Increase the safety of home appliance systems with intelligent AC control
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Today, at least 60% of household goods in the world use electronic devices. For the electrical equipment being 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 partly the result of increased demand for energy and water conservation, as well as increased consumer usability.
Performance and cost-effectiveness are always major challenges for home appliance manufacturers, and market globalization and standards have made it difficult to address these challenges. It is because of this that the control of the AC power source is constantly changing. The strong demand for differentiation in the appliance market is forcing manufacturers to improve the electrical performance of their systems and provide new features in terms of power savings or energy savings in normal or standby mode.
Performance and security improvements
When designing a control system, the designer's goal is to pursue high immunity to interference and increase robustness. The IEC61000-4 family of standards addresses electromagnetic compatibility requirements for AC power lines such as high voltage surges, fast transient shocks, and electrostatic discharge. This standard defines the level of anti-interference of the power control board and the specific standard value, and requires the gradual increase of the anti-interference ability and robustness of the home appliance system.
For example, in the design of the refrigerator, better food preservation performance and higher compressor efficiency are achieved by using digital control: a cabinet with a temperature difference of 3 °C can be reduced by 20%. A safety aspect of the washing machine is the ability to collect and analyze electrical and laundry parameters to avoid spillage or water shortage. With the help of the power control circuit, it is possible to command the stop of the heating device, open the water valve or open the drain pump.
Continuous improvement in solid state AC switches
On early boards, thyristors met tight, compact applications. SCRs are five times smaller than relays with currents less than 1 amp, but offer EMI-free switching performance, fast response times, and millions of switching cycle reliability, as well as lower power drivers. To further simplify the power supply design, the thyristor has been modified to remove the snubber 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 over their rated blocking voltage (VDRM/VRRM). Outside of this range, overvoltage will permanently degrade its switching performance: an uncontrolled overvoltage trigger will excite hot spots in its junction. Therefore, the thyristor must be protected with an external suppressor.
The most critical constraint is the voltage surge described in the IEC61000-4-5 standard. Surge is generally required to withstand 2kV 1.2/50us. 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 like a varistor to absorb the surge energy; Crowbar - the thyristor is safely connected, Surge energy is consumed into the load impedance.
Developed based on bi-face full planar technology, the new protected thyristor has excellent built-in overvoltage robustness, thus increasing system reliability. When the terminal voltage exceeds the avalanche voltage, the switch is reliably triggered to the short circuit mode. The voltage drops rapidly to a few volts and the overvoltage is converted to a current flowing through the switch. The planar process AC switch then resumes the blocking function at the end of the cycle, which is consistent with the IEC 60730 standard.
ACS achieves further improvements while achieving integration goals for reliability and ease of design. This new switch incorporates a gate level shifter that allows the MCU logic level drive to have higher resistance to electrical transients. For example, a 0.8A switch can guarantee 500V/us surge immunity, which is 10 times that of a thyristor with the same door sensitivity (IGT=10mA).
Since no noise suppressor is required, the design is simplified and the entire control can meet the IEC61000-4-4 standard. When performing the switch control of the water valve, a 0.8A AC switch can safely withstand the cutting operation, and the clamp can be used to absorb the induced energy of the load. The guaranteed switching energy capacity of the design must utilize a 28H high inductive load for critical test verification.
At present, the unique gate structure of the ACS switch makes the back electrical performance of the chip more stable, which is impossible in the previous thyristor architecture: the AC switch array can be packaged in a single package, specifically for dishwashers Centralized brake drive in the application.
Energy saving in the refrigerator
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 due to its leakage. This loss can be eliminated if the PTC is turned off with a solid-state AC switch after startup. Smooth temperature control reduces average input power by 20% and increases compressor on-and-off repetition rate by 50%.
The 10-year life of the compressor is equivalent to 270,000 cycles of on-off, which explains the significance of using solid-state technology. The new planar thyristors or ACSs provide the required off-state reliability due to their resistance to 2kV overvoltage and 200V/us transients. With system costs similar to those of electromechanical solutions, breakthroughs in solid state technology enable refrigerators or other refrigeration equipment to meet A+'s energy consumption standards, resulting in better food storage and no transient discharge and EMI interference.
AC switch and its control
With the application of system-in-package and power plane technology, it has been possible to imagine the use of micro-modules to combine AC switches with power control. Since there is a stable voltage behind the ACS chip, it can be replaced with a power IC for new functions such as fault detection or overload overheat protection.
The safety requirements for home appliances are enhanced in the UL and IEC standards. The home appliance controls the working state of the AC switch and detects the fault mode. At the proximity of the switch, a detection circuit is needed to sense the working state of the AC. Some serious load and appliance failures can be avoided, such as DC operation due to high inductive loads due to thyristor diode mode failure, and overheating of resistive loads, or overflow due to switch shorts. Innovative designs for small AC switches, such as ST's NeoS project, address these challenges by detecting switch failures in a cost-effective manner. An AC detector can be used to monitor all switch states and compare them to the driver to prevent major failures in the appliance.
Figure 1: Protected AC Switch: NeOS Example.
In addition, the switch's ability to implement thermal protection opens up a new way to prevent overload. It is known that the thermal state of the AC switch can be designed with a turn-off protection so that the switch can detect a fault if the electrical brake is mis-connected during the final assembly phase or if it is gradually damaged by a specific stress. The warning signal provides information to the control circuit, limiting the scope of repair to the brakes, not the entire device.
Figure 2: The switch fault detector prevents catastrophic events in appliances.
Such protection has proven to be viable through tests of severe electrical loads in harsh environments, such as cold air fans, drain pumps or heating devices. Switch detection and overload monitoring functions are now available in a cost-effective manner, paving the way for the development of remote maintenance equipment for home appliances.
Summary of this article
Product performance, power savings and resource-saving performance, as well as user safety and ease of use, have been improved through the development of robust and full-plane AC power switches and AC power control circuitry. These devices increase EMC performance during power-drive, and their overvoltage robustness and immunity to transients allow devices to meet and exceed international EMC standards.
They also improve the quality of reliability and application control while controlling material costs. Because they simplify the design of power control, design resources can be refocused on user-oriented features and system differentiation. The new generation of intelligent ACS breaks down barriers to discrete devices and integrates with power control to improve the safety of electrical loads and provides time-saving troubleshooting.
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