Integrating discrete power semiconductors, drivers, and controllers into a single package allows appliance manufacturers to simplify the design process, while ensuring a solid power electronics foundation for their products. In addition to speeding time-to-market, integration opens the door to new levels of design innovation to address longer term energy-saving initiatives. However, energy-efficient motor solutions require more complex electronic drive designs than those used traditionally. This overview illustrates three examples in which different power devices can be successfully integrated into a single module to satisfy a range of diverse needs.

Appliance drives

Different types of appliances use different drive solutions. Asynchronous motors using vector drives are commonly used in washing machines; brushless DC motors are employed for smaller motors in pumps and compressors; and switched-reluctance motors are turning up in vacuum cleaners. The different power levels of these motors require different dimensioning of the output stages. Furthermore, switched-reluctance motors require a different external switch topology from the three-phase inverter bridge.
Fig. 1 shows the block diagram of a generic household appliance application, focusing on the motor drive. The input power comes from the AC line input, and an auxiliary power supply uses a device called the Fairchild Power Switch to generate a 15 V power supply for the motor drive stage. The 15 V output is regulated down to 3.3 V with a linear regulator to provide power for the microcontroller or DSP circuit.
For the power stage, the AC line’s input voltage is rectified with a bridge rectifier. For higher power levels, which would benefit from power factor correction, the rectification and PFC function can be integrated into one module, as will be reviewed later. A three-phase inverter stage using three half-bridge drivers and six discrete IGBTs could be used to drive the motor.

Asynchronous drives

When considering the relative benefits of using modules versus discrete solutions for asynchronous motor drives, modules offer significant advantages. For example, Fairchild Semiconductor addresses the power stage in Fig. 1 using a range of Smart Power Modules called the Motion-SPM series. These modules integrate the functionality of three half-bridge drivers (FAN7380) and six IGBTs into one DIP package. The module is based on a ceramic, or DBC substrate for higher power levels. (In comparison, a typical discrete solution uses nine separate devices in this application: typically, six 7A or 12A IGBTs and, at these power levels, larger isolated TO-220 packages are necessary.)
Integrating all the components into one package reduces the space required for the resulting module. The advantages are not only its smaller package dimensions on the board (44 mm x 27 mm), but also its low height (7 mm, Mini-DIP versus 19 mm, TO220). By making the PCB smaller, this compact packaging allows the motor control function to be integrated into the panel controller in a washing machine or dishwasher.
Modules also can speed up the design process, since high-power routing can be standardized much more easily. In the case of modules with identical pinout, the power rating of the system can be changed by simply using modules with different power ratings. Motion-SPMs, for example, range from 3 A to 30 A in the same pinout, making changes caused by new motor power requirements or new thermal design much easier to handle. For high-voltage designs, certain design rules have to be met, like pin spacing. A standardized layout will reduce the number of time-consuming design checks.
The entire electrical and timing characteristics of the power stage in a module are much more precise, especially by removing the need for designing the correct gate drive for the power switches. Elimination of this extra design step has a significant and continuous impact on system efficiency, EMI generation, and safe operation. This internal design reduces design time and makes the lot-to-lot performance of the system much more similar. This can reduce the effort required elsewhere in the system to compensate for manufacturing variations, such as larger bus capacitors, and higher current ratings on input rectifiers.

Brushless DC control

In the appliance market, the power levels for brushless DC motors are much less than those for asynchronous motors in washing machines. In these designs, it is advantageous to consider using MOSFETs. To a first-order approximation, the conduction loss of an IGBT is proportional to the current; whereas the conduction loss of an MOSFET is proportional to the square of the current. With actual values of VCE(sat) and RDS (on) for comparable devices, it can be easily demonstrated that the conduction losses of a MOSFET at lower power levels are better than those of an IGBT.
It is interesting to note that the switching losses of a MOSFET in a motor drive application can be more than those for an IGBT –– contrary to what may first be believed –– because of the poor switching performance of the intrinsic body diode in a MOSFET. Even with an externally connected diode, the MOSFET body diode will conduct to some degree, resulting in extra switching losses when the diode is forced to turn off. These switching losses are caused by the reverse recovery characteristics of the diode.
The losses arise not only in the MOSFET body diode, but also as extra losses in the MOSFET, which is forcing the diode to commutate. An IGBT does not have this problem as it has no body diode. This is one of the reasons why IGBTs are used in motor drives even at low power levels, as the body diode effect outweighs the disadvantages of higher switch-off losses of an IGBT compared with a MOSFET. In addition, the body diode of a MOSFET can be disabled by an externally connected serial diode, which works against the original benefit of better conduction losses, and adds extra component count and cost. But, by suitable minority carrier lifetime control in the manufacture of the MOSFET, it is possible to improve the MOSFET recovery characteristics of the body diode (Irrm = 0.67 A, trr = 188 ns for 3 A peak device).
The Motion-SPM in Tiny-DIP (Fig. 2) uses such MOSFETs and is targeted to low power three-phase motor control applications. The extremely small package size (29 mm x 12 mm) and very low height (3.25 mm) allows designers to integrate the motor control unit into the body of the motor itself. The MOSFETs are dimensioned to have a wide reverse biased safe operating area (RBSOA) for maximum robustness.

Fig. 2. MOSFET module application example.

Switched-reluctance motors

A third type of motor, the switched-reluctance motor, is increasingly being used for vacuum cleaner applications. The benefit of this motor is its low manufacturing cost. Integrated solutions, such as a 50 A module from Fairchild, target single-phase-switched reluctance motor drives (Fig. 3). The output stage is different from a classical three-phase inverter topology, as each winding is individually activated. The emitter of the upper IGBT has a separate connection from the collector of the lower IGBT. The winding is connected between these two connections. The freewheel diode for the upper IGBT is connected between the high voltage bus and the collector of the lower IGBT, and the freewheel diode for the lower IGBT is connected between the emitter of the higher IGBT and the high voltage ground return.

PFC for high power

Fig. 4. PFC Module application example.

For higher power systems, such as air conditioning units, where PFC is used, Fairchild has developed a module specifically for PFC (Fig. 4). In a classical PFC boost circuit, between the AC input line and the high voltage output bus, there are three diode drops: two from either side of the half bridge rectifier and one from the boost diode itself. By using two IGBTs to replace the lower diodes in a full bridge rectifier circuit (Fig. 4), and by placing inductance in the AC path, it is possible to make two boost circuits, one of which is used in each half cycle. Instead of the three diode forward voltage drops, there is now one diode and one IGBT drop, which is usually less. This results in higher system efficiency, which is of particular importance in higher power systems.
This PFC module is designed to be switched at the line frequency and is rated at 11A. The power factor and input line harmonics are far better than with a standard bridge converter, but not as good as with a continuous conduction mode PFC. By changing the internal IGBT to a faster-switching device, the power factor and harmonics would be improved at the cost of slightly worse efficiency. A faster IGBT has a higher conduction loss than another from the same family.

Conclusion

Today’s power modules offer a range of functions to fit the specific requirements of consumer appliance systems. Offering the benefits of smaller size, increased reliability and most important of all, faster time to market, integrated modules help meet the increasing challenges of innovation in this competitive environment.