JPS6132794B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a pulse width modulation oscillator for controlling the output power of a static power conversion circuit of an induction heating cooker.

Structure of a conventional example and its problems FIG. 1A shows a conventional example of an induction heating cooker. In FIG. 1A, 1 is an oscillator, 2 is a voltage source of voltage V _r , 3 is a variable voltage source whose minimum output voltage is limited by the voltage source, and 4 is the output a of the oscillator 1 and the variable voltage source. The comparator connected to the output b of 3 constitutes a pulse width modulation oscillator. 5 is a switching element controlled by the output c of the comparator 4, 6 is a diode, 7 is an induction heating coil, 8 is a resonant capacitor, and 9 is a DC power supply, which constitutes a static power conversion circuit P. FIG. 1B is a waveform diagram showing the operation of FIG. 1A. In FIG. 1B, a, b, and c are the same signals as a, b, and c in FIG. 1A, respectively. V _L is the minimum value of the output voltage of the oscillator 1, and T is the H level pulse width of the output c of the comparator 4. As is clear from FIG. 1B, the pulse width T is controlled by the output b of the variable voltage source 3. As can be easily seen, the input power to the heating coil 7, ie the output power of the static power conversion circuit P, is an increasing function of the pulse width T. Therefore, the minimum value of the heating power of the induction heating cooker shown in FIG. 1A is determined by the minimum value of the pulse width T, that is, the minimum pulse width T _n . The minimum pulse width T _n is the pulse width T when the output voltage b is the minimum, that is, when b = V _r ,
There is a relationship between the minimum pulse width T _n and the minimum output voltage V _r as shown in FIG. 1C. Figure 1C
, the horizontal axis is the minimum voltage V _r , the vertical axis is the minimum pulse width T _n , and a solid line 10 shows the relationship between T _n and V _r . As can be seen from Figure 1C, V _r ≦V _L
In region I, T _n =0. This means that in the region I, the static power conversion circuit P stops. In general, in an induction heating cooker, it is desired that the minimum pulse width T _n be as small as possible in order to expand the heating power variable range and improve the heating performance. In order for P not to stop, T _n >0 is required. However, in this configuration, the variation in the oscillator output voltage minimum value V _L and the variable voltage source minimum output voltage V _r can be reduced to ±(Δ/100).
%, as shown in FIG. 2, in order to make T _n > 0, the design value of V _r is V _r0 , and the design value of V _L is _{V r0
0} > V _L0 +Δ(V _r0 +V _L0 ). Therefore, the minimum pulse width T _n
cannot be set below the value T _n0 for V _r0 = V _L0 −Δ(V _r0 + V _L0 ), and the minimum pulse width T
Under the worst condition of variation, _n is V _r = V _L0 + Δ(2V _r0
+V _L0 ), and the problem was that the value T _n ' for V _L =V _L0 -ΔV _L0 .

OBJECTS OF THE INVENTION The present invention solves these conventional problems and provides an induction heating cooker that expands the heating power variable range and improves the heating performance.

Structure of the Invention The induction heating cooker of the present invention has two
The oscillator has an oscillation output voltage based on one voltage of the reference power source, an oscillator, a variable voltage source, a comparator, and a static power conversion circuit. and the variable voltage source is limited by another voltage of the reference power supply. With this configuration, the minimum pulse width T _n of the comparator output is T _n > regardless of variations in the two voltage sources that constitute the reference power supply.
0 can be guaranteed, T _n can be set as small as desired, and the minimum heating power of the induction heating cooker can be set as small as desired.

DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 3A is an embodiment of the present invention. In FIG. 3A, E ₁ and E ₂ are voltage sources of voltages V _L and V _d, respectively, and are connected in series to form a reference power source that provides two reference voltages V _L and V _L +V _d . 11 is an oscillator whose minimum value of oscillation output voltage is limited by the reference voltage V _L ; 3 is the reference voltage V L;
variable voltage source with minimum output voltage limited at _L + V _d ;
A comparator 4 is connected to the output a of the oscillator 11 and the output b of the variable voltage source 3, and together with the reference power source constitutes a pulse width modulation oscillator. 5 is a switching element controlled by the output c of the comparator 4, 6 is a diode, 7 is an induction heating coil, 8 is a resonant capacitor, and 9 is a DC power supply, which constitutes a static power conversion circuit P. Figure 3B
3A is a waveform diagram showing the operation of FIG. 3A. Figure 3B
, a, b, and c are a, b, and c in Fig. 3 A, respectively.
This is the same signal as b and c. T indicates the H level pulse width of the output c of the comparator 4. FIG. 3C shows the minimum value T _n of the pulse width T and the voltage source
FIG. 2 is a characteristic diagram showing the relationship between voltage V _d of E ₂ . Third
As is clear from Figure C, if V _d >0, then T _n >0. This relationship holds true regardless of the voltage V _L of the voltage source E ₁ . Therefore, the variation in V _d can be reduced to ±(Δ/
100)% and Δ<1, the design value of V _d is V _d0
can be set as small as desired while guaranteeing T _n >0. That is, the design value T _n0 of T _n can be set as small as desired. FIG. 4 shows a specific circuit example of the pulse width modulation oscillator of the embodiment shown in FIG. 3A. Fourth
In the figure, 3, 4, 11, E ₁ , E ₂ are in Figure 3A
The functional parts a, b, and c with the same symbols are the signals with the same symbols in FIG. 3A. 12 is a constant current source, Q ₁ is PNP
The transistor _Q2 is an NPN transistor and constitutes a voltage limiter. 13 is a resistor, and 14 is a variable voltage source, which together with the voltage limiter constitute the variable voltage source 3 whose minimum output voltage is limited by the voltage V _d +V _L . The voltage limiter L is composed of a constant current source 15, a PNP transistor _Q3 , and an NPN transistor _Q4 . A Schmitt circuit S is composed of resistors 16, 17, and 18, a transistor _Q5 , a resistor 20, and a comparator 21. Said resistor 1
The connection points 6, 17, and 18 are connected to the voltage limiter L. Reference numeral 22 denotes a current source that becomes a constant current source with a current value I ₂ when the output of the comparator 21 is at the H level, and becomes an open state when the output of the comparator 21 is at the H level; and 23, a current source that has the current value I ₁
The constant current source 24 is a capacitor with a capacity c which determines the time constant of the oscillator 11 together with the current source 22 and the constant current source 23. The lower threshold of the Schmitt circuit S is set to V _L by the voltage limiter L. Therefore, the minimum value of the oscillation output voltage a is V _L
limited to. The minimum pulse width T _n of the output of the pulse width modulation oscillation circuit shown in FIG. 4 is expressed as follows.

T _n =I ₂ C/I ₁ (I ₂ - I ₁ ) V _d (1) In Fig. 4, transistor Q ₄ and resistor 16,
When the connection points 17 and 18 are disconnected, the Schmitt circuit L is no longer subject to voltage limitation by the external voltage source _E1 , and the threshold value of the Schmitt circuit L is determined by the resistors 16, 17, and 18.
The resistance values of resistors 16, 17, and 18 are set so that the lower value of the threshold value is equal to V _L , and the voltage of the voltage source E ₂ is set to V _d +V _L and the voltage of the voltage source E ₁ is set to When set to zero, the configuration becomes the same as the conventional example shown in FIG.
The minimum values of the oscillation output voltages are V _d +V _L and V _L , respectively, which are the same as those of the variable voltage source 3 and oscillator 1 in FIG. 4. The minimum pulse width T〓 _n is expressed as follows.

_T〓n = _I2C / _I1 ( _I2 - _I1 )( _Vr - _VL )...
...(2) Obviously, T _n in equation (1) and T〓 _n in equation (2) are the same value, but the variation in V _d , V _L , and V _r is calculated by ±(Δ/100).
%, the variation δT _n of T _n and the variation δT – _n of T〓 _n can be expressed as follows.

δT _n /T _n =Δ……(3) δT〓 _n /T〓 _n = Δ(V _r +V _L )/(V _r −V _L )=Δ(1+2V _L /V _r −V _L )…… …(4) From equations (3) and (4), it is clear that δT _n /T _n <δT〓
It can be seen that _n /T〓 _n . That is, the variation in the minimum pulse width of the output of the pulse width modulation oscillation circuit shown in FIG. 4 is smaller than the variation in the minimum pulse width of the output of the conventional pulse width modulation oscillation circuit shown in FIG. 1A. FIG. 5 is a characteristic diagram showing this effect, where the vertical axis is the rate of variation in the minimum pulse width, the horizontal axis is the set value of the minimum pulse width, and the solid line 25 is the fourth
The characteristics of the pulse width modulation oscillation circuit shown in the figure, the solid line 26, are the characteristics of the conventional pulse width modulation oscillation circuit shown at A in FIG. A significant effect can be seen when the minimum pulse width setting is small.

FIG. 6 shows another specific circuit example of the pulse width modulation oscillation circuit of the induction heating cooker shown in FIG. 3A. In FIG. 6, the same functional parts and the same parts as in FIG. 4 are given the same symbols. 27 and 28 are resistors, which together with the capacitor 24 determine the time constant of the oscillation circuit 11. 19 is a resistor, and _Q6 is a transistor.

Effects of the Invention As is clear from the above, the induction heating cooker of the present invention is particularly capable of converting the output voltage of the variable voltage source of the pulse width modulation oscillation circuit and the oscillation output voltage of the oscillation circuit into two DC voltages connected in series. By limiting the reference power source configured with a reference power source, the minimum output pulse width of the pulse width modulation oscillation circuit can be set as small as possible, and the accuracy of the minimum output pulse width can be increased. This makes it possible to achieve a gentler heat and improve the accuracy of the heat.

[Brief explanation of the drawing]

Fig. 1A is a block diagram showing the configuration of a conventional induction heating cooker, Fig. 1B is a waveform diagram showing the operation of the conventional example of Fig. 1A, Fig. 1C, and Fig. 2 are Fig. 1A. FIG. 3A is a block diagram showing an embodiment of the present invention, FIG. 3B is a waveform diagram showing the operation of the embodiment of FIG. 3A, and FIG. Figure 3 is a characteristic diagram showing the characteristics of the embodiment shown in A, Figure 4 is a circuit diagram showing a specific example of the main part of the embodiment shown in Figure 3 A, and Figure 5 is a characteristic diagram showing the effects of the specific example shown in Figure 4. Figure 6 is the third
FIG. 3 is a circuit diagram showing another specific example of the main part of the embodiment shown in FIG. A; E ₁ , E ₂ ... DC voltage source, 3 ... Variable voltage source,
4... Comparator, 11... Oscillator, P... Static power conversion circuit, L... Voltage limiter, S... Schmitt circuit.

Claims (1)

[Claims] 1. A reference power supply that provides two types of reference voltages, which is composed of two DC voltage sources connected in series, and one reference voltage of this reference power supply that is set to the maximum or minimum value of the oscillation output voltage. a variable voltage source whose output voltage is limited by another reference voltage of the reference power source; a pulse width modulation oscillation circuit including a comparator connected to the oscillator and the variable voltage source; An induction cooker with a static power conversion circuit connected to a modulated oscillator circuit. 2. The induction heating cooker according to claim 1, wherein the oscillator is comprised of a voltage limiting circuit and a Schmitt circuit.