Sponsored Article

Smart Power Modules for Power Factor Correction

Sponsored by Fairchild Semiconductor

by Vajapeyam Sukumar
Fairchild Applications Staff

The high prices of energy are leading to renewed interest in the more efficient motors driven by Variable Frequency Drives or VFDs. Many types of motors such as brushless DC motors, switched reluctance motors and even induction motors can be driven by VFDs that help synthesize the input AC voltage. This interest in efficient motors, in turn, has increased the interest in Fairchild's Smart Power Modules (SPM^TM). These modules integrate power switches, drive circuitry and protection functions in a low-cost, compact, and isolated module that directly interfaces to a microcontroller or DSP on one side and to the motor on the other. SPM is increasingly popular in high volume applications such as white goods and air conditioning systems.

Unlike power supplies, where Power Factor Correction circuits are required by norms such as the IEC 1000-3-2 in Europe, motor drives generally have little governmental regulation that requires the use of PFC. However, this situation will likely change as the concern for green becomes stronger. For the moment, the key reasons for incorporating power factor correction into motor drive circuits essentially are ease of design and marketing advantages. The maximum current that can be drawn from a 120Vac wall socket is pivotal in limiting the size of the motor. Generally speaking, in North America, these wall sockets are rated at 15A. However, the maximum current that can be drawn from the socket is limited to 12A by safety agency norms. The primary reason to use a PFC circuit in a motor drive circuit is the ability to accommodate a larger motor from the existing 120Vac wall socket.

Other reasons why a PFC circuit is used in motor control is the inherent universal voltage rating of the motor as well as the fact that the motor design is greatly simplified by the fact that the output voltage variations are greatly reduced. The motor input voltage after the PFC circuit is now high, resulting in lower currents which, in turn, reduces copper losses.

Two kinds of PFC circuits are popular in motor drives—partial PFC and the conventional PFC circuits. The partial PFC is a method of shaping the input current waveform by switching a power device once every half cycle. Partial PFC is, therefore very efficient but does not result in a very high power factor. The conventional PFC, on the other hand, is used to increase the PFC from around 70% for non-PFC designs to 99%. Both forms of Power Factor Correction are popular in motor control applications, especially if the motor drive modules and the PFC modules are placed in the same package, yielding a compact design. For example, the Mini SPM package shown in Figure 1 can house motor drive modules rated 600V and various current ratings ranging from 3A to 50A, depending on the type and the size of the silicon placed in these modules as well as the insulating material and its properties. Placing all the PFC circuit in the same module, we have a 40kHz switching, 600V conventional PFC circuit in the 2kW to 6kW range.

Fig. 1a.

Fig. 1a: The Mini DIP package is housed in a 44mm x 26.8mm and offers 2500Vac isolation ( for 1 minute) . Fairchild's Motion-SPMs range from 3A to 50A in this package and contain six IGBTs ( or MOSFETs), anti-parallel diodes and drivers for all power switches.

Placing both the PFC module and the Motion-SPM inverter in the same package as shown below in Figure 1b makes for a simple, elegant mechanical design.

Figure 1b.

Partial Switching Power Factor Correction Module

As mentioned above, since the IGBT in this module is switched at the frequency of the rectified line with a very low conduction voltage, a slow IGBT is appropriate for this type of module. The rectifiers used in the module are also optimized, not for recovery time but for the lowest forward drop. Partial PFC schemes are good to raise the power factor to up to the low to mid 90% range. The output capacitor should be designed to handle large ripple currents and the size of the passive components, especially the magnetics are large. This approach therefore is restricted to systems below 3kW. A typical application is a window unit air-conditioner.

Figure 2a.

Figure 2a : Typical voltage and current waveforms from a partial switch PFC circuit.

Fig. 2b : A typical schematic using Fairchild's FSAB20PH60 module. The thermistor shown in the top left is used to monitor the temperature inside the module and to feed the information to the MCU or DSP.

Figure 2b.

Conventional Power Factor Correction Module

The Conventional power factor correction Scheme is found not only in motor control applications but in other applications such as power supplies and lighting. The conventional power factor correction scheme can be computation intensive and usually a dedicated PFCIC such as the FAN4800 Continuous Current PFCIC is used in order to decrease the computational load on the MCU or DSP. For the power section, the use of a module can minimize the distance between the diode and the power switch. This, in turn, reduces inductance and therefore, decreases the power loss. These type of PFC circuits are suited for a wide range of power levels. Fairchild's FPxB series of smart power modules are suited for the 2kW to 6kW range and ideal for the higher power air conditioner market. The DBC isolating material is ideal for applications where thermal cycling is common. The addition of a NTC thermistor to monitor die temperatures within the module helps guard against thermal runaway. The incorporation of current sense resistors of low ohmic value within the module also allows for accurate current measurement. The two diodes on the high side of the module need to be very fast recovery diodes that also have a soft recovery to minimize EMI. The low side diodes can be optimized for cost and forward drop. Fast IGBTs used in the module can push the switching frequency of the module to over 20kHz.

Figure 3a.

Figure 3 a: Typical Voltage and Current Waveforms in a Conventional Power Factor Correction Scheme. Achieving at least 99% is possible.

Figure 3b : The internal schematic of Fairchild's FPAB50PH60, a 30A, 600V PFC module with IGBTs that switch over 20kHz and ultra-fast Stealth^TM diodes.

Figure 3b.

The advantage of a versatile, low cost, DBC-isolated, PFC Modules made of moulded plastic is its potential use in a wide variety of applications in the kilowatt range. By simply replacing one kind of IGBT with others or perhaps even charge-balanced mosfets such as Fairchild's 600V SuperFET^TM MOSFET, these modules can be targeted for any switching frequency from 100Hz to 100kHz.

FOR MORE INFORMATION

For more information on Fairchild's wide portfolio of integrated motor drive solutions, visit www.fairchildsemi.com/spm