JPH0648636B2 - Induction heating device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an induction heating device, and particularly to a domestic induction heating device.

(B) Conventional technology An induction heating device supplies high-frequency current to a heating coil in an inverter, generates a high-frequency alternating magnetic field with this heating coil, and induction-heats an object to be heated that is placed in the vicinity of this heating coil. is there. Such an induction heating device is shown, for example, in Japanese Patent Publication No. 53-44061. In such an induction heating device, an improper load is detected by comparing the AC input current and the set input power value.

C) Problems to be solved by the invention In such a device, the load condition appears remarkably at a high set input power, but at a low set input power level, there is almost no difference due to the load condition. Therefore, there is an inconvenience that the load cannot be surely discriminated when the set input level is low.

On the other hand, in such a device, a soft start circuit and an overcurrent protection circuit are provided, and it is necessary to restrict the operation of the device prior to the input power control. Therefore, when the protection circuit operates, there is a disadvantage that a predetermined load cannot be detected. Therefore, it is possible to make the load detection level variable on the assumption that the protection circuit operates, but in this case, there is a problem that the circuit configuration becomes very complicated.

D) Means for Solving the Problems In the present invention, a full-wave rectifier circuit for full-wave rectifying an AC power supply voltage, an induction heating coil connected to the full-wave rectification circuit, and an oscillating current flowing through the induction heating coil. In an induction heating device having an inverter circuit composed of a switching element for switching, an input power detection means for detecting an AC input current input to the full-wave rectification circuit, and an oscillating current for detecting an oscillating current in the inverter circuit. Comparing the detecting means, a calculating means for generating a load discriminating level in proportion to the signal level detected by the oscillating current detecting means, and the load discriminating level of the arithmetic means and the signal level of the input power detecting means. Accordingly, the load determining means for determining the suitability or the unsuitability of the load, which is disposed close to the induction heating coil, is provided.

E) Since the working input current and the oscillating current in the inverter are compared to determine whether the load is suitable or not, even if regulations are imposed by the protection circuit, the unsuitable load can be detected accurately. .

F) Embodiment FIG. 1 is a circuit block diagram of an embodiment of the induction heating device of the present invention, (1) is a full-wave rectification circuit for full-wave rectification by receiving commercial AC voltage, and (2) is this full-wave rectification. Circuit (1) A choke coil connected to the output end. (3) is a filter capacitor connected to the choke coil (2), (4) is an induction heating coil connected to the choke coil (2), and (5) is a resonance circuit with the induction heating coil (4). The resonance capacitor, (6) is a switching transistor connected in parallel to this resonance capacitor (5), (7) is a damper diode connected in anti-parallel to this switching transistor (6), and these filter capacitors (3 ) ~ The damper diode (7) forms the inverter circuit (8). (9) shows a drive circuit for controlling ON / OFF of the switching transistor (6),
Supplies the base voltage to the switching transistor (6). (10) compares the voltage across the heating coil (4) to detect the ON timing of the switching transistor (6), and supplies an ON pulse to the flip-flop circuit (11).
This is an N pulse generation circuit, and a signal is given to the drive circuit (9) from the flip-flop circuit (11) by this ON pulse, and the switching transistor (6) is turned on. (12) is an input power detection circuit which serves as input power detection means for receiving a signal from a first current transformer (13) for detecting an AC input current and outputting an input power level according to the input current, (14) Is an operating means for operating and setting the input power, and (16) is connected to a second current transformer (17) for detecting the resonance current in the inverter circuit (8), and a level signal corresponding to the magnitude of this resonance current is supplied. A resonance current detection circuit that serves as an oscillating current detection means to output, (18) is a level adjustment means for adjusting the magnitude of the level signal from this resonance current detection circuit (16), and these operation means (14), input Power detection circuit
(12), the level signal from the resonance current detection circuit (16) is A / D
Digital data Pref and Power Ip _{R in the} conversion circuit (19)
Is converted to. (20) is a subtracter for calculating Pref-Power (t) according to the t-th A / D conversion timing in the A / D conversion circuit (19), and (21) is this Pref-Power (t) Shown is an adder which adds new ON period data Pcon (t) to the data Pcon (t-1) one time before according to the time at which the switching transistor (6) should be turned ON, and shows the ON period data Pcon (t) Is given to the input side of the adder (21) via the delay circuit (22). (23) is the Pcon (t-
In place of 1), a soft circuit that gives low-level soft start data Soft to the adder (21) is shown, the output from the subtracter (20) is cut off at the start of this oscillation, and this Soft is data during the ON period. Is output from the adder (21). (24) is a current value setting device that generates a set resonance current value Iref that increases in accordance with the increase of Pref, that is, the setting input in the operating means (14), and (25) is the A / D conversion circuit (19). Iref-Ip according to the t-th A / D conversion timing of
A subtractor for calculating _R (t), and (26) is an adder for generating the limit data Ipcon (t) for limiting the resonance current, which is one time before transmitted through the delay circuit (27). Limit data Ipcon
(t-1) was added with Ircf-Ip _R (t) from the subtractor (25) calculates a new Ipcon (t). (28) is an ON start counter that starts counting by a signal from the flip-flop circuit (11), (29) is a comparator that compares the count value of the ON period counter (28) with the Pcon (t), Reference numeral 30) is a comparator for comparing the count value of the ON period counter (28) with Ipcon (t), and when one of these comparators (29) (30) is in agreement, OR The flip-flop circuit (11) is cleared via the gate (31), and the drive circuit (9) and ON
No signal output to the period counter (28). (32) is an arithmetic circuit that serves as an arithmetic means for linearly converting the data Ip _R (t) corresponding to the resonance current to calculate the improper load detection level P _LS (t), and (33) is this P _LS (t). ) And data Po corresponding to the above input power
A load discriminator serving as a load discriminating means for comparing wer (t) is shown. When Power < _PLS , the flip-flop circuit (11) is cleared via the OR gate (31). (34) is an overcurrent protection circuit for detecting whether or not the resonance current detected by the second current transformer (17) has exceeded a predetermined value, and when an overcurrent is detected, the OR gate (31 ) Is used to clear the flip-flop circuit (11).

In such an induction heating device, when the cooking pot, which is the load of the object to be heated, starts oscillating in the state of being placed close to the induction heating coil (4), first the ON pulse generation circuit (10) sends a signal to the flip-flop circuit (11). Is given. The flip-flop circuit (11) includes a drive circuit (9) and an ON period counter (28).
The switching transistor (6) is turned on, and the counting operation of the ON period counter (28) is started. This count value is set by the comparison circuit (29) (30)
n, compared with Ipcon. In the initial state of oscillation, as described above, low-level data Soft from the soft start circuit (23) via the adder (21) is transferred to the comparison circuit (29) by Pcon (0).
Therefore, when the count value of the ON period counter (28) becomes Soft, the flip-flop circuit (11) is cleared from the comparator (29) through the OR gate (31) and driven. Switching transistor due to circuit (9)
When (6) is turned off, the ON period counter (28) also stops counting and clears its content. After that, due to resonance in the inverter circuit (8), the switching transistor
The collector voltage of (6) rises once and then drops again.
The ON pulse generation circuit (10) detects such a fall of the collector voltage by reversing the voltage across the heating coil (4), and a signal is given to the flip-flop circuit (11). As a result, a signal is again emitted from the flip-flop circuit (11), the switching transistor (6) is turned on, and the ON period counter (28) starts counting.
Such operation is continued and the oscillation of the inverter circuit (8) is continued. Simultaneously with the start of this oscillation, the A / D conversion circuit (19)
Is started, the set input power value at the operating means (14),
The level signal from the input power detection circuit (12) and the level signal from the resonance current detection circuit (16) are digital data Pref.
(t), Power (t), and IP _R (t) are time-divisionally A / D converted. Further, such A / D conversion is performed in units of one cycle for each half wave of commercial AC. The subtractor (20) is thus A / D
Calculate Pref-Power (t) from the converted data and add
(21), and in the adder (21), the above-mentioned Prcf-Po is added to the ON period data Pcon (t-1) one time before which is given through the delay circuit (22).
wer (t) is added to form new ON period data Pcon (t). That is, here, the ON period data is sequentially corrected and Powe is corrected.
The ON period length is adjusted so that r becomes equal to Pref.
Therefore, after the oscillation is started, the ON period length of the switching transistor (6) is increased by this function, and Power becomes equal to Pref. After that, this ON period is maintained. on the other hand,
The data Iref-Ip _R (t) obtained by subtracting Ip _R (t) from Iref generated according to Pref by the subtracter (25) is made in accordance with the operation of the A / D conversion circuit (19) and added. Given to the vessel (26). Adder (26)
Is the previous limit data given via the delay circuit (27)
The above Iref-Ip _R (t) is added to IPcon (t-1) to form new restriction data Ipeon (t). That is, here the limit data
Ipcon is sequentially corrected so that Iref becomes equal to Ip _R (t).
It acts to adjust the N period length. However, when using a cooking pot made of highly magnetic material such as iron or holo, the resonance current in the inverter circuit (8) is small compared to the AC input power, and normally the ON period data Pcon is used. The ON period length of the switching transistor (6) is determined. However, when a stainless steel pot with weak magnetism is used, the resonance current of the inverter circuit (8) is relatively larger than that in the above case compared to the AC input current, and the limit data Ipcon
The ON period length of the switching transistor (6) is regulated by, and the output is substantially controlled by comparing Iref and Ip _R. In addition, the overcurrent protection circuit (34) performs switching when the resonance current in the inverter circuit (8) suddenly increases during such control, or when a match output is not output due to a failure of the comparison circuit (29) (30). The flip-flop circuit (11) is cleared by detecting an overcurrent flowing through the transistor (6) to turn off the switching transistor (6) for protection.

In addition to this oscillation operation, the load discriminator (33)
Load discrimination level P generated in the arithmetic circuit (32) in proportion to _R
Comparing _LS and Power, when P _LS > Power, it is judged as an inappropriate load, and the flip-flop circuit (11) is cleared.
Therefore, small loads such as knives and forks are placed close to the induction heating coil, and non-magnetic cooking pots are used.
_{When LS} > Power, inverter oscillation is stopped. Further, since the ratio of P _LS and Power is almost constant if the load is the same, as described above, even during the soft start period, the input power control and the output adjustment by the current control by IPcon overlap and Pref Even if a load that is not linear in the change in the power with respect to is properly or inappropriately determined, the determination can be reliably performed.

As an experimental example, the set value of input power Pref and the actual input power
The values of Power and the resonance current Ip _R of the inverter circuit (8) are shown in the table below.

In this table, φ is the diameter of the bottom of the pan (mm), t is the thickness of the pan (m
m).

Pref-Power characteristics are shown in Fig. 2 based on this table, and Ip _R-
Power characteristics are shown in FIG. In these figures, φ
200 holo pot, φ120 holo pot, φ110, t
1.1-8-18 stainless steel pan, φ200, t2.0-18-
The 8 stainless steel pans are indicated by a two-dot chain line, a dotted line, a solid line, and a one-dot chain line, respectively. As can be seen from the figure, it is difficult to separate the load at the low input level in Fig. 2 of the Prcf-Power characteristic, but in the Ip _R -Power characteristic of Fig. 3, it is possible to completely reduce the load from the low input level to the high input level by each pan. Separated and load discrimination level P _LS
= F (Ip _R ), which is linear with respect to the change in Ip _R , as described above, allows the arithmetic operation of the arithmetic circuit (32) to be performed easily. Further, by adjusting the level adjusting means (18), the straight line of the load discrimination level P _LS = f (Ip _R ) is relatively shifted,
It can be set to various load discrimination specifications.

Further, in this embodiment, the switching transistor (6) and the damper diode (7) are used as the resonance current of the inverter circuit.
Although the current is detected, the second current transformer (17) may be provided to detect the current flowing through the heating coil (4) as shown in FIG. With this structure, since the feedback current of the damper diode (7) is not detected, the resonance current can be detected more accurately.

G) Effect of the invention As described above, the induction heating device of the present invention compares the alternating input current with the oscillating current in the inverter to determine whether the load is suitable or not, so that from a high input level to a low input level. It is possible to completely detect the difference in the load state. Further, by the operation of various protection circuits, the predetermined load can be surely determined even when the input power control cannot be performed.

[Brief description of drawings]

FIG. 1 is a circuit block diagram of the induction heating device of the present invention, FIG. 2 is a set power-input power characteristic diagram, FIG. 3 is a resonance current-input current characteristic diagram, and FIG. 4 is another embodiment used in the present invention. It is a circuit diagram of an example inverter circuit. (1) …… Full wave rectification circuit, (4) …… Induction heating coil, (6)…
Switching transistor, (8) Inverter circuit, (13) (17) Current transformer, (12) Input power detection circuit, (14) Operating means, (16) Resonant current detection circuit , (19) …… A / D conversion circuit, (20) (25) …… subtractor, (2
1) (26) …… adder, (22) (27) …… delay circuit, (23) …… soft start circuit, (24) …… current value setter, (28) …… O
N period counter, (29) (30) …… comparator, (32) …… arithmetic circuit, (33) …… load discriminator, (34) …… overcurrent protection circuit.

Claims (1)

[Claims] 1. A full-wave rectification circuit for full-wave rectifying an AC power supply voltage, an inverter circuit including an induction heating coil connected to the full-wave rectification circuit and a switching element for flowing an oscillating current through the induction heating coil. In the induction heating device having :, an input power detecting means for detecting an AC input current input to the full-wave rectification circuit, an oscillating current detecting means for detecting an oscillating current in the inverter circuit, and the oscillating current detecting means. By calculating a load discrimination level in proportion to the signal level detected by, and comparing the load discrimination level of this computing means with the signal level of the input power detection means, the induction heating coil is placed close to the induction heating coil. And a load discriminating means for discriminating whether the load is suitable or not.