JP2010178533A - Power supply device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a power supply device that further improves the efficiency of power supply in operation with a light load. <P>SOLUTION: In the power supply device, an AC-DC converter includes: a first switching element 107; and a first control unit which controls the first switching element according to the output of the AC-DC converter and then controls the output of the AC-DC converter to a first voltage during the normal time, while a DC-DC converter includes: a second switching element 142; a second control unit which controls the second switching element according to the output of the DC-DC converter and then controls the output of the DC-DC converter to a second voltage which is lower than the first voltage during the normal time;and a third control unit by which, when a signal indicating low power consumption is input, the ON/OFF operation of the first switching element is brought into an intermittent operation state to control the output of the DC-DC converter to a third voltage which is lower than the second voltage. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a power supply apparatus suitable for an image forming apparatus such as a laser printer, and more particularly to improvement in efficiency at the time of low power consumption (corresponding to an energy saving mode of the image forming apparatus).

  FIG. 14 shows a basic circuit diagram of a conventional self-excited flyback power supply. Hereinafter, the configuration and operation of a conventional self-excited flyback power supply will be described with reference to FIG.

A DC input voltage is generated by a filter circuit 701, a rectifier circuit 702, and a smoothing capacitor 703 connected to the commercial AC 700. A first primary winding Np of a transformer 704 to which the generated DC input voltage is applied and a switching element 707 are connected in series. A starting resistor 705 is connected between the positive terminal of the DC input and the gate terminal of the switching element 707. When the voltage supplied from the DC input to the gate terminal of the switching element 707 through the starting resistor 705 rises, a drain current flows and a current flows through the primary winding Np.
As a result, the transformer 704 is excited and a voltage is induced in the auxiliary winding Nb which is the other primary winding. As a result, the gate voltage of the switching element 707 rises, the drain current of the switching element 707 further increases, and positive feedback is performed such that the gate voltage of the switching element 707 rises to the voltage of the auxiliary winding Nb. The gate voltage of the switching element 707 is supplied by the divided voltage of the resistor 711 and the resistor 708. On the other hand, the output of the auxiliary winding Nb is also supplied to an integrating circuit composed of a resistor 717 and a capacitor 718, and is connected so that the voltage across the capacitor 718 becomes the base voltage of the transistor 710. When the voltage across the capacitor 718 rises and the transistor 710 is turned on, the gate voltage of the switching element 707 is lowered via the resistor 709 to turn off the switching element 707.

  When the switching element 707 is turned off, back electromotive force is generated in each winding of the transformer 704, and current flows out from the secondary winding Ns through the secondary rectifier diode 720. This half-wave component current is smoothed by the capacitor 721 and becomes a DC output. When the energy stored in the transformer 704 becomes zero, the current also becomes zero. However, since the voltage is generated in the auxiliary winding Nb of the transformer 704 due to ringing due to the residual energy of the transformer, the switching element 707 becomes conductive again. Then, a current flows through the primary winding Np of the transformer 704, the voltage of the auxiliary winding Nb of the transformer 704 rises, and a voltage is applied to the gate terminal of the switching element 707, and at the same time, current supply to the capacitor 718 is started. .

  Thereafter, by repeating a series of operations as described above, the switching element 707 is repeatedly turned on and off. When the output voltage rises to the reference voltage of the shunt regulator 723 while repeating the above operation, the shunt regulator 723 is operated by the divided voltage of the resistors 724 and 725, and the photocoupler 714 is connected via the resistor 722. Current flows through As a result, the LED 714-b of the photocoupler 714 is turned on, and the impedance of the phototransistor 714-a in the photocoupler 714 decreases, so that the transistor 710 is turned on. Then, the switching element 707 is configured to output a constant voltage to the output terminal indicated by the arrow by a feedback operation in which the capacitor 718 is charged and turned off in a time shorter than the time in which the transistor 710 is turned on.

  This constant voltage is supplied into the apparatus as a power source for a motor or solenoid in a laser printer, or a power source for a high voltage power source or a drive system that requires a high DC voltage (for example, + 24V).

  On the other hand, the power supply voltage (for example, + 3.4V) to the control system such as the CPU and the ASIC is obtained by further reducing the drive system power supply voltage with a DC-DC converter.

  The operation of the DC-DC converter is shown below.

  In FIG. 14, 727 is a switching element, 728 is an inductor, 729 is a diode, 730 is an electrolytic capacitor, 732 is a comparator, and 738 is a Zener diode.

  The comparator 732 is supplied with the drive system power supply voltage. The comparator 732 compares the divided voltage of the detection resistors 735 and 739 and the voltage of the Zener diode 738. When the voltage of the resistor 739 is lower than the voltage of the Zener diode 738, the output of the comparator 732 is Lo. As a result, a voltage obtained by dividing the drive system power supply voltage by the resistor 726 and the resistor 731 is applied to the gate terminal of the switching element 727, and the switching element 727 is turned on. When the switching element 727 is turned on, current is supplied to the capacitor 730 via the inductor 728, and the voltage across the capacitor 730 increases. When the voltage rises, the voltage of the resistor 739 rises and becomes higher than the voltage of the Zener diode 738. Then, the comparator 732 becomes Hi (OFF) and the switching element 727 turns off. Since the inductor 728 tries to continue the current flow, the current flows in the closed loop including the diode 729, the inductor 728, and the capacitor 730, and the current stored in the inductor 728 is regenerated.

  In order to keep the comparator 732 OFF until the regeneration is completed, the diode is connected between the input terminal of the comparator 732 connected to the resistor 739 and the cathode terminal of the diode 729 so that the cathode terminal side of the diode 729 becomes the cathode. It may be inserted.

  However, in the above-described conventional power supply circuit, when the image forming apparatus is in a light load operation such as in an energy saving mode, the supply of the drive system power supply voltage is stopped by a load switch or the like, so that the load of the drive system voltage is extremely reduced. As a result, the time for turning on the switching element of the AC-DC converter (ON time) is shortened. When this happens, the switching frequency increases and the power loss associated with switching increases, so the efficiency decreases.

Various proposals have been made in the past to reduce the switching loss. Patent Documents 1 to 3 propose proposals for reducing the switching loss by extending the OFF time of the AC-DC converter by reducing the output of the AC-DC converter. Further, when such an intermittent operation is performed, an oscillation sound is generated from the switching transformer, and therefore, it is proposed in Patent Document 4 to perform the cycle of the intermittent operation outside the audible range. FIG. 15 is an example illustrating a voltage transition state of each unit when the output of the AC-DC converter is decreased.
Japanese Patent No. 29879992 Japanese Patent No. 3386016 JP-A-7-322610 Japanese Patent No. 3037050

  However, not only is the efficiency of the AC-DC converter reduced but also the efficiency of the DC-DC converter is a major problem during light load operation such as in an energy saving mode in a laser printer or a copier. That is, even in the DC-DC converter unit that generates the control system voltage from the drive system voltage, the current supply to each unit is interrupted and the circuit operation is stopped in the energy saving mode, so that the light load operation is performed. As described above, the efficiency of the DC-DC converter section also deteriorates during light load operation.

  Since the total efficiency as a power source is the product of the efficiency of the AC-DC converter and the efficiency of the DC-DC converter, there is a problem that the efficiency becomes about 20 to 30%. Further, since the drive voltage of the switching element of the subsequent DC-DC converter is lowered only by reducing the voltage of the AC-DC converter, there is a problem that the switching element of the DC-DC converter cannot be driven as it is.

  The present invention has been made under such circumstances, and an object of the present invention is to provide a power supply device capable of further improving the power supply efficiency at light load.

  In order to solve the above problems, in the present invention, the power supply device is configured as described in (1) below.

(1) A power supply device including an AC-DC converter and a DC-DC converter that receives the output of the AC-DC converter,
The AC-DC converter controls a first switching element that controls the output of the AC-DC converter by ON / OFF, and controls ON / OFF of the first switching element according to the output of the AC-DC converter. And a first control unit that controls the output of the AC-DC converter to the first voltage at normal time,
The DC-DC converter controls a second switching element that controls the output of the DC-DC converter by ON / OFF, and controls ON / OFF of the second switching element according to the output of the DC-DC converter. A second control unit that controls the output of the DC-DC converter to a second voltage lower than the first voltage at a normal time, a signal input unit that inputs a signal instructing low power consumption, and the signal When a signal instructing the low power consumption is input to the input unit, the ON / OFF operation of the first switching element of the AC-DC converter is set to an intermittent operation state according to the output of the DC-DC converter, And a third control unit that controls the output of the DC-DC converter to a third voltage lower than the second voltage.

  ADVANTAGE OF THE INVENTION According to this invention, the power supply device which can improve the power supply efficiency at the time of light load can be provided.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to an embodiment of a power supply device.

A “power supply device” that is Embodiment 1 will be described. FIG. 1 is a circuit diagram showing the configuration of this embodiment. The present embodiment is an example of a combination of an RCC AC-DC converter and a DC-DC converter. Specifically, the AC-DC converter outputs a first voltage in a normal state by ON / OFF control of the switching element, and the DC-DC converter in a normal state by ON / OFF control of the switching element. Outputs a second voltage lower than the first voltage. Furthermore, the DC-DC converter sets the ON / OFF operation of the switching element of the AC-DC converter to an intermittent operation state when a signal instructing low power consumption is input. This is an example of a power supply device that controls the output of the DC-DC converter to a third voltage lower than the second voltage.
This embodiment assumes a power supply device for a laser printer.

In FIG. 1, 100 is a commercial AC power source, 101 is a filter circuit, 102 is a diode bridge, 103 is a capacitor, and 104 is a switching transformer. Np is a primary winding of the transformer, Ns is a secondary winding of the transformer, and Nb is a bias winding (feedback winding) of the transformer. Reference numeral 105 denotes a starting resistor, 107 denotes a switching element, 108, 109, 111, 113, 116, and 117 denote resistors, 110 denotes a transistor, and 112 and 118 denote capacitors. 114-a is the phototransistor side of the photocoupler 114, and 114-b is the LED side of the photocoupler 114. Reference numerals 115 and 119 denote diodes. 121 is a secondary rectifier diode, 122 is an electrolytic capacitor, 123 to 126 are resistors, 128 is a Zener diode, 130 is a comparator, 131 is a resistor, and 132 is a load switch that performs output / output stop to a load.
Reference numeral 170 denotes an output terminal via the load switch 132, 171 denotes a control terminal of the load switch 132, 172 denotes an output terminal not via the load switch 132, and 173 denotes a GND terminal.

  A circuit of the DC-DC converter that receives the output voltage of the AC-DC converter and generates a lower voltage will be described. The DC-DC converter input terminal connected to the 172 terminal not passing through the load switch is 175, and the GND terminal of the DC-DC converter is 176. Reference numerals 140, 141, 146, 147, 148, 152, 154, and 157 are resistors, 142 is a switching element of a DC-DC converter, and a PchMOSFET is generally used in general. Reference numerals 143 and 144 denote diodes, 149 denotes a Zener diode, 150 denotes a smoothing capacitor, 151 and 156 denote comparators, 153 denotes a transistor, and 155-b denotes the LED side of the photocoupler 155. Further, 152 is a control terminal for turning on / off the transistor 153, 177 is an output terminal for a low voltage (for example, 3.4 V) generated by the DC-DC converter, and 178 is a GND terminal.

  When AC power is applied to the diode bridge 102 from the commercial AC power supply 100 via the filter circuit 101, both-wave rectification is performed by the diode bridge 102, and peak charging is performed by the capacitor 103. As a result, a DC voltage is generated across the capacitor 103.

  The operation of this embodiment will be described with reference to FIGS. 5 and 6 showing the voltage change of each part of the power supply apparatus. First, the operation in the case of normal time at startup (in the printer print mode) will be described. Normally, since the Hi signal is input to the terminal 174 or is in a high impedance state, the transistor 153 is turned OFF, and the LED 155-b of the photocoupler 155 does not emit light. The feedback operation in the AC-DC converter circuit is performed. That is, the Zener diode 128 is in an operating state.

  When a voltage is applied between the gate and source of the switching element 107 due to voltage division of the resistance 108 between the starting resistor 105 and the gate source, the switching element 107 is turned on, and a current starts to flow through the primary winding Np of the transformer 104. With this current, a voltage is generated in the auxiliary winding Nb in a direction to further increase the gate voltage, and a voltage is applied to increase the gate voltage of the switching element 107 via the resistor 111 while charging the capacitor 112. At the same time, the capacitor 118 is charged through the resistor 117. When charging of the capacitor 118 is started and the voltage across the capacitor 118 becomes equal to or higher than Vbe of the transistor 110, a base current flows and the transistor 110 is turned on. When the transistor 110 is turned on, the switching element 107 is turned off, and the drain-source voltage of the switching element 107 starts to rise. As a result, the voltage of the auxiliary winding Nb drops and starts to flow current in the reverse direction. A current flows in the secondary winding Ns in the direction in which the diode 121 is conducted, and the capacitor 122 is charged when the voltage becomes equal to or higher than the sum of the voltage of the capacitor 122 and the forward voltage of the diode 121. At the same time, the voltage appearing in the auxiliary winding Nb turns on the diode 119, passes a current in a direction to flow back through the resistor 116 and the diode 115, and discharges the capacitor 118. Since this causes the transistor 110 to be turned off, the gate voltage of the switching element 107 is then applied to the current supplied by the starting resistor 105 and the auxiliary winding Nb supplied via the resistor 111 and the capacitor 112. It depends on the current that flows.

  For the purpose of speeding up the switching element 107 OFF, as shown in FIG. 2, the resistor 111 may be divided, and a diode 141 may be connected in parallel to one of the divided resistors 140 in the direction of turning off the switching element 107. By making it as shown in FIG. 2, the gate voltage of the switching element 107 can be lowered more quickly and reliably.

  Since the energy stored in the transformer 104 moves to the capacitor 122 while the switching element 107 is OFF, the voltage of the winding Ns decreases with time. In this case, the output voltage on the auxiliary winding Nb side becomes small, so that the gate voltage of the switching element 107 rises due to the current flowing from the starting resistor 105. When the gate voltage of the switching element 107 becomes higher than the threshold value for turning on, the switching element 107 is turned on, and a current flows in the winding Np from the positive terminal of the capacitor 103 to the negative terminal of the capacitor 103 through the switching element 107. Flows. Since a current flows through the winding Nb in the direction of the capacitor 112, the resistor 111, the resistor 108, and the Nb winding from the Nb winding, the gate voltage of the switching element 107 further increases. Then, as described above, the capacitor 118 is charged by the voltage of the winding Nb and the resistor 117, and the switching element 107 is turned off by turning on the transistor 110. The energy stored in the transformer 104 during the ON period of the switching element 107 is stored in the capacitor 122 during the OFF period of the switching element 107, and the voltage across the capacitor 122 increases.

  As the voltage across the capacitor 122 rises, the voltage of the winding Ns during the OFF period of the switching element 107, the reverse voltage of the Nb winding, and the ringing voltage after discharging the transformer energy to the capacitor 122 increase. It will become. For this reason, in addition to the voltage increase due to the inflow current from the starting resistor 105, the gate voltage of the switching element 107 increases due to the increase of the ringing voltage, and the switching element 107 is turned on. In this case, the operation is performed in a stable OFF time, and the voltage of the capacitor 122 increases rapidly.

  As the voltage of the capacitor 122 rises, the comparator 130 operates by comparing the divided voltage of the resistors 124 and 125 with the reference voltage generated by the Zener diode 128, and the output of the comparator 130 is inverted to Lo. Then, the LED 114-b of the photocoupler 114 emits light. The phototransistor 114-a of the photocoupler 114 is connected to the resistor 113 and the base of the transistor 110. When the LED 114-b of the photocoupler 114 emits light, the phototransistor 114-a becomes conductive, so that the transistor 110 is turned on and switching is performed. The element 107 is turned off.

  On the contrary, when the voltage of the capacitor 122 becomes low, the comparator 130 is not inverted and the switching element is not turned off. Therefore, the switching element 107 is turned on until the maximum ON time determined by the capacitor 112 and the resistor 111. In this way, the voltage across the capacitor 122 is controlled so that the voltage across the resistor 125 is the same as the voltage across the Zener diode 128 (see 172 terminal voltage, 107 GS voltage in FIG. 5).

  In the energy saving mode of the laser printer, a signal instructing low power consumption is output from the control system of the laser printer to the power supply apparatus. A mechanism by which the control system outputs a signal instructing low power consumption will be described later.

When a signal indicating low power consumption is input to the terminal 174 and the terminal 174 is set to Lo, the transistor 153 is turned on. The divided voltage of the resistor 146 and the resistors 147 and 157 is compared with the voltage across the Zener diode 149 biased by the resistor 148. Lo. Accordingly, the LED 155-b of the photocoupler 155 is turned on and the transistor 155-a of the photocoupler 155 is turned on, so that the transistor 110 is turned on.
The zener voltage of the zener diode 149 is selected to be sufficiently lower than the zener voltage of the zener diode 128.

As a result, the switching element 107 of the AC-DC converter is turned OFF, and the voltage at the terminal 172 drops (see the 172 terminal voltage in FIG. 5). The DC-DC converter described later continues to operate during this period (see 142 terminal voltage in FIG. 5), and stably outputs a low voltage (for example, 3.4 V, the second voltage) (see FIG. 5). C150 Refer to both-end voltage).
When the voltage of the terminal 172 becomes sufficiently low and the output voltage of the subsequent DC-DC converter decreases and the value divided by the resistors 146 and 147 and 157 becomes lower than the voltage of the Zener diode 149, the comparator 156 Hi is output (OFF). As a result, the LED 114-b of the photocoupler 114 is turned off. As a result, the transistor 110 is turned off and the switching element 107 is turned on (refer to the voltage between 107 GS in FIG. 5).

  Although an example in which a Zener diode is used to provide a reference voltage has been introduced, in reality, a Zener diode that generates a low voltage generally has poor Zener voltage variation, Zener voltage dependency due to current, and temperature characteristics. For this reason, a Zener diode having a high Zener voltage may be used to divide by a resistor, or a shunt regulator may be used as a reference voltage.

  The operation of the DC-DC converter will be described below. 142 is a main switching element of the DC-DC converter, 145 is an inductor, 143 is a regenerative diode, 150 is a capacitor, 146 to 148 are resistors, 149 is a Zener diode, 157 is a resistor, and 151 and 156 are comparators.

  First, a normal description will be given. The comparator 151 compares the voltage of the Zener diode 149 with the voltage obtained by dividing the output voltage by the resistors 146 and 147 and the resistor 157. When the voltage of the resistor 1157 becomes lower than the Zener voltage of the Zener diode 149, the comparator 151 outputs Lo (ON), so that the gate voltage of the switch element (P-type FET) 142 is not supplied and the switching element 142 is turned ON. . For this reason, a current flows through the inductor 145, the capacitor 150 is charged, and the voltage of the capacitor 150 rises.

Next, when the voltage of the capacitor 150 rises and the voltage of the resistor 157 becomes higher than the voltage of the Zener diode 149, the comparator 156 outputs Hi (OFF), so that the gate voltage of the switching element 142 rises and the switching element 142 It becomes OFF. Since the inductor 145 tries to continue to pass current, the regenerative diode 143 is turned on, and the energy stored in the inductor 145 is charged in the capacitor 150. At this time, the diode 144 is turned on and the negative input terminal of the comparator 151 is lowered, so that the comparator 151 does not perform an inverting operation and secures an OFF time until the discharge of energy from the inductor 145 is completed.
By repeating the series of operations as described above, the DC-DC converter continues to oscillate (see 142 GS voltage in FIG. 5).

  When the laser printer shifts to the energy saving mode, a signal instructing low power consumption is input to the terminal 174, and the terminal 174 is set to Lo. The input to the terminal 174 is Hi or high impedance in the normal mode, and the transistor 153 is turned off because it is pulled up by the resistor 152. When the terminal 174 becomes Lo, the transistor 153 is turned on, the output of the comparator 156 is Lo, and the LED 155-b of the photocoupler 155 emits light when the voltage of the capacitor 150 is higher than 3.2V. When the LED 155-b emits light, the phototransistor 155-a of the photocoupler 155 becomes conductive, so that the transistor 110 is turned on and the primary side switching element 107 is turned off. At this time, the switching element 142 of the DC-DC converter is also stopped (OFF). Since no current is supplied to the capacitor 150 and the load current continues to flow, the voltage of the capacitor 150 decreases. The regenerative current of the inductor 145 ends and the current from the diode 144 disappears, the Zener voltage of the Zener diode 49 is restored, and the voltages of the resistors 147 and 157 that detect the output voltage become lower than the voltage of the Zener diode 149. Then, the comparator 156 outputs Hi again (OFF), and when the switching element 142 of the DC-DC converter is turned on and the LED 155-b of the photocoupler 155 is turned off, the switching element 107 of the AC-DC converter starts oscillation. . Then, the AC-DC converter starts to charge the capacitor 122, and the DC-DC converter starts to charge the capacitor 150 (see FIG. 5B and FIG. 6).

  Further, a high voltage transistor is connected in series with the start circuit (start resistance), and a photocoupler is connected between the base and emitter of the transistor so that the current consumed by the start resistance is stopped during the idle period of the switching element 107. You may do it. Such a connection example (a modification of the first embodiment) is shown in FIG. In FIG. 8, 106-a is a phototransistor of the photocoupler 106, and 106-b is an LED of the photocoupler 106. When the energy saving mode is selected by the laser printer, the Lo signal instructing low power consumption is input to the terminal 174, and the comparator 156 outputs Lo (ON) with the transistor 153 ON, the LED 155-b of the photocoupler 155 At the same time, the LED 106-b of the photocoupler 106 emits light, and the phototransistor 106-a of the photocoupler 106 is turned on. For this reason, since the transistor 158 connected in series to the starting resistor 105 is turned off and the current flowing through the starting resistor 105 is stopped, the power consumption in the power supply can be further reduced.

  FIG. 3 is a connection diagram of a laser printer control system and signal lines connected to the terminal 174. In FIG. 3, reference numerals 188 and 189 denote output transistors of the control circuit, and 190 denotes a CPU or ASIC. Reference numerals 185 to 187, 131, and 133 are resistors. The load switch 132 using the FET is turned on and off by the signal of the resistor 133 line, and the output of the drive system (+24 V) is turned on and off. 3, the output transistors 188 and 189 are in low impedance when the CPU 190 output is Hi, and the output transistors 188 and 189 are in high impedance state when the CPU 190 output is Lo.

  The operation when the laser printer enters the energy saving mode will be described with reference to the flowchart of FIG. When entering the energy saving mode, the laser printer first stops the load of the drive system in step 201 (abbreviated as S201 in the figure, the same applies hereinafter) so that the connection to the drive system load may be cut off. As for the signal from the control circuit, the output transistor 188 is turned off in step 202 to stop the supply of the first voltage, and the terminal output from the resistor 133 is set to the Hi or high impedance state. In this case, the FET of the load switch 132 is turned off, so that the first voltage is not output from the terminal 170 to the outside of the power supply device. Thereafter, at step 203, the logic section where the current load is large is cut off and stopped so that a light load operation is possible. In step 204, the transistor 189 is turned on, the terminal 184 becomes Lo, and a signal instructing low power consumption is input from the terminal 174 to the DC-DC converter. Then, the transistor 153 is turned on and a current flows through the LED 155-b of the photocoupler 155, and the operation of the comparator 156 of the DC-DC converter becomes effective.

  The comparator 156 is configured to compare the reference voltage of the Zener diode 149 with the voltages of the resistors 147 and 157, and outputs Lo (ON) when the voltages of the resistors 147 and 157 are higher than the reference voltage of the Zener diode 149. . Then, a current is supplied to the LED 155-b of the photocoupler 155 to cause the LED to emit light. The phototransistor 155-a on the light receiving side of the photocoupler 155 is connected so that the bias of the transistor 110 can be turned on and off. The transistor 153 is turned off, and the LED 155-b is turned off to turn off the transistor 110.

As described above, the comparator 156 compares the output voltage of the DC-DC converter with the reference voltage, and turns the first switching element 107 ON and OFF, whereby the third voltage is output as the output of the DC-DC converter. Control to get. At this time, the comparator 151 of the DC-DC converter always turns on the second switching element 142 because the detection voltage (voltage of 157) is always equal to or lower than the target voltage (voltage of the Zener diode 149). Operates to continue. Further, the feedback circuit for obtaining the first voltage (normal time) in the AC-DC converter has a resistance of the resistor 123 in order to pass an appropriate current to the LED 114-b of the photocoupler 114 at the first voltage. The value is taken large. For this reason, when the third voltage is reached, the current becomes insufficient, so that the LED 114-b cannot emit light, and the switching element 107 can operate with a long ON time. For this reason, the switching cycle can be set long, and the operation pause in the burst state with a long cycle according to the increase / decrease of 3.3V is entered, so that the pause for each switching can be taken long and the power supply efficiency is drastically increased. To rise.
FIG. 7 shows a graph of the efficiency of the power supply device according to this embodiment and the efficiency of the conventional power supply device.

  Next, returning from the energy saving mode will be described with reference to the flowchart of FIG. In step 205, the transistor 189 is turned off to stop the ON / OFF control of the starting resistor 105, and the output voltage of the AC-DC converter starts to increase toward the first voltage (+ 24V) which is the target voltage. After waiting for several ms until the output voltage of the AC-DC converter reaches the target voltage in step 206, when the output transistor 188 is turned on in step 208, the load switch 132 is turned on and the voltage is supplied from the terminal 170 to the drive system. If there is no element that malfunctions when a halfway voltage is applied to the load of the driving system, the first power supply voltage may not be turned on / off without the load switch.

  As described above, according to the present embodiment, the AC-DC converter is intermittently operated by the output voltage of the DC-DC converter, and in the modification of the present embodiment, the starting resistance is further turned ON / OFF, thereby -It is possible to avoid a decrease in efficiency during light load operation of the DC converter and the DC-DC converter. Further, by configuring in this way, the number of ON / OFF times of the switching element of the AC-DC converter is reduced, and the ON time is set to the longest ON time allowed as a power source, as represented by the start-up. It becomes possible.

  The circuit of the comparator 130 corresponds to the first control unit in the claims, the circuit of the comparator 151 corresponds to the second control unit in the claims, and the circuit of the comparator 156 in the claims. This corresponds to the third control unit. Further, the terminal 152 corresponds to a signal input unit referred to in the claims, and the transformer 142 corresponds to a transformer in the AC-DC converter referred to in the claims.

  A “power supply device” that is Embodiment 2 will be described. The present embodiment is an example in which a voltage supply unit that supplies a voltage to a driving circuit of a switching element of a DC-DC converter in a power supply device is provided particularly in the AC-DC converter. Here, the description overlapping with that of the first embodiment is omitted, and only a portion specific to the present embodiment will be described.

FIG. 9 is a circuit diagram showing the configuration of this embodiment. In FIG. 9, Ns2 is a sub power supply winding. Reference numeral 166 denotes a diode, and 167 denotes a capacitor. Many FETs used as switching elements of a DC-DC converter are Pch, a drain-source voltage of 30 V to 60 V, and a gate voltage of 4.5 V drive type. In this embodiment, since the output voltage from the AC-DC converter decreases from about 24 V to about 3.2 V, it is necessary to use a low gate threshold voltage FET such as a 2.5 V drive in the conventional configuration. In particular, it is difficult to produce a high drain-source voltage with a low gate threshold. Moreover, in order to lower the gate threshold, it is necessary to make the gate insulating layer thin, and the gate capacitance increases. However, when the gate capacity increases, there is a problem that the switching loss of the DC-DC converter increases.
In this embodiment, in order to apply a 4.5V drive type FET that is generally used, a sub power supply winding is provided and the rectified voltage is used.

  When the sub power supply winding is provided as described above, the sub power supply winding is wound so as to obtain a higher voltage as in the power supply device of FIG. Nch MOSFET can be used.

  A “power supply device” that is Embodiment 3 will be described. The present embodiment is an example in which a bootstrap circuit is used in a voltage supply unit that supplies a voltage to a drive circuit of a switching element of a DC-DC converter in a power supply device. Also here, the description overlapping with the first and second embodiments is omitted, and only the parts specific to the present embodiment will be described.

  FIG. 11 is a circuit diagram showing a configuration of the third embodiment. In FIG. 11, 1101 is a capacitor, 1102 is a diode bridge, 1103, 1105 to 1107 are resistors, 1103 is an NPN transistor, 1108 is a diode, and 1109 is a PNP transistor.

  When the transistor 1109 is turned on to turn off the FET 142-2 as a switching element, the gate voltage of the FET 142-2 rises to the voltage V1 at the terminal 175. At this time, current flows through the capacitor 1101 and the diode 1102, and the capacitor 1101 is charged with a voltage obtained by subtracting the voltage drop of the diode 1102 from the voltage at the terminal 175.

  Next, when the transistor 1109 is turned off to turn on the FET 142-2, the gate voltage of the FET 142-2 starts to drop toward the GND voltage of the terminal 176. At the same time, the collector side of the transistor 1109, which is one terminal of the capacitor 1101, is also at the GND potential, so that the diode 1102 terminal side, which is the other terminal of the capacitor 1101, is lowered by the voltage charged in the capacitor 1101, -Potential (-V1). At this time, since the transistor 1103 is turned on, the gate voltage of the FET 142-2 drops to −V1, so that the gate-source voltage of the FET 142-2 becomes approximately −2V1, and V1 <Vth (Vth is the gate threshold voltage of the FET 142-2). ) Can be driven.

  The FET drive circuit of this embodiment is merely an example, and the bootstrap circuit composed of the combination of the capacitor and the diode may be realized by other connections having the same effect.

  Even if the bootstrap circuit is operated when the voltage V1 of the terminal 175 is high, useless power is consumed and the withstand voltage of the FET 142-2 may be exceeded. In such a case, the bootstrap circuit may be configured to operate only when the voltage at the terminal 175 drops below a predetermined value. FIG. 12 shows such a circuit as a modification of the third embodiment.

  In FIG. 12, 1201 to 1203, 1205, 1206, 1209, 1210, 1212, 1214, 1215, 1218, 1220, 1221, 1223, 1224, 1226, 1228, and 1229 are resistors, and 1204 and 1225 are Zener diodes. 1208 and 1211 are diodes, 1207, 1216, 1217, 1222 and 1227 are transistors, and 1213 and 1219 are capacitors.

  When the input voltage from the terminal 175 is sufficiently higher than the Zener voltage of the Zener diode 1225 and the base-emitter voltage of the transistor 1222, the transistor 1222 is turned on, so that the transistor 1217 is turned off. For this reason, when the transistor 1216 is turned on to turn off the FET 142-2, the FET 142-2 is turned off, but the transistor 1217 is turned off, so that no current flows through the bootstrap circuit including the capacitor 1213 and the diode 1211. The capacitor 1213 is not charged.

  When the transistor 1216 is turned off to turn on the FET 142-2, the gate voltage of the FET 142-2 is discharged by the resistor 1205 and the resistor 1214. The Zener diode 1204 is inserted to protect the gate overvoltage of the FET 142-2, and is not necessary when the voltage of the resistor 1203 and the resistors 1205 and 1214 can be equal to or lower than the gate withstand voltage of the FET 142-2.

  Next, the operation when the low power consumption is instructed (energy saving mode) and the voltage at the terminal 175 becomes low will be described. When the voltage of the terminal 175 becomes lower than the voltage of the Zener diode 1225, the transistor 1222 is turned off and the transistor 1217 is turned on. When the transistor 1216 is turned on to turn off the FET 142-2, a current flows to the GND through the capacitor 1213-resistor 1212-diode 1211-transistor 1217, and the capacitor 1213 is charged with the power supply voltage, that is, the voltage at the terminal 175. Next, when the transistor 1216 is turned off to turn on the FET 142-2, the voltage at one terminal of the capacitor 1213 connected to the collector side of the transistor 1216 drops to the GND voltage (terminal 176 voltage). Therefore, the other terminal of the capacitor 1213, that is, the emitter side of the transistor 1207 becomes a negative potential due to the voltage stored in the capacitor 1213. Therefore, the transistor 1207 is turned on, and the gate voltage of the FET 142-2 is pulled to the potential of the other terminal of the capacitor 1213, that is, the emitter terminal side of the transistor 1207 through the resistor 1205 and the transistor 1207. As a result, a gate-source voltage equal to or higher than the voltage at the terminal 175 can be applied to the FET 142-2.

FIG. 13 shows the gate voltage of the FET 142-2 in each state in the circuit of FIG.
Here, the gate terminal voltage of the FET 142-2 and the collector terminal voltage of the transistor 1207 when the voltage of the terminal 175 is changed to 24V, 12V, and 6V are shown. The Zener voltage of the Zener diode 1225 is 15V, and the Zener voltage of the Zener diode 1204 for gate protection is also 15V.

First, when 24V is input to the terminal 175, the gate voltage is 15V or less due to the Zener diode 1204 for gate protection. In addition, the collector terminal voltage of the transistor 1207 is. The voltage is divided by the voltages of the resistors 1214 and 1206 and the voltages of the resistor 1205 and the gate protection Zener diode 1204.
Next, when the voltage at the terminal 175 reaches 16 V, the gate voltage becomes lower than the Zener voltage of the Zener diode 1204 for gate protection, so that the divided voltage of the resistor 1203 and the resistors 1205, 1206, and 1214 appears as the gate voltage as it is. Come.
It can be seen that when the voltage at the terminal 175 further decreases to 6V, the bootstrap circuit operates to increase the amplitude of about 4.5V to 9V.

  As described above, according to the present embodiment, when the input voltage decreases and becomes lower than the gate threshold voltage of the FET, or approaches the gate threshold voltage, and the desired ON resistance cannot be obtained in the FET, bootstrap is performed. By using the circuit to set the gate voltage to the GND potential or less, it is possible to use even an FET having a high gate threshold voltage. In addition, since the gate voltage becomes too large when the output voltage of the AC-DC converter is high simply by providing a bootstrap circuit, in the modification of this embodiment, the output voltage is lower than a predetermined voltage. Only the bootstrap circuit is operated.

  In this embodiment, an example using a Pch MOSFET is shown. However, it is possible to drive with an Nch MOSFET by adopting the same concept. Many NchMOSFETs have a low gate threshold voltage, and the chip size can be reduced to obtain the same ON resistance, which is advantageous in terms of cost.

  In the bootstrap circuit, it is necessary to limit the discharging time while securing the charging time. In particular, when the laser printer transitions from the normal mode to the energy saving mode, the input voltage of the DC-DC converter is lowered. Therefore, when no control is performed, the FET 142-2 which is a switching element of the DC-DC converter is used. The ON time becomes longer. If the ON time becomes too long, the capacitor 1213 is discharged, and the gate-source voltage of the FET 142-2 cannot be set to a desired voltage value. Therefore, the ON time is limited to control the voltage by the OFF time. Thus, the DC-DC converter is configured. Although not shown, there is a duty control that does not extend the ratio (duty) of the ON time and the OFF time to 100%. In addition to the ON time control method of simply limiting the ON time, the control may be performed by monitoring the voltage of the bootstrap circuit and controlling the ON time when the voltage is equal to or lower than a predetermined value.

  As described above, according to this embodiment, when the output voltage of the AC-DC converter is equal to or lower than a predetermined voltage, the bootstrap circuit sufficiently increases the drive voltage of the switching element of the DC-DC converter. Can be secured. Further, the operation of the bootstrap circuit can be ensured by the ON time limiting means for preventing the second switching element from being kept ON.

The circuit diagram which shows the structure of Example 1 Circuit diagram showing modification of gate drive circuit Circuit diagram showing the connection with the laser printer Flow chart showing operation when laser printer enters energy saving mode and returns to normal mode The figure which shows the voltage change of each part in Example 1. FIG. The figure which shows the voltage change of each part in Example 1. FIG. The figure which shows the efficiency of Example 1 and the efficiency of a prior art example The circuit diagram which shows the structure of a deformation | transformation of Example 1. FIG. Circuit diagram showing configuration of embodiment 2 Circuit diagram showing a modified configuration of the second embodiment Circuit diagram showing configuration of embodiment 3 Circuit diagram showing a modification of the bootstrap circuit The figure which shows the change of the gate voltage in the circuit of FIG. Circuit diagram showing configuration of conventional example The figure which shows the voltage change of each part in a prior art example

107 switching element 142 switching element 130 comparator 151 comparator 156 comparator

Claims (6)

A power supply device comprising: an AC-DC converter; and a DC-DC converter that receives the output of the AC-DC converter,
The AC-DC converter controls a first switching element that controls the output of the AC-DC converter by ON / OFF, and controls ON / OFF of the first switching element according to the output of the AC-DC converter. And a first control unit that controls the output of the AC-DC converter to the first voltage at normal time,
The DC-DC converter controls a second switching element that controls the output of the DC-DC converter by ON / OFF, and controls ON / OFF of the second switching element according to the output of the DC-DC converter. A second control unit that controls the output of the DC-DC converter to a second voltage lower than the first voltage at a normal time, a signal input unit that inputs a signal instructing low power consumption, and the signal When a signal instructing the low power consumption is input to the input unit, the ON / OFF operation of the first switching element of the AC-DC converter is set to an intermittent operation state according to the output of the DC-DC converter, And a third control unit for controlling the output of the DC-DC converter to a third voltage lower than the second voltage. The power supply device according to claim 1,
The AC-DC converter includes a voltage supply unit that supplies a voltage to a drive circuit of the second switching element. The power supply device according to claim 2,
The power supply apparatus, wherein the voltage supply unit uses a voltage of an auxiliary winding of a transformer in the AC-DC converter. The power supply device according to claim 1,
A power supply apparatus comprising a bootstrap circuit comprising a capacitor and a diode in a voltage supply section for supplying a voltage to a drive circuit for the second switching element. The power supply device according to claim 4,
In the DC-DC converter, in order to ensure the operation of the bootstrap circuit when a signal instructing the low power consumption is input to the signal input unit, the second switching element is kept on. A power supply device having an ON time limiting means for preventing The power supply device according to any one of claims 1 to 5,
A start resistor for starting the drive circuit of the first switching element; and a switch connected in series to the start resistor; and the third control unit also controls ON / OFF of the switch. Power supply.

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