Modern Induction Cooking Demands Compact and Efficient Solutions


Giuseppe DeFalco at Infineon Technologies AG

Discrete IGBTs are the preferred power switch for modern inverter-based induction cooking products due to their inherent efficiency. As energy costs continue to rise and consumer demand for ever-smaller cooking solutions increases, IGBT technology has to evolve to meet these demands.

In this article, Infineon Technologies describes the challenges in this sector and introduces a new technology that meets the technical and price challenges of this demanding market.

The basic principle of induction heating was discovered by Michael Faraday in 1831 and was further developed by Heinrich Lenz. During their experiments with magnetism and EMF they found that during the switching of the magnetic fields, heat was generated in the core.

Based upon this basic principle, induction hobs use a magnetic field to directly heat the cookware and, therefore, the food. The popularity of this style of cooking is increasing due to it being more energy efficient than gas hobs as only the cookware is heated. It is also quick and highly controllable as just the strength of the magnetic field needs to be changed to alter the level of heat.

The same induction technology is now being used in rice cookers where it allows for a better spread of heat than a standard heating element as well as instantaneous and precise temperature changes. A key benefit of induction-based cooking is that the cooking surface is fully sealed and therefore easier to clean, making the cooking process more hygenic than other methods.

As induction cooking devices become more popular, then consumer expectations increase, leading to greater challenges for designers. Efficiency is a key concern of many consumers, driven by rising energy costs worldwide and by the increased regulations strictness. Designers also have to be concerned with safety and reliability, ensuring that a product does not fail leaving the consumer with no means of cooking and damaging the supplier's reputation.

In order to differentiate their product portfolio, many manufacturers are offering advanced features such as enabling Wi-Fi control. While consumers assess these additional features, value and price are increasingly important criteria for selecting an induction-based cooking device.

At the heart of the induction cooker is an Insulated Gate Bipolar Transistor (IGBT) controlling the key power switching function. Therefore, the IGBT is a key factor in the efficiency, size, reliability and cost of the end product.

Clearly, the selection of the best IGBT solution is critical for induction cooker designers. In most cases designers will focus on key parameters such as the maximum collector current (IC) and the maximum collector-emitter voltage (VCE)to ensure that the IGBT is capable of controlling the required power (often up to 2100 W for cooking applications) as well as VCEsat and Eoff which arecritical for the operating efficiency of the device.

Choosing the optimum topology for the circuit is also important as the design needs to be simple, reliable and energy efficient. One of the most popular topologies used in induction cooking is the Single Ended Parallel Resonant Inverter (SEPR), despite its relatively limited power level.

Figure 1: The Quasi-Resonant single switch Inverter is commonly used in induction cooking applications

The topology consists primarily of a parallel inductor and capacitor resonant tank network along with a combined IGBT and diode and a small capacitor which improves EMI performance and, together with the diode, provides a path for the inductor's resonant current flow. The inverter is generally powered by a mains line voltage that is rectified but not significantly filtered, thus achieving close to unity power factor correction.

Typical operating frequencies are in the range of 20 to 60 kHz, thus avoiding the audible range entirely. The switching frequency is controlled, with soft-start operating at the higher frequencies and maximum power being delivered towards the lower part of the range. In general, the needs of induction cooking applications are simpler than motor drives as there is no need for hard switching capability, high short circuit ratings or special package types.

Soft-switching topologies significantly reduce the switching losses of the IGBT through the use of zero-voltage-switching (ZVS) or zero-current-switching (ZCS) modes of operation. In hard switching applications the voltage drop across the IGBT switch can cause considerable power losses. However, in soft-switching applications these losses are almost negligible thus improving the overall efficiency of the system.

The RC-E series

One of the latest reverse conducting IGBT devices to reach the market is the RC-E series from leading power semiconductor device manufacturer, Infineon Technologies AG. Based on the same application-specific technology as the world's leading discrete IGBT family (RC-H), the RC-E series is cost- and feature-optimized for resonant applications including low / mid-range induction cookers.

Infineon currently offer two devices in the RC-E series, the 15 A, 1200 V IHW15N120E1 and the 25 A, 1200 V IHW25N120E1. Similar to other reverse-conducting IGBTs from Infineon, the RC-E incorporates a monolithically-integrated reverse conduction free-wheeling diode within the IGBT itself, thus eliminating the need for an separate diode (usually co-packed with the IGBT) in soft-switching applications, making the RC-E series very easy to use.

Figure 2: The RC-E reverse conducting IGBT includes an integrated free-wheeling diode

Combining a fieldstop layer with the trench gate structure, the RC-E has a much improved saturation voltage and consumes very little energy at turn-off. The thinner substrates increase the conduction and switching performance over earlier NPT technology. When compared with co-packed diode solutions, the RC-E offers improved power density and, since the diode and IGBT use the same die area, the diode is rated at the full nominal current.

With low Eoff,  VF, Rth and Vce(sat), the RC-E is optimized for low switching and conduction losses, thus offering very similar performance to the market leader RC-H3 across a wide power range. The devices support the most common blocking voltage (1200 V) and are optimized for switching frequencies in the range of 18 to 40 kHz.


Figure 3: The RC-E family offers market leading performance

The power losses of the RC-E family are market-leading and, as shown above, substantially better than competitive devices. This lower loss level allows designers to easily achieve the key goals for induction cooking applications.

The lower losses mean that less energy is consumed while cooking, leading to lower operating costs for consumers. With less waste heat, the RC-E will run cooler, leading to greater reliability. As less cooling is required for a given power level, end products immediately become smaller as heatsinks reduce in size and, therefore, cost. In addition this contributes to better efficiency and increased lifetime of the IGBT.

As the RC-E series is packaged in the industry-standard TO-247 package replacement or upgrading of existing designs is very simple, allowing designers to improve specifications and costs of existing products with minimum effort and design risk.

Figure 4: The RC-E is packaged in the popular TO-247 package allowing easy replacement or upgrading

The complete range of discrete IGBTs for induction cooking comprises several series. The RC-H5 is ideal for high frequency range (>30 kHz) and offers the lowest losses for the highest efficiency.

While no driver IC is required in many induction cooking applications, the RC-E is ideally paired with the IRS44273L when a driver is required. The low voltage non-inverting gate driver features a current buffer stage and supports both MOSFETs and IGBTs. The monolithic construction is enabled by latch-immune CMOS technologies and driver inputs are compatible with either CMOS or LSTTL logic levels.

The E1 (RC-E) offers the best price / performance ratio of all devices. The 15 A version is ideal for lower power designs up to 1800 W. The RC-E is the ideal choice when price and performance are the key factors for the end product.

More information: Infineon    Source: Bodo's Power Systems, February 2017