Motors: Driving Progress (October 2006)
October 1, 2006
Motor drives in household appliances are becoming increasingly sophisticated to meet the challenges of higher efficiency, increased reliability, and lower cost. Recent developments in the motor drives and the power components that they contain are helping to fuel this trend.
The venerable induction motor has, for nearly a century, been the workhorse of the appliance industry. Many of the new variable-frequency drives use the induction motor as well. By contrast, some new appliances are using more efficient, more compact and lighter motors. These new kinds of appliance motors can effectively be divided into the brushless DC motor family and the switched-reluctance motor family.
Brushless DC motors with variable-frequency drives and high torque-to-weight ratios are being introduced in many household appliances. Spurring this increase interest in brushless DC Motors is the high price of energy, as well as the increase in the price of iron.
The main factor limiting wider adoption of these motors in more appliances has been cost and the complexity of drive design. Now, however, the popularity of brushless motors in non-appliance applications is steadily decreasing the cost of the brushless DC motor solutions. For example, the popularity of hybrid cars, which use brushless DC motors, will drive down the cost of these motors in the future.
Switched-reluctance motors have been popular in certain appliances such as vacuum cleaners and hand tools where the motor's noise and torque ripple are less of an issue. The switched-reluctance motors can boast high-torque and high-speed operation at a very low cost.
Both the brushless DC motors and the switched reluctance motors use a microcontroller or digital signal processors (DSP) to synthesize waveforms that are then amplified using power switches such as Power MOSFETs or IGBTs.
Types of drivesVariable-frequency drives can be designed in a number of different ways. The most popular low-frequency drive scheme for a typical three-phase motor is shown in Fig. 1.
If higher efficiency and performance are required, a PWM approach is used to produce a sinusoidal waveform. For further improvements in efficiency, a space-vector modulation scheme as shown in Fig. 2, can be used.
Smart power modulesThe last few years have seen the introduction of a number of smart power modules that have made the power interface between the microcontroller or DSP and the motors smaller and easier to design. The other main advantages of the module over a discrete implementation are decreased parasitic inductances and higher reliability, thanks to the use of sister dies for all the switches within the modules as well as the ease of testability.
These smart power modules comprise a drive circuit that can be interfaced directly to a low-voltage TTL or CMOS outputs of the microcontroller, as well as additional protection circuitry. A thermistor to monitor junction temperatures, logic to prevent accidental turn-on of both high-side and low-side switches, dead-time control and the appropriate drive wave-shaping circuitry to minimize EMI, are implemented in the modules. In a module, the driver ICs can be optimized to switch the power devices with minimal EMI and drive losses. A 3-phase drive module popular in the appliance industry is shown in Fig. 3.
For higher power ratings, the ceramic isolator in the module can be replaced by a DBC (Direct Bonding on Copper) isolator. This is a copper-aluminum oxide-copper sandwich (or in some cases Cu-AlN-Cu) that offers superior thermal resistance and temperature cycling reliability.
The advantage in space savings by the use of modules can result in the ability to mount the drive along the shaft of the motor itself and eliminate the need for an additional power board.
Lastly, the price point for smart power modules designed for motor-drive applications has dropped significantly in the last few years as their use in household appliances continues to grow. The smart power module is designed to maximize its flexibility, which allows it to be used for a wide variety of input voltages and power ranges.
High-voltage, bridge driversAnother recent trend that has enabled compact, low-cost modules to revolutionize motor drives is emergent high-voltage (600V) bridge-driver technology. Modern high-voltage bridge, gate drivers, such as the FAN7382, are the result of careful design to reduce the parasitic drain source capacitance internal to the high voltage IC process. This has resulted in drivers rugged enough to withstand negative voltages over -9 V. Positive and negative spikes on the power-supply voltage do not cause the driver to latch and lose gate control - a big difference from the gate drivers of the last decade.
Matched propagation delays below 50 ns make switching frequencies up to 100 kHz or 150 kHz feasible. The addition of common mode dV/dt noise cancellation circuitry within the IC also helped reduce the possibility of false turn-on, which in turn has made the power circuitry more rugged, as well as more compact, by eliminating additional filter components.
The lower quiescent currents of the modern ICs such as the FAN 7382 decrease operating temperature and, thereby, increase reliability. The big advantage of this is the reduction in board space and cost that comes from replacing the four isolated power supplies and opto-isolation circuitry between the microcontroller PC board and the power switch PC board that were very common in motor drives of a previous generation.
IGBTs: NPT vs. PTFor about two decades, the power-switching device of choice for motor drives has been the IGBT. The IGBT can be designed to minimize losses at a certain switching frequency. For the motor-drive industry, this has meant that specific IGBT families are targeted to switch at about 5 kHz for some consumer motors, about 20 kHz in many industrial motor applications, and even higher frequencies for non-motor applications.
Improvements in IGBT conduction voltage and turn-off energy per switching cycle have also contributed to reliable, low-cost modules. The last five years have seen vast improvements in the capabilities of the conventional IGBT and the popularization of the new non-punch-through IGBTs.
The NPT IGBT, while it looks similar to a conventional or punch-through IGBT, is manufactured differently. Unlike the MOSFET or the conventional IGBT, the NPT IGBT uses a lightly doped N-substrate, which then becomes the epitaxial region. The wafers that make up NPT IGBTs are processed by adding the P region and back metal regions, seen at the bottom of Fig. 5b. Then they are flipped over and the other layers of the IGBT are added.
The NPT IGBTs are generally not as fast or as low in conduction voltage [Vce(sat)] as their conventional counterparts. However, they are usually more rugged. The ability to withstand significantly longer periods in short circuit or over-current conditions makes them popular in motor-control applications.
Furthermore, by examining the switching waveforms of the two types of IGBTs, it can be seen that the EMI generated by the NPT IGBT is significantly less than the punch-through IGBT. The NPT IGBT has a fall time that essentially has a single slope. The fall time of the conventional IGBT, on the other hand, consists of a region where the dI/dt is rapid, but following this is a long tail where the rate of fall of current is low and the device losses are high.
In the high dI/dt region the EMI generated by the conventional IGBT is high and generally has the potential to affect the drive circuitry, often making it necessary to isolate the drive circuit from the power switches. Another advantage of the NPT IGBT is that it can be made to have a negative temperature coefficient to Vce(sat), a feature that is valuable when paralleling IGBTs.
Power MOSFETSIn addition to IGBTs, conventional or Superjunction (Charge-Balance MOSFETs) may be housed in the same package if switching frequencies of over 50kHz are required. Superjunction MOSFETs (known in the industry by trade names such as Fairchild's SuperFET) have improved the specific Rds(on) of power MOSFETs.
This new class of MOSFETs has been in the market for about a decade and they are more complicated and more expensive to manufacture than the conventional MOSFET. However, they replace the exponential rate of increase of RDS(on) with breakdown voltage by a linear figure making them attractive in lower power motor drive applications at 600V or higher. Conventional and Superjunction MOSFETs with fast anti-parallel body diodes are used especially in drives for motors less than 100 W.
MCUs and DSPsFollowing Moore's Law, the digital circuits that make up the ‘brains' of the motor drive have grown more powerful over time. Now, various low-cost DSPs and even microcontrollers are able to handle the computational challenges of the appliance motor-drive circuit. Furthermore, the DSPs and microcontrollers have started to resemble each other and both are increasingly designed with motor control in mind. Recently, low-cost 8-bit microcontrollers have proved capable enough to offer vector control of motors and are frequently targeted for the appliance industry.
The various advances in each component comprising smart power modules have, therefore, resulted in lower cost, more reliable modules. As important as the development of low-cost power modules has been various improvements that have significantly reduced EMI and increased safety, often eliminating the need for opto-isolation in many cases. These, in turn, have vastly improved the cost, size and reliability of motor drives and have helped manufacturers meet the challenge of developing more efficient appliances.
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