The STEVAL-KITXCB is ST’s first evaluation kit supporting the new Ki (pronounced “kee”) wireless power standard for cordless kitchen appliances requiring up to 2.2 kW, such as kettles, mixers, rice cookers, and more. Our reference design includes, among other things, microcontrollers, like the STM32G4, IGBTs, such as the STGWA50IH65DF, the ST25R3916 NFC readers, or the VIPer31 high-voltage converter. Engineers can, therefore, design cooktops with induction heating and wireless power transmission and tabletops that can power kitchen appliances cordlessly. The technology offers more than 92 % efficiency, like a power cord, and new safety protections to guard against foreign objects and overheating, as well as secure and robust authentication thanks to an STSAFE secure MCU.
How will Ki convince consumers?
Getting rid of the power cord in kitchens
In essence, Ki is Qi for the kitchen. Most consumers are now familiar with wirelessly charging their smartphones, even if they don’t call it Qi, the standard behind the technology. Just like Qi, Ki uses inductive charging, meaning that a current is applied to a coil on the cooktop or countertop to create a magnetic field. The coil on the receiver device then picks up that field, transforming it into an electrical current. Hence, while the principles remain the same, Ki must deal with far more power and new environments, such as appliances that also serve for induction cooking. Consequently, there are new safety mechanisms in place.
New safety mechanisms
To ensure that the transmitter and receiver communicate properly and prevent hazardous situations that may arise from the presence of a foreign object or an incompatible system, Ki relies on NFC communication. Consequently, the new standard implements a complex handshake mechanism that includes more guardrails and greater security.
For instance, the NFC card on the appliances tells the reader on the transmitter how much power it must send, which significantly simplifies power negotiations and helps reduce the risk of an overshoot. It also makes foreign object rejection much more straightforward since Ki can reject anything that doesn’t communicate properly with the reader. But NFC is also there for more than just the handshake, as it also provides auxiliary power to the appliance’s user interface at startup, implementing reliable energy harvesting before establishing the main power transfer.
Additionally, because Ki deals with high-power systems, compared to smartphones, it includes features that mitigate risks. For instance, power delivery must start within 60 ms, or the system will interpret this as a failure and stop sending power. The receiver can also dynamically change the load requested based on whether it needs more or less power. For instance, a blender will call for more power and immediately reduce the load once the machine stops blending, but it must remain on to power a screen or a light. And, as Qi charging requires higher switching frequencies (between 100 to 205 kHz) than an induction stove (between 20 to 75 kHz), engineers must include new hardware to support these features.
How will the STEVAL-KITXCB make Ki more appealing?
STM32G4 and ST25R3916
To tackle the need for precise frequency control and rapid load changes, the STEVAL-KITXCB includes the STM32G474RE, our mixed-signal MCU with high-resolution timers and a filter math accelerator (FMAC) to optimize digital power applications. Its Cortex-M4 at 170 MHz can ensure rapid and precise communication with the receiver and support drastic fluctuations in the power output, which can go from 50 W to 2.2 kW at any moment’s notice. The MCU also includes a high immunity against transient voltages and robust IOs, making it a prime candidate for the harsh environment of a Ki cordless kitchen station. Yet, the STM32G4 must also ensure the bill of materials remains low to foster democratization.
For instance, Ki includes a slotted communication technique that ensures the ST25R3916 NFC reader and card communicate well and the system can rapidly check for foreign objects. When the AC mains is close to 0 (zero-crossing), the system stops sending power for a very brief instant (1.5 ms) to enable NFC communication without interference. Then, the system resumes operation. Since the interruption is so short, the user is oblivious to it, as the appliance continues to work flawlessly. That’s how the Wireless Power Consortium can create new safeguards to protect users during operations. However, such a feature is only possible when using a device like the STM32G4, which is capable of rapid processing.
Power efficiency
The other critical aspect of the reference design is its efficiency. As many kitchen appliances may require up to 2,200 W, any power losses will significantly impact the system’s temperature and overall design costs. That’s why ST targeted more than 92 % efficiency, which is higher than what a traditional Qi wireless smartphone charger offers. To make this possible, the STEVAL-KITXCB uses a half-bridge topology and ST’s IGBTs with a trench gate field-stop to reduce switching losses and thus improve efficiency. Ultimately, there are also other ST devices in the STEVAL-KITXCB, making the reference design a testament to the comprehensive approach we are adopting to facilitate Ki adoption.
