JPH0255915B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to an induction heating device using a digital control circuit.

(b) Prior art An induction heating device consists of a heating coil connected to a DC power source, a resonant capacitor forming a resonant circuit forming the heating coil, and a switching element.The heating is achieved by controlling the switching element ON and OFF. A resonant current is generated in the coil, an alternating magnetic field is generated in the heating coil, and an object to be heated made of ferrous metal placed close to the heating coil is inductively heated. In such an induction heating device, the timing from when the switching element turns on until it turns off is RC
This was done using a time constant circuit.

However, in the RC time constant circuit, the time constant changes over time and temperature changes are large, and the switching element is turned on.
There was an inconvenience that the period could not be maintained accurately and the output would deviate from the set value. Furthermore, there is a problem in that even if the set power is the same, the heating output varies depending on the material and shape of the cooking utensil used.

For this reason, recently, as shown in Japanese Patent Application No. 59-24692, an induction heating device with a digital control configuration has been developed that maintains the ON period of the switching element accurately and eliminates the difference in heating output depending on the material and shape of the cooking utensil. It is being

An induction heating device having such a digital control configuration will be explained with reference to FIGS. 2 to 4. In these figures, Figure 2 is a circuit diagram of an inverter circuit used in such a digitally controlled induction heating device, where 1 is a full-wave rectifier circuit that full-wave rectifies the AC power supply voltage, and 2 is a circuit diagram of this full-wave rectifier. A chiyoke coil connected to the output terminal of the circuit 1, 3 a filter capacitor forming a filter circuit together with this chiyoke coil 2, 4 an induction heating coil connected to one end of this filter capacitor 3, and 5 resonating together with this induction heating coil 4. A resonant capacitor forming the circuit, 6 a switching element such as a transistor connected in parallel to the resonant capacitor 5, and 7 a damper diode connected antiparallel to this switching element. 8 is from the control circuit described later.
This shows a drive circuit that turns the switching element ON and OFF in response to ON and OFF signals.The input stage of this drive circuit 8 is provided with a differentiating circuit composed of a resistor 9 and a capacitor 10. , the fall is becoming more rapid. Reference numeral 11 is a current transformer provided on the AC power line, which detects an alternating current to this inverter circuit.

In such an inverter, the drive circuit 8
to the switching element 6 as shown in Fig. 3A.
When ON and OFF signals are applied, the current Ic shown in Figure B flows through this switching element, and the voltage V _CE between the 6 terminals of this switching element is as shown in Figure C.
fluctuate. At this time, a resonant current flows through the heating coil, generating a high-frequency alternating magnetic field, which is supplied to a cooking utensil 12 such as a pot made of iron or 18-8 stainless metal near the heating coil 4, and this cooking utensil 12 is Heated by induction. Incidentally, here, Vcon is a full-wave rectified voltage transmitted from the full-wave rectifying circuit via the choke coil 2.

FIG. 4 is a block diagram showing the control circuit of such an induction heating device, in which 13 is an A/D conversion circuit that converts the input current value detected by the current transformer 12 into digital input data _PAD ; 14
is synchronized with the sample signal that gives the timing for A/D conversion by this A/D conversion circuit 13 and the low potential part of the pulsating current obtained by full-wave rectification of the AC power supply voltage.
This figure shows a timing generation circuit that generates a MINT signal, and outputs a Sample signal and a MINT signal at predetermined timings of a rectified voltage obtained by full-wave rectification of an AC voltage. 15 receives input data (P _AD ) from the A/D conversion circuit 13, and determines whether small items such as a knife or fork are placed close to the heating coil 4 or no load where nothing is placed. This is an improper load detection circuit that detects a state, and issues a prohibition signal when the input data P _AD is less than a predetermined value.
16 is a power setting circuit in which power setting data Pref is set as a digital value according to the power to be output from this device; 17 is a power setting circuit that receives setting data Pref from this power setting circuit 16 and is input from the A/D conversion circuit 13; It shows a SUB circuit that receives data P _AD , and outputs a value Pref - P _AD obtained by subtracting input data P _AD from setting data Pref. 18 corresponds to the time to turn on the switching element 6.
ON period setting means in which ON period data Pcon is set, and the setting data is sent from the SUB circuit 17.
Difference data obtained by subtracting input data P _AD from Pref
The ON period data Pcon is adjusted in response to Pref-P _AD . Reference numeral 19 compares the DC voltage Vcon from the full-wave rectifier circuit with the terminal voltage _VCE of the switching element 6, and determines that it is "L" during the resonance period when _VCE > Vcon.
It is a resonance period detection circuit that outputs a level detection signal, and also serves as on-timing detection means for notifying the timing to turn on the switching element 6 by disappearance of the detection signal when V _CE <Vcon. Reference numeral 20 denotes an overcurrent protection setting circuit that measures the length of the resonance period based on the detection signal from the resonance period detection circuit 19, and sets limit data Icp for limiting the ON time of the switching element 6 according to the length of the resonance period. This prevents a large current from flowing to the switching element 6. 21 receives the detection signal from the resonance period detection circuit 19, starts counting operation from the end of this detection signal, and this count value becomes the above-mentioned value.
ON period data Pcon of the ON period setting means 18 or limit data Icp of the overcurrent protection setting circuit 20
22 is set when the detection signal from the resonance period detection circuit 19 ends, that is, when V _CE < Vcon, and the ON period counting circuit 22 outputs a coincidence signal when The flip-flop circuit is set by a match signal from 20, and the switching element 6 changes to "H" and "L" by this setting and resetting.
The control signal for controlling ON and OFF is sent to the drive circuit 8 above.
send to Further, the flip-flop circuit 22 is configured such that its operation is prohibited by the improper load detection signal from the improper load detection circuit 15.

In such a control circuit, during device operation, when the "L" level detection signal from the resonance period detection circuit 19 disappears and its output becomes "H" level, the flip-flop circuit 22 is set; From switching element 6
Sends an ON signal to the drive circuit 8. At the same time, due to the extinction of the detection signal, the ON period counting circuit 21 performs a counting operation, and when this value matches the input data Pcon in the ON period setting means 18, a reset signal is sent to the flip-flop circuit 22. As a result, the flip-flop circuit 22
is reset, and an OFF signal for the switching element 6 is sent to the drive circuit 8. That is, during normal operation, the ON period setting means 18 outputs the ON period.
The period data Pcon determines the ON period of the switching element.

On the other hand, the power setting data Pref set by the power setting circuit 16 and the input data P _AD obtained by A/D converting the input current value proportional to the AC input power by the A/D conversion circuit 13 are generated in accordance with the Sample signal. teSUB
This is transmitted to the circuit 17, and this SUB circuit 17 sets the data value Pref−P _AD , which is obtained by subtracting the input input data P _AD from this power setting data Pref, to the ON period setting means 1.
Send to 8. Upon receiving this input data Pref−P _AD,
The ON period setting means 18 adds Pref-P _AD to the initially set ON period input data Pcon to set a new one.
Use ON period data. This is the input data P _AD
When P is smaller than the power setting data Pref, the ON period data is increased to lengthen the ON period length of the switching element 6, and the input power is increased.On the other hand, when the input data P is smaller than the power setting data Pref, the ON period data is increased. It works to reduce Pcon, shorten the ON period length of switching element 6, and reduce input power. This kind of operation is the input data P _AD
is repeated until it matches the power setting data Pref. Therefore, the input power, which varies depending on the material, shape, and conductivity of the pot used, is automatically adjusted to remain constant.

Further, when the inverter is heated with a small load or no load, the input current value detected by the input current transformer 11 becomes low, and the data P _AD output from the A/D conversion circuit 13 becomes small. The inappropriate load detection circuit 15 receives such input data P _AD
It is detected that the value of is lower than a predetermined value, and the flip-flop circuit 22 is inhibited. Therefore, ON/OFF signals from the flip-flop circuit 22 to the driving circuit 8 of the switching element 6 are prohibited. still,
At this time, the predetermined value is linked to a power setting knob (not shown) so that it becomes larger when the power setting data Pref value of the power setting circuit 16 is set to a large value, and becomes smaller when the Pref value is set to a small value. It is preferable to change the

Furthermore, when a cooking utensil made of a non-magnetic material such as aluminum is placed close to the heating coil 4 and heated, the equivalent inductance of the heating coil 4 is lower than when a magnetic cooking utensil is used. Therefore, after the switching element 6 turns OFF, the switching element 6 terminal voltage V _CE becomes the DC voltage from the full-wave rectifier circuit 1.
The length of the resonance period in which the voltage is higher than Vcon becomes shorter. The resonance period detection circuit 19 detects this period, and the overcurrent protection setting circuit 20 limits the ON period according to the detected resonance period length. Limit input data
Decrease Icp value. Even if the ON period data Pcon of the numerical period setting means 18 is set to a large value so as to lengthen the ON period of the switching element 6, the count of the ON period counting circuit 21 will not exceed the limit data Icp of the overcurrent protection setting circuit 20. is limited to the value of
There is no risk that the ON period of the switching element 6 will be shortened and a large current will flow through the switching element.

(c) Problems to be solved by the invention However, in such an induction heating device,
As mentioned above, unsuitable loads are detected by detecting whether the output data P _AD from the A/D conversion circuit 13 is smaller than a predetermined value, so it can only be detected after the input power has risen sufficiently. There was a problem that unsuitable load could not be detected, it took time to detect unsuitable load, and undesired power consumption increased. In addition, input data from the A/D conversion circuit 13
It is necessary to determine a predetermined value for comparison with _PAD according to the set output, which has the drawback of complicating the circuit configuration.

SUMMARY OF THE INVENTION An object of the present invention is to use a simple circuit configuration to quickly detect an unsuitable load immediately after the start of oscillation when the input power does not rise sufficiently, in order to suppress undesired power consumption. .

(d) Means for solving the problem The present invention is a means for calculating the amount of change ΔPcos in ON period data, a means for calculating the amount of change ΔP _AD in input data P _AD , and a comparison between these ΔPcon and P _AD . and a comparison means for.

(E) Effect The amount of change ΔPcon in the ON period data and the amount of change ΔP _AD in the input data P _AD are compared by the comparison means, and if the magnitude of ΔP _AD is smaller than the magnitude of ΔPcon, the switching element is determined. prohibit the operation of That is, when using a cooking utensil made of magnetic metal, _as shown in FIG. When using cooking utensils consisting of
The amount of change ΔP _AD in input data relative to the amount of change ΔPcon in ON period data is small, and the operation of the switching element is prohibited. In this way, the suitability or unsuitability of the cooking utensil is detected immediately after the oscillation starts.

(F) Embodiment FIG. 1 is a block diagram of a control circuit which is a main part of the induction heating apparatus of the present invention, and the same inverter circuit as shown in FIG. 2 is used as the inverter circuit to be controlled. Also, in FIG. 1, the same parts as in FIG. 4 are given the same number. In the same figure,
The ON period setting means 18 includes an addition circuit 23 that outputs ON period data Pcon, and an ON period setting means 18 that outputs ON period data Pcon.
One period data Pcon, that is, the above MINT
A delay circuit 24 that delays one signal and outputs it as Pcond, an initial value setting circuit 25 that sets relatively small initial ON period data Pcon at the start of oscillation of this device, and data from this initial value setting circuit 25. Pcon o or data from delay circuit 24
Selection circuit 2 that selects either Pcon d or either
The output of the selection circuit 26 and the output from the SUB circuit 17 are input to the addition circuit 23. In other words, this ON period setting means 1
8, when the inverter oscillation starts, the initial ON period data Pcono from the initial value setting circuit 25 is
is selected by the selection circuit 26, and this data Pcono is outputted as ON period data Pcon via the addition circuit 23. Once oscillation is started, the ON period data Pcon output from the adder circuit 23 is delayed by the delay circuit 24 and given again to the adder circuit 23 via the selection circuit 26.
Difference data Pref−P _AD given from SUB circuit 17
is added and output as new ON period data. The operation inside this adder circuit 23 is
This is done in synchronization with the MINT signal. Reference numeral 27 indicates a minimum frequency guarantee circuit for guaranteeing the minimum frequency of this device, which limits the maximum value of the ON period data Pcon calculated and set by the adder circuit 23, and lowers the oscillation frequency of this device to the audible range. prevent it from entering. A SUB 28 receives the ON period data Pcon outputted from the adder circuit 23 and the ON period data Pcon d delayed by one data by the delay circuit 24.
It is a circuit, and the difference between this Pcon and Pcon d, that is,
Change in ON period data ΔPcon=Pcon−Pcon d
Calculate. 29 converts the input data P _AD outputted from the A/D conversion circuit 13 into one piece of data, that is,
A delay circuit delays one MINT signal and outputs it as P _AD d. 30 is a SUB circuit which receives the data P _AD d from the delay circuit 29 and receives input data P _AD from the A/D conversion circuit 13. , the amount of change in input data ΔP _AD =P _AD −P _AD d is calculated. 3
1 is ΔPcon from the above SUB circuits 29 and 30,
The comparison circuit receives ΔP _AD , and when |ΔPcon|>k|ΔP _AD | for an appropriate positive constant k, the flip-flop circuit 22 is inhibited to stop the switching operation of the switching element 6.

In such an induction heating device, as described above, at the start of oscillation, the ON period data Pcon is sequentially increased. Along with this, input data P _AD obtained by A/D converting the input current also increases. These are ON
Change amount ΔPcon of period data Pcon and input data
The P _AD change amount ΔP _AD is calculated and generated by the SUB circuits 28 and 30, and the comparator circuit 31 calculates k|ΔPcon|>|ΔP _AD |
You can check to see if you are used to it. Normally, when an appropriate load made of magnetic metal such as iron is used as a cooking utensil, as shown in A in Fig. 5 above, |ΔP _AD | is relatively large compared to |ΔPcon|
|ΔP _AD |, so the comparison circuit 31 does not inhibit the flip-flop circuit 22, and oscillation continues. On the other hand, if a cooking utensil made of a weakly magnetic metal such as Al is used as a load, or if oscillation occurs with a small object such as a knife or fork loaded or no load, then B and C in Figure 5. Like, |
Since |ΔP _AD | with respect to ΔPcon| is relatively small, k|ΔPcon|<|ΔP _AD |, and the comparison circuit 31 prohibits the flip-flop circuit 22 and stops the inverter oscillation.

Furthermore, if the cooking utensil is removed while the inverter oscillates using a suitable cooking utensil, the input signal detected by the current transformer 11 as shown in FIG. The amount of current decreases. This input current is applied to the A/D according to the Sample signal from the timing signal generation circuit 14.
The conversion circuit 13 converts it into input data P _AD . Therefore, the control circuit increases the input current.
Reset the ON period data Pcon in synchronization with the MINT signal. As a result, the input current also rises and the next
The input data P _AD converted at the sample timing increases. However, since oscillation occurs in a no-load state at this time, the amount of change ΔP _AD in input data does not become so large. Therefore, the relationship between the amount of change in ON period data ΔPcon and the amount of change in input data ΔP _AD is k
|ΔPcon|>|ΔP _AD |, and the flip-flop circuit 22 is inhibited from the comparator circuit 31.

(G) Effects of the Invention As described above, the induction heating device of the present invention compares the amount of change ΔPcon of ON period data with the amount of change ΔP _AD of input data, and stops the inverter oscillation when ΔP _AD is larger than ΔPcon. Therefore, the unsuitable load can be quickly detected immediately after the start of oscillation, and undesired power consumption can be reduced. Further, there is no need for a complicated circuit for determining abnormal load detection according to the set output, and abnormal load detection can be performed with a simple circuit configuration.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the control circuit of the induction heating device of the present invention, Fig. 2 is a circuit diagram of the inverter circuit, and Fig. 3 is a block diagram of the control circuit of the induction heating device of the present invention.
Figure A is a waveform diagram showing ON and OFF signals, Figure B is a waveform diagram showing the current flowing through the switching element, Figure C is a waveform diagram showing the switching element terminal voltage, and Figure 4 is a conventional induction heating device control. A block diagram of the circuit, Fig. 5 is a relationship diagram between the ON period of the switching transistor and input power to the inverter, and Fig. 6 is a timing diagram showing the operation when the cooking utensil is removed in the induction heating device of the present invention. It is. 1...Full wave rectifier circuit, 2...Chiyoke coil,
3... Filter capacitor, 4... Induction heating coil, 5... Resonant capacitor, 6... Switching element, 7... Damper diode, 8... Drive circuit, 11... Current transformer, 12... Cooking utensil, 13 ... A/D conversion circuit, 14 ... Timing generation circuit, 15 ... Improper load detection circuit, 16
...Power setting circuit, 17, 28, 30...
SUB circuit, 18... ON period setting means, 19...
Resonance period detection circuit, 20... Overcurrent protection setting circuit, 21... ON period counting circuit, 22... Flip-flop circuit, 31... Comparison circuit.

Claims (1)

[Claims] 1. A DC power supply made by full-wave rectification of AC power supply voltage, an induction heating coil coupled to this DC power supply, a resonant capacitor forming a resonant circuit together with this induction heating coil, and a resonant capacitor connected to the above resonant circuit and forming a resonant circuit. Induction heating consists of a switching element for generating a resonant current in the resonant circuit, and a diode connected in antiparallel to this switching element, and by controlling the switching element ON and OFF, a resonant current is generated in the resonant circuit. The device includes an input current detection means for detecting the AC input current, and an A/D for converting the current value detected by the input current detection means into digital data _PAD .
A conversion circuit, a power setting circuit that sets power setting data Pre _ref according to the input power as a digital value, and holds ON period data P _cpo corresponding to the time when the switching element should be turned on,
Input data P _AD from the A/D conversion circuit and power setting data P _ref from the power setting circuit are received, and the ON period data is corrected using the data P _AD and P _ref to set new ON period data. means, a counting circuit that starts counting when the switching element is turned ON, and gives an OFF signal to the switching element when the count reaches the ON period data value of the means _; Calculation means and the above input data P _{AD
ΔP cpo}
Comparison means for detecting an unsuitable load state by comparing P _AD to determine whether the magnitude of ΔP _AD is smaller than the magnitude of ΔP _cpo , and when the comparison means detects an unsuitable load state. The above switching element
An induction heating device consisting of means for prohibiting ON/OFF control.