IEC safety standards for electrical appliances permit system self testing, which can help save the cost of implementing sensors to detect abnormal operation. Software and firmware-based safety functions provided by microcontroller or system-on-chip vendors can further simplify the designer’s task
By Alberto Guerra, VP Strategic Marketing Development
and Ali Husain, System Design Manager, International Rectifier
Introduction: Appliance Safety Standards
Domestic appliances such as dishwashers and washing machines, as well as some types of non-household appliances such as dryers and commercial cleaning equipment, are required to meet basic and functional safety requirements as set out by the international standards IEC 60335-1 and IEC 60730-1.
The applicable standards create a distinction between basic safety and functional safety. Basic safety generally refers to risk of fire, electric shock or bodily injury, and is generally related to overheating of the motor and thermal stress on the winding insulation. In most cases these effects result from conditions such as jamming of the rotor leading to abnormally high current, or phase loss where one of the motor windings is disconnected or shorted out due to damage or degradation of insulation.
The traditional technique for preventing motor windings and insulation from overheating is to use a temperature sensor in the motor. This adds cost and increases system complexity. In addition, the chosen temperature sensor should be a component recognised by the certifying body. To avoid such costs and complications, phase loss detection, locked rotor detection and general overload detection are often implemented in software.
Functional safety refers to the risk associated with the normal operation of the product by the end user. In a dishwasher application, for example, the user may open the main door during the washing cycle to add or remove dishes. The inverter circuit controlling the main circulator pump needs to have a reliable motor speed signal to ensure that, by the time the door is open, the pump speed is reduced to avoid extremely hot water getting out of the washer. Similarly for a washing machine, if the door lock is released while the drum is still rotating, to allow access to clothes, this can create a hazard to the user’s arm. Hence motor speed control is considered within the scope of functional safety.
Modern practice, when implementing speed control, is to avoid the use of Hall sensors in order to save cost and complexity. Hall sensors also tend to be relatively unreliable, particularly at high temperatures. Sensorless speed control executed in software – and typically applied via a sinusoidal inverter drive to a Permanent Magnet (PM) motor so as to ensure high energy efficiency and low audible noise - is therefore becoming pervasive in modern appliances.
The precise safety standards that apply, in cases where any part of the basic safety functions is performed by software, are IEC 60335-1 Annex R and IEC 60730-1 Annex H Class B. To comply with these standards, the automatic control system for the appliance must include in its code all necessary elements to prevent unsafe operation without relying on any external redundant sensors or independent circuits. This can be achieved through the use of low-level selftest routines that periodically verify correct operation of the system.
Speeding-up Software Safety
Certification Vendors of microcontrollers optimised for use in appliances have begun providing ready-made self-test routines as software utilities, which may be added to the application code thereby saving significant software development effort. This approach can help speed up product testing and reduce the cost of achieving certification according to the IEC safety standards.
Microcontrollers positioned for use in domestic appliances often provide peripheral features such as timers, PWM blocks and ADCs needed to control the inverter driving the appliance motor. The Vector Control, or Field-Oriented Control (FOC), algorithm responsible for generating the motor-driving signals, however, is often implemented in software. A suitable algorithm may be provided by the microcontroller vendor, or the appliance designer may need to develop or source the algorithm independently. The appliance designer must also take care of other aspects of the motor controller, such as building and integrating the gate driver and power stage. These can be time consuming aspects of a project that also demand specialised design skills.
Alternatively, an appliance system-on-chip solution implementing a significant proportion of motor-control functions in configurable hardware can help designers overcome challenges such as developing FOC code and integrating the power stage.
An example of such a solution is IR’s IRMCK171, a One-Time Programmable (OTP) mixed-signal IC optimised for sensorless sinusoidal motor control in domestic and commercial appliances. This digital control IC is connected to an Intelligent Power Module (IPM) comprising an inverter power stage and gate driver built using HVIC technology, effectively creating a hardware chipset for appliance control. The ICs are part of IR’s iMotion™ integrated design platform, which provides everything needed to produce complete variablespeed motor-control subsystems for applications up to 2.2kW. Figure 1 shows how the functions of the digital control IC and integrated power module combine to control a permanent magnet motor.
The IRMCK171 features a 60MIPS, 8-bit 8051 microcontroller for hosting application-level functions, co-integrated with IR’s patented Motion-Control Engine (MCE™). The MCE implements an FOC algorithm in hardware. In addition to simplifying motor-control design, the hardware-based FOC also ensures faster execution resulting in improved motor torque and speed control. The 8051 microcontroller operates almost independently of the MCE and does not compete for system resources such as interrupts or internal registers.
Working in conjunction with the MCE is an Analog Signal Engine (ASE) that integrates all the signal conditioning and conversion circuits required for single current shunt, sensorless control of a PM motor.
Built-in IC Tests
To help designers accelerate safety certification in accordance with IEC 60335-1 Annex R and IEC 60730-1 Annex H Class B, the IRMCK171 is supplied with source-code level self-test routines.
For the 8051 part of the IC, the self-test routines are provided in the form of libraries, effectively presenting a set of function calls that implement the required 8051 power-up and periodic self-tests (safety checks). This permits designers to implement an IEC-compliant 8051 application with minimal effort. Figure 2 shows the power-up and periodic self tests implemented by software running on the 8051.
Power-up and periodic self tests are also required for the MCE. IR provides tests for the MCE that are built into firmware, rather than being supplied in library form. Since the MCE firmware is not usermodifiable, this approach provides additional safety assurance by eliminating any risk of programming errors. The MCE tests run in conjunction with the functions of the 8051 library, since the 8051 selftest library functions control and manage the MCE self-test functions automatically.
Of the two types of tests provided, the power-up tests execute once at system startup, immediately after power up or reset. Their purpose is to validate the basic functionality of the 8051 and MCE processors and memories. The periodic tests execute on a regular basis during normal runtime operation to monitor proper operation of system components, firmware and application software. Figure 3 lists the safety checks built into the MCE firmware.
Conclusion: Savings in Cost and Time
By taking advantage of pre-developed self-test routines, designers can simplify and shorten the processes that must be followed to obtain the required safety certifications from a recognised test house. When included as part of a dedicated appliance control platform based on a programmable mixed-signal IC that provides solutions to key motor control and power integration challenges, this approach can yield dramatic savings in development costs, project duration, and time to market.