The old saying that “a watched pot never boils” means that when impatiently waiting for something to happen, it seems to take forever – but that was before induction cooktops were invented. Now one can fill a pot of water, place it on an induction cooktop, turn on the energy and watch the water come to a boil within 30 seconds. Induction cooking uses electromagnetic energy to heat cookware made of magnetic material (steel, iron, nickel or various alloys). When the unit is turned on, the coils produce a high frequency alternating magnetic field that ultimately flows through the cookware. Molecules in the cookware move rapidly back and forth, causing the cookware to become hot and cook the food.

The market for induction cooking increased rapidly in recent years, mainly in Europe for induction cooktops and Asia for rice cookers. The technology is now experiencing a growth trend in the Americas, with CookTek as a leading provider of induction cooktops for commercial applications, using Infineon Technologies Insulated Gate Bipolar Transistors (IGBTs). High efficiency is driving this market towards high-performance, induction cooking products. In fact, when using an induction cooktop, over 90 percent of every dollar spent on energy goes right where it should go – into the pan. Gas delivers only 30 percent to the pan, and traditional electric about 65 percent.

Induction circuits

Enlarge this picture Fig. 1b. Reverse conducting-cell.

The most common circuit used today for induction cooking is the single-ended topology, which requires Infineon Technologies IGBTs with a blocking voltage of 1,200 V. The switching frequencies are in the range of about 25 kHz. It is obvious that only extremely low saturation voltage and low switching losses are suitable here in order to obtain a high efficiency. The Infineon TrenchStop™ technology is able to combine those two contradictory demands. This technology allows much thinner substrates and highly optimized doping schemes compared to a common Non-Punch-Through (NPT) technology. It is especially the fieldstop concept that reduces the saturation voltage effectively. The fieldstop principle is also applicable to thin wafer diodes.

A further development on the basis of the TrenchStop technology is the integration of a reverse conducting diode into the IGBT structure according to the cross sections, as seen in Fig. 1. This technology is called Reverse Conducting (RC), as these devices have a monolithic body diode for clamping reverse currents. The reverse conducting diode is formed using the intrinsic diode of the MOSFET structure with the IGBT P-layer shorted in specific areas to allow reverse current flow. This device is most applicable for soft switching applications such as induction cooking plates, microwave ovens, or rice cookers.

Fig. 2. Cross-section view of a TrenchStop-cell of a RC-IGBT, second generation. It is optimized for soft-switching applications, realizing a low saturation voltage and low switching losses.

On one hand, these RC-IGBTs combine a low saturation voltage and low switching losses of the IGBT. On the other hand, the integrated reverse diode has a low forward voltage drop and a low thermal resistance. Test results have achieved blocking voltages up to 1,600 V and static parameters and switching waveforms show the improvement of this technology.

The first products with this feature were introduced in 2003 with 600 V devices. Further implementations of this technology in 1,200 V devices followed in 2005. Now, Infineon Technologies offers a full range of voltage classes (600 V, 900 V, 1,000 V, 1,200 V and 1,600 V) with devices having the integrated reverse diode. Fig. 2 shows a cross-section view of a TrenchStop-cell of IHW20N120R2 (Inom = 30 A, VBRces = 1,200 V), one of Infineon’s second generation of RC-IGBTs.

Enlarge this picture Fig. 3a. Half-bridge topology for soft switching.

“Soft switching” topologies use external components such as inductors or capacitors in order to force either a low current or a low voltage during the switching transition. Design engineers use two major topologies, half-bridge and single-ended. These topologies dominate the market for induction cooking.

A half-bridge topology operates a resonant circuit shown in Fig. 3a, which consists of the inductance Lres and the capacitance Cres. Additionally, there is a snubber capacitance CS, which dominates the voltage rate of rise over the IGBT. The larger the capacitance CS, the slower the voltage increase dVce/dt during turn-off.

Enlarge this picture Fig. 3b. Typical waveform

The current shape in the resonant circuit is approximately sinusoidal. Both of the IGBTs take over the current for a half sine-wave period according to Fig. 3b. The IGBT is turned on during the conduction period of the diode, so that the turn-on energy of the IGBT is virtually zero. During turn-off, the snubber capacitor forces a slow increase of the collector-emitter voltage, so that the turn-off energy is also highly reduced.

A single-ended topology circuit and its typical waveforms are shown in Fig. 4. In contrast to the half-bridge topology, the current waveform is almost triangular (light blue) while the voltage waveform (dark blue) is sinusoidal. However, the IGBT is controlled in a way that it turns on when the voltage has reached the zero level again. This ensures a zero voltage switching during turn-on. The resulting voltage oscillation during turn-off leads to a low voltage level across the IGBT. The tail current therefore does not dominate the turn-off energy and the turn-off transition generates little losses.

Enlarge this picture Fig. 4. Single-ended topology for soft switching and typical waveform.

Infineon’s newest RC-IGBT IHW30N160R in 1,600 V/30 A addresses soft-switching applications such as induction cookers in single-ended topologies that need much higher breakdown voltages. The 1,200 V device is still the most common IGBT voltage class  used in these applications. A higher breakdown voltage gives the manufacturer a higher safety margin against over-voltage peaks that may occur for different reasons, such as net instabilities of the power source.

Enlarge this picture Fig. 4b

To account for a higher breakdown voltage, the thickness of the IGBT has to be increased to lower the electric field from the junction to the backside of the IGBT. This increase in thickness has two natural consequences, i.e. an increase in the on-state losses as well as the switching losses.

Enlarge this picture Fig. 5. VF vs. VCEsat of the new 1,600 V RC2-IGBT (IHW30N160R2), measured at nominal current of 30 A and at temperatures from 25 DegC up to 175 DegC.

Using TrenchStop-technology plus reverse conduction enables a device with low on-state losses (VF and VCEsat) and low switching losses. In Fig. 5, typical VF and VCEsat values of IHW30N160R are displayed for different temperatures. Comparing with typical values from competitors at 25 DegC yields the following for VCEsat:

• Competitor 1: 3.7V @ 40A.
• Competitor 2: 4V @ 40A.
• IHW30N160R2: 1.8V @ 30A, and 2.4V @ 60A.

In Fig. 6, measured Eoff values are displayed as a function of temperature for the whole class of RC-IGBTs. The trend shows with increasing breakdown voltage, the switching losses increase, which is due to the increasing thickness of the IGBT. As it was demonstrated for the 1,200 V RC-IGBTs, the TrenchStop-technology plus reverse conduction enables very effective devices, which is true for each voltage class.

One OEM’s experience

Enlarge this picture Fig. 6. Typical Eoff values of RC-IGBT’s for soft-switching conditions: voltage increase of 100 V per µs with a final voltage of 600 V, gate resistor of 15 V, collector current IC = 30 A.

CookTek designs and manufactures commercial induction equipment, primarily cooking and warming appliances. Fig. 7 shows several typical examples. On the lower right is a 240 V input dual element cooktop that provides up to 3,500 W of output power.

In the top left corner is a pizza delivery system. At the bottom of an insulated pizza transport bag is embedded a special energy storing and temperature sensing induction heating load pellet. The pellet consists of a special magnetic alloy that is encapsulated in a heat retentive plastic. The alloy activates the electromagnetic field produced by the charger, which in turn provides just the right amount of heat to be evenly dispersed throughout the pellet. When the pellet is fully heated, the induction charger automatically senses this, and the bag is now ready to maintain the perfect temperature for a hot pizza delivery time of up to 45 minutes.

The same induction technology is used in a variety of products, including multiple element cook tops and woks, as shown in Fig. 7.

Fig. 7. Sampling of CookTek induction products. At top left, a thermal pizza delivery system. Top right: single element wok style cooktop. Bottom left: single element cooktop. Bottom right: double element cooktop.

The key to CookTek’s products is their industry leading performance and reliability. For example, the induction inverters in these appliances operate with efficiencies in the mid-90 percent range. This level of efficiency is achieved with a parallel resonant tank inverter design that requires the IGBTs to conduct only the load energy replenishment current, and through a combination of zero voltage switching and low CESAT voltages. Infineon TrenchStop-technology IGBTs were carefully and specifically selected to provide the best performance in both of these areas. Efficiency with these parts in this design has been found to be so good that, for some products, the unit’s bottom plate suffices as the heat sink.

Commercial cooking applications demand exceptional reliability in the midst of highly stressful environments, in terms of ambient air temperature, humidity, grease, power line disturbances, etc. Infineon IGBTs have provided exceptional reliability by way of their excellent breakdown voltage characteristics and power handling capability. Outstanding device operating temperature ranges (Tjmax = 175 DegC) provide large thermal margins, even in highly stressful environments. Furthermore, the integrated reverse conduction diodes allow the elimination of external diodes, providing a cost reduction in many applications.

A watched pot does boil when induction-heating products are used. Reverse-conducting IGBTs with breakdown voltages in the range from 900 V to 1,600 V are used in induction-heating products. The advantages of lower switching losses and lower on-state losses by using Infineon Technologies TrenchStop-technology plus reverse conduction are found in CookTek induction heating products. Benefits of induction heating and new RC-IGBT technology are high efficiency (low power losses) and cost-effective induction-cooking systems.

For more information email: peter.stipan@infineon.com.