Thanks to the improved performance and efficiency they provide, the demand for inverterized drives and brushless DC (BLDC) motors continues to grow. These motors are widely used in consumer applications such as inverterized air conditioners, washers and fan motors. A BLDC-based inverter system is faster, quieter and more energy-efficient than conventional solutions such as brush DC motors or AC induction motors with on/off control. The key to enabling this progress is inverter technology, especially the integrated power module. Today, power module technology that integrates power switches and their gate-driving circuits can provide a compact, reliable and cost-effective inverter solution.
In addition to meeting the requirements of ever smaller, energy-efficient motors, another challenge for designers of home appliances is the ability to deliver power in a manner that the driver can handle appropriately. Power Factor Correction (PFC), EMI and thermal resistance are important considerations in applications that have to meet international standards for efficiency.
This article describes the benefits and techniques of applying “smart power” module technologies to two different motor drive applications. It gives two design examples of Fairchild Semiconductor’s Smart Power Module (SPMTM) technology and provides test results of induction motor designs for inverter systems. It also enumerates the many benefits of this technology and explains how these modules support the package and design requirements within the consumer market that requires cost-effectiveness and high performance .
Harmonic distortion in higher power applications
Harmonic current that is generated by a non-linear load deteriorates the power quality of a utility, and is a source of electrical interference for equipment that is connected to the point of common coupling. Today, this problem is regulated by domestic or international regulations, such as IEC61000-3-2, and compliance to these regulations is mandatory. To meet these regulatory requirements, it is necessary to have a pre-regulator instead of a diode rectifier to reduce the distortion of input current. Air conditioners, whose power ratings range from 1 to 4 kW, are classified as Class A equipment according to IEC61000-3-2 (input current is less than 16 Arms), and most of these air conditioners are adopting power factor correction (PFC) circuits as the pre-regulator to mitigate harmonic distortion.
PSC circuit approach to PFC
One of the more cost-effective approaches to PFC uses a “Partial Power Factor-Correcting Switching Converter” (PSC) circuit. With this method, the circuit topology is the same as that of the commonly used high-frequency PFC circuit, yet the switching frequency is just twice the utility frequency. Compared with the high-frequency PFC, the performance of a PSC circuit is limited––and not suitable for––high-power air conditioners over 3 kW since it limits harmonic regulation. However, most room air conditioners are below 3 kW, so this method provides acceptable performance as well as low EMI due to its low switching frequency. Moreover, because of its simplicity, the PSC method can be controlled by the microprocessor used for inverter control without requiring a dedicated control IC. For this reason, the PSC approach is popular for 1 to 3 kW room air conditioning systems, and its variants are widely adopted achieving approximately 97% of the input power factor.
New PSC module concept
To address Power Factor Correction requirements in 1-3 kW air conditioning systems, Fairchild Semiconductor has developed a new integrated PSC module, the PFC-SPM. This PSC module employs circuit topology similar to the high-frequency switching method but offers cost advantages for these specific applications.
Figure 1(a) is the external view of the PSC module’s package and Figure 1(b) shows the cross section diagram of the PSC. The PSC module is a direct-bonded-copper (DBC) based transfer-molded package. A typical lead-frame-based package, presents a difficulty when changing the circuit topology since the lead-frame has to be changed. A DBC substrate, on the other hand, allows designers to easily create a new topology without sacrificing cost. Moreover, due to the thickness (0.68mm) of the aluminum oxide isolation layer, it is possible to for the DBC-packaged module to offer very low thermal resistance while maintaining an isolation voltage of 2.5 kV for 1 minute.
Figure 1a. PSC Module. (a) package view
Figure 1b. PSC Module. (b) cross section diagram
Figure 1c Internal block diagram
By virtue of its DBC substrate, a PSC module can share the same package with Fairchild’s Motion -SPM series, which has a three-phase inverter topology. Sharing a package with the Motion-SPM device has potential advantages in the assembly of an inverter printed circuit board (PCB). For example, Figure 2 shows a Motion-SPM and a PSC module mounted on the same PCB board. Unlike discrete packages, such as a TO-220, smart power module packages are either non-standardized or they have their own packages. With a power module, the heat sink mounting can be very troublesome when different modules have to be placed on a single PCB, which may lead to decreased assembly productivity. The adoption of a PSC in the inverter PCB that uses a Motion-SPM can increase productivity.
Figure 2. PCB mounting with SPM3
Referring to the internal block diagram in Figure 1(c), the PSC module integrates two IGBTs for boost converter operation and four rectifier diodes as shown in Figure 1(b). This module also has an optional thermistor and a gate-driving low-voltage integrated circuit (LVIC) that provides a protection function. In the PSC circuit, the boost inductor and switch are installed at the AC input of the rectifier (Figure 3). The PSC module can be controlled by tying together the input signals IN(R) and IN(S) for IGBT Q1 and Q2.
Figure 3. Operation of PSC with Vin> 0