Power unit and image forming apparatus

Abstract

To efficiently supply power according to a load state without adding a new circuit.SOLUTION: A power supply control unit transits to a stop period when a switching operation of an FET is executed for predetermined frequencies in a switching period S603 YES, the power supply control unit transits to the switching period when it is determined that voltage to be output from a secondary wiring has dropped below target voltage on the basis of voltage to be output from a feedback unit in the stop period S606 YES, the power supply control unit changes time in which the FET is turned on in the switching period on the basis of length of the stop period measured by a measurement unit, and estimates a voltage value of AC voltage on the basis of the output voltage of a voltage dividing unit, length of the stop period, drop voltage per unit time in the stop period of a diode and a capacitor, division ratio of a voltage dividing resistor, and turn ratio of the number of turns of a primary winding and the number of turns of an auxiliary winding S607.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to a power supply device that converts AC voltage input from a commercial AC power source or the like into DC voltage, and to an image forming apparatus equipped with the power supply device. [Background technology]

[0002] It is known that switching power supplies that convert AC voltage input from a commercial AC power source or the like into DC voltage perform switching control according to the state of the load to which they supply power. That is, it is known that switching power supplies perform continuous switching control under heavy load conditions where the power supplied to the load is large, and perform intermittent switching control to improve power supply efficiency under light load conditions where the power supplied to the load is small. For example, in the switching power supply described in Patent Document 1, when performing intermittent switching control, the FB (feedback) voltage is referenced, and when the FB voltage reaches a predetermined voltage or higher, switching operation is restarted from a stopped state. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6700772 Summary of the Invention [Problem to be solved by the invention]

[0004] In order to perform switching control with high power efficiency, switching power supplies must accurately detect the state of the load to which power is being supplied. For example, some switching power supplies detect the load state on the secondary side based on the current flowing through a resistor connected to the primary winding of a transformer. However, the current value flowing through the resistor differs significantly between a heavy load state, in which a large amount of power is supplied to the load, and a light load state, in which a small amount of power is supplied to the load. Therefore, in order to accurately detect the load state, a range switching circuit is required that switches the detection range by changing the resistance value depending on the load state. Therefore, there is a demand for an inexpensive switching power supply that is highly efficient and can accurately detect the load state.

[0005] The present invention has been made under these circumstances, and an object of the present invention is to supply power efficiently in accordance with the load state without adding a new circuit. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, the present invention has the following configuration.

[0007] (1) A transformer including a primary winding, a secondary winding, and an auxiliary winding; a first switching element that supplies or cuts off power to the primary winding by switching operation; control means for controlling the switching operation; feedback means for feeding back a voltage output from the secondary winding; a first rectifying and smoothing unit that rectifies and smoothes an input AC voltage and outputs it to the primary winding; a second rectifying and smoothing unit that rectifies and smoothes a voltage induced in the auxiliary winding; and a voltage dividing unit that has a voltage dividing resistor and divides the voltage output from the second rectifying and smoothing unit with the voltage dividing resistor and outputs it to the control means, wherein the control means is capable of performing intermittent control that repeats a switching period in which the switching operation is performed and a stop period in which the switching operation is stopped, and the control means includes a measurement unit that measures the length of the stop period, and the control means transitions to the stop period after performing the switching operation of the first switching element a predetermined number of times, and transitions to the switching period if, during the stop period, the control means determines, based on the voltage output from the feedback means, that the voltage output from the secondary winding has dropped below a target voltage, and the control means changes the time for which the first switching element is on during the switching period based on the length of the stop period measured by the measurement unit, and the control means estimates a voltage value of the AC voltage based on the output voltage of the voltage divider unit, the length of the stop period, a voltage drop per unit time during the stop period of the second rectifying and smoothing unit, a voltage division ratio of the voltage divider resistors, and a turns ratio between the number of turns of the primary winding and the number of turns of the auxiliary winding.

[0008] (2) An image forming apparatus comprising an image forming unit that forms an image on a recording material, a control unit that controls the image forming unit, and a power supply device that supplies power to the image forming unit and the control unit, wherein the control unit is capable of switching between a print state in which the image forming unit forms an image on a recording material by controlling the image forming unit, a standby state that can be transitioned to the print state, and a sleep state in which power consumption is reduced, and the power supply device comprises a transformer having a primary winding, a secondary winding, and an auxiliary winding, and a power supply device that supplies power to the primary winding by switching operation. a first switching element for turning on or off the secondary winding, control means for controlling the switching operation, feedback means for feeding back a voltage output from the secondary winding, a first rectifying and smoothing section for rectifying and smoothing an input AC voltage and outputting the voltage to the primary winding, a second rectifying and smoothing section for rectifying and smoothing a voltage induced in the auxiliary winding, and a voltage dividing section having a voltage dividing resistor, for dividing the voltage output from the second rectifying and smoothing section with the voltage dividing resistor and outputting the voltage to the control means, and an image forming apparatus capable of performing intermittent control by repeating a switching period and a stop period in which the switching operation of the first switching element is stopped, the control means including a measurement unit that measures the length of the stop period, and during the switching period, the control means transitions to the stop period after performing the switching operation of the first switching element a predetermined number of times, and during the stop period, the control means transitions to the switching period if it determines, based on the voltage output from the feedback means, that the voltage output from the secondary winding has dropped below a target voltage, the control means changes a time for which the first switching element is turned on during the switching period based on the length of the stop period measured by the measurement unit, and the control means estimates a voltage value of the AC voltage based on the output voltage of the voltage divider unit, the length of the stop period, a voltage drop per unit time during the stop period of the second rectifying and smoothing unit, a voltage division ratio of the voltage divider resistor, and a turns ratio between the number of turns of the primary winding and the number of turns of the auxiliary winding. [Effects of the Invention]

[0009] According to the present invention, power can be supplied efficiently according to the load state without adding a new circuit. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a diagram showing the configuration of a laser beam printer according to first to fourth embodiments; [Figure 2] FIG. 1 shows the configuration of a power supply device according to first to fourth embodiments. [Figure 3] 1 is a circuit diagram showing a circuit configuration of a switching power supply device according to a first embodiment of the present invention; [Figure 4] 1 is a flowchart showing a switching control sequence of a power supply control unit during startup and intermittent switching operation in the first embodiment. [Figure 5] FIG. 1 is a diagram illustrating a voltage waveform of the switching power supply device according to the first embodiment. [Figure 6] 1 is a circuit diagram showing a circuit configuration of a switching power supply device according to a second embodiment of the present invention; [Figure 7] 10 is a flowchart showing a switching control sequence of a power supply control unit during startup and intermittent switching operation in a second embodiment. [Figure 8] FIG. 10 is a diagram illustrating a voltage waveform of the switching power supply device according to the second embodiment. [Figure 9] 10 is a flowchart showing a switching control sequence of a power supply control unit during startup and intermittent switching operation in a third embodiment. [Figure 10] FIG. 10 is a diagram illustrating a voltage waveform of the switching power supply device according to the third embodiment. [Figure 11] FIG. 10 is a circuit diagram showing the circuit configuration of a switching power supply device according to a fourth embodiment of the present invention. [Figure 12] 10 is a flowchart showing a switching control sequence of a power supply control unit during startup and intermittent switching operation in a fourth embodiment. [Figure 13] FIG. 10 is a diagram illustrating a voltage waveform of a switching power supply device according to a fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. [Example]

[0012] [Configuration of image forming device] FIG. 1 is a schematic cross-sectional view showing the configuration of a laser beam printer as an example of an image forming apparatus. The laser beam printer 10 (hereinafter referred to as printer 10) includes a photosensitive drum 11, a charging unit 12 that charges the photosensitive drum 11 to a uniform potential, and an exposure device 14 that irradiates the photosensitive drum 11 with laser light to form an electrostatic latent image on the surface of the photosensitive drum 11. The printer 10 also includes a developing unit 13 that develops the electrostatic latent image formed on the photosensitive drum 11 by applying toner to the electrostatic latent image, thereby forming a toner image. In the printer 10, the toner image formed on the photosensitive drum 11 is transferred by a transfer unit 15 to a sheet (not shown) that is a recording material fed from a cassette 18. The sheet with the transferred toner image is transported to a fixing unit 16. In the fixing unit 16, the toner image is fixed to the sheet by heating and pressing, and the sheet with the fixed toner image is discharged onto a tray 17. The image forming section for forming an image on a sheet is composed of a photosensitive drum 11, a charging section 12, a developing section 13, and a transfer section 15.

[0013] The printer 10 also includes a power supply 19, which supplies power to drive units such as motors and the engine control unit 20. The engine control unit 20, which serves as a control unit, includes a CPU (not shown) and a nonvolatile memory (not shown). The CPU controls the image formation operation by the image forming unit and the sheet conveyance operation according to a control program stored in the nonvolatile memory. When a print operation, which is a print state in which an image is formed on a sheet, is completed, the engine control unit 20 transitions the printer 10 to a standby state, which is a waiting state from which the printer 10 can transition to a print state. Furthermore, after transitioning to the standby state, when a predetermined time has elapsed, the engine control unit 20 transitions the printer 10 to a sleep state, which is an energy-saving mode, in order to reduce the power consumption of the printer 10 while it is in standby mode. In this way, the printer 10 has three states: a sleep state, a standby state, and a print state, and the engine control unit 20 can switch the printer 10 between each state.

[0014] [Power supply configuration] Fig. 2 is a diagram illustrating the schematic configuration of a power supply device according to this embodiment. As shown in Fig. 2, power supply device 19 includes a rectifying and smoothing unit that rectifies and smoothes AC voltage input from commercial AC power supply 100, and switching power supply device 200. The first rectifying and smoothing unit is composed of diode bridge 101 and smoothing capacitor 102. The AC voltage input from commercial AC power supply 100 is full-wave rectified by diode bridge 101 via current fuse 103 for circuit protection, and smoothed by smoothing capacitor 102 to generate DC voltage Vin. The potential on the lower side of smoothing capacitor 102 is designated as potential DCL, and the potential on the higher side is designated as potential DCH.

[0015] The DC voltage Vin, which is the charging voltage of the smoothing capacitor 102, is input to the switching power supply device 200. The switching power supply device 200 steps down the input DC voltage Vin to generate a DC voltage Vout, and outputs the generated voltage Vout to a load.

[0016] [Configuration of switching power supply] FIG. 3 is a circuit diagram showing the circuit configuration of a switching power supply device 200. In FIG. 3, the switching power supply device 200 includes an isolated transformer T1 having a primary winding P1 and an auxiliary winding P2 on its primary side and a secondary winding S1 on its secondary side. Note that in FIG. 3, the black circles on the transformer T1 indicate the winding direction of each winding. A field-effect transistor (hereinafter referred to as FET) 1, which serves as a first switching element, is connected in series to the primary winding P1 of the transformer T1. The switching operation of the FET 1 supplies or cuts off power to the primary winding, and energy is supplied from the primary winding P1 on the primary side of the transformer T1 to the secondary winding S1 on the secondary side. Meanwhile, a diode D11 and a capacitor C11, which serve as a rectifying and smoothing circuit that rectifies and smoothes the flyback voltage induced in the secondary winding S1, are provided on the secondary side of the transformer T1.

[0017] Furthermore, while FET1 is in the on state, a voltage (hereinafter referred to as the forward voltage) obtained by multiplying the input voltage Vin by the turns ratio (number of turns NP2 of the auxiliary winding P2 / number of turns NP1 of the primary winding P1) is induced in the auxiliary winding P2 on the primary side of the transformer T1. The voltage induced in the auxiliary winding P2 is rectified and smoothed by the diode D4 and capacitor C4, which form a second rectifying and smoothing section, to generate the power supply voltage V1.

[0018] (Feedback section) The switching power supply device 200 has a feedback unit 205, which is feedback means that feeds back voltage information about the voltage Vout output to a load connected to the power supply device, to the primary side of the transformer T1. The feedback unit 205 generates a voltage signal (hereinafter referred to as the FB terminal voltage) that is input to the FB terminal of the power supply control unit 201 based on the target voltage and the output voltage Vout. The feedback unit 205 has a shunt regulator IC5, a photocoupler PC5, and resistors R51, R52, and R53. The target voltage for the output voltage Vout is set by the reference voltage of the REF terminal (reference terminal) of the shunt regulator IC5, resistors R52, and resistor R53.

[0019] The REF terminal of shunt regulator IC5 receives a voltage obtained by dividing the output voltage Vout using resistors R52 and R53. Shunt regulator IC5 compares the voltage input to its REF terminal with a reference voltage. If the voltage input to its REF terminal is higher than the reference voltage, it becomes conductive, causing current to flow through the LED of photocoupler PC5. This causes the LED of photocoupler PC5 to light up, turning on the phototransistor. As a result, the charged voltage of capacitor C6 is discharged, causing the voltage at the FB terminal of power supply control unit 201 to drop.

[0020] On the other hand, when the voltage input to the REF terminal is equal to or lower than the reference voltage, the shunt regulator IC5 is turned off, and no current flows through the LED of the photocoupler PC5. Then, the LED of the photocoupler PC5 goes out, and the phototransistor turns off. As a result, the capacitor C6 is charged, and the charging voltage rises, causing the FB terminal voltage of the power supply control unit 201 to rise. The FB terminal voltage is a predetermined voltage value when the output voltage Vout is equal to the target voltage. When the output voltage Vout becomes larger than the target voltage, the FB terminal voltage becomes smaller than the predetermined voltage value. When the output voltage Vout becomes smaller than the target voltage, the FB terminal voltage becomes larger than the predetermined voltage value.

[0021] (Power supply control unit) In this embodiment, the power supply control unit 201 has an arithmetic control unit (e.g., a CPU, ASIC, etc.) that operates with a clock signal from an oscillator or the like, and also has a timer (measurement unit) that measures time, and a memory (not shown) that stores data, etc. A power supply voltage V2 generated by a DC / DC converter 204, which will be described later, is supplied to a VC terminal of the power supply control unit 201. The power supply control unit 201 outputs a control signal DS1 that controls the switching operation of FET1 to the FET drive unit 202 based on the FB terminal voltage. The control signal DS1 is a PWM signal of a predetermined frequency, and is controlled at a predetermined on-duty.

[0022] (FET driver) The FET driver 202 controls the switching of FET1 in response to a control signal DS1 output from the power supply controller 201. Specifically, the FET driver 202 generates a drive signal DL to be output to a gate terminal of FET1 in response to the control signal DS1 output from the power supply controller 201, and outputs the drive signal DL to the gate terminal of FET1. FET1 performs a switching operation in response to the drive signal DL output from the FET driver 202. A power supply voltage V1 generated from a voltage induced in an auxiliary winding P2 on the primary side of the transformer T1 is supplied to a VC terminal of the FET driver 202.

[0023] (DC / DC converter, starter circuit) DC / DC converter 204 is a three-terminal regulator or a step-down switching power supply device that generates power supply voltage V2 from power supply voltage V1 input to its VC terminal and outputs the generated power supply voltage V2 from its OUT terminal. Start-up circuit 203 is a three-terminal regulator or a step-down switching power supply device that generates power supply voltage V1 from DC voltage Vin input to its VC terminal and outputs the generated power supply voltage V1 from its OUT terminal. Start-up circuit 203 is a circuit that operates only when power supply voltage V1, generated from a voltage induced in auxiliary winding P2 of transformer T1, is equal to or lower than a predetermined voltage, and is used to supply power supply voltage V1 when switching power supply device 200 is started up.

[0024] [Power supply control unit control sequence] The power supply control unit 201 of this embodiment performs continuous switching operation, which continuously switches the FET1 when the printer 10 is in the printing state. When the printer 10 is in the standby state or sleep state, it performs intermittent switching operation. The state of the printer 10 is communicated to the power supply control unit 201 from the engine control unit 20 by a signal (not shown). The power supply control unit 201 switches between continuous switching and intermittent switching by a signal (not shown).

[0025] During continuous switching operation, the power supply control unit 201 uses the FB voltage to calculate the on-duty of the DS1 signal, and outputs a pulse signal of the calculated on-duty to the control signal DS1. Similar operations are repeated during the printing state.

[0026] 4 is a flowchart showing a control sequence of the output voltage Vout of the power supply control unit 201 during startup and intermittent switching operation in this embodiment. The process shown in FIG. 4 is executed by the power supply control unit 201, which is started when AC voltage is supplied to the switching power supply device 200 from the commercial AC power supply 100 and the power supply voltage V2 is generated by the startup circuit 203 and the DC / DC converter 204. In this embodiment, the power supply control unit 201 detects the state of the load connected to the power supply device by measuring the switching suspension period from when the switching operation of FET1 is stopped to when the switching operation is started based on the FB terminal voltage. Then, the power supply control unit 201 controls the intermittent switching operation of FET1 according to the detected load state.

[0027] In step (hereinafter referred to as S) 301, the power supply control unit 201 outputs a control signal DS1 in which an initial value of on-duty is set to the FET driving unit 202. Note that the on-duty refers to the percentage (%) of one pulse signal cycle of the control signal DS1, which is a pulse signal, that the pulse signal is in the on state. As described above, the FET driving unit 202 outputs a DL signal corresponding to the control signal DS1 to the gate terminal of FET1, which starts the switching operation of FET1, and a minute output voltage Vout is output to the secondary side of the transformer T1.

[0028] In S302, the power supply control unit 201 outputs a control signal DS1 with a value set larger than the previously set on-duty to increase the output voltage Vout. As a result, the FET1 performs a switching operation according to the control signal DS1, and the output voltage Vout output to the secondary side of the transformer T1 increases.

[0029] In S303, the power supply control unit 201 acquires the FB terminal voltage input to the FB terminal and determines whether the acquired FB terminal voltage is equal to or lower than the switching stop threshold for stopping the switching operation of FET1 (FB terminal voltage≦switching stop threshold?). As described above, the FB terminal voltage increases when the output voltage Vout is lower than the target voltage, and decreases when the output voltage Vout is higher than the target voltage. The threshold voltage in this case is a threshold voltage for stopping the switching operation of FET1 when the output voltage Vout is higher than the target voltage. If the acquired FB terminal voltage is equal to or lower than the switching stop threshold voltage, the power supply control unit 201 proceeds to S304, and if the acquired FB terminal voltage is higher than the switching stop threshold voltage, the power supply control unit 201 returns to S302.

[0030] In S304, the power supply control unit 201 stops the switching operation of FET1 and transitions to a switching suspension period, so sets the control signal DS1 output to the FET driving unit 202 to low level. In S305, the power supply control unit 201 resets and starts the timer to measure the time of the switching suspension period.

[0031] In S306, the power supply control unit 201 acquires the FB terminal voltage input to the FB terminal and determines whether the acquired FB terminal voltage is equal to or greater than the threshold voltage for starting the switching operation of FET1 (FB terminal voltage≧switching start threshold?). The threshold voltage in this case is the threshold voltage for starting the switching operation of FET1 when the output voltage Vout is lower than the target voltage. If the acquired FB terminal voltage is equal to or greater than the switching start threshold, the power supply control unit 201 proceeds to S307, and if the acquired FB terminal voltage is less than the threshold voltage, the power supply control unit 201 returns to S306.

[0032] In S307, the power supply control unit 201 stops the timer, and with reference to the timer, determines the on-duty of the control signal DS1 according to the length of the switching suspension period based on the acquired time information of the switching suspension period. The power supply control unit 201 may store in advance in a memory (not shown) data associating the duration of the switching suspension period with the on-duty of the control signal DS1, and determine the on-duty of the control signal DS1 based on the acquired time information of the switching suspension period.

[0033] In S308, the power supply control unit 201 outputs a control signal DS1 set at the on-duty determined in S307 to the FET drive unit 202, causing FET1 to perform a switching operation. In S309, the power supply control unit 201 outputs the control signal DS1 and determines whether FET1 has performed a predetermined number of switching operations. If the power supply control unit 201 determines that FET1 has performed a predetermined number of switching operations, it returns the process to S304. If the power supply control unit 201 determines that FET1 has not performed a predetermined number of switching operations, it returns the process to S308. Thereafter, the power supply control unit 201 repeats the processes of S304 to S309 to control the switching operation of FET1 so that an output voltage Vout corresponding to the load connected to the power supply device is output.

[0034] In this embodiment, the power supply control unit 201 stops the switching operation of FET1 after performing the switching operation of FET1 a predetermined number of times and enters a switching suspension period. Then, when the output voltage Vout decreases due to a load connected to the power supply device and the FB terminal voltage becomes equal to or greater than the switching start threshold, the power supply control unit 201 ends the switching suspension period and repeats the process of performing the switching operation of FET1 a predetermined number of times again. In this embodiment, the switching period during which FET1 performs the switching operation a predetermined number of times has a predetermined duration because the frequency and cycle of the control signal DS1 are constant. On the other hand, the switching suspension period continues until the FB terminal voltage becomes equal to or greater than the threshold voltage that starts the switching operation of FET1, so the switching suspension period varies depending on the load state. Furthermore, when the load is heavy, the power supply control unit 201 increases the on-duty of the control signal DS1 according to the duration of the switching suspension period, thereby increasing the energy supplied to the secondary side of the transformer T1. On the other hand, when the load is light, the power supply control unit 201 decreases the on-duty of the control signal DS1 according to the duration of the switching suspension period, thereby decreasing the energy supplied to the secondary side of the transformer T1.

[0035] [Switching operation of switching power supply] FIG. 5 shows voltage waveforms and signal waveforms in the switching power supply device 200. FIGS. 5(a) and 5(b) show voltage waveforms and signal waveforms when the load connected to the power supply device is small, with FIG. 5(a) showing the voltage waveform of the output voltage Vout and FIG. 5(b) showing the signal waveform (voltage waveform) of the control signal DS1. FIGS. 5(c) and 5(d) show voltage waveforms and signal waveforms when the load connected to the power supply device is large, with FIG. 5(c) showing the voltage waveform of the output voltage Vout and FIG. 5(d) showing the signal waveform (voltage waveform) of the control signal DS1. The vertical axis of FIGS. 5(a) to 5(d) represents voltage, and the horizontal axis represents time. To improve power supply efficiency, the power supply control unit 201 performs intermittent switching control (intermittent control) by repeating stop periods (periods 400 and 402) during which FET1 stops switching operation and switching periods (periods 401 and 403) during which FET1 performs switching operation.

[0036] (Switching operation when the load is small) The operation of the switching power supply device 200 when the load on the secondary side is small will be described with reference to Figures 5(a) and (b). A period 400 shown in Figure 5(b) is a period during which switching of FET1 is stopped, and the control signal DS1 is at a low level (a state in which the control signal DS1 is not output) as shown in Figure 5(b). During the period during which switching of FET1 is stopped, no energy (power) is supplied to the secondary side of the transformer T1, and therefore the output voltage Vout gradually decreases due to the load on the secondary side (the load connected to the power supply device) (Figure 5(a)).

[0037] The period 401 shown in FIG. 5(b) is the switching operation period of FET1. The control signal DS1 has a PWM waveform as shown in FIG. 5(b). The high-level period of one cycle of the control signal DS1 indicates the on-duty. As shown in FIG. 5(a), during the period 401, the output voltage Vout gradually increases. The on-duty of the control signal DS1 is determined by the process of S307 in FIG. 4 described above. In this embodiment, the on-duty of the control signal DS1 is increased in inverse proportion to the duration of the switching stop period (e.g., period 400), as the switching stop period becomes shorter, thereby increasing the amount of power (amount of energy) supplied to the secondary side of the transformer T1. As a result, by supplying a sufficiently large amount of power to the secondary side load during the switching operation period of FET1, high power supply efficiency can be maintained even when the load increases.

[0038] (Switching operation when the load is large) Next, the operation of the switching power supply device 200 when the load on the secondary side is large will be described using Figures 5(c) and (d). Period 402 shown in Figure 5(d) is the period during which switching of FET1 is stopped, and the control signal DS1 is at a low level as shown in Figure 5(d). During the period during which switching of FET1 is stopped, no power (energy) is supplied to the secondary side of the transformer T1, so the output voltage Vout gradually decreases due to the load connected to the power supply device (Figure 5(c)). Furthermore, because the load on the secondary side is larger than in Figures 5(a) and (b), the period during which switching is stopped (period 402) is shorter than period 400 in Figure 5(b) when the load is small.

[0039] Period 403 shown in FIG. 5(d) is the switching operation period of FET1, and the control signal DS1 has a PWM waveform as shown in FIG. 5(d). That is, because period 402, which is the switching stop period, is shorter than period 400 in FIG. 5(b), it is detected that the load on the secondary side is large. Therefore, as shown in FIG. 5(d), the on-duty of the control signal DS1 output during period 403 is larger than the on-duty of the control signal DS1 in FIG. 5(b), which has a small load. This allows the switching operation of FET1 to be performed efficiently even when the load on the secondary side is large.

[0040] Furthermore, when the load is large, the voltage rise in the output voltage Vout due to each switching operation of FET1 in response to the control signal DS1 is smaller than when the load is small. Therefore, in order to keep the voltage ripple (peak-to-peak voltage) of the output voltage constant regardless of the load, the number of switching operations of FET1 is increased when the load is large to compensate for the smaller voltage rise in the output voltage Vout. This reduces switching loss and enables efficient switching depending on the load.

[0041] As described above, the current load state can be detected by measuring the duration of the switching stop period of FET1. Then, the on-duty of the control signal DS1 is determined according to the measured duration of the switching stop period, and the switching of FET1 is controlled. As a result, the on-duty of the control signal DS1 can be varied according to the load state without providing the range switching circuit described above, and efficient power supply can be achieved.

[0042] As described above, according to this embodiment, power can be supplied efficiently in accordance with the load state without adding a new circuit. [Example]

[0043] In the second embodiment, a switching power supply device will be described that supplies a predetermined amount of power to a load even if the DC voltage Vin fluctuates, based on the duration of a switching stop period in intermittent switching control and the voltage of the DC voltage Vin applied to the primary side of a transformer. Note that the configuration of the printer 10, which is the image forming apparatus of this embodiment, is the same as that of the first embodiment, and the same devices will be designated by the same reference numerals, and description thereof will be omitted.

[0044] [Configuration of switching power supply] Fig. 6 is a circuit diagram showing the circuit configuration of a switching power supply device 200 of this embodiment. The circuit diagram shown in Fig. 6 differs from the circuit diagram shown in Fig. 3 of the first embodiment in that a VS terminal is added to the power supply control unit 201, and a resistive voltage divider circuit (voltage divider unit) that divides the power supply voltage V1 and inputs the divided voltage to the VS terminal is added. The other circuit configuration is the same as Fig. 3 of the first embodiment, and the same circuit components are described using the same reference numerals, and their description will be omitted here.

[0045] The forward voltage induced in the auxiliary winding P2 of the transformer T1 is rectified and smoothed by the diode D4 and the capacitor C4, and charged to the capacitor C4. The voltage charged in the capacitor C4 is divided by the voltage division ratio of the resistors R3 and R4, and input to the VS terminal of the power supply control unit 201. The forward voltage is the DC voltage Vin charged in the smoothing capacitor 102 multiplied by the turn ratio between the number of turns NP1 of the primary winding P1 of the transformer T1 and the number of turns NP2 of the auxiliary winding P2. Therefore, a voltage corresponding to the DC voltage Vin is input to the VS terminal; when the DC voltage Vin decreases, the voltage input to the VS terminal also decreases, and when the DC voltage Vin increases, the voltage input to the VS terminal also increases. Therefore, the power supply control unit 201 can detect the DC voltage Vin by detecting the voltage input to the VS terminal, and can detect the AC voltage because the DC voltage Vin is generated by full-wave rectifying the AC voltage input from the commercial AC power supply 100. Therefore, the power supply control unit 201 can efficiently supply power to the load even if the AC voltage input from the commercial AC power supply 100 fluctuates by determining the on-duty of the control signal DS1 according to the obtained input voltage of the VS terminal.

[0046] [Power supply control unit control sequence] The power supply control unit 201 of this embodiment performs continuous switching operation when the printer 10 is in a print state, as in embodiment 1, and performs intermittent switching operation when the printer 10 is in a standby state or sleep state. The continuous switching operation, and the continuous switching operation and intermittent switching operation are also similar to embodiment 1, and therefore a description thereof will be omitted.

[0047] Fig. 7 is a flowchart showing a control sequence of the output voltage Vout of the power supply control unit 201 during startup and intermittent switching operation in this embodiment. Similar to Fig. 4 of the first embodiment, the process shown in Fig. 7 is executed by the power supply control unit 201, which is started up when an AC voltage is supplied from the commercial AC power supply 100 to the switching power supply device 200 and the power supply voltage V2 is generated by the startup circuit 203 and the DC / DC converter 204.

[0048] The processes of S601 to S606 are the same as those of S301 to S306 shown in FIG. 4 of the first embodiment, and therefore their explanations will be omitted. In S607, the power supply control unit 201 stops the timer and acquires time information about the switching non-period by referring to the timer. Then, the power supply control unit 201 acquires the VS terminal voltage input to the VS terminal at the time the timer is stopped, corrects the acquired VS terminal voltage based on the time information about the switching non-period, and estimates the input voltage Vin based on the corrected VS terminal voltage. Then, the power supply control unit 201 determines the on-duty of the control signal DS1 based on the estimated input voltage Vin and the time information about the switching non-period. The processes of S608 and S609 are the same as those of S308 and S309 shown in FIG. 4 of the first embodiment, and therefore their explanations will be omitted.

[0049] [Switching operation of switching power supply] FIG. 8 shows voltage waveforms and signal waveforms in the switching power supply device 200. FIGS. 8(a), 8(b), and 8(c) show voltage waveforms and signal waveforms when the load connected to the power supply device is small. FIG. 8(a) shows the voltage waveform of the output voltage Vout, FIG. 8(b) shows the signal waveform (voltage waveform) of the control signal DS1, and FIG. 8(c) shows the voltage waveform of the VS terminal voltage input to the VS terminal. On the other hand, FIGS. 8(d), 8(e), and 8(f) show voltage waveforms and signal waveforms when the load connected to the power supply device is large. FIG. 8(d) shows the voltage waveform of the output voltage Vout, FIG. 8(e) shows the signal waveform (voltage waveform) of the control signal DS1, and FIG. 8(f) shows the voltage waveform of the VS terminal voltage input to the VS terminal. In FIGS. 8(a) to 8(f), the vertical axis represents voltage and the horizontal axis represents time. In this embodiment, as in the first embodiment, the power supply control unit 201 performs the following intermittent switching control to improve power supply efficiency. That is, the power supply control unit 201 performs intermittent switching control by repeating stop periods (periods 700 and 702) during which the FET1 stops switching operation and switching periods (periods 701 and 703) during which the FET1 performs switching operation.

[0050] (Switching operation when the load is small) The operation of the switching power supply device 200 when the secondary-side load is small will be described using FIGS. 8(a), 8(b), and 8(c). A period 700 shown in FIG. 8(b) is a period during which FET1 is not switching, and the control signal DS1 is at a low level (a state in which the control signal DS1 is not output) as shown in FIG. 8(b). During the period during which FET1 is not switching, no power is supplied to the secondary side of the transformer T1 or the auxiliary winding P2. Therefore, as shown in FIG. 8(a), the output voltage Vout gradually decreases due to the load on the secondary side (the load connected to the power supply device). The rate at which the output voltage Vout decreases is proportional to the magnitude of the load on the secondary side. During the period during which FET1 is not switching, no power is supplied to the auxiliary winding P2, and the voltage charged in the capacitor C4 is discharged as shown in FIG. 8(c). Therefore, the voltage at the VS terminal of the power supply control unit 201 also decreases. The voltage drop per unit time of the VS terminal voltage during the period when switching of the FET1 is stopped is determined by a time constant determined by the resistance values ​​of the resistors R3 and R4 and the capacitance of the capacitor C4.

[0051] The period 701 shown in FIG. 8(b) is the switching operation period of FET1. The control signal DS1 has a PWM waveform as shown in FIG. 8(b). The high-level period of one cycle of the control signal DS1 indicates the on-duty cycle. As shown in FIG. 8(a), during the period 701, the output voltage Vout gradually increases. When the switching operation of FET1 begins, power is supplied to the auxiliary winding P2. Therefore, as shown in FIG. 8(c), the VS terminal voltage increases to a voltage corresponding to the magnitude of the DC voltage Vin, i.e., a voltage obtained by multiplying the input voltage Vin by the turns ratio between the number of turns of the primary winding P1 of the transformer T1 and the number of turns of the auxiliary winding P2, and dividing this voltage by the voltage-dividing resistors R3 and R4. However, the VS terminal voltage begins to increase slightly after the start of the switching operation of FET1 due to a delay caused by the time constant determined by the resistance values ​​of resistors R3 and R4 and the capacitance of capacitor C4.

[0052] The ON duty of the control signal DS1 in the period 701 is determined by the process of S607 in Fig. 7. As described above, in this embodiment, the ON duty of the control signal DS1 (the proportion of the ON time during which FET1 is in the ON state) is determined not only in accordance with the duration of the switching stop period of FET1 but also in accordance with the DC voltage Vin estimated based on the acquired VS terminal voltage. The amount of power supplied to the secondary side of the transformer T1, i.e., to the load, is determined by the product of the DC voltage Vin and the ON time during which FET1 is in the ON state. Therefore, even if the DC voltage Vin fluctuates, by determining the ON duty of the control signal DS1 so that the product of the DC voltage Vin and the ON time during which FET1 is in the ON state remains constant, power can be supplied with high power efficiency.

[0053] However, because the charged voltage of capacitor C4 is discharged during the switching suspension period of FET1, the VS terminal voltage decreases as the switching suspension period elapses. Therefore, the power supply control unit 201 calculates the voltage drop per unit time of the VS terminal voltage in advance and stores it in memory (not shown), and acquires the VS terminal voltage at the end of the switching suspension period of FET1. The power supply control unit 201 can calculate the VS terminal voltage at the start of the switching suspension period by adding the voltage calculated by multiplying the voltage drop data per unit time acquired from the memory by the duration of the switching suspension period to the acquired VS terminal voltage. In this embodiment, the voltage drop per unit time of the VS terminal voltage is measured in advance and stored in memory. The power supply control unit 201 reads it out from the memory when correcting the VS terminal voltage, and corrects the acquired VS terminal voltage. The method for correcting the acquired VS terminal voltage is not limited to the above-described method, and may be calculated by calculation based on, for example, a time constant during discharge.

[0054] The power supply control unit 201 can estimate the DC voltage Vin at the start of the switching suspension period (which is also the end of the switching period of FET1) based on the VS terminal voltage at the start of the switching suspension period calculated by correcting the acquired VS terminal voltage. Also, similar to the first embodiment, the state of the load connected to the power supply device can be detected by detecting the duration of the switching suspension period of FET1. Then, the on-duty of the control signal DS1 is set according to the duration of the switching suspension period, and power is supplied to the load.

[0055] (Switching operation when the load is large) Next, the operation of the switching power supply device 200 when the load on the secondary side is large will be described using FIGS. 8(d), (e), and (f). A period 702 shown in FIG. 8(e) is a period during which switching of FET1 is stopped, and the control signal DS1 is at a low level as shown in FIG. 8(e). During the period during which switching of FET1 is stopped, power (energy) is not supplied to the secondary side of the transformer T1 and the auxiliary winding P2. Therefore, as shown in FIGS. 8(d) and (f), the output voltage Vout and the VS terminal voltage gradually decrease due to the load connected to the power supply device. Because the load on the secondary side (the load connected to the power supply device) is large, as shown in FIG. 8(e), the period 702 is shorter than the period 700 when the load is small as shown in FIG. 8(b). Because the switching stop period is short, the VS terminal voltage does not decrease below the VS terminal voltage when the load is small (FIG. 8(c)). The power supply control unit 201 corrects the VS terminal voltage acquired at the end of the switching suspension period using the duration of the switching suspension period and the voltage drop per unit time of the VS terminal voltage, using the same procedure as in the case of a small load described above, to estimate an accurate DC voltage Vin. As a result, even if the duration of the switching suspension period varies, accurate information about the DC voltage Vin can be estimated, enabling power supply with high power supply efficiency.

[0056] A period 703 shown in FIG. 8(e) is a switching operation period of FET1, and the control signal DS1 has a PWM waveform as shown in FIG. 8(e). That is, the duration of period 702, which is the switching stop period, is shorter than period 700 in FIG. 8(b), and therefore it is detected that the load on the secondary side is large. Therefore, as shown in FIG. 8(e), the on-duty of the control signal DS1 output during period 703 is larger than the on-duty of the control signal DS1 in FIG. 8(b), which has a small load. This allows the switching operation of FET1 to be performed with high power supply efficiency even when the load on the secondary side is large.

[0057] As described above, in this embodiment, as in the first embodiment, the current load state can be detected by measuring the duration of the switching suspension period of FET1. Then, the on-duty of the control signal DS1 is determined according to the measured duration of the switching suspension period, and the switching of FET1 is controlled. This makes it possible to realize an efficient power supply according to the load state without providing the range switching circuit described above. Furthermore, by correcting the acquired VS terminal voltage using the measured duration of the switching suspension period, it is possible to realize an efficient power supply according to the load state even if the voltage value of the AC voltage input from the commercial AC power supply 100 fluctuates.

[0058] In this embodiment and embodiment 1, the switching power supply device is of the flyback type, but the present invention is not limited to the flyback type and can be applied to other switching power supply device types, such as an LLC power supply using an LLC resonant circuit. Also, in this embodiment, the cycle (frequency) of the control signal DS1 is not changed, and the on-duty of the control signal DS1 is changed in accordance with the duration of the switching stop period of FET1, thereby performing switching control corresponding to the load on the secondary side. For example, the on-duty of the control signal DS1 may be fixed to a predetermined value, and the frequency of the control signal DS1 may be changed in accordance with the duration of the switching stop period, thereby performing switching control corresponding to the load on the secondary side and maintaining high power supply efficiency.

[0059] As described above, according to this embodiment, power can be supplied efficiently in accordance with the load state without adding a new circuit. [Example]

[0060] In the first and second embodiments, the intermittent switching control stops the switching operation of FET1 after a predetermined number of switching operations, regardless of the load state. In the third embodiment, an example is described in which the number of switching operations of FET1 is changed according to the duration of the switching stop period of FET1 in the intermittent switching control in order to suppress the generation of high-frequency noise. Note that the configuration of the printer 10, which is the image forming apparatus of this embodiment, is the same as that of the first embodiment, and the same reference numerals are used for the same devices, and the description thereof will be omitted.

[0061] [Configuration of switching power supply] The configuration of the switching power supply device of this embodiment is similar to that of the switching power supply device 200 of the second embodiment shown in FIG. 6, and the same circuit components are denoted by the same reference numerals as in FIG. 6, and description thereof will be omitted here.

[0062] [Power supply control unit control sequence] The power supply control unit 201 of this embodiment performs continuous switching operation when the printer 10 is in a print state, as in embodiment 1, and performs intermittent switching operation when the printer 10 is in a standby state or sleep state. The continuous switching operation, and the continuous switching operation and intermittent switching operation are also similar to embodiment 1, and therefore a description thereof will be omitted.

[0063] Fig. 9 is a flowchart showing a control sequence of the output voltage Vout of the power supply control unit 201 during startup and intermittent switching operation in this embodiment. Similar to Fig. 7 of the second embodiment, the processing shown in Fig. 9 is executed by the power supply control unit 201, which is started up when an AC voltage is supplied from the commercial AC power supply 100 to the switching power supply device 200 and the power supply voltage V2 is generated by the startup circuit 203 and the DC / DC converter 204.

[0064] The processes of S801 to S807 are the same as the processes of S601 to S607 shown in Fig. 7 of the second embodiment, and therefore description thereof will be omitted. In S808, the power supply control unit 201 calculates the duration of the switching period based on the time information of the switching stop period acquired in S807, and determines the number of times FET1 switches based on the duration of the switching period. In this embodiment, the number of times switching is determined based on the duration of the switching stop period so that the sum of the duration of the switching stop period and the duration of the switching period of FET1 remains the same even if the load state changes.

[0065] The process of S809 is the same as the process of S608 shown in Fig. 7 of the second embodiment, and therefore a description thereof will be omitted. In S810, the power supply control unit 201 outputs a control signal DS1 to determine whether FET1 has performed the switching operation the number of times determined in the process of S808. If the power supply control unit 201 determines that FET1 has performed the switching operation the number of times determined in the process of S808, it returns the process to S804. If the power supply control unit 201 determines that FET1 has not performed the switching operation the number of times determined in the process of S808, it returns the process to S808. Thereafter, the power supply control unit 201 repeats the processes of S804 to S810 to control the switching operation of FET1 so that an output voltage Vout corresponding to the load connected to the power supply device is output.

[0066] [Switching operation of switching power supply] FIG. 10 shows voltage waveforms and signal waveforms in the switching power supply device 200. FIGS. 10(a), 10(b), and 10(c) show voltage waveforms and signal waveforms when the load connected to the power supply device is small. FIG. 10(a) shows the voltage waveform of the output voltage Vout, FIG. 10(b) shows the signal waveform (voltage waveform) of the control signal DS1, and FIG. 10(c) shows the voltage waveform of the VS terminal voltage input to the VS terminal. On the other hand, FIGS. 10(d), 10(e), and 10(f) show voltage waveforms and signal waveforms when the load connected to the power supply device is large. FIG. 10(d) shows the voltage waveform of the output voltage Vout, FIG. 10(e) shows the signal waveform (voltage waveform) of the control signal DS1, and FIG. 10(f) shows the voltage waveform of the VS terminal voltage input to the VS terminal. Note that the vertical axis of FIGS. 10(a) to 10(f) represents voltage, and the horizontal axis represents time. In this embodiment, as in the first and second embodiments, the power supply control unit 201 performs the following intermittent switching control to improve power supply efficiency. That is, the power supply control unit 201 performs intermittent switching control that repeats stop periods (periods 900 and 902) during which the switching operation of FET1 is stopped and switching periods (periods 901 and 903) during which the switching operation is performed.

[0067] In this embodiment, in the process of S808 in FIG. 9 described above, the number of times FET1 is switched is changed depending on the length of the switching stop period. Specifically, the total time of the switching stop period of FET1 and the switching period of FET1 (hereinafter referred to as the burst period) is kept constant even if the state of the load connected to the power supply device fluctuates. Therefore, in this embodiment, the length of the switching period is determined depending on the length of the switching stop period so that the burst period is kept constant, and the number of times FET1 is switched is determined depending on the length of the switching period. As a result, the burst period can be kept constant so as not to become short.

[0068] Generally, when a frequency component obtained by multiplying the burst frequency, which is the inverse of the burst period, approaches the resonant frequency of the transformer T1, it is perceived by humans as a high-frequency sound. Generally, the lower the multiplication rate of the multiplied frequency component, the greater the energy. Therefore, when a frequency component with a lower multiplication rate approaches the resonant frequency of the transformer T1, the generated sound also becomes louder. On the other hand, as the load connected to the power supply device increases, the burst period becomes shorter and the burst frequency becomes higher. Therefore, in this embodiment, the switching of FET1 is controlled so that the burst period remains constant even when the load increases. As a result, the burst frequency does not approach the resonant frequency of the transformer T1, thereby suppressing the generation of high-frequency sound.

[0069] (Switching operation when the load is small) The operation of the switching power supply device 200 when the load on the secondary side is small will be described using Figures 10(a), (b), and (c). A period 900 shown in Figure 10(b) is a period during which switching of FET1 is stopped, and the control signal DS1 is at a low level (a state in which the control signal DS1 is not output) as shown in Figure 10(b). During the period during which switching of FET1 is stopped, no power is supplied to the secondary side of the transformer T1 and the auxiliary winding P2, so that the output voltage Vout gradually decreases as shown in Figure 8(a), and the voltage at the VS terminal of the power supply control unit 201 also decreases as shown in Figure 8(c).

[0070] A period 901 shown in Fig. 10(b) is a switching operation period of FET1, and the control signal DS1 has a PWM waveform as shown in Fig. 10(b), and the high-level period of one cycle of the control signal DS1 indicates the on-duty. As shown in Fig. 10(a), in a period 701, the output voltage Vout gradually increases, and the VS terminal voltage increases to a voltage corresponding to the magnitude of the DC voltage Vin, as shown in Fig. 10(c).

[0071] The on-duty of the control signal DS1 during period 901 is determined by the process of S807 in Fig. 9. In this embodiment, the on-duty of the control signal DS1 is determined based on the duration of the switching stop period of FET1 and the acquired VS terminal voltage. Furthermore, in the process of S809 in Fig. 9, in order to keep the burst period constant, the time obtained by subtracting the duration of the switching stop period from the burst period is set as the switching period, and the number of switching operations of FET1 is determined according to the duration of the switching period. This makes it possible to supply a large amount of power to the load during the switching period of FET1, and since the burst frequency does not approach the resonant frequency of the transformer T1, the generation of high-frequency noise can be suppressed.

[0072] (Switching operation when the load is large) Next, the operation of the switching power supply device 200 when the load on the secondary side is large will be described with reference to FIGS. 10(d), (e), and (f). A period 902 shown in FIG. 10(e) is a period during which FET1 is stopped from switching, and the control signal DS1 is at a low level as shown in FIG. 10(e). During the period during which FET1 is stopped from switching, the output voltage Vout and the VS terminal voltage gradually decrease as shown in FIGS. 10(d) and (f). Because the load on the secondary side (the load connected to the power supply device) is large, as shown in FIG. 10(e), the period 902 is shorter than the period 900 shown in FIG. 10(b) when the load is small. The power supply control unit 201 corrects the VS terminal voltage acquired at the end of the switching stop period using the duration of the switching stop period and the voltage drop per unit time of the VS terminal voltage, thereby estimating the DC voltage Vin.

[0073] A period 903 shown in Fig. 10(e) is a switching operation period of FET1, and the control signal DS1 has a PWM waveform as shown in Fig. 10(e), and the high-level period of one cycle of the control signal DS1 indicates the on-duty. As shown in Fig. 10(d), during the period 903, the output voltage Vout gradually increases, and the VS terminal voltage increases to a voltage corresponding to the magnitude of the DC voltage Vin, as shown in Fig. 10(f). Note that the total time (burst period) of the periods 902 and 903 shown in Fig. 10(e) is the same as the total time (burst period) of the periods 900 and 901 in Fig. 10(b) described above.

[0074] The on-duty of the control signal DS1 during period 903 is determined by the process of S807 in Fig. 9. In this embodiment, the on-duty of the control signal DS1 is determined based on the duration of the switching stop period of FET1 and the acquired VS terminal voltage. Furthermore, in the process of S809 in Fig. 9, in order to keep the burst period constant, the time obtained by subtracting the duration of the switching stop period from the burst period is set as the switching period, and the number of switching operations of FET1 is determined according to the duration of the switching period. This makes it possible to supply a large amount of power to the load during the switching period of FET1, and since the burst frequency does not approach the resonant frequency of the transformer T1, the generation of high-frequency noise can be suppressed.

[0075] This embodiment describes a method of performing intermittent switching control so that the burst period remains constant and does not fluctuate even when the load connected to the power supply device increases. For example, a different control method from the method of this embodiment may be used, such as extending the burst period by extending the switching period longer than the shortened switching stop period as the load increases. Furthermore, because the duration of period 902, which is the switching stop period when the load is large, is shorter than the duration of period 900 when the load is small, the predicted secondary-side load value increases, and therefore the control signal DS1 is set to have a large on-duty. This allows sufficient power to be supplied even when the load connected to the power supply device increases.

[0076] As explained above, similarly to the above-mentioned first and second embodiments, the current load state can be detected by measuring the switching stop time. In this embodiment, the switching period (burst period) of FET1 that suppresses high frequency noise is determined in advance, and the switching operation period and switching stop period are allocated within that period to determine the number of times FET1 switches. Therefore, by changing the number of times FET1 switches according to the measured switching stop period and performing control to keep the burst period constant, high frequency noise can be suppressed regardless of the load state.

[0077] As described above, according to this embodiment, power can be supplied efficiently in accordance with the load state without adding a new circuit. [Example]

[0078] In the first to third embodiments, the present invention is applied to a flyback switching power supply. In the fourth embodiment, the present invention is applied to an active clamp switching power supply. The configuration of the printer 10, which is the image forming apparatus of this embodiment, is the same as that of the first embodiment, and the same devices are designated by the same reference numerals, and the description thereof will be omitted.

[0079] [Configuration of switching power supply] FIG. 11 is a circuit diagram showing the circuit configuration of a switching power supply device 200 of this embodiment. The circuit diagram shown in FIG. 11 differs from the circuit diagram shown in FIG. 6 of the second embodiment in that the following circuit is added to the primary side of the transformer T1. That is, on the primary side of the switching power supply device 200, a circuit in which a voltage clamp capacitor C2 and a second switching element FET2 are connected in series is connected in parallel to the primary winding P1. Furthermore, with the addition of FET2, circuits and signal lines for controlling FET2 are added to the power supply control unit 201 and the FET drive unit 202. Furthermore, to reduce losses when FET1 and FET2 are switched off, a voltage resonance capacitor C1 is connected in parallel to FET1. Diodes D1 and D2 are body diodes of FET1 and FET2, respectively.

[0080] (Power supply control unit) Based on the FB terminal voltage, the power supply control unit 201 outputs a control signal DS1 that drives FET1 and a control signal DS2 that drives FET2. The power supply control unit 201 outputs the control signal DS1 to the FET drive unit 202 to drive FET1. When FET1 subsequently turns off, after a dead time during which both FET1 and FET2 are in the off state, the power supply control unit 201 outputs a control signal DS2 so that FET2 turns on. When FET2 turns off, after a dead time, the power supply control unit 201 outputs a control signal DS1 so that FET1 turns on again. The other circuit configuration of the power supply control unit 201 is the same as that shown in FIG. 6 of the second embodiment, and the same circuit components are designated by the same reference numerals and will not be described again here.

[0081] (FET driver) The FET driver 202 generates and outputs drive signals DL and DH to the gate terminals of FET1 and FET2 in response to control signals DS1 and DS2 output from the power supply controller 201. To drive FET2, a power supply voltage is supplied to the VH terminal of the FET driver 202 by a charge pump circuit formed of a capacitor C5 and a diode D5. When the control signal DS1 is set to a high level, the FET driver 202 sets the drive signal DL to a high level, which is output to the gate terminal of FET1, thereby turning on FET1. When the control signal DS2 is set to a high level, the FET driver 202 sets the drive signal DH to a high level, which is output to the gate terminal of FET2, thereby turning on FET2. The other circuit configuration of the switching power supply 200 is the same as that shown in FIG. 6 of the second embodiment. The same circuit components are designated by the same reference numerals, and a description thereof will be omitted here.

[0082] [Power supply control unit control sequence] The power supply control unit 201 of this embodiment performs continuous switching operation when the printer 10 is in a print state, as in embodiment 1, and performs intermittent switching operation when the printer 10 is in a standby state or sleep state. The continuous switching operation, and the continuous switching operation and intermittent switching operation are also similar to embodiment 1, and therefore a description thereof will be omitted.

[0083] Fig. 12 is a flowchart showing a control sequence of the output voltage Vout of the power supply control unit 201 during startup and intermittent switching operation in this embodiment. Similar to Fig. 7 of the second embodiment, the processing shown in Fig. 12 is executed by the power supply control unit 201, which is started up when an AC voltage is supplied from the commercial AC power supply 100 to the switching power supply device 200 and the power supply voltage V2 is generated by the startup circuit 203 and the DC / DC converter 204.

[0084] The processing of S1101 to S1109 is the same as the processing of S601 to S609 shown in Fig. 7 of the second embodiment, and therefore description thereof will be omitted. In S1110, the power supply control unit 201 calculates the time for turning on the clamp FET (FET2) based on the time information of the switching stop period acquired in S1107, and outputs a control signal DS2. After outputting the control signal DS2, the power supply control unit 201 returns the processing to S1104 and stops the switching operations of FET1 and FET2.

[0085] [Switching operation of switching power supply] FIG. 13 shows voltage waveforms and signal waveforms in the switching power supply device 200. FIGS. 13(a), 13(b), 13(c), and 13(d) show voltage waveforms and signal waveforms when the load connected to the power supply device is small. FIG. 13(a) shows the voltage waveform of the output voltage Vout, FIG. 13(b) shows the signal waveform (voltage waveform) of the control signal DS1, FIG. 13(c) shows the signal waveform (voltage waveform) of the control signal DS2, and FIG. 13(d) shows the voltage waveform of the VS terminal voltage input to the VS terminal. Meanwhile, FIGS. 13(e), 13(f), 13(g), and 13(h) show voltage waveforms and signal waveforms when the load connected to the power supply device is large. FIG. 13(e) shows the voltage waveform of the output voltage Vout, FIG. 13(f) shows the signal waveform (voltage waveform) of the control signal DS1, FIG. 13(g) shows the signal waveform (voltage waveform) of the control signal DS2, and FIG. 13(f) shows the voltage waveform of the VS terminal voltage input to the VS terminal. 13(a) to 13(h), the vertical axis represents voltage, and the horizontal axis represents time. In this embodiment, as in the above-described embodiments, the power supply control unit 201 performs the following intermittent switching control to improve power supply efficiency. That is, the power supply control unit 201 performs intermittent switching control that repeats stop periods (periods 1200 and 1202) during which FET1 stops switching operation and switching periods (periods 1201 and 1203) during which switching operation is performed.

[0086] (Switching operation when the load is small) The operation of the switching power supply device 200 when the load on the secondary side is small will be described using Figures 13(a), (b), (c), and (d). A period 1200 shown in Figure 13(b) is a period during which switching of FET1 and FET2 is stopped, and the control signals DS1 and DS2 are at a low level (a state in which the control signals DS1 and DS2 are not output) as shown in Figures 13(b) and (c). During the period during which switching of FET1 and FET2 is stopped, no power is supplied to the secondary side of the transformer T1 and the auxiliary winding P2, so that the output voltage Vout gradually decreases as shown in Figure 13(a), and the voltage at the VS terminal of the power supply control unit 201 also decreases as shown in Figure 13(d).

[0087] 13(b) is the switching period of FET1 and FET2, and the control signals DS1 and DS2 have PWM waveforms as shown in Figures 13(b) and 13(c), and the high-level period of one cycle of the control signals DS1 and DS2 indicates the on-duty. As shown in Figure 13(a), in period 1201, the output voltage Vout gradually increases, and the VS terminal voltage increases to a voltage corresponding to the DC voltage Vin, and remains at that voltage during the switching period of FET1 and FET2.

[0088] As shown in Figures 13(b) and 13(c), the control signal DS2 is output first during the switching period. In the active clamp method of this embodiment, the clamp FET (FET2) is turned on briefly at the end of the switching period and then stopped. This allows a sufficient voltage to be charged to the clamp capacitor C2. Therefore, at the start of the switching period, simply turning on FET2 is sufficient to supply power to the secondary side of the transformer T1. However, if FET1 were turned on first instead of FET2 in this state, excessive voltage would be supplied to the clamp capacitor C2, which could cause damage to FET1 and FET2. Therefore, in this embodiment, when the switching period begins, FET2 is driven before FET1.

[0089] After that, the control signals DS1 and DS2 alternate between high and low states, with dead time (not shown) between them. After the switching operations of FET1 and FET2 have been performed a predetermined number of times, the control signal DS2 is set to high for a predetermined period of time, turning on FET2 and allowing current to flow through FET2 to the clamp capacitor C2. This reduces energy consumption compared to flowing current through the body diode D2 of FET2. After that, FET2 is turned off before the current flowing through FET2 switches from the drain terminal to the source terminal.

[0090] (Switching operation when the load is large) The operation of the switching power supply device 200 when the load on the secondary side is large will be described using Figures 13(e), (f), (g), and (h). A period 1202 shown in Figure 13(f) is a period during which switching of FET1 and FET2 is stopped, and the control signals DS1 and DS2 are at a low level (a state in which the control signals DS1 and DS2 are not output) as shown in Figures 13(f) and (g). During the period during which switching of FET1 and FET2 is stopped, no power is supplied to the secondary side of the transformer T1 and the auxiliary winding P2, so the output voltage Vout gradually decreases as shown in Figure 13(e), and the voltage at the VS terminal of the power supply control unit 201 also decreases as shown in Figure 13(h).

[0091] The period 1203 shown in FIG. 13(f) is the switching period of FET1 and FET2. The control signals DS1 and DS2 have PWM waveforms as shown in FIG. 13(f) and (g). The high-level period of one cycle of the control signals DS1 and DS2 indicates the on-duty. As shown in FIG. 13(e), during the period 1203, the output voltage Vout gradually increases, and the VS terminal voltage increases to a voltage corresponding to the DC voltage Vin. This voltage remains constant throughout the switching period of FET1 and FET2. As shown in FIG. 13(f) and (g), after the control signal DS2 first goes high, the control signals DS1 and DS2 alternate between high and low states, with dead time (not shown) between. At this time, the duration of the period 1202 is shorter than the duration of the period 1200, so the on-duty of the control signal DS1 in FIG. 13(f) is large. In this embodiment, the proportion of the on-duty is increased by lengthening the high-level time of the control signal DS1. This makes it possible to supply a sufficiently large amount of power to the load on the secondary side (the load connected to the power supply device) during the switching period of FET1 and FET2, and to supply power with high power efficiency regardless of the load.

[0092] Then, after performing the switching operations of FET1 and FET2 a predetermined number of times, the control signal DS2 is set to high level for a predetermined time, thereby setting FET2 to the on state and allowing current to flow to the clamp capacitor C2 via FET2, which is efficient. Furthermore, by changing the final on time of FET2 in proportion to the on duty changed according to the duration of the switching stop period, FET2 can be turned off just before the current flowing through FET2 switches from the drain terminal to the source terminal, even if the load on the secondary side fluctuates.

[0093] As described above, in the active clamp switching power supply of this embodiment, the load state is detected by measuring the switching stop period during intermittent switching operation. Then, by changing the switching control of FET1 and FET2 according to the detected load state, it is possible to realize a power supply with high power supply efficiency. Furthermore, in intermittent switching operation, the on time of the clamp FET (FET2) is changed at the end of the switching period according to the detected load state, and the switching operation of FET2 is performed, thereby further improving power supply efficiency.

[0094] In this embodiment, as in embodiment 2, the DC voltage Vin is estimated based on the acquired VS terminal voltage, thereby making it possible to determine the optimal on-time of FET1 in the switching period according to the DC voltage Vin. Furthermore, since the amount of power supplied from the primary side to the secondary side of the transformer is determined by the product of the DC voltage Vin and the on-time of FET1, efficient power supply is possible by determining the on-time of FET1 so that the product of the DC voltage Vin and the on-time of FET1 is constant.

[0095] In this embodiment, the ON duty of the control signal DS1 is changed according to the duration of the switching non-period, but this is not limiting, and power supply efficiency can also be maintained by increasing the ON time of FET1 according to the duration of the switching non-period. Also, the ON duty of the control signal DS2 may be decreased according to the duration of the switching non-period.

[0096] As described above, according to this embodiment, power can be supplied efficiently in accordance with the load state without adding a new circuit. [Explanation of symbols]

[0097] 1 FET (field effect transistor) 201 Power supply control unit 205 Feedback Section T1 transformer C4 capacitor D4 diode

Claims

1. a transformer having a primary winding and a secondary winding; a first switching element that supplies or cuts off power to the primary winding by switching operation; a feedback means for feeding back a signal based on a voltage output from the secondary winding of the transformer; a control means for controlling the switching operation and performing intermittent control that alternately repeats a switching period in which the switching operation is performed and a stop period in which the first switching element is in an off state; Equipped with when the control means detects that the value of the voltage is lower than a threshold value based on the signal, the control means controls the intermittent control so that the intermittent control transitions from the stop period to the switching period; when the intermittent control is performed in the order of a first stop period, a first switching period, a second stop period, and a second switching period, and the second stop period is shorter than the first stop period, the control means controls the switching operation in the second switching period so that an on-duty in the second switching period is larger than an on-duty in the first switching period; When the second stop period is longer than the first stop period, the control means controls the switching operation in the second switching period so that an on-duty in the second switching period is smaller than an on-duty in the first switching period.

2. A power supply device as described in claim 1, characterized in that when the second stop period is shorter than the first stop period, the control means controls the switching operation in the second switching period so that the on-duty in the second switching period is greater than the on-duty in the first switching period, and the switching period of the first switching element in the first switching period is equal to the switching period of the first switching element in the second switching period.

3. The transformer has an auxiliary winding, a first rectifying and smoothing unit that rectifies and smoothes an input AC voltage and outputs the rectified AC voltage to the primary winding; a second rectifying and smoothing unit that rectifies and smoothes the voltage induced in the auxiliary winding; a voltage dividing unit having a voltage dividing resistor, dividing the voltage output from the second rectifying and smoothing unit by the voltage dividing resistor, and outputting the divided voltage to the control means; Further provided with 2. The power supply device according to claim 1, wherein the control means determines an on-duty ratio in the second switching period based on the voltage supplied from the voltage dividing section.

4. 4. The power supply device according to claim 3, wherein the control means determines an on-duty ratio in the second switching period based on the voltage supplied from the voltage dividing unit at the start of the second stop period.

5. 2. The power supply device according to claim 1, wherein the control means varies a length of the second switching period in accordance with a length of the second stop period so that a sum of a length of the first switching period and a length of the first stop period is equal to a sum of a length of the second switching period and a length of the second stop period, and changes a number of switching operations of the first switching element in the second switching period in accordance with the varied length of the second switching period.

6. 6. The power supply device according to claim 5, wherein a frequency component obtained by multiplying a frequency that is the reciprocal of the sum of the length of the second switching period and the length of the second stop period is deviated from a resonant frequency of the transformer.

7. The power supply device a second switching element connected in parallel to the primary winding of the transformer and controlled by the control means; a capacitor connected in series to the second switching element and connected in parallel with the primary winding of the transformer together with the second switching element; Further provided with During the switching period, the control means performs the switching operation of alternately turning on or off the first switching element and the second switching element with a dead time in between, during which both the first switching element and the second switching element are turned off, and during the stop period, the control means stops the switching operation; 2. The power supply device according to claim 1, wherein the control means, when transitioning from the switching period to the stop period, turns on the second switching element before transitioning to the stop period, and when transitioning from the stop period to the switching period, also turns on the second switching element before transitioning to the switching period.

8. 8. The power supply device according to claim 7, wherein the control means determines a time for which the second switching element is turned on when the switching period transitions to the stop period based on the length of the stop period.

9. an image forming section that forms an image on a recording material; a control unit that controls the image forming unit; the power supply device according to claim 1 , which supplies power to the image forming unit and the control unit; Equipped with The image forming apparatus is characterized in that the control unit is capable of switching between a print state in which the image forming unit is controlled to form an image on the recording material, a standby state in which the image forming unit can be transitioned to the print state, and a sleep state in which the intermittent control is performed.

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