JPS6122437B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an induction heating cooker, and an object of the present invention is to detect the voltage waveform of an inverter circuit and determine the magnitude of the load.

Conventionally, induction heating cookers using an inverter circuit have a method of connecting a microresistance to the input of the inverter and detecting the voltage drop in order to distinguish between the pot being heated and small objects such as knives and spoons.
Alternatively, a method of comparing the inverter input level and a set value using a current transformer is adopted. However, these methods have the disadvantage that when varying the output of the inverter, the inverter input current also changes, so the set value to be compared must be changed, making the circuit configuration complex. Another drawback is that if the input current from the power supply fluctuates, this will cause an error and make it impossible to make accurate determinations. On the other hand, as another method, it is possible to use changes in the Q value of the inverter's output coil to determine the load based on the magnitude of the resonance voltage. Value correction is required.

The present invention solves these problems by providing a circuit that detects the operating voltage of the inverter and determines the load size by knowing its time width, and when the output is variably controlled or It was able to operate stably and accurately when the input fluctuated.

Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 shows an inverter circuit and a load discrimination circuit, E is a DC power supply, _L1 is a heating coil connected to the positive terminal side +B of the DC power supply E,
_T1 is a switching transistor connected in series with this heating coil _L1 , and has a collector connected to the heating coil _L1 and an emitter connected to the negative terminal side of the power source E. C ₁ is connected in parallel with heating coil L ₁ 〓〓〓〓〓
A resonant capacitor forms a single resonant circuit 1 with the heating coil L ₁ . _D1 is a diode connected anti-parallel to the switching transistor _T1 , and together with the switching transistor _T1 constitutes the switching block 2, and is driven at an ultra-audible frequency. The single resonant circuit 1 and switching block 2 constitute an inverter circuit. 3
The insulating top plate 4 is placed on the heatable coil L ₁ .
It is a cooking pot placed through the. This cooking pot 3
is made of a magnetic material.
Then, a magnetic field generated by the high frequency alternating current applied to the heating coil _L1 is added to this, and the cooking pot 3
This generates eddy currents, and heating occurs due to iron loss. Reference numeral 5 denotes an oscillation drive circuit connected to the base of the switching transistor _T1 via a resistor _R1 , in which a superaudible frequency oscillator is used to control on/off of the switching transistor _T1 . The drive signal is denoted by B. 6 is a negative voltage detection circuit which receives as input the voltage signal C of the connection terminal between the collector of the switching transistor _T1 and the cathode of the diode _D1 , and when the signal C becomes a voltage lower than the negative electrode of the DC power supply E, A signal D is generated at the output terminal.
7 is a load discrimination circuit which takes this output signal D and the output B of the oscillation drive circuit 5 as two inputs and compares the pulse time widths of both signals; 8 is this load discrimination circuit;
This is a prohibition circuit that stops the oscillation drive of the inverter circuit by the output F of the inverter circuit.

FIG. 2 specifically shows the negative voltage detection circuit 6 and the load discrimination circuit 7, where 9 is an input terminal to which a low voltage power supply +V is applied, and 10 is a DC power supply E.
Another power input terminal connected to the negative terminal of 11
is the terminal to which the voltage signal C is input, and the diode
Connected to the cathode of D ₂ . R ₂ and T ₂ are a resistor and a transistor connected in series between input terminals 9 and 10, and the base of this transistor T ₂ is
Connected to the anode of diode D ₂ . _R3 is
This is a resistor inserted between the input terminal 9 and the base of the transistor _T2 . The output D of the negative voltage detection circuit 6 of the above lecture is taken out from the collector of the transistor T ₂ and applied to the load discrimination circuit 7 at the next stage. Next, the configuration of this load discrimination circuit 7 will be explained. 9 and 10 are input terminals into which the same power supply as mentioned above is input, and between these two input terminals 9 and 10,
Transistor T ₃ , resistor R ₄ , variable resistor R ₅ respectively
and another transistor T ₄ are connected in series. The signal B from the oscillation drive circuit 5 is input to the base of the transistor _T3 , and the signal D is input to the base of the other transistor _T4 , thereby performing a switching operation. C ₂ is a capacitor inserted between the connection point 12 of resistor R ₄ and variable resistor R ₅ and the power input terminal 10, and G is this connection point 1.
An output signal F is obtained by a comparator circuit having two inputs: the signal E of No. 2 and the voltage signal of the low potential side power supply input terminal 10. This output signal F is applied to the inhibit circuit 8 and stops the operation of the inverter circuit when it reaches a high level.

Next, the operation will be explained with reference to FIG. In the figure,
A to F indicate the signal waveforms in the case of suitable loads, A' to F' indicate the signal waveforms in the case of small loads, waveforms A and A' indicate the current waveforms flowing through the heating coil _L1 , and B, B' to F ，
F′ represents the current or voltage signal waveform at each point mentioned above. First, to describe the case of heating a suitable load using waveforms A to F, when the switching transistor _T1 becomes conductive at time _t1 , a current i _T flows from the DC power source E through the heating coil _L1 . , excite the heating coil L ₁ . When this transistor T ₁ is cut off at the subsequent time t ₂ , the electromagnetic energy given to the heating coil L ₁ until then becomes an oscillating current i _LC due to the resonance capacitor C ₁ , and the current flows through the switching block 2 . Not flowing. The resonance voltage at this time is shown by waveform C. At time t ₃ , when the resonant voltage C falls below zero, the diode D ₁ becomes conductive and a current i _D flows into the DC power supply E through the heating coil L ₁ .
This current i _D is the time it takes for the resonant voltage C to return to zero.
Flows up to _t4 . At time _t4 , signal B from the oscillation drive circuit 5 is applied to the transistor _T1 , and the inverter is driven to oscillate again. Here, the area I from time t ₁ to _{t 2} shown in waveform A is the energy flowing out from the DC power source E, while the area from time t ₃ to _{t 4} is, on the contrary, returned to the DC power source E. It is a powerful energy. By observing these two areas, it is possible to know the energy absorbed or consumed by the load during the period from time _t1 to time _t4 , and the size of the load can be determined by the ratio of both areas. . Note that this ratio remains constant regardless of changes in heating output or power supply voltage. The ratio of the above areas is the same as the ratio of time t ₁ to _{t 2} and time t ₃ to _{t 4} .
Ru. This _is because _{the heating coil}
This is because only L ₁ is involved. Here, the time _t1 to _t2 corresponding to the area is the conduction time of the transistor _T1 , the time _t3 to _t4 corresponding to the area is the conduction time of the diode _D1 , and the signal waveform C is the conduction time of the transistor _T1 .
~t ₂ , the saturation voltage v _T of transistor T ₁ , time
From _t3 to _t4 , the forward voltage of the diode _D1 is _vD . Since this forward voltage v _D is lower than the negative electrode of the power supply, the negative voltage detection circuit 6 outputs the signal D during this time period t ₃ to _{t 4} . That is, the voltage v _D is applied to the base of the transistor T ₂ through the diode D ₂ and lowers the base voltage below the threshold voltage, so that the collector output signal D of the transistor T ₂ becomes high level. On the other hand, the signal B from the oscillation drive circuit 5 is applied to the base of the transistor T ₃ of the load discrimination circuit 7, and at its high level, the transistor T _{3 becomes conductive, and the resistor T 3} becomes conductive.
A charging current is flowing through _R4 to capacitor _C2 . After that, due to the arrival of the above signal D, the transistor
When T ₄ becomes conductive, this charging voltage is discharged through variable resistor R ₅ . Here, the time width t ₁ to t ₂ is T _A , t ₃
If ~t ₄ is T _B and the ratio K (= TB/TA) between the two is the discrimination point, when KT _A > T _B , it is an appropriate load, and when KT _A < T _B , it is a small load, and the variable resistor R ₅ If the ratio with the resistor _R4 is selected to be K, the capacitor voltage can always be kept somewhat charged (charged amount > discharged amount) when the load is appropriate, as shown in waveform E. I can do it. In this state, the output of the comparison circuit G is always held at a low level. Therefore, the inhibition circuit 8 does not operate, and the inverter circuit normally oscillates and is heated by induction.

However, when a small load such as a knife or spoon is placed on the top plate 4, the power is not easily absorbed by the load, and the feedback current i _D flowing through the diode _D1 increases, causing the areas ′ and ′ shown in waveform A′ to increase. The ratio becomes smaller. That is, the current i
The flow time t ₃ ′ to _{t 4} of _D ′ becomes longer, and the capacitor C ₂
Discharge until the amount of discharge becomes equal to the amount of charge.
Therefore, at time _t5 between time _t3 ' and time _t4 ', the discharge voltage becomes zero (waveform E'), and the output F' of comparator circuit G changes to a high level. This high-level pulse activates the prohibition circuit 8, causing the inverter circuit to stop oscillating.
Heating operation stops.

As explained above, when the induction heating cooker of the present invention detects the load of small items, it calculates the ratio of the energy flowing out from the DC power supply and the remaining feedback energy absorbed by the load, and then detects the load. Since it detects aptitude and unsuitability, it has the following effects.

Even when adjusting the heating output by adjusting the ON period of the switching element, it is possible to stably determine whether the load is suitable or not, without having to correct the set value that serves as a comparison standard, in response to current changes in the heating coil. be done.

Similarly, there is no need to correct the set value for variations in power supply voltage.

Since load discrimination is performed at a high speed comparable to the oscillation cycle of the inverter circuit, the inverter immediately stops when a small object is detected, so the small object will not be heated, and safety is extremely high.

Manufacturing costs are low because there is no power consumption associated with conventional microresistance methods, and there is no transformer required when using a current transformer.

Since load discrimination is performed by comparing time widths, signal detection can be performed digitally and is accurate.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the induction heating cooker of the present invention, FIG. 2 is a circuit diagram of the main part thereof, and FIG. 3 is a signal waveform diagram for explaining the operation of the same example. 4...Heating coil, 1...Single resonance circuit, 2...
...Switching block, 4...Top plate, 3...Cooking pot, 5...Oscillation drive circuit, 6...
Negative voltage detection circuit, 7...Load discrimination circuit, 8...Prohibition circuit, G...Comparison circuit. 〓〓〓〓〓

Claims (1)

[Claims] 1. A DC power supply, a heating coil connected to this DC power supply, a resonant capacitor that forms a resonant circuit with this heating coil, a switching element for generating a resonant current in this resonant circuit, and a switching element connected in antiparallel to this switching element. It has a switching block composed of connected diodes and an oscillation drive circuit that controls on/off the switching element, and this oscillation drive circuit drives an inverter circuit composed of the resonance circuit and the switching block. In the induction heating cooker, means for detecting the conduction period of the switching element based on the output of the oscillation drive circuit, means for detecting the voltage between terminals of the switching element to detect the conduction period of the diode, and both of the detection means. An induction heating cooker comprising load determining means for comparing the outputs of the inverter circuit and determining the load of the inverter circuit.