JPH08138849A - Inverter control method - Google Patents

Abstract

PURPOSE: To provide an inverter control method by which a switching element can be reliably turned off before an electric current reaches zerocross time in translocation control in an inverter. CONSTITUTION: An inverter control method is provided with transistors 13 and 14 to reverse the direction of an electric current flowing to an induction heating coil 19, a current transformer 20 to detect the electric current of the induction heating coil 19 and a coil current phase discriminating circuit 21 to discriminate a phase of the electric current flowing to the induction heating coil 19 by output electric current of the current transformer 20. The transistors 13 and 14 are constituted so as to be turned off by a discriminating result of the coil current phase discriminating circuit 21.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter control method.

[0002]

2. Description of the Related Art An electromagnetic cooker induction-heats an object to be heated by the heating output of an induction heating coil. Therefore, the electromagnetic cooker is equipped with an inverter that controls the heating output of the induction heating coil. As this inverter, there is a transistor system of induction heating which uses a series resonance circuit composed of an induction heating coil and a capacitor connected in series with the induction heating coil as a load.
-190082.

FIG. 10 is a schematic configuration diagram of an inverter of this type. Between the positive electrode terminal 1a and the negative electrode terminal 1b of the DC power supply 1, a series circuit of transistors 2 and 3 which are switching elements is provided. Transistor 2
A series circuit of a resonance capacitor 4 and a resonance capacitor 5 is interposed between the collector of the capacitor and the emitter of the transistor 3. The connection intermediate point between the resonance capacitors 4 and 5 is connected via the induction heating coil 6 to the connection intermediate point between the emitter of the transistor 2 and the collector of the transistor 3. A flywheel diode 7 having an anode on the emitter side is interposed between the emitter and collector of the transistor 2, and a flywheel diode 8 having an anode on the emitter side is interposed between the emitter and collector of the transistor 3.

In this inverter, the transistor 2 and the transistor 3 are alternately turned on and off to cause a resonance current to flow in the induction heating coil 6. Then, when one transistor 2 (or 3) is turned off and a predetermined rest time (dead time) elapses, the other transistor 3
(Or 2) is turned on so that the transistors 2 and 3 are simultaneously turned on and the DC power supply 1 is not short-circuited to perform a commutation operation.

The transistors 2 and 3 are turned off before the current in the induction heating coil 6 reaches the zero crossing time point t ₀ as shown in FIG. 11 (a). This is because when the transistor 2 or 3 is turned off after the current of the induction heating coil 6 has passed the zero crossing time point t ₀ , the resonance current continues to flow through the flywheel diode, and the current of the induction heating coil 6 is as shown in FIG.
This is because the effective current value changes as shown in (b) and the effective current value decreases and the input current of the induction heating coil 6 decreases as compared with the case where the transistor 2 or 3 is turned off before the zero crossing time point t ₀ .

Normally, this inverter performs control to lengthen the on time of the transistor when the input current decreases, but if the on time is lengthened in the state as shown in FIG. 11 (b), the input current will decrease and the heating will increase. The output cannot be controlled normally. Therefore, it is necessary to surely turn off the transistors 2 and 3 before the current of the induction heating coil 6 reaches the zero crossing time point t ₀ .

By the way, the current of the induction heating coil 6 is shown in FIG.
If the transistor 2 is turned off before the zero crossing time is reached as shown in (a), the collector voltage of the transistor 3, which is the voltage of the inverter, decreases as shown in Fig. 12 (b), and after a lapse of a predetermined rest time, the transistor While the transistor 3 is turned on and the transistor 2 is turned off, the collector voltage of the transistor 2 remains lowered. Then transistor 3
Is turned off, the collector voltage of the transistor 3 rises. And when the transistors 2 and 3 are turned off,
The phase difference γ between the current of the induction heating coil 6 which is the current of the inverter and the zero crossing time point t ₀ is measured.

When the inverter is started, the inverter is started at a high frequency, and then the operating frequency is gradually lowered with the resonance frequency as a target while phase comparison between the voltage and current of the inverter is performed. And the measured phase difference γ is
When γ ≦ γ _ad (where γ _ad is the lower limit of the phase difference γ), the operation of lowering the operating frequency is stopped. In this way, the time difference at the time of each zero crossing of the inverter voltage and current
The (phase difference) is reduced to a few μsec to prevent the effective current value of the induction heating coil 6 from decreasing.

[0009]

As described above, in the conventional inverter control method, it is necessary to detect the off time of the transistor and the zero crossing time of the current in the induction heating coil. Therefore, the voltage and current of the inverter are detected. It must be detected accurately. However, the inverter has 200
Since a high DC voltage obtained by rectifying an AC voltage of V is supplied, a circuit element that withstands the high voltage is required for the detection unit, and the control circuit of the inverter cannot be inexpensively constructed. Further, since it is necessary to detect both the voltage and the current of the inverter, this also raises a problem such as an increase in cost.

In view of the above problems, the present invention provides a method of controlling an inverter that detects only the current of the inverter and can surely turn off the transistor before the current of the induction heating coil reaches the zero crossing point. With the goal.

[0011]

According to a first aspect of the present invention, there is provided an inverter control method, in which commutation control is performed on a switching element for turning on / off a current in a resonance circuit including a coil and a resonance capacitor connected in series with the coil. In a method of controlling an inverter, a current detection unit that detects a current of the coil, a first voltage related to a detection current of the current detection unit, and the first voltage
A voltage generation unit that generates a second voltage with a delayed voltage and a comparison unit that compares the first voltage and the second voltage are provided, and the switching element is turned off according to the comparison result of the comparison unit. It is characterized by

The control method of the inverter according to the second invention is
In a method of controlling an inverter, which commutates a switching element that turns on and off a current of a resonance circuit that includes a coil and a capacitor that is connected in series to the coil, a current detection unit that detects a current of the coil is provided, and the switching is performed. After the element is turned on, the switching element is turned off when the detected current of the current detection unit becomes a predetermined value or less.

An inverter control method according to the third invention is
In a method of controlling an inverter, which commutates a switching element that turns on and off a current of a resonance circuit that includes a coil and a capacitor that is connected in series to the coil, a current detection unit that detects a current of the coil is provided, and the switching is performed. After the element turns on and the detected current exceeds a predetermined value,
It is characterized in that the switching element is turned off when it becomes a predetermined value or less.

[0014]

In the first aspect of the present invention, the first voltage generated by detecting the current of the coil and the second voltage delayed from the first voltage are generated to generate the phase difference between the first voltage and the second voltage. It corresponds to the period from the time when the switching element is turned off to the time when the coil current crosses zero. The first voltage and the second voltage are compared in magnitude, and the switching element is turned off according to the comparison result. As a result, only the coil current can be detected and the switching element can be turned off before the coil current reaches the zero crossing point.

In the second invention, the current of the coil is detected.
After the switching element is turned on, the switching element is turned off when the detected current becomes equal to or less than a predetermined value after a lapse of a predetermined time. As a result, since there is no circuit for delaying the voltage, the OFF time of the switching element is stabilized even if the operating frequency changes.

In the third invention, the coil current is detected.
After the switching element is turned on, when the detected current becomes equal to or higher than the predetermined value and then becomes equal to or lower than the predetermined value, the switching element is turned off. As a result, since there is no circuit for delaying the voltage, the OFF time of the switching element is stabilized even if the operating frequency changes.

[0017]

The present invention will be described in detail below with reference to the drawings showing the embodiments thereof. FIG. 1 is a block diagram showing a main configuration of an electromagnetic cooker controlled by the inverter control method according to the present invention. The three-phase AC power supply terminals T _U , T _V , and T _W are separately connected to the AC input terminals 10 a, 10 b, and 10 c of the three-phase rectification bridge 10. A series circuit of a choke coil 11 and a smoothing capacitor 12 is provided between the positive side DC output terminal 10d and the negative side DC output terminal 10e of the three-phase rectification bridge 10. Smoothing capacitor
A series circuit including a transistor 13 and a transistor 14 is connected to 12 in parallel. A flywheel diode 15 having an anode connected to the emitter is interposed between the emitter and collector of the transistor 13.

A flywheel diode 16 having an anode connected to the emitter is interposed between the emitter and collector of the transistor 14. A series circuit of a resonance capacitor 17 and a resonance capacitor 18 is interposed between the collector of the transistor 13 and the emitter of the transistor 14. The connection intermediate point between the resonance capacitors 17 and 18 is connected to the connection intermediate point between the emitter of the transistor 13 and the collector of the transistor 14 via an induction heating coil (hereinafter referred to as a heating coil) 19. The output current of the current transformer 20 that detects the current of the heating coil 19 is input to the coil current phase determination circuit 21.

Output from the coil current phase discrimination circuit 21
, Heating coil current phase angle θ, and heating coil current phase
Determination of the current phase angle θ + π, which is the angle θ plus a half-cycle phase angle π
The different signal is input to the on-phase limiting circuit 22. On-phase control
Switching inversion signal #S output from the limit circuit 22_SWIs
Input to the voltage controlled oscillator 23. Three-phase power supply terminal T_V (T
_W) To the AC input terminal 10b (10c)
The output current of the current transformer 30 (31) is the heating output of the heating coil 6.
Heating output signal S to be determined_HControl signal circuit 24
Is input to Control signal output from the control signal circuit 24
S_CIs input to the voltage controlled oscillator 23. Voltage controlled oscillator
Reset signal S output from 23_RTurns on the phase-limited
It is input to the path 22 and the two-phase division circuit 25. Two-phase split circuit 25
Drive signal S output from_D1Is the drive circuit 26 and on
The drive signal S is input to the phase limiting circuit 22._D2Drive circuit 27
And on-phase limiting circuit 22.

FIG. 2 is a block diagram showing the configurations of the ON phase limiting circuit 22 and the coil current phase determining circuit 21. The reset signal S _R from the voltage controlled oscillator 23 is input to the input terminal B of the one-shot multivibrator 22a. The input terminal A of the one-shot multivibrator 22a is grounded, and the enable terminal CD is connected to the power supply E. A capacitor C ₀ is provided between the capacitor connecting terminal T ₁ and the capacitor connecting terminal T ₂ , and a connection midpoint between the capacitor connecting terminal T ₂ and the capacitor C ₀ is connected via a resistor R ₀ to a power source E.
Connected to The output signal of the output terminal Q is the reset terminal R of the D flip-flop 22b and the D flip-flop 22c.
Is input to the reset terminal R of. D flip-flop 22
The drive signal S _D1 from the two-phase division circuit 25 is input to the input terminal D of b.
Is input, and the inverted output signal of its inverted output terminal #Q is OR
It is input to one input terminal of the circuit 22d, and the set terminal S is grounded.

At the input terminal D of the D flip-flop 22c
Is the drive signal S from the two-phase division circuit 25. _D2Is entered and the
The inverted output signal of the inverted output terminal #Q is the other input terminal of the OR circuit 22d.
It is input to the child and the set terminal S is grounded. OR circuit 22d
From switching inversion signal #S _SWIs output. Current transformer
A resistor R is placed between the 20 secondary winding terminals.₁Is interposed and the resistance R₁
One terminal is connected to the base of transistor 21a
Side terminal has resistance R₂Grounded through the resistor R₃Through
It is connected to the DC power source E.

The collector of the transistor 21a is connected to the power supply E, the emitter is connected to the positive input terminal + of the comparator 21b, and is grounded via the resistor R ₄ . The emitter of the transistor 21a is connected to the comparator 21b via the resistor R.
Of the negative input terminal-, and the negative input terminal-is grounded via the capacitor C. An output terminal of the comparator 21b is connected to the power supply E via a resistor R _5, directly to the inverter
Connected to the input terminal of 21c. The output terminal of the inverter 21c is connected to the clock terminal CK of the D flip-flop 22b and the input terminal of the inverter 21d. The output terminal of the inverter 21d is connected to the clock terminal CK of the D flip-flop 22c.

FIG. 3 shows a voltage controlled oscillator 23 and a two-phase dividing circuit.
FIG. 25 is a block diagram showing a configuration of 25. The control signal S _C from the control signal circuit 24 is input to the positive input terminal of the comparator 23a. The negative input terminal of the comparator 23a - is grounded through a parallel circuit of a transistor 23b and the capacitor C ₁ of the common emitter is connected to the power source E through the resistor R _10. The output terminal of the comparator 23a is connected to one input terminal of the OR circuit 23c, and is connected to the power source E via the resistor R ₁₁ . The ON phase limiting circuit 22 is connected to the other input terminal of the OR circuit 23c.
The switching inversion signal #S _{SW from} is input.

The output terminal of the OR circuit 23c is connected to the set terminal S of the SR flip-flop 23d. Output terminal Q of the SR flip-flop 23d via a resistor R ₁₂ transistor 23
Connected with the base of b. The inverting output terminal #Q of the SR flip-flop 23d is connected to the input terminal of the inverter 23e via the resistor R _12, and the input terminal of the inverter 23e and the resistor R ₁₂ are connected.
_The midpoint of connection with ₁₂ is grounded via a capacitor C ₂ . The output terminal of the inverter 23e is the SR flip-flop 23.
It is connected to the reset terminal R of d.

The inverting output terminal #Q of the SR flip-flop 23d and the clock terminal CK of the D flip-flop 25a are
It is connected to each one input terminal of the AND circuit 25b and AND circuit 25c, and to the input terminal B of the one-shot multivibrator 22a of the ON phase limiting circuit 22. The output terminal Q of the D flip-flop 25a is connected to the other input terminal of the AND circuit 25b, and the inverting output terminal #Q is the input terminal D of the D flip-flop 25a.
And the other input terminal of the AND circuit 25c. The drive signal S _D1 is output from the output terminal of the AND circuit 25b. The drive signal S _D1 is input to the drive circuit 26 and the on-phase limiting circuit 22. The drive signal S _D2 is output from the output terminal of the AND circuit 25c. The drive signal S _D2 is input to the drive circuit 27 and the on-phase limiting circuit 22.

Next, the control operation of the inverter of the electromagnetic cooker configured as described above will be described with reference to FIG. 4 which shows a timing chart of signals of respective parts. Turn on transistors 13 and 14,
When a current flows through the heating coil 19 by controlling the off state, the current transformer 20 detects the current. The output current of the current transformer 20 is the resistance R ₁
And a voltage corresponding to the output current of the current transformer 20 is obtained between both terminals of the resistor R ₁ . The voltage obtained by adding the offset voltage generated by the resistor R ₂ to this voltage is input to the base of the transistor 21a. As a result, the emitter potential of the transistor 21a changes according to the output current of the current transformer 20 like the voltage V _A shown in FIG. 4 (a). Further, this voltage V _A is delayed by an RC filter composed of a resistor R and a capacitor C to generate a voltage V _B.

Then, the voltage V _A is input to the positive input terminal + of the comparator 21b and the voltage V _B is input to the negative input terminal − of which the magnitude comparison is performed. Thus, the current detected by the current transformer 20, that is, the phase angle θ and the phase angle θ + π of the inverter current are detected.
Then, at the time of the phase angle θ, the output of the comparator 21b is L
The clock terminal CK of the D flip-flop 22b rises to the H level, but if the drive signal S _D1 is still at the H level while the transistor 13 is turned on at this time, the D flip-flop 22b is inverted. Output terminal #Q is L
And the switching inversion signal #S _SW changes to the level shown in Fig. 4 (e).
As shown in (3), the switching inversion signal #S _SW is input to the set terminal S of the RS flip-flop 23d of the voltage controlled oscillator 23.

Then, the inverting output terminal #Q of the RS flip-flop 23d is slightly delayed due to the delay of the circuit operation and inverted to the L level as shown in FIG. 4 (f), and the voltage controlled oscillator 23 resets the L level. The signal S _R is output. This reset signal S _R is the AND circuit 25b, 25c of the two-phase division circuit 25.
The output of the AND circuit 25b is inverted to the L level, the drive signal S _D1 becomes the L level, and the transistor 13 is turned off before the zero cross point of the inverter current. After that, the output of the inverter 23e of the voltage controlled oscillator 23 is inverted to the H level when the quiescent time (dead time) of the predetermined time (about 1 μs) has elapsed, and the inverted output terminal #Q of the SR flip-flop 23d is again set to the H level. Then, the reset signal S _R becomes H level as shown in FIG. 4 (f). Then, the inverting output terminal #Q of the D flip-flop 25a becomes H level, the H level drive signal S _D2 is output from the AND circuit 25c, and the transistor 14 is turned on.

Further, as shown in FIG. 4 (c), the phase angle θ + π
At the time of detecting the switching inversion signal #S
_SW inverts to L level as shown in Fig. 4 (e), and the drive signal S
_D2 becomes L level, and the transistor 14 is turned off before the zero crossing point of the inverter current.

Further, during the idle period due to the reset signal S _{R in which} the inverting output terminal #Q of the SR flip-flop 23d in the voltage controlled oscillator 23 is at the L level, the output terminal Q of the one-shot multivibrator 22a1 of the ON phase limiting circuit 22 is It becomes H level as shown in FIG. 4 (g) for a period of several μs. Then, during this H-level period, both the D flip-flops 22b and 22c that have detected the phase advance of the drive signal S _D1 are reset. If reset is performed for a few μs here, if noise occurs near the phase angle θ, θ + π of the output current of the current transformer 20, which is the inverter current, it is input to the clock terminal CK of the D flip-flops 22b, 22c. It is possible to avoid an adverse effect due to the vibration of the signal that is generated.

The one-shot multivibrator 22
While the output terminal Q of a is at H level,
It should be shorter than the ON period of 13,14. in this way,
Only the current flowing through the heating coil 19 is detected, and the transistors 13 and 14 can be reliably turned off before the zero crossing point of the current flowing through the heating coil 19 based on the detected current waveform.

By the way, in the above-described inverter control method, the voltage V _B generated by the RC filter of the resistor R and the capacitor C is used to turn off the transistor before the current of the heating coil crosses the zero crossing point. Therefore, when the frequency of the inverter changes, the amplitude and phase of the voltage V _B change, and the time when the transistors 13 and 14 turn off may change. In that case, it can be solved by the following inverter control method.

FIG. 5 is a block diagram showing another configuration of the electromagnetic cooker controlled by the inverter control method according to the present invention. The three-phase AC power supply terminals T _U , T _V , and T _W are separately connected to the AC input terminals 10 a, 10 b, and 10 c of the three-phase rectification bridge 10. Positive side DC output terminal 10d of the three-phase rectifier bridge 10,
A series circuit of a choke coil 11 and a smoothing capacitor 12 is interposed between the negative side DC output terminal 10e and the negative side DC output terminal 10e. To the smoothing capacitor 12, a series circuit of a transistor 13 and a transistor 14 is connected in parallel. A flywheel diode 15 having an anode connected to the emitter is interposed between the emitter and collector of the transistor 13.

A flywheel diode 16 having an anode connected to the emitter is interposed between the emitter and collector of the transistor 14. Flywheel diode
The 15, the snubber capacitor C _S1 is connected in parallel, the flywheel diode 16, snubber capacitor C _S2 are connected in parallel. A series circuit of a resonance capacitor 17 and a resonance capacitor 18 is interposed between the collector of the transistor 13 and the emitter of the transistor 14. An intermediate connection point between the resonance capacitors 17 and 18 is connected to an intermediate connection point between the emitter of the transistor 13 and the collector of the transistor 14 via an induction heating coil (hereinafter referred to as a heating coil) 19. The secondary winding of the current transformer 20 that detects the current in the heating coil 19 is connected to the positive input terminal + of the comparator 63 and the negative input terminal of the comparator 64.

The positive input terminal + of the comparator 63 and the negative input terminal − of the comparator 64 are grounded via the resistor R ₃₀ . An output terminal of the comparator 63, the resistance through the R ₃₁ is connected to the positive input terminal + of the comparator 64, connected point between the positive input terminal of a resistor R ₃₁ and the comparator 64 is grounded through a resistor R _32. The voltage division ratio of the resistors R ₃₁ and R ₃₂ is the voltage ir to be compared with the voltage of the current i.
It is selected to have a current i corresponding to the current I of the heating coil 19 which is sufficient to complete the charging and discharging of the snubber capacitors C _S1 and C _S2 during the idle periods of 13,14.

The comparators 63 and 64 and the resistors R ₃₁ and R _{32 form a} current level detector 65.
The output terminal of the comparator 64 is connected to the input side of the level conversion circuit 66, and its output side is connected to the input side of the inverter 67
Connected to one input terminal of D circuit 68. The output side of the inverter 67 is connected to one input terminal of the AND circuit 69. The output terminals of the AND circuits 68 and 69 are separately connected to one input terminal of the OR circuit 70 and the other input terminal thereof, and the output terminal thereof is connected to one input terminal of the AND circuit 71.

The error amplifier 72, to which the output command signal ref from the output command generator (not shown) is input, is a current transformer for detecting the current flowing from the three-phase power supply terminals T _V and T _W to the AC input terminals 10b and 10c. The feedback signal fb corresponding to the current of the heating coil 19, which is the output current of 30,31, is input. The error signal err output by the error amplifier 72 is input to the voltage controlled oscillator 73, and the switching signal s output by the AND circuit 71 is input to the voltage controlled oscillator 73. The timing signal tm output from the voltage controlled oscillator 73 is input to the one-shot circuit 74 and the flip-flop 75. The output signal of the flip-flop 75 is input to the pause time generation circuit 76.

Flip-flop 75 and pause time generation circuit 76
And constitute a drive signal generator 77. The drive signal gu output from the drive signal generator 77 is the drive circuit 78 and the AN.
The drive signal gl is input to the other input terminal of the D circuit 68, and is input to the drive circuit 79 and the other input terminal of the AND circuit 69. The drive signals gu and gl output from the drive circuits 78 and 79 are the transistors 13 and 1.
Input to each of the 4 bases separately. The determination valid signal ce output from the one-shot circuit 74 is input to the other input terminal of the AND circuit 71. One-shot circuit 74, inverter 67, AN
The D circuits 68, 69, 71 and the OR circuit 70 form a level determination unit 80.

Next, FIG. 6 showing a timing chart of signals at various parts in the control operation when operating with a lag phase current by the control method of the inverter of the electromagnetic cooker configured as described above is shown in FIG.
It will be explained together. Connect a three-phase power supply (not shown) to the three-phase rectifier bridge 10 to turn on the transistors 13 and 14 alternately.
When turned off, the direction of the current flowing from the three-phase rectifying bridge 10 to the heating coil 19 is reversed depending on whether the transistors 13 and 14 are on or off. Then, the heating output is obtained from the heating coil 19.

Now, the error amplifier 72 outputs the error signal err by the output command signal ref and the feedback signal fb, and the voltage control oscillator 73 outputs the timing signal tm shown in FIG. 6 (a). The drive signal generator 77 is shown in FIG. 6 (b) in which the pause time is added based on the timing signal tm.
The drive signals gu and gl shown in (c) are output and input to the drive circuits 78 and 79. Then, the transistors 13 and 14 are driven by the output signals of the drive circuits 78 and 79.

A current I flows in the heating coil 19 by driving the transistors 13 and 14, and the current I is detected by the current transformer 20.
The output current i of 20 flows through the resistor R _30, the AC voltage generated across the resistor R ₃₀ is changed according to the output current i of the current transformer 20. The output of the comparator 63 becomes H level when the output current i is higher than the ground potential, and becomes L level when the output current i is lower than the ground potential, and outputs a voltage synchronized with the output current i as shown in FIG. 6 (d). , The voltage is divided by resistors R ₃₁ and R ₃₂ to generate a voltage ir, which is input to the positive input terminal + of the comparator 64. As a result, the comparator 64 outputs the output current i
6 and the voltage ir are compared, and the output voltage l'of the comparator 64 becomes H level while the voltage due to the current i is smaller than the voltage ir as shown in FIG. 6 (e). This signal l'is input to the level conversion circuit 66 and converted to a predetermined level, and its output signal l becomes as shown in FIG. 6 (f).

The output signal l is input to the inverter 67, and the inverted signal #l shown in FIG. 6 (g) is input to the AND circuit 69. The drive signals gu and gl shown in FIGS. 6 (b) and 6 (c) are input to the AND circuits 68 and 69, and when the logic of the AND circuits 68 and 69 is established, the output signal gu × l of the AND circuits 68 and 69 is input. , gl × #l
Changes as shown in FIGS. 6 (h) and 6 (i). These output signals gu × l and gl × # l are input to the AND circuit 71 via the OR circuit 70, while falling at the rising edge of the timing signal tm shown in FIG. The output judgment valid signal ce shown in FIG. 6 (j) is input to the AND circuit 71. As a result, when the logic of the AND circuit 71 is established, the H level switching signal s is output.

However, the logic of the AND circuit 71 is not established depending on the signals shown in FIGS. 6 (h), (i), and (j), and the switching signal s is not output as shown in FIG. 6 (k). The transistors 13 and 14 are driven by the timing signal tm and surely turn off the transistors 13 and 14 before the current flowing through the heating coil 19, that is, the output current i of the current transformer 62 reaches the zero crossing point.

In this way, the output current i of the current transformer 20 which is the current of the heating coil 19 as shown in FIG.
When the timing signal tm is delayed, the switching signal s does not rise to the H level, so that the transistor 13, in the generation cycle of the timing signal tm of the voltage controlled oscillator 73 based on the level of the error signal err. Continue the 14 ON / OFF switching operation.

Next, the control operation in the case of operating with the current of the inverter close to the same phase as the timing signal tm will be described with reference to FIG. 7 which shows a timing chart of signals of respective parts. The timing signal tm, the drive signals gu and gl, the voltage based on the current i, and the voltage ir are generated as shown in FIGS. 7 (a), 7 (b), 7 (c) and 7 (d), as described above. And the output signal l'of the comparator 64
Becomes H level while the voltage due to the current i is smaller than the voltage ir as shown in FIG. 7 (e). This output signal l'is input to the level conversion circuit 66, and its output signal l is shown in FIG.
It is converted to a predetermined level as shown in (f). Further, the output signal l is inverted by the inverter 67, and the output signal #l of the inverter 67 is inverted as shown in FIG. 7 (h).

The output signal gu × l of the AND circuit 68, to which the output signal 1 and the drive signal gu shown in FIG. 7B are input, is shown in FIG.
It becomes H level as shown in (h). Further, the output signal gl × # l of the AND circuit 68 to which the output signal #l and the drive signal gl shown in FIG. 7 (c) are input is as shown in FIG. It becomes H level as shown in i).

These output signals gu × l and gl × # l are input to one input terminal of the AND circuit 71 via the OR circuit 70, while rising at the rising edge of the timing signal tm shown in FIG. 7 (a). The determination valid signal ce shown in FIG. 7 (j) output from the down-shot and one-shot circuit 74 is input to the other input terminal of the AND circuit 71. As a result, the output signal from the AND circuit 71 is gu × l or gl ×
The switching signal s shown in FIG. 7 (k) is output which rises at each rising edge of #l and falls at the falling edge of the judgment valid signal ce. Then, the switching signal s is input to the voltage controlled oscillator 73, and the voltage controlled oscillator 73 generates the timing signal tm. That is, the operating frequency does not drop any more regardless of the level of the error signal err. Therefore, good heating control can be performed without further reducing the phase delay of the current flowing through the heating coil 19.

FIG. 8 is a block diagram showing another structure of the electromagnetic cooker controlled by the control method of the inverter according to the present invention. The output signal of the comparator 63 of the current level detector 65 is input to the level conversion circuit 90. Level conversion circuit 90
The output signal id of is input to one input terminal of the EXOR circuit 91.
The output signal 1 of the level conversion circuit 66 is input to the other input terminal of the EXOR circuit 91. The output signal Id + 1 of the EXOR circuit 91 is input to the clock terminal CK of the RS flip-flop 92.

The input terminal D of the RS flip-flop 92 is connected to the power source E, and the set terminal S is grounded. The reset terminal R has a timing signal output from the voltage controlled oscillator 73.
tm is input. The determination valid signal ce output from the output terminal Q is input to the other input terminal of the AND circuit 71. These level conversion circuit 90, EXOR circuit 91 and RS flip-flop
The determination valid signal generator 100 is composed of 92. The configuration other than the determination valid signal generation unit 100 is the same as the configuration other than the one-shot circuit 74 in FIG. 5, and the same components are designated by the same reference numerals.

Next, the control operation of the inverter in this electromagnetic cooker is shown in FIG.
It will be explained together. 9 (a), (b), (c), (d), (e), (f), (g),
Timing signal tm, drive signal gu, drive signal g shown in (h)
The voltage l and the voltage ir by the current i, the output signal l, the output signal #l, the output signal gr × l, and the output signal gl × # l are output as in the case of FIG. The RS flip-flop 92 in the judgment valid signal generator 100 is reset each time the timing signal tm shown in FIG. 9 (a) from the voltage controlled oscillator 73 is input to the reset terminal R and output from the output terminal Q. The determination valid signal ce becomes L level.

The output terminal Q is inverted to the H level at the time when the clock terminal CK rises, but the clock terminal CK shows the polarity of the voltage by the current i shown in FIG. 9 (i). The level-converted signal id shown in Fig. 9 (i)
9 is a comparison result of the voltage and the voltage ir by the current i.
Output signal of EXOR circuit 91 to which signal l shown in (e) is input
Id + 1 is input, and the output signal Id + l falls when the voltage due to the current i becomes smaller than the voltage ir as shown in FIG. 9 (j), and rises when it becomes larger.

That is, the output terminal Q of the RS flip-flop 92
Becomes H level when the current i exceeds the current ir as shown in FIG. 9 (i). As a result, the judgment valid signal ce is
As shown in FIG. 9 (k), the period from the time when the voltage due to the current i becomes equal to or higher than the voltage ir until the operation of the transistor is switched to H level.

Therefore, when the current of the heating coil 19 is operating in a state close to the same phase as the timing signal tm, the voltage controlled oscillator 73 operates according to the switching signal s shown in FIG. 9 (l).
The timing signal tm shown in (a) will be generated.
Therefore, regardless of the level of the error signal err, the operating frequency does not drop further. Therefore, the phase delay of the current flowing through the heating coil 19 does not decrease further, and good heating control can be achieved.

Although not shown, when the inverter is operating with a lagging current as compared with the timing signal, the switching signal s does not become H level, as described above, so that the voltage based on the level of the error signal err. At the generation cycle of the timing signal tm of the control oscillator 73, the transistors 13 and 14 are turned on,
Continue off operation.

In these control operations, the current level of the load is directly detected and the transistor is turned on / off based on the current level. Therefore, the load current is kept below a predetermined value regardless of the operating frequency of the inverter. If it happens, the operation of the transistor can be reliably switched. Therefore, the snubber capacitor in the charged state is not short-circuited by the transistor, and the loss in the transistor can be suppressed to the minimum. In this embodiment, the case where the inverter control method is applied to the electromagnetic cooker has been described, but the same effect can be obtained by being applied to an inverter other than the electromagnetic cooker.

[0056]

As described above in detail, according to the inverter control method of the first invention, it is not necessary to detect only the inverter current and the high voltage of the inverter.
The inverter control circuit can be realized at low cost.

According to the inverter control method of the second invention or the third invention, after the current of the inverter is detected and the switching element is turned on for a predetermined time,
Since the operation of the switching element is switched when the detected current falls below a predetermined value or after the current of the inverter exceeds a predetermined value and then falls below the predetermined value, the inverter control circuit is inexpensive. Can be realized. Further, since the filter composed of the resistor and the capacitor is not used to determine the turning-off time of the switching element, it is possible to stabilize the turning-off time of the switching element even if the operating frequency of the inverter changes. Produce the effect.

[Brief description of drawings]

FIG. 1 is a block diagram showing a main configuration of an electromagnetic cooker controlled by an inverter control method according to the present invention.

FIG. 2 is a block diagram showing configurations of an on-phase limiting circuit and a coil current phase determining circuit.

FIG. 3 is a block diagram showing configurations of a voltage controlled oscillator and a two-phase division circuit.

FIG. 4 is a timing chart of signals of respective parts.

FIG. 5 is a block diagram showing another configuration of the electromagnetic cooker controlled by the inverter control method according to the present invention.

FIG. 6 is a timing chart of signals of respective parts.

FIG. 7 is a timing chart of signals of respective parts.

FIG. 8 is a block diagram showing another configuration of the electromagnetic cooker controlled by the inverter control method according to the present invention.

FIG. 9 is a timing chart of signals of respective parts.

FIG. 10 is a schematic configuration diagram of an inverter.

FIG. 11 is a current waveform diagram of the inverter.

FIG. 12 is an explanatory diagram of timing for turning off a transistor.

[Explanation of code] 13,14 Transistor 17,18 Resonant capacitor 19 Induction heating coil 20 Current transformer 21 Coil current phase discrimination circuit 21b Comparator 22 On phase limiting circuit 22a One shot multivibrator 22b, 22c D flip-flop 23 Voltage controlled oscillator 25 Two-phase split circuit R resistor C capacitor

Claims (3)

[Claims] 1. A method for controlling an inverter, which commutates a switching element for turning on and off a current of a resonance circuit composed of a coil and a resonance capacitor connected in series with the coil, comprising: a current detection for detecting the current of the coil. Comparing the first voltage and the second voltage with a voltage generation unit that generates a first voltage related to the detection current of the current detection unit and a second voltage that is a delay of the first voltage. And a control unit for turning off the switching element according to a comparison result of the comparison unit. 2. A method of controlling an inverter, which commutates a switching element for turning on and off a current of a resonance circuit composed of a coil and a capacitor connected in series to the coil, comprising: a current detecting section for detecting the current of the coil. The method for controlling an inverter, comprising: turning on the switching element, when the current detected by the current detection unit becomes equal to or less than a predetermined value after a lapse of a predetermined time after the switching element is turned on. 3. A method of controlling an inverter, which commutates a switching element for turning on / off a current of a resonance circuit composed of a coil and a capacitor connected in series to the coil, comprising: a current detecting section for detecting the current of the coil. A control method for an inverter, comprising: a switching element which is turned on when the current detected by the switching element being turned on reaches a predetermined value or more and then becomes lower than a predetermined value.