User interface – Display – There are several options available for the type of displays to be used in such a design. Selection of the display depends on the product target price and feature set needed. The most widely used display for an induction cooker design is an LED segment display or LCD glass but sometimes the OEM may prefer to use a graphic LCD display that requires an external display controller.
An LED segment display can be implemented with a few additional IOs and a timer for periodically updating the display. LCD glass can be implemented using similar resources but needs some special firmware to be able to generate different drive levels.
User interface – Buttons – There is growing demand for capacitive sensing based touch-sense buttons because of their sleek look and the fact that they don’t experience wear and tear, making them a preferred technology over mechanical buttons. Capacitive sensing-based touch buttons have to meet the following requirements:
Need to reject spurious detection due to water droplets (Water- proofing) – In an induction cooker, water will be spilled over the sensors. Thus, waterproofing becomes a key requirement for the design to ensure that there is no false triggering in the presence of water.
A Robust solution to work in noisy environments – In a microwave oven, huge amount of current flows in the system, so a lot of switching noise should be expected from the power supply. Variation in power supply gets coupled to the capacitive sensors and can impact the reliability of the UI if not designed properly. This requirement has to be addressed by following schematic / layout techniques in order to ensure a clean supply for the system.
SoCs have on-chip peripherals and features that can address all or most of the above mentioned requirements. For instance, the CY8C28645 is an SoC provided by Cypress which is capable of capacitive sensing and implementing other control functions as well. Figure 5 provides the block diagram of the induction cooker design using an SoC.
For pan-detection, when the pan is placed on the cooker top, the inductance of the resonant converter (L*) is increased. This effectively decreases the switching frequency. The sync control signal shown in Fig 5 is the zero crossing signal of the resonant converter voltage. The sync control pulses are counted for a fixed period. Thus the number of sync control pulses would be less when the pan is present compared to when the pan is absent. If the Pan is not present, the IGBT is turned OFF.
This device has a dual-channel with dedicated hardware that can scan two sensors at the same time. This helps in providing a faster response to a user touch. Also, as CPU intervention is less due to hardware implementation and less processing overhead due to dual channels, this allows the CPU to perform other tasks.
Features of this sensing technology helps us to achieve the requirements listed above:
Water proofing using Shield electrode
– Figure 6 shows a typical layout of a capacitive sensor printed circuit board (PCB). Instead of connecting the hatched pattern surrounding the sensor (blue area – representation of hatch pattern) to ground, for water proofing designs, it is connected to a shield electrode.
This electrode is driven by the same signal, which is connected to the sensors. Thus, when a water droplet falls on the sensor because of no potential difference between the hatched pattern and sensor, no additional capacitance gets coupled on the sensor, which in turn helps in achieving water proofing. When the sensors are completely submerged with water, then guard sensors gets activated, which can be used to cut-off all the sensors in the firmware.
Robust operation in noisy environments
– In any system where measurement is dependent on some reference, any variation in the reference impacts the accuracy and reliability of the measurement. For example, the power supply generally carries significant noise/ripples due to variations in load current and digital switching. By following proper schematic and layout guidelines, ripples in the power supply can be reduced; however, it is very difficult to remove them completely. If the reference is driven from +ve line of power supply, it may cause false alarms in capacitive sensing. Sensing techniques used in this device switches the sensors between GND and VREF (generated internally), thus any change in the power supply will not cause false triggers, provided they are within the specified operating limit of the device. However, while designing the PCB, ground traces must be laid out properly to ensure there is no ground bouncing. Figure 7 provides the implementation of capacitive sensing technique inside this device.
In this technique, the sensor capacitance switches between the integrating capacitor (Cmod) voltage and GND using two non-overlapping clocks Sw1 and Sw2. Output of the comparator in this circuit serves two purposes:
- Modulates the current source in order to maintain the voltage at make the voltage level at Cmod equal to the Vref.
- Acts as an enable signal for the timer via a flip flop.
A continuous bit stream gets generated at the comparator’s output, which thus measures the duration for which flip flop output is high in a given time duration. Because of the nature of this technique any signal on the input gets integrated for n number of clock cycles (n is determined by resolution), thus if any high frequency noise gets coupled on the input gets averaged out because of the integrating effect of this technique.
y: In today’s time, user safety is the critical requirement in home appliances. In case of any fault in the electronics, appliance must be shut down to avoid any unpleasant instance. There are safety certifications that are mandatory for home appliances to be sold in several countries. For example, Class B certification as specified in IEC60730 standard. To meet all the test conditions like IOs are working fine, ADC is working fine etc. will need additional hardware to connect various references to the pins and ADC inputs to ensure their proper working. Flexible routing and significant resources like references, inbuilt peripherals that are yielded by SoC, makes its easy, economical and less time consuming to implement various tests to ensure safe operation of the appliance.
In the competitive market where appliances need to be feature-rich and need higher user safety considerations, an integrated solution becomes the best choice for OEMs. SoCs available in the market are constantly allowing the OEMs to integrate more and more features into their design that too by keeping the overall cost of the system down. However, this approach of integrating the whole system holds good for smaller appliances like induction cooker or microwave oven but appliances like washing machine or refrigerator will still need a dedicated control for some specific functions like FOC motor control
, which will require an additional device in the system to perform those tasks.
About the Authors
Pushek Madaan is currently working with Cypress Semiconductor India Pvt. Ltd. as a Senior Application Engineer. His interests lie in designing Embedded system applications in C and assembly languages, working with analog and digital circuits, developing GUIs in C# and, above all, enjoying adventure sports. Pushek can be reached at firstname.lastname@example.org.
Sachin Gupta is working as Product Marketing Engineer with Cypress Semiconductor. He holds B.Tech in Electronics and Communication from Guru Gobind Singh Indraprastha University, Delhi. He has several years of experience in mixed signal applications development.