Understanding how modern induction cookers work

STMicroelectronics Pty Ltd

By Akshat Jain and Ranajay Mallik, AMG Central Lab, STMicroelectronics, India
Friday, 21 April, 2017


Cooker

The induction cooking craze has been getting lots of buzz, even though the technology has been around for about a century. The induction cooktop is quite popular, both in domestic and commercial usage, and is considered one of the advanced technological innovations in the field of cooking.

Induction cooking is a type of electric cooking that uses magnetic coils to heat cookware. The beauty of induction cooking is that the cooking surface itself remains cool while heat is generated within the cookware. Cooking by means of induction cooktop is quick and highly energy efficient as compared to other conventional methods; on top of this, the lack of open flame makes it safer too.

How does it work?

Induction cooking is the process of generating heat directly in a ferromagnetic utensil/pot/cookware by means of electromagnetic induction and subsequent generation of eddy currents. Electromagnetic induction principle was discovered back in 1831 by Michael Faraday. Electromagnetic induction refers to a phenomenon by which electric current is generated in a closed circuit by the fluctuation of current in another circuit placed next to it.

The ferromagnetic cookware is to be positioned on top of an induction cooktop. Below a ceramic or glass surface lies a resonant coil. The induction cooktop and cookware can be considered as a transformer in which cookware act as shorted secondary (load). An alternating is made to flow through the resonant coil, which leads to generation of oscillating magnetic field. The magnetic field induces an electric current inside the cookware.

The induction cooktop only works with cookware made of certain materials that have specific properties. In order to be heated by the magnetic field, the cookware has to be made of a ferromagnetic material, such as stainless steel or iron.

Advantages of induction cooking

The induction cooktop will consume power as long as the cookware is present on top of it. Unlike a gas burner or electrical stove, the induction cooktop is incapable of producing heat on its own. In the case that the induction cooker is made to operate while no cookware is present on top of it or the cookware is removed while induction cooktop is in operation, the resonant coil sees as if there is no load (open circuit) and there will be no energy transfer. In the case of no cookware, the induction cooktop enters into sleep mode in order to have minimum standby power consumption (<1 W).

Which resonant power conversion topology is used for induction cooking?

Generally, semiconductor devices are used as a switching element in various power converters. Insulated gated bipolar transistors (IGBT) are used in induction cooking systems. For power conversion, ‘soft switching’ techniques are preferred over ‘hard switching’ in order to have minimum switching losses. In soft switching, the voltage or current is manipulated to become zero across the resonant switch at the moment of switching. Soft switching can be further categorised in two methods: zero-voltage switching (ZVS) and zero-current switching (ZCS).

Both ZVS and ZCS switching methods have their pros and cons, as well as application-specific use cases. The voltage or current administered to the switching circuit can be made zero by using the resonance created by an LC resonant circuit. This sort of converter is known as ‘resonant converter’ topology. The two main resonant power conversion topologies used in induction cooktops are quasi-resonant converters and half-bridge resonant converters.

Parameters  Quasi-resonant (QR) converter Half-bridge (HB) resonant converter
Switching circuits requirements One IGBT required Two IGBTs required
Heat sink and PCB size Relatively small Relatively large
Price Relatively less Relatively more
Switching circuit stability Relatively unstable Stable
Power rating <2000 W <5000 W
IGBT voltage rating range 1200–1350 V 600–650 V
Market Domestic Commercial

Table 1: Comparison between quasi-resonant and half-bridge resonant converters.

Control algorithms

Induction cooktops work on the principle of an LC resonant converter. The resonant frequency not only depends on resonant tank circuit, but also on the size and material of the cookware/utensil. This causes the system to have an oscillating resonant frequency. In order to control the power delivery of the system (transferred to utensils/cookware), input mains voltage and current through IGBT are monitored by microcontroller and the system switching frequency is adjusted.

Quasi-resonant topology-based induction cooker

The 1.8 kW quasi-resonant induction cooktop system has been developed using an STGWT20IH125DF 1250 V, 20 A IH series trench gate field-stop IGBT and a STM8S003F3 value line 8-bit microcontroller from STMicroelectronics. The system includes a comprehensive safety mechanism to handle voltage transients and inconsistent cookware/utensils. This system is shown in Figure 1.

Figure 1.

Half-bridge topology-based induction cooker

The 3.5 kW half-bridge induction cooktop system evaluation board has been developed using the STGW40H65DFB HB series 650 V, 40 A high-speed, trench gate field-stop IGBT, an L6491 high-voltage high- and low-side 4 A gate driver and an STM32F072 32-bit MCU from STMicroelectronics. As above, the system incorporates a comprehensive safety mechanism to handle voltage transients and inconsistent cookware/utensil.  The system is shown in Figure 2.

Figure 2.

Features Quasi-resonant (QR) induction cooker Half-bridge (HB) resonant induction cooker
Input voltage 150 V to 280 VAC 150 V to 280 VAC
Power levels Eight power levels up to 1.8 kW Eight power levels up to 3.5 kW
Soft start Yes Yes
Power consumption (sleep mode) 0.5 W 0.5 W
IGBT voltage rating 1250 V 650 V

Table 2: Comparison between quasi-resonant and half-bridge resonant induction systems.

Conclusion

Induction cooking is a promising technology for those seeking sustainability in the kitchen, and is clearly a better choice over standard electric or the expense and carbon footprint of installing a gas line.

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