Conventionally, a typical driving system of inverter is a Royer's circuit (for example, see the patent literature 1). FIG. 8 shows an example of the basic circuit of Royer's circuit.
In FIG. 8, a transformer 1 has two primary windings L1, L2 and a single secondary winding L3, and a negative electrode of DC power source 2 is connected to a mutually connected end of the primary windings L1, L2, and a positive electrode of DC power source 2 is connected to the other end of the primary winding L1 via a switch SW1 while being connected to the other end of the primary winding L2 via a switch SW2.
As shown in FIG. 9(a), the switch SW1 and the switch SW2, both of which are initially turned off, are alternately turned on and off during the time T1, T2, allowing to alternately flow positive and negative input current in the primary windings L1, L2 of transformer 1, whereby an output signal as shown in FIG. 9(b) can be obtained in the secondary winding L3 of transformer 1.
According to the driving system of this Royer's circuit, even if the input current may instantaneously become zero during operation, the operation of circuit is not completely suspended.
Further, another typical driving system of inverter is PWM (Pulse Width Modulation) (see FIGS. 10(a), 10(b)). This is a system in which the pulse widths T1, T2 of input current in the cycle T of on-and-off operations of switches SW1, SW2 is adjusted to allow the intermittent driving of circuit, whereby the secondary output voltage is controlled.
That is, the pulse width of the secondary output voltage is changed by adjusting T1 and T2 which are on-time of the switches SW1 and SW2 in FIG. 8, thereby adjusting an effective output voltage. Thus, if the input current of transformer 1 is within the range of increasing in proportion to the time T1, T2 and the magnetic field of transformer 1 is within the range of saturation, the longer the time T1, T2 (that is, the higher the duty), the greater the effective value of output voltage becomes.
Hereby, PWM is a system in which input current is intermittently driven for the purpose of “control of output voltage”.