11 May Induction Cooker Reference Design with Digital Control of Induction Heating
Combined with the digital control requirements of induction heating and analyzing the system’s hardware and software designs, a corresponding digital control induction cooker is made. The system can make the voltage phase lead to the current phase when the working frequency of the half-bridge circuit is higher than the resonant frequency where the converter can work in a resistive-inductive load state. As a result, the switching loss reduces, and the working efficiency of the induction cooker will improve.
Keywords: induction cooker coil design; induction cooker reference design; induction cooker circuit design
With the rapid development of power electronics technology, induction heating technology has been developed and applied for induction cooker reference design in many fields. In the design of induction cooker, induction heating technology has also been applied, which has the advantages of high efficiency, energy-saving, and environmental protection, that has attracted people’s attention.
However, after the induction heating for the induction cooker is adopted, in the case of a high operating frequency, a large energy loss is generated. Thus, the work of the induction cooker will be a security threat. Therefore, the design of a digitally controlled induction heating for induction cooker should be strengthened to maintain the safe and reliable operation of the induction cooker.
1. Analysis of digital control requirements for induction heating induction cooker
From a structural point of view, an induction cooker consists of induction cooker coil design, a control circuit, a metal casing, etc. It is necessary to use a 20-40 kHz alternating current heating coil to generate a high-frequency alternating magnetic field. Placing metal cookware on the induction cooker can induce eddy currents, which can cause the cookware to generate loss and heat.
In principle, after the alternating current generated by the induction heating power supply that passes through the coil of induction cooker, an alternating magnetic field will be generated. The placement of the magnetically permeable object in cutting the alternating magnetic field (lines of the magnetic field) can generate an alternating current, ie; the eddy current and inside the object.
Under the action of the eddy current, the atoms inside the object will move randomly at high speed, collide and rub against each other, resulting in the generation of heat energy. Compared with induction cookers that use heat conduction methods such as electric heating sheets and electric heating rings for slow heating, induction heating for induction cookers is more efficient. However, the induction heating for the induction cooker needs to use a diode rectifier bridge and a filter circuit to process the input alternating current to obtain direct current. The direct current needs to be loaded into the induction coils on both ends of the load through the half-bridge inverter circuit so that the magnetic field is generated.
The induction heating power supply, includes a rectifier, filter, inverter, converter, controller, and load. The load is composed of the induction cooker coil design and heated parts, just like a transformer. In the case of high operating frequency, the converter load of the half-bridge inverter circuit is capacitive impedance, and the switching devices will be turned on, causing the diode to generate a large reverse recovery current, resulting in large device loss. If the load of the converter is inductive, the switching device can achieve soft switching, but the safety of the converter cannot be guaranteed, so it will also cause a large loss in the system. To ensure the safe operation of the induction cooker, the resonant converter must be an inductive load, so it is necessary to realize the digital control of the half-bridge resonant inverter circuit.
2. Design of Induction Heating Induction Cooker with Digital Control
2.1 Overall design idea of the system
Combined with the digital control requirements of the induction heating for induction cooker, the DSP chip can be used to design the digital control system of the induction heating for induction cooker. The induction heating for the induction cooker adopts the power frequency adjustment method to adjust the power output. The inverter part adopts a full-bridge inverter, the rectifier part adopts a single-phase uncontrolled rectifier bridge, and the control part adopts a DSP chip to realize digital control. During the working process of the induction cooker, single-phase AC output will be realized. After bridge uncontrolled rectification and large capacitor filtering, a smooth DC voltage can be obtained, and then the voltage will be supplied to the inverter. The inverter adopts a full-bridge inverter and adopts the thermistor with a negative temperature coefficient to suppress the surge current generated at the moment of system power-on.
2.2.1 Design of Inverter Rectifier Module
In the design of the system inverter rectifier module (in the single-phase bridge rectifier circuit), the average current of each diode is only half of the average value of the load current, and half of the AC input voltage is used through the cycle. Taking into account the grid voltage fluctuation and the reverse voltage that the diode bears, a single-phase rectifier bridge should also be selected (KBPC3510), and its rated current and voltage are 35A and 1,000 V respectively.
After filtering by a large capacitor, the resulting DC voltage will reach 264V, the resonant power will reach the maximum, and the inverter output maximum current will be 12.6A. Therefore, when selecting the switching device, it is necessary to complete the selection of the IMBH60D-100 IGBT with a rated current of 60A, and its rated voltage is 1,000V. By paralleling snubber capacitors, snubber resistors, and fast recovery diodes, the switching traces can be improved, and the voltage stress can be reduced, ensuring the reliable operation of the switching devices.
2.2.2 System Power Supply Module Design
In the actual induction cooker reference design, it is necessary to strengthen the design of the system power supply module. The system power supply consists of 2 parts: (1) 220V input AC voltage, and (2) output +5V voltage. After the AC voltage is input, and after rectification and filtering by the transformer, a voltage of 9V can be obtained to supply power for the MV7805 chip, so that it can output a voltage of +5V, which is the DSP chip power supply.
The output +5 V voltage flows through the power supply chip TPS767D318, and the +1.8V and +3.3V required by the DSP can be obtained. The power chip used is a linear low-dropout LDO high-precision digital regulated power supply, which is specially designed for DSP and can provide two voltage outputs. Each channel of the chip can provide 1 A DC, so the power supply requirements of the DSP can be met. With a 1.8 V output voltage, the DSP core can be powered. Using the 3.3 V output voltage, it can provide power for chip FLASH programming, I/O simulation, ADC simulation, etc.
2.2.3 Design of System Detection and Conditioning Module induction cooker circuit design
The design of induction heating for induction cooker is realized by using a digital control system. The detection of current, voltage, and other signals in the induction cooker circuit design is also realized through signal control. To this end, it is necessary to complete the design of the system detection and conditioning module to use sensors to detect current and voltage. After the current output by the sensor is converted by the measuring resistance, a voltage signal was obtained. According to the gain and resistance of the operational amplifier, the voltage can be determined to ensure that the output bipolar signal is in the range of -3.3 to +3.3 V.
In a level-polarity-shifting circuit, an operational amplifier can scale a bipolar signal to a unipolar signal. The use of a low-pass filter can screen out high-order harmonic signals in the AC input current. Therefore, the switching frequency can be effectively adjusted, and current detection and conditioning can be realized.
2.2.4 System Fault Protection Module Design
The working environment of the induction heating for the induction cooker is relatively poor, and it is easily disturbed by external factors. For example, when the power grid fluctuates, it is easy to cause the drive circuit or the control circuit to malfunction causing the output circuit of the induction cooker to short-circuit. If the current in the circuit is too large, the switching device will be damaged. Considering that the current changes quickly, high current and high voltage often appear in the switching device, so it is necessary to realize the fault protection power design and realize the rapid detection of the overcurrent circuit. In the actual design, 2SD315A needs to be used to detect the DC bus and self-test to strengthen the double protection of the line. Once a fault occurs, hardware and software can be used to strengthen line protection at the same time to complete fault detection in time.
2.3 System software design
From the point of view of system software design, it is mainly necessary to realize the system main program design and it’s also necessary to design subroutines. Since the system has many functions and complex logical relationships, the modular design method is also adopted to design each function as a level module such as: display module, A/D conversion module, software phase-locked loop module and fault interrupt processing module.
In the process of system work, different functions can be realized by calling different modules. Judging from the main program of the system, after starting to work, the system will conduct a self-check to confirm whether the work is normal and whether it can be shut down normally. If it is not normal, it will alarm.
After confirming that everything is normal, the system will supply power to the main circuit, prompting the induction heating induction cooker to start at the maximum frequency and achieve frequency tracking. Through the detection of parameters such as current and voltage, the monitoring of each parameter is realized to determine whether there is overcurrent, overvoltage or undervoltage in the system. If necessary, the switching device will be turned off and an alarm will be issued. If there is no fault inside the system, it is necessary to determine whether the current, voltage and other values exceed the set value range. When the range is exceeded, the power supply needs to be cut off. If the range is not exceeded, the average power needs to be sent, and the working time calculation is completed according to the given power value.
When the working time is 0, the switching device needs to be turned off, otherwise, the detection of parameters such as current and power supply needs to be continued. By adopting this measure, the induction heating induction cooker can always be kept in a good working state, so as to ensure the stable operation of the induction cooker.
Through research, it can be found that in the process of induction cooker reference design, a digitally controlled induction heating for the induction cooker is necessary to analyze the working principle and digital control requirements.
This can also help to realize a reasonable design of the induction cooker digital control system and use scientific methods to realize the system hardware and the software design to ensure that the system works steadily and that the induction cooker is always in good working condition and maintains high work efficiency.