JPH0470753B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an electromagnetic induction heater that heats a cooking pot by electromagnetic induction.

<Prior Art> As is well known, this type of electromagnetic induction heater includes a heating coil that generates magnetic flux for induction heating,
A switching element is provided to control the oscillation of a resonant circuit including the heating coil. As one method for controlling the heating output, the oscillation output itself is maintained at a predetermined value by controlling the on-width of the switching element, and the oscillation is intermittent and the time during oscillation and the time when oscillation is stopped are controlled. It is known that the heating output is controlled to a set value depending on the ratio between the heating output and the heating output.

In this method, when restarting oscillation from a state in which oscillation is stopped, the on-width of the switching element is not suddenly set to a predetermined value, but is gradually brought closer to a predetermined value from a small value. In this case, the oscillation output is small until the predetermined on-width is reached, resulting in a small average heating output.To avoid this, set the on-width to a large value so that the average heating output does not match the set value. I'm trying to make it happen. Note that a method may be adopted in which the on-width at which the set heating output is obtained is estimated from the current of the heating coil, and the on-width is increased all at once.

<Problems to be Solved by the Invention> As described above, when the on-width of the switching element is increased, it becomes necessary to use a switching element with a large withstand voltage or switching current, which increases the cost. Also, if the ON width is narrow, the switching element will turn ON with voltage remaining.
However, this noisy state exists for a long period of time, resulting in a noisy electromagnetic induction heater.

In addition, in the method of raising the power all at once, if the power supply voltage fluctuates when estimating the on-width, the heating output also fluctuates, and an excessive voltage is applied to the switching element, resulting in destruction.

The present invention focuses on these points, and aims to shorten the time until a predetermined on-width is reached, and to obtain a set heating output without increasing the on-width more than necessary. It is something that

<Means for solving the problem> A time ratio control circuit that alternately outputs an oscillation start signal and an oscillation stop signal with time distribution such that a preset heating output is obtained; an on-time determining circuit that determines a preliminary oscillation period with a small output on-width for estimating the type of pot based on the output of the on-time determining circuit; and an on-time determining circuit that receives the output from the on-time determining circuit and switches the on-width signal through a drive circuit. an on-time control circuit that supplies the current to the heating coil, an AC/DC conversion circuit that converts the current flowing through the heating coil into a DC voltage and extracts it, and when the preliminary oscillation period has elapsed, the load is controlled by the preliminary oscillation output from the AC/DC conversion circuit. The oscillation output is controlled by an on-time determination signal output circuit that estimates and outputs the on-width required to obtain an output close to a predetermined oscillation output, and an on-time determination signal output circuit that estimates and outputs the on-width required to obtain an output close to a predetermined oscillation output. It consists of a coil current limiting circuit.

<Function> In the present invention, the load is estimated by preliminary oscillation with a small on-width, and based on the result, the on-width is increased at once to a value close to the value at which a predetermined oscillation output is obtained, and thereafter the oscillation output feedback is The on-width is controlled to a value that allows a predetermined oscillation output to be obtained. Therefore, the time required to reach a predetermined on-width is shortened, the on-width does not become larger than necessary, and even if the power supply voltage fluctuates, excessive voltage is not applied to the switching element. Not at all.

<Example> The illustrated example will be described below. FIG. 1 is a circuit diagram, and FIG. 2 is a time chart showing the operation.

In Fig. 1, a rectifier bridge 1 is connected to the input section that receives commercial power, and the output of the rectifier bridge 1 is connected to a choke coil 2 and a smoothing capacitor 3.
It is becoming smoother. A heating coil 4 that generates magnetic flux for induction heating and a resonant capacitor 5 are connected in series between both terminals of the smoothing capacitor 3 to form a resonant circuit. , a switching element 6 and a diode 7 are connected in parallel to form a switching circuit.

A transistor is used as the switching element 6, and the output of the drive circuit 10 is applied to the base of the transistor.
Switching element 6 is turned on according to the output of 0.
By turning off, the resonant circuit is caused to oscillate,
The current flowing through the heating coil 4 generates magnetic flux for induction heating. Note that the switching element 6 may also be a semiconductor element such as a gate turn-off switch or a unidirectional three-terminal thyristor.

A current transformer 8 for detecting the current of the heating coil 4 is coupled to the current-carrying path of the heating coil 4.
The detection output of the current transformer 8 is converted into a DC signal by an AC/DC conversion circuit 9 and sent to an on-time determining circuit 11 and a coil current limiting circuit 12, respectively. The on-time determining circuit 11 is a circuit that determines the on-width of the switching element 6, which varies depending on the type of pot, and the coil current limiting circuit 12 is a circuit that limits the oscillation output. The outputs of the on-time determining circuit 11 and the coil current limiting circuit 12 are sent to the on-time control circuit 16 via diodes 13 and 14, respectively. The output of each circuit 11 and 12 is
The signals are added in an analog manner by an OR circuit and then input to the on-time control circuit 16.

On the other hand, the setting value of the heating setting volume 19, which is operated by the user to set the heating output to a desired value, is determined by the time ratio control circuit 18 at a ratio such that the heating output is set by the heating setting volume 19. , which alternately outputs an oscillation start signal H and an oscillation stop signal L, and the signals are sent to the on-time determination signal output circuit 17 and the on-time control circuit 16.
sent to each. On-time determination signal output circuit 17
is a circuit that controls the timing of determining the on-time, and its output is sent to the on-time determining circuit 11 and the coil current limiting circuit 12, respectively.

Next, the operation will be explained with reference to the time chart shown in FIG.

When the power supply voltage is applied, the time ratio control circuit 1
8 outputs an oscillation start signal H and an oscillation stop signal L at a ratio according to the setting value of the heating setting volume 19. First, when the oscillation start signal H is output,
The preliminary oscillation period Ta for estimating the on-width that will result in different set outputs depending on the pan is a period in which a signal voltage that sets the on-width to Ton ₁ is output from the on-time determining circuit 11 to the on-time control circuit 16. Therefore, a signal with an on-width of Ton ₁ is output from the on-time control circuit 16, sent to the switching element 6 via the drive circuit 10, and heated by the heating coil 4.
A resonant circuit consisting of the resonant capacitor 5 and the resonant capacitor 5 oscillates. When oscillation occurs, a high frequency current flows through the heating coil 4, and the current is detected by the current transformer 8.
The AC/DC conversion circuit 9 outputs a DC voltage proportional to the current value.

When the preliminary oscillation period Ta has elapsed, the on-time determination signal output circuit 17 outputs an on-time determination signal to the on-time determination circuit 11. Since there is a proportional relationship between the output voltage of the AC/DC conversion circuit 9 when this signal is output and the ON width of the set output as shown in FIG. Estimate the on-width that will result in the set output from the relationship, and select an on-width that is slightly smaller than that on-width.
A signal voltage that becomes Ton ₂ is output to the on-time control circuit 16. As a result, the oscillation is switched to the main oscillation, and the oscillation output of the resonant circuit is
Increases to a value equivalent to Ton ₂ . Note that Ton ₂ is set to, for example, 80% of the estimated on-width, but this value can be arbitrarily selected depending on the circumstances.

On the other hand, this oscillation output is transmitted through current transformer 8 and AC/DC
Feedback is provided from the conversion circuit 9 to the coil current limiting circuit 12. While the oscillation start signal H is being input, the output from the coil current limiting circuit 12 gradually increases until the output of the AC/DC conversion circuit 9 reaches a value Vp corresponding to a predetermined oscillation output. The output of the on-time determining circuit 11 is OR-processed in an analog manner, and the on-time controlling circuit 1
6 is input. As a result, the on-width of the switching element 6 quickly increases to a predetermined on-width, and from then on, the oscillation stop signal L is transmitted to the time ratio control circuit 1.
The main oscillation is maintained at a predetermined oscillation output until output from 8.

When the oscillation stop signal L is output from the time ratio control circuit 18, the ON signal is no longer output from the on-time control circuit 16, and oscillation is stopped. At the same time, the output of the on-time determination signal output circuit 17 is L.
level, and the output from the coil current limiting circuit 12 disappears. Therefore, the on-time control circuit 16
In order to estimate a predetermined on-width in preparation for the next restart of oscillation, only the signal whose on-width is Ton ₁ is input from the on-time determining circuit 11. Then, when the oscillation start signal H is output from the time ratio control circuit 18 again, oscillation is restarted by the operation described above, and from then on, oscillation and oscillation stop are performed at a time ratio according to the set heating output. repeated.

The operation of this embodiment is as described above, and even if the power supply voltage is low during preliminary oscillation and the on-width is estimated to be greater than the predetermined oscillation output, the main oscillation is, for example, 80%. Therefore, even if the power supply voltage is restored, the ON width will not become abnormally wide. Since the main oscillation starts from the predetermined 80%, the time required to reach 100% is short. Furthermore, even if the current in the heating coil 4 fluctuates due to fluctuations in the power supply voltage, the coil current limiting circuit 12 adjusts to maintain the set output, so there is no risk of abnormal voltage being applied to the switching element 6. do not have.

<Effects of the Invention> As described above, the present invention estimates the load by performing preliminary oscillation with a small on-width, increases the on-width at once to near a predetermined on-width, and then gradually increases the on-width. It is controlled to have a predetermined ON width. Therefore, the time it takes for the on-width of the switching element to reach a predetermined value is short, and it is easy to prevent the on-width from exceeding a predetermined value, making it possible to use switching elements with small withstand voltages and switching currents. It is possible to reduce the cost and reduce noise generation. Furthermore, even if the power supply voltage fluctuates, it is difficult for excessive voltage to be applied to the switching element, which also reduces costs and improves reliability.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is a time chart explaining the operation of the same, and FIG.
The figure is a diagram showing the relationship between the output voltage of the AC/DC conversion circuit and the on-width of the set output. 4...Heating coil, 5...Resonance capacitor, 6
...Switching element, 8...Current transformer, 9...
AC/DC conversion circuit, 11... On time determination circuit, 12... Coil current limiting circuit, 16... On time control circuit, 17... On time determination signal output circuit, 18... Time ratio control circuit, 19... ...Heating setting volume.

Claims (1)

[Claims] 1 Equipped with a heating coil that generates magnetic flux for induction heating and a switching element that controls the oscillation of a resonant circuit that includes this heating coil, and controls the ON width of the switching element to maintain the oscillation output at a predetermined value, In an electromagnetic induction heater that uses a switching element to intermittent the oscillation of a resonant circuit and control the heating output to a set value depending on the time ratio between oscillation and stop, the time distribution is such that a preset heating output can be obtained. a time ratio control circuit that alternately outputs an oscillation start signal and an oscillation stop signal, and a preliminary oscillation period with a small output ON width for estimating the type of pot using the output from the time ratio control circuit. an on-time determining circuit that receives the output from the on-time determining circuit and supplies an on-width signal to the switching element via a drive circuit, and an on-time control circuit that converts the current flowing through the heating coil into a DC voltage. The load is estimated using the AC/DC conversion circuit to be extracted and the preliminary oscillation output from the AC/DC conversion circuit after the preliminary oscillation period has elapsed, and from this, the ON width necessary to obtain an output close to the predetermined oscillation output is calculated. an on-time determination signal output circuit that determines and outputs the on-time determination signal;
An electromagnetic induction heater comprising a coil current limiting circuit that limits oscillation output by feeding back the output from an AC/DC conversion circuit.