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Posted on 06 August 2019

Unique PFC Controller for Design Challenges of Packaged Air Conditioner

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Air conditioners are an important appliance in almost every residential and commercial building. While both window-type and duct-type air conditioners are common in the market today, duct-type air conditioners are becoming more main stream today due to their ease of installation.

By Wen Chien, BLDC Product Line Marketing Manager, Fairchild Semiconductor

However, duct work passing through walls can at times ruin the interior design aspects of a space. This is creating a new concept in air conditioning, the duct-less packaged air conditioner. Unlike the duct-type that only provides cooling air within a room or a floor, the packaged air conditioners need to cover the entire building and thus its power rating is much higher than duct-type air conditioners. Because of this high power rating, inverterizing is the trend in order to accelerate energy savings. Inverterized motors will however cause worse power factor (PF) and harmonics on power grid, therefore a power factor correction (PFC) controller is consequently needed. Conventional passive PFC and single channel active PFC are not able to meet the requirements for PFC in a packaged air conditioner for various reasons such as size, heat dissipation and efficiency management. To address this challenge, Fairchild has introduced a new PFC controller which is interleaved to not only meet these requirements but also to include other features which are unique for the packaged air conditioner industry.

Development Trends of Air Conditioners

Air conditioners are a popular home appliance in developed countries – as well as developing countries – in recent years, with annual production quantities increasing significantly, now up to more than 100 million units annually according to statistic data from several market research companies. With the significant volume annually produced, air conditioners inevitably face more stringent energy saving requirements, and the acceleration of moving to inverter type compressors that use permanent magnet synchronous motors (PMSM) / brushless DC (BLDC) motors is the evident proof.

The data indicate that more than 40% of motors have been inverterized today worldwide. The down side is that inverter compressors will deteriorate power factor, resulting in more power drawn from the AC outlet than the actual demand, and pollute the power line grid with harmonics. Therefore, a corresponding PFC measure needs to be taken. For window-type and duct-type air conditioners, passive PFC was used. However, the current trend is moving to a single-channel boost continuous conduction mode (CCM) PFC due to stricter regulations.

Window-type air conditioners hanging on the wall are not considered aesthetic and duct-type air conditioners require drilling holes along the duct path, not always possible in existing structures without ruining the interior design. This affects each living room and bed room as each normally will have a window-type unit or need a duct created.

Packaged air conditioners are, therefore, pursued better option for many applications. Packaged air conditioners centralize all of the required coolant energy in one unit and it is placed in the backyard as indicated in figure 1.

Packaged air conditioner unit and the house

Its installation happens prior to construction for interior decoration and thus the pipes are hidden. Since the packaged air conditioner unit has to provide cooling for the whole building, its power rating is certainly much higher than a normal room air conditioner. The power rating could range from 5 kW to 7.5 kW, requiring all of the electronic components controlling the BLDC/PMSM compressor to have a higher power rating level.

Bottleneck of PFC Controller in Packaged Air Conditioner

When the power rating of an application is required to be in the range of 5 kW to 7.5 kW, the front-end PFC controller design and component selection will be challenging. Today’s single-channel boost CCM PFC is not applicable in this situation either in terms of inductance size or electronic power components. The PMSP/BLDC motor or compressor’s DC bus voltage after the bridge rectifier is up to 311 V at 220 V AC line input. The current drawn and flowing through the IGBT and/or fast recovery diode will be 24 A. The selection of high current devices is very challenging with very limited choices. Its heat dissipation design is another challenging area when considering the current solutions available in the market.

Benefits of Interleaved Topology

In the past, the interleaved boost PFC topology was introduced primarily for the power supply application in the personal computer industry. Figure 2 shows simple diagrams of the single-channel and the interleaved topology.

Single Channel Boost CCM PFC and Interleaved Two Channels Boost CCM PFC

As shown in figure 2, the current flowing through the inductor is split into two paths, thus the current flowing through each device in interleaved topology is halved and its heat dissipation, I×Vdrop, is halved as well in active devices such as switching MOSFETs and fast recovery diodes. Moreover, the heat dissipation in the inductors will be not just one half but one quarter. The heat source in an inductor is derived from Ohm’s law – that is I2×R. When flowing current is reduced to one half, its heat will consequently only be one quarter. This big reduction in heat can bring a big advantage on inductance design, by which the heat dissipation surface of the inductor design can be reduced, greatly reducing the inductance volume. Figure 3 shows a real example of inductance volume comparison of 2.5 kW design in terms of single channel versus a 3-channel interleaved option.

Comparison of Inductor Volume

The reduction percentage is up to 60%.

Just imagine how large the inductance volume will be at 7.5 kW without using interleaved topology, and it clearly becomes quite challenging to have an inductor for the power rating. Based on the experiences of designing PFC circuits, 2.5 kW~3kW per channel is optimal in terms of inductance design and the easy selection of active MOSFETs, IGBT and diodes. For a 7.5 kW design, the 3-channel topology should be best suited.

Other Features Required for Packaged Air Conditioners Besides PFC, whose importance has been addressed in the second section of this article, some other features are less noticed but also required to achieve greater energy-savings. A configurable and adjustable boost output voltage is very helpful to optimize system efficiency on the fly. Under some circumstances, input AC line voltage might drop for a while due to instability and the conversion efficiency of the air conditioner will be deteriorated due to a wider gap between the input AC voltage and the boost output voltage. The configurable and adjustable feature can be used to improve the efficiency with the assistance from an MCU which can monitor the AC line change all the time.

A packaged air conditioner is not required to operate at full power all the time although it provides cooling air for several rooms in a building. The air conditioner will work at light load when the on/off controllers in some rooms are not switched on. The PFC efficiency of an interleaved topology is not optimized if all of the interleaved channels are enabled at light load. Figure 4 illustrates the relationship between load, channel and efficiency.

Interleaved PFC efficiency vs. Load vs. enabled Channel Number

To manually switch on channels vs. load power by an MCU is not always easy due to the lack of load power information. An automatic mechanism such as the black curve shown in Figure 4 will be much anticipated.

World’s First 3-Channel PFC Controller Solves All the Design Pains at Once

The FAN9673Q is an interleaved Boost CCM PFC controller from Fairchild which is designed for high power industrial applications such as packaged air conditioner, high power motor, UPS, Welders, etc. The main features are:

  • Worldwide first 3-channel interleaved PFC controller to drive up to 9 kW
  • Two modes of channel on/off: controlled by MCU or automatic on/off by internal mechanism
  • AC input peak detection let the user easily capture AC line information
  • Configurable-and-adjustable output voltage help optimize system performance
  • Differential current sensing reduces cost of current transformer by using simple resistor with good ground noise immunity

These features are unique and an ideal fit for packaged air conditioner design requirements and the challenges addressed in previous paragraphs.

Packaged air conditioners are the future star in the industry but still face many design challenges as the power ratings increase. To meet all the challenges at once, the FAN9673Q is designed with the highest integration by using an interleaved topology and many unique features to power PFC designs and help designers develop amazing products.

To learn more about FAN9673Q please visit: www.fairchildsemi.com

 

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