Easy-to-use inductive cooking surfaces (hobs) are gaining acceptance by consumers as they are becoming more affordable. They are significantly safer, without flames or other direct heat sources on the hob, and they have better overall performance, including faster heating time.
Even though induction technology is well founded and proven, the design of equipment to drive the inductive plate—causing metal pots to heat up—requires designers to understand a wide variety of physical principles and design techniques. Lying below the surface of the relatively simple block diagram of an inductive hob, the required techniques include several distinct areas of analog- and digital signal processing, electrical protection, and isolation.
For example, safety standards require isolation between the user interface and the power supply. There are three primary loci of isolation:
A safe system must meet a minimum of two of these isolation requirements. This article will discuss innovative solutions that allow isolation of the IGBT gate drivers and the user interfaces.
Description of the System
Figure 1. Inductive heating system.
The inductor current waveform is created by a high-efficiency switched dc power supply and a pair of IGBT switches. The switches are driven by a microcontroller, which responds to a feedback loop that forces conditions monitored by sensors to correspond to settings established by the user—and to remain within safe limits.
The main sensor, a transformer in series with the inductive plate, monitors the value of the current through the inductive plate in order to maintain the appropriate current value for the selected cooking level. This prevents damage to the power stage—the inductive plate and IGBTs—by decreasing the current level as necessary to avoid an overcurrent condition.
Since the inductance and capacitance of the inductive plate, pan, and transformer constitute a resonant LC circuit, one might think that the induction frequency could be determined by setting the values of L and C. Unfortunately, the inductance and capacitance values, and hence the resonant frequency, depend on the size, shape, and material of the pan that is being used. Thus, the different heating levels selected through the user interface can’t be set by fixed frequencies. A more effective way to set these working levels is based on a current measurement, which provides a measure of the dissipated power. The feedback loop allows the microcontroller to adjust the current level to correspond to the chosen heating level. The microcontroller adjusts the frequency of the pulse-width-modulated (PWM) waveform to adapt to the pan. The inductive-hob designer, already knowing the current that corresponds to each required heating level, simply programs the microcontroller to adjust the PWM frequency to provide the appropriate current for each heating level.
The frequency of the PWM signal that drives the IGBTs will typically range from about 20 kHz to 100 kHz. Considering that IGBTs have slower turn-off characteristics than MOSFETs, the switching frequency is limited to a few tens of kilohertz. The PWM signal from the microcontroller has a fixed duty cycle (say 50%); its frequency will be adjusted depending on the power required for the heating level selected by the user.
Due to the high voltages that can be generated in high-current inductive circuitry, it is important to provide electrical isolation at critical points in the system. In particular, it is essential to isolate the power stage of the inductive hob from the microcontroller and other digital circuitry. One way to do this is to use isolated IGBT drivers. A series of low-cost isolated gate-driver circuits, based on ADI’s innovative iCoupler® technology, has many advantages compared to traditional isolation solutions.
Galvanic isolation is a means of preventing current from flowing directly between two communicating circuits. There are two major motivations for using isolation. The first is to protect people and equipment where there is the possibility of exposure to high operating voltages or current surges. The second is to avoid ground loops and disruptive ground currents where interconnections involve differing ground potentials. In both cases, isolation techniques prevent current flow but allow for data- or power flow between the two circuits.
iCoupler technology (Figure 2) is a transformer-based approach to isolation. Integrating microtransformers and electronic circuitry, it has all the advantages of optocoupler-, discrete-transformer-, and semiconductor technologies—but without the disadvantages of optocouplers and discrete transformers. Optocoupler limitations include excessive power consumption, large timing errors, data-rate limitations, and sensitivity to temperature. In iCoupler-based products, insulation to meet the requirements of safety agencies is achieved through the use of a 20-µm-thick polyimide insulation layer between the transformer coils. It is capable of achieving an isolation rating of greater than 5 kV rms. This technology uses patented refresh circuitry, which updates the output to correspond correctly to the input state when input-signal transitions are not present, thus avoiding the inherent inability of discrete transformers to achieve correct dc levels.
Figure 2. The anatomy of iCoupler technology.
iCoupler technology provides benefits in five key areas:
Isolation of IGBTs Using iCoupler
Figure 3. Functional diagram of the ADuM1233.
The input circuit’s power is provided by an isolated power supply and may require one or more stages of voltage conversion. A 5-V power supply is required for the microcontroller and the rest of the system, and the IGBT circuit requires 15 V for efficient operation. The iCoupler-isolated gate drivers must supply up to 100 mA of peak drive current, so an additional gain stage is required, as shown in Figure 4.
Figure 4. Driving the IGBTs using iCoupler isolation.
Because of the importance of the timing relationship between the two channels—with the IGBTs being driven by PWM signals in antiphase—the speed, stability, and reliability of the iCoupler technology are especially advantageous compared to LEDs and photodiodes. The curves in Figure 5 show that the propagation delays on the rising edges of the two channels are matched to about 100 ps—and on the falling edges to better than 1 ns—over the 12-V to 18-V output- and 4.5-V to 5.5-V input power-supply ranges.
Figure 5. Propagation-delay channel matching as a function
of power-supply voltage.
The resulting timing margins ensure fully complementary switching of the IGBTs, improving the efficiency of the power stage and the overall system.
As noted, the ADuM1233 offers true galvanic isolation between the input circuitry and the outputs of the device, and between the two output circuits. Each isolated output can operate at up to ±700 V with respect to the input, thus supporting the negative voltages of the low-side power supply (–HV in Figure 4). The difference between the high- and low-side supply rails (+HV and –HV) must not be greater than 700 V; however, this is compatible with the voltage rails typically used for powering inductive cooking.
Isolation of the User Interface
with iCoupler Technology
Integrated isolation solutions available in iCoupler technology reduce space requirements and design complexity at low cost. The ADuM1250 shown in Figure 6, and ADuM1251, embody true bidirectional isolation and incorporate a buffer to eliminate glitches and lockup. This degree of comprehensive integration limits the required external components to two bypass capacitors and two pairs of pull-up resistors (specified in the I2C standard)—and provides an I2C interface at low cost. Details on applying these devices can be found in AN-913 Application Note Isolating PC Interfaces.
Figure 6. ADuM1250 dual hot swappable I2C isolator.
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