Power supply device and image forming apparatus

Abstract

To better optimize overcurrent protection operation in accordance with changes in output voltage.SOLUTION: A power supply device includes a control unit 200 for switching an output voltage Vout to a first voltage or a second voltage lower than the first voltage and a switching element 34 for applying a bias current to resistors 7 and 8. The switching element 34 applies no bias current to the resistors 7 and 8 when the output voltage Vout is the first voltage and applies a bias current to the resistors 7 and 8 when the output voltage Vout is switched from the first voltage to the second voltage.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a power supply device and an image forming apparatus, and more particularly to a power supply device having a function of reducing consumption of power input from an AC power source, for example. [Background technology]

[0002] In recent years, there has been an increasing demand for reducing power consumption in the standby state of devices, and a method has been adopted in which the output voltage of a power supply device is changed to a voltage lower than the voltage during normal operation in the standby state to reduce power consumption. Also, in order to avoid a continuation of an excessive current output in an abnormality such as a short circuit in the output of the power supply device, many devices adopt a configuration that detects the operating current of a switching element on the primary side and provides overcurrent protection (see, for example, Patent Document 1). That is, a configuration has been disclosed that detects the output current in the standby state to reduce the switching current during switching operation, and reduces the number of switching operations by making the switching an intermittent operation, thereby reducing power consumption. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5341627 Summary of the Invention [Problem to be solved by the invention]

[0004] However, a flyback power supply, which is one type of switching power supply, has the following problem. In a configuration in which overcurrent protection operates by detecting the current of a switching element on the primary side, if the output voltage on the secondary side is set to a low voltage, the output current at which overcurrent protection operates increases inversely proportional to the output voltage. Also, in the above-mentioned conventional example, if the output voltage is reduced in the standby state, the output current during overcurrent protection operation increases as described above.

[0005] The present invention has been made under these circumstances, and has an object to further optimize the overcurrent protection operation in response to changes in output voltage. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, the present invention has the following configuration. (1) A transformer having a first winding on the primary side and a second winding on the secondary side, and a No. 1 A switching element; A control signal is output to the first switching element. The above No. 1 Controlling switching elements to conductive or non-conductive death a control means for controlling an output voltage output from the second winding; No. 1 A current detection means for detecting a current flowing through a switching element, and a determination means for determining whether or not an overcurrent has occurred based on the result of detection by the current detection means. ,before a switching means for switching the output voltage between a first voltage and a second voltage lower than the first voltage; a second switching element that switches a path through which a bias current flows based on the control signal; Equipped with when the output voltage is the first voltage, the bias current is not applied to the current detection means, and when the output voltage is switched from the first voltage to the second voltage by the switching means, the second switching element switches the path so that the bias current is applied to the current detection means. A power supply device comprising: (2) An image forming apparatus comprising: an image forming unit for forming an image on a recording medium; and the power supply device according to (1) above. Effect of the Invention

[0007] According to the present invention, the overcurrent protection operation can be further optimized in response to changes in the output voltage. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing a circuit of a power supply unit according to a first embodiment of the present invention; [Diagram 2] FIG. 1 shows operational waveforms of the first embodiment. [Diagram 3] FIG. 1 is a diagram showing the relationship between the output voltage and the output current in the first embodiment. [Figure 4] FIG. 13 is a diagram showing the circuit of a power supply unit according to a second embodiment of the present invention. [Diagram 5] FIG. 13 is a diagram showing operational waveforms of the second embodiment. [Figure 6]FIG. 11 is a diagram showing a circuit of a power supply unit according to a third embodiment of the present invention. [Figure 7] FIG. 13 is a diagram showing operation waveforms of the third embodiment. [Figure 8] FIG. 13 is a diagram showing a circuit of a power supply unit according to a fourth embodiment of the present invention. [Figure 9] FIG. 13 is a diagram showing a configuration of an image forming apparatus according to a fifth embodiment of the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings by way of examples. EXAMPLES

[0010] <Power supply> FIG 1 is a circuit diagram showing the configuration of a power supply device 100 according to a first embodiment. An AC power supply 1 is a power supply that supplies an AC voltage Vac. A bridge diode 2, which is a rectifier circuit, rectifies the AC voltage Vac. A smoothing capacitor 3 smoothes the voltage rectified by the bridge diode 2 and outputs a voltage Vdc. The bridge diode 2 and the smoothing capacitor 3 together form an example of a rectifying and smoothing means. Here, the higher voltage of the smoothing capacitor 3 is designated as DCH, and the lower voltage is designated as DCL.

[0011] The switching transformer (hereinafter referred to as the transformer) 4 is an example of a transformer. The voltage Vdc output from the smoothing capacitor 3 is connected to the first winding 4a on the primary side, and converts the output from the second winding 4b on the secondary side into an output voltage Vout, which is a DC voltage. The output voltage from the third winding 4c on the primary side generates a forward output voltage Vfw (forward voltage) as the operating power supply for the power supply IC5. The voltage Vdc from the smoothing capacitor 3 is applied to the first winding 4a of the transformer 4 by the conductive and non-conductive operation of the switching element 6 connected to the first winding 4a, and the second winding 4b and the third winding 4c output a voltage proportional to the number of turns of the first winding 4a.

[0012] The power supply IC5, which is a control means, is an IC that controls the switching operation of controlling the switching element 6 to be conductive or non-conductive. The switching element 6, such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), repeats the switching operation of being conductive or non-conductive. The power supply IC5 outputs a DRV signal to a gate terminal (control terminal) of the switching element 6 to control the switching element 6. The diode 10 rectifies the output voltage from the second winding 4b. The rectified voltage is smoothed by the capacitor 9 and output as the output voltage Vout.

[0013] The photocoupler 16 feeds back the state of the output voltage Vout from the second winding 4b to the power supply IC5. The state of the output voltage Vout is transmitted to the power supply IC5 as a feedback signal by driving the photocoupler 16 so that the output voltage Vout becomes a predetermined voltage by the divided voltage of the resistors 12, 13, and 14 and the regulator 15. The resistor 17 is a current limiting resistor of the photocoupler 16. The power supply IC5 controls the conductive and non-conductive operation of the switching element 6 according to the input feedback signal (hereinafter referred to as FB). The switching element 18 such as a MOSFET is connected to both ends of the resistor 14 and operates as a switch that switches the voltage applied to the regulator 15 by switching the voltage division ratio of the resistors 12, 13, and 14. When the switching element 18 is in a conductive state, the output voltage Vout is Vout_H, and when it is in a non-conductive state, the output voltage Vout is Vout_L. The magnitude relationship of the output voltages is Vout_H>Vout_L.

[0014] A capacitor 21 is connected to the third winding 4c of the transformer 4 via a diode 20, and when the switching element 6 is conductive, the forward output voltage Vfw is half-wave rectified via the diode 20 and charged into the capacitor 21. The capacitor 21 is connected to the VCC of the power supply IC5, and supplies operating power to the power supply IC5.

[0015] Resistor 7 detects the current value of primary winding 4a when switching element 6 is conductive, and functions as a current detection means. Power supply IC5 detects the current of primary winding 4a as IS via resistor 8, and controls the switching operation of switching element 6 together with FB. Power supply IC5 compares an internal current detection threshold voltage Vth with the IS signal, and when the voltage of the IS signal is lower than the current detection threshold voltage Vth, it keeps switching element 6 conductive, and when the voltage of the IS signal reaches the current detection threshold voltage Vth, it puts switching element 6 in a non-conductive state. Note that PGND, which is the reference voltage of power supply IC5, is connected to DCL.

[0016] The resistor 30 and the diode 31 are connected so as to apply the DRV of the power supply IC 5 as a bias current to the resistors 7 and 8. The photocoupler 32 is connected to the output voltage Vout via the resistor 33, and is also connected to a switching element 34 such as a MOSFET. The switching element 34 operates as a switch for switching the operation of the photocoupler 32. When the switching element 34 is in a conductive state, it does not apply a bias current to the resistors 7 and 8, and when it is in a non-conductive state, it applies a bias current. Here, the photocoupler 32 and the switching element 34 are included in an application means for applying a bias current to the resistors 7 and 8.

[0017] When starting a switching operation, the power supply IC5 inputs operating power via the ST terminal connected to the smoothing capacitor 3 (DCH). As the voltage Vdc of the smoothing capacitor 3 rises, the voltage input to the ST terminal as operating power charges the capacitor 21 via the inside of the power supply IC5. The forward output voltage Vfw of the capacitor 21 rises, and the power supply IC5 starts operating. When the output voltage from the third winding 4c of the transformer 4 charges the capacitor 21 via the diode 20, the supply of operating power via the ST terminal stops. In addition, the power supply IC5 operates or stops by detecting the voltage Vdc of the smoothing capacitor 3 via the ST terminal. Although FIG. 1 shows a configuration in which the third winding 4c is connected, the power supply IC5 may be configured to operate by inputting operating power via the ST terminal, or may not have the third winding 4c.

[0018] The output voltage Vout is input to the VCC of the control unit 200, and supplies the operating power for the control unit 200. The control unit 200 is a CPU (Central Processing Unit) that controls switching of the output voltage Vout supplied to the load 11. It is configured to integrate a ROM (Read Only Memory) that holds operating procedures and data, and a RAM (Random Access Memory) that temporarily holds information, and may be, for example, a one-chip microcomputer. The control unit 200 is grounded to the reference potential GND.

[0019] The load 11 is connected so that the output voltage Vout of the power supply device 100 is input thereto. The output voltage Vout changes according to a signal (hereinafter referred to as CHG signal) output from a CHG terminal of the control unit 200. For example, when the CHG signal is in a high level state, the switching element 18 is in a conductive state, and Vout_H of the output voltage Vout is input to the load 11, enabling it to handle a heavy load state in which power consumption is large. In addition, when the switching element 34 is in a conductive state and the photocoupler 32 is in an on state, no bias current is applied to the resistors 7 and 8.

[0020] On the other hand, when the CHG signal is in a low level state, the switching element 18 is in a non-conductive state, and the output voltage Vout changes to Vout_L, which is lower than Vout_H. The control unit 200 functions as a switching means for switching the output voltage Vout to a first voltage (Vout_H, described later) or a second voltage (Vout_L, described later) lower than the first voltage. The load 11 receives the low output voltage Vout, Vout_L, and is capable of handling a light load state in which power consumption is small. At the same time, the switching element 34 is in a non-conductive state, and the photocoupler 32 is in an off state, thereby applying a bias current to the resistors 7 and 8. As described above, when the output voltage Vout is the first voltage, no bias current is applied to the resistors 7 and 8, and when the output voltage Vout is switched from the first voltage to the second voltage, a bias current is applied to the resistors 7 and 8. In the first embodiment, a bias current according to the DRV signal (control signal) is applied to the resistors 7 and 8. An example of the load 11 may be an information processing device (not shown) such as a computer, or an image input / output device such as an image scanner or a printer, and is assumed to be a unit that consumes power.

[0021] The power supply device 100 may not be able to supply power depending on the power consumption of the load 11. For example, when the load 11 is short-circuited and the power supply IC 5 detects an overcurrent multiple times within a predetermined period, the output voltage Vout may be reduced to protect the power supply device 100 and the load 11, and the power supply device 100 may be stopped to stop the power supply. The power supply IC 5 functions as a determination means for determining whether or not an overcurrent occurs based on the result detected by the resistor 7. When the current detected by the resistors 7 and 8 reaches a current detection threshold voltage Vth, which is a predetermined value, the power supply IC 5 makes the switching element 6 non-conductive. When the output voltage Vout is the second voltage and the switching element 6 becomes non-conductive multiple times within a predetermined time, the power supply IC 5 determines that an overcurrent has occurred.

[0022] <Protection operation> FIG. 2 is a diagram showing the operation of the power supply device 100 and the control of the control unit 200 shown in FIG. 1, and the protective operation of the power supply device 100 when the power supply IC5 detects an overcurrent multiple times within a predetermined period. FIG. 2(a) shows a DRV signal output from the power supply IC5 to the gate terminal of the switching element 6, and when it is at a high level, the switching element 6 is conductive (ON), and when it is at a low level, the switching element 6 is non-conductive (OFF). FIG. 2(b) shows the voltage of the IS terminal of the power supply IC5 (hereinafter referred to as the IS terminal voltage), and the current detection threshold voltage Vth and Vbias, which will be described later, are shown by dashed lines. FIG. 2(c) shows a CHG signal output from the CHG terminal of the control unit 200, and when it is at a high level, it is shown as High, and when it is at a low level, it is shown as Low. FIG. 2(d) shows the output voltage Vout, and Vout_H and Vout_L are shown by dashed lines. The horizontal axis indicates time, and t1 to t10 indicate time. Also shown are the output voltage Vout as the power consumed by the load 11 in a heavy load state (times t1 to t4), a light load state (times t4 to t8), and a state in which the power supply IC 5 detects an overcurrent multiple times within a predetermined period (from time t8).

[0023] During a period ΔT1-2 from time t1 to time t2, the DRV signal is in an ON state and the switching element 6 is in a conductive state. This is a period during which the IS terminal voltage of the power supply IC5 rises with an increase in the current value of the first winding 4a of the transformer 4. The CHG signal of the control unit 200 is in a high level state, and the output voltage Vout is controlled by the power supply IC5 to be Vout_H. At time t2, the IS terminal voltage reaches the current detection threshold voltage Vth set inside the power supply IC5, the DRV signal is turned OFF, and the switching element 6 is turned non-conductive. Thereafter, the IS terminal voltage decreases until time t3. At time t3, the DRV signal is turned ON and the switching element 6 is turned on. Thereafter, the same operation is repeated from time t1.

[0024] At time t4, the CHG signal of the control unit 200 shown in FIG. 1 is switched to a low level state, and the output voltage Vout drops to Vout_L. At time t5, the output voltage Vout becomes Vout_L, and during the period ΔT5-6 until time t6, the DRV signal is turned on, and the switching element 6 is turned on. During this period ΔT5-6, the DRV signal output is supplied as a bias current to the resistors 7 and 8 via the resistor 30 and the diode 31 shown in FIG. 1. Therefore, the IS terminal voltage rises to Vbias, and the current of the first winding 4a of the transformer 4 is superimposed at time t5. At time t6, the IS terminal voltage reaches the current detection threshold voltage Vth, the DRV signal is turned off, and the switching element 6 is turned off. At the next time t7, the DRV signal is turned on, and the switching element 6 is turned on. After that, the same operation is repeated from time t5.

[0025] (Occurrence of overcurrent) After time t8, if the load 11 is short-circuited and the power supply IC 5 detects an overcurrent multiple times within a predetermined period, the time interval Δt between the OFF and ON states of the DRV signal becomes shorter. If the IS terminal voltage reaches the current detection threshold voltage Vth multiple times during the period OCP_Tdet from time t8 when the IS terminal voltage reaches the current detection threshold voltage Vth to time t9, the power supply IC 5 stops switching operation to protect the power supply device 100 and the load 11. As a result, the output voltage Vout drops after time t10.

[0026] <Relationship between output voltage and output current> Fig. 3 is a graph showing the relationship between the output voltage Vout and the output current Iout of the power supply device 100. In Fig. 3, the horizontal axis shows the output current Iout, and the vertical axis shows the output voltage Vout. Also, white circles indicate a heavy load, and white squares indicate a light load. When the load 11 is in a heavy load state, the power supply device 100 sets the output voltage Vout to Vout_H and operates within the current range I_H. When the output voltage Vout is Vout_H, if the output current exceeds Iocp_H, the output voltage Vout drops due to the protective operation of the power supply device 100.

[0027] Further, when the load 11 is in a light load state, the power supply device 100 operates within the current range I_L with the output voltage Vout set to Vout_L which is lower than Vout_H. When the output voltage Vout is Vout_L, a bias current is supplied to resistors 7 and 8, the IS terminal voltage rises to Vbias, and the current in the first winding 4a of transformer 4 is superimposed. Therefore, when the output current Iocp_L (<Iocp_H) which is smaller than the heavy load state exceeds, the output voltage Vout decreases due to the protection operation of the power supply device 100.

[0028] As described above, according to the first embodiment, while achieving power saving by changing the output voltage according to the state of the load, the detection voltage of the switching current is changed according to the output voltage. Thereby, an optimized configuration can be realized for the current value for protecting the power supply device and the load according to the output voltage.

[0029] As described above, according to the first embodiment, the overcurrent protection operation can be further optimized according to the change in the output voltage.

Embodiment

[0030] <Power supply device> FIG. 4 is a circuit diagram showing the configuration of the power supply device 110 as the second embodiment. This is an example in which the bias power supply applied by the resistor 30 and the diode 31 shown in FIG. 1 is the forward output voltage Vfw which is the output from the third winding 4c of the transformer 4. The forward output voltage Vfw is a voltage proportional to the voltage Vdc obtained by rectifying and smoothing the AC voltage Vac which is the input voltage to the power supply device 110. Further, since the voltage Vdc obtained by rectifying and smoothing the AC voltage Vac which is the input voltage is input to the power supply IC 5 via the ST terminal, the power supply IC 5 can detect the input voltage. The other circuit configurations are the same as those in FIG. 1. Also, the relationship between the output voltage Vout and the output current Iout of the power supply device 110 shown in FIG. 3 is the same.

[0031] <Protection operation> 5(A) and (B) are diagrams showing the operation of the power supply device 110 and the control of the control unit 200 shown in FIG. 4, and the protective operation of the power supply device 110 when the power supply IC 5 detects an overcurrent multiple times within a predetermined period. In FIG. 5(A) and (B), (a) shows the forward output voltage Vfw. Note that (b) to (e) show waveforms similar to those in FIG. 2(a) to (d). The horizontal axis indicates time, and t11 to t20 and t21 to t30 indicate time. The forward output voltage Vfw shown in FIG. 5(A) and (B) is a voltage proportional to the voltage Vdc obtained by rectifying and smoothing the AC voltage Vac, which is the input voltage to the power supply device 110. The voltage V_H in (A) is higher than the voltage V_L in (B), and the AC voltage Vac in (A) is higher than that in (B). Also shown are a heavy load state (times t11 to t14, t21 to t24) and a light load state (times t14 to t18, t24 to t28) as the power consumed by the load 11. Furthermore, the output voltage Vout in a state in which the power supply IC 5 detects an overcurrent multiple times within a predetermined period (from time t18, from time t28) is also shown.

[0032] (When Vfw=V_H) In the period ΔT1-2_H from time t11 to t12 in FIG. 5(A), the DRV signal is in the ON state and the switching element 6 is in the conductive state. This is the period during which the IS terminal voltage of the power supply IC5 rises with the rise in the current value of the first winding 4a of the transformer 4. The same applies after time t13. At time t14, the CHG signal of the control unit 200 shown in FIG. 4 is switched to a low level state, the forward output voltage Vfw is applied to the resistor 30, and a bias current is supplied to the resistors 7 and 8 via the diode 31. In the second embodiment, a bias current according to the voltage Vdc rectified by the bridge diode 2 and output from the smoothing capacitor 3 is applied to the resistors 7 and 8. That is, a bias current according to the voltage induced in the third winding 4c is applied to the resistors 7 and 8. The voltage induced in the third winding 4c is the forward output voltage Vfw.

[0033] As a result, the IS terminal voltage rises to Vbias_H. At time t15, the DRV signal turns ON, and the switching element 6 turns conductive. During a period ΔT5-6_H until time t16, the current of the first winding 4a of the transformer 4 is superimposed. At time t16, the IS terminal voltage reaches the current detection threshold voltage Vth, the DRV signal turns OFF, and the switching element 6 turns non-conductive. At the next time t17, the DRV signal turns ON, the switching element 6 turns conductive, and the same operation is repeated.

[0034] (Occurrence of overcurrent) If the load 11 becomes short-circuited after time t18 and the power supply IC5 detects an overcurrent multiple times within a predetermined period, the interval between the OFF and ON states of the DRV signal becomes shorter. If the IS terminal voltage reaches the current detection threshold voltage Vth multiple times during the period OCP_Tdet in which the IS terminal voltage reaches the current detection threshold voltage Vth, the power supply IC5 stops the switching operation to protect the power supply device 110 and the load 11. Here, the period OCP_Tdet is the period from time t18 to time t19. As a result, the output voltage Vout decreases after time t20.

[0035] (When Vfw=V_L) In the period ΔT1-2_L from time t21 to t22 in FIG. 5B, the DRV signal is in the ON state and the switching element 6 is in the conductive state. This is the period during which the IS terminal voltage of the power supply IC5 rises with the rise in the current value of the first winding 4a of the transformer 4. The same applies after time t23. At time t24, the CHG signal of the control unit 200 shown in FIG. 4 is switched to the low level state, the forward output voltage Vfw is applied to the resistor 30, and a bias current is supplied to the resistors 7 and 8 via the diode 31. Therefore, the IS terminal voltage rises to Vbias_L. In the period ΔT5-6_L until time t26, the current of the first winding 4a of the transformer 4 is superimposed. At time t26, the IS terminal voltage reaches the current detection threshold voltage Vth, the DRV signal is turned OFF, and the switching element 6 is turned non-conductive. Voltage V_L (B), which is proportional to the input voltage, is lower than voltage V_H (A), and voltage Vdc of smoothing capacitor 3 is also low. Therefore, period ΔT5-6_L during which the DRV signal is in the ON state is longer than period ΔT5-6_H (ΔT5-6_L>ΔT5-6_H). At the next time t27, the DRV signal turns ON, switching element 6 turns conductive, and the same operation is repeated.

[0036] (Occurrence of overcurrent) If the load 11 becomes short-circuited after time t28 and the power supply IC5 detects an overcurrent multiple times within a predetermined period, the interval between the OFF and ON states of the DRV signal becomes shorter. If the IS terminal voltage reaches the current detection threshold voltage Vth multiple times during the period OCP_Tdet in which the IS terminal voltage reaches the current detection threshold voltage Vth, the power supply IC5 stops switching operation to protect the power supply device 110 and the load 11. Here, the period OCP_Tdet is the period from time t28 to time t29. As a result, the output voltage Vout decreases after time t30.

[0037] As described above, according to the second embodiment, the output voltage from the forward winding of the transformer, which is proportional to the input voltage, is used as the bias power supply while the output voltage is reduced according to the load state to save power. This makes it possible to realize an optimized configuration for the current value that protects the power supply device and the load, even when the input voltage changes and the output voltage is reduced.

[0038] As described above, according to the second embodiment, the overcurrent protection operation can be further optimized in response to changes in the output voltage. EXAMPLES

[0039] <Power supply> FIG. 6 is a circuit diagram showing the configuration of a power supply device 120 as a third embodiment. According to the third embodiment, FIG. 6 shows a configuration in which the photocoupler 32, the resistor 33, and the switching element 34 shown in FIG. 1 are not required. In addition, the winding start position of the third winding 4c of the transformer 4 is configured to be opposite to the winding start position shown in FIG. 1, and a flyback output voltage Vfb is output. In addition, a constant voltage diode 70 (Zener diode) having a Zener breakdown voltage characteristic (voltage Vz) is connected to the capacitor 21, and a switching element 71 is connected to the DCL between the resistor 30 and the diode 31. More specifically, the switching element 71 has a gate terminal connected to the anode terminal of the constant voltage diode 70, a source terminal connected to the DCL, and a drain terminal connected to the connection point between the resistor 30 and the diode 31. The constant voltage diode 70 has a cathode terminal connected to the high potential side of the capacitor 21. In the third embodiment, the constant voltage diode 70 and the switching element 71 are included in the application means.

[0040] The capacitor 21 is charged with the flyback output voltage Vfb from the third winding 4c, which is proportional to the output voltage Vout of the second winding 4b, after being half-wave rectified by the diode 20. When the flyback output voltage Vfb smoothed by the capacitor 21 exceeds the voltage Vz of the constant voltage diode 70 and the switching element 71 is in a conductive state, the bias current from the DRV signal is not applied to the resistors 7 and 8. When the flyback output voltage Vfb is low and the switching element 71 is in a non-conductive state, a bias current is applied to the resistors 7 and 8. In this way, in the third embodiment, the bias current from the DRV signal is applied to the resistors 7 and 8 in accordance with the output voltage Vout. That is, the bias current from the DRV signal is applied to the resistors 7 and 8 in accordance with the voltage induced in the third winding 4c. The voltage induced in the third winding 4c is the flyback voltage. The other circuit configurations are the same as those in FIG. 1. The relationship between the output voltage Vout and the output current Iout of the power supply device 120 shown in FIG. 3 is also the same.

[0041] <Protection operation> Fig. 7 is a diagram showing the operation of the power supply device 120 and the control of the control unit 200 shown in Fig. 6, and the protective operation of the power supply device 120 when the power supply IC5 detects an overcurrent multiple times within a predetermined period. Fig. 7(e) shows the flyback output voltage Vfb proportional to the output voltage Vout, and Vfb_H and Vfv_L are shown by dashed lines. The operation waveforms in Fig. 7(a) to (d) are the same as those in Fig. 2(a) to (d). The horizontal axis indicates time, and t41 to t50 indicate times.

[0042] At time t44, the CHG signal of the control unit 200 shown in FIG. 6 is switched to a low level state, and the output voltage Vout starts to drop to Vout_L. Also, the flyback output voltage Vfb proportional to the output voltage Vout starts to drop to Vfb_L. At time t45, the output voltage Vout becomes Vout_L, and the flyback output voltage Vfb also drops to Vfb_L. During the period ΔT5-6 until time t46, the DRV signal is turned on, and the switching element 6 is turned on. During this period ΔT5-6, the voltage of the DRV signal is applied, and is supplied as a bias current to the resistors 7 and 8 via the resistor 30 and the diode 31 shown in FIG. 1. Therefore, the IS terminal voltage rises to Vbias, and the current of the first winding 4a of the transformer 4 is superimposed at time t45. At time t46, the IS terminal voltage reaches the current detection threshold voltage Vth, the DRV signal is turned off, and the switching element 6 is turned off. At the next time t47, the DRV signal turns ON, the switching element 6 turns ON, and the same operation is repeated.

[0043] (Occurrence of overcurrent) If the load 11 becomes short-circuited after time t48 and the power supply IC5 detects an overcurrent multiple times within a predetermined period, the time interval between the OFF and ON states of the DRV signal becomes shorter. If the IS terminal voltage reaches the current detection threshold voltage Vth multiple times during the period OCP_Tdet in which the IS terminal voltage reaches the current detection threshold voltage Vth, the power supply IC5 stops switching operation to protect the power supply device 120 and the load 11. Here, the period OCP_Tdet is the period from time t48 to time t49. As a result, the output voltage Vout decreases after time t50.

[0044] As described above, according to the third embodiment, the output voltage is changed according to the load state to achieve power saving, while the output voltage from the flyback winding of the transformer, which is proportional to the output voltage, is detected and the detection voltage of the switching current is changed. This makes it possible to realize an optimized configuration for the current value that protects the power supply device and the load according to the output voltage.

[0045] As described above, according to the third embodiment, the overcurrent protection operation can be further optimized in response to changes in the output voltage. EXAMPLES

[0046] <Power supply> FIG. 8 is a circuit diagram showing the configuration of a power supply device 130 as a fourth embodiment. FIG. 8 shows a configuration in which a constant voltage diode 35 having a Zener breakdown voltage characteristic (voltage Vz) is connected to the output voltage Vout and the resistor 33 in the configuration shown in FIG. 1, thereby eliminating the need for the switching element 34. That is, the constant voltage diode 35 and the photocoupler 32 are included in the application means. The constant voltage diode 35 is arranged to detect whether the output voltage Vout exceeds the voltage Vz. When the output voltage when the load 11 is in a heavy load state is Vout, the output voltage Vout exceeds the voltage Vz of the constant voltage diode 35, and the photocoupler 32 is turned on. Then, the bias current from the DRV signal output flows into the DCL via the photocoupler 32, and is not applied to the resistors 7 and 8.

[0047] When the load 11 is in a heavy load state, the output voltage Vout is low, and the photocoupler 32 is in an off state, a bias current is applied to the resistors 7 and 8. The other circuit configurations are the same as those in Fig. 1. The relationship between the output voltage Vout and the output current Iout of the power supply device 130 shown in Fig. 3 is also the same.

[0048] As described above, according to the fourth embodiment, it is possible to realize a configuration in which the current value for protecting the power supply device and the load according to the output voltage is optimized by detecting the output voltage supplied to the load while saving power by changing the output voltage according to the load state. Note that in other embodiments, a configuration using a constant voltage diode 35 may be used instead of the switching element 34, in place of the configuration using the switching element 34.

[0049] As described above, according to the fourth embodiment, the overcurrent protection operation can be further optimized in response to changes in the output voltage. EXAMPLES

[0050] <Image forming device> FIG. 9 is a diagram showing an image forming apparatus 201, which is a laser beam printer in which a power supply device 100 is disposed. The power supply device 100 may be the power supply device 110, 120, or 130, and the following description will be given using the power supply device 100 as an example. The control unit 200 is a control unit that controls the image forming apparatus 201, and is, for example, a CPU. The control unit 200 is an integrated unit that includes a ROM that stores procedures and data for controlling the operation of the power supply device 100 and the image forming apparatus 201, a RAM that temporarily stores information, or a unit that individually connects circuits having equivalent functions. The recording medium 202 is a recording medium for recording an image, such as paper, which records and stores an image. The pickup roller 203 is a roller that feeds the recording medium 202. The registration roller (hereinafter referred to as the registration roller) 204 is a roller that aligns the leading edge of the recording medium 202.

[0051] The image forming unit 205 is an example of an image forming means, and forms an image on the conveyed recording medium 202 by a known electrophotographic process. The irradiation unit 205a projects an image in the electrophotographic process onto the photosensitive drum 205b as a latent image. The development unit 205c develops the latent image formed on the photosensitive drum 205b as a toner image. The transfer member 205d transfers the toner image formed on the photosensitive drum 205b onto the recording medium 202.

[0052] The fixing section 300 is an example of an image heating means, and stably fixes an unfixed toner image formed on the recording medium 202. The heating member 301 is a member that generates heat when power is supplied from the AC power source 1. The film member 300a transfers the heat of the heating member 301 to the recording medium 202. The pressure member 300b presses the recording medium 202 and the toner image together with the heat transferred from the heating member 301 by the film member 300a, thereby fixing them. The discharge roller 206 discharges the recording medium 202 to the top of the image forming apparatus 201. The discharge tray 207 stacks the discharged recording medium 202. 208 indicates a state in which the recording medium 202 on which the image has been formed and fixed is discharged and stacked. The image forming operation by the image forming apparatus 201 has been described above.

[0053] Motor 210 drives pickup roller 203, which transports recording medium 202, registration roller 204, and image forming unit 205. Motor 211 drives fixing unit 300. Motors 210 and 211 consume power output from power supply device 100. Note that pickup roller 203, registration roller 204, discharge roller 206, and motors 210 and 211 are an example of a transport means for recording medium 202.

[0054] The image forming apparatus 201 transitions to a low power consumption sleep state in which the power supply device 100 is operating and the image forming apparatus 201 is stopped. The image forming apparatus 201 then transitions to a print state in which the image forming apparatus 201 is operating and printing an image on the recording medium 202. When the power supply device 100 is installed in the image forming apparatus 201, the "light load state" described in the above embodiment becomes a "sleep state" and the "heavy load state" becomes a "print state."

[0055] For example, as described in the first embodiment, in the standby state in which the image forming apparatus 201 is in a sleep state, the control unit 200 outputs a low-level CHG signal to the power supply device 100 to lower the output voltage Vout and turn off the photocoupler 32. This applies a bias current to the current detection resistor 7 and the bias resistor 8. As a result, while the image forming apparatus 201 is in a sleep state, when the output current exceeds Iocp_L as shown in Fig. 3, the output voltage Vout is lowered by the protective operation of the power supply device 100. Then, the power supply device 100 and the image forming apparatus 201 can be protected with an output current smaller than that in the state in which the output voltage Vout is Vout_H.

[0056] As described above, according to the fifth embodiment, the detection voltage of the switching current is changed according to the output voltage of the power supply while saving power of the image forming apparatus. As a result, when power is supplied to the image forming apparatus, an optimized configuration can be realized for the current value that protects the power supply device and the image forming apparatus, which is a load.

[0057] As described above, according to the fifth embodiment, the overcurrent protection operation can be further optimized in response to changes in the output voltage. [Explanation of symbols]

[0058] 4. Transformer 5 Power IC 6 Switching elements 7 Current Sensing Resistor 8 Bias resistors 200 Control section

Claims

1. a transformer having a first winding on a primary side and a second winding on a secondary side; a first switching element connected to the first winding; a control means for controlling the first switching element to be conductive or non-conductive by outputting a control signal to the first switching element, thereby controlling an output voltage output from the second winding; a current detection means for detecting a current flowing through the first switching element; a determination means for determining whether or not an overcurrent occurs based on the result detected by the current detection means; a switching means for switching the output voltage between a first voltage and a second voltage lower than the first voltage; a second switching element that switches a path through which a bias current flows based on the control signal; Equipped with When the output voltage is the first voltage, the bias current is not applied to the current detection means; When the output voltage is switched from the first voltage to the second voltage by the switching means, the second switching element switches the path so that the bias current is applied to the current detection means. A power supply device comprising:

2. A bias current applying means for applying the bias current to the current detecting means, 2. The power supply device according to claim 1, wherein the applying means applies the bias current corresponding to the control signal to the current detecting means.

3. the transformer has a third winding on the primary side, 3. The power supply device according to claim 2, wherein the applying means applies the bias current to the current detecting means in response to a voltage induced in the third winding.

4. 4. The power supply device according to claim 3, wherein the voltage induced in the third winding is a flyback voltage.

5. An application means for applying the bias current to the current detection means; A rectifying and smoothing means for rectifying and smoothing an AC voltage; Equipped with 2. The power supply device according to claim 1, wherein the applying means applies the bias current corresponding to the voltage output from the rectifying and smoothing means to the current detecting means.

6. A bias current applying means for applying the bias current to the current detecting means, the transformer has a third winding on the primary side, 2. The power supply device according to claim 1, wherein the applying means applies the bias current corresponding to the voltage induced in the third winding to the current detecting means.

7. 7. The power supply device according to claim 6, wherein the voltage induced in the third winding is a forward voltage.

8. the control means brings the first switching element into the non-conducting state when the current detected by the current detection means reaches a predetermined value; 8. The power supply device according to claim 1, wherein the determination means determines that the overcurrent has occurred when the first switching element becomes non-conductive a plurality of times within a predetermined time period when the output voltage is the second voltage.

9. an image forming means for forming an image on a recording medium; A power supply device according to any one of claims 1 to 7; An image forming apparatus comprising:

10. the control means brings the first switching element into the non-conducting state when the current detected by the current detection means reaches a predetermined value; 10. The image forming apparatus according to claim 9, wherein the determining unit determines that the overcurrent has occurred if the first switching element becomes non-conductive a plurality of times within a predetermined time when the output voltage is the second voltage.

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