Power supply device

Abstract

To make it possible to suppress noise and stabilize output voltage even if a load current increases when an office machine equipped with a power supply device is in a sleep mode.SOLUTION: A power supply device includes a rectifier circuit that rectifies an AC voltage input from an AC power source, a smoothing circuit that smooths the rectified voltage, and a transformer circuit that transforms the smoothed voltage, and converts the voltage transformed by the transformer circuit into DC and supplies the DC voltage to a load. The smoothing circuit has a first capacitor, a switch element connected in series to the first capacitor, and a second capacitor connected in parallel to the first capacitor and the switch element and having a smaller capacitance than the first capacitor. The power supply device includes a load current detection circuit that detects a current corresponding to a load current flowing through the load, and a switching circuit that, when the detected current detected by the load current detection circuit is equal to or greater than a predetermined threshold value, turns on the switch element to connect the first capacitor.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a power supply device that rectifies and smoothes an AC voltage, transforms the transformed voltage, and supplies the resulting DC voltage to a load. In particular, the present invention relates to a power supply device that can suppress noise and stabilize the output voltage while reducing power loss due to a smoothing capacitor. [Background technology]

[0002] Energy-saving electrical appliances are being required to consume less power in order to comply with the International Energy Star Program (Energy Star), an environmental labeling system. Because of this system, office equipment manufacturers are competing with each other over the TEC value, which is an index (evaluation standard) of equipment power consumption. The TEC value is the amount of electricity (kWh) consumed by a multifunction device or printer in a conceptual week (five days of alternating operation and sleep / off + two days of sleep / off).

[0003] This TEC value is greatly affected not only by the operating power but also by the power consumed during sleep (when operating in sleep mode). Recently, the power consumed during sleep has become smaller, for example, to 0.5W or less. However, the power consumed during operation is high, ranging from 200 to 500 W, depending on the performance of the office equipment. In particular, aluminum electrolytic capacitors (hereinafter referred to as "electrolytic capacitors") used as smoothing capacitors have the characteristic of generating leakage current, and the larger the capacitance, the larger the leakage current tends to be. In addition, there is a large individual variation and it is temperature dependent. Therefore, if an electrolytic capacitor is connected during sleep, it leads to a decrease in efficiency due to loss in the electrolytic capacitor.

[0004] Patent Document 1 describes a power supply device that improves efficiency degradation caused by loss in the electrolytic capacitor. As shown in FIG. 16, the power supply device described in Patent Document 1 comprises an electrolytic capacitor, a switch element connected in series with the electrolytic capacitor, and a film capacitor (or ceramic capacitor) connected in parallel with the electrolytic capacitor and the switch element and having a smaller capacitance than the electrolytic capacitor. When the calculation unit receives a load operation command, it transitions from sleep mode to operating mode and starts operating the load after bringing the switch element into a conductive state.

[0005] With this invention, the loss due to leakage current of the electrolytic capacitor during sleep can be suppressed because the electrolytic capacitor is disconnected, while the film capacitor (or ceramic capacitor) functions as a means for noise suppression. Furthermore, when the calculation unit receives a command to stop the operation of the load, the load state transitions from a state consuming a certain amount of power to a state consuming relatively little power, and the load operation is started after a certain time has elapsed since the switching element was switched from a non-conductive state to a conductive state, thereby achieving stable operation. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2014-192952 A Summary of the Invention [Problem to be solved by the invention]

[0007] According to the power supply device described in the above Patent Document 1, it is considered that the noise regulation during operation (operation such as printing) and the sleep efficiency during sleep are satisfactory. However, in recent years, there has been a demand for a function to quickly determine commands such as printing even during sleep, but there was a risk that this could not be fully addressed. Specifically, the power supply circuit is required to deal with an instantaneous peak load, that is, when a request for a print operation is received even during sleep, the load current value increases instantaneously. In the power supply device described in Patent Document 1, when an instantaneous peak load (or a mode in which power consumption during sleep is large) occurs, the current flowing through the load (hereinafter simply referred to as "load current") increases rapidly, but since the calculation unit receives an operation command for the load, the electrolytic capacitor with a relatively large capacitance at that time is not in a connected state, so there is a problem that the regulation of noise, especially terminal noise, and the stability of the output voltage cannot be satisfied.

[0008] In other words, the division into operating and sleep (low power consumption) modes is determined by the device's mode (load operation / stop operation command), but if the electrolytic capacitor is connected or disconnected using a switch element based on this division, the electrolytic capacitor cannot be instantly connected even if the current value suddenly increases during sleep. In such cases, noise cannot be sufficiently suppressed using only a film capacitor (or ceramic capacitor) with a relatively small capacitance, which can result in problems such as malfunction of the control IC that controls the main switching element that switches the voltage applied to the primary winding of the transformer, generation of a ripple voltage higher than the specified level in the voltage supplied to the secondary side after transformation by the transformer, and malfunction of the equipment.

[0009] The present invention has been made to solve such problems, and aims to suppress noise and stabilize the output voltage while reducing power loss, even if the current flowing to the load increases during sleep mode. [Means for solving the problem]

[0010] That is, the present invention is a power supply device comprising a rectifier circuit that rectifies an AC voltage input from an AC power source, a smoothing circuit that smoothes the voltage rectified by the rectifier circuit, and a transformer circuit that transforms the voltage smoothed by the smoothing circuit, and converts the voltage transformed by the transformer circuit into DC and supplies it to a load, wherein the smoothing circuit has a first capacitor, a switch element connected in series to the first capacitor, and a second capacitor connected in parallel to the first capacitor and the switch element and having a smaller capacitance than the first capacitor, and further comprises a load current detection circuit that detects a current corresponding to the load current flowing through the load, and a switching circuit that turns on the switch element to connect the first capacitor when the detected current detected by the load current detection circuit is equal to or greater than a predetermined threshold, and turns off the switch element to disconnect the first capacitor when the detected current is less than the predetermined threshold.

[0011] With this configuration, regardless of the mode of the device, the load current detection circuit detects a current corresponding to the load current and switches the connection state of the first capacitor (a smoothing capacitor with a larger capacitance than the second capacitor) in response to the load current. Therefore, when the load current is small (during sleep mode), the first capacitor is disconnected, reducing power loss. On the other hand, even if the load current suddenly increases during sleep mode, the load current detection circuit quickly detects this and connects the first capacitor, thereby suppressing noise and stabilizing the output voltage.

[0012] The transformer circuit of the present invention comprises a transformer having a primary winding, a first secondary winding and a second secondary winding, a main switching element connected in series to the primary winding of the transformer, and a control unit that controls the on / off of the main switching element, and is characterized in that the second secondary winding, whose induced voltage changes depending on the load current, functions as a load current detection circuit, and the induced voltage of the second secondary winding is output to a switching circuit. With this configuration, an induced voltage in the secondary winding according to the load current is output to the switching circuit, so that the capacitor can be connected or disconnected depending on the value of the induced voltage.

[0013] Furthermore, the switching circuit of the present invention is characterized in that it has a Zener diode electrically connected in series to the control terminal of the switch element, and when the detection current is equal to or greater than a predetermined threshold value, the Zener diode becomes conductive and a voltage is applied to the control terminal of the switch element to turn on the switch element. With this configuration, the first capacitor can be connected or disconnected depending on the voltage value of the induced voltage in the secondary winding that corresponds to a predetermined current value of the load current.

[0014] Furthermore, the switching circuit of the present invention has a first resistor and a third capacitor connected in series between the output of the smoothing circuit and the control terminal of the switch element, and is characterized in that when an AC voltage is applied, the voltage smoothed by the smoothing circuit via the first resistor and the third capacitor is applied to the control terminal of the switch element, thereby turning on the switch element for a predetermined period of time. With this configuration, the first capacitor can be connected even when the AC power supply is turned on, and the output voltage can be stabilized.

[0015] Furthermore, the fourth capacitor and the contacts of the relay connected in series are connected in parallel to the AC power supply, the second secondary winding of the transformer circuit is connected to one end of an excitation coil of the relay, and the other end of the excitation coil of the relay is connected to one end of a current path of a switch element, and when the switch element is turned on, the relay is excited, the contacts of the relay are turned on, and the fourth capacitor is connected in parallel to the AC power supply. By configuring it in this manner, when the load current increases, the fourth capacitor is connected, thereby suppressing noise in the AC power supply, and when the load current is low, the fourth capacitor is disconnected, thereby suppressing losses due to the fourth capacitor.

[0016] The power supply device also has a shunt resistor that detects the load current, and the switching circuit applies a voltage corresponding to the voltage generated across the shunt resistor to the control terminal of the switch element, thereby switching the switch element on and off in response to the load current.

[0017] With this configuration, the load current can be detected using a shunt resistor, and the first capacitor can be connected when the current value increases even during sleep mode, thereby reducing noise and stabilizing the output voltage. Effect of the Invention

[0018] According to the present invention, it is possible to reduce power loss, while suppressing noise and stabilizing the output voltage by instantly connecting a capacitor even if the current flowing through the load increases during sleep mode. [Brief description of the drawings]

[0019] [Figure 1] 1 is a circuit diagram of a power supply device according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a circuit diagram of the transformer circuit shown in FIG. [Diagram 3] 2 is a circuit diagram of a switching circuit shown in FIG. 1. [Figure 4] FIG. 13 is a diagram showing the relationship between the load current and the induced voltage in the second secondary winding. [Diagram 5] 11 is a diagram showing the stability of the output voltage when the load current of the power supply device according to the embodiment increases. FIG. [Figure 6] FIG. 11 is a diagram showing the stability of the output voltage when the load current of a conventional power supply device increases. [Figure 7] FIG. 1 is a circuit diagram of a transformer circuit in a conventional power supply device. [Figure 8] FIG. 11 is a circuit diagram of a switching circuit in a conventional power supply device. [Figure 9] 5A and 5B are diagrams illustrating the stability of the output voltage when the power supply device according to the embodiment is turned on. [Figure 10] 1 is a diagram illustrating the stability of the output voltage when the power supply of a conventional power supply device is turned on. [Figure 11] 5 is a waveform diagram showing the stability of the output voltage when the power supply device according to the embodiment is turned on. FIG. [Figure 12] 1 is a waveform diagram showing the stability of the output voltage when the conventional power supply device is turned on. [Figure 13]FIG. 4 is a circuit diagram of a power supply device according to another embodiment of the present invention. [Figure 14] FIG. 13 is a circuit diagram of a power supply device according to yet another embodiment of the present invention. [Figure 15] FIG. 15 is a circuit diagram of the switching circuit shown in FIG. [Figure 16] FIG. 1 is a circuit diagram of a power supply device described in a prior art document. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] A power supply device according to an embodiment of the present invention will be described with reference to FIGS. 1 to 3. FIG. Fig. 1 is a circuit diagram of a power supply device 1 according to an embodiment of the present invention. Fig. 2 is a circuit diagram of a transformer circuit 11 shown in Fig. 1, and Fig. 3 is a circuit diagram of a switching circuit 12 shown in Fig. 1. This power supply device 1 comprises a transformer circuit 11 which converts an AC power source AC into a DC voltage which is rectified by a diode bridge D1 (corresponding to the rectifier circuit of the present invention) and smoothed by capacitor C1 (corresponding to the first capacitor of the present invention) and capacitor C2 (corresponding to the second capacitor of the present invention) via a switching circuit 12 which does not perform any operation on the DC voltage, and a voltage V2 generated by the transformer circuit 11 which is rectified and smoothed by diode D4 and capacitor C5 and supplied to a load as an output voltage VOUT. Capacitor C1 is connected in series with switch element Q1, and a gate voltage VG is supplied to the gate (control terminal) of switch element Q1 from switching circuit 12. An induced voltage VN (detailed circuit description will be given later) is also supplied from transformer circuit 11 to switching circuit 12. Capacitors C1 and C2 correspond to the smoothing circuit of the present invention.

[0021] The transformer circuit 11 includes a transformer T1 having a primary winding P to which a rectified and smoothed voltage VS is supplied and two secondary windings, a first secondary winding S and a second secondary winding N, a main switching element Q2 connected in series with the primary winding P, and an output of a control IC that controls the main switching element Q2 is connected to the gate of the main switching element Q2. Although not shown, the induced voltage of the second secondary winding N is rectified and smoothed and is supplied as a power supply voltage for the control IC. Also, the induced voltage VN of the second secondary winding N is output to the switching circuit 12.

[0022] The control IC is a well-known integrated circuit for switching power supplies, and although not shown, when the power is turned on, a rectified voltage is supplied until the induced voltage VN of the second secondary winding N rises, and the control IC controls the main switching element Q2 so that the output voltage VOUT on the secondary side is kept constant by a feedback signal of the output voltage VOUT. The control IC corresponds to the "control unit" of the present invention. Although MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are used for the switch element Q1 and the main switching element Q2, other elements that can be turned on and off, such as IGBTs, may also be used. The induced voltage V2 of the first secondary winding S is output after being rectified and smoothed by the diode D4 and capacitor C5 shown in Fig. 1. The load current value is the current value when the output voltage VOUT is supplied to the load.

[0023] A voltage (smoothed voltage) VS smoothed by a smoothing circuit is supplied to a switching circuit 12 shown in FIG. A resistor R1 (corresponding to the first resistor of the present invention), a capacitor C3 (corresponding to the third capacitor of the present invention) and a resistor R2 are connected in series between the positive terminal to which the smoothed voltage VS is applied and the GND terminal. The connection point between the capacitor C3 and the resistor R2 is connected to one end of the resistor R3, the other end of the resistor R3 is connected to the anode of the Zener diode D2, and the cathode of the Zener diode D2 is connected to the cathode of the diode D3. The induced voltage VN of the second secondary winding N is supplied from the transformer circuit 11 to the anode of the diode D3. One end of the resistor R2 is a connection point between the capacitor C3 and the resistor R3, and a gate voltage VG is connected to the gate of the switch element Q1. The other end of the resistor R2 is connected to GND.

[0024] With the above configuration, the switching circuit 12 is supplied with the induced voltage VN of the second secondary winding N from the transformer circuit 11, and the on / off of the switch element Q1 is controlled by the voltage value. The induced voltage VN of the second secondary winding N supplied from the transformer circuit 11 changes in voltage depending on the load current of the secondary winding S.

[0025] Figure 4 shows the relationship between the load current and the induced voltage VN of the second secondary winding N. As the load current increases, the induced voltage VN increases. Note that the load current shown in Figure 4 is measured up to 3.0A because the maximum instantaneous peak load current is 3.0A even in sleep mode. In this embodiment, the second secondary winding N of the transformer T1 corresponds to the "load current detection circuit" of the present invention, and detects the magnitude of the load current by the induced voltage of the second secondary winding according to the load current, and turns on and off the switch element Q1. In other words, the detection current is a current according to the load current that flows due to the induced voltage of the second secondary winding N that functions as the load current detection circuit.

[0026] As an example, a transformer T1 can be used in which the primary winding P has a winding of 40 T (turns), the first secondary winding S has a winding of 3 T, and the second secondary winding N has a winding of 12 T. If the number of turns of the transformer T1 is different, the measured values ​​will differ from those in Figure 4. In that case, even if the induced voltage changes due to a transformer with a different number of turns, the voltage value of the on / off operation of the switch element Q1 can be adjusted by adjusting the constants of the voltage value of the Zener diode D2 and the resistor R3, etc.

[0027] Furthermore, if the device in which the power supply device 1 is used is a printer, it will often be configured with two outputs, a 5V output and a 24V output, as shown in Table 1. Table 1 shows an example of each voltage output used in a printer and the maximum current value at each status. Each status can be classified into two modes (sleep mode and non-sleep mode). It is also assumed that a printer using the power supply device 1 of the present invention has five operating statuses. In this embodiment, no distinction is made between the 5V output and the 24V output, and the currents of the two outputs are collectively treated as the load current.

[0028] [Table 1] In this embodiment, when the load current becomes 0.1 A, the capacitor C1 is connected. Therefore, the current (detection current) that flows due to the induced voltage of the second secondary winding N when the load current is 0.1 A corresponds to the "predetermined threshold value" of the present invention. If the load current is 0.1 A, in this embodiment, the induced voltage VN is 14 V as shown in FIG. 4. This is the voltage value of the induced voltage corresponding to the load current.

[0029] Here, "connecting a capacitor" means connecting a capacitor between the positive terminal of the rectified voltage on the primary side of the transformer and the GND terminal, and also means connecting a capacitor in parallel to the AC power source AC. In addition, normal sleep mode refers to a state in which a minimum amount of current is flowing and signals such as print instructions can be received; standby mode refers to a state in which printing operations can be performed immediately, for example, by pressing the "start printing" button and printing can be performed at any time; light load mode refers to a state in which operations other than printing, such as scanning functions and data transfer / save operations, and operating mode refers to the printer's printing operation. Capacitor C1 is connected when the current increases in sleep mode due to receiving a print instruction (when receiving a print instruction), when in non-sleep mode, during standby, light load, and during operation. Note that in the present invention, capacitor C1 is turned on and off depending on the magnitude of the load current, so that even in standby, if the load current falls below a specified current value, capacitor C1 is disconnected.

[0030] As a result, the power supply unit 1 is equipped with a function that quickly processes command signals for printing, etc. even during sleep mode, and because capacitor C1 is connected to the power supply unit 1, it is possible to satisfy noise regulations, especially terminal noise, and output stability even if the load current increases suddenly.

[0031] Next, the fact that the output voltage is stable even if the load current increases during sleep will be described with reference to FIGS. Fig. 5 is a diagram showing the stability of the output voltage when the load current of the power supply device according to the embodiment of the present invention increases, and Fig. 6 is a diagram showing the stability of a conventional power supply device. Each diagram shows (1) the output voltage (voltage between VOUT and ground), (2) the load current, and (3) the VG voltage (the voltage applied to the gate terminal of the switch element Q1). The measurement points of the measurement data in FIG. 6 are the same as those in FIG. 5, but the transformer circuit and the switching circuit are the transformer circuit 21 and the switching circuit 22 shown in FIG. 7 and FIG.

[0032] FIG. 7 shows a transformer circuit in a conventional power supply device, and FIG. 8 shows a switching circuit in a conventional power supply device. The transformer circuit 21 shown in Fig. 7 does not output an induced voltage VN of the second secondary winding N as shown in Fig. 2. In the switching circuit 22 shown in Fig. 8, a resistor R11, a switch S1, and a resistor R12 are connected in series between the positive terminal to which the smoothed voltage VS is applied and the GND terminal, but the induced voltage VN from the transformer circuit 21 as shown in Fig. 3 is not connected to the connection point between the switch S1 and the resistor R12. A gate voltage VG output to the gate of the switch element Q1 is connected to the connection point between the switch S1 and the resistor R12. The switch S1 is turned on when the capacitor C1 is connected by an external signal.

[0033] Although the explanation is out of order, Fig. 5 shows that the load current increases from less than 0.1 A to 3 A, and the load current exceeds a predetermined threshold, causing the switching circuit 12 to operate and connect the capacitor C1, and the output voltage VOUT is stable due to the connection of the capacitor C1. On the other hand, Fig. 6 shows that the switch S1 shown in Fig. 8 remains off, so the capacitor C1 is not connected, and the output voltage is not stable.

[0034] 3 has a function of stabilizing the output voltage at the time of power-on by connecting the capacitor C1 when the power is turned on, in addition to the function of turning on and off the switch element Q1 depending on the magnitude of the load current. This function is achieved by the configuration of the resistor R1, capacitor C3, and resistor R2 inserted between the positive terminal to which the smoothed voltage VS is applied and the GND terminal.

[0035] Fig. 9 is a diagram for explaining the stability of the output voltage when the power supply device according to the embodiment is turned on. Four waveforms are shown in Fig. 9, from the top: (A) AC power supply voltage AC, (B) rectified voltage (both ends of capacitor C2), (C) Q1 gate voltage (voltage VG), and (D) output voltage VOUT. The bottom shows the connection state of capacitor C1 (E).

[0036] When the AC power supply voltage is turned on (A), the rectified voltage is immediately output as a half-wave rectified waveform (B). Here, resistor R1, capacitor C3, and resistor R2 of the switching circuit 12 are connected in series between the positive terminal of the smoothed voltage VS and the GND terminal. Therefore, during the initial predetermined time until the capacitor C3 is charged, the voltage at one end of resistor R2 (the other end of resistor R2 is connected to GND) divided by resistors R1 and R2 is applied to the gate of switch element Q1 (C), switching element Q1 is turned on, and capacitor C1 is connected (E). By connecting capacitor C1, the half-wave rectified voltage is smoothed (B), and the rectified voltage stabilizes at a voltage value close to the peak value of the half-wave waveform.

[0037] At almost the same time, the control IC of the transformer circuit 11, to which the rectified voltage is supplied, starts operating, the main switching element Q2 starts turning on and off, and the output voltage rises (D). As the output voltage rises, the output of the second secondary winding N of the transformer T1 is supplied to the Q1 gate voltage, which gradually increases (C).

[0038] Therefore, since the capacitor C1 is connected for a predetermined initial time after the AC power supply AC is turned on, the output voltage rises stably. The predetermined time period of the present invention corresponds to the time period from when the power is turned on until the output of the second secondary winding N of the transformer T1 is supplied. In contrast to this, in the case of the switching circuit 22 of the conventional power supply device shown in FIG. 8, the output voltage VOUT rises as shown in FIG. 10, and the output voltage is not stable when the power supply rises. Figure 10 is a diagram explaining the stability of the output voltage when the conventional power supply device is turned on. Each waveform shown in Figure 10 is a diagram of the waveform at the same measurement point shown in Figure 9, and although the explanation is omitted, the difference from Figure 9 is that the capacitor C1 is not connected at the time of turning on the AC power supply, but is connected with a delay of about two and a half cycles after the AC power supply is turned on.

[0039] The cause of this instability is that the switching circuit 22 shown in Fig. 8 does not have the capacitor C3 of the switching circuit 12 shown in Fig. 3, and the switch S1 is turned on and off by an external signal. Also, it is not connected to apply the induced voltage VN from the secondary winding N of the transformer T1. The capacitor C1 is connected only after the output voltage rises and the external signal is turned on.

[0040] As described above, the stabilization of the output voltage during the rise of the AC power supply has been explained by comparing the difference between the switching circuit of the present invention and the conventional circuit with reference to FIGS. 11 and 12 are waveform diagrams showing the output stability of the power supply device according to the embodiment and the conventional circuit when the power is turned on, and are actual measurement examples. The three waveforms in each diagram and the measurement points of the power supply device 1 are the same as those in Fig. 5 and Fig. 6, but (4) the waveform of the AC power supply voltage is added. The stability of the output voltage shown in FIGS. 11 and 12 is almost the same as that described with reference to FIGS.

[0041] The switching circuit 12 of the power supply unit 1 connects or disconnects the capacitor C1 based on a predetermined current value of the load current, thereby stabilizing the output voltage when the load suddenly increases even during sleep mode, and also stabilizing the output voltage when the power supply starts up.

[0042] Fig. 13 is a circuit diagram of a power supply device according to another embodiment. The power supply device 100 shown in Fig. 13 is obtained by adding a circuit to the power supply device 1 shown in Fig. 1, which connects a capacitor C4 (corresponding to the fourth capacitor of the present invention) in parallel with the AC power supply AC, and connects or disconnects the capacitor C4 depending on the load current. Since the power supply device 1 shown in FIG. 1 is the same as the power supply device 1 shown in FIG.

[0043] Although constant connection of capacitor C4 to the AC power supply AC is effective in suppressing noise, it is connected even when the power supply device is almost unloaded, and this causes power loss due to capacitor C4. Therefore, when the current according to the load current of the power supply device 100 is below a predetermined threshold, the loss can be suppressed by disconnecting capacitor C4.

[0044] The circuit added to the power supply device 1 shown in Fig. 1 to the power supply device 100 shown in Fig. 13 is a circuit that connects the induced voltage VN of the transformer circuit 11 to the drain of the switch element Q1 (corresponding to one end of the current path in the present invention) via the exciting coil of the relay RL1 and a diode D6. In addition, a common mode choke coil L1 is inserted between the AC power supply AC and the diode bridge D1. When the current corresponding to the load current exceeds a predetermined threshold and the switch element Q1 turns on, the relay RL1 also turns on. Because the contact of the relay RL1 and the capacitor C4 are connected in series, when the relay RL1 turns on, the capacitor C4 is connected to the AC power source AC, making it possible to suppress noise generated in the AC power source AC, and when the current corresponding to the load current is less than the predetermined threshold, the capacitor C4 is disconnected, making it possible to suppress loss in the capacitor C4.

[0045] Fig. 14 is a circuit diagram of a power supply device according to yet another embodiment, which differs from the power supply device 100 shown in Fig. 13 in the method of detecting the load current and in part of the circuit of the switching circuit 32. 1 and 13, the induced voltage VN of the second secondary winding N of the transformer T1 changes depending on the load current, and the voltage value is input to the switching circuit 12. On the other hand, in the power supply device 200 shown in Fig. 14, a resistor R4 (shunt resistor) for current detection is inserted in series in the secondary output of the transformer T1, the voltage generated across the resistor is amplified by an operational amplifier OP1, and the signal is insulated by a photocoupler PC1 and connected to the switching circuit 32. The load current detection circuit of the embodiment shown in FIG. 14 is composed of a resistor R4 (shunt resistor), an operational amplifier OP1, and a photocoupler PC1. A current corresponding to the load current is converted into a voltage, and the switch element Q1 is turned on and off depending on the voltage value.

[0046] Fig. 15 is a circuit diagram of the switching circuit 32. The switching circuit 32 shown in Fig. 15 does not have components equivalent to the Zener diode D2 and the diode D3 of the switching circuit 12 shown in Fig. 3, and a resistor R23 of the switching circuit 32 is connected to the emitter of the transistor of the photocoupler PC1. In addition, resistor R21, capacitor C23, and resistor R22 are connected in series between the positive terminal to which the smoothed voltage VS is applied and the GND terminal. This is the same as the circuit shown in Figure 3, and performs the same function when the power is turned on. One end of resistor R22 is the connection point with capacitor C23 and resistor R23, and the voltage VG is output to the control terminal of switch element Q1. The collector of the transistor of photocoupler PC1 is connected to the output part of induced voltage VN of transformer circuit 11 via diode D5.

[0047] A current corresponding to the load current flowing through resistor R4 is converted to a voltage, which is amplified by operational amplifier OP1, and a current flows through the diode of photocoupler PC1. A current based on the induced voltage VN of the transformer circuit flows through the transistor of photocoupler PC1 via diode D5. When a current flows through the transistor of photocoupler PC1, the gate voltage VG of switch element Q1 of power supply device 200 rises, and when the load current becomes equal to or exceeds a predetermined threshold, switch element Q1 turns on. When the switch element Q1 is turned on, the capacitor C4 is also connected in addition to the capacitor C1. This suppresses terminal noise and stabilizes the output voltage.

[0048] The present invention connects or disconnects the first capacitor depending on the magnitude of the load current, thereby suppressing power loss while at the same time connecting the first capacitor to suppress noise caused by the increase in current even if the load current increases suddenly during sleep mode, thereby stabilizing the output voltage. [Explanation of symbols]

[0049] 1, 100, 200...power supply unit, 11, 21...transformer circuit, 12, 22, 32...switching circuit.

Claims

1. A power supply device comprising: a rectifier circuit that rectifies an AC voltage input from an AC power supply; a smoothing circuit that smoothes the voltage rectified by the rectifier circuit; and a transformer circuit that transforms the voltage smoothed by the smoothing circuit, the power supply device converting the voltage transformed by the transformer circuit into DC and supplying the DC voltage to a load, the smoothing circuit includes a first capacitor, a switch element connected in series to the first capacitor, and a second capacitor connected in parallel to the first capacitor and the switch element and having a capacitance smaller than that of the first capacitor; a load current detection circuit that detects a current corresponding to a load current flowing through the load; a switching circuit that turns on the switch element to connect the first capacitor when the detected current detected by the load current detection circuit is equal to or greater than a predetermined threshold, and turns off the switch element to disconnect the first capacitor when the detected current is less than the predetermined threshold.

2. the transformer circuit includes a transformer having a primary winding, a first secondary winding, and a second secondary winding, a main switching element connected in series to the primary winding of the transformer, and a control unit that controls the on / off of the main switching element; 2. The power supply device according to claim 1, wherein the second secondary winding, whose induced voltage changes depending on the load current, functions as the load current detection circuit, and the induced voltage of the second secondary winding is output to the switching circuit.

3. 3. The power supply device according to claim 2, wherein the switching circuit has a Zener diode electrically connected in series to a control terminal of the switch element, and when the detected current is equal to or greater than a predetermined threshold, the switching circuit applies a voltage to the control terminal of the switch element such that the Zener diode becomes conductive and the switch element is turned on.

4. 4. The power supply device according to claim 3, wherein the switching circuit has a first resistor and a third capacitor connected in series between the output of the smoothing circuit and the control terminal of the switch element, and when the AC voltage is applied, the voltage smoothed by the smoothing circuit is applied to the control terminal of the switch element via the first resistor and the third capacitor, thereby turning on the switch element for a predetermined period of time.

5. 5. The power supply device according to claim 2, wherein a fourth capacitor and a contact of a relay connected in series are connected in parallel to the AC power supply, a second secondary winding of the transformer circuit is connected to one end of an exciting coil of the relay, the other end of the exciting coil of the relay is connected to one end of a current path of the switch element, and when the switch element is turned on, the relay is excited, the contact of the relay is turned on, and the fourth capacitor is connected in parallel to the AC power supply.

6. the load current detection circuit has a shunt resistor for detecting a current corresponding to the load current; 2. The power supply device according to claim 1, wherein the switching circuit applies a voltage corresponding to a voltage generated across the shunt resistor to a control terminal of the switch element, thereby switching the switch element on and off in accordance with the load current.

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