Multi-output DC/DC converter

This application claims the priority of Korean Patent Application No. 2008-30384 filed on Apr. 1, 2008, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

1. Field of the Invention

The present invention relates to a multi-output DC/DC converter, and more particularly, to a multi-output DC/DC converter capable of controlling power-conversion switching in synchronization with a frequency of one output voltage among multiple output voltages in an LLC resonant DC/DC converter.

2. Description of the Related Art

Power supplies, particularly, DC/DC converters have been commonly used for information devices such as personal computers, and home electronic appliances such as air-conditioners and audio-visual devices.

The DC/DC converter may provide multiple output voltages according to an electronic product to which the DC/DC converter is applied.

A general multi-output DC/DC converter converts an input DC voltage to an AC voltage through a single transformer, and rectifies the converted AC voltage to output multiple DC voltages. However, if a voltage level of one DC voltage among multiple DC voltages is changed by a single transformer, it affects to other DC voltages, causing the general multi-output DC/DC converter to fail to maintain cross-regulation. In order to maintain the cross-regulation, a multi-output DC/DC converter employs a buck chopper circuit at an output terminal.

FIG. 1 is a diagram illustrating a configuration of a related art multi-output DC/DC converter.

Referring to FIG. 1 , in the related art multi-output DC/DC converter, a switching unit 11 converts an input DC voltage Vin into an AC voltage by alternate switching, and sends the converted AC voltage to a primary winding L 1 of a single transformer 12 .

First and second secondary windings L 2 and L 3 respectively send AC voltages to first and second rectification units 13 and 14 depending on the preset ratio of the number of turns of the primary winding L 1 to the corresponding secondary winding. The first and second rectification units 13 and 14 rectify the AC voltages to output DC voltages.

In detail, the first rectification unit 13 outputs a first DC voltage Vs, and the second rectification unit 14 sends a DC voltage to a buck chopper circuit 15 . The buck chopper circuit 15 switches the DC voltage from the second rectification unit 14 under the control of a controller 15 a, and outputs a second DC voltage Va having a lower voltage level than a voltage level of the DC voltage.

The buck chopper circuit 15 includes a switch S, a freewheeling diode D, and a filter L.

However, the related art multi-output DC/DC converter has a limitation of low power conversion efficiency because it performs primary voltage conversion and then re-converts the voltage through the buck chopper circuit 15 . Also, the buck chopper circuit 15 causes an increase in number of components, thereby increasing a manufacturing cost.

An aspect of the present invention provides a multi-output DC/DC converter capable of controlling power-conversion switching in synchronization with a frequency of one output voltage among multiple output voltages without using a buck chopper circuit, which is constructed at an output terminal, in an LLC resonance DC/DC converter.

According to an aspect of the present invention, there is provided a multi-output direct-current (DC)/DC converter including: a power conversion circuit performing alternate switching on an input DC voltage to output multiple DC voltages including a first DC voltage and a second DC voltage each having a preset voltage level; and a control circuit controlling the alternate switching of the power conversion circuit in synchronization with a preset resonant frequency.

The control circuit may include: a frequency control unit controlling the resonant frequency according to a result of comparing a voltage level of a preset first reference voltage with a voltage level of the first DC voltage; a saw-tooth wave generation unit generating a saw-tooth wave according to the resonant frequency from the frequency control unit; a duty control unit comparing the saw-tooth wave from the saw-tooth wave generation unit with a difference between a voltage level of a preset second reference voltage with a voltage level of the second DC voltage to control a switching duty of the power conversion circuit; and a switching control unit providing first and second switching signals for controlling the alternate switching of the power conversion circuit according to the switching duty controlled by the duty control unit.

The frequency control unit may include: a first error amplifier comparing the voltage level of the preset first reference voltage with the voltage level of the first DC voltage to amplify a result of the comparison according to a preset amplification rate; a first resistor setting the amplification rate of the first error amplifier according to a preset resistance value; and a frequency modulator setting the resonant frequency according to the result of the comparison amplified by the first error amplifier.

The duty control unit may include: a second error amplifier comparing the voltage level of the preset second reference voltage with the voltage level of the second DC voltage to amplify a result of the comparison according to a preset amplification rate; a second resistor setting the amplification rate of the second error amplifier according to a preset resistance value; a comparator comparing the saw-tooth wave from the saw-tooth generation unit with the result of the comparison amplified by the second error amplifier; and a duty modulator setting a switching duty of the power conversion circuit according to the result of the comparison from the comparator.

The power conversion circuit may include: a switching unit including first and second switches performing alternate switching under the control of the control unit; first and second conversion units each converting a voltage switched from the switching unit into an AC voltage having a voltage level determined according to a preset ratio of the number of turns in primary and secondary windings; and first and second rectification units rectifying AC voltages from the first and second conversion units to output the first DC voltage and the second DC voltage, respectively.

The first conversion unit may be an inductor-inductor-capacitor (LLC) resonant conversion unit including a resonant capacitor and a resonant inductor connected in series to a power input terminal of the first switch, and a magnetizing inductor connected in parallel to the first switch. The second conversion unit may be an LLC resonant conversion unit including a resonant capacitor and a resonant inductor connected in series to a power input terminal of the second switch, and a magnetizing inductor connected in parallel to the second switch.

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 diagram illustrating a configuration of a related art multi-output DC/DC converter;

FIG. 2 is a diagram illustrating a configuration of a multi-output DC/DC converter according to an exemplary embodiment of the present invention;

FIG. 3 is a diagram illustrating a configuration of a control circuit employed in a multi-output DC/DC converter, according to an exemplary embodiment of the present invention;

FIG. 4 is a waveform diagram illustrating the operation of a multi-output DC/DC converter according to the exemplary embodiment of the present invention; and

FIG. 5 is a diagram illustrating a configuration of a multi-output DC/DC converter according to another exemplary embodiment of the present invention.

Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

FIG. 2 is a diagram illustrating a configuration of a multi-output DC/DC converter according to an exemplary embodiment of the present invention.

Referring to FIG. 2 , a multi-output DC/DC converter according to an exemplary embodiment of the present invention includes a power conversion circuit 100 , and a control circuit 200 . The power conversion circuit 100 alternately switches on and off an input DC voltage (Vin) to output multiple DC voltages including first and second DC voltages (Vo 1 /Vo 2 ). The control circuit 200 controls switching of the power conversion circuit 100 .

The power conversion circuit 100 may include a switching unit 110 , first and second conversion units 120 and 130 , and first and second rectification units 140 and 150 .

The switching unit 110 may include first and second switches M 1 and M 2 . The first and second switches M 1 and M 2 are electrically connected together in series and electrically connected in parallel to an input DC power terminal. The first and second switches M 1 and M 2 perform alternate switching in response to respective switching signals SW 1 and SW 2 from the control circuit 200 . AC voltages (Vds 1 /Vds 2 ) switched from the switching unit 110 are sent to the first and second conversion units 120 and 130 .

The first conversion unit 120 may be an inductor-inductor-capacitor (LLC) resonant conversion unit including a first resonant capacitor Cr 1 and a first resonant inductor Lr 1 connected together in series, and a first magnetizing inductor Lm 1 connected in parallel to the first switch M 1 . The second conversion unit 130 may also be an LLC resonant conversion unit including a second resonant capacitor Cr 2 and a second resonant inductor Lr 2 connected together in series, and a second magnetizing inductor Lm 2 connected in parallel to the second switch M 2 . The first conversion unit 120 may further include a transformer including a primary winding L 1 and a secondary winding L 2 having a preset ratio of the number of turns of the primary winding L 1 to the secondary winding L 2 . The second conversion unit 130 may also include a transformer including a primary winding L 3 and a secondary winding L 4 having a preset ratio of the number of turns in the primary and secondary windings L 3 and L 4 . Each of the first and second magnetizing inductors Lm 1 and Lm 2 may be formed only as leakage inductance of corresponding one of the transformers.

The AC voltages switched from the switching unit 110 are converted into AC voltages having voltage levels according to the preset ratio of the number of turns of each of the first and second conversion units 120 and 130 . The converted AC voltages are sent to the first and second rectification units 140 and 150 .

The first and second rectification units 140 and 150 rectify the AC voltages from the first and second conversion units 120 and 130 , and output first and second DC voltages Vo 1 and Vo 2 , respectively. A rectification device of each of the first and second rectification units 140 and 150 may include at least one diode D 5 for half-wave rectification, or a bridge diode including a plurality of diodes D 1 to D 4 for full-wave rectification. The respective first and second DC voltages Vo 1 and Vo 2 from the first and second rectification units 140 and 150 are sent to the control circuit 200 .

A power conversion circuit employed in a multi-output DC/DC converter according to the present invention may include the first and second conversion units 120 and 130 each including one transformer, but may include a conversion unit including one transformer to reduce a circuit area (see 120 ′ of FIG. 5 ).

FIG. 5 is a diagram illustrating a configuration of a multi-output DC/DC converter according to another exemplary embodiment of the present invention.

Referring to FIG. 5 , a power conversion circuit 100 ′ of the multi-output DC/DC converter may include a power conversion circuit 100 ′ including a switching unit 110 ′, a conversion unit 120 ′, and first and second rectification units 130 ′ and 140 ′. The switching unit 110 ′ and the first and second rectification units 130 ′ and 140 ′ illustrated in FIG. 5 have identical electrical functions to those of the switching unit 110 and the first and second rectification units 140 and 150 illustrated in FIG. 2 . Therefore, the description thereof will be omitted.

The conversion unit 120 ′ of the multi-output DC/DC converter of FIG. 5 includes one transformer. Thus, the transformer includes one primary winding L 1 , and first and second secondary windings L 2 and L 3 . The first secondary winding L 2 sends the first rectification unit 130 ′ an AC voltage converted according to the preset ratio of the number of turns of the primary winding L 1 to the first secondary winding L 2 . The second secondary windings L 3 sends the second rectification units 140 ′ an AC voltage converted according to the ratio of the number of turns of the primary winding L 1 to the second secondary winding L 3 . The conversion unit 120 ′ may also be an LLC resonant conversion unit including a resonant capacitor Cr and a resonant inductor Lr connected together in series, and a magnetizing inductor Lm connected in parallel to the first switch M 1 . The magnetizing inductor Lm may be formed only as leakage inductance of the transformer.

The control circuit 200 includes a frequency control unit 210 receiving the first DC voltage Vo 1 , a saw-tooth wave generation unit 220 , a duty control unit 230 receiving the second DC voltage Vo 2 , and a switching control unit 240 . A detail configuration of the control circuit 200 will now be described with reference to FIG. 3 in detail.

FIG. 3 is a diagram illustrating a configuration of a control circuit employed in a multi-output DC/DC converter, according to an exemplary embodiment of the present invention.

FIG. 3 shows the detailed configuration of the control circuit 200 employed in the multi-output DC/DC converter, according to an exemplary embodiment of the present invention.

The frequency control unit 210 of the control circuit 200 includes a first error amplifier 211 amplifying a difference between a voltage level of a first DC voltage Vo 1 and a voltage level of a preset first reference voltage Vref 1 , a first resistor 212 setting a rate of error amplification of the first error amplifier 211 according to a preset resistance value, and a frequency modulator 213 setting a frequency according to a comparison result from the first error amplifier 211 . A frequency signal from the frequency modulator 213 is sent to the saw-tooth wave generation unit 220 .

The saw-tooth wave generation unit 220 generates a saw-tooth wave synchronized with a frequency signal from the frequency modulator 213 . The saw-tooth wave is sent to the duty control unit 230 .

The duty control unit 230 includes a second error amplifier 231 , a second resistor 232 , a comparator 233 , and a duty modulator 234 . The second error amplifier 231 amplifies a difference between a voltage level of a second DC voltage Vo 2 and a voltage level of a preset second reference voltage Vref 2 . The second resistor 232 sets a rate of error amplification of the second error amplifier 231 according to a preset resistance value. The comparator 233 compares a voltage level of the saw-tooth wave from the saw-tooth wave generation unit 220 with a voltage level of a comparison result from the second error amplifier 231 . The duty modulator 234 sets a switching duty according to a comparison result from the comparator 223 . A duty signal from the duty modulator 234 is sent to the switching control unit 240 .

The switching control unit 240 sends first and second switching signals SW 1 and SW 2 controlling switching of the first and second switches M 1 and M 2 according to the duty signal from the duty modulator 234 .

FIG. 4 is a waveform diagram illustrating the operation of a multi-output DC/DC converter of FIG. 2 .

Referring to FIGS. 2 through 4 , the detailed operation of the multi-output DC/DC converter according to the exemplary embodiment of the present invention will now be described.

The first and second switches M 1 and M 2 are alternately switched on and off under the control of the control circuit 200 , and thus are respectively operated at duty cycles of D and 1 -D. Thus, when the first and second switches M 1 and M 2 are in an OFF state, voltages Vds 1 and Vds 2 are respectively applied to the first and second switches M 1 and M 2 .

A charging voltage of the first resonant capacitor Cr 1 is adjusted through the alternate switching of the first and second switches M 1 and M 2 , thereby controlling a voltage being applied to the primary winding L 1 of the first conversion unit 120 . Thus, a first DC voltage Vo 1 is formed through the secondary winding L 2 of the first conversion unit 120 and the diodes D 1 and D 4 or D 2 and D 3 of the first rectification unit 140 .

The first DC voltage Vo 1 is precisely formed through the frequency control unit 210 modulating a switching frequency.

The charging voltage of the second resonant capacitor Cr 2 is adjusted through the alternate ON/OFF switching of the first and second switches M 1 and M 2 , thereby controlling a voltage being applied to the primary winding L 3 of the second conversion unit 130 . Thus, a second DC voltage Vo 2 is formed through the secondary winding L 4 and the diode D 5 of the second rectification unit 150 .

The multi-output DC/DC converter according to the exemplary embodiment of the present invention includes the frequency control unit 210 controlling an output voltage based on the first DC voltage Vo 1 , and the duty control unit 230 controlling an output voltage based on the second DC voltage Vo 2 .

The duty control unit 230 performs pulse-width modulation in synchronization with a switching frequency of the frequency control unit 210 changed based on the first DC voltage Vo 1 .

In the frequency control unit 210 of the control circuit 200 , the first error amplifier 211 compares a voltage level of a preset first reference voltage Vref 1 with a voltage level of a first DC voltage Vo 1 divided by a preset resistance value. Thereafter, the frequency modulator 213 of the first control unit 210 sends the saw-tooth wave generation unit 220 a frequency signal having a frequency being varied according to a comparison result from the first error amplifier 211 .

The saw-tooth wave generation unit 220 forms a saw-tooth wave synchronized with the variable frequency of the frequency signal. In the duty control unit 230 , the second error amplifier 231 compares a voltage level of a preset second reference voltage Vref 2 with a voltage level of a second DC voltage Vo 2 . Thereafter, the comparator 233 of the duty control unit 230 compares the saw-tooth wave from the saw-tooth wave generation unit 220 with an error result from the second error amplifier 231 . Then, the duty modulator 234 of the duty control unit 230 forms a duty signal and sends it to the switching control unit 240 . The switching control unit 240 sends first and second switching signals SW 1 and SW 2 to the switching unit 110 according to the duty signal, thereby controlling switching of the first and second switches M 1 and M 2 .

In more detail, a charging voltage of the first resonant capacitor Cr 1 is adjusted through the alternate ON/OFF switching of the first and second switches M 1 and M 2 to control a voltage being applied to the primary winding L 1 of the first conversion unit 120 . Accordingly, the first DC voltage Vo 1 is formed through the secondary winding L 2 of the first conversion unit 120 and the diodes D 1 and D 4 or D 2 and D 3 of the first rectification unit 140 .

If the voltage level of the first DC voltage Vo 1 increases, a switching frequency is increased through the frequency control unit 210 modulating the switching frequency, thereby reducing the charging voltage of the first resonant capacitor Cr 1 . Accordingly, the voltage being applied to the primary winding L 1 of the first conversion unit 120 is reduced, which causes a decrease in voltage formed at both ends of the secondary winding L 2 . Therefore, the increased voltage level of the first DC voltage Vo 1 is reduced and maintained at a normal state.

The first rectification unit 140 may include a DC blocking capacitor Cb. The DC blocking capacitor Cb is used in order to prevent a resonant current generated at the first conversion unit 120 from passing through a path just for the diodes D 1 and D 4 or D 2 and D 3 of the first rectification unit 140 when asymmetry of a duty generated based on the second DC voltage Vo 2 occurs

When the voltage level of the second DC voltage Vo 2 changes because of a change in load of the second DC voltage Vo 2 , a duty-ON cycle of a switch directly associated with the second DC voltage Vo 2 is controlled. An operating duty D, which is directly associated with the second DC voltage Vo 2 is applied to the first switch M 1 . If the voltage level of the second DC voltage Vo 2 increases, the operating duty D is reduced because of the pulse-width modulation control. For this reason, the first DC voltage Vo 1 is affected to change a gain curve, so that the voltage level of the first DC voltage Vo 1 increases.

As the voltage level of the first DC voltage increases, the switching frequency increases because of the control of the frequency control unit 210 . The increase in the switching frequency induces voltage drop of the second DC voltage Vo 2 . Accordingly, the first and second DC voltages Vo 1 and Vo 2 having desired voltage levels are obtained simultaneously.

The operation of the multi-output DC/DC converter according to the embodiment of FIG. 5 is the same as the operation illustrated in FIG. 4 , except for a current Ipri 2 flowing to the primary winding L 3 of the second conversion unit 130 . Thus, the description thereof will be omitted.

As described so far, frequency control and duty control synchronized with the frequency control can be performed simultaneously at the time of power conversion. Accordingly, a buck chopper circuit used in a related art multi-output DC/DC converter is unnecessary. Thus, power loss due to switching loss and conduction loss of the buck chopper circuit can be reduced. A manufacturing cost corresponding to the number of components of the buck chopper circuit can be saved.

When the multi-output DC/DC converter according to the embodiment of the present invention uses a single transformer as in the related art, the same circuit area and the same number of components are required as in the related art multi-output DC/DC converter. Also, when the multi-output DC/DC converter according to the embodiment of the present invention uses two transformers, the power can be handled separately by the two transformers whereas in the related art, the entire power is handled by a single transformer. Accordingly, miniaturization of each transformer can be achieved. In the case of the control circuit 200 , the aforementioned functions thereof can be integrated into one integrated circuit (IC). Thus, the number of components and the manufacturing cost are prevented from increasing.

According to the present invention, the multi-output DC/DC converter according to the embodiments of the present invention does not use a buck chopper circuit to maintain cross regulation between multiple output voltages. Also, alternate switching for power conversion is controlled in synchronization with a first DC voltage among multiple output voltages, thereby increasing the efficiency of the power conversion. Also, the manufacturing costs can be saved by omitting the buck chopper circuit and thus reducing the number of components being used therein.

While the present invention has been shown and described in connection with the exemplary 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.