JPS6131957B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an induction heating cooker using a single ended pushpable (hereinafter abbreviated as SEPP) inverter.

Conventionally, as a drive circuit for this type of induction heating cooker,
A high-frequency inverter is known that utilizes a series resonant circuit consisting of an induction heating coil and a resonant capacitor, and connects a switching element in parallel to the resonant capacitor to configure a high-frequency inverter. In a cooker having such a configuration, the oscillation frequency of the inverter changes depending on the on period of the switching element and the resonance period of the series resonant circuit. By changing this frequency, and in particular by controlling the on-period of the switching element, the input to the load is adjusted. In such a frequency control type cooking appliance, when configured with multiple mouths, a problem arises in that noise is generated. That is, when adjacent heating ports are operated at the same time, the oscillation frequency of the inverter naturally changes due to the difference in the material of the heated pan or the difference in the assumed input. The above-mentioned noise is generated when the magnetic fields from each heating port interfere with each other and according to the frequency difference between the two, and tends to increase as the frequency difference becomes larger. Such noise generation makes users feel uncomfortable and is a cause of lowering the product value.

The present invention has been made in consideration of these circumstances, and is capable of keeping the oscillation frequency of the inverter constant and adjusting the input under this condition, and is particularly applicable to multi-mouth induction cooking devices. Although beneficial,
There is no problem in applying it to a single-serve induction heating cooker.

In order to achieve the above object, the present invention uses a single end pushable (SEPP) inverter, and in this SEPP inverter, one end of a load circuit consisting of an induction heating coil and a resonant capacitor is connected to the high potential side of the power supply. It becomes connected. In this respect, it differs from a typical conventional SEPP inverter, which has one end of its load circuit connected to the low potential side (usually ground potential).

The present invention also includes adding an input reduction function that detects when an excessive current flows through the switching elements constituting the SEPP inverter and reduces the input to the load, and also adds an input reduction function that reduces the input to the load after entering the input reduction state. The purpose is to add a stop function that stops the oscillation of the inverter when excessive current is detected.

Another object of the present invention is to add an input reduction function that detects the input current of the AC power source that is the drive source of the SEPP inverter and reduces the input when excessive input is applied to the load.

FIG. 1 shows the configuration of a SEPP inverter 1 used in an embodiment of the present invention, where Q ₁ and Q ₂ are a first transistor and a second transistor, respectively, which are a first switching element and a second switching element, and both are NPN transistors. type transistors are used and are connected in series between the DC power supplies. As the first and second switching elements, in addition to transistors, GTOs and the like can also be used. D ₁ and D ₂ are freewheel diodes connected in antiparallel to the first and second transistors Q ₁ and Q ₂ , 2 is a load circuit connected in parallel to the first transistor Q ₁ , and the induction heating coil L ₁ and a resonant capacitor _C1 . A cooking pot made of ferrous metal is placed close to the induction heating coil _L1 .

Figure 2 shows its operating waveform diagram, and shows the first and second waveforms.
On/off signals A and B are applied to the bases of the transistors Q ₁ and Q ₂ , respectively. First, when the second transistor Q ₂ is turned on by the signal B, the drive current I ₁ flows through the induction heating coil L ₁ , the resonant capacitor C ₁ and the second transistor Q _{2 , and the second transistor Q 2} is turned off and the second transistor Q ₂ is turned on. 1 When transistor Q ₁ turns on, induction heating coil L ₁ , resonant capacitor
A circulating current I ₂ flows through C ₁ and diode D ₁ . When this circulating current I ₂ becomes zero, load circuit 2
The current flowing through the first transistor Q ₁ ,
A drive current I ₃ flows through the resonant capacitor C ₁ and the induction heating coil L ₁ . Subsequently, the second transistor Q ₂ is turned on again and the first transistor Q ₁ is turned off, but the circulating current I ₄ flows through the diode D ₂ , the resonant capacitor C ₁ and the induction heating coil L ₁ for a while. In FIG. 3, the on-off period ratio of the first transistor Q ₁ is made equal, and on the other hand, the on-period ratio of the second transistor Q ₂ is made equal to that of the first transistor Q _{1 .}
The current value can be arbitrarily controlled by setting the off period of the first transistor _Q1 as the maximum and zero as the minimum. First transistor Q ₁
Since the emitter potential changes unstablely, its duty control is difficult and requires a complicated circuit, but the second transistor Q ₂
Since the emitter potential is fixed at a low potential (earth potential), duty control is easy. Therefore, when starting the oscillation, it is possible to easily start the second transistor _Q2 from a state where the ON period is short, and the burden on the transistor due to the large current that tends to occur during starting and the surge voltage when cutting off the current can be reduced. 4 and 5 show an embodiment of the present invention. In Fig. 4, 1 is the aforementioned SEPP inverter, AC is an alternating current power supply, 3 is a power switch, 4 is a rectifier circuit consisting of a diode bridge, CH is a chiyoke coil, and _C2 is _a smoothing capacitor. A voltage across the terminals is applied to the SEPP inverter 1. 5 is induction heating coil L ₁
A cooking pot that serves as a load and is placed close to the cooking pot and is made of ferrous metal. 6 and 7 are first and second transistors
The first and second drive circuits apply drive signals to make Q ₁ and Q ₂ conductive alternately, and the drive frequency thereof is set to a constant high frequency, for example, 20 KHz. 8
is a duty control circuit that controls the operation of the first and second drive circuits 6 and 7. The cycle is a certain period, for example, one second, and the input to the load is controlled by changing the ratio of on and off periods during this period. Ru. In this example, the main input control is performed by this duty control circuit 8. CT ₁ is a first current transformer installed on the emitter side line of the second transistor Q ₂ and serves as a first current detection circuit;
CT ₂ is a second current transformer provided in one line of the AC power supply 2 and serves as a second current detection circuit, and obtains current detection signals Ip ₁ and Ip ₂ , respectively.

In FIG. 5, COM ₁ is a comparator, and a side terminal voltage V ₁ of a smoothing capacitor C ₂ is divided by resistors R ₁ , R ₂ and R ₃ and applied to its side reference terminal. Further, a detection signal from the first current transformer _CT1 is converted into a voltage by a resistor _R4 and inputted to the side signal terminal. FF ₁ is a flip-flop to which the output of the comparator COM ₁ is input to the 11 set terminal, and the "L" pulse B' synchronized with the rise of the ON signal of the first transistor Q ₁ is input to the set terminal.
Including this flip-flop _FF1 , flip-flops _FF2 and _FF3 , which will be described later, are all triggered by an "L" level pulse. Q ₃
The set output of flip-flop FF ₁ is connected to resistor R ₆
The collector is connected to the constant voltage Vcc terminal via the resistor _Q7 , and the emitter is grounded. 9 is a transistor
A time constant circuit connected in parallel between the collector and emitter of _Q3 , consisting of a resistor _R8 and a capacitor _C3 . COM ₂ is a comparator to which the capacitor C ₃ terminal voltage of this time constant circuit 9 is input to the side signal terminal, and the voltage Vcc is inputted to the side reference terminal after being divided by resistors R ₉ and R ₁₀ . . FF ₂ is a flip-flop to which a trigger pulse synchronized with the duty ion signal is input to the set terminal, and the output of the comparator COM ₂ is applied to the reset terminal; 10 is the set output signal of this flip-flop FF ₂ and the flip-flop.
FF ₁ is an AND gate that takes the set output signal of 1 as its 2 inputs, 11 is a monostable multivibrator that becomes operational when the output of AND gate 10 is at the "H" level, and 7 is a monostable multivibrator that receives a pulse signal from monostable multivibrator 11. The above-mentioned second drive circuit operates as follows, and its output is applied to the base of the second transistor _Q2 . FF ₃ is a flip-flop _{in which} a trigger pulse synchronized with the duty ion signal is input to the set terminal, and the output signal of comparator COM ₁ is input to the reset terminal _. The emitter of this transistor is connected to a constant voltage Vdd terminal, and the collector is connected to a monostable multivibrator 11 via a time constant circuit 12. Time constant circuit 1
2 is composed of resistors R ₁₂ and R ₁₃ and a capacitor C ₄ and determines the "H" level period of the monostable multivibrator 11. The monostable multivibrator 11 is given a pulse signal B'' synchronized with the fall of the drive signal B of the first transistor _Q1 as a starting signal. ₁ is turned off, an "H" level signal with a time width determined by the time constant circuit 12 is output, and this signal is used as the second
Used as ON signal C for transistor _Q2 .
_Q5 is another transistor connected in parallel between the collector and emitter of transistor _Q4 , and 14 is a second transistor.
A rectifier circuit that rectifies the detection signal of current transformer CT ₂ , C ₅ is a smoothing capacitor, and COM ₃ is a comparator where the side terminal voltage of smoothing capacitor C ₅ is input to its side signal terminal. is applied after being divided by resistors R ₁₄ and R ₁₅ . 15
is a time constant circuit composed of a resistor _R16 and a capacitor _C6 , and the output of the comparator _COM3 is inputted thereto.
The output of time constant circuit 15 is applied to the base of transistor _Q5 via resistor _R17 .

Next, the operation will be explained. In FIG. 6, period _T1 indicates a state in which heating operation is being performed for a normal load. That is, first, a pulse signal synchronized with the rise of the duty ion signal A is generated.
Flip-flop FF ₂ ,
FF ₃ is set. In this case, the voltage conversion value of the detection signal Ip ₁ of the first current transformer CT ₁ is determined by the comparator.
Since the voltage at the COM ₁ side reference terminal is not reached, its output D remains at the "H" level. Therefore, the output E of the flip-flop _FF1 is "H", the transistor _Q3 is on, and the output G of the time constant circuit 9 is "L".
The output of comparator COM ₂ is “H”, flip-flop
The output I of FF ₂ becomes "H". This "H" level signal I causes the monostable multivibrator 11 to
It becomes operational. In other words, the monostable multivibrator 11 is
becomes “H” level, and this “H” level period is determined by the time constant of the time constant circuit 12. In the case of normal heating, this "H" level period is set to be equal to or slightly shorter than the off period of the first transistor _Q1 . In this case, flip-flop FF ₃ is in the set state, so its output F is
"H" therefore transistor _Q4 is in the off state.
Also, since the detection signal level of the second current transformer CT ₂ is low, the output J of the comparator COM ₃ is "H",
The output K of the time constant circuit 15 is "H", and the transistor _Q5 is in an off state. Thus, the monostable multivibrator 11 oscillates at the maximum time constant of the time constant circuit 12, and the second drive circuit 7 operates.
SEPP inverter 1 performs oscillation drive. Note that the output B of the first drive circuit 6 has an on/off period fixed in a one-to-one ratio, and is formed by an oscillator or the like.

Next, a case will be explained in which an aluminum pot (hereinafter referred to as an aluminum pot) is used as a load. In this case, a phenomenon occurs in which an excessive current flows through the second transistor _Q2 , which causes destruction of the second transistor _Q2 . This is the same even when there is no load or when heating a small object. Note that at this time, the AC input current is small. The signal waveform in this state is shown in Figure 6.
Shown in _T2 . When the current flowing through the second transistor _Q2 exceeds the current value set based on its rated current value, the detection voltage level of the first current transformer _CT1 exceeds the reference level and the output D of the comparator _COM1
Generates an “L” level pulse. Both flip-flops FF ₁ and FF ₃ are reset by this "L" level pulse. By resetting flip-flop _FF3 , its set output signal F changes to the "L" level, and transistor _Q4 is turned on. This transistor Q ₄ acts as an impedance element and reduces the time constant of the time constant circuit 12.
Therefore, the "H" level period at the output of the monostable multivibrator 11 is shortened, and the on period of the second transistor _Q2 is shortened (waveform C).

On the other hand, the reset of the flip-flop _FF1 turns off the transistor _Q3 , and charging of the capacitor _C3 forming the time constant circuit 9 begins. Since the flip-flop FF ₁ is set by the signal B' synchronized with the rise of the ON signal B of the first transistor Q ₁ , charging of the capacitor C ₃ occurs when the first transistor Q ₁ is turned ON after detecting an overcurrent. It will be carried out until the end of time. Therefore, this charging period becomes longer if the current flowing through the second transistor _Q2 reaches the predetermined value earlier, and becomes shorter in the opposite case. When this charging of the capacitor _C3 is repeated and the voltage level G reaches the reference terminal voltage _Vref1 of the comparator _COM2 , the output H of the comparator _COM2 changes to the "L" level. Therefore, the output I of the AND gate 10 is inverted to "L" level, and the operation of the monostable multivibrator 11 is prohibited. As a result, the second transistor _Q2 is fixed off, and the oscillation of the SEPP inverter 1 is stopped. This stop period is indicated by _T3 in FIG. In this way, when a small load such as an aluminum pot or knife is heated, or when there is no load, the on period of the second transistor _Q2 is first shortened, and the input to the load is reduced. If an excessive current is still detected in such a state, the oscillation of the SEPP inverter 1 is stopped. In this case, depending on the material or size of the load, there may be a state in which heating continues in a low input state.

Next, a case opposite to the above case, that is, a case where the AC input current is large and the current flowing through the second transistor _Q2 is small, will be explained with reference to FIG. This phenomenon is seen in iron pots whose bottoms are lined with a layer of copper. In this case, even if the input is assumed to be 1500W, in reality, for example, 1800W of power may be supplied, which not only prevents accurate input control, but also causes the fuse to blow and the breaker to shut off. This may occur, and the heating operation itself may become impossible. In such a case,
When the detected current value of the second current transformer _CT2 reaches a value determined based on a preset maximum value of input power, the output signal J of the comparator _COM3 changes to "L" level. The capacitor _C6 of the time constant circuit 15, which has been in a charged state up to this point, is then gradually discharged, and when the voltage K first drops to a predetermined value Vrefz, the transistor _Q5 is turned on. This transistor _Q5 acts as an impedance element that reduces the time constant of the time constant circuit 12, shortens the on period of the second transistor _Q2 drive signal C, and acts to reduce the input applied to the load. In this way, supply of excessive power is suppressed, and power can be supplied at a value close to the set value.

Here, the significance of the time constant circuit 15 will be described. In this example, after an excessive input current is detected by the second current transformer CT ₂ , the transistor
The time constant circuit 15 delays the period _T3 until _Q5 turns on and lowers the input. If this time constant circuit 15 is not provided, when an excessive input current is detected, the "L" level of the comparator COM ₃ is inverted, and the transistor Q _{5 is} immediately turned off.
is turned on, shortening the on period of the second transistor _Q2 . When the input is thereby reduced, the excessive current disappears immediately after that, and the output of the comparator COM ₃ returns to the "H" level. Then, the transistor _Q5 is turned off again and the on period of the second transistor _Q2 becomes longer. Then, excessive input current is detected again and the above-described operation is repeated.
When the input power changes in this way for each oscillation cycle of the SEPP inverter 1, a beat sound is generated, causing discomfort to the user. The time constant circuit 15 overcomes this drawback by delaying the response time of the transistor Q ₅ to the detection signal from the second current transformer CT ₂ and averaging the detection signal to lengthen the input power change cycle. This prevents the beat sound from occurring.
When input control is performed by duty control as in this example, the maximum input is calculated at 100% duty.
It can be set to 1500W and 750W at 50% duty, but in reality, the rise of the AC input is gradual, so the on period is lengthened by a few percent, for example 750W at 52% duty, and 300W at 24% duty.
If you set it like this, you can obtain more accurate input power.

As mentioned above, the induction heating cooker of the present invention has a second
The first current detection circuit detects the current flowing through the switching element, and when it reaches an excessive current value, first lower the input to the load, and if the excessive current value continues to be detected, the inverter oscillates. Since the switching element is stopped, the switching element can be protected and wasteful consumption of power can be prevented. In this case, since the heated load is an inappropriate load, an alarm device may be added to notify this. Furthermore, since the present invention is provided with a second current detection circuit that detects AC input current, it is possible to detect a load that cannot be detected by the first current detection circuit alone, and it is possible to detect loads that cannot be detected by only the first current detection circuit. This eliminates the drawback that proper heating operation cannot be performed due to input.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of the main part for explaining an embodiment of the present invention, Figs. 2 and 3 are waveform diagrams of the same example, Fig. 4 is a circuit diagram showing the SEPP inverter section and power supply section, Fig. 5 6 is a circuit diagram showing a control portion, and FIGS. 6 and 7 are waveform diagrams for explaining the operation. 1... SEPP inverter, L ₁ ... induction heating coil, 6, 7... first and second drive circuit, 8... duty control circuit, 11... monostable multivibrator.

Claims (1)

[Scope of Claims] 1. An AC power supply, a rectifier circuit that rectifies the AC power supply current, a first switching element on the high potential side of the power supply converted to DC by the rectification circuit, and a second switching element on the low potential side. a switching circuit comprising elements connected in series; a resonant circuit comprising an induction heating coil and a resonant capacitor connected in parallel to the first switching element; a drive circuit that alternately conducts the first and second switching elements; a first current detection circuit that detects the current flowing through the second switching element, and when the first current detection circuit detects a current that is greater than a current value determined based on the rated current of the second switching element, the detection circuit output signal; a first input reduction circuit that shortens the conduction period of the second switching element, when the current detected by the first current detection circuit is equal to or greater than the predetermined current value when the input reduction circuit is in an operating state; An induction heating cooker comprising a stop circuit that stops the operation of the drive circuit. 2. An AC power source, a rectifier circuit that rectifies the AC power current, a first switching element connected in series to the high potential side of the power source converted to DC by the rectifier circuit, and a second switching element connected in series to the low potential side. a resonant circuit comprising an induction heating coil and a resonant capacitor connected in parallel to the first switching element, and a series circuit comprising an induction heating coil and a resonant capacitor connected in parallel to the first and second switching elements. a circuit, a drive circuit that alternately conducts the first and second switching elements, a first current detection circuit that detects the current flowing through the second switching element, and a current determined based on the rated current of the second switching element. a first input resistance circuit that shortens the conduction period of the second switching element by an output signal of the first current detection circuit when the first current detection circuit detects a current exceeding a current value; a stop circuit that stops the operation of the drive circuit when the current detected by the first current detection circuit is equal to or higher than the predetermined current value; A second current detection circuit that detects when the current is equal to or higher than a determined current value, and a second input reduction circuit that shortens the conduction period of the second switching element using a detection output signal of the second current detection circuit. Induction heating cooker.