The present invention relates to the improvement of a DC-DC converter which converts the primary direct current (DC) voltage to the secondary DC voltage through the switching of the primary DC voltage by switching transistors, the voltage conversion by a transformer, and the rectification of the secondary voltage of the transformer.
FIG. 1A shows a prior DC-DC converter having a single transistor, and FIG. 1B shows the operational waveform in that DC-DC converter. In FIG. 1A, the reference numeral 1 is a transformer for converting an alternate voltage, 2 is a DC power source or a battery, 3 is a rectifier/smoother circuit, having the pair of diodes D.sub.1 and D.sub.2, the inductance L and the capacitance D.sub.1. Also, 4 is a load, and Tr is a transistor for the switching of the current in the primary winding of the transformer. The primary winding 1.sub.1 of the transformer 1 is connected to the power source 2 through the collector-emitter path of the transistor Tr. The output voltage of the DC power source 2 is supposed to be V volts. The secondary winding 1.sub.2 of the transformer 1 is connected to the load through the rectifier/smoother circuit 3. The transistor TR repeats the ON status and the OFF status according to the pulse signal applied to the base electrode (B) of the transistor Tr, and then, the transformer 1 is excited substantially by the alternate current (AC) voltage. During the time t.sub. 1 when the transistor Tr is in ON status, the load current flows through the diode D.sub.1 in the rectifier/smoother circuit 3, and during the time t.sub.2 when the transistor Tr is in OFF status, the load current flows through the diode D.sub.2 in the rectifier/smoother circuit 3. Thus, the continuous direct current is supplied to the load 4.
In FIG. 1A, the voltage V.sub.ce between the collector and the emitter of the transistor Tr is almost zero when the transistor Tr is ON, and said voltage V.sub.ce is V+V.sub.R, which is the sum of the voltage (V) of the power source 2 and the flyback voltage V.sub.R induced in the primary winding of the transformer 1 when the transistor Tr is OFF. Therefore, the waveform of the voltage V.sub.ce is shown in FIG. 1B, and it should be noted that the maximum value of the voltage V.sub.ce is V+V.sub.R. Therefore, the withstand voltage between the collector and the emitter of the transistor Tr must be considerably high and must be higher than V+V.sub.R. And said withstand voltage must be extremely high when the input voltage V is obtained by rectifying the commercial alternate current power source.
However, a transistor with a high withstand voltage has generally the disadvantage that the switching speed is rather slow. The recent DC-DC converter operates in high speed for providing a small size of an apparatus and the high efficiency, that slow switching speed of a transistor with a high withstand voltage is a serious disadvantage.
Another prior DC-DC converter is a full-bridge type DC-DC converter which has a transformer and two pairs of switching transistors connected to the transformer. Each pair of switching transistors are alternately turned ON and OFF, then, the bipolar current is provided in the primary winding of the transformer from the DC power source, and the secondary voltage induced on the secondary winding of the transformer is rectified by the rectifier/smoother circuit and is applied to the load. Accordingly, the both terminals of the primary winding of the transformer are subject to be disconnected at the same time from the primary power source by the switching transistors, and it is the disadvantage that the potential of the primary winding is floating and is unstable when the primary winding is disconnected from the power source. That is to say, although the switching transistors disconnects the transformer from the primary power source, the transformer is still connected to the primary power source through a shield member surrounding the transformer, or the capacitance between the transformer and the ground. If the variance of the stray or the change of the stray capacitance et al changes, the potential of the transformer changes. If the change of the potential of the transformer is large, the flyback voltage generated when the switching transistors are in OFF status is superposed on the power source line through the diodes which are connected parallel to the switching transistors for protecting the same from the high flyback voltage. Further, the switching transistors are subject to be supplied the voltage higher than the allowable withstand voltage of the transistors, due to said change of the potential of the transformer.