In a cordless kitchen the used appliances are cordless, so this power needs to be transferred over a distance of at least 4 cm, at high efficiency. Therefore an inductive power system needs to be installed with an inductive power transmitter below the kitchen countertop and an inductive power receiver inside the appliance. Next to the inductive power system the cordless appliance should also be equipped with a communication system.
There are many transmitter topologies which can satisfy the functional requirements (like amount of power transferred, high efficiency, etc.) of the inductive power system. But satisfying the non-functional requirements (like EMI) most often takes several design iterations.
Presently, a transmitter based on a series resonance principle will be the first choice of the designer because it is relatively simple to understand and can satisfy the functional requirements of the system easily. The aim of the article is to introduce a new transmitter topology for the cordless kitchen system that can intrinsically minimise the problems associated with the present topology.
Wireless power in the kitchen worktop
An alternating current flowing through a coil produces its own alternating magnetic field. If another coil is placed in the proximity of the first one then, this alternating magnetic field gets linked to it. Such an arrangement of the coils is called as an inductive coupler.
In a cordless kitchen the inductive coupler consists of a transmitter coil and a mutually coupled receiver coil. The transmitter coil is located beneath the kitchen counter top, whereas, the receiver coil is located inside an electrical appliance (figure 1). As shown in figure 1, the system operates with the help of two channels: a power channel and a communication channel. The power transfer is executed by the power channel, whereas the information or data exchange is done using the communication channel.
The power transfer and the communication is done at a coil to coil distance of 4 cm. In order to get the maximum system efficiency, the principle of resonance is used in the inductive coupler. To achieve this resonant nature, different combinations of interconnections of both the coils (L) and the capacitors (C) can be used. These different combinations are known as topologies. Each topology has its own operating principle with advantages as well as disadvantages, depending on the application.
Present systems and its drawback
Figure 2 shows the topology which is used in the present inductive power system. This system has the series resonance principle applied in the transmitter as well as in the receiver side. In figure 2, Vin is the DC voltage obtained after rectifying the mains voltage. The transmitter is nothing but the AC-AC conversion stage comprising Vin, an inverter stage and a resonant tank comprising of the series connection of the transmitter coil (L1) and a capacitor (C1).
On the other hand, the receiver comprises of the receiver coil (L2) and a capacitor (C2) connected in series with the load (RL). If the transmitter (Tx) is powered, in the absence of the coupling (k), it has its natural resonant frequency (fTx). Similarly, in the absence of the coupling (k), the receiver (Rx) has its natural resonant frequency (fRx).
In practice, the fTx and fRx are chosen to be equal  and the coupling factor (k) between the transmitter coil (L1) and the receiver coil (L2) is in the order of 0.1 to 0.35. Therefore, the whole inductive power system has its own natural resonant frequency or frequencies (fr). For low coupling factors the fTx and fRx are merged into a single resonant frequency fr. For higher coupling factors the fTx and fRx frequencies are moved away from each other, resulting in fr1 and fr2.
Optimum operating point
The inverter stage of the transmitter can be operated at different operating points (fop). Choosing the optimal fop for the inverter is an important task because the high frequency AC voltage is applied to the transmitter coil, with the help of the power switches. Thus, one has to make sure that Zero Voltage Switching (ZVS) for the power switches is applied, avoiding high dV/dt’s in the inverter stage. Therefore, in this case, the optimal fop is chosen where the efficiency is maximum and input current (IL1) is slightly lagging the inverter bridge voltage (VLH,RH). With the help of the mathematical analysis, one can determine the optimal operating frequency for the inverter stage of the transmitter, using the particular values of the circuit components shown in figure 2 .
In the present topology, with the help of a frequency control loop the fop can be varied to achieve the maximum efficiency for the power transfer. Furthermore, at a particular fop the amount of power transfer can be controlled with the help of the duty cycle (D) control. The present system satisfies the functional requirements easily however it is much harder to meet the non-functional requirements. Figure 3 shows the voltage across the transmitter coil, when fop < fr1, fop = fr1 and fop > fr1 at D = 100 %.
It can be clearly noted that high dV/dt’s are observed in all the three cases. These high dV/dt’s may create a changing electric field between the node which carries the high dV/dt and surrounding conducting components, which has some kind of capacitive coupling to the environment. This can cause common mode currents to flow from the system to the environment and back via the mains, causing common mode noise emission. There are many protective means of reducing this electromagnetic interference (EMI) but, often these require additional and expensive components, like filters.
Requirements of the system
In the cordless kitchen system, the transmitter (Tx) remains the same and the receiver (Rx) changes, depending on which appliance is located on the kitchen counter top. Thus, it is beneficial if the drawback of the cordless kitchen system is minimized by modifying only the transmitter side. Thus, in the further studies different topologies for the transmitter are analysed keeping the receiver part the same.
The basic requirement from the new transmitter topology is to meet all the non-functional requirements intrinsically without compensating for the efficiency.
As mentioned earlier, the duty cycle control might result in the loss of ZVS of the power switches resulting in high dV/dt’s at the bridge nodes. The change in the input current w.r.t. time (dI/dt) as well as the change in the voltage across the transmitter coil w.r.t. time (dV/dt) may create electromagnetic interference (EMI) problems. This gives rise to the first requirement of the system: having a low EMI. Thus, one can summarize the requirements for the transmitter topology as: functional requirements:
Power transfer up to 2.4 kW,
- Efficiency > 90 %,
- Simple control system.
The most important non-functional requirement:
- Low EMI
Based on these requirements a new topology is identified and analysed in further sections.