Isolated boost converter

This patent application claims the benefit of U.S. Provisional Patent Application No. 62/860,650 filed Jun. 12, 2019 which is incorporated by reference herein.

The present disclosure relates to an isolated boost converter, and more particularly to an isolated boost converter with a high boost ratio.

The statements in this section merely provide background information related to the present disclosure and do not necessarily constitute prior art.

Please refer to FIG. 1 , which shows a circuit block diagram of a conventional non-isolated boost converter. If a high voltage conversion ratio (boost ratio) is required, for example more than 10 times, two or more than two boost (DC-to-DC) converters are used to implement the requirement of high boost ratio. As shown in FIG. 1 , a first DC boost converter and a second DC boost converter are used. In which, a voltage gain of one of the boost converters is about 4. However, disadvantages of the common non-isolated boost converters are higher circuit component costs and lower conversion efficiency.

Please refer to FIG. 2 , which shows a circuit diagram of a conventional push-pull converter. The push-pull converter has an advantage of electrical isolation by an isolated transformer. By adjusting the number of coil turns of the isolated transformer, the high boost ratio (voltage conversion ratio) of the push-pull converter can be implemented. For example, a 380-volt output voltage can be achieved by converting a 40- to 60-volt input voltage with the adjusted coil turns. If the voltage conversion ratio is too high, however, excessive number of the secondary-side coil turns causes an increase in the leakage inductance of the isolated transformer, resulting in higher stress on the switch and/or the diode. Further, the components with higher rated voltage need to be used and the turned-on resistance of the switch becomes larger cause increased circuit costs, reduced efficiency, and larger transformer volume.

Please refer to FIG. 3 , which shows a circuit diagram of a conventional half-bridge converter. It's similar to the push-pull converter shown in FIG. 2 , including the change of the voltage conversion ration by adjusting the transformer, and the disadvantages of the push-pull converter. Moreover, the push-pull converter shown in FIG. 2 and the half-bridge converter shown in FIG. 3 both use hard switching of the switches and the first- and third-quadrant operations of the transformers. The quadrant here refers to the main operation area of the transformer in the B-H curve (hysteresis phenomenon is ignored).

Please refer to FIG. 4 and FIG. 5 , which show circuit diagrams of a conventional full-bridge LLC resonant converter and a conventional half-bridge LLC resonant converter, respectively. The switches of the two kind of LLC resonant converters can operate under ZVS (zero voltage switching) condition. By adjusting the number of coil turns of the isolated transformer, the high voltage conversion ratio of the LLC resonant converter can be implemented. If the voltage conversion ratio is too high, however, excessive number of the coil turns causes the increased coil turns of the transformer, increased volume of the transformer, and reduced efficiency. Moreover, the full-bridge LLC resonant converter shown in FIG. 4 and the half-bridge LLC resonant converter shown in FIG. 5 provide the first- and third-quadrant operations of the transformers.

Please refer to FIG. 6 , which shows a circuit diagram of a conventional full-bridge converter. The switches of the full-bridge converter can operate under ZVS condition by phase control techniques. In order to implement the high boost ratio (voltage conversion ratio), however, the excessive number of the coil turns causes larger transformer volume, higher stress of the semiconductor switch, and lower conversion efficiency.

In the above several common isolated converters, the isolated transformer thereof operate in the first quadrant and the third quadrant. The present disclosure proposes an isolated converter with high boost ratio that can operate in the first quadrant and the third quadrant. The high boost ratio of the isolated converter can be implemented by charging capacitors to establish a high output voltage, thereby effectively reducing the number of coil turns of the isolated transformer. Accordingly, it is to increase efficiency of the boost circuit, reduce circuit component costs, and reduce occupied volume due to the reduction of the number of coil turns of the isolated transformer.

In order to solve the above-mentioned problems, an isolated boost converter is provided. The isolated boost converter includes a transformer, a first bridge arm, a second bridge arm, and a boost circuit. The transformer includes a secondary side having a secondary side first contact and a secondary side second contact. The first bridge arm has a first diode and a second diode; a cathode of the first diode is coupled to a DC positive output contact, an anode of the first diode is coupled to a first bridge arm upper contact, an anode of the second diode is coupled to a DC negative output contact, and a cathode of the second diode is coupled to a first bridge arm lower contact. The second bridge arm has a third diode and a fourth diode; a cathode of the third diode is coupled to the DC positive output contact, an anode of the third diode is coupled to a second bridge arm upper contact, an anode of the fourth diode is coupled to the DC negative output contact, and a cathode of the fourth diode is coupled to a second bridge arm lower contact. The boost circuit includes four diodes and at least two capacitors. Anodes of the two diodes are mutually coupled to a first contact and cathodes of the two diodes are coupled to the first bridge arm upper contact and the second bridge arm upper contact, respectively. Cathodes of the two diodes are mutually coupled to a second contact and anodes of the two diodes are coupled to the first bridge arm lower contact and the second bridge arm lower contact, and the second contact is coupled to the first contact. The at least two capacitors are coupled to the secondary side first contact and the secondary side second contact.

Accordingly, the isolated boost converter is provided to increase efficiency of the boost circuit, reduce circuit component costs, and reduce occupied volume due to the reduction of the number of coil turns of the isolated transformer.

In order to solve the above-mentioned problems, an isolated boost converter is provided. The isolated boost converter includes a first bridge arm, a second bridge arm, and a boost circuit. The transformer includes a secondary side having a secondary side first contact, a secondary side second contact, and a center-tapped contact. The first bridge arm has a first diode and a second diode; a cathode of the first diode is coupled to a DC positive output contact, an anode of the first diode is coupled to a first bridge arm upper contact, an anode of the second diode is coupled to a DC negative output contact, and a cathode of the second diode is coupled to a first bridge arm lower contact. The second bridge arm has a third diode and a fourth diode; a cathode of the third diode is coupled to the DC positive output contact, an anode of the third diode is coupled to a second bridge arm upper contact, an anode of the fourth diode is coupled to the DC negative output contact, and a cathode of the fourth diode is coupled to a second bridge arm lower contact. The boost circuit include four diodes and four capacitors. Anodes of the two diodes are mutually coupled to a first contact and cathodes of the two diodes are coupled to the first bridge arm upper contact and the second bridge arm upper contact, respectively. Cathodes of the two diodes are mutually coupled to a second contact and anodes of the two diodes are coupled to the first bridge arm lower contact and the second bridge arm lower contact, and the second contact is coupled to the first contact and the center-tapped contact. The four capacitors are coupled to the secondary side first contact and the secondary side second contact.

Accordingly, the isolated boost converter is provided to increase efficiency of the boost circuit, reduce circuit component costs, and reduce occupied volume due to the reduction of the number of coil turns of the isolated transformer.

It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the present disclosure as claimed. Other advantages and features of the present disclosure will be apparent from the following description, drawings and claims.

The present disclosure can be more fully understood by reading the following detailed description of the embodiment, with reference made to the accompanying drawings as follows:

FIG. 1 is a circuit block diagram of a conventional non-isolated boost converter.

FIG. 2 is a circuit diagram of a conventional push-pull converter.

FIG. 3 is a circuit diagram of a conventional half-bridge converter.

FIG. 4 is a circuit diagram of a conventional full-bridge LLC resonant converter.

FIG. 5 is a circuit diagram of a conventional half-bridge LLC resonant converter.

FIG. 6 is a circuit diagram of a conventional full-bridge converter.

FIG. 7 is a single-bus circuit structure of boosting voltage of an isolated boost converter according to a first embodiment of the present disclosure.

FIG. 8A is a schematic diagram of current paths of a first-quadrant operation in FIG. 7 .

FIG. 8B is a schematic diagram of current paths of a third-quadrant operation in FIG. 7 .

FIG. 9A is a single-bus circuit structure of boosting voltage of the isolated boost converter according to a second embodiment of the present disclosure.

FIG. 9B is a single-bus circuit structure of boosting voltage of the isolated boost converter according to a third embodiment of the present disclosure.

FIG. 10 is a dual-bus circuit structure of boosting voltage of the isolated boost converter according to the present disclosure.

FIG. 11A is a schematic diagram of current paths of a first-quadrant operation in FIG. 10 .

FIG. 11B is a schematic diagram of current paths of a third-quadrant operation in FIG. 10 .

FIG. 12 is a circuit diagram of a full-bridge converter having the single-bus circuit structure of boosting voltage in FIG. 7 .

FIG. 13 is a circuit diagram of a half-bridge converter having the single-bus circuit structure of boosting voltage in FIG. 7 .

FIG. 14 is a circuit diagram of a half-bridge LLC resonant converter having the single-bus circuit structure of boosting voltage in FIG. 7 .

FIG. 15 is a circuit diagram of a full-bridge LLC resonant converter having the single-bus circuit structure of boosting voltage in FIG. 7 .

FIG. 16 is a circuit diagram of a push-pull converter having the single-bus circuit structure of boosting voltage in FIG. 7 .

FIG. 17 is a circuit diagram of a full-bridge converter having the dual-bus circuit structure of boosting voltage in FIG. 10 .

FIG. 18 is a circuit diagram of a half-bridge converter having the dual-bus circuit structure of boosting voltage in FIG. 10 .

FIG. 19 is a circuit diagram of a half-bridge LLC resonant converter having the dual-bus circuit structure of boosting voltage in FIG. 10 .

FIG. 20 is a circuit diagram of a full-bridge LLC resonant converter having the dual-bus circuit structure of boosting voltage in FIG. 10 .

FIG. 21 is a circuit diagram of a push-pull converter having the dual-bus circuit structure of boosting voltage in FIG. 10 .

Reference will now be made to the drawing figures to describe the present disclosure in detail. It will be understood that the drawing figures and exemplified embodiments of present disclosure are not limited to the details thereof.

Please refer to FIG. 7 , which shows a single-bus circuit structure of boosting voltage of an isolated boost converter according to a first embodiment of the present disclosure. The isolated boost converter includes a transformer T 1 , a first bridge arm Lb 1 , a second bridge arm Lb 2 , and a boost circuit 10 . The transformer T 1 includes a secondary side having a secondary side first contact Ps 1 and a secondary side second contact Ps 2 . The first bridge arm Lb 1 has a first diode D 1 and a second diode D 2 . A cathode of the first diode D 1 is coupled to a DC positive output contact +Vbus, an anode of the first diode D 1 is coupled to a first bridge arm upper contact Pu 1 , an anode of the second diode D 2 is coupled to a DC negative output contact −Vbus, and a cathode of the second diode D 2 is coupled to a first bridge arm lower contact Pw 1 . The second bridge arm Lb 2 has a third diode D 3 and a fourth diode D 4 . A cathode of the third diode D 3 is coupled to the DC positive output contact +Vbus, an anode of the third diode D 3 is coupled to a second bridge arm upper contact Pu 2 , an anode of the fourth diode D 4 is coupled to the DC negative output contact −Vbus, and a cathode of the fourth diode D 4 is coupled to a second bridge arm lower contact Pw 2 .

The boost circuit 10 includes two diodes D 5 , D 6 that are mutually coupled by their anodes, two diodes D 7 , D 8 that are mutually coupled by their cathodes, and at least two capacitors C 1 -C 4 . Specifically, the anodes of the diodes D 5 , D 6 are coupled to a first contact P 1 , and cathodes of the diodes D 5 , D 6 are coupled to the first bridge arm upper contact Pu 1 and the second bridge arm upper contact Pu 2 , respectively. The cathodes of the diodes D 7 , D 8 are coupled to a second contact P 2 , and anodes of the diodes D 7 , D 8 are coupled to the first bridge arm lower contact Pw 1 and the second bridge arm lower contact Pw 2 , respectively. The second contact P 2 is coupled to the first contact P 1 . The at least two capacitors C 1 -C 4 are coupled to the secondary side first contact Ps 1 and the secondary side second contact Ps 2 .

As shown in FIG. 7 , the number of the at least two capacitors C 1 -C 4 is four, including a first capacitor C 1 , a second capacitor C 2 , a third capacitor C 3 , and a fourth capacitor C 4 . Two ends of the first capacitor C 1 are coupled to the secondary side first contact Ps 1 and the first bridge arm upper contact Pu 1 . Two ends of the second capacitor C 2 are coupled to the secondary side second contact Ps 2 and the second bridge arm upper contact Pu 2 . Two ends of the third capacitor C 3 are coupled to the secondary side first contact Ps 1 and the first bridge arm lower contact Pw 1 . Two ends of the fourth capacitor C 4 are coupled to the secondary side second contact Ps 2 and the second bridge arm lower contact Pw 2 . For the boost circuit 10 shown in FIG. 7 , the boost circuit 10 is a symmetrical circuit along a reference line which is the connection between the first contact P 1 and the second contact P 2 .

The symmetrical diode-capacitor circuit, which is coupled to the secondary side of the transformer T 1 , is proposed to charge the capacitors through the secondary side coil turns, thereby reducing coil turns, increasing conversion efficiency, and reducing occupied volume. Further, the symmetrical diode-capacitor circuit can be used in a variety of isolated converters capable of operating both in a first quadrant and a third quadrant.

Please refer to FIG. 8A , which shows a schematic diagram of current paths of a first-quadrant operation in FIG. 7 . When the isolated boost converter operates in the first quadrant, a main current Ic 1 flows through the secondary side of the transformer T 1 , the first capacitor C 1 , the first diode D 1 , the DC positive output contact +Vbus, the DC negative output contact −Vbus, the fourth diode D 4 , the fourth capacitor C 4 , and the secondary side. At this condition, the secondary side of the transformer T 1 are connected in series to the first capacitor C 1 and the fourth capacitor C 4 to establish a high output voltage. In particular, the first capacitor C 1 and the fourth capacitor C 4 are equivalent to be connected in series since the first diode D 1 and the fourth diode D 4 are turned by forward bias voltages.

A pre-charge current Ic 2 flows through the secondary side of the transformer T 1 , the third capacitor C 3 , a seventh diode D 7 , a sixth diode D 6 , the second capacitor C 2 , and the secondary side. The second capacitor C 2 and the third capacitor C 3 are charged by the pre-charge current Ic 2 . In particular, since the sixth diode D 6 and the seventh diode D 7 are turned on by forward bias voltages, the second capacitor C 2 and the third capacitor C 3 are equivalent to be connected in series.

Please refer to FIG. 8B , which shows a schematic diagram of current paths of a third-quadrant operation in FIG. 7 . When the isolated boost converter operates in the third quadrant, a main current Ic 1 flows through the secondary side of the transformer T 1 , the second capacitor C 2 , the third diode D 3 , the DC positive output contact +Vbus, the DC negative output contact −Vbus, the second diode D 2 , the third capacitor C 3 , and the secondary side. At this condition, the secondary side of the transformer T 1 are connected in series to the second capacitor C 2 and the third capacitor C 3 to establish a high output voltage. In particular, the second capacitor C 2 and the third capacitor C 3 are equivalent to be connected in series since the second diode D 2 and the third diode D 3 are turned by forward bias voltages.

A pre-charge current Ic 2 flows through the secondary side of the transformer T 1 , the fourth capacitor C 4 , an eighth diode D 8 , a fifth diode D 5 , the first capacitor C 1 , and the secondary side. The first capacitor C 1 and the fourth capacitor C 4 are charged by the pre-charge current Ic 2 . In particular, since the fifth diode D 5 and the eighth diode D 8 are turned on by forward bias voltages, the first capacitor C 1 and the fourth capacitor C 4 are equivalent to be connected in series.

Please refer to FIG. 9A , which shows a single-bus circuit structure of boosting voltage of the isolated boost converter according to a second embodiment of the present disclosure. As shown in FIG. 9A , the number of the at least two capacitors C 1 -C 4 is two, including the first capacitor C 1 and the second capacitor C 2 . Two ends of the first capacitor C 1 are coupled to the secondary side first contact Ps 1 and the first bridge arm upper contact Pu 1 . Two ends of the second capacitor C 2 are coupled to the secondary side second contact Ps 2 and the second bridge arm upper contact Pu 2 . The secondary side first contact Ps 1 is coupled to the first bridge arm lower contact Pw 1 , and the secondary side second contact Ps 2 is coupled to the second bridge arm lower contact Pw 2 . Similarly, for the boost circuit 10 shown in FIG. 9A , the boost circuit 10 is a symmetrical circuit along a reference line which is the connection between the first contact P 1 and the second contact P 2 . In comparison with the first embodiment shown in FIG. 7 , the number of the capacitors shown in FIG. 9A is less than that shown in FIG. 7 , that is, the number of capacitors shown in FIG. 9A is half that shown in FIG. 7 . Since the first capacitor C 1 is coupled to the fourth capacitor C 4 in series, the first capacitor C 1 is used to replace the in-series first capacitor C 1 and fourth capacitor C 4 . Also, since the second capacitor C 2 is coupled to the third capacitor C 3 in series, and the second capacitor C 2 is used to replace the in-series second capacitor C 2 and the third capacitor C 3 .

As shown in FIG. 9A , when the isolated boost converter operates in the first quadrant, a main current (not shown) flows through the secondary side of the transformer T 1 , the first capacitor C 1 , the first diode D 1 , the DC positive output contact +Vbus, the DC negative output contact −Vbus, the fourth diode D 4 , and the secondary side; a pre-charge current (not shown) flows through the secondary side of the transformer T 1 , the seventh diode D 7 , the sixth diode D 6 , the second capacitor C 2 , and the secondary side. When the isolated boost converter operates in the third quadrant, a main current (not shown) flows through the secondary side of the transformer T 1 , the second capacitor C 2 , the third diode D 3 , the DC positive output contact +Vbus, the DC negative output contact −Vbus, the second diode D 2 , and the secondary side; a pre-charge current (not shown) flows through the secondary side of the transformer T 1 , the eighth diode D 8 , the fifth diode D 5 , the first capacitor C 1 , and the secondary side.

Please refer to FIG. 9B , which shows a single-bus circuit structure of boosting voltage of the isolated boost converter according to a third embodiment of the present disclosure. As shown in FIG. 9B , the number of the at least two capacitors C 1 -C 4 is two, including the third capacitor C 3 and the fourth capacitor C 4 . Two ends of the third capacitor C 3 are coupled to the secondary side first contact Ps 1 and the first bridge arm lower contact Pw 1 . Two ends of the fourth capacitor C 4 are coupled to the secondary side second contact Ps 2 and the second bridge arm lower contact Pw 2 . The secondary side first contact Ps 1 is coupled to the first bridge arm upper contact Pu 1 , and the secondary side second contact Ps 2 is coupled to the second bridge arm upper contact Pu 2 . Similarly, for the boost circuit 10 shown in FIG. 9B , the boost circuit 10 is a symmetrical circuit along a reference line which is the connection between the first contact P 1 and the second contact P 2 . In comparison with the first embodiment shown in FIG. 7 , the number of the capacitors shown in FIG. 9B is less than that shown in FIG. 7 , that is, the number of capacitors shown in FIG. 9B is half that shown in FIG. 7 . Since the first capacitor C 1 is coupled to the fourth capacitor C 4 in series, the fourth capacitor C 4 is used to replace the in-series first capacitor C 1 and fourth capacitor C 4 . Also, since the second capacitor C 2 is coupled to the third capacitor C 3 in series, and the third capacitor C 3 is used to replace the in-series second capacitor C 2 and the third capacitor C 3 .

As shown in FIG. 9B , when the isolated boost converter operates in the first quadrant, a main current (not shown) flows through the secondary side of the transformer T 1 , the first diode D 1 , the DC positive output contact +Vbus, the DC negative output contact −Vbus, the fourth diode D 4 , the fourth capacitor C 4 , and the secondary side; a pre-charge current (not shown) flows through the secondary side of the transformer T 1 , the third capacitor C 3 , the seventh diode D 7 , the sixth diode D 6 , and the secondary side. When the isolated boost converter operates in the third quadrant, a main current (not shown) flows through the secondary side of the transformer T 1 , the third diode D 3 , the DC positive output contact +Vbus, the DC negative output contact −Vbus, the second diode D 2 , and the third capacitor C 3 , and the secondary side; a pre-charge current (not shown) flows through the secondary side of the transformer T 1 , the fourth capacitor C 4 , the eighth diode D 8 , the fifth diode D 5 , and the secondary side.

Please refer to FIG. 10 , which shows a dual-bus circuit structure of boosting voltage of the isolated boost converter according to the present disclosure. In the same concept of the symmetrical diode-capacitor circuit, the dual-bus circuit structure is proposed for the specific application, such as a dual-bus output of the UPS's power converter. In comparison with the single-bus circuit structure of boosting voltage of the isolated boost converter shown in FIG. 7 , the dual-bus circuit structure of boosting voltage of the isolated boost converter shown in FIG. 10 further includes a center-tapped contact Pct. Therefore, balance charging and discharging operation of the dual-bus output voltage can be implemented through the center-tapped structure. Specifically, the isolated boost converter includes a transformer T 1 , a first bridge arm Lb 1 , a second bridge arm Lb 2 , and a boost circuit 10 . The transformer T 1 includes a secondary side having a secondary side first contact Ps 1 and a secondary side second contact Ps 2 . The first bridge arm Lb 1 has a first diode D 1 and a second diode D 2 . A cathode of the first diode D 1 is coupled to a DC positive output contact +Vbus, an anode of the first diode D 1 is coupled to a first bridge arm upper contact Pu 1 , an anode of the second diode D 2 is coupled to a DC negative output contact −Vbus, and a cathode of the second diode D 2 is coupled to a first bridge arm lower contact Pw 1 . The second bridge arm Lb 2 has a third diode D 3 and a fourth diode D 4 . A cathode of the third diode D 3 is coupled to the DC positive output contact +Vbus, an anode of the third diode D 3 is coupled to a second bridge arm upper contact Pu 2 , an anode of the fourth diode D 4 is coupled to the DC negative output contact −Vbus, and a cathode of the fourth diode D 4 is coupled to a second bridge arm lower contact Pw 2 .

The boost circuit 10 includes two diodes D 5 , D 6 that are mutually coupled by their anodes, two diodes D 7 , D 8 that are mutually coupled by their cathodes, and four capacitors C 1 -C 4 . Specifically, the anodes of the diodes D 5 , D 6 are coupled to the center-tapped contact Pct, and cathodes of the diodes D 5 , D 6 are coupled to the first bridge arm upper contact Pu 1 and the second bridge arm upper contact Pu 2 , respectively. The cathodes of the diodes D 7 , D 8 are coupled to the center-tapped contact Pct, and anodes of the diodes D 7 , D 8 are coupled to the first bridge arm lower contact Pw 1 and the second bridge arm lower contact Pw 2 , respectively. The four capacitors C 1 -C 4 are coupled to the secondary side first contact Ps 1 and the secondary side second contact Ps 2 .

As shown in FIG. 10 , the number of the at least two capacitors C 1 -C 4 is four, including a first capacitor C 1 , a second capacitor C 2 , a third capacitor C 3 , and a fourth capacitor C 4 . Two ends of the first capacitor C 1 are coupled to the secondary side first contact Ps 1 and the first bridge arm upper contact Pu 1 . Two ends of the second capacitor C 2 are coupled to the secondary side second contact Ps 2 and the second bridge arm upper contact Pu 2 . Two ends of the third capacitor C 3 are coupled to the secondary side first contact Ps 1 and the first bridge arm lower contact Pw 1 . Two ends of the fourth capacitor C 4 are coupled to the secondary side second contact Ps 2 and the second bridge arm lower contact Pw 2 . For the boost circuit 10 shown in FIG. 10 , the boost circuit 10 is a symmetrical circuit.

Please refer to FIG. 11A , which shows a schematic diagram of current paths of a first-quadrant operation in FIG. 10 . When the isolated boost converter operates in the first quadrant, a main current Ic 1 flows through the secondary side of the transformer T 1 , the first capacitor C 1 , the first diode D 1 , the DC positive output contact +Vbus, the DC negative output contact −Vbus, the fourth diode D 4 , the fourth capacitor C 4 , and the secondary side. At this condition, the secondary side of the transformer T 1 are connected in series to the first capacitor C 1 and the fourth capacitor C 4 to establish a high output voltage. In particular, the first capacitor C 1 and the fourth capacitor C 4 are equivalent to be connected in series since the first diode D 1 and the fourth diode D 4 are turned by forward bias voltages.

A pre-charge current Ic 2 flows through the secondary side of the transformer T 1 , the third capacitor C 3 , a seventh diode D 7 , a sixth diode D 6 , the second capacitor C 2 , and the secondary side. The second capacitor C 2 and the third capacitor C 3 are charged by the pre-charge current Ic 2 . In particular, since the sixth diode D 6 and the seventh diode D 7 are turned on by forward bias voltages, the second capacitor C 2 and the third capacitor C 3 are equivalent to be connected in series.

Please refer to FIG. 11B , which shows a schematic diagram of current paths of a third-quadrant operation in FIG. 10 . When the isolated boost converter operates in the third quadrant, a main current Ic 1 flows through the secondary side of the transformer T 1 , the second capacitor C 2 , the third diode D 3 , the DC positive output contact +Vbus, the DC negative output contact −Vbus, the second diode D 2 , the third capacitor C 3 , and the secondary side. At this condition, the secondary side of the transformer T 1 are connected in series to the second capacitor C 2 and the third capacitor C 3 establish a high output voltage. In particular, the second capacitor C 2 and the third capacitor C 3 are equivalent to be connected in series since the second diode D 2 and the third diode D 3 are turned by forward bias voltages.

A pre-charge current Ic 2 flows through the secondary side of the transformer T 1 , the fourth capacitor C 4 , an eighth diode D 8 , a fifth diode D 5 , the first capacitor C 1 , and the secondary side. The first capacitor C 1 and the fourth capacitor C 4 are charged by the pre-charge current Ic 2 . In particular, since the fifth diode D 5 and the eighth diode D 8 are turned on by forward bias voltages, the first capacitor C 1 and the fourth capacitor C 4 are equivalent to be connected in series.

Please refer to FIG. 12 , which shows a circuit diagram of a full-bridge converter having the single-bus circuit structure of boosting voltage in FIG. 7 . The symmetrical diode-capacitor circuit shown in FIG. 7 is applied to the full-bridge converter having the single-bus circuit structure to achieve the DC-to-DC converter with high boost ratio and high efficiency.

Moreover, the symmetrical diode-capacitor circuit shown in FIG. 9A and that shown in FIG. 9B are also applied to the full-bridge converter having the single-bus circuit structure (no detailed circuit diagram is shown).

Please refer to FIG. 13 , which shows a circuit diagram of a half-bridge converter having the single-bus circuit structure of boosting voltage in FIG. 7 . The symmetrical diode-capacitor circuit shown in FIG. 7 is applied to the half-bridge converter having the single-bus circuit structure to achieve the DC-to-DC converter with high boost ratio and high efficiency.

Moreover, the symmetrical diode-capacitor circuit shown in FIG. 9A and that shown in FIG. 9B are also applied to the half-bridge converter having the single-bus circuit structure (no detailed circuit diagram is shown).

Please refer to FIG. 14 , which shows a circuit diagram of a half-bridge LLC resonant converter having the single-bus circuit structure of boosting voltage in FIG. 7 . The symmetrical diode-capacitor circuit shown in FIG. 7 is applied to the half-bridge LLC resonant converter having the single-bus circuit structure to achieve the DC-to-DC converter with high boost ratio and high efficiency.

Moreover, the symmetrical diode-capacitor circuit shown in FIG. 9A and that shown in FIG. 9B are also applied to the half-bridge LLC resonant converter having the single-bus circuit structure (no detailed circuit diagram is shown).

Please refer to FIG. 15 , which shows a circuit diagram of a full-bridge LLC resonant converter having the single-bus circuit structure of boosting voltage in FIG. 7 . The symmetrical diode-capacitor circuit shown in FIG. 7 is applied to the full-bridge LLC resonant converter having the single-bus circuit structure to achieve the DC-to-DC converter with high boost ratio and high efficiency.

Moreover, the symmetrical diode-capacitor circuit shown in FIG. 9A and that shown in FIG. 9B are also applied to the full-bridge LLC resonant converter having the single-bus circuit structure (no detailed circuit diagram is shown).

Please refer to FIG. 16 , which shows a circuit diagram of a push-pull converter having the single-bus circuit structure of boosting voltage in FIG. 7 . The symmetrical diode-capacitor circuit shown in FIG. 7 is applied to the push-pull converter having the single-bus circuit structure to achieve the DC-to-DC converter with high boost ratio and high efficiency.

Moreover, the symmetrical diode-capacitor circuit shown in FIG. 9A and that shown in FIG. 9B are also applied to the push-pull converter having the single-bus circuit structure (no detailed circuit diagram is shown).

Please refer to FIG. 17 , which shows a circuit diagram of a full-bridge converter having the dual-bus circuit structure of boosting voltage in FIG. 10 . The symmetrical diode-capacitor circuit shown in FIG. 10 is applied to the full-bridge converter having the dual-bus circuit structure to achieve the DC-to-DC converter with high boost ratio and high efficiency.

Please refer to FIG. 18 , which shows a circuit diagram of a half-bridge converter having the dual-bus circuit structure of boosting voltage in FIG. 10 . The symmetrical diode-capacitor circuit shown in FIG. 10 is applied to the half-bridge converter having the dual-bus circuit structure to achieve the DC-to-DC converter with high boost ratio and high efficiency.

Please refer to FIG. 19 , which shows a circuit diagram of a half-bridge LLC resonant converter having the dual-bus circuit structure of boosting voltage in FIG. 10 . The symmetrical diode-capacitor circuit shown in FIG. 10 is applied to the half-bridge LLC resonant converter having the dual-bus circuit structure to achieve the DC-to-DC converter with high boost ratio and high efficiency.

Please refer to FIG. 20 , which shows a circuit diagram of a full-bridge LLC resonant converter having the dual-bus circuit structure of boosting voltage in FIG. 10 . The symmetrical diode-capacitor circuit shown in FIG. 10 is applied to the full-bridge LLC resonant converter having the dual-bus circuit structure to achieve the DC-to-DC converter with high boost ratio and high efficiency.

Please refer to FIG. 21 , which shows a circuit diagram of a push-pull converter having the dual-bus circuit structure of boosting voltage in FIG. 10 . The symmetrical diode-capacitor circuit shown in FIG. 10 is applied to the push-pull converter having the dual-bus circuit structure to achieve the DC-to-DC converter with high boost ratio and high efficiency.

Although the present disclosure has been described with reference to the preferred embodiment thereof, it will be understood that the present disclosure is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the present disclosure as defined in the appended claims.