to spurious turn-on induced by high dV/dt values. This ensures robust performance even at high dV/dt switching conditions.

In terms of forward voltage, switching energy and rms current versus frequency characteristics, trench IGBTs offer improved performance compared to the planar IGBTs.

In addition, the DS trench IGBT has several features that make it more robust in motion-control applications. The smooth turn-off characteristics of the trench IGBTs under short-circuit conditions reduce the voltage spikes and stress on the IGBT.

Under short-circuit conditions, the trench IGBT does not exhibit gate overcharging and, consequently, the overcurrent spike. This significantly reduces the stress on the IGBT and improves the reliability of the inverter. The wider and squarer reverse-bias safe-operating-area (RBSOA) characteristics of the trench IGBT ensure safe switching under severe overload conditions, further improving the inverter's robustness (Fig. 7). This, along with a high peak turn-off capability and a good short-circuit rating, makes it easier to design robust inverters with trench IGBTs.

In addition to the improved RBSOA characteristics, DS trench IGBTs allow operating temperatures as high as 175°C, high peak turn-off capability, a positive collector-to-emitter saturation-voltage temperature coefficient, a short-circuit rating of 5 µs and high immunity to turn-on induced by excessive dV/dt.

By achieving these high-performance targets for 600-V rated power-switching applications, DS trench IGBTs remove thermal design as a significant barrier to the development of variable-speed motion controls in the cost-sensitive domestic-appliance market. The expense of complex heat management measures has been one factor holding back this important step forward. Combined with emerging configurable ICs for digital control and highly integrated analog signal ICs, discrete and modular IGBT solutions create a convenient platform for rapid configuration of variable-speed motor controllers offering high performance at low consumer-price points.

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