16 May How Does Induction Cooking Works?
Summary: The load of the induction cooker is the pan that is used to heat it. When the size and material of the cooker change, the resonant frequency of the main circuit of the induction cooker will also change, which will often cause overvoltage damage to the power tube IGBT. Therefore, it is necessary to adjust the output power of the induction cooker in time when the cooker changes, and protect the power tube IGBT.
A pan detection circuit is also required. In the past, the detection circuit of pots and pans has deficiencies. This paper proposes an accurate detection method for pots – the detection method of the number of pulses.
So, how does an induction cooking works? Find out more below.
Key words: induction cooker; cooker; single chip computer; pulse number detection method, Impulse Examination Method.
Abstract: The load of the induction cooker that is used to heat is a boiler. The changing of the size and the material, resulting in the main circuit with the electrical resonance of induction cooker flap the frequency variety. So, the adjustment of the variety often will cause damage to the power tube IGBT as external pressure. With this, the adjustment of the electric circuit is needed. Once after examining the electric circuit containing shortage, this text is put forward to examine the method about a pot of induction cooker according one-chip computer —– impulse examination method.
An induction cooker, also known as an induction cooktop, uses the principle of high-frequency electromagnetic induction to heat. The induction cooker converts the AC power of the main into DC power through a bridge rectifier. This will be converted into an alternating current with a frequency of 20-40KHz by a voltage resonant converter. This High-frequency alternating current is coupled to the bottom of the pot through a coil wound on a circular plane. At the bottom of the pot, a vortex-like current is generated, which makes the pot heat up quickly and increases the temperature of the pot. The role of food in a hot pot is shown in Figure 1.
Figure 1: Principle of Induction Cooker
Because the pot itself is the heat source, there is no loss of heat conduction and has no circuit connection with the power supply, so it’s safe. This is one of the advantages of complete, energy-saving, high-efficiency and simple induction cooker. And this is normally how induction cooking works.
The pot of the induction cooker is the load itself. When the size and material of the pot change, the equivalent inductance of the load will also change. This causes the resonant frequency of the main circuit of the induction cooker to change. In this way, the output power of the induction cooker will be unstable, which will often cause the power tube IGBT to be overvoltage and get damaged. Especially when there is no pot and no load, the Q value of the LC oscillation circuit becomes larger. At this time, if it is in a high-power continuous heating state, and the power consumption drops significantly, the power will be abnormally high voltage on the collector of the tube, which will cause the power tube to burn out. In this case, the main circuit must be in a low-power detection state. So the control circuit of a pot detection circuit is implemented to control whether the pulse width modulation stage outputs switching pulses.
So, when the size and material of the cookware is change to adjust the output power of the induction cooker, and to protect the power tube IGBT, a cookware detection circuit is required. The task of the pot detection circuit is to check whether there is a pot on the induction cooker. If there is a suitable pot, continuous heating work or heat preservation work can be carried out. if there is no pot being used or the pot is not properly selected or placed, and there are small items on the stove, the continuous operation will be stopped. When the heating continues, a no-pot alarm will sound to remind the user that there is no pot being placed. If a pot is placed in the middle of no pot alarm, then Continue heating work.
The current detection method is used for the detection of induction cooker pots. To further know about how does induction cooking works, continue reading.
Detector Reason: As shown in Figure 2, if there is no pot, the heating coil has no load, and there is no power consumption.
Therefore, the main circuit has only a small input current, which is sensed and sampled by the current transformer. The current is very small, and the voltage U6 obtained by the rectification of D20~D23 is also very low. Therefore, on a single chip, the voltage detected by the machine will be low as well. A pot-less assay U60 can be determined experimentally. Voltage threshold, if the U6 voltage is less than this when the control line is turned on for more than 50ms. If the critical value is set, it is considered to be no pot, and then the output of no pot will detect the PWM duty cycle. Disconnect the control line, and open the control line every 2 seconds to check if the CRU voltage is high. At the critical value of CRU, it is considered that there is a pot, and the original working state is restored. If no pot is detected for several consecutive times, it will automatically shut down. In the same way, it is possible to detect small items or non-compliant pots.
Figure 2: Current detection circuit
There’s more to it when it comes to how an induction cooking works. Now, there will be a problem when checking the pot in this way: the same pot is high voltage. It is judged to have a pot under voltage, but it may be judged to be no pot under low voltage, so different current limits must be set for high and low voltages. If no separate current limit is set, misjudgment may occur, that is, a smaller pot is detected as no pot at 170V, but there is a pot at 250V. Therefore, it is necessary to judge the power supply voltage at that time to make a judgment about whether there is a pot or not.
To overcome the above shortcomings, a new detection method can be used— Pulse Punch detection method. That is to let the IGBT work for several MS, the coil disk, and the resonant capacitor. The oscillation occurs, although the natural frequency of the LC loop increases when there is no pot.
The load consumes very little energy when the pot is loaded, and the energy is concentrated in the oscillating circuit.
If the time is long, the oscillation period will be long. When there is a pot, the energy of the coil plate will be consumed quickly, so the oscillation is also fast, and the oscillation period is long. Sampling can be used to determine the single unit time.
The number of high-voltage pulses obtained by the tablet machine is used to determine whether there is a pot. Of course, it also needs to measure the number of pulses to judge whether there is a pot in a unit time by an experimental method critical value.
Figure 3: Pulse count detection circuit
The pulse detection method can use the circuit shown in Figure 3 to complete: the gated control tube. The high voltage pulse of the collector is divided by R7, R8, and R33 and sent to IC4 LM339.
The inverting input terminal of the ÿ pin of the comparator and its non-inverting input terminal is connected to a fixed level. So, a square wave is generated at the output that is synchronized with the high voltage pulse at the collector of the gated tube.
The single-chip computer judges whether the high-voltage pulses in unit time there is a pot and whether the pot is suitable, to carry out the next work instruction. The number of pulses detected by this pot detection method has nothing to do with the local voltage. Whether the voltage is 170V or 250V, the number of pulses is only related to the oscillation period. In this way, the program design is also simplified, and the pot inspection is very reliable
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