Increase safety in home appliance systems with intelligent AC control

　　Today, at least 60% of household items in the world use electronic devices. For the electrical appliances sold, the upgrade from the original electromechanical to digital control has been completed, and the current system architecture is implemented around microprocessors, discrete transistors and high-voltage thyristors. This change is also to some extent the result of the growing demand for energy and water conservation, as well as increased consumer ease of use.

　　Performance and cost-effectiveness have always been the main challenges for home appliance equipment manufacturers, and the globalization and standards of the market have made it more difficult to meet these challenges. This is why the control of AC power is constantly changing. The strong demand for differentiation in the home appliance market forces manufacturers to improve the electrical performance of the system and provide new features to save power or energy in normal operation or standby mode.

Performance and security improvements

　　When designing a control system, the designer's goal is to pursue high anti-interference performance and increase robustness. The IEC61000-4 series of standards covers electromagnetic compatibility requirements for AC power lines, such as high voltage surges, fast transient shocks, and electrostatic discharge. The standard defines the level and specific standard values ​​of anti-interference for power control boards, and requires a gradual increase in the anti-interference capability and robustness of home appliance systems.

　　For example, in refrigerator design, better food preservation and higher compressor efficiency can be achieved by using digital control: that is, a 3°C temperature difference in the cabinet can reduce power consumption by 20%. A safety task in washing machines is to be able to collect and analyze electrical and laundry parameters to avoid overflow or lack of water. With the help of the power control circuit, it is possible to command to stop the heating device, open the water valve or turn on the drain pump.

Continuous Improvements in Solid-State AC Switches

　　In early circuit boards, thyristors met the needs of compact and simple applications. The volume of thyristors is 5 times smaller than that of relays with currents less than 1 amp, but they provide switching performance without EMI interference, fast response time, reliability of millions of switching cycles, and low power consumption. In order to further simplify the power supply design, the thyristor was improved by removing the buffer circuit in parallel with the standard thyristor. The designer only needs to consider the rectification parameter (dI/dt)C selected according to the load shutdown current.

　　However, thyristors are only reliable within their rated blocking voltage (VDRM/VRRM). Outside this range, overvoltage will permanently deteriorate their switching performance: an uncontrolled overvoltage trigger will excite hot spots in their junction area. Therefore, thyristors must be protected with external suppressors.

　　The most critical constraint is the voltage surge described in the IEC61000-4-5 standard. It is usually required to be able to withstand a 2kV 1.2/50us surge. In order to be able to withstand this 40 joule surge, the following two main methods can be used: clamping - using an external voltage suppressor such as a varistor to absorb the surge energy; crowbar - the thyristor is safely turned on and the surge energy is consumed in the load impedance.

　　The new protected thyristor is developed based on bi-face full planar technology and has excellent built-in overvoltage robustness, thus improving system reliability. When the terminal voltage exceeds the avalanche voltage, the switch is reliably triggered to short-circuit mode. The voltage quickly drops to a few volts and the overvoltage is converted into a current flowing through the switch. The planar AC switch then resumes its blocking function at the end of the cycle, which is consistent with the IEC60730 standard.

　　ACS has achieved further improvements while achieving the integration goals of reliability and ease of design. This new switch integrates a gate level shifter, which makes the MCU logic level drive more immune to electrical transient interference. For example, the 0.8A switch can guarantee a surge resistance of 500V/us, which is 10 times that of a corresponding thyristor with the same gate sensitivity (IGT=10mA).

　　由于不需要任何的噪声抑制器，从而简化了设计，而且整个控制能够满足IEC61000-4-4标准。在执行水阀的开关控制时，一个0.8A交流开关能够安全地承受切断操作，利用箝位来吸收负载的感应能量。设计所保证的开关能量的容量必须利用一个28H的高感性负载来进行苛刻的测试验证。

　　Now, the unique gate architecture of the ACS switch allows for more robust electrical performance on the back side of the chip, which was not possible with previous thyristor architectures: an array of AC switches can be packaged in a single package specifically for centralized brake actuation in applications like dishwashers.

Energy saving in refrigerators

　　Electrical control improves compressor efficiency by eliminating starter leakage and providing better temperature control. The starter is a positive temperature coefficient resistor (PTC) that continuously absorbs 2.5W of energy generated by its leakage. This loss can be eliminated if the PTC is turned off with a solid-state AC switch after starting. Smooth temperature control can reduce average input power by 20% and increase the compressor's on-off repetition rate by 50%.

　　A 10-year compressor life is equivalent to 270,000 on-off cycles, which explains the significance of using solid-state technology. With the ability to withstand 2kV overvoltage and 200V/us transient shocks, the new planar thyristor or ACS provides the required off-state reliability. With a system cost similar to that of electromechanical solutions, breakthroughs in solid-state technology enable refrigerators or other refrigeration equipment to meet A+ energy consumption standards, bring better food storage effects, and no transient discharge and EMI interference.

AC switch and its control

　　With the application of system-level packaging and power plane technology, people can already imagine combining AC switching and power control with micromodules. Since there is a stable voltage behind the ACS chip, it can be replaced with a power IC in the next step to realize new functions, such as fault detection or overload and overheat protection.

　　Safety requirements for home appliances have been enhanced in UL and IEC standards. Appliance controls monitor the operating status of AC switches and detect failure modes. A detection circuit is needed to sense the operating status of the AC close to the switch. Some serious load and appliance failures can be avoided, such as DC operation of highly inductive loads due to thyristor diode mode failure, overheating of resistive loads, or flooding due to switch short circuits. Innovative designs of small AC switches, such as ST's NeoS project, address these challenges by detecting switch failures in a cost-effective manner. Using an AC detector, all switch states can be monitored and compared with the driver to prevent major failures in home appliances.

Figure 1: Protected AC switch: NeOS example.

　　In addition, the switch's ability to implement thermal protection opens up a new way to protect against overloads. Knowing the thermal state of the AC switch allows a cut-off protection to be designed, so that if an electrical brake is misconnected during final assembly or gradually deteriorates under certain stresses, the switch can detect the fault. An early warning signal provides information to the control circuit, limiting the scope of repair to the brake rather than the entire device.

Figure 2: Switch fault detectors protect home appliances from catastrophic events.

　　This type of protection has been proven to be feasible by testing demanding electrical loads such as cooling fans, drain pumps or heating components in various harsh environments. Switch detection and overload monitoring functions can now be implemented in a cost-effective manner, paving the way for the development of remote maintenance devices for home appliances.

Conclusion

　　By developing robust and fully planar AC power switches and AC power control circuits, product performance, power and resource saving performance, as well as user safety and ease of use have been improved. These devices increase the electromagnetic compatibility performance of the power drive process, and their overvoltage robustness and transient interference resistance enable the equipment to meet and exceed international EMC standards.

　　While controlling bill of materials costs, they also improve reliability and the quality of application control. Because they simplify the design of power control, design resources can be refocused on user-oriented functions and system differentiation functions. The new generation of intelligent ACS breaks the barriers of discrete devices and integrates with power control to improve the safety performance of electrical loads and provide time-saving fault diagnosis functions.