A Kind of Examine Method about Pot of Induct Cooker According One-chip Computer
Abstract: The load of the induction cooker is a boiler that is used to heat. The changing of the size and the material, resulting in the main circuit with electrical resonance of induction cooker flap the frequency variety, often will cause the damage of the power tube IGBT as external pressure. so the adjustment of the exportation power for induction cooker is necessary when have variety the pot ,and have a method to examining the electric circuit. The former examining electric circuit contain shortage, this text put forward a kind of examine method about pot of induction cooker according one-chip computer ----- impulse examination method.
Induction cooker also known as induction range, is the use of high frequency electromagnetic induction principle heating.The induction cooker transforms the AC power supply of the mains into direct current through the bridge rectifier, and then into the alternating current with a frequency of 20-40khz through the voltage resonance converter.This high frequency ALTERNATING current is coupled to the bottom of the pot through the coil coil wound on the circular plane, which generates a vortex current at the bottom of the pot to heat the pot quickly and play a role of heating the food in the pot, as shown in Figure 1.Because the pot provides itself is heat source, without the loss of heat conduction, do not have the connection on the road with power supply again, so it has the advantage of safety, energy saving, efficient, convenient.
The pot of induction cooker is load, when the size that provides when the pot, material produces change, the equivalent inductance of load can produce change, cause induction cooker to master loop resonant frequency change, the output power of such induction cooker can be unstable, regular meeting causes power tube IGBT overpressure to damage.Especially when there is no pot and no load, the Q value of the LC oscillation circuit increases. At this time, if the power consumption drops significantly in the state of high-power continuous heating, abnormal high pressure will appear at the collector of the power tube to burn out the power tube. In this case, the main circuit must be in the detection state of low power.Therefore, the boiler detection circuit is set up in the control circuit to control whether the pulse width modulation level output switching pulse.
Figure 1: Principle of induction cooker
So when the size of the pot, material changes, timely adjustment of the output power of the induction cooker, protect the power tube IGBT, you need to pot detection circuit.
The task of the cooker detection circuit is to check whether there is a pot on the induction cooker, if there is a suitable pot, it will carry out continuous heating work or heat preservation work;If there is no pot, and pot selection or improper placement, there are small items on the stove surface, etc., will stop continuous heating and issued no pot alarm sound, prompting the user now no pot, after a number of time will do shutdown processing;If a pot is placed in the middle of the no pot alarm, then Continue the heating work.
Present induction cooker cooker provides detection is to use current detection method.Detection principle: As shown in Figure 2, if there is no pot, the heating coil has no load, so there is no power consumption, so the main circuit has only a very small input current, the current sampled through the current transformer is very small, and the voltage U6 obtained through d20-D23 rectification is also very low, that is, the voltage detected by the MCU will be very low.Can be determined through the experiment a pot detection U60 voltage critical value, if when the control line open more than 50ms, U6 voltage is less than this critical value, it is considered that there is no pot, after the output pot detection PWM duty ratio, off the control line, every 2 seconds to open the control line detection once, if the C RU voltage is higher than the C RU critical value,That there is a pot, restore the original working state;If no pot is detected for several times, the machine will shut down automatically.Small objects or POTS that do not meet requirements can be detected using the same method.
Figure2 Current detection circuit
However, there is a problem with checking POTS in this way: the same pot is judged as having POTS at high voltage, but may be judged as having no POTS at low voltage, so different current limits must be set at high voltage and low voltage.If the current limit is not set separately, misjudgment may occur, that is, a smaller pot is detected as no pot at 170V, but as a pot at 250V.Therefore, we must judge the power supply voltage at that time to make the judgment of whether there is a pot.
In order to overcome the above shortcomings, a new detection method - pulse detection method can be adopted.Is to make the IGBT work a number of mS, the coil plate vibration and resonant capacitance, although the natural frequency of the LC circuit in the absence of pan was increased, but there is no pot load very little energy consumption, energy is gathered in the oscillation circuit, charge and discharge time is long, the oscillation cycle is long, have a pot coil plate soon run out of energy, so the oscillation is also fast, the oscillation cycle is long.It can determine whether there is a pot by sampling and judging the number of high pressure pulses obtained by the single chip microcomputer per unit time.Of course, it is also necessary to measure a critical value of pulse number to judge whether there is a pot or not in unit time by experimental method.
Figure 3 Pulse count detection circuit
Pulse detection test can be completed by the circuit shown in Figure 3: the high voltage pulse of the gate collector passes through the partial voltage of R7, R8 and R33 to the ⑩ reverse input terminal of the COMPARator of IC4 LM339, whose in-phase input terminal is connected with a fixed level.This generates a square wave signal at the output synchronized with the high voltage pulse at the gate collector.Microcontroller according to the unit time of the high pressure pulse value to judge whether there is a pot, and put the pot is appropriate, so as to carry out the next work instructions.The number of pulses detected by this method has nothing to do with the local voltage, no matter the voltage is 170V or 250V, the number of pulses is only related to the oscillation cycle, so the program design is simple, and the pot is very reliable.