JPH01206868A - Power source equipment - Google Patents

Abstract

PURPOSE:To reduce switching loss, by matching the ON/OFF timing of a switching means for driving the primary winding with resonance characteristic of a transformer in synchronization with the zero-cross point timing of flyback voltage. CONSTITUTION:In a power source system for a zerographic image forming machine, output from a commercial power source CP is rectified directly through a diode bridge and fed through a capacitor C3 to the primary winding T1 of a switching transformer T. The power supply is controlled by means of a MOSFET Q1 through a control IC Q2. Secondary windings T3-T7 are provided in the transformer T in order to feed power to respective loads. Output voltage form the winding T7 is detected through a voltage detecting circuit 6 and fed through a photo-coupler Q5 to the error amplifier 3 in the IC Q2. Oscillation frequency of an oscillator 2 is controlled by a transistor Q6 through external- mounted resistors R1, 2 and a capacitor C2. By such an arrangement, OFF interval of the FET Q1 can be matched with the resonant waveform of the transformer T.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a power supply device, in particular, a DC voltage generated in the secondary winding of the transformer by switching on and off the DC voltage applied to the primary winding of the transformer. This invention relates to a power supply device that rectifies alternating current (AC) in flyback mode and supplies power to a load.

[Prior Art] Conventionally, in electrophotographic image forming apparatuses such as copying machines and laser beam printers, voltage resonance switching regulators for supplying power to chargers and other loads have been configured as shown in FIG. ing. In the figure, the symbol T2
is a switching transformer, switching transformer T2
A low voltage S voltage Vcc is supplied to the primary winding. This negative side voltage supply is controlled by a switching transistor 104 connected together with a protection diode 105 and a resonance capacitor 106 between one end of the winding and the ground potential. The output of the secondary side of the switching transformer T2 is supplied to the load via the rectifying diode 108 or as it is.

The load current is measured by a resistor 10 connected between the secondary circuit and ground.
7 and is fed back to the driving ICQ2 of the switching transistor 104.

The control claw I CQ2 has an error amplifier 103, an oscillator 102, and a PWM (pulse width modulation circuit) 101.
The load current is controlled to be constant by controlling the drive pulse width of the switching transistor 104 using the control ICQ2.

[Problems to be solved by the invention] However, in the conventional structure described above, the cut-off time on the primary side is determined by changes in the resonant frequency caused by variations in the transformer. There is a problem in that it is not possible to perform precise control.

In addition, switching power supplies use a drive system that detects the load current and load voltage and changes them to desired values or controls them to a constant value. Due to the variations, it is difficult to reduce switching losses under all operating conditions.

The object of the present invention is to solve the above problems.

[Means for Solving the Problems] In order to solve the above-mentioned problems, in the present invention, the DC voltage applied to the primary winding of the transformer is interrupted by a switching means, so that the secondary winding of the transformer is In a power supply device that rectifies alternating current generated in a transformer in a flyback mode to supply power to a load, the power supply device includes means for detecting a zero-crossing timing of a flyback voltage generated in the transformer, and a zero-crossing timing of the flyback voltage being detected by the detecting means. A configuration is adopted in which a pulse control means for forcibly advances the edge of the drive pulse of the switching means when the switching means is activated.

[Function] According to the above configuration, the on/off timing of the switching means that drives the primary winding can be adapted to the resonance characteristics of the transformer in synchronization with the timing of the zero cross point of the flyback voltage. Even if there are variations in components or changes in power supply conditions, power can always be supplied with small switching loss.

[Example] Hereinafter, the present invention will be described in detail based on an example shown in the drawings.

FIG. 1 is a circuit diagram of the power supply system of an electrophotographic image forming apparatus that employs the present invention. In FIG. It is directly rectified by diode D4 and input to capacitor C3. Capacitor C
3 is the voltage across the primary winding T of the switching transformer T.
l is supplied.

This power supply is controlled by a switching transistor Q1 composed of a MOSFET or the like as in the conventional example described above. The source of switching transistor Q1
A protection diode D1 and a resonance capacitor C1 are connected between the drains. Switching control of the switching transistor Ql is controlled by a control ICQ2. -The next-side control circuit will be explained in detail later.

The switching transformer T is provided with secondary windings T3 to T7, each of which supplies power to a predetermined load. The secondary winding T3 supplies power to the lamp 7 (fluorescent lamp) for illuminating the document, and the output AC of the secondary @ wire T3 is supplied to the lamp 7 via the ballast L1. The light amount of the lamp 7 is controlled by a known lighting control section 5. One ends of the secondary windings T3 to T7 are commonly grounded.

Diodes D5 to D are installed in the secondary windings T4 to T6, respectively.
8. A rectifying/smoothing circuit including capacitors 04 to C7 is connected, and supplies high voltage having a predetermined polarity to the charger, developer, etc., respectively.

In addition, a diode D8 and a capacitor C are connected to the secondary winding T7.
A rectifier/smoothing circuit consisting of 7 is connected, and this output is 24
Supplied to V motors and other low voltage loads. Secondary winding T
7 is grounded at its midpoint, and a rectifier Φ smoothing circuit consisting of a diode DIO and a capacitor C8 is connected to the forward winding section on the opposite side, and the 5V output formed by this is sent to a control system such as a microprocessor. supplied,
Only this 5V output is driven in forward mode, and all other windings are driven in flyback mode.

The output voltage of the winding T7 is detected by the voltage detection circuit 6, and the LED of the photocoupler Q5 is turned on with the amount of light corresponding to this voltage. The voltage for this LED is supplied from the 5v output. The output of the photocoupler Q5 is input to an error amplifier 3 consisting of an operational amplifier of the control ICQ2. A threshold voltage corresponding to a predetermined load voltage is supplied to this error amplifier, and its error output is inputted together with the output of an oscillator 2 to an operational amplifier 1 forming an operational amplifier.

The control ICQ2 has a built-in power supply section P, and receives power from the diode D4 via the resistor R9 at startup. When the operation of the switching transformer T becomes stable, power is supplied to the power supply section P from the secondary winding T2. The output of the secondary winding T2 is rectified and smoothed by a diode D3 and a capacitor C9, and then supplied to the power supply section P. At this time, this power supply voltage is stabilized by the Zener diode 11.

The phototransistor of the photocoupler Q5 mentioned above is supplied with power from this power supply section P.

The oscillation frequency of oscillator 2 is determined by a resistor R1 externally connected to the IC.
Controlled by R2 and capacitor C2. The circuit shown on the lower right side of the oscillator 2 is a circuit that changes the oscillation frequency by controlling the time constant. The output of the secondary winding T2 is connected to the transistor Q3 via the diode D2 and resistor R2.
is input to the emitter of The base of this transistor Q3 is controlled by transistor Q4.

That is, the emitter and base of transistor Q3 are coupled by bias resistor R7, and transistor Q3
The base of is connected to the collector of transistor Q4.

The collector of transistor Q3 and the base of transistor Q4 are connected to a common potential on the primary side via a resistor R8. The emitter of transistor Q4 is grounded to this common potential, and the base of transistor Q4 is connected to the output of operational amplifier 1 of control ICQ2 via resistor R4.

The emitter of transistor Q3 is connected to the base of transistor Q6 via resistor R5.

The emitter of transistor Q6 is connected to a common potential on the primary side. Transistor Q6 functions to change the combined resistance of resistors R1 and R2, which determines the oscillation frequency of oscillator 2, by changing its conductivity.

Next, the operation in the above configuration will be explained.

When power is supplied to the circuit shown in FIG. 1 from the commercial AC power supply CP, power is supplied to the control ICQ2 via the resistor R9.
Switching of the switching transistor Q1 is started: When the 0 oscillation is stabilized, an output transformed according to the winding ratio is obtained in each secondary winding, and the control IC Q2 is turned to the secondary winding! i
Receives power from T2. The configuration of the secondary winding is as shown above, with secondary windings T3 to T6 and 24 for high voltage output.
The V-system secondary winding T7 is controlled in flyback mode. Further, the 5V side of the secondary winding T7 for sequence control is controlled in forward mode. According to such a configuration, by controlling the oscillation duty ratio of the switching transistor Q1, the voltage of each flyback output can be controlled as desired, and the 5V output in the forward mode is not affected by this PWM control. do not have. Thereby, for example, in a standby state, it is possible to lower the high voltage output to the charger and the exposure voltage by the resistor 7 without affecting the sequence control system.

As mentioned above, controlling the flyback output changes the resonant frequency of the switching transformer T, so this must be optimally controlled. Figure 2 explains this operation, and connects the connection point a in Figure 1. -e voltage waveform is shown. That is, FIG. 2 shows, from the top, the oscillation waveform of the oscillator 2, the output waveform of the operational amplifier 1, the flyback voltage waveform of the transformer T, the zero-cross detection signal of the flyback voltage used inside the control ICQ2, and the oscillation frequency of the oscillator 2. The control signal for switching is shown.

The switching transistor Ql depends on the waveform at the connection point S.
is driven, but when the switching transistor Q1 is turned off, a flyback voltage as shown in the third stage of FIG. 2 is generated at the connection point C due to resonance between the primary winding TI of the switching transformer T and the capacitor C1. Also, the output point d of the diode D2 has the polarity of the diode and the secondary winding T.
Since a forward voltage is generated from the polarity of 2, a voltage waveform as shown in the figure is generated. This waveform can be used as a detection signal for the zero-crossing point of the flyback voltage.

Transistor Q3. Since Q4 performs a thyristor operation based on the signals at connection points i and d, the waveform at connection point e, that is, the base of transistor Q6 changes as shown in the bottom row of FIG. The transistor Q6 becomes conductive when the signal at the connection point e becomes high level, and is cut off when the signal becomes low level. As a result, if the frequency of the 0 oscillator, in which the oscillation time constant of oscillator 2 changes, is inversely proportional to the combined resistance of resistors R1 and R2, when transistor Q6 is turned on, the oscillation frequency becomes low, and when transistor Q6 is turned off, the frequency decreases. It gets expensive. Therefore, the output waveform of the oscillator 2 changes as shown in the top row of FIG.

As described above, when the flyback voltage reaches oV, the oscillation frequency of the oscillator 2 is controlled to be high, and therefore the timing at which the output of the oscillator 2 passes the threshold of the operational amplifier l becomes earlier, that is, the on-timing of the switching transistor Ql. Since the off-period of the switching transistor Ql can be brought closer to the resonant waveform of the transformer, optimal control without switching loss can be performed. Therefore, efficient power can be supplied to each load, and the transformer can be made smaller and lighter when achieving the same power capacity as the conventional one.

In order to control the oscillation frequency of the oscillator 2 of the control ICQ 2 in the above configuration, a configuration as shown in FIG. 3 can also be used. Figure 3 shows only a part of Figure 1, and differs from Figure 1 in that the collector of transistor Q6 is connected to the connection point of series-connected resistors R1 and R2. Since the influence of the collector capacitance can be reduced and the oscillation frequency of the oscillator 2 can be rapidly changed, more ideal oscillation control is possible.

4 to 6 show configurations for controlling the oscillation frequency that can be implemented when the oscillator of FIG. 1 is constructed using logic circuits.

In FIG. 4, numerals 502 and 503 are comparators,
Each - and child terminal is connected to a connection point of resistors R51-R53 connected in series between power supply voltage Vcc and ground potential. The output of the comparator 502 is connected to the reset terminal of the flip-flop 501 through the comparator 503.
The output of OR gate 504 is input to OR gate 504.
The output of 4 is input to the set terminal of flip-flop 501. The oscillation frequency switching signal obtained at the connection point e in FIG. 1 is input to one input terminal of the OR gate 504.

The output of flip-flop 501 is connected to the base of transistor 51. Transistors 51 and 52 are connected in series between power supply voltage Vcc*ground potential, and the base of transistor 52 is connected to transistor 53 whose base and collector are connected. The emitter of transistor 53 is grounded, and the power supply voltage Vcc is input to its collector via resistor R54. Transistor 51.
The emitter-collector connection point 52 is connected to the other end of a capacitor C51 whose one end is grounded.

The connection point of the capacitor C51 with the transistor is connected to the + and one input terminals of the comparators 502 and 503. In such a configuration, the flip-flop 5
Transistor 51.5 by reset of 01
The charging and discharging of the capacitor C51 is controlled via the capacitor C51, and an oscillation operation is performed. When the terminal voltage of capacitor C51 rises above the terminal voltage of resistor R52, the flip-flop is reset and discharge of capacitor C51 is started.

When the potential of the capacitor C51 becomes lower than the voltage determined by the resistor R53, the flip-flop 501 is set by the comparator 503 and charging of the capacitor C51 is started.Oscillation is performed by repeating 0 or more.The 0 oscillation frequency is determined by the resistor R51~ Determined by R53 and capacitor C51.

Furthermore, by inputting the oscillation frequency switching signal e to the OR gate 504, the rise of the oscillation waveform determined by the flip-flop 501 can be accelerated. This is done by forcibly advancing the rise timing of the waveform rather than by changing it, so as shown in Figure 5, the oscillation waveform is suddenly started, and the rise timing of the PWM output waveform is brought closer to the zero-crossing point of the flyback voltage. Can be done. As a result, the off-period of the switching transistor can be made closer to the resonance waveform of the transformer than in the previous embodiment, and more ideal switching it can be achieved.
j control becomes possible.

In the circuit shown in FIG. 4, the oscillation frequency switching signal is inputted using the OR gate 504, but it may also be inputted via the transistor 64 as shown in FIG.
The emitter of 4 is connected to the output point of capacitor C51,
The collector is electric! The zero frequency switching signal connected to the IX voltage Vcc is input to the base of transistor 64. Even with such a configuration, the same operation as in the case of FIG. 4 is possible.

Further, as shown in FIG. 7, an OR gate 704 is provided between the transistor 51 and the flip-flop 501, and this
Even if a frequency switching signal is input through the R gate 704, the same effect as described above can be obtained.

In FIG. 1, the zero cross point of the flyback voltage is detected from the rectified output of the secondary winding T2, but the configuration of this part can also be changed as shown in FIG. 8. In FIG. 8, one end of the secondary winding T2 is connected to a common potential on the primary side, and the other end is connected to a capacitor C92. Capacitor C
A diode D92 and a Zener diode Z are connected between the output side of 92 and the common potential.
D91 is connected. According to such a configuration, it is possible to detect the zero-crossing point of the drain voltage of the source or drain of the switching transistor Ql from the connection point between the capacitor C92 and the diode D92, and use this signal to perform frequency switching control in the same manner as above. can be done.

The above configuration can be applied to a power supply section of an electronic device other than an image forming apparatus.

[Effects of the Invention] As is clear from the above, according to the present invention, one of the transformers
In a power supply device that rectifies the alternating current generated in the secondary winding of a transformer in a flyback mode and supplies power to a load by intermittent DC voltage applied to the secondary winding by a switching means, A configuration is adopted that includes means for detecting zero-crossing timing of the back voltage, and pulse control means for forcibly advancing the edge of the drive pulse of the switching means when the zero-crossing timing of the flyback voltage is detected by the detecting means. Therefore, the switching means that drives the primary winding is turned on/off in synchronization with the timing of the zero-crossing point of the flyback voltage.
Since the off-timing can be adapted to the resonance characteristics of the transformer, power can always be supplied with small switching loss even if there are variations in components or changes in power supply conditions, making it possible to improve the efficiency of the power supply or This has excellent effects such as making it possible to reduce the size and weight of the power supply section and therefore the device in which it is employed.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a power supply device adopting the present invention, Figure 2 is a waveform diagram showing the operation of the circuit in Figure 1, and Figure 3 is a circuit diagram showing a modification of the frequency control circuit in Figure 1. Figure 4 is the first
5 is a circuit diagram showing a modification of the frequency switching circuit and oscillator shown in FIG. 5. FIG. 5 is a waveform diagram showing the operation of the circuit in FIG. 4.
7 and 7 are circuit diagrams showing different modifications of the configuration of FIG. 4, FIG. 8 is a circuit diagram showing a different configuration for detecting flyback voltage, and FIG. 9 is a circuit diagram of a conventional switching regulator. FIG. 2 is a circuit diagram showing the configuration of FIG. l...Operational amplifier 2...Oscillator 3...Error amplifier 7...Lamp L1...Ballast Ql...Switching transistor Q2...Control IC Fig. 3 Fig. 4 1/44 old a Sun Nagisa rJ'> Wholesale Y Kihi Keibe Tsutomu Figure 5 Figure 6 Figure 7 Concave

Claims (1)

[Claims] 1) In a power supply device that rectifies the AC generated in the secondary winding of the transformer in a flyback mode by intermittent DC voltage applied to the primary winding of the transformer using a switching means, and supplies power to the load. A means for detecting a zero-crossing timing of a flyback voltage generated in a transformer, and a pulse control means for forcibly advancing an edge of a driving pulse of the switching means when the zero-crossing timing of a flyback voltage is detected by the detecting means. A power supply device characterized by: