JPH09199268A - Induction heating cooking utensil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent generation of a noise when a pan is up/down operated, by performing control so as to prevent even any condition from exceeding voltage or current specification of a switching element. SOLUTION: Full wave rectification of an AC power source 1 is performed in a rectifying bridge 2 and capacitors 3, 6, heating coil 4 generating a high frequency magnetic field, a switching element 5 for supplying a high frequency current to the heating coil 4 and an inverter circuit having a diode 7 are connected to the rectifying bridge 2. A chopping wave generating circuit 9, generating serrate wave based on an output of the inverter circuit, and a control circuit 12, performing output control based on output heating set by a user by an output adjusting volume 13 and information from a current transformer 8, are connected to an input port of a drive pulse generating circuit 11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an induction heating cooker for heating an object to be heated such as an iron pan by an eddy current generated by a high frequency magnetic field from a heating coil to cook the object.

[0002]

2. Description of the Related Art Conventionally, as a switching element of an induction heating cooker, a transistor having a withstand voltage specification of a collector-base voltage V _CE of several hundreds V to 1 kV has been used, so that the withstand voltage of V _CE is usually used. Do not exceed the specifications
It is used with some margin. Further, when an abnormal mode occurs, an overvoltage stop circuit is provided as a means for detecting the voltage of the switching element so as not to exceed the withstand voltage specification (rated voltage) of V _CE .
The output stops the driving of the switching element so that V _CE of the switching element does not exceed the rated voltage.

[0003]

However, in the above-described conventional induction heating cooker, the above-mentioned stop function operates even if the load is abnormal, for example, the pan is lowered after being lifted during heating. , Stop, heating is repeated. As a result, there is a problem in that an abnormal noise is generated from the pan each time and the user feels uncomfortable. The reason is as follows. For example, put the pot in the normal position and
When heating at 00V-1200W, the pulse-on width of the switching element is 20μs and V _CE is 700V.
Reach If a switching element having a withstand voltage specification of V _CE of 1000 V is used, the operating voltage of the overvoltage stop circuit is generally set to about 900 V with a margin.

If the user once lifts the pan during cooking of stir-fried food, the induction heating coupling between the heating coil and the pan becomes poor. Therefore, if the user wants to maintain 1200 W, the pulse-on width of the switching element is widened. The control circuit will operate. The pulse-on width of the switching element spreads in proportion to the height at which the pan is lifted, but can reach 30 μs.

In this state, if the user lowers the pot to the original position, the control circuit naturally operates so as to reduce the pulse-on width to 20 μs again, but since the response of the control circuit is not good, the induction heating coupling is not possible. Even if it improves, heating is performed with a pulse-on width that is, for example, 1.5 times wider (30 μs remains). In this state, since the withstand voltage specification of V _CE is exceeded, the overvoltage stop circuit operates to suddenly stop heating.
If stopped, V _CE will drop, and heating will be started again. At this time, if the pulse-on width is wide, it is stopped again. This operation is repeated until the control circuit responds correctly and returns to an appropriate pulse-on width (about 25 μs or less), at which time an abnormal noise is generated from the pan.

The present invention has been made in view of the above-mentioned conventional problems, and it is controlled so as not to exceed the voltage or current specifications of the switching element in any state, and when the pan is raised or lowered. Another object of the present invention is to provide an induction heating cooker that prevents the generation of abnormal noise from the pan.

[0007]

According to the present invention, there is provided a capacitor, a heating coil for generating a high frequency magnetic field, and a switching element for supplying a high frequency current to the heating coil while being connected to a rectified power source. An inverter circuit, a drive pulse generation circuit that applies a drive pulse to the switching element to turn on / off the switching element, a triangular wave generation circuit that generates a triangular wave, and a voltage applied to the switching element exceeds a predetermined value. In the case of an overvoltage stop circuit that outputs a signal for stopping the generation of the drive pulse to the drive pulse generation circuit, and a control circuit that controls the drive pulse generation circuit so that the output heating set by the user is obtained. An induction heating cooker comprising: Is determined by the driving pulse output from the drive pulse generating circuit based on the magnitude relationship between the voltage and the output voltage of the control circuit, the control circuit on-time of the switching element at the maximum output heating T _A
For controlling the drive pulse output from the drive pulse generation circuit so that the ON time of the switching element does not exceed the time T _M obtained by multiplying the ON time T _A by a predetermined coefficient. It is an induction heating cooker characterized by the following.

According to a second aspect of the present invention, in the first aspect, when the ON time of the switching element becomes shorter than the stored ON time T _A while heating at the maximum output,
The control circuit clears the stored ON times T _A and T _M , newly stores the shortened ON time as T _A , and calculates T _M based on the ON time T _A to control heating. It is an induction heating cooker characterized by the following.

According to a third aspect of the present invention, in addition to the second aspect, a power source voltage reading circuit that reads the power source voltage at the time of maximum output heating and transmits the power source voltage to the control circuit is provided, and while heating at the maximum output, When the ON time of the switching element becomes shorter than the stored ON time T _A and the power supply voltage becomes higher than the rated voltage at the same time, the control circuit clears the stored ON times T _A and T _M , and shortens. the on-time T _a together with the on-time is newly stored as T _a
The induction heating cooker is characterized in that T _M is calculated based on the above, and heating control is performed, and when the power supply voltage becomes equal to or lower than the rated voltage, heating control is performed with the cleared on-time T _A and T _M. is there.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of an induction heating cooker according to the present invention will be described below with reference to the drawings.
FIG. 1 is a block diagram showing the configuration of an embodiment of the induction heating cooker according to the present invention. As shown in FIG. 1, in the induction heating cooker according to the present embodiment, the AC power source 1 is full-wave rectified by the rectifying bridge 2, the capacitors 3 and 6, and the heating coil 4 that generates a high-frequency magnetic field in the rectifying bridge 2. ,
An inverter circuit having a switching element 5 and a diode 7 for flowing a high frequency current is connected to the heating coil 4.

The square wave drive pulse generated by the drive pulse generating circuit 11 is applied to the switching element 5 to turn on / off the switching element 5, so that a high frequency current is passed through the heating coil 4 to generate a high frequency magnetic field. , A pan or the like placed above the heating coil 4 is induction-heated for cooking. Further, a triangular wave generating circuit 9 for generating a triangular wave based on the output of the inverter circuit, and an output adjusting volume 1
Control circuit 1 for performing output heating set by the user via 3 and output control based on information from the current transformer 8.
And 2 are connected to the input port of the drive pulse generation circuit 11. Further, an overvoltage stop circuit 10 that outputs a signal for stopping the generation of the drive pulse when the voltage applied to the switching element 5 exceeds a predetermined value is also connected to the input port of the drive pulse generation circuit 11.

Next, the operation of the induction heating cooker having the above structure will be described. FIG. 2 shows a time chart of voltage and current of each element and circuit shown in FIG. A is the voltage waveform of the capacitor 3, B is the collector voltage waveform of the switching element 5, C is the output voltage waveform of the triangular wave generation circuit 9, and
D is the output voltage waveform of the control circuit 12, and E is the output waveform of the drive pulse generation circuit 11. Also, when the solid line is set for maximum output heating and the pot is placed at the upper position of the cooker, the dashed line is set for maximum output heating and the pot is shifted upward or sideways from the upper position of the cooker. When placed, the broken line indicates the maximum on-time of the switching element, and the pot is placed on the cooker in the upper position, and the chain double-dashed line sets the maximum output heating and the power supply voltage is set high to put the pot on the cooker. The figures show the cases when placed in the upper position. The horizontal axis represents time t.

First, when the output voltage C of the triangular wave generation circuit 9 becomes smaller than the output voltage D of the control circuit 12, the output E of the drive pulse generation circuit 11 is in the H state and the switching element 5 is turned on. Then, the collector voltage (V _CE ) B of the switching element 5 becomes 0V. When the switching element 5 is turned on, the current waveform I _C of the switching element 5 rises in a triangular wave shape as shown by the solid line. Here, the time during which the switching element 5 is in the ON state (ON time) is T _A1 .

When the output voltage C of the triangular wave generating circuit 9 becomes larger than the output voltage D of the control circuit 12, the output E of the drive pulse generating circuit 11 is in the L state and the switching element 5 is turned off. When the switching element 5 is turned off, its collector voltage V _CE rises once and then falls as shown by the solid line B. The time constant of this rise and fall is
It is almost determined by the L value of the heating coil 4 (a pot is placed) and the time constants of the capacitors 3 and 6. Further, when the collector voltage (V _CE ) B becomes smaller than the voltage A of the capacitor 3, a trigger is applied inside the triangular wave generating circuit 9, and the voltage waveform C of the triangular wave generating circuit 9 once sharply drops and then starts rising again. At this time, the output E of the drive pulse generation circuit 11 is again in the H state, and the above operation is repeated thereafter.

As is clear from the above description, the ON time T _A1 of the switching element 5 is such that the output voltage C of the triangular wave generating circuit 9 is smaller than the output voltage D of the control circuit 12, that is, the driving pulse generating circuit 11 is rectangular. It is defined as the time that the wave drive pulse is at the H level. On the other hand, the off time T _B1 of the switching element 5 is the time during which the output voltage C of the triangular wave generation circuit 9 is higher than the output voltage D of the control circuit 12, that is, the time when the square wave drive pulse of the drive pulse generation circuit 11 is at the L level. Is defined as It should be noted that the output DC voltage D of the control circuit 12 may be increased in order to lengthen the time during which the drive pulse is at the H level (hereinafter referred to as the drive pulse ON time). Also, the time during which the drive pulse exists at the L level (hereinafter referred to as the drive pulse off time)
Is the same value if the same pot is placed in the same position.

Here, the user puts the pan in a predetermined position,
When set to maximum output heating (eg 1200W),
In the present embodiment, as shown in FIG. 2, the peak value of the applied voltage V _CE of the switching element 5 is about 700 V and the peak value of the current is about 60 A. In this state, when the user lifts the pan, the coupling between the heating coil 4 and the pan deteriorates, and the input current decreases. Therefore, the control circuit 12
The output DC voltage D of is increased to lengthen the drive pulse ON time, thereby attempting to restore the input current.

However, when the user lifts the pot considerably, even if the input current has not reached the level set by the user, if the drive pulse ON time becomes too long, the collector voltage V _CE becomes too high and the overvoltage stops. The operating level V _CEM of the circuit 10 may be reached. Therefore, the drive pulse ON time is set so as not to be longer than a certain value. In the case of this embodiment, it is set to 30 μs.

As described above, the drive pulse ON time is 30 μm.
If the pan is suddenly lowered from a position where it is considerably lifted while operating at s, the drive pulse on time is 30 μs.
As it is, the collector voltage V _CE may exceed the withstand voltage specification for a moment because it is heated for a moment. Therefore, the overvoltage stop circuit 10 operates to suddenly stop the heating, and as a result, an abnormal noise is generated from the pan.

In the induction heating cooker of the present invention, the user sets the maximum output heating (for example, 1200 W) through the output control volume 13 and the drive pulse ON time when the induction heating is performed, that is, the switching element 5 is turned on. The control circuit 12 stores the ON time T _A, and the ON time of the switching element 5 is a predetermined coefficient (1 or more) to this T _A until the pot is removed or the heating is stopped.
The drive pulse of the drive pulse generation circuit 11 is controlled so as not to exceed the time T _M obtained by multiplying by.
Therefore, the withstand voltage specification of the switching element 5 is not exceeded.

In this embodiment, the predetermined coefficient to be multiplied by the ON time T _A of the switching element 5 is 1.25.
Is set to The reason is that there is a temperature rise in the pot,
Moreover, even if the pot is slightly displaced, the drive pulse ON time is extended only about 1.2 times, so 1.25 times is a reasonable coefficient.

Further, as described above, in this embodiment,
The drive pulse ON time T _A1 at the time of maximum output heating is 20
Since it is μs, T _M is 25 μs (= 20 μs × 1.2).
5). In this state, even if the pot is suddenly lowered from the lifted position, the drive pulse ON time remains at 25 μs or is controlled to be shorter than that. Therefore, the collector voltage V _CE of the switching element 5 is momentarily overvoltage rather than withstand voltage specification. Since the operating voltage value V _CEM of the stop circuit 10 is not reached, no abnormal noise is generated from the pot and the switching element 5 is not destroyed.

In the induction heating cooker having the above structure, if the pot is accidentally heated at the maximum output while the pot is erroneously placed at a considerably shifted position at the start of cooking, the drive pulse ON time T _A1 during heating is widened. , 25 μs in some cases. However, even if the pot is placed correctly in this state, the control circuit 12 stores 25 μs as T _A , and the calculated T _M is 31.25 μs (= 25 μs × 1.25).
Will be. Therefore, the drive pulse ON time is 31.
If the pot is suddenly lowered from that position after lifting the pot while operating for 25 μs, it will heat for a moment while the drive pulse ON time remains at 31.25 μs, and the collector voltage V _CE of the switching element 5 will not withstand. May exceed specifications. As a result, the overvoltage stop circuit 10 operates and the heating is suddenly stopped, so that an abnormal noise is generated from the pan.

Therefore, the ON time T _A stored in the ON time of the switching element 5 during heating at the maximum output is stored.
T _A when it becomes shorter, the control circuit 12 has stored
And T _M is clear, that the heating control seeking T _M by multiplying a predetermined coefficient to the on-time T _A with newly stores the shortened ON time as T _A. By controlling the heating in this way, after the heating is started, when the pot is properly placed and the driving pulse ON time changes to the shorter one (20 μs), the control circuit 12 stores the stored T _A (25 μs) and T _M. (31.25 μs) is cleared, T _A is stored as the shorter ON time (20 μs), and T _M (25
μs = 20 μs × 1.25) is calculated and heating is controlled. Therefore, the collector V _CE does not exceed the withstand voltage specification even for a moment, and does not reach the operating voltage value V _CEM of the overvoltage stop circuit 10, so that no abnormal noise is generated from the pot and the switching element 5 is not destroyed. .

On the other hand, if the voltage of the AC power supply 1 becomes high after the control circuit 12 newly stores T _A and T _M as described above, the drive pulse ON time T _A1 naturally becomes short.
Even if the pot is placed correctly, the control circuit 12 will misunderstand. That is, according to the above example, the control circuit 12 clears the newly stored T _A (20 μs) and TM (25 μs), and then sets the erroneous shorter ON time (for example, 15 μs) as T _A. Remember and T _M (1
Since 8.7 μs = 15 μs × 1.25) is calculated and the heating is controlled, the initially set maximum output heating (1200 W in the above example) is lost. The reason why T _A1 naturally becomes shorter as the power supply voltage becomes higher is that the voltage of the capacitor 3 rises as the power supply voltage becomes higher, so that the current I _C of the switching element 5 is increased as indicated by the chain double-dashed line in FIG. This is because the gradient of becomes steeper and the time required to reach a predetermined current value becomes shorter.

Therefore, the induction heating cooker of the present invention is shown in FIG.
As shown in, a power supply voltage reading circuit 14 for reading the voltage of the AC power supply 1 at the time of maximum output heating and transmitting it to the control circuit 12 is further provided, and the ON time of the switching element 5 is stored while heating at the maximum output. If the power supply voltage becomes higher than the rated voltage at the same time as the ON time T _A that was
According to the above example, the control circuit 12 clears T _A (20 μs) and TM (25 μs), stores the shorter ON time (for example, 15 μs) as T _A , and
_M (18.75 μs = 15 μs × 1.25) is calculated and heating is controlled. As a result, the collector V _CE does not exceed the withstand voltage specification for a moment, and does not reach the operating voltage value V _CEM of the overvoltage stop circuit 10, so that no abnormal noise is generated from the pot and the switching element 5 may be destroyed. There is no. Also, when the power supply voltage returns to the rated voltage or less, the cleared T _A (20 μs) and T _M (= 25 μs) are stored again and the heating control is performed, so the initially set maximum output heating (1200 W) is obtained. To be

[0026]

As described above, according to the induction heating cooker of the present invention, the control circuit stores the ON time of the switching element when the induction heating is performed at the maximum output, and always stores the ON time in the ON time. The heating value is controlled so that the value obtained by multiplying it by the predetermined coefficient does not exceed the ON time of the switching element.Therefore, even when a load abnormality occurs by moving the pan up / down or left / right, the current of the switching element / Since it does not exceed the voltage specifications, it is highly reliable, and since the overvoltage stop circuit does not operate and abnormal noise is generated from the pan, the user can use it with peace of mind without any discomfort. Further, according to the induction heating cooker of the present invention, since the collector voltage of the switching element does not exceed the withstand voltage specification even for a moment,
It is also possible to omit the overvoltage stop circuit, which allows
An induction heating cooker can be provided at low cost.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of an induction heating cooker according to the present invention.

FIG. 2 is a time chart of voltage and current of each element and circuit shown in FIG.

[Explanation of symbols]

 1 AC Power Supply 2 Rectifier Bridge 3 Capacitor 4 Heating Coil 5 Switching Element 6 Capacitor 7 Diode 8 Current Transformer 9 Triangular Wave Generation Circuit 10 Overvoltage Stop Circuit 11 Drive Pulse Generation Circuit 12 Control Circuit 13 Output Adjustment Volume 14 Power Supply Voltage Reading Circuit

Claims (3)

[Claims] 1. An inverter circuit, which is connected to a rectified power source, has a capacitor, a heating coil for generating a high-frequency magnetic field, and a switching element for flowing a high-frequency current through the heating coil, and turns on / off the switching element. A drive pulse generating circuit for applying a drive pulse to the switching element, a triangular wave generating circuit for generating a triangular wave, and for stopping the generation of the drive pulse when the voltage applied to the switching element exceeds a predetermined value. An induction heating cooker including an overvoltage stop circuit that outputs the signal of 1 to the drive pulse generation circuit, and a control circuit that controls the drive pulse generation circuit so that the output heating set by the user is obtained. , The ON time of the switching element is the output voltage of the triangular wave generation circuit and the output voltage of the control circuit The control circuit stores the ON time T _A of the switching element at the time of maximum output heating, which is determined by the drive pulse output from the drive pulse generation circuit based on the magnitude relationship, and the ON time of the switching element is ON. An induction heating cooker characterized by controlling a drive pulse output from the drive pulse generation circuit so as not to exceed a time T _M obtained by multiplying the time T _A by a predetermined coefficient. 2. When the ON time of the switching element becomes shorter than the stored ON time T _A during heating at the maximum output, the control circuit causes the stored ON times T _A and T _M. 2. The heating time is controlled by clearing the ON time, newly storing the shortened ON time as T _A , and calculating T _M based on the ON time T _A.
The induction heating cooker described. 3. A power supply voltage reading circuit for reading the power supply voltage at the time of maximum output heating and transmitting it to the control circuit, wherein the ON time of the switching element is stored while heating at the maximum output. When the power supply voltage becomes higher than the rated voltage at the same time as the ON time becomes shorter than the ON time T _A , the control circuit clears the stored ON times T _A and T _M , and newly stores the shortened ON time as T _A. In addition, T _M is calculated based on the on-time T _A to control heating, and when the power supply voltage becomes equal to or lower than the rated voltage, heating is controlled by the cleared on-time T _A and T _M. The induction heating cooker according to claim 2.