JP6961421B2 - Power supply and image forming equipment - Google Patents

Description

The present invention relates to a power supply device and an image forming device, and more particularly to a switching power supply device using an active clamp method for an isolated converter using a flyback transformer.

In a switching power supply that converts an AC voltage into a DC voltage such as a commercial power supply, it is required to improve the efficiency of the switching power supply in order to reduce the power consumption of the switching power supply. Here, the efficiency of the switching power supply is represented by the ratio of the power output by the switching power supply to the power supplied to the switching power supply.

As a means for improving the efficiency of the switching power supply, for example, as described in Patent Document 1, a method of using synchronous rectification for the secondary side output of the switching power supply has been proposed.

Japanese Patent No. 3152016

However, the switching power supply using the active clamp method for the isolated converter using the flyback transformer has the following problems. That is, when synchronous rectification is used for the secondary side output of the switching power supply, there is a problem that it is difficult to detect the optimum timing for turning off the switching element used for the secondary side rectification. If the timing for turning off the switching element used for synchronous rectification of the secondary output is not the optimum timing, the power supply efficiency will decrease. That is, when the switching element used for synchronous rectification is turned off early, the efficiency is lowered because the rectification is performed by the diode. On the other hand, if the off timing of the switching element used for synchronous rectification is late, a current flows back from the secondary side to the primary side, causing noise or a large loss, which may cause a failure.

The present invention has been made under such circumstances, and an object of the present invention is to improve the efficiency of a switching power supply using an active clamp method.

In order to solve the above-mentioned problems, the present invention includes the following configurations.

(1) A transformer having a primary winding and a secondary winding, a first switching element connected in series with the primary winding of the transformer, and a first switching element connected in parallel with the primary winding of the transformer. A second switching element, a capacitor connected in series with the second switching element and connected in parallel with the first winding of the transformer together with the second switching element, and induced in the secondary winding. Based on the rectifying means for rectifying the voltage, the feedback means for outputting the feedback voltage to the primary side according to the voltage induced in the secondary winding of the transformer, and the first A switching element and a first control means for controlling the on / off of the second switching element are provided, and the first control means turns off both the first switching element and the second switching element. A power supply device that performs a switching operation in which the first switching element and the second switching element are alternately turned on or off with a dead time to be caused, and the rectifying means is induced in the secondary winding. It has a third switching element for rectifying a voltage, an inductor connected in series with the third switching element, and a second control means for controlling the switching operation of the third switching element. The third switching element connected in series and the voltage detecting means for detecting the voltage across the inductor are provided, and the second control means turns the third switching element from off to on and then turns the third switching element on. It is controlled so as to keep the on state of the third switching element regardless of the voltage detected by the voltage detecting means for one time, and after the first time elapses, it is detected by the voltage detecting means. The third switching element is controlled to be turned from on to off according to the voltage, and during the first time, the voltage detected by the voltage detecting means due to the on of the second switching element becomes negative. A power supply device characterized in that it is longer than the period from when the detected voltage becomes the minimum value and shorter than the on-time of the second switching element.
(2) A transformer having a primary winding and a secondary winding, a first switching element connected in series with the primary winding of the transformer, and a first switching element connected in parallel with the primary winding of the transformer. A second switching element, a capacitor connected in series with the second switching element and connected in parallel with the first winding of the transformer together with the second switching element, and induced in the secondary winding. Based on the rectifying means for rectifying the voltage, the feedback means for outputting the feedback voltage to the primary side according to the voltage induced in the secondary winding of the transformer, and the first A switching element and a first control means for controlling the on / off of the second switching element are provided, and the first control means turns off both the first switching element and the second switching element. A power supply device that performs a switching operation in which the first switching element and the second switching element are alternately turned on or off with a dead time to be caused, and the rectifying means is induced in the secondary winding. It has a third switching element for rectifying a voltage, an inductor connected in series with the third switching element, and a second control means for controlling the switching operation of the third switching element. The third switching element connected in series and the voltage detecting means for detecting the voltage across the inductor are provided, and the second control means turns the third switching element from on to off and then turns the third switching element on. It is controlled so as to keep the off state of the third switching element regardless of the voltage detected by the voltage detecting means for two hours, and after the second time elapses, it is detected by the voltage detecting means. The third switching element is controlled to be turned from off to on according to the voltage, and the second time is from when the second switching element is turned off until the current flowing to the secondary side becomes zero. A power supply device characterized by being longer than the period of 1 and shorter than the on-time of the first switching element.
(3) An image forming apparatus including an image forming means for forming an image on a recording material and a power supply device according to the above (1) or (2).

According to the present invention, the efficiency of a switching power supply using the active clamp method can be improved.

Schematic diagram of the power supply circuit of the first embodiment Explanatory drawing of inductor L12 of Example 1 Explanatory drawing of control method of power supply circuit of Example 1 Schematic diagram of the power supply circuit of the second embodiment Explanatory drawing of control method of power supply circuit of Example 2 Schematic diagram of the power supply circuit of the third embodiment The figure which shows the image forming apparatus of Example 4.

Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings by way of examples.

[Power supply]
FIG. 1 is a circuit diagram showing an outline of a switching power supply circuit using the active clamp method of the first embodiment. The AC power supply 10 such as a commercial power supply outputs an AC voltage, and the voltage rectified by the bridge diode BD1 which is a full-wave rectifying means is input to the switching power supply circuit 100. The smoothing capacitor C3 is used as a smoothing means for the rectified voltage, and the potential on the low side of the smoothing capacitor C3 is DCL and the potential on the high side is DCH. The switching power supply circuit 100 outputs an output voltage Vout from the input voltage Vin charged in the smoothing capacitor C3 to the isolated secondary side. In the first embodiment, the switching power supply circuit 100 outputs a constant voltage of, for example, 24V as an example of the output voltage Vout.

The switching power supply circuit 100 has an insulated transformer T1 having a primary winding P1 and an auxiliary winding P2 on the primary side and a secondary winding S1 on the secondary side. Energy is supplied to the primary winding P1 to the secondary winding S1 of the transformer T1 by a switching operation described later with reference to FIG. The auxiliary winding P2 of the transformer T1 is used to rectify and smooth the forward voltage of the input voltage Vin applied to the primary winding P1 by the diode D4 and the capacitor C4 to supply the power supply voltage V1.

On the primary side of the switching power supply circuit 100, a field effect transistor (hereinafter referred to as FET) 1 which is a first switching element is connected in series to the primary winding P1 of the transformer T1. The capacitor C2 for voltage clamping and the FET 2 which is the second switching element are connected in series. The voltage clamp capacitors C2 and FET2 connected in series are connected in parallel to the primary winding P1 of the transformer T1. On the primary side of the switching power supply circuit 100, a control unit 101 is provided as a first control means for the FET 1 and the FET 2. The voltage resonance capacitor C1 connected in parallel with the FET 1 is provided in order to reduce the loss when the FET 1 and the FET 2 are switched off. The capacitance between the drain terminal and the source terminal of the FET 1 may be used without providing the capacitor C1 for voltage resonance. In order to facilitate the operation of turning on the switching element at the zero voltage described with reference to FIG. 3, the capacitor C1 for voltage resonance is selected to have a smaller capacitance than the capacitor C2 for voltage clamping. On the secondary side of the switching power supply circuit 100, the FET 12, the diode D12, which is the third switching element which is the rectifying means on the secondary side of the flyback voltage, which is the voltage induced in the secondary winding S1 of the transformer T1, It has an inductor L12 and a control unit 200. The diode D1 of the first embodiment is a body diode of the FET1. Similarly, the diode D2 is the body diode of the FET 2. Similarly, the diode D12 is the body diode of the FET 12.

The voltage induced in the secondary winding S1 of the transformer T1 is smoothed by the capacitor C11 which is a smoothing means on the secondary side, and is output as an output voltage Vout. The secondary side of the switching power supply circuit 100 has a feedback unit 115 that outputs a feedback voltage as a feedback means for feeding back information according to the output voltage Vout output to the secondary side to the primary side (FIG. FIG. Middle, dotted frame).

(Control unit 101)
The control unit 101 is a circuit that controls the gate drive signal DL and the gate drive signal DH based on the voltage signal (feedback voltage) (hereinafter referred to as FB terminal voltage) input from the feedback unit 115 to the FB terminal. The gate drive signal DL is a signal for controlling the gate voltage of the FET 1, and the gate drive signal DH is a signal for controlling the gate voltage of the FET 2. The power supply voltage V1 generated by the auxiliary winding P2 is supplied between the VC terminal and the G terminal of the control unit 101. When the switching between the FET 1 and the FET 2 is started, the power is supplied from the auxiliary winding P2. The VS terminal of the control unit 101 is supplied with power necessary for starting the switching power supply circuit 100 from the start resistor R6, and is used as a power supply means until switching is started. Further, in order to drive the FET 2, a power supply voltage V1 is supplied between the VH terminal and the GH terminal by a charge pump circuit composed of a capacitor C5 and a diode D5.

(Control unit 200)
The control unit 200, which is the second control means, is a control circuit for rectifying the secondary side current I2 in the direction of the arrow in FIG. An output voltage Vout is supplied as a power supply voltage between the VC terminal and the G terminal of the control unit 200. In the first embodiment, the source terminal of the FET 12 is connected to the GND (ground) side of the secondary winding S1, one end of the inductor L12 is connected to the drain terminal of the FET 12, and the secondary winding S1 is connected to the other end of the inductor L12. It is connected. The voltage detection unit 201, which is a voltage detecting means, detects the timing of transitioning the ON state and the OFF state of the FET 12 by detecting the voltage across the series circuit of the FET 12 and the inductor L12. The voltage across the series circuit of the FET 12 and the inductor L12 is detected by the control unit 200 as a detection voltage V2 applied between the S terminal and the G terminal of the control unit 200. The first embodiment is characterized in that the voltage is detected by sandwiching the inductor L12. The gate drive unit 202 outputs the gate drive signal DS2 of the FET 12 based on the detection result of the voltage detection unit 201. The control method by the control unit 200 will be described later with reference to FIG.

(Inductor L12)
FIG. 2 is a diagram showing an example of a method for forming the inductor L12. FIG. 2 shows an example of a method of forming an inductor without using a coil. FIG. 2 illustrates the FET 12, the gate terminal (G terminal), drain terminal (D terminal), and source terminal (S terminal) of the FET 12. In FIG. 2A, the inductance is formed by passing the D terminal of the FET 12 between the magnetic materials 50. The magnetic material 50 is a magnetic material formed in a bead shape with a hole in the center. Further, in FIG. 2B, the D terminal of the FET 12 is connected to the transformer T1 via the pattern of the conductive layer 51 on the substrate 52 (on the substrate), and the pattern of the D terminal of the FET 12 and the conductive layer 51 It forms an inductance. The board 52 is a board on which a power supply circuit such as an FET 12 or a transformer T1 is mounted. By using the methods shown in FIGS. 2A and 2B, the inductance can be formed without increasing the circuit scale and cost. Further, a coil may be used for the inductor L12.

(Feedback section 115)
Returning to the description of FIG. The feedback unit 115 is used to control the output voltage Vout to a predetermined constant voltage. The voltage value of the output voltage Vout is set by the reference voltage of the reference terminal REF of the shunt regulator IC5, the resistor R52, and the resistor R53. When the output voltage Vout becomes higher than a predetermined voltage (24V in this case), a current flows from the cathode terminal K of the shunt regulator IC5, and the secondary side diode of the photocoupler PC5 becomes conductive via the pull-up resistor R51. As a result, the primary transistor of the photocoupler PC5 operates, and the electric charge is discharged from the capacitor C6. Therefore, the FB terminal voltage of the control unit 101 drops. On the other hand, when the output voltage Vout becomes lower than 24V, the secondary side diode of the photocoupler PC5 becomes non-conducting. As a result, the transistor on the primary side of the photocoupler PC5 is turned off, and a constant current flows from the FB terminal of the control unit 101 to the capacitor C6, so that the FB terminal voltage of the control unit 101 rises.

In this way, the feedback unit 115 changes the FB terminal voltage of the control unit 101 according to the fluctuation of the output voltage Vout. The control unit 101 can perform feedback control for controlling the output voltage Vout to a predetermined constant voltage by detecting the FB terminal voltage input from the feedback unit 115. The control unit 101 controls the output voltage Vout by controlling the on-duty of the FET 1 according to the FB terminal voltage by using a control method such as PWM.

[Control method of switching power supply circuit 100]
FIG. 3 is an explanatory diagram of a control method of the switching power supply circuit 100 using the active clamp method by the control unit 101 and the control unit 200. In FIG. 3, (i) is a diagram showing the gate drive signal DL of the FET 1, and (ii) is a diagram showing the gate drive signal DH of the FET 2. In FIG. 3, (iii) is a diagram showing the drain current of the FET 1, and (iv) is a diagram showing the voltage between the drain terminal and the source terminal of the FET 1. In FIG. 3, (v) is a diagram showing the secondary side current I2, (vi) is a diagram showing the gate drive signal DS2 of the FET 12, and (vi) is a diagram showing the detection voltage V2 of the voltage detection unit 201. The horizontal axis shows time.

(Control method of control unit 101)
The control unit 101 repeatedly turns the FET 1 and the FET 2 on and off alternately with a dead time (the period of [4] and [5] in FIG. 3) for turning off both the FET 1 and the FET 2. The operation using the FET 2 and the capacitor C2 for voltage clamping (hereinafter referred to as active clamping operation) will be described with reference to FIGS. 3 (i) to 3 (v).

As shown in FIGS. 3 (i) and 3 (iii), the current flowing through the primary winding P1 of the FET 1 and the transformer T1 linearly increases during the period [3] in which the gate drive signal DL of the FET 1 is on. When the FET1 transitions from the on state to the off state, during the dead time period [5] shown in FIG. 3, the voltage resonance operation of the voltage resonance capacitor C1 and the transformer T1 causes the FET1 to be as shown in FIG. 3 (iv). The voltage between the drain terminal and the source terminal rises.

In the active clamp type switching power supply circuit 100, the voltage rise speed between the drain terminal and the source terminal of the FET 1 can be suppressed by the action of the capacitor C1 for voltage resonance, so that the loss at the time of switching off of the FET 1 can be reduced. .. Further, in the active clamp type switching power supply circuit 100, a voltage clamp capacitor C2 having a larger capacitance than the voltage resonance capacitor C1 is connected. Therefore, the surge voltage due to the leakage inductance of the transformer T1 that occurs when the FET 1 is turned off can be suppressed, and the withstand voltage required for the switching elements (FET1, FET2, FET12) can be suppressed.

As shown in FIGS. 3 (ii) and 3 (iv), the voltage resonance operation by the voltage clamping capacitor C2 and the transformer T1 continues during the period [6] in which the gate drive signal DH of the FET 2 is on. When the voltage between the drain terminal and the source terminal of the FET 1 becomes large, the secondary side current I2 is output from the transformer T1 as shown in FIG. 3 (v). When the FET 2 transitions from the on state to the off state, during the dead time period [4] shown in FIG. 3, the voltage resonance operation of the voltage resonance capacitor C1 and the transformer T1 causes the FET 1 to be as shown in FIG. 3 (iv). The voltage between the drain terminal and the source terminal drops. The capacitance of the voltage resonance capacitor C1 is sufficiently lower than the capacitance of the voltage clamp capacitor C2. Therefore, when the FET 2 is turned off, the voltage between the drain terminal and the source terminal of the FET 1 drops to zero volt or less, and a current flows in the forward direction of the body diode D1 of the FET 1. Transitioning the FET 1 to the on state while a current flows through the body diode D1 of the FET 1 is called zero voltage switching, and the loss when the switching of the FET 1 is turned on can be reduced.

(Control method of control unit 200)
As shown in FIG. 3 (v), the control unit 200 detects a state in which the secondary side current I2 flows in the direction of the arrow shown in FIG. 1 and controls the FET 12 to be turned on. This will be described with reference to FIGS. 3 (v) to (vii).

First, the control of transitioning the FET 12 to the on state will be described. As shown in FIG. 3 (v), when the secondary side current I2 flows through the body diode D12 of the FET 12, a voltage drop occurs due to the forward voltage (for example, 0.6 V) of the diode, and in FIG. 3 (vii). During the period of [1] shown, the detection voltage V2 of the voltage detection unit 201 becomes a negative voltage. When the control unit 200 detects that the detection voltage V2 is equal to or less than a predetermined threshold value Von (below the first threshold value), which is the first threshold value, as shown in FIG. 3 (vi), the gate drive signal DS2 of the FET 12 Is set to a high level, and the FET 12 is transitioned to the on state. The control unit 200 keeps the FET 12 on until the detection voltage V2 becomes equal to or higher than a predetermined threshold value Voff (second threshold value or higher), which is the second threshold value. In the first embodiment, the zero voltage is set as the threshold value Voff. Since the on-resistance loss of the FET 12 is smaller than the loss due to the forward voltage of the diode, the efficiency of the switching power supply circuit 100 can be improved by controlling the FET 12 to be turned on. The predetermined threshold Voff is larger than the predetermined threshold Von (Voff> Von).

Next, a method of detecting the timing of transitioning the FET 12 to the off state, which is a feature of the first embodiment, will be described. In the period of [4] of FIG. 3, when the FET 2 transitions from the on state to the off state by the control of the control unit 101 described above, the secondary side current I2 is caused by the voltage resonance operation by the voltage resonance capacitor C1 and the transformer T1. Will decrease sharply. When the secondary side current I2 decreases, a voltage is generated in the inductor L12.
Voltage generated in inductor L12 = -L12 inductance x dI2 / dt ... Equation (1)

Therefore, in the period of [2] shown in FIGS. 3 (v) and 3 (vii), the detection voltage V2 is a predetermined threshold voltage Voff even though the secondary side current I2 is flowing in the direction of the arrow in FIG. It can be detected that the above (zero voltage or more in the first embodiment) is satisfied. Then, the FET 12 can be turned off.

By the way, it takes a predetermined switching period to make the FET 12 transition from the on state to the off state. Therefore, if control is performed to turn off the FET 12 after detecting that the secondary side current I2 has become zero, the following problems occur. That is, the timing for turning off the FET 12 may not be in time, and a backflow current may flow from the capacitor C11 to the primary circuit via the transformer T1. When a backflow current is generated, there is a problem that the power supply efficiency of the switching power supply is lowered.

Further, as a method of preventing the backflow current to the primary side circuit, a method of setting the voltage of a predetermined threshold value Voff to a negative voltage value lower than the zero voltage can be considered. However, when the voltage value of the threshold value Voff is lowered, the FET 12 is turned off even though a large current is flowing in the secondary side current I2 when a switching element having a low on-resistance value is used for the FET 12. The problem of closing up arises. However, if the on-resistance value of the FET 12 is increased in order to prevent this problem, there is a problem that the on-resistance loss of the FET 12 increases and the efficiency of the switching power supply decreases.

Further, a method of notifying the control unit 200 on the secondary side of the timing of turning off the FET 12 from the control unit 101 on the primary side via an insulating circuit for communication (photocoupler, transformer, etc.) is also conceivable. However, the method using an insulated circuit for communication has a problem that the circuit scale and cost increase and the power consumption by the insulated circuit increases.

In the switching power supply circuit 100 described in the first embodiment, the FET 12 can be controlled at an appropriate timing even when an FET having a low on-resistance value is used for the FET 12 without increasing the circuit scale and cost. Then, the efficiency of the switching power supply circuit 100 can be improved.

As described above, the control unit 200 can detect the timing of turning on / off the FET 12 of the synchronous rectifier circuit with a simple circuit configuration by using the active clamping operation of the control unit 101 and the inductor L12. can. Then, the power efficiency of the switching power supply using the active clamp method can be improved.

[Configuration of switching power supply circuit]
Next, the switching power supply circuit 300 of the second embodiment will be described. The same reference numerals are given to the same configurations as in the first embodiment, and the description thereof will be omitted. The switching power supply circuit 300 shown in FIG. 4 has a control unit 301 which is a second control means having a timer unit 303, a resistor R12 constituting a CR filter (snubber circuit), and a capacitor C12. Different from the configuration. The snubber circuit including the resistor R12 and the capacitor C12 is connected in parallel with the series circuit of the FET 12 and the inductor L12. The voltage detection unit 201 of the second embodiment detects the voltage across the series circuit of the FET 12 and the inductor L12 as the detection voltage V2 by detecting the voltage smoothed by the snubber circuit in which the resistor R12 and the capacitor C12 are connected in series. doing.

(Timer section 303)
The timer unit 303 of the control unit 301 has a timer for a forced on time that forcibly holds the on state for a predetermined time when the gate drive signal DS2 is changed from the off state to the on state. Further, the timer unit 303 also has a timer for a forced off time for forcibly holding the off state for a predetermined time when the gate drive signal DS2 is changed from the on state to the off state.

FIG. 5 is a diagram showing waveforms of each part in the second embodiment, and (i) to (vii) are the same graphs as (i) to (vii) in FIG. [7] in FIG. 5 shows the forced on time, which is the first time. In the forced on time shown in [7], the gate drive signal DS2 can be held in the on state even when the detection voltage V2 of the voltage detection unit 201 rises above the threshold value Voff. As shown in FIG. 5 [6], in the switching power supply using the active clamp method, the secondary side current I2 slowly increases due to the action of the voltage clamping capacitor C2 (FIG. 5 (v)). Therefore, at the timing immediately after the FET 12 is turned on and before the current value of the secondary side current I2 increases, the detection voltage V2 becomes close to zero voltage, and the detection voltage V2 becomes equal to or higher than the threshold value Voff due to noise or the like, resulting in erroneous detection. There is a problem that will end up. In the second embodiment, by using the forced on time by the timer unit 303 of the control unit 301, the FET 12 is appropriate even when the secondary side current I2 is small or when a switching element having a low on-resistance value is used by the FET 12. It can be controlled at the right timing.

In the forced off time, which is the second time shown in FIG. 5 [8], the gate drive signal DS2 can be held in the off state even when the detection voltage V2 of the voltage detection unit 201 drops below the threshold value Von. It is possible to prevent the detection voltage V2 from becoming equal to or less than the threshold value Von due to the noise when the switching of the FET 2 is turned off and the noise when the switching of the FET 1 is turned on, resulting in erroneous detection and turning on the FET 12.

(CR filter (snubber circuit))
By using the CR filter formed by the resistor R12 and the capacitor C12 when detecting the detection voltage V2, it is possible to prevent the control unit 301 from malfunctioning due to external noise or the like. By the way, in the conventional synchronous rectifier circuit that does not use the inductor L12, if a CR filter is used to detect the detection voltage V2, the timing of transitioning the FET 12 from the on state to the off state is delayed. Therefore, a backflow current may flow from the secondary side capacitor C11 to the primary side circuit via the transformer T1. However, as described in the first embodiment, the control unit 301 has a feature that the timing of turning off the FET 12 can be detected before the secondary current I2 becomes zero during the period [2] of FIG. ing. Therefore, even when the CR filter is used as shown in FIG. 4, it is possible to prevent the problem that the backflow current flows to the primary side circuit of the switching power supply circuit 300. The CR filter can also be used in the switching power supply circuit 100 of the first embodiment and the switching power supply circuit 500 of the third embodiment described later.

In this way, by using the timer unit 303 and the CR filters (C12, R12), it is possible to prevent the control unit 301 from malfunctioning, and the timing of turning the FET 12 of the synchronous rectifier circuit on and off can be set to a simple circuit configuration. Can be detected with. Then, the power efficiency of the switching power supply using the active clamp method can be improved.

Next, the switching power supply circuit 500 of the third embodiment will be described. The same reference numerals are given to the same configurations as in the first embodiment, and the description thereof will be omitted. The switching power supply circuit 500 shown in FIG. 6 differs from the configuration of the first embodiment in that a synchronous rectifier circuit is provided at a terminal on the output voltage Vout side of the secondary winding S1. The auxiliary winding S2, the diode D21, and the capacitor C21 are circuits used to supply the power supply voltage of the control unit 200. The directions of the detection voltage V2 and the secondary side current I2 detected by the voltage detection unit 201 are as shown in FIG. More specifically, in the third embodiment, the source terminal of the FET 12 is connected to the secondary winding S1, one end of the inductor L12 is connected to the drain terminal of the FET 12, and the output voltage Vout is output from the other end of the inductor L12. ..

Similarly, in the switching power supply circuit 300 of the second embodiment, the synchronous rectifier circuit may be provided at the terminal on the output voltage Vout side of the secondary winding S1. As described above, the synchronous rectifier circuit may be provided at the terminal on the GND side of the secondary winding S1 or at the terminal on the Vout side. As described above, according to the third embodiment, the efficiency of the switching power supply using the active clamp method can be improved.

The switching power supply circuits 100, 300, and 500, which are the power supply devices described in the first to third embodiments, are switching that supplies power to, for example, a low-voltage power supply of an image forming apparatus, that is, a drive unit such as a controller (control unit) or a motor. It can be applied as a power supply. Hereinafter, as an example, a configuration of an image forming apparatus to which the switching power supply circuit 100 of the first embodiment is applied will be described.

[Configuration of image forming apparatus]
As an example of the image forming apparatus, a laser beam printer will be described as an example. FIG. 7 shows a schematic configuration of a laser beam printer, which is an example of an electrophotographic printer. The laser beam printer 700 includes a photosensitive drum 311 as an image carrier on which an electrostatic latent image is formed, a charging unit 317 (charging means) that uniformly charges the photosensitive drum 311, and an electrostatic latent image formed on the photosensitive drum 311. A developing unit 312 (developing means) for developing an image with toner is provided. Then, the toner image developed on the photosensitive drum 311 is transferred to a sheet (not shown) as a recording material supplied from the cassette 316 by a transfer unit 318 (transfer means), and the toner image transferred to the sheet is transferred to the fixing device 314. And discharge to tray 315. The photosensitive drum 311, the charging unit 317, the developing unit 312, and the transfer unit 318 are image forming units. Further, the laser beam printer 700 includes a switching power supply circuit 100. The image forming apparatus to which the switching power supply circuit 100 can be applied is not limited to the one illustrated in FIG. 7, and may be, for example, an image forming apparatus including a plurality of image forming portions. Further, the image forming apparatus may include a primary transfer unit that transfers the toner image on the photosensitive drum 311 to the intermediate transfer belt and a secondary transfer unit that transfers the toner image on the intermediate transfer belt to the sheet.

The laser beam printer 700 includes a controller 320 that controls an image forming operation by an image forming unit and a sheet conveying operation, and the switching power supply circuit 100 supplies electric power to, for example, the controller 320. Further, the switching power supply circuit 100 supplies electric power to a driving unit such as a motor for rotating the photosensitive drum 311 or for driving various rollers or the like that convey the sheet. As described above, according to the fourth embodiment, the efficiency of the switching power supply can be improved even in the image forming apparatus equipped with the switching power supply using the active clamp method.

101 Control unit 115 Feedback unit 200 Control unit C2 Capacitor for voltage clamp FET1 First switching element FET2 Second switching element FET12 Third switching element L12 Inductor T1 Transformer

Claims (9)

A transformer with primary and secondary windings,
A first switching element connected in series with the primary winding of the transformer,
A second switching element connected in parallel to the primary winding of the transformer,
A capacitor connected in series with the second switching element and connected in parallel with the primary winding of the transformer together with the second switching element.
A rectifying means for rectifying the voltage induced in the secondary winding and
A feedback means that outputs a feedback voltage to the primary side according to the voltage induced in the secondary winding of the transformer.
A first control means for controlling on or off of the first switching element and the second switching element based on the feedback voltage.
With
The first control means alternately turns on or off the first switching element and the second switching element with a dead time for turning off both the first switching element and the second switching element. A power supply that performs switching operations
The rectifying means is a switching of a third switching element for rectifying a voltage induced in the secondary winding, an inductor connected in series with the third switching element, and the third switching element. It has a second control means for controlling the operation,
A voltage detecting means for detecting a voltage across the third switching element and the inductor connected in series is provided.
The second control means keeps the third switching element on state for the first time after turning the third switching element from off to on, regardless of the voltage detected by the voltage detecting means. After the first time has elapsed, the third switching element is controlled to be turned from on to off according to the voltage detected by the voltage detecting means.
The first time is longer than the period from when the voltage detected by the voltage detecting means becomes negative due to the on of the second switching element until the detected voltage becomes the minimum value, and , A power supply device characterized in that it is shorter than the on-time of the second switching element. A transformer with primary and secondary windings,
A first switching element connected in series with the primary winding of the transformer,
A second switching element connected in parallel to the primary winding of the transformer,
A capacitor connected in series with the second switching element and connected in parallel with the primary winding of the transformer together with the second switching element.
A rectifying means for rectifying the voltage induced in the secondary winding and
A feedback means that outputs a feedback voltage to the primary side according to the voltage induced in the secondary winding of the transformer.
A first control means for controlling on or off of the first switching element and the second switching element based on the feedback voltage.
With
The first control means alternately turns on or off the first switching element and the second switching element with a dead time for turning off both the first switching element and the second switching element. A power supply that performs switching operations
The rectifying means is a switching of a third switching element for rectifying a voltage induced in the secondary winding, an inductor connected in series with the third switching element, and the third switching element. It has a second control means for controlling the operation,
A voltage detecting means for detecting a voltage across the third switching element and the inductor connected in series is provided.
The second control means keeps the third switching element off for a second time after turning the third switching element from on to off, regardless of the voltage detected by the voltage detecting means. After the second time has elapsed, the third switching element is controlled to turn from off to on according to the voltage detected by the voltage detecting means.
The second time is longer than the period from when the second switching element is turned off until the current flowing to the secondary side becomes zero, and shorter than the on time of the first switching element. A power supply that features. The second control means turns on the third switching element when the voltage detected by the voltage detecting means becomes equal to or less than the first threshold value, and the voltage detected by the voltage detecting means is higher than the first threshold value. The power supply device according to claim 1 or 2 , wherein the third switching element is turned off when the threshold value becomes higher than the second threshold value. The third switching element is a field effect transistor having a body diode.
The voltage detecting means is characterized in that when the third switching element is in the off state, the voltage drop of the body diode due to a forward current flowing through the body diode is detected by the voltage detecting means. The power supply device according to claim 3. In the voltage detecting means, when the third switching element is in the ON state, the first control means causes the second switching element to transition from on to off to reduce the current flowing through the inductor. The power supply device according to claim 3 or 4 , wherein the voltage generated in the inductor is detected. The third switching element is a field effect transistor and
Equipped with a magnetic material with holes
The power supply device according to any one of claims 1 to 5 , wherein the inductor is formed by passing a drain terminal of the field effect transistor through the hole. The third switching element is a field effect transistor and
A substrate on which the field effect transistor is mounted is provided.
The drain terminal of the field effect transistor is connected to the transformer via a pattern of a conductive layer on the substrate.
The power supply device according to any one of claims 1 to 6 , wherein the inductor is formed by a pattern of the drain terminal of the field effect transistor and the conductive layer. A snubber circuit in which a resistor and a capacitor are connected in series, including the third switching element connected in series and a snubber circuit connected in parallel with the inductor.
The power supply device according to any one of claims 1 to 7 , wherein the voltage detecting means detects a voltage smoothed by the snubber circuit. An image forming means for forming an image on a recording material,
The power supply device according to any one of claims 1 to 8.
An image forming apparatus comprising.

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