FIG. 13 shows a conventional power conversion device similar to the switching power supply device described in Japanese Patent No. 3387456 (see paragraphs 24–37 and FIGS. 1–2 thereof). This circuit incorporates a “flyback converter,” where a main switching element 2 and a sub-switching element 3 are repeatedly turned on and off alternately. The excitation energy accumulates in an insulating transformer 4 when the main switching element 2 is in the on-state and discharges when the main switching element 2 is in the off-state to supply DC power to a load.
A DC power supply 1 is obtained by subjecting the AC power supply voltage to smoothing rectification. The main switching element 2 and the sub-switching element 3 can be MOSFETs or the like. The insulating transformer 4 has coils 4a–4e. The elements 5a and 5b are diodes, and elements 6 and 10 are capacitors. A main control circuit 30 controls the main switching element 2 to turn on and off, to keep the value of an input output voltage of the device (voltage of the capacitor 6) constant. A sub-control circuit 40 controls the sub-switching element 3 to turn on and off using opposite logic of the control of the main switching element 2. The sub-control circuit 40 comprises the coil 4e of the transformer 4, capacitors 41, 45, resistors 42, 44, a bead 43 as a type of inductor, and a transistor 46. Each end of the capacitors 41, 45 is connected to both ends of the coil 4e. 
This conventional technology operates as follows. During the start-up process, a voltage is applied to the gate of the main switching element 2 via a resistor (not shown) for star-up inside the main control circuit 30 to turn on the main switching element 2. Turning on the main switching element 2 generates a voltage of the same polarity in the coils 4a and 4d of the transformer, and the excitation energy is accumulated within the coil 4a. At the same time, the voltage generated in the coil 4c is subjected to smoothing rectification by the diode 5b and the smoothing capacitor 6, and then supplied to a load. When the main control circuit 30 turns off the main switching element 2, the excitation energy accumulated within the coil 4a is discharged as electrical energy via the coil 4b, and is subjected to smoothing rectification by the diode 5a and the smoothing capacitor 6, and then supplied to a load. When the whole exciting energy accumulated within the coil 4a is discharged via the coil 4b, a voltage of the same polarity as the voltage generated during the start-up process is generated in the coil 4d, and the main switching element 2 is turned off by the main control circuit 30. In this manner, the electrical energy is supplied to a load in accordance with the on-off operation of the main switching element 2.
The operation of the sub-switching element 3 follows. As the main switching element 2 is turned off, a voltage of a polarity opposite to that generated when the main switching element 2 is in the on-state is generated in the coil 4e within the sub-control circuit 40. This voltage is applied to the gate of the sub-switching element 3 via the capacitor 41, the resistor 42, and the bead 43 to turn on the sub-switching element 3. Accordingly, the capacitor 10 absorbs the energy accumulated in leakage inductance of the transformer 4, and prevents a surge voltage from applying to the main switching element 2. Furthermore, the leakage inductance of the transformer 4 and the capacitor 10 resonate serially, discharging the energy absorbed by the capacitor 10 to the load via the transformer 4, the diode 5a, and the smoothing capacitor 6.
The voltage generated in the coil 4e is applied to a series circuit of the resistor 44 and the capacitor 45 configuring a time-constant circuit, and the capacitor 45 is charged. When the voltage of the capacitor 45 reaches a threshold voltage of the transistor 46, the transistor 46 is turned on. Upon turning on the transistor 46, there is no longer the difference in potential between the gate and the source of the sub-switching element 3, the voltage applied to the gate of the sub-switching element 3 disappears from the coil 4e, and the sub-switching element 3 is turned on rapidly. Here, the time between when the voltage is generated in the coil 4e and when the voltage of the capacitor 45 reaches the threshold voltage of the transistor 46 is the time constant of the time-constant circuit made up of the resistor 44 and the capacitor 45. FIG. 14 shows a frame format of a waveform of current passing through the diode 5a in the conventional technology shown in FIG. 13.
FIG. 15 shows another conventional technology, similar to the switching power supply device described in Japanese Patent Application Laid-Open No. H11-285248 (see paragraphs 34–47 and FIGS. 1 and 3 thereof).
In FIG. 15, the transformer 4′ has only coils 4a, 4b, and 4e. The sub-control circuit 60 comprises the coil 4e, resistors 61, 63, a capacitor 62, a diode 64, a transistor 65, and a Zener diode 66. A cathode of the Zener diode 66 is connected to the gate of the sub-switching element 3. Moreover, it includes a diode bridge 50 for obtaining a DC power supply from an AC power supply via a smoothing capacitor 11. It should be noted that the control circuit (main control circuit) of the main switching element 2 is omitted from the figure.
The operation of this conventional technology is described with reference to FIG. 16. In FIG. 16, ID2 is the drain current of main switching element 2, VDS2 is drain-source voltage of the main switching element 2. Va is voltage of the coil 4e of the transformer 4′, and its polarity is such that the positive electrode is in the direction in which voltage is generated from the beginning of winding to the end of winding. VGS3 is the gate-source voltage of the sub-switching element 3, and ID3 is the drain current of the sub-switching element 3. ID0 is the current of the diode 5a. Im shown in dashed lines is the excitation current of the transformer 4′.
First, the period t=t1−t2 in FIG. 16 is a period during which the main switching element 2 is in the on-state, and the excitation energy is accumulated in the coil 4a of the transformer 4′. At the same time, negative voltage Va expressed in the following Equation 1 is generated in the coil 4e: Va=−(double-end voltage of smoothing capacitor 11)×(winding number of coil 4e)/(winding number of coil 4a)  (1).
During this period, the gate-source voltage VGS3 of the sub-switching element 3 is reversely biased to a forward voltage of the Zener diode 66, and the sub-switching element 3 is in the off-state.
Next, during the period t=t2−t3, the main switching element 2 is turned off at t2 and is in the off-state during t2−t3, where the current that has flowed in the coil 4a of the transformer 4′ is then transferred to the path of the body diode of the sub-switching element 3- capacitor 10- coil 4a. During this period, the voltage Va of the coil 4e is inverted from the negative to the positive. The value of the voltage Va at this moment is substantially expressed in the following Equation 2:Va=+(double-end voltage of smoothing capacitor 6)×(winding number of coil 4e)/(winding number of coil 4b)  (2).
As a result, a gate-source capacity of the sub-switching element 3 is charged via the resistor 63, and when the voltage VGS3 exceeds drive threshold voltage of the sub-switching element 3, the sub-switching element 3 is turned on. Moreover, the capacitor 10 absorbs the energy accumulated in the leakage inductance of the transformer 4′ during the period when the drain current ID3 of the sub-switching element 3 is negative. Thus, a surge voltage is not generated in the drain-source voltage VDS2 of the main switching element 2.
Furthermore, the leakage inductance of the transformer 4′ and the capacitor 10 resonate serially, discharging the energy absorbed by the capacitor 10 during the period when the drain current ID3 of the sub-switching element 3 is positive to the load via the transformer 4′, the diode 5a, and the smoothing capacitor 6. The time constant of the time-constant circuit made up of the resistor 61 and the capacitor 62 is set such that the transistor 65 is turned off and the sub-switching element 3 is turned off at the time when this energy is discharged completely.
During the period t=t3−t4, the transistor 65 is still in the on-state, and the sub-switching element 3 is in the off-state. During this period, the energy accumulated in the transformer 4′ during the period t1−t2 is discharged to the load via the smoothing capacitor 6 to the diode 5a. The main switching element 2 is turned on at the time t=t4, and the same operation is repeated thereafter.
The above-described conventional technologies have drawbacks in that, if the DC power supply voltage (voltage of the DC power supply 1 in FIG. 13 or voltage rectified by the diode bridge 50 in FIG. 15 (double-end voltage of the smoothing capacitor 11)) changes significantly, loss within the circuit increases, and the conversion efficiency of the device decreases. For example, in the conventional technology shown in FIG. 15, the AC input voltage can differ by countries, and if the double-end voltage of the smoothing capacitor 22 via the diode bridge 50 changes significantly, the value of the negative voltage generated in the coil 4e of the transformer 4′ as shown in Equation 1 also changes significantly. Here, the value of the resistor 63 that charges the gate-source capacity of the sub-switching element 3 needs to be set such that the gate-source voltage exceeds the drive threshold voltage during the period in which the drain current of the sub-switching element 3 flows negatively. The set value is generally in the order of several tens through hundreds ohm (Ω).
Therefore, the following problems can occur. Specifically, when the double-end voltage of the smoothing capacitor 11 is high, the current passing through the resistor 63 and the Zener diode 66 increases at the period t=t1−t2, loss generated from these parts increase, and the conversion efficiency of the device decreases. Furthermore, during the period t=t3−t4, positive voltage generated in the coil 4e of the transformer 4′ is shorted by the transistor 65 via the resistor 63, generating loss in the resistor 63 and the transistor 65, and further decreasing the conversion efficiency of the device. Another problem is that, when the drain-source voltage has not reached zero when the main switching element 2 is turned on, a surge current is generated as indicated in the drain current waveform ID2 in FIG. 16, thus increasing the noise or switching loss. The same problems can occur in the conventional technology shown in FIG. 13.
Moreover, if the voltage generated in the coil 4e changes in accordance with the DC power supply voltage, the switching frequency of each of the switching elements 2, 3 or an on-off duty ratio is changed, contributing to the increase of loss and worsening the efficiency of the device. In addition, another problem is that in the conventional technology shown in FIG. 13, the transformer 4 includes a large number of coils. Thus, it is rather difficult to reduce the size and weight of the entire device.
Accordingly, there still remains a need to improve conventional power conversion devices. The present invention addresses this need.