Patent Application
20130064566
1. Field of the Invention
The present invention relates to a power supply apparatus and an image forming apparatus, and more specifically, to a technique for reducing voltage drops caused by impedance between output of the power supply apparatus and a power supply destination and reducing power consumption under light load.
2. Description of the Related Art
Conventionally, switching power supply systems such as an AC/DC converter and a DC/DC converter are known as types of power supply apparatus. Also, a control system such as described below is known as a method for controlling output voltage of a switching power supply system. For example, a voltage (hereinafter referred to as a comparison voltage) defined from the output voltage of a power supply apparatus based on a voltage ratio corresponding to a resistance ratio is compared with a reference voltage of a feedback circuit section, a differential amplifier signal generated by amplifying a potential difference between the comparison voltage and the reference voltage is fed back to the switching circuit section, and thereby the output voltage is controlled to be constant. For example, a switching power supply of a flyback type such as shown in
Techniques for correcting the voltage drops caused by the cable interconnecting the power supply apparatus and the power consumption section include, for example, a technique proposed in Japanese Patent Application Laid-Open No. H04-261358. On a secondary side of the power supply apparatus, the technique proposed in Japanese Patent Application Laid-Open No. H04-261358 develops a first output voltage connected to the power consumption section and a second output voltage not connected to the power consumption section. A specific method involves amplifying a potential difference between the first output voltage and the second output voltage and adding the potential difference to a reference voltage of a feedback circuit section, where the first output voltage causes a load current to flow and causes voltage drops while the second output voltage does not cause a load current to flow and does not cause any voltage drop.
However, since the technique proposed in Japanese Patent Application Laid-Open No. H04-261358 requires two output voltages, there are problems of complex configuration and increased cost. Also, with the conventional technique shown in
A purpose of the present invention is to enable reducing voltage drops caused by impedance on a path which interconnects a power supply apparatus and a power consumption section as well as reducing power consumption under light load.
Another purpose of the present invention is to provide a power supply apparatus comprising: a transformer; a switching section adapted to drive a primary side of the transformer; a feedback section adapted to compare a voltage obtained by dividing an output voltage output from a secondary side of the transformer between a first resistor and a second resistor with a reference voltage and feed back an output based on a comparison result, the first resistor and the second resistor being connected in series; a control section provided on the primary side of the transformer and adapted to control operation of the switching section based on the output from the feedback section; and a resistor adapted to separate a first ground and a second ground from each other on the secondary side of the transformer, the first ground being located on a load side supplied with the output voltage while the second ground being located closer to the transformer than is the first ground, wherein the second resistor is connected to the second ground and the reference voltage is connected to the first ground.
Still another object of the present invention is to provide an image forming apparatus comprising: an image forming section adapted to form an image; a control section adapted to control operation of the image forming section; and a power supply adapted to supply electric power to the control section, wherein the power supply comprises: a transformer; a switching section adapted to drive a primary side of the transformer; a feedback section adapted to compare a voltage obtained by dividing an output voltage output from a secondary side of the transformer between a first resistor and a second resistor with a reference voltage and feed back an output based on a comparison result, the first resistor and the second resistor being connected in series; a control section provided on the primary side of the transformer and adapted to control operation of the switching section based on the output from the feedback section; and a resistor adapted to separate a first ground and a second ground from each other on the secondary side of the transformer, the first ground being located on a load side supplied with the output voltage while the second ground being located closer to the transformer than is the first ground, wherein the second resistor is connected to the second ground and the reference voltage is connected to the first ground.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings.
A configuration and an operation of the present invention will be described below. However, note that the embodiments described below are only exemplary and not intended to limit the technical scope of the present invention. Now, embodiments of the present invention will be described in detail below with reference to the accompanying drawings.
Modes for carrying out the present invention will be described in detail below with reference to embodiments. Although a circuit configuration of a flyback type will be described in the embodiments, this is not intended to limit the application range of the present invention. Specifically, the present invention is also applicable to DC/DC converters, current resonant converters, forward converters, and the like.
A first embodiment will be described.
On the secondary side, the power supply apparatus is equipped with a diode 201 adapted to rectify a secondary-side output of the transformer 113, an electrolytic capacitor 202 adapted to store secondary-side power, and a coil 203 and an electrolytic capacitor 204 adapted to further rectify and smooth the voltage after passage through the diode 201. Also, on the secondary side, the power supply apparatus is equipped with an upper resistor 205 of a voltage divider and a lower resistor 206 of a voltage divider adapted to generate a comparison voltage from the output voltage, a regulator IC 207 adapted to provide a reference voltage (predetermined reference voltage) of a feedback circuit section and serve as a differential amplifier circuit, and a resistor 211 adapted to detect a secondary-side current. Also, commercial AC power is input to Vin_H and Vin_L and a voltage full-wave rectified through a rectifier diode bridge (not shown) is applied to charge a primary smoothing electrolytic capacitor 100 with a DC voltage.
Operation according to the present embodiment will be outlined below. However, most part of the operation is in common with operation of a circuit according to a conventional technique shown in
The feedback circuit section (feedback circuit) compares the comparison voltage (REFERENCE) generated by the upper resistor 205 of the voltage divider (first resistor) and the lower resistor 206 of the voltage divider (second resistor) with the reference voltage (REF) of the regulator IC 207, where the comparison voltage is proportional to the output voltage. The regulator IC 207 amplifies a potential difference (comparison result) between the compared voltages, drives a transistor in the regulator IC 207, and thereby causes a current to flow between a cathode and an anode (hereinafter referred to as “between CATHODE and ANODE”) of the regulator IC 207. That is, a current proportional to the potential difference between the comparison voltage and the reference voltage flows between CATHODE and ANODE of the regulator IC 207 from the output voltage by passing through a current limiting resistor 210 and the photo coupler 107. Besides, the feedback circuit section includes a phase compensation circuit made up of a resistor 208 and capacitor 209.
An operation of a primary-side circuit including the transformer 113 will be described. Basic operating waveforms of the primary-side circuit are shown in
The FB terminal voltage of the power supply IC 101 changes with an FB terminal current released from the power supply IC 101 as well as with operation of a secondary-side feedback circuit and operation of the photo coupler 107. When the output voltage of the power supply apparatus falls, a current Ic flowing through a transistor section of the photo coupler 107 decreases and the FB terminal voltage rises. Conversely, when the output voltage of the power supply apparatus rises, the current Ic flowing through the transistor section of the photo coupler 107 increases and the FB terminal voltage falls. Therefore, when the switching FET 102 is turned off, releasing the energy accumulated in the core from the secondary winding of the transformer 113, the output voltage rises and the FB terminal voltage of the power supply IC 101 falls accordingly.
Unlike a turn ratio between the primary winding and the secondary winding, a turn ratio between the primary winding and auxiliary winding of the transformer 113 is set so as to provide a VCC voltage needed for the power supply IC 101. An induced electromotive force corresponding to the counter-electromotive force of the primary side is generated also on the auxiliary winding, developing a voltage proportional to the secondary winding. The power supply IC 101 feeds the voltage generated on the auxiliary winding to a BOTTOM terminal and thereby detects that the energy release from the secondary winding of the transformer 113 has finished. When the energy release from the secondary winding of the transformer 113 is finished, the “out” terminal voltage of the power supply IC 101 goes High again, and then the series of operations described above is repeated.
In the series of operations described above, the period during which the “out” terminal voltage of the power supply IC 101 remains High, i.e., the ON duty, depends on a difference between the FB terminal voltage of the power supply IC 101 and the reference voltage (not shown) in the power supply IC. In this case, the higher the FB terminal voltage of the power supply IC 101, the larger the ON duty.
The above is the operation common to both of the present embodiment and the conventional technique. Next, characteristic part of the present invention implemented in the present embodiment will be described.
The present embodiment differs from the conventional technique in the following points. First of all, the secondary-side current detection resistor 211 adapted to detect a secondary-side current is included additionally. Second, the reference voltage in the regulator IC 207 of the feedback circuit section is grounded to GND1 on the downstream side of the secondary-side current detection resistor 211 (on the side of power consumption section). On the other hand, the lower resistor 206 of the voltage divider is connected at one end to the upper resistor 205 of the voltage divider and connected at the other end to GND2 on the upstream side of the secondary-side current detection resistor 211 (on the side of the transformer 113: on the transformer side). In this respect, the conventional technique shown in
It is a feature of the present embodiment that the output voltage under light load can be set low because the present embodiment undergoes less changes in the output voltage near the power consumption section than the conventional technique in
Changes in load current and changes in output voltage according to the conventional technique of
Vo=Vin×(R205+R206)/R206 (1)
In the power supply apparatus of
Here, the output voltage Vo expressed by Eq. (1) is a potential difference between the output voltage connected with the upper resistor R205 of the voltage divider and GND1 connected with the lower resistor 206 of the voltage divider. Therefore, in the case of the conventional technique shown in
Vos=Vo−{Is−(Zc1+Zc2)} (2)
As indicated by a broken line in
The above is the changes in load current and changes in output voltage according to the conventional technique. Next, “changes in load current and changes in output voltage” according to the present embodiment will be described.
Changes in load current and changes in output voltage according to the present embodiment are shown in
Vd=Vo−Vos=Is×(Zc1+Zc2) (3)
On the other hand, under no-load conditions in which the load current of the power consumption section is substantially zero, the output voltage Vo is given by the equation below.
Vo=Vin×(R205+R206)/R206 (4)
Eq. (4) is equal to Eq. (1) which gives the output voltage of the conventional technique. If the load current Is flows through the secondary-side current detection resistor 211, since the lower resistor 206 that can be defined by the resistance ratio in the voltage divider is grounded to the upstream side of the secondary-side current detection resistor 211 (on the side of the transformer 113), the comparison voltage Vin changes by Is×R211=Vri. Thus, if the output voltage at this time is Vo′, then
Vo′=(Vin+Vri)×(R205+R206)/R206 (5)
At this time, there is a relationship of Vo<Vo′ between the no-load output voltage Vo expressed by Eq. (4) and the output voltage Vo′ produced when the load current Is expressed by Eq. (5) flows, and the difference between the output voltages Vo and Vo′ varies with the current flowing through the secondary-side current detection resistor 211, i.e., with the load current Is.
To keep the output voltage Vos near the power consumption section constant regardless of the values of load current as shown in
Vo′=Vo+Vd (6)
This is because the value of the secondary-side current detection resistor 211 is set based on the line impedances Zc1 and Zc2 as well as on the voltage ratio defined by the resistance ratio between R205 and R206 of the voltage divider. Detailed settings of the secondary-side current detection resistor 211 are shown below.
To begin with, it follows from Eqs. (5) and (6) that the output voltage Vo′ produced when the load current Is flows is
Vo′=Vo+Vd=(Vin+Vri)×(R205+R206)/R206={Vin×(R205+R206)/R206}+{Vri×(R205+R206)/R206}
It can be seen from Eq. (4) that the first term on the right side of the above equation is equal to the no-load output voltage Vo. Thus,
Vo′=Vo+{Vri×(R205+R206)/R206} (7)
That is, the potential difference Vd between the output voltage Vo of the power supply apparatus and the output voltage Vos near the power consumption section is given by
Vd=Vri×(R205+R206)/R206 (8)
Thus, it can be seen that the potential difference Vd and the voltage Vri across the secondary-side current detection resistor 211 are proportional to each other.
The line impedances Zc1 and Zc2 and the secondary-side current detection resistor 211 are put between the output of the power supply apparatus and the power consumption section, and the same load current to be consumed by the power consumption section flows through these components. Therefore, substituting Eq. (3) into Eq. (8) gives the value (resistance value) of the secondary-side current detection resistor 211 as follows:
Is×(Zc1+Zc2)=(Is×R211)×(R205+R206)/R206 (Is×R211)=Is×(Zc1+Zc2)×R206/(R205+R206) R211=(Zc1+Zc2)×R206/(R205+R206) (9)
That is, the resistance value of the secondary-side current detection resistor 211 can be determined from the line impedances Zc1 and Zc2 as well as the voltage ratio defined by the resistance ratio between R205 and R206 of the voltage divider using Eq. (9). According to the present embodiment shown in
Zc1≅33 mΩ (an AWG18 wire with 485 mm: impedance of another circuit)
Zc2≅32 mΩ (an AWG18 wire with 485 mm: impedance of another circuit)
R211≅24 mΩ (R205: 3.83 kΩ/R206: 2.21 kΩ)
where AWG is the unit of thickness of a cable core wire, i.e., size of cross-sectional area.
Consequently, the output voltage Vo of the power supply apparatus changes as follows according to the load current of the power consumption section and the line impedances of the cables such that the output voltage Vos near the power consumption section will be constant.
When load current is zero [A]: voltage corresponding to Eq. (4)
When load current is n [A]: voltage corresponding to Eq. (5)
Thus, according to the present embodiment, the secondary-side current detection resistor 211 is installed in series on a feedback route of a secondary-side current path of the power supply apparatus and the lower resistor 206 of the voltage divider used to generate the comparison voltage is grounded on the upstream side of the secondary-side current detection resistor 211 (on the side of the transformer 113). Also, the reference voltage of the feedback circuit section is grounded on the downstream side of the secondary-side current detection resistor 211 (on the side of power consumption section). Furthermore, the secondary-side current detection resistor 211 is determined based on the line impedances Zc1 and Zc2 as well as on the voltage ratio defined by the resistance ratio between R205 and R206 of the voltage divider. Consequently, the output voltage can be set near the standard lower limit value without consideration of voltage drops caused by line impedance under heavy load, and power consumption under light load can be reduced, where the output voltage is determined by the reference voltage of the feedback circuit section and the comparison voltage. That is, the present embodiment enables reducing voltage drops caused by impedance on the path which interconnects the power supply apparatus and power consumption section as well as reducing power consumption under light load.
Next, a second embodiment will be described.
In addition to the technique of the first embodiment, the second embodiment further concerns a protection circuit adapted to protect equipment from overvoltage and overcurrent conditions.
Conventionally, power supply apparatus are generally equipped with a circuit adapted to detect overcurrent, overvoltage, and other abnormal conditions of an output section, and thereby protect the entire equipment. For example, Japanese Patent Application Laid-Open No. H11-215690 discloses a technique for implementing a protection circuit against overcurrent using a potential difference produced across a resistor inserted in series with a current path. Available techniques for protection circuits against overcurrent include a technique, such as disclosed in Japanese Patent Application Laid-Open No. H11-215690, which configures a protection circuit on the secondary side and a technique which configures a protection circuit on the primary side. The protection circuit configured on the primary side sometimes has higher dispersion in the values of protection current to be detected than the protection circuit configured on the secondary side. In the case of a power supply apparatus (so-called AC/DC converter) which accepts as input a voltage obtained by rectifying an AC voltage, variations (ripple) in the AC voltage are superimposed on a primary-side voltage. Consequently, in relation to the values of the secondary-side load current to be detected, the values of primary-side current has dispersion proportional to variations in the primary-side voltage. Therefore, protection circuits against overcurrent allow higher-accuracy detection when configured on the secondary side as described in Japanese Patent Application Laid-Open No. H11-215690. Also, Japanese Patent Application Laid-Open No. 2000-156972 describes a circuit configuration which, in order to provide protection against overvoltage, compares an output voltage of a power supply apparatus with a Zener voltage of a Zener diode and interrupts operation of the power supply apparatus when an overvoltage condition is detected.
With the conventional configuration shown in
Thus, it is desired that protection against overvoltage and overcurrent is enabled by the same protection circuit configured on the secondary side not affected by variations in the AC voltage.
A circuit configuration diagram according to the present embodiment is shown in
According to the present embodiment, abnormal conditions such as an overvoltage condition in the output voltage of the power supply apparatus and an overcurrent condition in the output of the power supply apparatus can be detected by the same protection circuit section configured with the Zener diode 213 on the secondary side. In particular, even in an overcurrent condition, detection accuracy equivalent to that described in Japanese Patent Application Laid-Open No. 2000-156972 is available without being affected by the AC voltage. Consequently, the present embodiment features a reduced mounting area and reduced cost compared to the conventional technique. The operation of the protection circuit section will be described below in separate parts: “operation of protection circuit section under overvoltage condition” and “operation of protection circuit section under overcurrent condition.”
If the output voltage according to the present embodiment is denoted by Vo, the output voltage Vo is given by the equation below as in the case of the output voltage of the power supply apparatus according to the first embodiment.
Vo=(Vin+Vri)×(R205+R206)/R206 (10)
Incidentally, the comparison voltage Vin in Eq. (10) stabilizes at a value equal to the reference voltage REF of the regulator IC 207.
Now, if it is assumed that a maximum load current in the power consumption section of equipment equipped with the power supply apparatus according to the present embodiment is α [A], the output voltage Vo_α of the power supply apparatus is given by
Vo
—
α={Vin+(α×R211)}×(R205+R206)/R206 (11)
The power supply apparatus according to the present embodiment is configured such that the output voltage Vo_α will be lower than the upper limit value Vmax of the standard voltage (Vo_α<Vmax). Also, the Zener voltage Vz of the Zener diode 213 of the protection circuit is set to a sufficiently higher voltage than the upper limit value Vmax of the standard voltage (Vz>>Vmax) to allow for abnormal conditions such as a failure of the power supply apparatus and peripheral circuitry thereof. That is, the three voltages satisfy the following relationship.
Vo
—
α<Vmax<<Vz (12)
Incidentally, the reason why the Zener voltage Vz is set sufficiently higher than the upper limit value Vmax of the standard voltage as indicated by Vmax<<Vz is as follows. That is, the reason is to prevent the protection circuit from malfunctioning due to ringing, switching noise of the power supply apparatus itself, or external noise which can occur under operating conditions such as the start of operation of the power supply apparatus or abrupt changes in the load current of the power consumption section.
Now, suppose the photo coupler 107 of the feedback circuit section shown in
At time (A) in
If the maximum load current in the power consumption section of equipment equipped with the power supply apparatus according to the present embodiment is α [A], the output voltage Vo_α of the power supply apparatus is given by Eq. (11) as described above. Similarly, the relationship among the output voltage Vo_α, the upper limit value Vmax of the standard voltage of the equipment, and the Zener voltage Vz of the Zener diode 213 is as described with reference to Eq. (12).
Now, suppose some failure occurs in the power consumption section, resulting in an overcurrent condition (α+n [A]) with a maximum current of α [A] or above. Then, output voltage Vo_α+n becomes
Vo
—
α+n={Vin+((α+n)×R211)}×(R205+R206)/R206 (13)
Therefore, when configured according to the first embodiment, the output voltage according to the present embodiment rises in proportion to increases in the load current. Consequently, if the output voltage Vo_α+n at this time exceeds the Zener voltage Vz, the protection circuit configured with the Zener diode 213 comes into operation, interrupting power supply to the secondary side as with the operation of the above-described protection circuit against overvoltage conditions. Specifically, if an excessive current flows, satisfying the condition:
dVn=(n×R211)×(R205+R206)/R206>dVz (14)
where (i) dVn=Vo_α+n−Vo_α,
(ii) dVz=Vz−Vo_α,
the protection circuit comes into operation and interrupts the power supply to the secondary side.
In this way, the present embodiment can detect any overvoltage and overcurrent conditions due to a failure of the power supply apparatus and its peripheral circuitry using the same protection circuit section made up of the Zener diode 213 and the photo coupler 214 and stop power supply from the power supply apparatus. The reason why the present embodiment allows overvoltage and overcurrent conditions to be detected using the same circuit is that the output voltage near the power supply apparatus is proportional to the load current as described with reference to
It should be noted that although the protection circuit according to the present embodiment is configured with a Zener diode, this is not intended to limit the application range of the present invention. Specifically, the present invention is also applicable when the protection circuit is configured with an active element such as a comparator or transistor or a passive element such as a resistor. That is, the present embodiment enables reducing voltage drops caused by impedance on the path which interconnects the power supply apparatus and the power consumption section as well as reducing power consumption under light load. Furthermore, protection against overvoltage and overcurrent is enabled by the same protection circuit configured on the secondary side not affected by variations in the AC voltage.
Next, a third embodiment will be described.
The power supply apparatus described in the first and second embodiments are applicable as power supplies to a controller (control section) of an image forming apparatus. A configuration of an image forming apparatus to which the power supply apparatus according to the first or second embodiment is applied will be described below.
A laser beam printer will be described as an example of an image forming apparatus. A schematic configuration of a laser beam printer which is an example of an electrophotographic printer is shown in
The laser beam printer 300 is equipped with a controller (not shown) adapted to control image forming operation of the image forming section and sheet transport operation, and the power supply apparatus described in the first or second embodiment supplies electric power, for example, to the controller. That is, the power consumption section according to the first and second embodiments corresponds to the controller. The power supply apparatus and the controller are interconnected, for example, via a cable, and the power supply apparatus attached to the image forming apparatus according to the present embodiment can reduce voltage drops caused by the line impedance of the cable. Also, the image forming apparatus according to the present embodiment can reduce power consumption in a standby state for power savings. Also, the image forming apparatus equipped with the power supply apparatus according to the second embodiment enables protection against overvoltage and overcurrent using the same protection circuit configured on the secondary side not affected by variations in AC voltage.
Thus, the present embodiment enables reducing voltage drops caused by impedance on the path which interconnects the power supply apparatus and the power consumption section as well as reducing power consumption under light load.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2011-197125, filed Sep. 9, 2011, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-197125 | Sep 2011 | JP | national |
