The present invention relates generally to a power circuit. More particularly, the invention relates to a forward type power circuit which converts a direct current power into an alternating current power by switching operation and further converts the alternating current power into a stable direct current power.
As such type of switching power circuits, there are single transistor forward type and double transistor forward type power circuits. The former circuit is illustrated in FIG. 6. An alternating current power supply e.sub.1 is converted into a direct current through a full-wave rectification by a rectifier RC.sub.1, and then smoothed by a capacitor C.sub.1. The smoothed direct current output is supplied to a series circuit of a primary winding N.sub.1 of a transformer TR.sub.1 and a switching MOS transistor Q.sub.1. By controlling switching of this transistor Q.sub.1, the direct current power is inverted into the alternating current power. Thereafter, the alternating current power is again converted into the direct current power through a rectification circuit constituted of diodes D.sub.3 and D.sub.4. This direct current power is smoothed and stabilized by a choke coil L.sub.1 and a capacitor C.sub.3 and stabilized by a PWM control circuit (not shown), and thus becomes a desired direct current voltage to be derived between both ends 1-2 of the capacitor C.sub.3.
A tertiary winding N.sub.3 is provided in the transformer TR.sub.1. By connecting the tertiary winding N.sub.3 to a diode D.sub.1, an exciting energy (shown as an exciting current i.sub.3) induced in the transformer TR.sub.1 upon turning OFF of the transistor Q.sub.1 is regenerated toward the rectifier RC.sub.1 through the diode D.sub.1 for protecting the switching transistor Q.sub.1 from being damaged.
FIG. 7 shows a operational waveform of the power circuit of FIG 6, in which are shown the alternating current input voltage e.sub.1, a voltage VC.sub.1 between both ends of the capacitor C.sub.1, and an input direct current i.sub.1, respectively.
Since it is required to minimize a ripple component of a full-wave rectified voltage waveform shown by the broken line of (b) of FIG. 7, and to compensate temporary drop of the alternating current voltage e.sub.1, the capacitor C.sub.1 is desired to have sufficiently large capacity. As a result, the voltage VC.sub.1 at both ends of the capacitor C.sub.1 becomes a direct current at a level in the vicinity of the peak of the full-wave rectified waveform, as shown in FIG. 7(b). However, the output current i.sub.1 of the rectifier RC.sub.1 has a waveform with substantially small duty cycle (T.sub.1 /T=D), and, on the other hand, with substantially high peak value, as shown in FIG. 7(c).
FIG. 8 shows a double transistor forward type power circuit, in while like elements to those in FIG. 6 are identified by the like reference numerals. In the shown circuit, the switching MOS transistor Q.sub.1, the primary winding N.sub.1 and a switching MOS transistor Q.sub.2 are connected in series in this order between both ends of the capacitor C.sub.1. A diode D.sub.5 is inserted between a Junction between the transistor Q.sub.1 and the primary winding N.sub.1 and a negative voltage terminal of the rectifier RC.sub.1. Also, a diode D.sub.6 is inserted between a Junction between the transistor Q.sub.2 and the primary winding N.sub.1 and a positive voltage terminal of the rectifier RC.sub.1.
An exciting energy (shown as the exciting current i.sub.3) induced in the primary winding N.sub.1 while both transistors Q.sub.1 and Q.sub.2 are OFF, is regenerated toward the rectifier RC.sub.1 by a loop formed by the diode D.sub.5, the winding N.sub.1 and the diode D.sub.6, the switching transistors Q.sub.1 and Q.sub.2 are protected from being damaged.
With this circuit, the desired direct current voltage can be attained between the terminals 1-2 by simultaneously turning both transistors Q.sub.1 and Q.sub.2. The signal waveforms at respective part during operation of this circuit are identical to those in FIG. 7. Therefore, the output current i.sub.1 of the rectifier RC.sub.1 has substantially small duty cycle and substantially high peak value.
As set forth above, in the conventional forward type power circuit, for minimizing the ripple component of the direct current voltage after rectification and smoothing and for assuring a stable operation of a DC--DC converter at the later stage by compensating the temporary drop of the alternating current voltage, it becomes necessary to apply the alternating current having substantially small duty cycle and substantially large peak value. As a result, high level high harmonic current may flow through a power distribution system to affect by heating of the powering capacitor or a power transistor. Also, a power factor (a ratio of an effective power versus a null power) of the power circuit can be lowered.