JP5495679B2 - Power circuit - Google Patents

Description

  The present invention relates to a power supply circuit.

  In various electronic devices, a power supply circuit that supplies electric power for driving the electronic device is provided. As this type of power supply circuit, a multiple output control power supply apparatus disclosed in Patent Document 1 is exemplified.

  This type of apparatus is configured as shown below in order to enable double output control with low conversion loss. That is, the PWM control circuit performs PWM control on the primary winding side transistor. Further, a synchronization signal synchronized with the pulse output from the PWM control circuit is output to the D flip-flop circuit on the secondary winding side. The data of the D flip-flop circuit is changed according to the synchronization signal output to the D flip-flop circuit. At that time, the transistor circuit on the secondary winding side in which no emitter voltage is applied is turned on and off.

JP-A-6-169568

  By the way, in general, the turns ratio of the transformer of the power supply circuit is set so that the transformer can be operated with little loss when receiving the supply of power having a voltage value within a predetermined range. Yes. For this reason, when power having a voltage value outside the predetermined range is supplied, the transformer cannot operate with little loss, so that the conversion efficiency of the voltage value is lowered and an ideal transformation ratio cannot be obtained. There is a problem.

  In the device disclosed in Patent Document 1, the multi-output control with low conversion loss is enabled by turning on and off the transistor circuit on the secondary winding side in a state where the emitter voltage is not applied.

  However, in the device disclosed in Patent Document 1, on / off of the transistor circuit on the secondary winding side in a state where no emitter voltage is applied is controlled in order to enable double output control with low conversion loss. However, the transformer turns ratio does not change. Therefore, when the transformer is supplied with power having a voltage value that is out of the range of the voltage value that can operate in a low loss state, the conversion efficiency of the voltage value is lowered, and an ideal transformation ratio is obtained. The problem of not being solved is not solved.

  The present invention has been made to solve the above problem, and even when the transformer is supplied with power having a voltage value outside the range of voltage values that can operate in a state with little loss, the voltage value is An object of the present invention is to provide a power supply circuit capable of obtaining an ideal transformation ratio without lowering the conversion efficiency.

A power supply circuit according to one aspect of the present invention includes a transformer including a primary winding and a secondary winding, a winding number changing unit that changes the number of turns of the primary winding, and a rectifier circuit that rectifies the supplied AC power. A voltage boosting circuit that boosts the voltage value of the rectified AC power to a preset voltage value, a smoothing circuit that smoothes the boosted AC power to obtain DC power, and is smoothed by the smoothing circuit A first control unit that feedback-controls the booster circuit so that the voltage value of the DC power becomes the preset voltage value in response to voltage feedback, and the DC obtained by the smoothing circuit Power is configured to be supplied to the primary winding via a primary power line, and the winding number changing unit determines the number of turns of the primary winding according to the voltage value of the power supplied to the primary winding. Change and the transformer is the primary The power supplied to the wire is transmitted to the secondary winding by electromagnetic induction, the primary winding is divided into a plurality of sections, and the winding number changing unit is a voltage of the power among the plurality of sections. The section corresponding to the value is made conductive, and the winding number changing unit is provided corresponding to each of the plurality of sections, and operates to make each of the plurality of sections conductive, and the power A second control unit that operates one of the plurality of switching elements according to a voltage value; and a Zener diode having a cathode connected to the primary power line and an anode connected to the second control unit. The primary winding includes a first primary winding and a second primary winding, and the plurality of switching elements are a first switching element that operates to conduct the first primary winding; The above And a second switching element that operates to conduct the first primary winding and the second primary winding, and the second control unit is a voltage of DC power supplied to the primary power line. When the value is less than the breakdown voltage of the Zener diode, a pulse having a predetermined duty ratio is output to the first switching element, the first switching element is turned on / off in synchronization with the pulse, and the primary side When the voltage value of the DC power supplied to the power line exceeds the breakdown voltage of the Zener diode, a pulse having a predetermined duty ratio is output to the second switching element, and the second switching element is synchronized with the second pulse. The switching element is turned on / off .

  According to this configuration, since the number of turns of the primary winding of the transformer is switched according to the voltage value of the power supplied to the primary winding, an ideal turn ratio according to the voltage value of the supplied power is obtained. It is done.

  As a result, an ideal turns ratio can be obtained even when power of a voltage value in a range in which the transformer cannot operate with a small loss when the number of turns of the primary winding is fixed is supplied. The ideal transformation ratio can be obtained without lowering the conversion efficiency of the voltage value. Therefore, even if the range of the voltage value of the supplied power reaches a wide range, the transformer can operate in a state with little loss, so that a reduction in power that can be supplied to the load on the secondary winding side can be prevented. .

  According to this configuration, the number of turns of the primary winding can be appropriately switched by a simple control of conducting a section corresponding to the voltage value of the power supplied to the primary winding among a plurality of sections in the primary winding. it can.

  According to this configuration, the number of turns of the primary winding can be appropriately switched by a simple control of operating any one of the plurality of switching elements according to the voltage value of the power supplied to the primary winding.

  According to this configuration, the voltage value of the rectified AC power is boosted to a preset voltage value by the booster circuit, and the boosted AC power is smoothed by the smoothing circuit to become DC power. Then, the DC power is supplied to the primary winding.

  Thus, since the voltage value of the rectified AC power is boosted to a preset voltage value, the primary winding is set in advance even when the range of the voltage value of the supplied AC power is wide. DC power having a predetermined voltage value is supplied. As a result, on the secondary winding side, DC power having a constant voltage value can be obtained regardless of the range of the voltage value of the supplied AC power. Therefore, the power supply device according to this configuration can be used in countries around the world where the voltage value of the supplied AC power is different.

In the above-described configuration, an energy saving control unit that executes an energy saving mode for inputting the rectified AC power to the smoothing circuit without boosting the voltage value of the AC power rectified by the rectifier circuit by the booster circuit. Ru can be configured to include.

  According to this configuration, when the energy saving mode is executed, the rectified AC power is input to the smoothing circuit without being boosted. For this reason, the smoothing circuit transmits to the primary winding DC power in which the voltage value of the supplied AC power is reflected as it is.

  According to this configuration, as described above, an ideal turns ratio corresponding to the voltage value of the supplied power can be obtained. Therefore, even in the energy saving mode, the voltage value of the supplied AC power is Even if the direct-current power reflected as it is is transmitted to the primary winding, the transformer can operate with little loss.

  According to the present invention, an ideal turns ratio can be obtained even when power having a voltage value in a range in which the transformer cannot operate with a small loss when the number of turns of the primary winding is fixed is supplied. Therefore, an ideal transformation ratio can be obtained without reducing the conversion efficiency of the voltage value. Therefore, even if the range of the voltage value of the supplied power reaches a wide range, the transformer can operate in a state with little loss, so that a reduction in power that can be supplied to the load on the secondary winding side can be prevented. .

It is the figure which showed an example of the power supply circuit which concerns on one Embodiment of this invention. It is the figure which showed an example of the connection state between each of the 1st and 2nd transistor, and a primary winding. It is the figure which showed typically the 1st primary winding, the 2nd primary winding, and the secondary winding. 3 is a diagram illustrating an example of a correlation between a voltage value of supplied DC power and the number of turns of a primary winding 20. FIG. It is the figure which showed the reference example of the power supply circuit.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a diagram showing an example of a power supply circuit according to an embodiment of the present invention.

  A power supply circuit 1 shown in FIG. 1 is configured to convert AC power supplied from an AC power supply 12 into DC power and supply the DC power to a load 11. Therefore, the power supply circuit 1 includes a transformer 2, a Zener diode 3, first and second switching circuits 40 and 41, a PWM control unit (control unit) 42, a diode bridge (rectifier circuit) 5, and a PFC provided as a booster circuit. A circuit (power factor control circuit) 6, a PFC control unit 7, a primary side smoothing capacitor (smoothing circuit) 8, a rectifier diode 9, a secondary side smoothing capacitor 10, and an energy saving control unit 13 are provided.

  The transformer 2 includes a primary winding 20 and a secondary winding 21, and transmits DC power supplied to the primary winding 20 to the secondary winding 21 by electromagnetic induction. The Zener diode 3 is provided to detect whether or not the voltage value of the DC power supplied to the primary power line L1 exceeds a preset voltage value. The first and second switching circuits 40 and 41 switch between conduction and non-conduction of the first primary winding 200 and the second primary winding 201. The PWM control unit (control unit) 42 performs PWM control on the first and second switching circuits 40 and 41.

  The diode bridge 5 rectifies AC power supplied from the AC power supply 12 to generate a pulsating flow. The PFC circuit 6 boosts the voltage value of the AC power to a predetermined voltage value while aligning the process of changing the voltage value and current value of the AC power input as the pulsating current. Here, as the PFC circuit 6, for example, a step-up PFC circuit can be used.

  The PFC control unit 7 receives the feedback of the voltage between the terminals of the primary side smoothing capacitor 8 so that the voltage value of the DC power supplied from the primary side smoothing capacitor 8 to the primary winding 20 becomes a predetermined voltage value. Then, the PFC circuit 6 is controlled (feedback control).

  In order to cause the PFC control unit 7 to perform such feedback control, a series circuit including resistors 45 and 46 is connected in parallel with the primary side smoothing capacitor 8 in the primary side power supply line L1. The series circuit including these resistors 45 and 46 steps down the voltage across the terminals of the primary side capacitor 8 by resistance division. The inter-terminal voltage stepped down by such resistance division is input to the PFC control unit 7. Accordingly, the PFC control unit 7 can perform feedback control of the voltage value of the DC power supplied from the primary side smoothing capacitor 8 to the primary winding 20 in response to feedback of the voltage between the terminals of the primary side smoothing capacitor 8. .

  Here, an IC for a PFC circuit can be used as the PFC control unit 7. The primary side smoothing capacitor 8 receives the AC power output as a pulsating flow from the PFC circuit 6 and charges and discharges the DC power to the primary winding 20.

  The rectifier diode 9 rectifies DC power transmitted as a pulse to the secondary winding 21 of the transformer 2. The reason why DC power is transmitted as a pulse to the secondary winding 21 of the transformer 2 will be described later. The secondary side smoothing capacitor 10 smoothes the pulse rectified by the rectifier diode 9 to generate DC power.

  The energy saving control unit 13 outputs a control signal to the PFC control unit 7 to execute the energy saving mode. The PFC control unit 7 does not operate the PFC circuit 6 when receiving a control signal instructing execution of the energy saving mode. At this time, the AC power rectified by the diode bridge 5 is not boosted by the PFC circuit 6 and is maintained as the DC voltage by the primary side smoothing capacitor 8 while maintaining the voltage value as it is.

  Hereinafter, the basic operation of the power supply circuit 1 will be described while detailing the function of each component described above. First, the diode bridge 5 receives AC power supplied from the AC power supply 12. Here, the diode bridge 5 receives, for example, AC power within an effective value range of 85V to 276V.

  The diode bridge 5 rectifies the received AC power to generate a pulsating flow. Here, the pulsating flow means a current whose flow direction is constant and whose magnitude changes periodically. The AC power converted to pulsating current is input to the PFC circuit 6. Then, the AC power input to the PFC circuit 6 is the AC power in which the process of changing the voltage value and the current value is uniform and the voltage value is boosted to a predetermined voltage value, and the primary power Output to line L1.

  The AC power output to the primary side power line L1 is smoothed by charging / discharging of the primary side smoothing capacitor 8 to become DC power, and is output to the primary side power line L1. With such a function of the primary side smoothing capacitor 8, DC power is supplied to the primary side power line L1. Thereby, DC power is supplied to the primary winding 20. The DC power is also output to the source terminals of a first transistor 40 and a second transistor 41, which will be described later. Thereby, the source voltage is applied to the first transistor 40 and the second transistor 41.

  The cathode terminal of the Zener diode 3 is connected to the primary power line L1. On the other hand, the anode terminal is connected to the PWM control unit 42. As a result, if the voltage value of the DC power supplied to the primary power line L <b> 1 is equal to or higher than the breakdown voltage specific to the Zener diode 3, the reverse current of the Zener diode 3 flows to the PWM control unit 42. On the other hand, the reverse voltage does not flow when the voltage value of the DC power supplied to the primary power line L1 is less than the breakdown voltage.

  The PWM control unit 42 detects that the voltage value of the DC power supplied to the primary power line L1 exceeds the breakdown voltage of the Zener diode 3 by detecting the reverse current. On the other hand, when the reverse current is not detected, the PWM control unit 42 detects that the voltage value of the DC power supplied to the primary power line L1 is less than the breakdown voltage of the Zener diode 3.

  The PWM control unit 42 is connected to the gate terminal of the first transistor 40 via the resistor 43, and is connected to the gate terminal of the second transistor 41 via the resistor 44. Here, in the present embodiment, the first transistor 40 and the second transistor 41 are constituted by N-channel MOSFETs.

  In each of the first transistor 40 and the second transistor 41, each source terminal is connected to the primary power line L1. Since DC power is supplied to the primary power line L1 as described above, a source voltage is applied to each source terminal.

  In the transformer 2, the primary winding 20 is partitioned into a first primary winding 200 and a second primary winding 201. The first primary winding 200 has ends a and b, and the second primary winding 201 has ends b and c. Here, the end b is an end that the first primary winding 200 and the second primary winding 201 have in common. The drain terminal of the first transistor 40 is connected to the end b of the first primary winding 200. The drain terminal of the second transistor 41 is connected to the end c of the second primary winding 201.

  The first transistor 40 functions as a first switching element that operates to make the first primary winding 200 conductive. Further, the second transistor 41 functions as a second switching element that operates to make the first primary winding 200 and the second primary winding 201 conductive. The PWM control unit 42 outputs a pulse having a predetermined duty ratio in order to cause each of the first transistor 40 and the second transistor 41 to function as the first and second switching elements.

  The first transistor 40 and the second transistor 41 receive a pulse output from the PWM control unit 42. A gate voltage is applied when the pulse is at a high level, and a current flows between the drain terminal and the source terminal. In this way, when the current flows between the drain terminal and the source terminal, the first primary winding 200 and the primary winding 20 that is the first primary winding 200 and the second primary winding 201. It becomes a conductive state.

  Thus, the first primary winding 200 and the primary winding 20 are in a conductive state when the pulse output from the PWM control unit 42 is at a high level. Therefore, the longer the period during which the pulse output from the PWM control unit 42 is at the high level, the longer the period in which the pulse is in the conductive state. On the other hand, the shorter the period at the high level, the shorter the period in the conductive state. Therefore, the PWM control unit 42 controls the period in which the conduction state is established by increasing or decreasing the duty ratio of the pulse.

  When the first primary winding 200 and the primary winding 20 are in a conductive state, the first primary winding 200 and the primary winding 20 have AC power (that is, direct current) smoothed by the primary-side smoothing capacitor 8. Power). Here, as described above, the period in which the first primary winding 200 and the primary winding 20 are in the conductive state correlates with the period in which the pulse is at the high level. DC power is supplied to the line 20 in synchronization with the pulse. Therefore, DC power is supplied to the first primary winding 200 and the primary winding 20 in a pulsed manner.

  The DC power supplied as pulses to the first primary winding 200 and the primary winding 20 is transmitted to the secondary winding 21 by electromagnetic induction. Thus, since the DC power supplied as a pulse to the primary winding side 20 is transmitted to the secondary winding 21, the transformed DC power is transmitted to the secondary winding 21 in a pulse shape.

  The DC power transmitted in a pulse form to the secondary winding 21 is rectified by the rectifier diode 9 and smoothed by the secondary side smoothing capacitor 10 to be converted to DC power. The DC power thus generated by being smoothed by the secondary side smoothing capacitor 10 is output to the secondary side power line L2. The DC power output to the secondary power line L2 is supplied to the load 11.

  Here, it is known that the voltage value of the DC power supplied to the load 11 increases as the period of the high level of the DC power transmitted to the secondary winding 21 in a pulse shape is longer. Therefore, the PWM control unit 42 controls the voltage value of the DC power supplied to the load 11 by increasing or decreasing the duty ratio of the pulses output to the first transistor 40 and the second transistor 41.

  As described above, the power supply circuit 1 that performs such basic operation includes the winding number changing unit 4 that changes the number of turns of the primary winding 20 in accordance with the voltage value of the DC power supplied to the primary winding 20. . The winding number changing unit 4 applies pulses (pulse-like) to the Zener diode 3, the first transistor 40 and the second transistor 41, the PWM control unit 42, and the gate terminals of the first transistor 40 and the second transistor 41. Resistors 43 and 44 for outputting the voltage signal of the above.

  In the winding change unit 4, each of the first transistor 40 and the second transistor 41 and the primary winding 20 are connected as shown in FIG. 2, for example. FIG. 2 is a diagram illustrating an example of a connection state between each of the first transistor 40 and the second transistor 41 and the primary winding 20.

  As shown in FIG. 2, the first primary winding 200 and the second primary winding 201 are connected in series at an end b shared by the first primary winding 200 and the second primary winding 201. It is connected to the. Thus, the primary winding 20 is a double winding.

  A primary power line L1 is connected to the end a of the first primary winding 200. Further, the drain terminal of the first transistor 40 is connected to the end b shared by the first primary winding 200 and the second primary winding 201 as described above. Further, the drain terminal c of the second transistor 41 is connected to the end c of the second primary winding 201 as described above.

  Further, the gate terminals of the first transistor 40 and the second transistor 41 are connected to the PWM control unit 42 as described above. Further, the source terminals of the first transistor 40 and the second transistor 41 are connected to the primary power line L1 as described above.

  The operation of the winding number changing unit 4 will be described below.

(1) When the voltage value of the DC power supplied to the primary power line L1 is less than the breakdown voltage of the Zener diode 3, the voltage value of the DC power supplied to the primary power line L1 becomes the breakdown voltage of the Zener diode 3. When not satisfied, the reverse current of the Zener diode 3 does not arrive at the PWM control unit 42. In this state, the PWM control unit 42 outputs a pulse having a predetermined duty ratio only to the transistor 40, and turns on / off the transistor 40 in synchronization with the pulse. At this time, the PWM control unit 42 does not output a pulse to the transistor 41. As a result, conduction and non-conduction of only the first primary winding 200 are switched, but the second primary winding 201 remains non-conduction.

  As described above, when the voltage value of the DC power supplied to the primary power line L1 is less than the breakdown voltage of the Zener diode 3, the conduction and non-conduction of only the first primary winding 200 are switched. Primary winding 201 remains non-conductive.

(2) When the voltage value of the DC power supplied to the primary power line L1 exceeds the breakdown voltage of the Zener diode 3, the voltage value of the DC power supplied to the primary power line L1 exceeds the breakdown voltage of the Zener diode 3 Sometimes, the reverse current of the Zener diode 3 arrives at the PWM control unit 42. In response to this, the PWM control unit 42 outputs a pulse having a predetermined duty ratio only to the transistor 41, and turns on / off the transistor 41 in synchronization with the pulse. At this time, the PWM control unit 42 does not output a pulse to the transistor 40. Thereby, conduction | electrical_connection and non-conduction of the whole primary winding 20 which is the 1st primary winding 200 and the 2nd primary winding 201 are switched.

  Thus, when the voltage value of the DC power supplied to the primary power line L1 exceeds the breakdown voltage of the Zener diode 3, the conduction and non-conduction of the entire primary winding 20 are switched. Therefore, when the voltage value of the DC power supplied to the primary power line L1 exceeds the breakdown voltage of the Zener diode 3, the number of turns of the primary winding 20 is the number of turns of the primary winding 20 itself.

  Next, advantages of the power supply circuit 1 according to the present embodiment will be described with reference to FIGS. FIG. 3 is a diagram schematically showing the first primary winding 200, the second primary winding 201, and the secondary winding 21. FIG. 4 is a diagram illustrating an example of the correlation between the voltage value of the supplied DC power and the number of turns of the primary winding 20. FIG. 5 is a diagram showing a reference example of the power supply circuit.

  As shown in FIG. 3, in the transformer 2, the number of turns of the first primary winding 200 is “NP1”. The number of turns of the second primary winding 201 is “NPn”. Further, the number of turns of the secondary winding 21 is “NS”.

  Here, the turn ratio “NP1 / NS” between the turn number “NP1” of the first primary winding 200 and the turn number “NS” of the secondary winding 21 is a predetermined first value that does not satisfy the set voltage value. The winding ratio is such that an ideal transformation ratio is obtained when DC power having a voltage value is supplied. The turn ratio “(NP1 + NPn) / NS” between the number of turns “NP1 + NPn” of the primary winding 20 and the number of turns “NS” of the secondary winding 21 is a predetermined second voltage value exceeding the set voltage value. The turn ratio is such that an ideal transformation ratio is obtained when DC power is supplied.

  In such a transformer 2, the range of the DC power voltage value in which the transformer 2 can operate with little loss is assumed to be 200 V or more. In this case, when a set voltage value of 200 V is set as a set voltage value that becomes a boundary where the number of turns of the primary winding 20 is changed, the number of turns changing process shown below is performed in the transformer 2.

  That is, as shown in FIG. 4, when the voltage value of the DC power supplied to the primary power line L1 is the second voltage value (for example, 276 V) exceeding the set voltage value (200) V, the above-mentioned Thus, since the number of turns of the primary winding 20 is the number of turns of the primary winding 20 itself, the number of turns is “NP1 + NPn”. On the other hand, when the voltage value of the DC power supplied to the primary power line L1 is the first voltage value (for example, 140V) that is less than the set voltage value (200) V, as described above, the primary winding 20 Is the number of turns of the first primary winding 200, and thus the number of turns is “NP1”.

  Thus, the power supply circuit 1 according to the present embodiment has an ideal turns ratio depending on whether the voltage value of the DC power supplied to the primary power line L1 exceeds or does not exceed the set voltage value (200) V. So that the number of turns of the primary winding 20 is changed.

  On the other hand, in the power supply circuit 100 shown in the reference example of FIG. 5, the number of turns of the primary winding 20 remains the same as the number of turns of the primary winding 20 itself. For this reason, when DC power having a voltage value outside the range of voltage values at which an ideal turns ratio can be obtained is input to the primary winding 20, the following problems occur.

  For example, depending on how far the voltage value of the DC power input to the primary winding 20 is out of the range of voltage values at which an ideal turns ratio can be obtained, the transformation ratio obtained by the transformer 2 is ideal. In order to approach the transformation ratio, it is necessary to adjust the period during which the pulse output from the PMW control unit 42 is at a high level. For this purpose, an IC capable of setting various high-level periods is necessary, which increases costs.

  In addition, when DC power having a voltage value outside the range of voltage values at which an ideal turns ratio can be obtained is input to the primary winding 20, the transformer 2 cannot operate with little loss. There is a problem that the conversion efficiency of the voltage value is lowered and an ideal transformation ratio cannot be obtained. At that time, the DC power supplied to the load 11 decreases.

  In contrast, in the power supply circuit 1 according to the present embodiment, the number of turns of the primary winding 20 of the transformer 1 is switched according to the voltage value of the power supplied to the primary winding 20, so An ideal turns ratio corresponding to the voltage value can be obtained.

  Thereby, an ideal winding ratio can be obtained without requiring an IC capable of setting various high-level periods, and an ideal transformation ratio can be obtained. As a result, even if the range of the voltage value of the supplied power reaches a wide range, the transformer 2 can operate with little loss, so that the direct current power that can be supplied to the load 11 on the secondary winding 21 side is reduced. Can be prevented.

  In the power supply circuit 1 according to the present embodiment having the above advantages, during execution of the energy saving mode, the AC power rectified by the diode bridge 5 is not boosted to a constant voltage value by the PFC circuit 6 as described above. Therefore, the primary winding 20 of the transformer 2 is supplied with DC power that maintains the voltage value in the supplied state. At that time, the voltage value of the DC power supplied to the primary winding 20 of the transformer 2 may fluctuate greatly.

  Thus, even when the voltage value of the DC power supplied to the primary winding 20 of the transformer 2 fluctuates greatly, the number of turns of the primary winding 20 so that an ideal turn ratio can be obtained according to the voltage value. Therefore, the power supply circuit 1 according to the present embodiment is particularly useful during execution of the energy saving mode.

DESCRIPTION OF SYMBOLS 1 Power supply circuit 2 Transformer 3 Zener diode 4 Turn number change part 5 Diode bridge 6 PFC circuit 7 PFC control part 8 Primary side smoothing capacitor 13 Energy saving control part 20 Primary winding 200 1st primary winding 201 2nd primary winding 21 Secondary winding 40 First transistor 41 Second transistor 42 PWM controller

Claims (2)

A transformer including a primary winding and a secondary winding;
A winding number changing unit for changing the number of turns of the primary winding;
A rectifier circuit for rectifying the supplied AC power;
A booster circuit that boosts the voltage value of the rectified AC power to a preset voltage value;
A smoothing circuit that smoothes the boosted AC power to obtain DC power;
A first control unit that feedback-controls the booster circuit so that the voltage value of the DC power becomes the preset voltage value upon receiving feedback of the voltage smoothed by the smoothing circuit;
The DC power obtained by the smoothing circuit is configured to be supplied to the primary winding via a primary power line ,
The winding number changing unit changes the number of turns of the primary winding according to the voltage value of the power supplied to the primary winding,
The transformer transmits the power supplied to the primary winding to the secondary winding by electromagnetic induction,
The primary winding is divided into a plurality of sections;
The winding number changing unit conducts a section according to a voltage value of the power among the plurality of sections,
The winding number changing unit is
A plurality of switching elements that are provided corresponding to each of the plurality of sections and that operate to conduct each of the plurality of sections;
A second control unit that operates any one of the plurality of switching elements according to a voltage value of the power ;
A Zener diode having a cathode connected to the primary power line and an anode connected to the second controller;
The primary winding comprises a first primary winding and a second primary winding;
The plurality of switching elements operate to conduct the first switching element that is activated to conduct the first primary winding, and the first primary winding and the second primary winding. A second switching element,
The second control unit outputs a pulse with a predetermined duty ratio to the first switching element when a voltage value of DC power supplied to the primary power line is less than a breakdown voltage of the Zener diode. The first switching element is turned on / off in synchronization with the pulse, and when the voltage value of the DC power supplied to the primary power line exceeds the breakdown voltage of the Zener diode, a predetermined value is applied to the second switching element. A power supply circuit that outputs a pulse having a duty ratio of 2 and turns on and off the second switching element in synchronization with the pulse . It further comprises an energy saving control unit that executes an energy saving mode for inputting the rectified AC power to the smoothing circuit without boosting the voltage value of the AC power rectified by the rectifying circuit by the boosting circuit. The power supply circuit according to claim 1.

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