Efficient AC

Energy efficiency in appliances is becoming a hot topic. Several countries have now enacted legislation aimed at boosting the efficiency of products such as clothes washers, water heaters, and in particular air conditioners. Air conditioners are a special target because they are becoming more popular in Japan, Europe, and emerging countries such as China and India.

In the U.S., for example, the Dept. of Energy recently issued energy-efficiency standards for numerous appliance products. Similar initiatives have been in force in Europe and Japan. In this regard, the residential air-conditioning market (about 35 million units manufactured worldwide) is by its nature a "high-impact product" for energy-efficiency programs. An air conditioner must have a SEER of at least 10 to be sold in the U.S. Higher efficiency models have a SEER of 11, 12, 13 or 14. (SEER is Seasonal Energy Efficiency Rating. It is the most commonly used measure of the efficiency of consumer central-airconditioning systems. EER, or Energy Efficiency Rating, is the most commonly used measure of efficiency for commercial air-conditioning systems.)

U.S. legislation on air conditioners and heat pumps establishes more stringent minimum efficiency standards that become effective January 23, 2006. These standards are tough to hit without the use of advanced technology and are quite challenging for manufacturers. Without the use of more advanced electronic technology these standards may be potentially impractical. Such technology includes variable-speed drives powering either a standard ac induction or brushless dc compressor.

Most variable-speed drives have used electronic inverters for controlling the motor. These inverters (as well as linear and switch-mode power supplies) contain a bridge rectifier/capacitor frontend. But the front end presents a highly nonlinear load to the main line: The input bulk capacitor charges only toward peaks of the voltage sine wave and thus induces a peak of current in the load.

This nonsinusoidal pulse current contains harmonic multiples of the fundamental line frequency. And each of these harmonics has a significant energy content.

These combined effects of only drawing line current during voltage peaks and high harmonic content contribute to realizing a circuit that has an extremely poor power factor.

Now consider what happens when a multitude of similar systems are on the same ac lines and all draw current at the same time. This phenomenon reduces ac network capacity, in essence aggravating any energy problems on the power grid. So to head off the problem, electronic motion controls of this type must use an input section that incorporates a power-factor correction circuit.

POWER FACTOR LIMITS
The IEC standard EN/IEC61000-3-2 spells out the maximum allowable harmonicson the ac line for various classes of equipment. The standard applies to all products up to 16 A/phase and classifies all motor-driven equipment as Class A. Thus air conditioners fall into Class A of the standard as well and must exhibit harmonics that fall below the levels spelled out there.

Various methods have been adopted by the industry to keep harmonic currents down to allowable levels. The most straightforward uses a passive PFC (power-factor-correction) topology, where a simple inductor directly connects in series with the line. But this approach has many limitations when applied with the power levels of in-room air-conditioning units. The size and weight of the inductor can be appreciable, for example. And the PFC performance is not first rate.

Active-circuit topology provides far more-effective PFC. However, an active PFC circuit is complex, requiring selection of numerous components which may impact the overall efficiency of the system.

Advanced semiconductors and IC packaging techniques can help solve these problems. There is a trend in power inverters to integrate all power semiconductors into a single package. The approach can easily be extended to the input converter to better manage power.

Recently what is called a Bridgeless approach to PFC topology has emerged that, though not necessarily revolutionary, is definitely innovative. Appliance manufacturers in China, Japan, and Korea are now using this approach. The topology is being adopted in various inverter designs and several semiconductor manufacturers are fielding power integrated solutions employing the technique.

The "bridgeless" label comes from the absence of a traditional rectifier bridge in the circuit. The new design employs IGBTs (insulated-gate bipolar transistors) in a manner that eliminates the need for bulky inductors and capacitors that would otherwise be required in the input. (As a quick review, IGBTs combine the best of conventional bipolar transistors and FETs. Like FETs, they only require a voltage across the base to conduct. But like conventional bipolars,-they conduct current efficiently through their collector/emitters. They are significantly more efficient and easier to control than any other power semiconductors. IGBTs are commonly available with ratings up to 1,200 A and about 1,700 V.)

A Bridgeless approach offers several advantages compared with a conventional PFC circuit. It is more efficient and uses one less diode in the power circuit. Diodes across IGBTs in a Bridgeless circuit do not need to have a fast recovery time because they conduct high-frequency-current components, and therefore can be chosen with lower VF ( forward on voltage). The IGBTs also operate more efficiently. The IGBTs in the circuit each only carry half the total load current. This allows use of smaller heat sinks and IGBTs made with dies smaller than if the devices handled 100% of the current. Use of smaller IGBTs, in turn, reduces the gate drive requirements.

Also worth notice is that the IGBTs in the Bridgeless topology can be driven at a low switching frequency (typically twice the mains frequency) or at high frequency in continuous mode operation, providing optimum selection of IGBT speed.

It is useful to review how the Bridgeless circuit actually operates. It may be helpful to picture the topology as a fullwave bridge with IGBTs in both bottom legs and diodes in both upper legs. The ac line connection is to the center of the bridge through two inductors. The output is tapped from the top and bottom.

Circuit operation becomes clear by considering what happens first during the positive ac half cycle and then the negative ac half cycle. When the ac input voltage goes positive, the gate of one IGBT is driven high and current flows from the ac mains through an inductor and the IGBT. The inductor begins to store energy. When the IGBT turns off, energy in the inductor releases as current flows through one of the diodes, through the load and returns through the body diode of the other IGBT back to the input mains.

During the off time, current through the inductor (that during this time discharges its energy) flows into the boost diode. This closes the circuit through the load.

Operation during the negative half cycle is symmetrical to what takes place during the positive cycle. A point to note is that the two IGBTs can be driven simultaneously because the presence of freewheeling diodes that recirculate the current during the opposite polarity cycle.

APPLICATION TESTS
There have been tests to evaluate the efficiency of "bridgeless" input converters. One such check simulated a 1,200-W power application (typical for a 12,000 Btu/hr air-conditioning system). A dedicated-gate driver circuit powered the IGBT switches. A 50-kHz variable-duty cycle generator served as the input signal.

The absolute best performances came using the silicon technology from International Rectifier. IGBT power switches were two IRGB20B06UPD1 Warp2 series. The rectifier portion used four 8ETX06 devices optimized for the lowest recovery time and minimal recovery current.

Technicians measured the total input converter losses and efficiency under the assumption that input voltage varied from a minimum of 95 Vrms to a maximum of 265 Vrms and 400-Vdc constant bus voltage. Tests took place with a fixed switching frequency of 50 kHz.

The overall losses under these conditions must be considered worst case: Tests took place with switches operated at constant duty cycle across the range of input voltages for the preset bus voltage. In a normal application the duty cycle (for continuous-mode operation) varies to follow the sinusoidal input current. Thus the overall losses (switching and conduction) are significantly lower.

ON THE HORIZON
As with all power-switching circuits and regulators, circuit layout is critical with this topology. An integrated power module that houses the input converter topology, current-sensing circuitry, and the gate driver is a way to help electronic engineers face the challenges of powermanagement issues.

It is possible today to integrate all functions including power management of a typical driver for air conditioning into just two power integrated hybrid ICs (advanced power modules). Future improvements in power ICs, HVIC drivers, thermomechanical features and package-integration density will help bring more-efficient motor-drive solutions to appliances.

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IR-SA Integrated Technologies,
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