JP2016082713A - Power source device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce power consumption with a simple configuration.SOLUTION: A power source device includes: a transformer T401; a bridge diode DA401 that rectifies and smooths an AC voltage; a capacitor C401; a main switching element Q401 that is connected to a primary winding Np of the transformer T401 to control supply and interruption of a current to the primary winding Np; a switching element Q402 that controls a switching operation of the main switching element Q401; a starting resistance R401; a switching element Q602 that carries out connection and disconnection of the starting resistance R401 and main switching element A401; and a Zener diode ZD101 that controls the switching element Q602 on the basis of a voltage generated in a feedback winding.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a power supply device that supplies power to a device having a plurality of operation states for power saving, and an image forming apparatus including the power supply device, and more particularly to a flyback power supply device.

  In general, a switching power supply device such as a flyback converter operating in a discontinuous mode generates a DC output voltage by rectifying and smoothing an alternating voltage generated in a secondary winding of a transformer. In some cases, a step-down converter circuit that generates a DC voltage (for example, DC 3.3 V or DC 5 V) from the DC output voltage (for example, DC 24 V) of the transformer may be disposed after the secondary side rectifying and smoothing circuit. The switching power supply device is not only a unit that directly uses a DC output voltage (for example, DC 24V) of a transformer, such as a motor, but also a DC voltage (for example, DC3. 3V or DC5V). Further, with respect to the DC output voltage of DC 3.3V or DC 5V, a certain amount of power is consumed in order for the device to maintain the standby state. On the other hand, regarding the DC output voltage of 24V DC, the power consumption is generally small when the device is in the standby operation because the drive source such as the motor is not operating.

  In consideration of the conversion efficiency of the step-down converter circuit, the conversion efficiency increases as the DC output voltage of the transformer input to the step-down converter circuit is smaller. Therefore, since the operation of a drive source such as a motor is not necessary during the device standby operation, a technique is adopted in which the DC output voltage of the transformer is reduced and the conversion efficiency of the step-down converter circuit is improved. However, in the method of reducing the DC output voltage, a plurality of control circuits must be arranged, leading to an increase in the cost of the device. Further, for example, Patent Document 1 proposes a circuit configuration that reduces the loss due to the starting resistance by separating the starting resistance after the start of continuous oscillation, as a technique for improving the power conversion efficiency of the switching power supply device, particularly during the device standby operation. Yes.

  FIG. 5 is a circuit diagram showing a circuit configuration of a ringing choke converter, which is a conventional flyback converter that operates in a discontinuous mode, as an example of a conventional switching power supply apparatus. FIG. 6 is a diagram showing an operation waveform of each part of the circuit of FIG. Details of FIGS. 5 and 6 will be described later. In the ringing choke converter of FIG. 5, after the main switching element Q401 starts continuous oscillation due to the voltage from the feedback winding Nb, all the current flowing from the starting resistor R401 to the resistor R402 becomes a reactive current, which increases the efficiency of the switching power supply device. It becomes a factor to reduce. Therefore, as a method for avoiding this, a method of separating the starting resistor R401 from the input voltage Vin after the main switching element Q401 starts continuous oscillation has been proposed. FIG. 7 is a circuit diagram in which a circuit configured by this method is added to the switching power supply apparatus of FIG. 5. In the switching power supply apparatus of FIG. 7, after the main switching element Q401 starts continuous oscillation, the reactive current by the starting resistor R401 is obtained. Can be reduced. Details of the circuit of FIG. 7 will be described later.

Japanese Patent No. 3798289

  However, the switching power supply device configured to disconnect the starting resistor has the following problems. That is, in the circuit of the switching power supply device shown in FIG. 7, an expensive element such as a photocoupler IC 601 or a peripheral circuit is added to perform conduction / non-conduction control of the switching element Q601 that controls the current flowing through the starting resistor R401. Yes. Therefore, there is a problem that the circuit configuration is expensive.

  The present invention has been made under such circumstances, and an object thereof is to reduce power consumption with a simple configuration.

  In order to solve the above-described problems, the present invention has the following configuration.

  (1) a transformer having a primary winding, a secondary winding, and a feedback winding; and a first rectifying and smoothing unit that rectifies and smoothes an AC voltage to generate a DC voltage and supplies the DC voltage to the primary winding of the transformer; A switching element connected to the primary winding of the transformer and controlling supply and interruption of current to the primary winding, control means for controlling a switching operation of the switching element, and starting oscillation of the switching element A starting resistance for switching, a switching means for connecting and disconnecting the starting resistance and the switching element, and a first feedback means for controlling the switching means based on a voltage generated in the feedback winding, A power supply device comprising:

  (2) An image forming apparatus comprising: an image forming unit that forms an image on a recording material; and the power supply device according to (1).

  According to the present invention, power consumption can be reduced with a simple configuration.

1 is a circuit diagram illustrating a configuration of a switching power supply device according to a first embodiment. The figure which shows the operation | movement waveform of each part of the switching power supply device of Example 1. The circuit diagram which shows the structure of the switching power supply apparatus of Example 2. Schematic diagram of the image forming apparatus of Example 4 Circuit diagram showing configuration of conventional switching power supply device The figure which shows the operation waveform of each part of the switching power supply device of a prior art example Circuit diagram showing configuration of conventional switching power supply device

  First, the configuration and operation of a conventional flyback converter that operates in a discontinuous mode will be described for comparison with a switching power supply device of an embodiment to be described later.

[Basic operation and problem of switching power supply]
(Configuration of ringing choke converter)
A ringing choke converter (hereinafter simply referred to as RCC) shown in FIG. 5 will be described as an example of a flyback switching power supply device that operates in a discontinuous mode. The insulating transformer T401 (hereinafter also referred to as a transformer T401) includes an input-side primary winding Np, an output-side secondary winding Ns, and a primary-side feedback winding Nb. The feedback winding Nb is a drive winding for the switching element Q402 that controls conduction and non-conduction of the control terminal of the main switching element Q401. The input voltage Vin is a DC voltage obtained by rectifying an AC input voltage from the AC voltage source AC by a bridge diode DA401 and smoothing it by an aluminum electrolytic capacitor C401 (hereinafter referred to as a capacitor C401), and is also a voltage between terminals of the capacitor C401. . The (+) side of the input voltage Vin (hereinafter referred to as the (+) side that is the positive terminal of the capacitor C401) is connected to one end of the primary winding Np on the (−) side of the input voltage (hereinafter referred to as the negative terminal of the capacitor C401). (Pointing to the (−) side) is connected to the current outflow terminal of the main switching element Q401. The input voltage Vin is applied between one end of the primary winding Np and the current outflow terminal of the main switching element Q401.

  The feedback winding Nb is connected to the same polarity as the primary winding Np and to a different polarity from the secondary winding Ns. A starting resistor R401 is connected between the (+) side of the input voltage Vin and the control terminal of the main switching element Q401. A resistor R402 is connected between the control terminal of the main switching element Q401 and the (−) side of the input voltage Vin. Then, by dividing the input voltage Vin by the starting resistor R401 and the resistor R402, a voltage sufficient for the main switching element Q401 to conduct is generated. A capacitor C402 and resistors R403 and R404 are connected between the control terminal of the main switching element Q401 and the terminal on the same polarity side as the primary winding Np of the feedback winding Nb. A diode D401 whose feedback winding Nb side is the cathode terminal side is connected to both ends of the resistor R404, and the turn-on and turn-off speeds of the main switching element Q401 are adjusted.

  The switching element Q402 as control means is provided for controlling the conduction and non-conduction of the main switching element Q401 that controls the supply and interruption of the current to the primary winding Np. The current inflow terminal of the switching element Q402 is connected to the control terminal of the main switching element Q401, and the current outflow terminal is connected to the (−) side of the input voltage Vin. Further, a capacitor C403 is connected between the control terminal of the switching element Q402 and the current outflow terminal. A resistor R405 is connected between the terminal on the same polarity side as the primary winding Np of the feedback winding Nb and the control terminal of the switching element Q402, and the resistor R405 and the capacitor C403 constitute a time constant circuit.

  A resistor R406 is connected between the current inflow terminal of the phototransistor of the photocoupler IC401 and the control terminal of the main switching element Q401 to limit the current flowing through the photocoupler IC401. The current outflow terminal of the phototransistor of the photocoupler IC 401 is connected to the control terminal of the switching element Q402. An anode terminal of a rectifying diode D402 is connected to a side opposite to the primary winding Np of the secondary winding Ns of the insulating transformer T401. An electrolytic capacitor C404 is connected between the cathode terminal side of the diode D402 and the primary winding Np and the same polarity side of the secondary winding Ns to smooth the alternating voltage rectified by the diode D402.

  The DC output voltage Vout (hereinafter also referred to as output voltage Vout) is divided by resistors R407 and R408, and the divided voltage is input to the detection terminal (+ terminal) of the operational amplifier IC402. The operational amplifier IC 402 compares the divided voltage input to the detection terminal with the reference voltage input to the inverting input terminal (− terminal). The operational amplifier IC 402 changes the voltage of the output terminal, which is a detection signal, based on the comparison result, and controls the current flowing through the light-emitting diode of the photocoupler IC 401 via the resistor R409. The resistor R410 and the capacitor C405 are connected between the non-inverting input terminal (also a detection terminal) of the operational amplifier IC402 and the output terminal, and are added to prevent oscillation during control by performing phase compensation.

(Operation of ringing choke converter)
The main switching element Q401 becomes conductive when a voltage divided by the starting resistor R401 and the resistor R402 is applied to the control terminal. When the main switching element Q401 becomes conductive, the input voltage Vin is applied to the primary winding Np, and a voltage having a positive polarity on the same polarity side as the primary winding Np is induced in the feedback winding Nb. At this time, a voltage is also induced in the secondary winding Ns. However, since the induced voltage is a voltage with the anode terminal side of the diode D402 being negative, the voltage is not transmitted to the secondary side.

At this time, the excitation current I1p is proportional to time, and after the conduction time ton of the main switching element Q401 has elapsed, it becomes the excitation current I1p according to the following equation (1). Here, Lp is a primary inductance of the insulating transformer T401.

The current flowing through the primary winding Np is only the exciting current I1p of the insulating transformer T401, and energy proportional to the square of the exciting current I1p is stored in the insulating transformer T401. In the insulation transformer T401, energy Pin is stored according to the following equation (2). Here, f is the oscillation frequency (Hz) of the RCC, specifically, the frequency of the switching operation for turning on and off the main switching element Q401.

  Thereafter, the capacitor C403 of the time constant circuit constituted by the resistor R405 and the capacitor C403 is charged with electric charge from the feedback winding Nb. When the voltage across the capacitor C403 becomes higher than the threshold value for turning on the switching element Q402, the switching element Q402 becomes conductive. Since switching element Q402 becomes conductive, the voltage at the control terminal of main switching element Q401 decreases, and main switching element Q401 becomes nonconductive.

  At this time, a voltage having a polarity opposite to that at the time of start-up is generated in each winding of the insulating transformer T401, and a voltage having a positive polarity on the anode terminal side of the diode D402 is generated in the secondary winding Ns. The stored energy is rectified and smoothed and transmitted to the secondary side. Then, according to the above-described equation (2), when all of the energy stored in the insulating transformer T401 is transmitted to the secondary side, the main switching element Q401 becomes conductive again. This is because when the transmission of energy to the secondary side ends, the control terminal of the main switching element Q401 is again applied with a voltage in the positive direction from the capacitor C402 that is C-coupled (capacitively coupled) by backswing. It is.

  Since a large amount of current flows through the phototransistor of the photocoupler IC 401 when the output voltage Vout is high, a large amount of current is supplied to the capacitor C403 when the output voltage Vout is high, and the charging time of the capacitor C403 is shortened. By shortening the charging time of the capacitor C403, the time from the non-conducting state of the switching element Q402 to the conducting state is shortened. In other words, this means that the conducting time ton, which is the time of the conducting state of the main switching element Q401, is short. It shows that it becomes. As the conduction time ton of the main switching element Q401 is shortened, the energy stored in the insulating transformer T401 is reduced, and the output voltage Vout is lowered. On the other hand, when the output voltage Vout is low, the operation is reversed. By performing such an operation, the RCC performs a constant voltage operation so that the output voltage Vout becomes a predetermined voltage. The above is the basic operation in the ringing choke converter (RCC).

(Overvoltage protection circuit)
Further, the operational amplifier IC403 in FIG. 5 compares the input voltage of the non-inverting input terminal (+ terminal) detected from the Zener diode ZD401 via the resistor R411 with the reference voltage input to the inverting input terminal (−terminal). The operational amplifier IC403 controls the current flowing through the light emitting diode (LED) of the photocoupler IC401 via the resistor R412 according to the comparison result with the reference voltage. Therefore, when the operational amplifier IC403 detects an overvoltage of the output voltage Vout that is an input voltage of the non-inverting input terminal, the operational amplifier IC403 increases the voltage of the output terminal. As a result, the current flowing through the light emitting diode of the photocoupler IC 401 increases, and the current flowing through the phototransistor also increases. As a result, the capacitor C403 is rapidly charged, so that the switching element Q402 is changed from the non-conductive state to the conductive state, and the main switching element Q401 is changed to the non-conductive state. This prevents the output voltage Vout from becoming an abnormal voltage, and the operational amplifier IC403 is arranged for the purpose of performing an overvoltage protection operation. Note that the Zener voltage of the Zener diode ZD401 is a voltage detected as an overvoltage.

(Operating waveform of ringing choke converter)
FIG. 6 shows waveforms at various parts of the RCC of FIG. 6A shows the waveform of the voltage (drain-source voltage) Vds between the current inflow terminal and the current outflow terminal of the main switching element Q401, and FIG. 6B shows the waveform of the current Ip flowing through the main switching element Q401. Is shown. FIG. 6C shows the waveform of the current Is flowing through the rectifying diode D402. FIG. 6A shows the maximum value (Vds (max)) (broken line) of the drain-source voltage Vds, the off time toff that is the non-conduction time of the main switching element Q401, and the on time ton that is the conduction time. The oscillation period T (= 1 / f) is also shown. The oscillation frequency f is calculated by 1 / (ton + toff). FIG. 6B also shows a peak current Ip (max) (broken line) of the current Ip. In addition, the horizontal axis of each figure of FIG. 6 shows time (t).

[Starting resistance switching circuit]
In the ringing choke converter (RCC) described in FIG. 5, the voltage induced in the feedback winding Nb is applied to the control terminal of the main switching element Q401. As a result, after the main switching element Q401 starts continuous oscillation, all of the current flowing from the starting resistor R401 to the resistor R402 becomes a reactive current, which causes a reduction in the efficiency of the switching power supply device. As a method for avoiding this, there has been proposed a method of separating the starting resistor R401 from the input voltage Vin after the main switching element Q401 starts continuous oscillation. FIG. 7 is a circuit diagram of a switching power supply device including a starting resistance switching circuit which is a circuit for this purpose. Hereinafter, a circuit configuration and operation for disconnecting the starting resistor R401 will be described with reference to FIG. In FIG. 7, circuits similar to those in FIG. 5 are denoted by the same reference numerals as those in FIG. 5, and description thereof is omitted.

(Configuration of starting resistor switching circuit)
First, the circuit configuration of the starting resistance switching circuit which is a switch means will be described. In FIG. 7, a current inflow terminal of a switching element Q601 (first transistor) composed of an npn type transistor is connected to one end of a starting resistor R401, and a current outflow terminal is connected to one end of a resistor R402. The control terminal of the switching element Q601 is connected to the (+) side of the input voltage Vin (the (+) side of the capacitor C401) via the resistor R601. It is assumed that the resistance value of the resistor R601 is larger than the resistance value of the starting resistor R401. The current inflow terminal of the switching element Q602 is connected to the control terminal of the switching element Q601, and the current outflow terminal of the switching element Q602 is connected to one end of the resistor R402. A resistor R602 is connected between the current inflow terminal and the control terminal of the switching element Q602 (second transistor) formed of a pnp transistor. The control terminal of the switching element Q602 is connected to the current inflow terminal of the phototransistor of the photocoupler IC601, and the current outflow terminal of the phototransistor of the photocoupler IC601 is the (−) side of the input voltage Vin (the (−) side of the capacitor C401). It is connected to the. The anode terminal of the light emitting diode of the photocoupler IC 601 is connected to the high potential side of the output voltage Vout via the resistor R603, and the cathode terminal of the light emitting diode is connected to the current inflow terminal of the switching element Q603. The current outflow terminal of the switching element Q603 is connected to the low potential side (ground side) of the output voltage Vout, and a potential determining resistor R604 is connected between the control terminal of the switching element Q603 and the low potential side of the output voltage Vout. Has been. The control terminal of the switching element Q603 is connected to, for example, a control unit (not shown) of a device including the power supply device via a resistor R605, and an output signal from the device control unit is used as an output signal from the device control unit. Accordingly, switching element Q603 is controlled.

(Operation of starting resistor switching circuit)
Next, the operation of the starting resistance switching circuit will be described. The device control unit can set the switching element Q603 to the conductive state or the non-conductive state by changing the level of the output signal. For example, when the ringing choke converter (RCC) is activated and continuous oscillation is reached by the voltage induced in the feedback winding Nb, the device control unit sets the output signal to a high level. As a result, the switching element Q603 is turned on, a forward current flows through the light emitting diode of the photocoupler IC 601, and the switching element Q602 is turned on. As a result, the switching element Q601 becomes non-conductive and no current flows through the starting resistor R401, and the starting resistor R401 is not connected to the control terminal of the main switching element Q401 (disconnected). As a result, the reactive current due to the starting resistor R401 can be reduced.

  In addition, a current flows through the resistance R601 to the control terminal of the main switching element Q401. For example, if the resistance value of the resistor R601 is twice that of the starting resistor R401, the power consumption of the resistor R601 is half that of the starting resistor R401, and the power consumption can be reduced. However, in this circuit, conduction and non-conduction control of the switching element Q601 is performed using an expensive element such as the photocoupler IC 601, and there is a problem that the cost of components constituting the circuit is high.

[Circuit configuration and operation of power supply unit]
1 is a circuit diagram illustrating a configuration of a switching power supply device according to a first embodiment. The difference between this embodiment and the above-described conventional example is that in FIG. 1 of this embodiment, a negative voltage detection circuit including a diode D101, a capacitor C101, and a resistor R101 is provided in the feedback winding Nb. is there. Further, in FIG. 1, the negative voltage detection circuit as the first feedback means is connected to the control terminal of the switching element Q601 constituting the starting resistance switching circuit described above via the Zener diode ZD101 which is a constant voltage diode. Yes. Further, in FIG. 1, a feedback circuit (second circuit) that is provided on the secondary side of the transformer T401 and directly detects the output voltage Vout and feeds back to the primary side of the transformer T401 in FIGS. Feedback means) has been removed. Hereinafter, the circuit configuration and operation of FIG. 1 will be described. In FIG. 1, the same circuits as those in FIGS. 5 and 7 described above are denoted by the same reference numerals, and description thereof is omitted.

(Circuit configuration)
In FIG. 1, a cathode terminal of a diode D101 is connected to a terminal having the same polarity as the primary winding Np of the feedback winding Nb. The anode terminal of the diode D101 is connected to a terminal having a polarity different from that of the primary winding Np of the feedback winding Nb via a capacitor C101 connected in series. A resistor R101 is connected to both ends of the capacitor C101. The electric charge accumulated in the capacitor C101 is discharged, and the potential generated at both ends of the capacitor C101 is stabilized by balancing with the charge charged from the feedback winding Nb. I am trying. The anode terminal of the diode D101 is connected to the anode terminal of the Zener diode ZD101, and the cathode terminal of the Zener diode ZD101 is connected to the control terminal of the switching element Q602. The voltage Vb of the feedback winding Nb generated on the anode terminal side of the diode D101 is relative to the potential on the (−) side of the input voltage Vin due to the rectifying action of the diode D101 according to the following equation (3). The electric potential will drop to Here, Vout in Equation (3) is the output voltage on the secondary side of the transformer T401, Nb is the number of turns of the feedback winding Nb, and Ns is the number of turns of the secondary winding Ns.

  From equation (3), the voltage Vb generated at the anode terminal of the diode D101 of the feedback winding Nb is the output voltage Vout, the number Nb of the feedback winding Nb, and the number of the secondary winding Ns. And Ns. The switching power supply device of FIG. 1 of the present embodiment is characterized in that the output voltage Vout is controlled using the voltage Vb generated in the feedback winding Nb.

(Circuit operation)
When the input voltage Vin is applied to the switching element Q601 connected to the starting resistor R401, a current is supplied to the control terminal of the switching element Q601 via the resistor R601, and the switching element Q601 becomes conductive. When switching element Q601 becomes conductive, a voltage obtained by dividing input voltage Vin by starting resistor R401 and resistor R402 is applied to the control terminal of main switching element Q401. As a result, when the main switching element Q401 becomes conductive, the input voltage Vin is applied to the primary winding Np of the transformer T401. The conduction time of main switching element Q401 is controlled by the time until switching element Q402 becomes conductive. The time until switching element Q402 is turned on is determined by a time constant circuit including resistor R405 and capacitor C403. When the time determined by the time constant circuit is reached, switching element Q402 becomes conductive, and conversely, main switching element Q401 changes to a non-conductive state. When the main switching element Q401 becomes non-conductive, the diode D402 becomes conductive due to the voltage generated in the secondary winding Ns by the energy accumulated in the transformer T401, and the electrolytic capacitor C404 is charged, whereby the output voltage Vout Will rise.

  When the output voltage Vout increases, the voltage Vb determined by the above equation (3) also increases in the negative voltage direction. Hereinafter, the increase in the absolute value of the negative voltage is referred to as “rising in the negative voltage direction” or “increasing in the negative voltage direction”. When the voltage Vb further increases in the negative voltage direction and the potential charged in the capacitor C101 becomes higher than the Zener voltage of the Zener diode ZD101, the Zener diode ZD101 becomes conductive and current flows. For the Zener diode ZD101, for example, a component having a Zener voltage that is substantially the same voltage as the voltage Vb when the output voltage Vout in Equation (3) is set to DC24V is selected. When a current flows through the Zener diode ZD101, a current flows from the control terminal of the switching element Q602, and the switching element Q602 becomes conductive. On the other hand, when switching element Q602 becomes conductive, switching element Q601 shifts to a non-conductive state. As a result, the starting resistor R401 is disconnected from the control terminal of the main switching element Q401. Therefore, when the switching element Q601 is in a conductive state, the current flowing from the (+) side of the input voltage Vin to the resistor R402 via the starting resistor R401 flows through the resistor R601 when the switching element Q601 is in a non-conductive state. . Since the resistance value of the resistor R601 is larger than that of the starting resistor R401, the voltage applied to the control terminal of the main switching element Q401 decreases due to voltage division with the resistor R402, and the main switching element Q401 becomes nonconductive. Since the non-conducting state of the switching element Q601 is maintained while the switching element Q602 is in the conducting state, the main switching element Q401 also maintains the non-conducting state, and the main switching element Q401 does not oscillate.

  When the load current flows to the secondary load, the output voltage Vout decreases, and when the voltage Vb generated in the feedback winding Nb decreases in the negative voltage direction, the potential charged in the capacitor C101 decreases. Hereinafter, the decrease in the absolute value of the negative voltage is referred to as “decrease in the negative voltage direction”. As a result, when the potential charged in the capacitor C101 becomes lower than the Zener voltage of the Zener diode ZD101, the Zener diode ZD101 is turned off. Since no current flows to the control terminal of the switching element Q602 via the Zener diode ZD101, the switching element Q602 transitions to a non-conduction state. When switching element Q602 transitions to a non-conducting state, a current is supplied from the (+) side of input voltage Vin to the control terminal of switching element Q601 via resistor R601, and switching element Q601 enters a conducting state. When switching element Q601 transitions to the conductive state, starting resistor R401 is connected to the control terminal of main switching element Q401, and current flows to resistor R402 via starting resistor R401. The voltage obtained by dividing the input voltage Vin by the starting resistor R401 and the resistor R402 is again applied to the control terminal of the main switching element Q401, the main switching element Q401 becomes conductive, and the oscillation operation of the main switching element Q401 is performed. Is resumed.

[Power supply operation waveforms]
FIG. 2 shows the waveform of each part in the circuit of the switching power supply device of this embodiment shown in FIG. Vds (Q401) in FIG. 2A is the waveform of the voltage (drain-source voltage) Vds between the current inflow and current outflow terminals of the main switching element Q401, and Ip (Q401) in FIG. The waveform of the current Ip flowing through the main switching element Q401 is shown. Further, Is (D402) in FIG. 2C is a waveform of the current Is flowing through the rectifying diode D402, and Vgs (Q401) in FIG. 2D is applied to the control terminal of the main switching element Q401. The waveform of the voltage Vgs is shown. Further, Vout in FIG. 2 (e) represents the waveform of the output voltage Vout, and Vb in FIG. 2 (f) represents the waveform of the voltage Vb generated in the negative voltage detection circuit.

  “Vds (max)” (broken line) on the vertical axis in FIG. 2A indicates the maximum value of the drain-source voltage Vds. In the horizontal axis of FIG. 2A, “ton” indicates an on-time that is a conduction time of the main switching element Q401, “toff” indicates an off-time that is a non-conduction time, and “T = 1 / “f” indicates the oscillation period T (= 1 / f) of the main switching element Q401. “Ip (max)” (broken line) on the vertical axis in FIG. 2B indicates the peak current value of the current Ip. “Vgs (cutoff)” (broken line) on the vertical axis in FIG. 2D indicates a threshold voltage at which the main switching element Q401 becomes conductive. “Vout (mean)” (broken line) on the vertical axis in FIG. 2E indicates the average voltage of the output voltage Vout. “Vb (min)” (broken line) in FIG. 2F indicates the lowest voltage (the absolute value of the voltage is minimum) in the negative voltage direction of the voltage Vb generated in the negative voltage detection circuit. In addition, the horizontal axis of each figure of FIG. 2 shows time (t).

  The output voltage Vout increases with the oscillation of the main switching element Q401, and decreases during the non-conduction period of the main switching element Q401. As the output voltage Vout decreases, the voltage Vb decreases in the negative voltage direction, and it can be seen that oscillation starts again by the main switching element Q401. Further, a resonance vibration waveform is observed at the control terminal of the main switching element Q401 due to the leakage inductance of the transformer T401 after the diode D401 is turned on. The oscillation period of the main switching element Q401 varies depending on the load current. When the load current increases, the number of oscillations per unit time increases, and when the load current decreases, the number of oscillations per unit time decreases. By controlling the number of oscillations as the load current increases or decreases, the output voltage Vout can be maintained within a predetermined voltage range.

  As described above, the output voltage Vout can be controlled by controlling the conduction / non-conduction state of the switching element Q601 using the voltage Vb generated in the feedback winding Nb. Further, when the switching element Q601 is in a non-conductive state, the starting resistor R401 can be disconnected from the circuit. With the circuit configuration shown in FIG. 1, the output voltage Vout required for the device and the step-down converter is transferred to the input side (primary side) of the transformer T401 without using a transmission element such as a photocoupler and an expensive element such as an operational amplifier as in the conventional example. ) Can be generated only by the control of the circuit. Furthermore, since the number of components can be reduced, the mounting area of the components can be reduced, and as a result, the size of the power supply device can be reduced and the cost of the power supply device can be reduced. In this embodiment, the circuit format using the ringing choke converter has been described. However, the present invention can be applied to other circuits, for example, a circuit format such as a flyback system using a dedicated IC regardless of the polarity of the feedback winding. be able to.

  As described above, according to the present embodiment, power consumption can be reduced with a simple configuration.

[Circuit configuration and operation of power supply unit]
FIG. 3 is a circuit diagram illustrating a configuration of the switching power supply device according to the second embodiment. The difference between the present embodiment and the first embodiment is that in FIG. 3 of the present embodiment, the control circuit unit that controls the conduction time of the main switching element Q401 according to the output voltage and the control that controls the negative voltage detection circuit described above. This is the addition of a part switching circuit. The control circuit unit includes a photocoupler IC302, an operational amplifier IC303, resistors R305 to R309, and a capacitor C301. On the other hand, the control unit switching circuit includes a photocoupler IC 301, a switching element Q301, and resistors R301 to R304.

(Circuit configuration of control unit switching circuit)
A circuit configuration of the control unit switching circuit as setting means will be described. A current inflow terminal of a phototransistor that is a light receiving side element of the photocoupler IC 301 is connected to one end of a capacitor C101 via a resistor R301, and a current outflow terminal is connected to the other end of the capacitor C101. The anode terminal of the light emitting diode (LED) of the photocoupler IC301 is connected to the (+) terminal of the electrolytic capacitor C404 having the same potential as the output voltage Vout via the resistor R302, and the cathode terminal is the current inflow terminal of the switching element Q301. It is connected to the. The current outflow terminal of the switching element Q301 (third transistor) is connected to the low potential side (ground side) of the output voltage Vout. A resistor R303 is connected between the control terminal of the switching element Q301 and the current outflow terminal in order to determine the potential. The control terminal of the switching element Q301 is connected to a device control unit (not shown) via a resistor R304, and the on / off state of the switching device Q301 is controlled by an output signal from the device control unit.

(Operation of control unit switching circuit)
Next, the operation of the control unit switching circuit will be described. When the output signal from the device control unit is at a high level, the switching element Q301 is turned on, and the phototransistor IC 301 is turned on, so that the phototransistor is also turned on. As a result, both ends of the capacitor C101 are connected via the resistor R301. Therefore, even if the output voltage Vout increases, the voltage Vb generated in the negative voltage detection circuit, that is, the potential charged in the capacitor C101 is not increased. , Almost zero. As a result, the Zener diode ZD101 is turned off, so that no current flows through the control terminal of the switching element Q602. Therefore, the switching element Q602 is turned off and the switching element Q601 is turned on. Therefore, the starting resistor R401 is connected to the control terminal of the main switching element Q401, and a current always flows from the starting resistor R401 to the resistor R402 via the switching element Q601.

  On the other hand, when the terminal that outputs the output signal of the device control unit is in a high impedance state, the switching element Q301 becomes non-conductive due to the resistor R303 connected between the control terminal of the switching element Q301 and the current outflow terminal. Since the switching element Q301 becomes non-conductive, no current flows through the LED of the photocoupler IC 301. Therefore, the LED becomes non-conductive, and the phototransistor also becomes non-conductive. Thus, both ends of the capacitor C101 are not connected via the resistor R301 as in the case where the output signal from the device control unit is at a high level. Therefore, when the output voltage Vout increases, the voltage Vb generated on the anode terminal side of the diode D101 also increases in the negative voltage direction, and the potential charged between the terminals of the capacitor C101 also increases. When the charging potential of the capacitor C101 becomes higher than the Zener voltage of the Zener diode ZD101, the Zener diode ZD101 becomes conductive. As a result, since a current flows through the control terminal of switching element Q602, switching element Q602 is turned on, while switching element Q601 is turned off. Then, when switching element Q601 is turned off, starting resistor R401 is disconnected from the control terminal of main switching element Q401. Therefore, when the switching element Q601 is in a conductive state, the current flowing from the (+) side of the input voltage Vin to the resistor R402 via the starting resistor R401 flows through the resistor R601 when the switching element Q601 is in a non-conductive state. . Since the resistance value of the resistor R601 is larger than that of the starting resistor R401, the voltage applied to the control terminal of the main switching element Q401 decreases due to voltage division with the resistor R402, and the main switching element Q401 becomes nonconductive.

  Thereafter, when the output voltage Vout decreases due to the flow of a load current to the secondary load, and the voltage Vb generated in the feedback winding Nb decreases in the negative voltage direction, the Zener diode ZD101 becomes non-conductive. Since no current flows to the control terminal of the switching element Q602 via the Zener diode ZD101, the switching element Q602 transitions to a non-conduction state. When switching element Q602 transitions to a non-conducting state, a current is supplied from the (+) side of input voltage Vin to the control terminal of switching element Q601 via resistor R601, and switching element Q601 enters a conducting state. When switching element Q601 transitions to the conductive state, starting resistor R401 is connected to the control terminal of main switching element Q401, and the collector current of switching element Q601 flows to resistor R402 via starting resistor R401. Then, the voltage obtained by dividing the input voltage Vin by the starting resistor R401 and the resistor R402 is again applied to the control terminal of the main switching element Q401, the main switching element Q401 becomes conductive, and the oscillation operation is resumed. Thus, it can be said that the control unit switching circuit is a circuit that switches between valid and invalid of the output voltage control by the negative voltage detection circuit.

(Control circuit part)
The control circuit unit for controlling the conduction time of the main switching element Q401 according to the output voltage added in the present embodiment has the same function as the feedback control circuit of FIGS. 5 and 7 of the conventional example described above. . That is, the photocoupler IC 302 and the operational amplifier IC 303 in FIG. 3 correspond to the photocoupler IC 401 and the operational amplifier IC 402 in FIGS. Also, the resistors R305 and R309 and the resistors R306 and R307 for dividing the output voltage in FIG. 3 correspond to the resistors R409, R406, R407, and R408 in FIGS. 5 and 7, respectively. Further, the phase compensation resistor R308 and the capacitor C301 in FIG. 3 also correspond to the resistor R410 and the capacitor C405 shown in FIGS. Since the feedback control circuit of FIGS. 5 and 7 has been described above, the description of the control circuit unit of FIG. 3 is omitted.

  As described above, with the circuit configuration described above, for example, if the output of the device control unit is set to a state other than a high level, the voltage Vb generated in the feedback winding Nb described in the first embodiment is used. The conduction / non-conduction state of the switching element Q601 can be controlled. Thereby, when the switching element Q601 is in a non-conductive state, the starting resistor R401 can be disconnected from the circuit, and the main switching element Q401 is also in a non-conductive state, so that the output voltage Vout can be controlled. Therefore, for example, the voltage of the output voltage Vout when the Zener diode ZD101 becomes conductive is set to the same voltage as the Zener voltage (first voltage) of the Zener diode ZD401 of the overvoltage protection circuit of FIGS. Thereby, when the output of the device control unit is set to, for example, a high impedance state, the negative voltage detection circuit can perform the same operation as the overvoltage protection circuit, and the overvoltage required in the circuits of FIGS. The protection circuit can be deleted. As a result, the number of components can be reduced without using an expensive element such as a photocoupler, so that the cost of the power supply device can be reduced while the power supply device can be downsized by reducing the mounting area. .

  As described above, according to the present embodiment, power consumption can be reduced with a simple configuration.

  In the third embodiment, the control target voltage of the output voltage Vout by the negative voltage detection circuit and the control target voltage by the operational amplifier IC303 are set to different voltages in the switching power supply device of FIG. And the example which switches a control target voltage according to the operation state of the apparatus provided with a switching power supply device is demonstrated. Specifically, in the first and second embodiments, the control target voltage is set to DC24V, but in this embodiment, for example, the control target voltage by the negative voltage detection circuit is DC10V, and the control target voltage using the operational amplifier IC303 is DC24V. Set to.

  Further, a step-down converter circuit (not shown) that generates a voltage (for example, 3.3 V, etc.) necessary for the device control unit is disposed in the subsequent stage, which is the secondary side of the transformer T401 of the switching power supply device of this embodiment. Yes. In general, the step-down converter circuit is more efficient when the input voltage is lower. Further, in a device provided with the switching power supply device of the present embodiment, for example, office equipment such as an image forming apparatus, when comparing power consumption when an image forming operation is performed and when the device is in a standby state, There is a big difference. In this embodiment, since the power consumption greatly changes depending on the state of the device as described above, and the drive source such as the motor is stopped when the device is in the standby state, a relatively high direct-current output voltage such as 24 VDC is used. This is particularly useful for a switching power supply provided in a device that is not required.

  In this embodiment, when the device is in the standby state in FIG. 3, the control by the negative voltage detection circuit is enabled by setting the terminal that outputs the output signal of the device control unit to the high impedance state, and the output voltage Vout is DC10V. It is controlled to be (second voltage). As a result, the power consumption of the device can be reduced by improving the efficiency of the step-down converter disposed in the subsequent stage when the device is on standby. On the other hand, when the device is in a normal operation state, such as during an image forming operation, the control by the operational amplifier IC 303 is enabled by outputting a high-level output signal from the device control unit, and the output voltage Vout is 24 VDC. Control to be. Thereby, when the device is in the normal operation state, it is possible to provide a switching power supply device that can reduce the fluctuation of the output voltage Vout due to the fluctuation of the load current and maintain the desired output voltage Vout.

  As described above, according to the present embodiment, power consumption can be reduced with a simple configuration.

  The power supply apparatus described in the first to third embodiments can be applied as, for example, a low-voltage power supply for an image forming apparatus, that is, a power supply that supplies power to a drive unit such as a controller (control unit) or a motor. Hereinafter, the configuration of the image forming apparatus to which the power supply devices of Embodiments 1 to 3 are applied will be described.

[Configuration of Image Forming Apparatus]
A laser beam printer will be described as an example of the image forming apparatus. FIG. 4 shows a schematic configuration of a laser beam printer which is an example of an electrophotographic printer. The laser beam printer 500 includes a photosensitive drum 511 as an image carrier on which an electrostatic latent image is formed, a charging unit 517 (charging unit) that uniformly charges the photosensitive drum 511, and an electrostatic latent image formed on the photosensitive drum 511. A developing unit 512 (developing unit) that develops an image with toner is provided. The toner image developed on the photosensitive drum 511 is transferred to a sheet (not shown) as a recording material supplied from the cassette 516 by a transfer unit 518 (transfer unit), and the toner image transferred to the sheet is transferred to the fixing unit 514. Then, the toner is fixed and discharged onto the tray 515. The photosensitive drum 511, the charging unit 517, the developing unit 512, and the transfer unit 518 are image forming units. The laser beam printer 500 includes the power supply device 550 described in the first to third embodiments. The image forming apparatus to which the power supply device 550 of the first to third embodiments can be applied is not limited to the one illustrated in FIG. 4, and may be an image forming apparatus including a plurality of image forming units, for example. Further, the image forming apparatus may include a primary transfer unit that transfers a toner image on the photosensitive drum 511 to an intermediate transfer belt and a secondary transfer unit that transfers the toner image on the intermediate transfer belt to a sheet.

  The laser beam printer 500 includes a controller 520 that controls an image forming operation by the image forming unit and a sheet conveying operation, and the device control unit of the first to third embodiments corresponds to the controller 520. The power supply device 550 described in the first to third embodiments supplies power of an output voltage Vout (DC 24 V) to a driving unit such as a motor for rotating the photosensitive drum 511 or driving various rollers that convey the sheet. To do. Further, the power supply device 550 described in the third embodiment supplies power such as DC 3.3 V to the controller 520 through the step-down converter circuit. Further, the controller 520 outputs a high-level control signal indicating a normal operation state to the control unit switching circuit of the second and third embodiments according to the operation state of the apparatus, for example, during an image forming operation, and in a standby state. Set the terminal that outputs the control signal to the high impedance state.

  As described above, according to the present embodiment, power consumption can be reduced with a simple configuration.

Q401 Main switching element Q402 Switching element Q602 Switching element R401 Starting resistor T401 Insulating transformer ZD101 Zener diode

Claims (17)

A transformer having a primary winding, a secondary winding and a feedback winding;
A first rectifying / smoothing means for rectifying and smoothing an AC voltage to generate a DC voltage and supplying the DC voltage to the primary winding of the transformer;
A switching element that is connected to the primary winding of the transformer and controls the supply and interruption of current to the primary winding;
Control means for controlling the switching operation of the switching element;
A starting resistor for starting oscillation of the switching element;
Switch means for connecting and disconnecting the starting resistor and the switching element;
First feedback means for controlling the switch means based on a voltage generated in the feedback winding;
A power supply apparatus comprising: The switch means includes a first transistor connected to the starting resistor, and a second transistor,
When the second transistor is off, the first transistor is turned on, the starting resistor is connected to the switching element,
2. The power supply device according to claim 1, wherein when the second transistor is on, the first transistor is turned off, and the starting resistor is not connected to the switching element. Having one end connected to the positive terminal of the first rectifying and smoothing means and the other end connected to the current inflow terminal of the second transistor;
The starting resistor has one end connected to the positive terminal of the first rectifying and smoothing means,
The first transistor has a current inflow terminal connected to the other end of the starting resistor, a current outflow terminal connected to a control terminal of the switching element, and a control terminal connected to the other end of the resistor. The power supply device according to claim 2.   The power supply device according to claim 3, wherein the second transistor has a current inflow terminal connected to the other end of the resistor, and a current outflow terminal connected to a control terminal of the switching element.   The power supply device according to claim 3, wherein a resistance value of the resistor is larger than the starting resistance. The first feedback means includes a diode, a capacitor, and a constant voltage diode,
The diode and the capacitor are connected in series,
The cathode terminal of the diode is connected to the same polarity side as the primary winding of the feedback winding,
The anode terminal of the diode is connected to one end of the capacitor,
The other end of the capacitor is connected to the primary winding and a different polarity side of the feedback winding,
6. The power supply device according to claim 2, wherein the constant voltage diode is connected to a control terminal of the second transistor. A second rectifying / smoothing means for generating a DC voltage by rectifying and smoothing the voltage generated in the secondary winding of the transformer;
The constant voltage diode becomes conductive when the voltage charged in the capacitor becomes a voltage generated in the feedback winding when the DC voltage generated by the second rectifying and smoothing means is a predetermined voltage, The power supply device according to claim 6, wherein the second transistor is turned on. A second rectifying / smoothing means for generating a DC voltage by rectifying and smoothing the voltage generated in the secondary winding of the transformer;
Second feedback means for detecting a DC voltage generated by the second rectifying and smoothing means and outputting a detection signal corresponding to the detected DC voltage to the control means;
Setting means for setting whether the control of the switch means by the first feedback means is valid or invalid;
The power supply device according to claim 6, further comprising:   9. The power supply apparatus according to claim 8, wherein the control unit stops the operation of the switching element based on a detection signal from the second feedback unit. The setting means includes a third transistor and a photocoupler,
The third transistor is connected to a light emitting diode of the photocoupler;
The phototransistor of the photocoupler has a current inflow terminal connected to one end of the capacitor of the first feedback means, and a current outflow terminal connected to the other end of the capacitor. 9. The power supply device according to 9. When the third transistor is off, the switch means is controlled by the first feedback means,
11. The power supply device according to claim 10, wherein when the third transistor is on, the switch means is not controlled by the first feedback means.   The constant voltage diode is conductive when a voltage charged in the capacitor becomes a voltage generated in the feedback winding when a DC voltage generated by the second rectifying and smoothing means is higher than a predetermined voltage. 12. The power supply device according to claim 10, wherein the second transistor is turned on and the second transistor is turned on.   The constant voltage diode is conductive when the voltage charged in the capacitor becomes a voltage generated in the feedback winding when the DC voltage generated by the second rectifying / smoothing means is lower than a predetermined voltage. 12. The power supply device according to claim 10, wherein the second transistor is turned on and the second transistor is turned on. Image forming means for forming an image on a recording material;
A power supply device according to any one of claims 1 to 13,
An image forming apparatus comprising: Image forming means for forming an image on a recording material;
A controller for controlling the image forming means;
A power supply device according to claim 12,
With
The controller turns on the third transistor when the power supply device detects an overvoltage of the DC voltage generated by the second rectifying and smoothing means, and turns on the third transistor when the overvoltage is not detected. An image forming apparatus characterized in that three transistors are turned off. Image forming means for forming an image on a recording material;
A controller for controlling the image forming means;
A power supply device according to claim 13,
With
The image forming apparatus, wherein the controller controls the third transistor in accordance with a state of the image forming apparatus.   17. The image formation according to claim 16, wherein the controller turns on the third transistor when the image forming unit forms an image, and turns off the third transistor when the image forming unit does not perform image formation. apparatus.

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