Components for induction cookers
Energy efficiency in the kitchen
Induction cookers are extremely energy efficient. EPCOS supplies a full range of components, including film capacitors, chokes, transformers, thermistors and varistors.
Induction cookers are the true king among cookers. Their high efficiency, increasingly important in times of rapidly rising energy costs, greatly distinguishes them from their conventional counterparts. With an efficiency rate of close to 90 percent, they are far superior to gas cookers (40 percent) or conventional electric cookers (50 to 60 percent). Leading European manufacturers of kitchen appliances such as BSH Bosch Siemens Hausgeraete, Fagor and Arcelik now manufacture these high-tech appliances in series.
Induction cookers achieve this high efficiency by means of a sophisticated technology that heats up only special cookware that has a ferromagnetic base. The induction cooker itself produces no heat, but generates an alternating magnetic field in the cooking zone via a coil operated at a frequency of 20 to 50 kHz. This field in turn generates eddy currents in the base of the cookware, thus producing the required heat. This effect is further reinforced by hysteresis losses that generate more heat. The cookware acts like a short-circuited secondary winding of a transformer. Figure 1 shows the basic configuration of the power circuit of an induction cooker.
EMC input filters
Due to the relatively high frequencies and steep switching edges of the IGBTs, sufficient EMC filtering is required. Figure 2 shows the circuit diagram of the filter used. The B32923A2105M* series with a capacitance of 1 µF is particularly suitable for the two X2 capacitors. Its outstanding feature is a low insertion height of only 22 mm. Among the Y1 capacitors, which suppress symmetrical interference and transients, EPCOS offers the B81122C1222M* type with 2.2 nF and the B81122A1153M* type with 15 nF. EPCOS currently offers the most space-saving X2 and Y1 capacitors on the market.
EPCOS also implements an extensive range of power chokes in normal and power compensation variants in its B82726* series.
|FIG. 2: BASIC CIRCUIT DIAGRAM OF THE EMC INPUT FILTER|
The input section also contains the key protective functions of inrush current limiting and overvoltage protection (Fig. 2).
The current limiter performs two important functions:
- First, because the intermediate circuit capacitors are still uncharged at the moment of turn-on, direct application of the power would cause high charging currents to flow for a brief period, thus endangering the rectifier. An inrush current limiter (ICL) based on a leaded disk NTC thermistor reduces the inrush current to a tolerable level. These thermistors are cold at the moment of turn-on and thus have a relatively high resistance. As the current flows through them, they heat up and their resistance drops. Thanks to this behavior, they not only protect the rectifier but also prevent the power fuse from blowing. In induction cookers and other household appliances, this task may be performed by the ICL of the B57153S* series, for example.
- Second, to protect the electronics from overvoltages, the use of varistors is recommended. For this function, EPCOS offers the B7221* series that covers all typical power voltages.
Figure 3 shows a selection of EPCOS protective components typically used in the input circuit of induction cookers.
|FIG. 3: EPCOS PROTECTIVE COMPONENTS|
The basic circuit diagram of the power circuit that supplies the induction coil is shown in Figure 4. In addition to the IGBT power switches, this subassembly contains capacitors with various functions.
|FIG. 4: POWER CIRCUIT OF INDUCTION COOKERS|
Because switching of the induction coil produces high induction-voltage peaks, the collector-emitter path of the IGBT must be suitably protected. This function is performed by snubber capacitors: numerous types are available from EPCOS. In addition to the leaded versions of the B32652A1153K* series, special types with strap terminals are available that can be screwed directly onto the power semiconductors of frequency converters. These capacitors feature extremely low ESR and ESL values. As with all film capacitors from EPCOS, their self-healing design greatly improves their reliability.
Intermediate circuit capacitors
Intermediate circuit (DC link) capacitors are needed to smooth the rectified AC voltage and to minimize ripple voltages. As a rule, aluminum electrolytic capacitors with snap-in or screw terminals are used for this purpose in industrial electronics applications, such as rectifiers and power supply systems. In the case of induction cookers, however, film capacitors are often preferred for space reasons. EPCOS offers a wide range of capacitance and voltage values for this purpose in its B32564* series. These capacitors are also distinguished by low parasitic ESR and ESL values, depending on their type.
Operation of induction coils requires resonant capacitors to be connected with them in series. Here too, film capacitors are used. The B32653* series was designed for this purpose.
Temperature protection and control
Suitable temperature monitoring is required to prevent thermal overload of the IGBT power switches. A fast and heat resistant surface mount temperature sensor is used in induction cookers: K560 with aluminium housing and K1560 with ceramic housing
Main Features and Benefits:
- NTC thermistor potted into aluminium/ceramic case with cable outlet
- Temperature range: -10°C to 250°C, short-term peak temperature up to 280°C
- Short response time in air stream
- High insulation voltage
- Virtually no influence by electromagnetic energy
- Flange housing for good thermal coupling to hot surface
|FIG. 5: SENSOR REGULATES COOKING TEMPERATURE|
Electronic components from EPCOS also perform key functions in the control and operating units of induction cookers. Thus, SMT inductors and transformers are used in DC/DC converters. Single-ended aluminum electrolytic capacitors or multilayer varistors are other indispensable components.