# Patent

## Title
Power supply device having improved power conversion efficiency in light load conditions

## Abstract

There is provided a phase shift full bridge (PSFB) type power supply device controlling a switching on time of lagging leg switches according to a load state. The power supply device includes a power supply unit supplying preset DC power by switching input power using a full bridge by a phase shift method; and a control unit controlling a switching time of a switch of the full bridge according to a load state in which the DC power is received from the power supply unit.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the priority of Korean Patent Application No. 10-2012-0151307 filed on Dec. 21, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power supply device having improved power conversion efficiency in light load condition.
2. Description of the Related Art
Recently, various types of electronic devices, such as computers, display devices, various control devices, and the like that meet various requirements have been used domestically, commercially and industrially.
These electronic devices have essentially adopted a power supply device supplying required driving power so as to perform various operations while meeting various requirements.
Meanwhile, in order to meet the requirements of high power specifications, in the foregoing power supply device, a full bridge converter capable of reducing a switching loss by implementing zero voltage switching using a phase shift control has been prevalently used.
The foregoing full bridge converter may include two leading leg switches and two lagging leg switches, similarly to the invention disclosed in the following related art document.
However, in case of the leading leg switch, the zero voltage switching can be made in the overall load region, but in case of the lagging leg switch, as the load is decreased, the zero voltage switching may not be smoothly performed, and thus, the power conversion efficiency may be drastically degraded in a light load region.
RELATED ART DOCUMENT
(Patent Document 1) Korean Patent Laid-Open Publication No. 10-2001-0095453
SUMMARY OF THE INVENTION
An aspect of the present invention provides a phase shift full bridge (PSFB) type power supply device controlling a switching time of lagging leg switches according to a load state.
According to an aspect of the present invention, there is provided a power supply device, including: a power supply unit supplying preset DC power by switching input power using a full bridge by a phase shift method; and a control unit controlling a switching time of a switch of the full bridge according to a load state in which the DC power is received from the power supply unit.
The switch of the full bridge of the power supply unit may include: first and second leading leg switches; and first and second lagging leg switches.
The control unit may control a switching time of at least one of the first and second lagging leg switches according to the load state.
The control unit may delay a switching on time of at least one of the first and second lagging leg switches according to the load state.
The control unit may include: a comparison unit comparing received load state information with a preset reference load state; and a signal generation unit generating first and second leading leg switching control signals and first and second lagging leg switching control signals and delaying a switching on time of at least one of the first and second lagging leg switching control signals according to a comparison result of the comparison unit.
The power supply unit may include: a transformer having a primary winding receiving power switched by the switch of the full bridge and a secondary winding electrically isolated from and magnetically coupled to the primary winding to form a preset winding ratio therewith, transforming the switched power according to the winding ratio and outputting the transformed power; and an output unit rectifying and stabilizing the transformed power from the secondary winding of the transformer and outputting the stabilized power.
The output unit may include: first and second rectifying switches connected to one end and the other end of the secondary winding, respectively; and an inductor and a capacitor stabilizing the power rectified by the first and second rectifying switches.
According to another aspect of the present invention, there is provided a power supply device, including: a power supply unit supplying preset DC power by switching input power using a full bridge by a phase shift method and rectifying and stabilizing transformed switching power; and a control unit controlling a switching time of a switch of the full bridge by determining a load state in which the DC power is received from the power supply unit, based on resonance between the switch of the full bridge and an output capacitor and a parasitic inductor of a rectifying switch.
The control unit may include: a comparison unit comparing received load state information with a reference load state set according to the resonance between the switch of the full bridge and the rectifying switch; and a signal generation unit generating first and second leading leg switching control signals and first and second lagging leg switching control signals and delaying a switching on time of at least one of the first and second lagging leg switching control signals according to a comparison result of the comparison unit.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a circuit diagram schematically illustrating a power supply device according to an embodiment of the present invention;
FIG. 2 is a circuit diagram schematically illustrating a control unit employed in the power supply device according to the embodiment of the present invention;
FIG. 3 is a graph illustrating signal waveforms of main components of the power supply device according to the embodiment of the present invention;
FIGS. 4A and 4B are diagrams illustrating resonance between switches of a full bridge and an output capacitor and a parasitic inductor of rectifying switches in a power supply unit employed in the power supply device according to the embodiment of the present invention; and
FIG. 5 is a graph illustrating zero voltage switching in the power supply device according to the embodiment of the present invention in light load condition.



DETAILED DESCRIPTION OF THE EMBODIMENTS
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
In the drawings, the same or like reference numerals will be used to designate the same or like elements.
FIG. 1 is a circuit diagram schematically illustrating a power supply device according to an embodiment of the present invention.
Referring to FIG. 1, a power supply device 100 according to an embodiment of the present invention may include a power supply unit 110 and a control unit 120.
The power supply unit 110 may include a full bridge 111, a transformer 112, and an output unit 113.
The full bridge 111 may include first and second leading leg switches S1 and S2 and first and second lagging leg switches S3 and S4. The first and second leading leg switches S1 and S2 and the first and second lagging leg switches S3 and S4 of the full bridge 111 may individually carry out switching on/off that may be controlled by a phase shift method.
The transformer 112 may have a primary winding Np disposed on a primary side and a secondary winding Ns electrically isolated from and magnetically connected to the primary winding Np and disposed on a secondary side. The primary winding Np and the secondary winding Ns form a preset winding ratio, and may transform power switched by the full bridge 111 according to the winding ratio and output the transformed power.
The output unit 113 may include first and second rectifying switches SR1 and SR2 individually rectifying the transformed power from the secondary winding Ns, and an inductor Lo and a capacitor Co stabilizing the power rectified by the first and second rectifying switches SR1 and SR2.
The control unit 120 may provide switching control signals SS1, SS2, SS3, and SS4 controlling the switching of the first and second leading leg switches S1 and S2 and the first and second lagging leg switches S3 and S4 of the full bridge 111 according to a load state.
FIG. 2 is a circuit diagram schematically illustrating a control unit employed in the power supply device according to the embodiment of the present invention.
Referring to FIGS. 1 and 2, the control unit 120 of the power supply device 100 according to the embodiment of the present invention may include a comparison unit 121 and a signal generation unit 122.
The comparison unit 121 may include a comparator OP comparing a preset reference load state with detected load state information, and a stabilization circuit R1, R2, and C1 stabilizing a comparison result signal of the comparator OP.
The signal generation unit 122 may generate the switching control signals SS1, SS2, SS3, and SS4 controlling the switching of the first and second leading leg switches S1 and S2 and the first and second lagging leg switches S3 and S4. In this case, the signal generation unit 122 may delay a switching on time of at least one of the lagging leg switching control signals SS3 and SS4 controlling the switching of the first and second lagging leg switches S3 and S4 according to the comparison result signal of the comparison unit 121.
FIG. 3 is a graph illustrating signal waveforms of main components of the power supply device according to the embodiment of the present invention.
Referring to FIGS. 1 and 3, when the switching on time of the lagging leg switch is delayed (t0→t1), resonance between the switches of the full bridge 111 and the parasitic inductor LR disposed on the primary side and the output capacitor Co disposed on the secondary side may be generated.
Therefore, as illustrated, waveforms of switch voltages Vds3 and Vds4 of the lagging leg switches S3 and S4 of the full bridge 111 on the primary side may exhibit resonance and waveforms of primary side current ipri, magnetization inductance current iLm, and currents isr1 and isr2 and voltages Vsr1 and Vsr2 of the first and second rectifying switches SR1 and SR2 may also exhibit resonance.
FIGS. 4A and 4B are diagrams illustrating resonance between switches of a full bridge and a parasitic inductor and an output capacitor in a power supply unit employed in the power supply device according to the embodiment of the present invention.
With reference to FIGS. 4A and 4B, it can be appreciated that switch voltages Vds and Vgs of the full bridge 111 are increased and reduced according to a difference in load current. Therefore, in the case that the load current is above predetermined load current, the switching on time of the lagging leg switch is maintained as before, while in the case that the load current is below the predetermined load current, the switching on time of the lagging leg switch is delayed, thereby reducing switching loss.
FIG. 5 is a graph illustrating zero voltage switching in the power supply device according to the embodiment of the present invention in light load condition.
With reference to FIG. 5, it can be appreciated that when a load state of the power supply device according to the embodiment of the present invention corresponds to a preset light load state, the first lagging leg switching control signal SS3 is delayed. Although not illustrated, the second lagging leg switching control signal SS4 may be delayed in the same manner.
As described above, according to the embodiment of the present invention, the switching on time of the lagging leg switch may be controlled according to a load state. That is, the switching on time of the lagging leg switch is delayed in light load condition to thereby suppress a reduction in power conversion efficiency, while the switching on time of the lagging leg switch is maintained as before in heavy load condition to thereby maintain power conversion efficiency.
As set forth above, according to embodiments of the present invention, a reduction in power conversion efficiency in light load can be suppressed by controlling a switching on time of at least one of lagging leg switches according to a load state.
While the present invention has been shown and described in connection with the embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.

## Claims

1. A power supply device, comprising:
a power supply unit supplying preset DC power by switching input power using a full bridge by a phase shift method; and
a control unit controlling a switching time of a switch of the full bridge according to a load state in which the DC power is received from the power supply unit;
wherein the switch of the full bridge of the power supply unit includes:
first and second leading leg switches; and
first and second lagging leg switches;

wherein the control unit maintains a switching on time of at least one of the first and second lagging leg switches as before, in the case that the load current is above a predetermined load current; and
wherein the control unit delays a switching on time of at least one of the first and second lagging leg switches, in the case that the load current is below the predetermined load current;
wherein the switching on time is a point in time of switching on the first or second lagging leg.

2. The power supply device of claim 1, wherein the control unit includes:
a comparison unit comparing received load state information with a preset reference load state; and
a signal generation unit generating first and second leading leg switching control signals and first and second lagging leg switching control signals and delaying a switching on time of at least one of the first and second lagging leg switching control signals according to a comparison result of the comparison unit.

3. The power supply device of claim 1, wherein the power supply unit includes:
a transformer having a primary winding receiving power switched by the switch of the full bridge and a secondary winding electrically isolated from and magnetically coupled to the primary winding to form a preset winding ratio therewith, transforming the switched power according to the winding ratio and outputting the transformed power; and
an output unit rectifying and stabilizing the transformed power from the secondary winding of the transformer and outputting the stabilized power.

4. The power supply device of claim 3, wherein the output unit includes:
first and second rectifying switches connected to one end and the other end of the secondary winding, respectively; and
an inductor and a capacitor stabilizing the power rectified by the first and second rectifying switches.

5. A power supply device, comprising:
a power supply unit supplying preset DC power by switching input power using a full bridge by a phase shift method and rectifying and stabilizing transformed switching power; and
a control unit controlling a switching time of a switch of the full bridge by determining a load state in which the DC power is received from the power supply unit, based on resonance between the switch of the full bridge and an output capacitor and a parasitic inductor of a rectifying switch;
wherein the switch of the full bridge of the power supply unit includes
first and second leading leg switches; and
first and second lagging leg switches;

wherein the control unit maintains a switching on time of at least one of the first and second lagging leg switches as before, in the case that the load current is above a predetermined load current; and
wherein the control unit delays a switching on time of at least one of the first and second lagging leg switches, in the case that the load current is below the predetermined load current;
wherein the switching on time is a point in time of switching on the first or second lagging leg.

6. The power supply device of claim 5, wherein the control unit includes:
a comparison unit comparing received load state information with a reference load state set according to the resonance between the switch of the full bridge and the rectifying switch; and
a signal generation unit generating first and second leading leg switching control signals and first and second lagging leg switching control signals and delaying a switching on time of at least one of the first and second lagging leg switching control signals according to a comparison result of the comparison unit.

7. The power supply device of claim 5, wherein the power supply unit includes:
a transformer having a primary winding receiving power switched by the switch of the full bridge and a secondary winding electrically isolated from and magnetically coupled to the primary winding to form a preset winding ratio therewith, transforming the switched power according to the winding ratio and outputting the transformed power; and
an output unit rectifying and stabilizing the transformed power from the secondary winding of the transformer and outputting the stabilized power.

8. The power supply device of claim 7, wherein the output unit includes:
first and second rectifying switches connected to one end and the other end of the secondary winding, respectively; and
an inductor and a capacitor stabilizing the power rectified by the first and second rectifying switches.