SoI Smartens Motor Management

Energy costs and ecological pressures are influencing the migration from traditional motor technologies to more efficient inverter-controlled motors. Josef Abels looks at the technical challenges.

Consumers want their domestic appliances to be cheap to run, quiet, vibration free and to have all the latest features and controls. When it comes to motor control these customer demands are more likely to met by the implementation of inverter-based designs.

This growing demand for inverter controls makes it even more important to simplify the relatively complex design processes, to reduce time to market and per-unit cost. Designers have several choices, including discrete MOSFETs or IGBTs with separate peripherals, including high-side and low-side circuitry, regulation and protection. A number of compact power modules are also entering the market, which can be quickly combined with function specific companion chips. But the ultimate goal of a single-chip solution has proved more elusive.

The challenges of handling high voltages in a compact monolithic IC have prevented such a convenient option from reaching the desks of power designers. But Silicon on Insulator (SoI) fabrication technology now allows high voltage transistors to be cost-effectively fabricated on the same substrate as low voltage circuitry such as decoder logic. This makes single-chip, high voltage motor driver ICs possible. It will also allow induction motors to permeate a wider range of applications, including low-cost products that previously would not warrant such sophisticated motor technology.

SOI ADVANTAGES
Conventional bulk silicon fabrication technology forces designers to use junction isolation to isolate individual circuit elements. But devices such as forward biased diodes and minority carrier injection devices cannot then be integrated on the same substrate as a high voltage power transistor such as an IGBT or power MOSFET.

By contrast, high voltage SoI fabrication allows the various circuit elements to be isolated by a thin layer of silicon dioxide, which is a perfect insulator. Not only does this allow almost any type of active device to be integrated alongside power transistors rated up to 500V or higher, but it also allows all devices to be packed together very densely on the substrate. Leakage currents are also greatly reduced, as are parasitic effects. By comparison, junction isolation techniques introduce parasitic capacitances and do not prevent leakage currents from flowing.

Toshiba's SoI power ICs have silicon dioxide trenches that isolate the IGBTs from low-voltage CMOS circuitry. These trenches can be made very thin, enabling compact and highly integrated power devices. In fact, the separation area between high voltage devices is reduced to around 0.7% of that required for a bulk silicon device of comparable current rating. This allows an equivalent current density of approximately 100A/cm2. The active circuit elements are also isolated from the silicon substrate by a thick layer of silicon dioxide. Figure 1 shows a cross-section of the SoI power IC structure.

SINGLE-CHIP INVERTERS
By exploiting the latest phase of Toshiba's SoI technology for high voltage applications, it is now possible to build cost-effective single chip inverters with built-in power outputs. These make it much easier to implement a variety of home appliance motor control applications in the 1-2A arena, including fan motor drivers for air-conditioning systems and water heaters, inverter refrigerators or inverter pumps.

SoI also benefits designers using power modules because it allows the driver IC stage to be integrated into the module itself, reducing the component count, raising reliability, and simplifying the design process. Such modules typically offer operation of up to 20A, making them suitable for applications like drum motor drives.

The transfer mould intelligent power modules (TM-IPM) from Toshiba, for example, have low saturation and high-speed IGBTs, and are rated for 600V operation and can be supplied with current ratings of 10A, 15A or 20A. Each module includes several protection circuits and a control IC three-phase inverter system. The control IC is capable of driving a high-side arm directly without photocouplers.

One of the latest products to exploit Toshiba's SoI technology is the TPD4104K single chip inverter IC. This device has high-voltage PWM technology and accepts logic signals from the MPU or motor controller. The device can drive up to 2A through integrated IGBT outputs. All necessary 3-phase decode logic, high-side and low-side drivers, fast recovery diodes (FRDs), over-current and under-voltage protection and thermal shutdown circuitry are included in the 23pin HZIP package.

The device can handle voltages up to 500V. It accepts logic inputs between –0.5V to +7V and also includes a built-in 7V regulator for the 3-phase decode circuitry. The IC is rated up to 2A for continuous operation, or 3A under pulse load, and is suitable for operation from 240VAC mains and higher voltages. As shown in Figure 2, the device only requires logic inputs from a host MPU or external controller to provide a complete motor control solution.

Of course, a raw PWM signal can introduce electrical and acoustic noise resulting from its fast switching. Where this cannot be tolerated it is solved using an external microprocessor running software to manipulate the PWM signal into a sinusoidal motor coil current waveform. However, this can now be implemented in hardware using a companion chip such as the TB6551F 3-phase, full sine wave PWM brushless motor controller.

Figure 3 shows a functional block diagram of the TB6551F, which is designed to operate with an external power IGBT module such as the TPD4104K.

In this configuration, the TBP6551F generates a full sine wave PWM output without an external microcontroller. Using this IC, a low-noise brushless DC motor drive is implemented quickly and without sacrificing the efficiency and low power benefits of PWM control.

The TB6551F features a built-in triangular wave generator, while an integrated lead angle control function allows the designer to move the lead angle between 0 and 58° in 32 separate steps. This ability to optimise lead angle electronically, rather than using conventional mechanical adjustment, allows designers to tune their application for optimum efficiency, facilitating the use of smaller, lower cost, low power motors. A dead-time function can be set for 2.6µs or 3.8µs to ensure safe operation of the power IGBTs in a push-pull configuration. Over-current protection is also provided on-chip.

The transfer mould IPM (TM-IPM) is expected to beome an important device in high-efficiency, energy-saving inverter systems. The SoI process is again a key enabler of this emerging product category. It has allowed 4th generation low saturation voltage trench gate IGBTs and fast recovery diodes in a three-phase, full bridge configuration to be combined with built-in intelligence including all high- and low-side driver circuitry as well as protection against under-voltage, short circuit and over-temperature.

Transfer moulding is a low-cost packaging technology that enables a very compact component outline. A high thermal conduction resin is used to maximise heat dissipation from the integrated IGBTs, and it is easy to attach an external heatsink to the package.

Combined with the high levels of integration made possible by SoI, transfer moulding has resulted in new TM-IPMs such as Toshiba's new M1G10J504H that puts 10A/600V switching capability into a package measuring 63mm x 23mm x 10.5mm, and allows designers to minimise external circuitry. As shown in figure 4, the M1G10J504H integrates six IGBTs and is particularly suited to washing machines and other next-generation appliances requiring cost-effective, efficient and quiet three-phase motor drive solutions. It can be driven directly by a PWM signal and can be used without a current transformer to detect the phase current.

SoI technology has enabled a significant breakthrough in the design of highly integrated inverters and intelligent power modules for motor drive applications. This high level of integration has certainly simplified the design of motor drives and controls. It is expected that future development will focus on increasing the current and voltage handling capabilities of single chip inverters and TM-IPMs. For inverters, sinusoidal output capabilities will also migrate on-chip to enable low noise single chip driver solutions. More devices compatible with Hall sensor inputs as well as sensor-less designs will emerge.

TM-IPM development will focus on driving up voltage and current ratings while also reducing overall package size, without sacrificing thermal performance. Future TM-IPMs in Toshiba's MIG series are set to achieve a 60% reduction in mounting area while also extending current handling capability to 20A.

As sophisticated motor technologies become the mainstream for domestic appliances, designers will value the forthcoming next-generation motor driver solutions. Emerging, highly integrated single chip inverters and TM-IPMs save design time and implementation costs and also boost reliability.

Silicon on Insulator technology
Conventional bulk silicon fabrication technology forces designers to use junction isolation to isolate individual circuit elements. But devices such as forward biased diodes and minority carrier injection devices cannot then be integrated on the same substrate as a high voltage power transistor, such as an IGBT or power MOSFET.

By contrast high voltage SoI fabrication allows the various circuit elements to be isolated by a thin layer of silicon dioxide, which is a perfect insulator. Not only does this allow almost any type of active device to be integrated alongside power transistors rated up to 500V or higher, but it also allows all devices to be packed together very densely on the substrate. Leakage currents are also greatly reduced, as are parasitic effects. By comparison, junction isolation techniques introduce parasitic capacitances and do not prevent leakage currents from flowing.

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