JP2014054038A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent degradation of the power-supply characteristics of a switching power supply without depending on an input voltage range or an output voltage variable range.SOLUTION: A switching power supply 1 having an LLC resonant circuit and switch elements Q1 and Q2 detects error between a voltage value of an output voltage and a target value of the output voltage determined by a power supply voltage of a reference voltage source Vref, and passes a current according to the detection result to a phototransistor PC2. Further, a control circuit 10 controls proportional gain according to the oscillation frequency of the switching power supply 1 and controls the on-off frequency of the switch elements Q1 and Q2 on the basis of the controlled proportional gain and a voltage at a terminal P1.

Description

  The present invention relates to a switching power supply.

  Conventionally, a switching power supply having a resonant circuit has been proposed (see, for example, Patent Documents 1 and 2).

[Configuration of Switching Power Supply 100]
FIG. 7 is a circuit diagram of a switching power supply 100 according to a conventional example. The switching power supply 100 includes a transformer T, a primary circuit connected to the primary winding W1 of the transformer T, and a secondary circuit connected to the secondary windings W2 and W3 of the transformer T.

  First, the primary side circuit will be described. The primary side circuit includes switch elements Q1 and Q2 configured by N-channel MOSFETs, a control circuit 110, a capacitor C1, an inductor L, and a phototransistor PC2.

  The switch elements Q1 and Q2 form a half bridge circuit, and the input terminal IN is connected to the drain of the switch element Q1. An input terminal GND connected to a reference potential source is connected to the source of the switch element Q2. One end of the primary winding W1 is connected to the source of the switch element Q1 and the drain of the switch element Q2 via the capacitor C1 and the inductor L. Capacitor C1, inductor L, and primary winding W1 form an LLC resonant circuit. An input terminal GND is connected to the other end of the primary winding W1.

  The terminal P2 of the control circuit 110 is connected to the gate of the switch element Q1, and the terminal P3 of the control circuit 110 is connected to the gate of the switch element Q2. The control circuit 110 is connected to the input terminal GND and to the collector of the phototransistor PC2 at the terminal P1. The input terminal GND is connected to the emitter of the phototransistor PC2.

  Next, the secondary circuit will be described. The secondary circuit includes diodes D1 and D2, capacitors C2 and C3, resistors R1 to R4, a reference voltage source Vref, an error amplifier AMP, and a photodiode PC1.

  One end of the secondary winding W2 is connected to the anode of the diode D1, and the other end of the secondary winding W3 is connected to the anode of the diode D2. The output terminal OUT1 is connected to the cathode of the diode D1 and the cathode of the diode D2. The output terminal OUT2 is connected to the other end of the secondary winding W2 and one end of the secondary winding W3. The output terminal OUT1 and the output terminal OUT2 are connected through a capacitor C2 and are connected through resistors R3 and R4.

  The output terminal OUT1 is connected to one end of the resistor R3, one end of the resistor R4 is connected to the other end of the resistor R3, and the output terminal OUT2 is connected to the other end of the resistor R4. An inverting input terminal of the error amplifier AMP is connected to the other end of the resistor R3 and one end of the resistor R4, and an output terminal of the error amplifier AMP is connected via the capacitor C3 and the resistor R2. The positive terminal of the reference voltage source Vref is connected to the non-inverting input terminal of the error amplifier AMP, and the output terminal OUT2 is connected to the negative electrode of the reference voltage source Vref.

  The output terminal of the error amplifier AMP is connected to the cathode of the photodiode PC1, and the anode of the photodiode PC1 is connected to the output terminal OUT1 via the resistor R1. The photodiode PC1 is provided in a pair with the phototransistor PC2, and the photodiode PC1 and the phototransistor PC2 constitute a photocoupler.

[Operation of Switching Power Supply 100]
In the switching power supply 100 having the above configuration, the control circuit 110 controls the on / off frequency of the switching elements Q1 and Q2 according to the voltage of the terminal P1, and the input voltage input from the input terminals IN and GND is predetermined. And output from the output terminals OUT1 and OUT2 as output voltages.

  The voltage at the terminal P1 changes according to the output voltage. Specifically, the error amplifier AMP detects an error voltage that is an error between the voltage value of the output voltage and the target value of the output voltage determined by the power supply voltage of the reference voltage source Vref, and the amount of light corresponding to the detection result is detected. From the photodiode PC1 to the phototransistor PC2. The phototransistor PC2 allows a current corresponding to the amount of light received from the photodiode PC1 to flow from the control circuit 110 toward the input terminal GND. According to this, the voltage at the terminal P1 changes according to the error voltage.

JP 2004-343855 A JP 2007-252144 A

  FIG. 8 is a diagram illustrating gain phase characteristics of the switching power supply 100. In FIG. 8, the vertical axis represents gain or phase, and the horizontal axis represents the response frequency of the switching power supply 100.

  In a switching power supply having a resonance circuit such as the switching power supply 100, the proportional gain becomes maximum at the lower limit of the input voltage as shown in FIG. For this reason, in order to stabilize the output voltage, a negative feedback control constant is set at the lower limit of the input voltage. However, if the input voltage range is wide, the proportional gain decreases as shown in FIG. 8 during normal operation or at the upper limit of the input voltage, which may deteriorate power supply characteristics.

  Therefore, in order to suppress the above-described deterioration of the power supply characteristics, it is conceivable to take measures such as providing a power factor correction circuit (boost converter) to narrow the input voltage range. However, even when this measure is taken, the above-described deterioration of the power supply characteristics cannot be suppressed because the effect of the boost converter is lost during a power failure.

  Further, as another problem, even when the output voltage is varied similarly to the input voltage fluctuation, the proportional gain fluctuation occurs. In this case, the proportional gain becomes maximum at the upper limit of the output voltage. However, even in this case, if the variable range of the output voltage is wide, the proportional gain decreases at the time of normal operation or at the lower limit of the output voltage, similarly to the case where the negative feedback control constant is set at the upper limit of the input voltage. In some cases, power supply characteristics deteriorate.

  In view of the above-described problems, an object of the present invention is to suppress deterioration of power supply characteristics of a switching power supply regardless of an input voltage range or an output voltage variable range.

The present invention proposes the following items in order to solve the above-described problems.
(1) The present invention relates to a resonance circuit (e.g., equivalent to an LLC resonance circuit formed by the capacitor C1, the inductor L, and the primary winding W1 in FIG. 1) and a switch element (for example, the switch element Q1, FIG. Q2) is a switching power supply (e.g., equivalent to the switching power supply 1 in FIG. 1), and control means (e.g., in the control circuit 10 in FIG. 1) that controls the proportional gain according to the oscillation frequency of the switching power supply. A switching power supply characterized in that

  According to this invention, the control means for controlling the proportional gain according to the oscillation frequency of the switching power supply is provided in the switching power supply having the resonance circuit and the switch element. For this reason, by controlling the proportional gain according to the oscillation frequency of the switching power supply, the proportional gain can be controlled according to the input voltage or the output voltage. Therefore, it is possible to suppress the proportional gain from fluctuating according to the fluctuation of the input voltage or the output voltage, and it is possible to suppress the deterioration of the power supply characteristics of the switching power supply regardless of the input voltage range or the output voltage variable range.

  (2) The present invention relates to the switching power supply of (1), wherein the control means is configured to output the switching power supply based on an output voltage or a voltage corresponding to the output voltage (for example, corresponding to the voltage at the terminal P1 in FIG. 1). Oscillation frequency calculation means for obtaining the oscillation frequency (for example, equivalent to the oscillation frequency calculation unit 11 in FIG. 2) and correction means for controlling the proportional gain according to the oscillation frequency obtained by the oscillation frequency calculation means (for example, FIG. 2). Switch element control for controlling the on / off frequency of the switch element based on the output voltage or the voltage according to the output voltage and the proportional gain controlled by the correction means. And a switching power supply characterized by comprising means (e.g., corresponding to the switch element control unit 13 in FIG. 2).

  According to the present invention, in the switching power supply of (1), the control means is provided with the oscillation frequency calculation means, the correction means, and the switch element control means. Then, the oscillation frequency calculation means obtains the oscillation frequency of the switching power supply based on the output voltage or the voltage corresponding to the output voltage, and the correction means controls the proportional gain according to the oscillation frequency obtained by the oscillation frequency calculation means. It was decided to. Further, the switch element control means controls the on / off frequency of the switch element based on the output voltage or the voltage corresponding to the output voltage and the proportional gain controlled by the correction means. For this reason, the proportional gain can be controlled according to the oscillation frequency of the switching power supply, and the same effects as described above can be obtained.

  (3) In the switching power supply according to (2), the oscillation frequency calculation unit holds in advance information indicating a relationship between the oscillation frequency and a proportional gain. A switching power supply has been proposed in which a proportional gain value corresponding to an oscillation frequency is obtained.

  According to the present invention, in the switching power source of (2), the oscillation frequency calculation means holds in advance information indicating the relationship between the oscillation frequency of the switching power source and the proportional gain, and this information is used for switching. The value of the proportional gain corresponding to the oscillation frequency of the power supply was determined. Therefore, the proportional gain can be controlled according to the oscillation frequency of the switching power supply. Therefore, the same effects as those described above can be achieved.

  (4) In the switching power supply according to (3), the relationship between the oscillation frequency for which the oscillation frequency calculation means holds information in advance and the proportional gain is linear or nonlinear. Proposed power supply.

  According to the present invention, in the switching power source of (3), the relationship between the oscillation frequency of the switching power source in which the oscillation frequency calculating means holds information in advance and the proportional gain is linear or nonlinear. For this reason, the proportional gain can be changed linearly or nonlinearly with respect to the oscillation frequency of the switching power supply, and the same effect as described above can be obtained.

  (5) In the switching power supply according to any one of (2) to (4), the present invention relates to the output voltage or a voltage corresponding to the output voltage: a voltage value of the output voltage and a target value of the output voltage. A switching power supply characterized by an error voltage has been proposed.

  According to the present invention, any one of the switching power supplies of (2) to (4) has an output voltage or a voltage corresponding to the output voltage, which is an error voltage between the voltage value of the output voltage and the target value of the output voltage. It was supposed to be. For this reason, the oscillation frequency of the switching power supply can be obtained based on the error between the voltage value of the output voltage and the target value, and the same effect as described above can be obtained.

  (6) The present invention provides the switching power supply according to any one of (2) to (5), wherein the control means is provided on a primary side of the switching power supply, and outputs the output voltage or a voltage corresponding to the output voltage. There is proposed a switching power supply comprising transmission means (for example, equivalent to the photodiode PC1 and the phototransistor PC2 in FIG. 1) for transmitting a signal from the secondary side to the primary side of the switching power supply.

  According to this invention, in the switching power supply of any one of (2) to (5), the control means is provided on the primary side of the switching power supply, and the output voltage or the voltage corresponding to the output voltage is supplied to the secondary side of the switching power supply. A transmission means for transmitting a signal from the primary side to the primary side is provided. For this reason, even when the control means is on the primary side of the switching power supply, the output voltage or a voltage corresponding to the output voltage can be transmitted to the control means, and the same effects as described above can be obtained. it can.

  (7) In the switching power supply according to any one of (2) to (5), the control unit is provided on the secondary side of the switching power supply, and controls the on / off frequency of the switch element. A switching power supply, wherein a control signal is transmitted to the switch element via a transmission means (for example, equivalent to the insulating circuit 31 in FIG. 6) for transmitting a signal from the secondary side to the primary side of the switching power supply. Has proposed.

  According to this invention, in the switching power supply of any one of (2) to (5), the control means is provided on the secondary side of the switching power supply, and the control means controls the on / off frequency of the switch element by this control means. The signal is transmitted to the switch element via a transmission means for transmitting a signal from the secondary side to the primary side of the switching power supply. For this reason, even when the control means is on the secondary side of the switching power supply, the on / off frequency of the switch element can be controlled by the control means, and the same effects as described above can be obtained. .

  (8) In the switching power supply according to any one of (2) to (5), the control unit controls the proportional gain when the input voltage is equal to or lower than a predetermined threshold value. A switching power supply is proposed.

  According to the present invention, in any of the switching power supplies of (2) to (5), the proportional gain is controlled by the control means when the input voltage is equal to or lower than a predetermined threshold value. For this reason, when the input voltage is equal to or lower than a predetermined threshold, the same effect as described above can be obtained.

  (9) In the switching power supply according to any one of (1) to (8), the present invention includes a power factor correction circuit (e.g., equivalent to the power factor correction circuit 20 in FIG. 5) at the input portion of the switching power supply. We propose a switching power supply characterized by

  According to the present invention, in any one of the switching power supplies of (1) to (8), the power factor correction circuit is provided at the input portion of the switching power supply. For this reason, during the period when electric power is supplied to the power factor correction circuit, the power factor correction circuit can suppress the deterioration of the power supply characteristics of the switching power supply.

  According to the present invention, it is possible to suppress the deterioration of the power supply characteristics of the switching power supply regardless of the input voltage range or the output voltage variable range.

1 is a circuit diagram of a switching power supply according to a first embodiment of the present invention. It is a block diagram of a control circuit with which the switching power supply is provided. It is a figure which shows the gain phase characteristic of the said switching power supply. It is a circuit diagram of the switching power supply concerning a 2nd embodiment of the present invention. It is a circuit diagram of the switching power supply which concerns on 3rd Embodiment of this invention. It is a circuit diagram of the switching power supply which concerns on 4th Embodiment of this invention. It is a circuit diagram of the switching power supply concerning a prior art example. It is a figure which shows the gain phase characteristic of the said switching power supply.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the constituent elements in the following embodiments can be appropriately replaced with existing constituent elements, and various variations including combinations with other existing constituent elements are possible. Accordingly, the description of the following embodiments does not limit the contents of the invention described in the claims.

<First Embodiment>
[Configuration of Switching Power Supply 1]
FIG. 1 is a circuit diagram of a switching power supply 1 according to the first embodiment of the present invention. The switching power supply 1 is different from the switching power supply 100 according to the conventional example shown in FIG. 7 in that a control circuit 10 is provided instead of the control circuit 110. In the switching power supply 1, the same components as those of the switching power supply 100 are denoted by the same reference numerals, and the description thereof is omitted.

  FIG. 2 is a block diagram of the control circuit 10. The control circuit 10 includes an oscillation frequency calculation unit 11, a correction unit 12, and a switch element control unit 13.

[Operation of switching power supply 1]
In the switching power supply 1, the control circuit 10 controls the proportional gain according to the oscillation frequency of the switching power supply 1, and the switching elements Q 1 and Q 2 are turned on / off based on the controlled proportional gain and the voltage at the terminal P 1. Control the frequency.

  Specifically, the control circuit 10 first causes the oscillation frequency calculation unit 11 to calculate the output voltage target value determined based on the voltage of the terminal P1, that is, the output voltage and the power supply voltage of the reference voltage source Vref. The oscillation frequency of the switching power supply 1 is obtained based on the error voltage that is the error of.

  Next, the control circuit 10 controls the proportional gain according to the oscillation frequency of the switching power supply 1 obtained by the oscillation frequency calculation unit 11 by the correction unit 12. Specifically, the correction unit 12 stores in advance a calculation formula indicating the relationship between the oscillation frequency of the switching power supply 1 and the proportional gain. Then, by substituting the oscillation frequency of the switching power supply 1 into this calculation formula, the value of the proportional gain corresponding to the oscillation frequency of the switching power supply 1 is obtained, and the proportional gain is controlled to the obtained value.

  Next, the control circuit 10 controls the on / off frequency of the switch elements Q1 and Q2 by the switch element control unit 13 based on the proportional gain controlled by the correction unit 12 and the voltage at the terminal P1.

  The above calculation formula is a mathematical formula in which the relationship between the oscillation frequency of the switching power supply 1 and the proportional gain is linear or non-linear, and is used for, for example, P control, PI control, and PID control. It is a mathematical formula.

  When the relationship between the oscillation frequency of the switching power supply 1 and the proportional gain is linear, for example, the proportional gain is decreased linearly as the oscillation frequency of the switching power supply 1 increases.

  Further, when the relationship between the oscillation frequency of the switching power supply 1 and the proportional gain is nonlinear, for example, if the oscillation frequency of the switching power supply 1 is less than a predetermined threshold, the proportional gain is set to the first value. To. On the other hand, if the oscillation frequency of the switching power supply 1 is equal to or higher than a predetermined threshold value, the proportional gain is set to a second value smaller than the first value. According to this, when the oscillation frequency of the switching power supply 1 is equal to or higher than the above threshold, the proportional gain is reduced as compared with the case where the oscillation frequency is lower than the above threshold. That is, the proportional gain changes nonlinearly with whether or not the oscillation frequency of the switching power supply 1 is equal to or higher than the above threshold value.

  Here, there is a correlation between the oscillation frequency of the switching power supply 1 and the input voltage. For this reason, by controlling the proportional gain according to the oscillation frequency of the switching power supply 1, the proportional gain can be controlled according to the input voltage.

  FIG. 3 is a diagram illustrating gain phase characteristics of the switching power supply 1. In FIG. 3, the vertical axis represents the gain or phase, and the horizontal axis represents the response frequency of the switching power supply 1.

  When the proportional gain is controlled according to the oscillation frequency of the switching power supply 1 as described above, the proportional gain can be controlled according to the input voltage. As a result, as shown in FIG. The reduction of the proportional gain at the upper limit of the voltage is suppressed compared to the case shown in FIG.

  According to the above switching power supply 1, the following effects can be produced.

  The switching power supply 1 controls the proportional gain according to the oscillation frequency of the switching power supply 1. For this reason, it can suppress that a proportional gain fluctuates according to the fluctuation | variation of an input voltage, and can suppress degradation by the input voltage range about the power supply characteristics of the switching power supply 1, such as a sudden load change characteristic and a regulation characteristic. According to this, the capacity of the capacitor C2 can also be reduced.

Second Embodiment
[Configuration of Switching Power Supply 1A]
FIG. 4 is a circuit diagram of a switching power supply 1A according to the second embodiment of the present invention. The switching power supply 1A is different from the switching power supply 1 according to the first embodiment of the present invention shown in FIG. 1 in the connection of the phototransistor PC2. In the switching power supply 1A, the same components as those of the switching power supply 1 are denoted by the same reference numerals, and the description thereof is omitted.

  A voltage source V1 is connected to the collector of the phototransistor PC2, and a terminal P1 of the control circuit 10 is connected to the emitter of the phototransistor PC2.

  According to the above switching power supply 1 </ b> A, the same effects as those described above that can be achieved by the switching power supply 1 can be achieved.

<Third Embodiment>
[Configuration of Switching Power Supply 1B]
FIG. 5 is a circuit diagram of a switching power supply 1B according to the third embodiment of the present invention. The switching power supply 1B is different from the switching power supply 1 according to the first embodiment of the present invention shown in FIG. 1 in that a power factor correction circuit 20 is provided in the input section. In the switching power supply 1B, the same components as those of the switching power supply 1 are denoted by the same reference numerals, and the description thereof is omitted.

  The power factor correction circuit 20 forms a boost converter and operates when the input voltage of the switching power supply 1B is higher than a predetermined threshold. On the other hand, when the input voltage of the switching power supply 1B is less than or equal to the threshold value, the control circuit 10 controls the proportional gain according to the oscillation frequency of the switching power supply 1B as described above, and the input voltage of the switching power supply 1B exceeds the threshold value. If it is high, the control of the proportional gain corresponding to the oscillation frequency of the switching power supply 1B is stopped.

  For this reason, when the input voltage is higher than the threshold, the power factor correction circuit 20 boosts the input voltage and narrows the input voltage range. On the other hand, when the input voltage is equal to or lower than the threshold value (for example, during a power failure), the power factor correction circuit 20 does not operate, but the control circuit 10 controls the proportional gain according to the oscillation frequency of the switching power supply 1B.

  According to the above switching power supply 1B, in addition to the above-described effects that the switching power supply 1 can exhibit, the following effects can be achieved.

  When the input voltage is higher than the threshold, the switching power supply 1B can narrow the input voltage range by the power factor correction circuit 20 and suppress the deterioration of the power supply characteristics of the switching power supply 1B.

<Fourth embodiment>
[Configuration of Switching Power Supply 1C]
FIG. 6 is a circuit diagram of a switching power supply 1C according to the fourth embodiment of the present invention. The switching power supply 1C differs from the switching power supply 1 according to the first embodiment of the present invention shown in FIG. 1 in place of the control circuit 10, the phototransistor PC2, the photodiode PC1, and the resistor R1. 31 is different. In the switching power supply 1C, the same components as those of the switching power supply 1 are denoted by the same reference numerals and description thereof is omitted.

  The control circuit 10A is provided on the secondary side of the switching power supply 1C. Specifically, the control circuit 10A is directly connected to the output terminal of the error amplifier AMP. Further, the control circuit 10A is connected to the gate of the switch element Q1 and the gate of the switch element Q2 via the insulating circuit 31.

[Operation of switching power supply 1C]
In the switching power supply 1 </ b> C, the control circuit 10 </ b> A controls the proportional gain according to the oscillation frequency of the switching power supply 1 </ b> C, similarly to the control circuit 10. Then, a control signal corresponding to the controlled proportional gain and the voltage at the terminal P1 is transmitted to the switch elements Q1 and Q2 via the insulating circuit 31 to control the on / off frequency of the switch elements Q1 and Q2. To do.

  According to the above switching power supply 1 </ b> C, the same effects as those described above that can be achieved by the switching power supply 1 can be achieved.

  The present invention is not limited to the above-described embodiment, and various modifications and applications can be made without departing from the gist of the present invention.

  For example, in each of the above-described embodiments, the half-bridge circuit is provided in the primary circuit, but the present invention is not limited thereto, and for example, a full-bridge circuit may be provided.

  In each of the above-described embodiments, the transformer T has a so-called center tap. However, the present invention is not limited to this.

  Further, in each of the above-described embodiments, the primary side circuit is provided with the LLC resonant circuit as the resonant circuit. However, the present invention is not limited to this.

  In all the embodiments described above, the proportional gain is controlled in accordance with the oscillation frequency as the input voltage varies. However, the present invention is not limited to this. For example, even when the output voltage is varied, the proportional gain can be controlled in accordance with the oscillation frequency. In this case, it is possible to suppress the proportional gain from fluctuating according to the fluctuation of the output voltage, and it is possible to suppress the deterioration of the power supply characteristics of the switching power supply regardless of the output voltage variable range.

  Moreover, although the example which provided the power factor improvement circuit 20 in the above-mentioned 3rd Embodiment was demonstrated, the power factor improvement circuit 20 can be provided also in other embodiment.

1, 1A, 1B, 1C, 100; switching power supply 10, 10A, 110; control circuit 11; oscillation frequency calculation unit 12; correction unit 13; switch element control unit 20; power factor improvement circuit 31; insulation circuit AMP; PC1; Photodiode PC2; Phototransistor Q1, Q2; Switch element T; Transformer

Claims (9)

A switching power supply having a resonant circuit and a switch element,
A switching power supply comprising control means for controlling a proportional gain according to an oscillation frequency of the switching power supply. The control means includes
Based on an output voltage or a voltage corresponding to the output voltage, an oscillation frequency calculation means for obtaining an oscillation frequency of the switching power supply
Correction means for controlling the proportional gain according to the oscillation frequency obtained by the oscillation frequency calculation means;
Switch element control means for controlling the on / off frequency of the switch element based on the output voltage or a voltage according to the output voltage and a proportional gain controlled by the correction means, The switching power supply according to claim 1. The oscillation frequency calculating means includes
Information indicating the relationship between the oscillation frequency and the proportional gain is held in advance,
3. The switching power supply according to claim 2, wherein a value of a proportional gain corresponding to the oscillation frequency is obtained using the information.   4. The switching power supply according to claim 3, wherein a relationship between the oscillation frequency for which the oscillation frequency calculation unit holds information in advance and a proportional gain is linear or nonlinear.   5. The switching power supply according to claim 2, wherein the output voltage or a voltage corresponding to the output voltage is an error voltage between a voltage value of the output voltage and a target value of the output voltage. . The control means is provided on the primary side of the switching power supply,
The switching power supply according to any one of claims 2 to 5, further comprising a transmission means for transmitting the output voltage or a voltage corresponding to the output voltage from a secondary side to a primary side of the switching power supply.   The control means is provided on the secondary side of the switching power supply, and includes a transmission means for transmitting a control signal for controlling the on / off frequency of the switching element from the secondary side to the primary side of the switching power supply. The switching power supply according to claim 2, wherein the switching power supply is transmitted to the switch element.   The switching power supply according to claim 1, wherein the control unit controls the proportional gain when an input voltage is equal to or lower than a predetermined threshold value. The switching power supply according to any one of claims 1 to 8,
A switching power supply comprising a power factor correction circuit at an input section of the switching power supply.

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