i1=i2=(id+is)/2 …(1)
i3=i4=(id-id)/2 …(2)
Therefore the continuity of each switching current (i1-i4) is established by connecting Ld and Ls because each value is derived by id and is. This phenomenon is used for an overlapping commutation ZCS and its ZCS achieves the soft switching without auxiliary switch and circuit. Ld of input DC reactor is not connected in fact, because the stray inductance of DC branch is used as Ld. In consequence the conducting loss and the maximum voltage of switch are suppressed.
Accordingly the proposed inverter suppress the switching stress, such as the maximum current of switch and the maximum voltage of switch. Moreover a simple circuitry is realized, in order that the soft switching is achieved without adding auxiliary switches and auxiliary circuit and Ld is not connected.
Fig. 6 Effects on Vsmax according to changing p
Fig. 7 Effects on Ismax according to changing p
4. CIRCUIT CHARACTERISTICS
4. 1 Method of numerical analysis
The values of inductance and resistance are changed according to temperature rise of induction heating load. And the circuit parameters are changed. Therefore the circuit characteristics are analyzed in order to drive the proposed inverter by optimum parameters. In the circuit analysis of the proposed inverter, the normalized parameters as described in Table. 1 are provided in order to generalize and to give flexibility in the circuit design. The numerical analysis of each modes is analyzed from transient state to steady state by 4th order Runge-Kutta method.
4.2 Effects of reactor ratio β and capacitor ratio p
As mentioned above, the circuit characteristics is changed by temperature rise of induction heating load. Especially the values of inductance and resistance of induction heating load are remarkably changed in the neighbourhood of Curie temperature. Consequently, the circuit parameters, such as the reactor ratio of α and β, the normalized frequency of μ and the normalized resistance of λ, are changed while driving PM reduction system. Moreover the circuit characteristics, such as the ZCS operation region switching stress and output power, are much affected depending on circuit parameters. Therefore the reactor ratio of β and capacitor ratio of p are investigated for the circuit characteristics. Particularly, the parameters which ensure the wide ZCS operation region and suppress the switching stress such as the maximum voltage of switch of Vsmax and the maximum current of switch of Ismax are examined.
From previous analysis(3), it is already clarified that the circuit characteristics are hardly influenced in the case of the value of a less than 0.5 in the proposed inverter. In this paper, in order that the stray inductance of DC branch is utilized to the input DC reactor of Ld and is very small the circuit characteristics are analyzed at α=0.05 as constant.
Fig. 6 and Fig. 7 show the effects on Vsmax and Ismax acconding to changing β and p, respectively . In these figures, the values of the normalized frequency of μ and the normalized resistance of λ are fixed, and output power is constant as 2[kW] by varying input voltage. In Fig. 6 and Fig. 7, both of Vsmax and Ismax are suppressed according to enlarging β. And from Fig. 6 and Fig. 7, it is indicated that Ismax is increased, although Vsmax is suppressed when p is enlarged In order to enlarge p, Cp as shown in Fig .3 is enlarged. Therefore this phenomenon is caused, in order that current of load flows through Cp and bypasses the load.
Through the previous experiment, it is already known that the value of Lo tend to increase until temperature of induction heating load becomes Curie temperature, and that their values are instantly decreased at Curie temperature and are stable after exceeding Curie temperature. Therefore in the circuit design, it is necessary to give consideration to changing β.
