Power supply apparatus

A PFC circuit generates substantially sinusoidal current that is in-phase with AC input voltage to compensate for a power factor. A DC-DC converter converts the voltage of a signal outputted from the PFC circuit to determined voltage. A current change detection section detects an amount of change in current outputted from the DC-DC converter, compares the detected amount of change in current with a threshold set in advance, and if the detected amount of change in current is larger than the threshold, outputs a gain switching signal for raising the gain of voltage outputted from the PFC circuit. The current change detection section outputs a voltage control signal for raising the voltage outputted from the PFC circuit to a determined value. A voltage setting section sets the voltage outputted from the PFC circuit to the determined value on the basis of the voltage control signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-240127, filed on Nov. 20, 2013, the entire contents of which are incorporated herein by reference.

FIELD

The embodiment discussed herein is related to a power supply apparatus.

BACKGROUND

In order to suppress harmonics of an input signal, usually a PFC (Power Factor Correction) circuit is used in a switching power supply which performs AC (Alternating Current)-DC (Direct Current) conversion.

A PFC circuit generates current which is in-phase with AC input voltage and which is substantially sinusoidal. By doing so, the PFC circuit suppresses harmonics and improves a power factor.

Formerly the following technique was proposed. If a detected value of current outputted from a DC-DC converter which converts voltage outputted from a PFC circuit to determined voltage is smaller than a set value, then the operation of the PFC circuit is stopped.

If there is a load fluctuation, such as a sudden increase in load current, during the operation of a switching power supply, voltage outputted from a PFC circuit may fall and become smaller than a determined value. This causes a decrease in the efficiency of the switching power supply.

SUMMARY

According to an aspect, there is provided a power supply apparatus including a PFC circuit configured to generate current that is in-phase with AC input voltage and that is substantially sinusoidal to compensate for a power factor, a DC-DC converter configured to convert voltage of a signal outputted from the PFC circuit to determined voltage, and a current change detection section configured to detect an amount of a change in current outputted from the DC-DC converter and output, at the time of the detected amount of the change in current being larger than a threshold, a gain switching signal for raising gain of voltage outputted from the PFC circuit.

DESCRIPTION OF EMBODIMENTS

Embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals refer to like elements throughout.FIG. 1illustrates an example of the structure of a power supply apparatus. A power supply apparatus1includes a PFC circuit1a, a DC-DC converter1b, a current change detection section1c, and a voltage setting section1d.

The PFC circuit1agenerates current which is in-phase with AC input voltage and which is substantially sinusoidal to compensate for a power factor. The DC-DC converter1bconverts the voltage of a signal outputted from the PFC circuit1ato determined voltage.

The current change detection section1cdetects an amount of a change in current outputted from the DC-DC converter1band compares the detected amount of the change in current with a threshold set in advance. If the amount of the change in current is larger than the threshold, then the current change detection section1coutputs a gain switching signal for raising the gain of voltage outputted from the PFC circuit1a, and feeds back the gain switching signal to the PFC circuit1a.

Furthermore, the current change detection section1coutputs a voltage control signal for raising the voltage outputted from the PFC circuit1ato a determined value. The voltage setting section1dsets voltage outputted from the PFC circuit1ato a determined value on the basis of a voltage control signal.

As has been described, with the power supply apparatus1an amount of a change in current outputted from the DC-DC converter1blocated at a stage behind the PFC circuit1ais detected. If the amount of the change in current is larger than the threshold, then control is exercised for raising the gain of the PFC circuit1aby a certain amount. This checks a decrease in the efficiency of a switching power supply caused at the time of load fluctuations.

The contents of the operation of a PFC circuit and problems to be solved will now be described by the use ofFIGS. 2 through 7before the details of an embodiment will be described. First the cause of the generation of harmonics will be described by the use ofFIGS. 2 and 3.

FIG. 2illustrates an example of the structure of a rectifying and smoothing circuit. A rectifying and smoothing circuit100indicates an input section of a switching power supply not including a PFC circuit, and includes a full-wave rectifier101and a smoothing capacitor C0.

The full-wave rectifier101is formed of a diode bridge including diodes D1through D4. Components, such as a DC-DC converter, located at stages behind the rectifying and smoothing circuit100are not illustrated inFIG. 2.

Each element is connected in the following way. A positive terminal (+) of an input power supply VI (commercial power supply) is connected to an anode of the diode D1and a cathode of the diode D2. A negative terminal (−) of the input power supply VI is connected to an anode of the diode D3and a cathode of the diode D4.

A cathode of the diode D1is connected to a cathode of the diode D3, one end of the capacitor C0, and one end of a load Rm. An anode of the diode D2is connected to an anode of the diode D4, the other end of the capacitor C0, and the other end of the load Rm. The load Rm corresponds to, for example, an information processing apparatus such as a server.

The operation of the rectifying and smoothing circuit100is as follows. The full-wave rectifier101full-wave-rectifies AC voltage Vin inputted from the input power supply VI. The smoothing capacitor C0smooths a signal outputted from the full-wave rectifier101.

It is possible to make AC-DC conversion by the use of these basic components included in the rectifying and smoothing circuit100. However, if only these components are used, the waveform of current Iin is distorted at the time of applying the AC voltage Vin.

FIG. 3illustrates harmonics. InFIG. 3, a vertical axis indicates voltage and current and a horizontal axis indicates time. In the rectifying and smoothing circuit100, the AC voltage Vin inputted from the input power supply VI is rectified by the full-wave rectifier101and is applied to the smoothing capacitor C0.

Current does not flow to the smoothing capacitor C0during a period during which the AC voltage Vin is lower than terminal voltage of the smoothing capacitor C0. When the AC voltage Vin rises and exceeds the terminal voltage of the smoothing capacitor C0, charging the smoothing capacitor C0begins and current flows to the smoothing capacitor C0.

As a result, as illustrated inFIG. 3, the waveform of the AC voltage Vin looks like a sine wave. However, the waveform of input current Iin1is not a sine wave and is distorted. Time during which the input current Iin1flows is short compared with time during which the AC voltage Vin is applied. In addition, a peak value of the input current Iin1is high and the waveform of the input current Iin1is pointed.

The waveform of the input current Iin1includes a component whose frequency is higher than the frequency (which is usually 50 or 60 Hz) of a fundamental wave and whose frequency is integer times as high as the frequency of the fundamental wave. A component whose frequency is integer times as high as the frequency of the fundamental wave is referred to as a harmonic.

As stated above, if the structure of the rectifying and smoothing circuit100is adopted, the waveform of input current is distorted and discontinuous harmonics are generated. Furthermore, if a harmonic having a frequency integer times 50 or 60 Hz flows to a commercial power supply, power transmission and distribution equipment or the like may be damaged.

The operation of a PFC circuit for improving a power factor will now be described by the use ofFIGS. 4 and 5. If the difference in phase between AC voltage and current is Φ, then a power factor is given by cos Φ (alternatively, a power factor may be defined as the ratio of effective power to apparent power).

If both of the waveform of voltage and the waveform of current are sine waves, then the difference in phase between them is 0. As a result, a power factor is 1 and an ideal state is realized. On the other hand, as the waveform of current is distorted with respect to the waveform of voltage, a power factor becomes smaller than 1. Therefore, making a power factor approach 1 leads to suppressing harmonic current.

FIG. 4illustrates an example of the structure of a rectifying and smoothing circuit. A rectifying and smoothing circuit100aindicates an input section of a switching power supply including a PFC circuit, and includes a full-wave rectifier101, a PFC circuit102, and a smoothing capacitor C0.

The PFC circuit102is a step-up PFC circuit including a step-up coil L0, a switching element S0, a diode D0, and a control IC (Integrated Circuit)102a. Furthermore, an n-channel MOSFET (Metal-Oxide Semiconductor Field Effect Transistor) is used as the switching element S0. Components, such as a DC-DC converter, located at stages behind the rectifying and smoothing circuit100aare not illustrated inFIG. 4.

Each element is connected in the following way. A positive terminal (+) of an input power supply VI is connected to an anode of a diode D1and a cathode of a diode D2. A negative terminal (−) of the input power supply VI is connected to an anode of a diode D3and a cathode of a diode D4.

A cathode of the diode D1is connected to a cathode of the diode D3and one end of the step-up coil L0. The other end of the step-up coil L0is connected to a drain of the switching element S0and an anode of the diode D0.

A cathode of the diode D0is connected to one end of the capacitor C0and one end of a load Rm. An anode of the diode D2is connected to an anode of the diode D4, a source of the switching element S0, the other end of the capacitor C0, and the other end of the load Rm.

The control IC102ais connected to a gate of the switching element S0and outputs a switching control signal for turning on or off the switching element S0.

FIG. 5illustrates a state in which harmonics are suppressed. InFIG. 5, a vertical axis indicates voltage and current and a horizontal axis indicates time. In the PFC circuit102, high-speed switching of the switching element S0is performed to control the amount of current which flows from the diode D0to the smoothing capacitor C0.

By incorporating the above PFC circuit102, current Iin2which is in-phase with AC voltage Vin and which is substantially sinusoidal is generated. This makes it possible to suppress harmonics and improve a power factor.

There is a restriction on harmonic current for all power supplies which output 75 watts or more of power. The value of harmonic current to be suppressed by PFC circuits is provided by IEC 61000-3-2 of IEC (International Electrotechnical Commission).

Problems to be solved by the embodiment will now be described by the use ofFIGS. 6 and 7.FIG. 6indicates a change in the efficiency of a switching power supply. InFIG. 6, a vertical axis indicates efficiency (%) and a horizontal axis indicates output power (W). Furthermore, graphs k1through k4indicate cases where voltage outputted from a PFC circuit is 370 V, 387 V, 400 V, and 420 V respectively.

One of the characteristics of a switching power supply is efficiency. Efficiency is an index which indicates how input power is utilized without being wasted (how much input power is not transformed into heat in an apparatus but is conducted as output power), and is expressed by the ratio of output power to input power. Accordingly, high efficiency means that the amount of heat generated is small and that wasteful transformation of electric power into heat is small.

As illustrated inFIG. 4, usually a PFC circuit is a step-up circuit. With a step-up PFC circuit an increase in output voltage causes an increase in switching current which flows through a FET that is a switching element.

In this case, an increase in switching current leads to a great loss (large amount of heat generated), so the efficiency of a switching power supply decreases. Accordingly, in order to check a decrease in the efficiency of the switching power supply, voltage outputted from the PFC circuit is decreased and switching current is decreased.

InFIG. 6, the same output power is generated in the cases which is indicated by the graphs k1through k4and in which voltage outputted from the step-up PFC circuit is 370 V, 387 V, 400 V, and 420 V respectively. As can be seen fromFIG. 6, as voltage outputted from the step-up PFC circuit increases, the efficiency decreases. As voltage outputted from the step-up PFC circuit decreases, the efficiency increases.

As indicated inFIG. 6, lower voltage outputted from the PFC circuit leads to the improvement of the efficiency of the switching power supply. Accordingly, it is desirable to make voltage outputted from the PFC circuit as low as possible in a determined range, specifically, in a range of voltage inputted to a DC-DC converter located at a stage behind the PFC circuit.

FIG. 7indicates a fall in output voltage at the time of load fluctuations. InFIG. 7, a vertical axis indicates voltage (V) outputted from a PFC circuit and a horizontal axis indicates time (sec). A range H indicates a range of voltage inputted to a DC-DC converter located at a stage behind the PFC circuit.

InFIG. 7, the input voltage range H is 340 to 420 V. Input voltage in this range meets the operation specification of the DC-DC converter and the efficiency of a switching power supply does not decrease.

A graph g1indicates a change in output voltage at the time of operating the switching power supply with output voltage from the PFC circuit initially set to 400 V in the input voltage range H for the DC-DC converter.

In addition, a graph g2indicates a change in output voltage at the time of operating the switching power supply with output voltage from the PFC circuit initially set to 370 V in the input voltage range H for the DC-DC converter.

In both of the cases of the graphs g1and g2, in the beginning there are no load fluctuations. At time ta load fluctuations, such as a sudden increase in load current, take place. In the case of the graph g1, output voltage set to 400 V falls to 350 V at the time ta at which the load fluctuations take place. However, even when output voltage falls to 350 V, output voltage is in the input voltage range H, that is to say, in a tolerance range.

In the case of the graph g2, on the other hand, output voltage set to 370 V falls to a value close to 320 V at the time ta at which the load fluctuations take place. This value close to 320 V is lower than the lower limit value 340 V of the input voltage range H. Output voltage from the PFC circuit (input voltage to the DC-DC converter) which is lower than the lower limit value of the tolerance range does not meet the operation specification of the DC-DC converter.

As stated above, when the switching power supply is operated with output voltage from the PFC circuit set to a low value in the input voltage range H for the DC-DC converter, the output voltage from the PFC circuit may become lower than the lower limit value of the tolerance range due to sudden load fluctuations. This decreases the efficiency of the switching power supply.

The embodiment described below is given in view of the above problems. There is provided a power supply apparatus which checks a decrease in the efficiency at the time of load fluctuations.

A power supply apparatus according to an embodiment will now be described in detail.FIG. 8illustrates an example of the structure of a power supply apparatus. A power supply apparatus1-1includes an EMI (ElectroMagnetic Interference) and rectifying circuit10, a PFC circuit20, a smoothing capacitor C0, a DC-DC converter30, a current change detection circuit40, and a voltage setting circuit50.

In addition, the PFC circuit20is a step-up PFC circuit and includes resistors R1and R2, a step-up coil L0, a switching element S0, a diode D0, and a PFC control circuit20a. Furthermore, the PFC control circuit20aincludes an input current control circuit21, an output voltage control circuit22, and a PWM (Pulse Width Modulation) circuit23.

Connections around the passive elements are made in the following way. One end of the resistor R1is connected to one output end of the EMI and rectifying circuit10and one end of the coil L0. One end of the resistor R2is connected to the other end of the resistor R1and an input end (input voltage detection terminal) of the input current control circuit21. The other end of the resistor R2is connected to GND.

The other end of the coil L0is connected to a drain of the switching element S0and an anode of the diode D0. A cathode of the diode D0is connected to one end of the capacitor C0, an input end of the voltage setting circuit50, and one input end of the DC-DC converter30.

A gate of the switching element S0is connected to an output end (switching control signal output terminal) of the PWM circuit23. A source of the switching element S0is connected to the other output end of the EMI and rectifying circuit10, an input end (input current detection terminal) of the input current control circuit21, the other end of the capacitor C0, and the other input end of the DC-DC converter30.

The EMI and rectifying circuit10full-wave-rectifies AC voltage Vin inputted from an input power supply VI and reduces noise caused by EMI. The PFC circuit20generates current which is in-phase with the AC voltage Vin and which is substantially sinusoidal to compensate for a power factor.

The smoothing capacitor C0smooths a voltage signal outputted from the PFC circuit20. The DC-DC converter30converts direct-current voltage outputted from the smoothing capacitor C0to determined voltage and supplies it to a load Rm.

The current change detection circuit40monitors current (DC-DC output current) outputted from the DC-DC converter30, detects an amount of change in the current, and generates a gain switching signal and a voltage control signal on the basis of a detection result. The voltage setting circuit50sets a voltage value to be outputted from the PFC circuit20on the basis of the voltage control signal.

The input current control circuit21detects input voltage Vi and input current Iin via the EMI and rectifying circuit10and generates a control signal c1for generating current which is in-phase with the AC voltage Vin and which is substantially sinusoidal.

The output voltage control circuit22sets gain on the basis of the gain switching signal by, for example, switching resistance division ratio, and generates a control signal c2for making voltage outputted from the PFC circuit20equal to the voltage value set by the voltage setting circuit50. The PWM circuit23outputs a switching control signal c3on the basis of the control signals c1and c2to exercise switching control (on-off control) of the switching element S0.

The degree of control exercised in the output voltage control circuit22over the differential between a target value and an actual value is determined by its gain. As the gain becomes higher, capability to correct deviation from the target value improves. Accordingly, if the gain of, for example, the output voltage control circuit22is high, the deviation of output voltage from the target value is corrected immediately and the output voltage is set to the correct value.

The operation of the power supply apparatus1-1will now be described by the use of timing charts.FIG. 9is a timing chart of the operation of the power supply apparatus.

It is assumed that a load fluctuation causes an increase in load current. At this time output current from the DC-DC converter30also increases. The current change detection circuit40recognizes the increase in load current by monitoring the output current from the DC-DC converter30.

(S1) The current change detection circuit40detects an amount of a change (di/dt) in the output current from the DC-DC converter30in the interval from the time t1to the time t2. The current change detection circuit40then compares the detected amount of the change in the output current with a threshold Th set in advance.

(S2) If the amount of the change in the output current is larger than the threshold Th, then the current change detection circuit40outputs a gain switching signal for increasing gain currently set in the PFC circuit20by a certain amount.

In this case, the current change detection circuit40continues outputting a gain switching signal for a time longer by time d1than the interval during which the DC-DC output current is changing, that is to say, until time (t2+d1). Gain is increased by the certain amount in the PFC circuit20while the gain switching signal is being applied.

(S3) The increase in load current leads to a drop in output voltage from the PFC circuit20. However, the gain switching signal is being supplied from the current change detection circuit40to the PFC circuit20. Accordingly, output control is exercised by increasing gain.

As a result, even when the increase in load current takes place, the drop in output voltage from the PFC circuit20is limited to ΔV. Even when initial output voltage V0from the PFC circuit20decreases by the voltage ΔV, gain is increased in the PFC circuit20so that output voltage from the PFC circuit20will fall within the input voltage range H for the DC-DC converter30.

As stated above, if an amount of a change in the DC-DC output current is larger than the threshold, then the current change detection circuit40recognizes that a load fluctuation has taken place, and outputs a gain switching signal to control a drop in output voltage from the PFC circuit20caused by the load fluctuation.

This reliably checks a drop in voltage caused by the initial load fluctuation (first load fluctuation) which takes place after a state in which there is no load fluctuation. Accordingly, a decrease in the efficiency of the switching power supply caused by this load fluctuation is checked.

There is no load fluctuation and the increase in load current stops. Accordingly, the increase in the output current from the DC-DC converter30also stops and the output current from the DC-DC converter30is constant.

(S4) The current change detection circuit40discontinues outputting the gain switching signal at the time (t2+d1), and returns gain in the PFC circuit20to an original value.

(S5) The current change detection circuit40outputs a voltage control signal to the PFC circuit20at the time (t2+d1). The voltage control signal is used for making the PFC circuit20output voltage at a determined value Vr.

(S6) The output voltage from the PFC circuit20is rising to the determined value Vr during the interval from the time t2to the time (t2+d1).

It is assumed that a load fluctuation causes a decrease in load current and that the output current from the DC-DC converter30decreases.

(S7) The current change detection circuit40detects an amount of a change (di/dt) in the output current from the DC-DC converter30in the interval from the time t3to the time t4. The current change detection circuit40then compares the detected amount of the change in the output current with the threshold Th set in advance.

(S8) If the amount of the change in the output current is not larger than the threshold Th, then the current change detection circuit40does not output a gain switching signal.

(S9) The current change detection circuit40sets the output voltage from the PFC circuit20to the determined value Vr during the interval from the time (t2+d1) to the time t5by applying the voltage control signal to the PFC circuit20.

As stated above, the current change detection circuit40discontinues outputting a gain switching signal. Taking this opportunity, the current change detection circuit40outputs a voltage control signal for a certain period of time for raising the output voltage from the PFC circuit20to a determined value. This controls a decrease in the output voltage from the PFC circuit20caused by a load fluctuation (second load fluctuation) which may take place in the certain period of time.

As a result, a decrease in the output voltage from the PFC circuit20caused by the load fluctuation (second load fluctuation) which may take place after the initial load fluctuation (first load fluctuation) which takes place after a state in which there is no load fluctuation is also checked effectively.

FIG. 10is a timing chart of the operation of the power supply apparatus.FIG. 10indicates an interval TO ofFIG. 9.

In the interval from the time t6to the time t7, it is assumed that a load fluctuation causes an increase in load current and that the output current from the DC-DC converter30also increases. Furthermore, in the interval of the time t8to t9, it is assumed that a load fluctuation causes a decrease in load current and that the output current from the DC-DC converter30also decreases. It is assumed that a time interval between the time t7and the time t8after the increase in load current before the decrease in load current is shorter than a time interval between the time t2and the time t3indicated inFIG. 9.

(S11) The current change detection circuit40detects an amount of a change (di/dt) in the output current from the DC-DC converter30in the interval from the time t6to the time t7and compares the detected amount of the change in the output current with the threshold Th set in advance.

(S12) The amount of the change in the output current from the DC-DC converter30in the interval from the time t6to the time t7is larger than the threshold Th, so the current change detection circuit40outputs a gain switching signal to the PFC circuit20. In this case, the current change detection circuit40continues outputting a gain switching signal for a time longer by the time d1than the interval during which the output current from the DC-DC converter30is changing.

(S13) The current change detection circuit40detects an amount of a change (di/dt) in the output current from the DC-DC converter30in the interval from the time t8to the time t9and compares the detected amount of the change in the output current with the threshold Th set in advance.

(S14) The amount of the change in the output current from the DC-DC converter30in the interval from the time t8to the time t9is larger than the threshold Th, so the current change detection circuit40outputs a gain switching signal to the PFC circuit20. In this case, the current change detection circuit40continues outputting a gain switching signal for a time longer by the time d1than the interval during which the output current from the DC-DC converter30is changing.

The gain switching signals are being supplied from the current change detection circuit40to the PFC circuit20. Accordingly, output control is exercised by increasing gain. In this case, the width of the gain switching signals is longer by the time d1than the interval during which the output current from the DC-DC converter30is changing.

If the width of the gain switching signals is equal to the interval during which the output current from the DC-DC converter30is changing, the following problem arises. As indicated inFIG. 10, an increase and a decrease in load current may take place in a short period of time. In such a case, the gain switching signals are comb-like high-frequency pulses. However, gain control exercised by the use of high-frequency pulses may cause a circuit malfunction or noise.

Therefore, in this embodiment a gain switching signal whose width is longer by a certain time than an interval during which the output current from the DC-DC converter30is changing is generated and outputted. As indicated inFIG. 10, an increase and a decrease in load current may take place in a short period of time. Even in such a case, it is possible to exercise gain control while stabilizing the operation of the PFC circuit20.

The structure of the current change detection circuit40will now be described.FIG. 11illustrates an example of the structure of the current change detection circuit. The current change detection circuit40includes a differential circuit41, a peak hold circuit42, a comparator43, a reference power supply Vref, expansion circuits44aand44b, and an edge detection circuit45. One-shot circuits (monostable multivibrators) or the like may be used as the expansion circuits44aand44b.

The differential circuit41includes a capacitor C11and a resistor R11. The peak hold circuit42includes a diode D11, a capacitor C12, and a resistor R12.

Each element is connected in the following way. Current outputted from the DC-DC converter30is inputted to one end of the capacitor C11. The other end of the capacitor C11is connected to one end of the resistor R11and an anode of the diode D11. The other end of the resistor R11is connected to GND.

A cathode of the diode D11is connected to one end of the capacitor C12, one end of the resistor R12, and an input terminal (+) of the comparator43. The other end of the capacitor C12and the other end of the resistor R12are connected to GND.

An input terminal (−) of the comparator43is connected to a positive-side power supply terminal of the reference power supply Vref and a negative-side power supply terminal of the reference power supply Vref is connected to GND. An output terminal of the comparator43is connected to an input end of the expansion circuit44a.

An output end of the expansion circuit44ais connected to an input end of the edge detection circuit45. Furthermore, a gain switching signal is outputted from the output end of the expansion circuit44a. An output end of the edge detection circuit45is connected to an input end of the expansion circuit44b. A voltage control signal is outputted from an output end of the expansion circuit44b.

The differential circuit41differential-filters (high-pass-filters) DC-DC output current to generate a current change signal. The peak hold circuit42holds a peak value of the current change signal for a certain period of time and outputs a peak hold signal.

The comparator43compares the peak hold signal with reference voltage and outputs an H-level signal for an interval for which the level of the peak hold signal is higher than the level of the reference voltage. The width of the H-level signal outputted from the comparator43is expanded by a certain time by the expansion circuit44aand is then outputted as a gain switching signal.

The edge detection circuit45detects the falling of the gain switching signal and outputs a pulse signal at the time of the falling of the gain switching signal. The expansion circuit44bexpands the width of the pulse signal and outputs a pulse signal with required width as a voltage control signal.

In the above description, the current change detection circuit40is made up of analog circuits. However, the current change detection circuit40may be realized by firmware. That is to say, a microcomputer or the like may be used and programs for differential filtering, a threshold comparison process, and the like may be generated.

The structure of the voltage setting circuit50will now be described.FIG. 12illustrates an example of the structure of a voltage setting circuit. A voltage setting circuit50aincludes a photocoupler51for insulation transmission, a switching element Sw1, and resistors R3through R6.

The DC-DC converter30includes a transformer and a primary side and a secondary side of the transformer are electrically insulated from each other. Accordingly, the photocoupler51is used to perform signal transmission while maintaining insulation.

A voltage control signal is inputted to an input end of the photocoupler51. One end of the resistor R3is pulled up and the other end of the resistor R3is connected to a gate of the switching element Sw1and one output end of the photocoupler51.

One end of the resistor R5is connected to the cathode of the diode D0, one end of the capacitor C0, and one input end of the DC-DC converter30. The other end of the resistor R5is connected to an input end of the PFC control circuit20a, one end of the resistor R4, and one end of the resistor R6.

The other end of the resistor R4is connected to a drain of the switching element Sw1. The other end of the resistor R6is connected to GND, a source of the switching element Sw1, and the other output end of the photocoupler51.

When a voltage control signal outputted from the current change detection circuit40is inputted to the photocoupler51, the photocoupler51converts the inputted voltage control signal into light. A light receiving element is driven by the light and continuity takes place. As a result, an H-level signal is outputted. When a voltage control signal is not inputted to the photocoupler51, continuity does not take place in the photocoupler51.

FIG. 13illustrates voltage division by resistors.FIG. 13illustrates two-stage voltage division by resistors in the voltage setting circuit50a. The switching element Sw1is in an OFF state for a disable interval for which a voltage control signal is not outputted from the current change detection circuit40.

In this case, output voltage V1from the PFC circuit20is divided by the resistors R5and R6and voltage obtained by the division is set voltage. That is to say, set voltage Vs obtained at this time is (R6×V1)/(R5+R6).

On the other hand, the switching element Sw1is in an ON state for a enable interval for which a voltage control signal is outputted from the current change detection circuit40. In this case, a combined resistor obtained by connecting the resistors R4and R6in parallel and the resistor R5are connected in series and voltage obtained by division is set voltage. Combined resistance Rc obtained by connecting the resistors R4and R6in parallel is Rc=(R4×R6)/(R4+R6), so set voltage Vs=(Rc×V1)/(R5+Rc).

As has been described, in order to obtain set voltage, different voltage division ratios realized by the use of resistors are used in the voltage setting circuit50a, at the time of dividing output voltage from the PFC circuit, for an enable interval for which a voltage control signal is outputted from the current change detection circuit40and a disable interval for which a voltage control signal is not outputted from the current change detection circuit40. As a result, two voltage values are set by switching. In addition, voltage switching is performed by simple circuit structure.

FIG. 14illustrates an example of the structure of a voltage setting circuit. A voltage setting circuit50bincludes a photocoupler51for insulation transmission, a pulse width modulator52, a filter53, resistors R3and R5, and a variable resistor Rv.

A voltage control signal is inputted to an input end of the pulse width modulator52. An output end of the pulse width modulator52is connected to an input end of the photocoupler51. One end of the resistor R3is pulled up and the other end of the resistor R3is connected to one output end of the photocoupler51and one end of a resistor Rf in the filter53. The other output end of the photocoupler51is connected to GND.

The other end of the resistor Rf is connected to one end of a capacitor Cf in the filter53and a variable control terminal of the variable resistor Rv. The other end of the capacitor Cf is connected to GND. One end of the resistor R5is connected to the cathode of the diode D0, one end of the capacitor C0, and one input end of the DC-DC converter30. The other end of the resistor R5is connected to an input end of the PFC control circuit20aand one end of the variable resistor Rv. The other end of the variable resistor Rv is connected to GND.

When a voltage control signal is outputted from the current change detection circuit40to the pulse width modulator52, the pulse width modulator52generates and outputs a pulse-width-modulated signal (PWM signal) while the voltage control signal is being outputted from the current change detection circuit40.

The photocoupler51receives the PWM signal and converts the received PWM signal into light. A light receiving element is driven by the light and continuity takes place. As a result, an H-level signal is outputted. When a PWM signal is not inputted to the photocoupler51, continuity does not take place in the photocoupler51.

The filter53low-pass-filters the signal outputted from the photocoupler51into a direct-current signal and inputs it to the variable control terminal of the variable resistor Rv. The variable resistor Rv changes its resistance value according to the width of an H-level signal outputted from the filter53.

As has been described, the voltage setting circuit50bgenerates a PWM signal by pulse-width-modulating a voltage control signal, and changes according to the pulse width of the PWM signal a resistance value used for dividing output voltage from the PFC circuit. By doing so, various voltage values are set.

As a result, various voltage values are flexibly set by switching. Furthermore, voltage switching is performed by simple circuit structure. In addition, voltage value switching is finely performed, so an overshoot is reduced.

Simulation results will now be described.FIG. 15indicates simulation results of the efficiency of switching power supplies. InFIG. 15, a vertical axis indicates efficiency (%) and a horizontal axis indicates output power (W) from a PFC circuit. A graph g11is an efficiency characteristic curve (375 V) of the switching power supply according to this embodiment. A graph g12is an efficiency characteristic curve (400 V) of a conventional switching power supply.

As can be seen fromFIG. 15, when output power is, for example, P1(W), the efficiency of the switching power supply according to this embodiment in which output voltage from the PFC circuit is set to 375 V is 0.25 percent higher than that of the conventional switching power supply in which output voltage from the PFC circuit is set to 400 V.

FIG. 16indicates simulation results of power factors. InFIG. 16, a vertical axis indicates a power factor and a horizontal axis indicates output power (W) from a PFC circuit. A graph g21is a power factor characteristic curve (375 V) of the switching power supply according to this embodiment. A graph g22is a power factor characteristic curve (375 V) of a conventional switching power supply. A graph g23is a power factor characteristic curve (400 V) of the conventional switching power supply.

An 80 Plus Titanium condition is not met in a region r. As can be seen fromFIG. 16, however, the power factor of the switching power supply according to this embodiment does not fall within the region r and meets the 80 Plus Titanium condition.

“80 Plus” is one of the standards for the power conversion efficiency of power supply units for computers (certification standard given to electrical apparatus which realize an AC-DC conversion efficiency of 80 percent or greater). Furthermore, the 80 Plus standard has grades according to the degree of AC-DC conversion efficiency and a certification standard which is referred to as “Titanium” and which is the highest grade at this point of time is given to power supply units having an AC-DC conversion efficiency of 98 percent or greater.

The embodiment has been described. However, each component described in the embodiment may be replaced with another unit having the same function. In addition, any other component or process may be added.

According to an aspect, a decrease in the efficiency of a power supply is checked.