JPH10201230A - Dc high voltage power supply drive circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent generation of parasitic vibration and noise by forming a self-excited oscillation circuit which positively feeds back an electromotive force generated in a base coil of a voltage boosting transformer to the base of power transistor via capacitor, resistor and junction type transistor. SOLUTION: When a negative electromotive force generated in a base coil TB, of a voltage boosting transformer T is reduced, a current IB flowing into the base of a junction type transistor 18 from a resistor r7 responding to the PWM signal increases against a negative current flowing to the base from the collector of the junction type transistor 18 via a resistor r8 and this condition is maintained until the positive base current IB starts to flow into the base of the power transistor 19. When the positive base current IB starts to flow to the base of the power transistor 19, the power transistor 19 enters the same ON condition as that in the initial stage and thereby the power transistor repeats the ON and OFF conditions for the self-excited oscillating operation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage control technique for a DC high-voltage power supply, and more particularly to a self-oscillation output voltage of a DC high-voltage power supply mainly composed of a step-up transformer, a power transistor and a high-voltage rectifying / smoothing circuit. The present invention relates to an improvement in a drive circuit of a DC high-voltage power supply that can linearly follow a duty ratio of a PWM signal.

[0002]

2. Description of the Related Art In recent years, copiers (copy), laser printers (LBP), faxes (FAX), and the like have become widespread as products to which electrophotographic technology is applied. FIG. 2 shows an image forming section of a copying machine among the products to which the electrophotographic technique is applied. In FIG. 2, reference numeral 1 denotes a photosensitive drum, 2 denotes an exposure device that forms a latent image on the surface of the photosensitive drum 1 charged by a corona charger 3, and 4 denotes a visible image of the latent image. A transfer unit 5 transfers the visible image to recording paper. Further, reference numeral 6 denotes a fixing device for fixing the visible image transferred to the recording paper, and reference numeral 7 denotes a static eliminator for separating the recording paper from the photosensitive drum 1 and removing the residual charge of the photosensitive drum 1.

In order to form an electrostatic image in the copying machine, a DC high voltage power supply is always required. In particular, in the image forming section of the copying machine shown in FIG. 2, a corona charger, a developing device and a transfer device are used. Etc. are used.

Conventionally, for example, a drive circuit for a DC high-voltage power supply used in an image forming apparatus such as a copying machine has been used as shown in FIG. In FIG.
8 is connected in series with the input winding T _I of the step-up transformer T, indicates a DC power source for supplying power to the step-up transformer T, r ₁ is connected to the output terminal of the pulse width modulation circuit 9 to be described later, is inserted resistor to set the base current of the power transistor 10 to turn on / off the voltage applied to the input winding T _I of the step-up transformer T. Also, r ₂ is
Is connected to a connection point between the base resistor r ₁ and the power transistor 10, the resistance provided to bypass discharge the leakage current and the accumulated charge between the base and emitter of the collector-base of the power transistor 10, This is for speeding up the malfunction and off operation of the power transistor 10.

[0005] Then, 11 rectifier _D 1, is constituted by _{D 2} and capacitor _C 1, _{C 2,} a high voltage rectification of a so-called voltage doubling method for smoothing a voltage outputted from the output winding _{T O} of the step-up transformer T R denotes a control object controlled by the output voltage from the high-voltage rectifying / smoothing circuit 11, for example, a load of an exposure unit or the like in an image forming unit of a copying machine.

Next, the configuration of the pulse width modulation circuit 9 will be described with reference to FIG. The pulse width modulation circuit 9 includes, as shown in FIG.
And a comparison voltage generation circuit 13 that generates a comparison voltage to be compared with the output voltage of the triangular wave oscillation circuit 12, and two output voltages output from the triangular wave oscillation circuit 12 and the comparison voltage generation circuit 13, A voltage comparison circuit 14 for outputting a rectangular voltage wave corresponding to the comparison result.

The triangular wave oscillation circuit 12 includes, for example, an open collector output comparator CP1.
One end of each of three resistors R1, R2, and Rx is connected to a non-inverting input terminal of the comparator CP1. The other end of the resistor R1 is connected to the constant voltage power supply Vcc of the triangular wave oscillation circuit 12, the other end of the resistor R2 is connected to GND, and the other end of the resistor Rx is connected to the output terminal of the comparator CP1.

The output terminal of the comparator CP 1 is connected to one end of the resistor Rx and two pull-up resistors R 3 and Rt, respectively. The other end of the resistor R 3 is connected to the constant voltage power supply of the triangular wave oscillation circuit 12. Vcc, and the other end of the resistor Rt is connected to the inverting input terminal of the comparator CP1.
Each is connected. One end of a timing capacitor Ct is connected to the inverting input terminal of the comparator CP1 in addition to the resistor Rt, and the other end of the timing capacitor Ct is connected to GND.

As shown in the following equations (1) and (2), the non-inverting input terminal of the comparator CP1 of the triangular wave oscillation circuit 12 has V ' _SH [when the output from the comparator CP1 is "H". V] is applied and when the output from the comparator CP1 is "L", V '
A voltage of _SL [V] is applied.

[0010]

(Equation 1)

Next, the oscillation operation of the triangular wave oscillation circuit 12 will be described. When the output from the comparator CP1 is "H", the timing capacitor Ct is charged via the pull-up type resistor R3 and the resistor Rt, and gradually.
The voltage between the terminals increases. At this time, the comparator CP
Since the voltage of V ′ _SH [V] is applied to the non-inverting input terminal of No. 1, the comparator CP1 remains “H” until the voltage between the terminals of the timing capacitor Ct exceeds V ′ _SH [V] due to charging. Maintain output. When the voltage between the terminals of the timing capacitor Ct exceeds V ′ _SH [V], the output of the comparator CP1 is inverted from “H” to “L”.

When the output of the comparator CP1 becomes "L", the discharge of the timing capacitor Ct is started via the resistor Rt and the comparator CP1. With this discharge, the non-inverting input terminal of the comparator CP1 V _'SL
A voltage of [V] is applied. For this reason, the voltage between the terminals of the timing capacitor Ct is discharged to V ′ _SL [V].
Until the voltage becomes lower, the “L” output of the comparator CP1 is maintained. When the voltage between the terminals of the timing capacitor Ct becomes lower than V ′ _SL [V], the comparator CP1
Is inverted from "L" to "H", and charging of the timing capacitor Ct is started again.

As described above, the timing capacitor Ct
Repeats charging and discharging at a constant cycle, and the voltage between its terminals becomes a triangular voltage wave. The maximum value of this voltage wave is V '
_SH [V], and the minimum value is _V'SL [V]. Then, this triangular wave voltage wave is input as an output voltage of the triangular wave oscillation circuit 12 to a voltage comparison circuit 14 described later.

The comparison voltage generation circuit 13 shown in FIG. 3 includes a variable resistor VR connected between a constant voltage power supply Vcc and GND,
And a capacitor CI for stabilizing the output voltage output from the variable resistor VR. When the slider S of the variable resistor VR is moved (up and down in this figure), the value of the comparison voltage output from the comparison voltage generation circuit 13 is changed.
That is, when the slider S of the variable resistor VR moves upward, the comparison voltage value increases, and when the slider S of the variable resistor VR moves downward, the comparison voltage value decreases.

The voltage comparison circuit 14 includes an open collector output comparator CP2 and a pull-up resistor R4. The output voltage of the triangular wave oscillation circuit 12 is applied to the inverting input terminal of the comparator CP2,
The comparison voltage of the comparison voltage generation circuit 13 is applied to the non-inverting input terminal. Therefore, when the comparison voltage of the comparison voltage generation circuit 13 is higher than the output voltage of the triangular wave oscillation circuit 12, “H” is output from the PWM terminal of the voltage comparison circuit 14; L ”is output.

As described above, the voltage wave output from the pulse width modulation circuit 9 is a rectangular wave of “H” or “L”, and its frequency matches the frequency of the output voltage of the triangular wave oscillation circuit 12. Further, the duty ratio of the voltage wave output from the pulse width modulation circuit 9 is changed by increasing and decreasing the comparison voltage generated by the comparison voltage generation circuit 13. That is, when the comparison voltage is increased, the duty ratio of the rectangular wave output from the pulse width modulation circuit 9 increases, and conversely, when the comparison voltage is reduced, the duty ratio of the rectangular wave decreases.

As described above, the PWM signal having the "H" and "L" voltage values adjusted to the predetermined duty ratio is supplied from the output terminal of the pulse width modulation circuit 9 to the resistor r shown in FIG.
₁ is input to the base of the power transistor 10. The power transistor 10 includes a resistor r ₁
Is turned on when the voltage value input to the base through "H" is "H", and turned off when it is "L".

[0018] As a result, the input winding _{T I} of the step-up transformer T
The, when on of the power transistor 10, thus flowing the collector current controlled by the current flowing through the base of the power transistor 10, wherein the output winding T _O of the step-up transformer T, is boosted in accordance with a predetermined turns ratio In addition, a voltage value is induced which is proportional to the ON time of the power transistor 10, in other words, proportional to the duty ratio of the PWM signal. Thereafter, the output voltage of the output winding T _O is smoothed by the high voltage rectifying and smoothing circuit 11, an exposure device or the like constituting the image forming unit of the copying machine or the like, controls the load R is a controlled object.

[0019]

However, in the conventional DC high voltage power supply drive circuit, the power transistor 10 is driven by an external drive signal (PWM signal).
Is directly driven, that is, a separately excited oscillation circuit system is employed. Therefore, the resonance frequency and PW determined by the conditions of the step-up transformer T, the high-voltage rectifying and smoothing circuit 11 and the load R are determined.
In some cases, the frequencies of the M signals do not match, which results in parasitic vibration and noise.

Further, in a case where there arises the load fluctuation due to humidity or temperature changes, the output in the output voltage waveform of the winding T _O and spike voltage and ringing voltage large amount occurs, and its waveform is disturbed I was In particular, the spike voltage, since its size is directly added to the output voltage of the output winding T _O, the output voltage of the output winding T _O is to the duty ratio of the PWM signal, the relationship of linear (directly proportional) not to, not only the variable range of the output can duty ratio is limited, depending on the selected duty ratio, the spike voltage and ringing voltage to the supply voltage and interfering input winding T _I to the step-up transformer T Since the flowing current may be an overcurrent, load control of an object to be controlled, for example, an exposure device of a copying machine, has been unstable. Therefore, because the have a snubber circuit in parallel with the input winding T _I of the step-up transformer T in order to remove the spike voltage and the like corresponding to inserted, the device itself increases in size, the number of parts is increased It was uneconomical.

The present invention has been made in view of the above problems, and has as its object to perform unstable operation due to interference between the resonance frequency of a booster circuit including a booster transformer and a high-voltage rectifying / smoothing circuit and the frequency of a PWM signal. It is an object of the present invention to provide a small and inexpensive DC high-voltage power supply driving circuit that eliminates the use of a snubber circuit by linearly changing the output voltage of a DC high voltage with respect to a PWM signal.

[0022]

According to a first aspect of the present invention, there is provided a driving circuit for a DC high-voltage power supply which is driven by a DC power supply, has an input winding, an output winding, and a base winding, and has an output winding. A step-up transformer for outputting an AC high voltage from a line, a high-voltage rectifying / smoothing circuit comprising a rectifier and a capacitor connected to the output winding, and a power transistor for turning on / off a voltage applied to an input winding of the step-up transformer A current control means interposed between a base winding of the step-up transformer and a base of the power transistor to limit a base current of the power transistor, thereby providing a resonance frequency of a step-up circuit including the step-up transformer and a high-voltage rectifying / smoothing circuit. And a synchronous drive circuit for matching the on / off timing of the power transistor, and a control circuit for controlling the current control means. And wherein the door.

In the driving circuit for a DC high voltage power supply according to the present invention, the current control means for limiting the base current of the power transistor includes a junction type transistor, a collector between the junction type transistor and a base winding of the step-up transformer. And a resistor and a capacitor interposed therebetween.

The current limit value of the current control means corresponding to the duty ratio of the PWM signal in the driving circuit of the DC high voltage power supply according to the third aspect is a PWM generated by a serial DA conversion circuit using a primary low-pass filter including a resistor and a capacitor. It is characterized in that the saturation current of the collector current of the junction transistor is controlled by an analog signal voltage corresponding to the duty ratio of the signal to set the saturation current.

According to a fourth aspect of the present invention, there is provided a synchronous drive circuit in the DC high voltage power supply drive circuit, wherein the electromotive force generated in the base winding of the step-up transformer is supplied to the base of the power transistor via a capacitor, a resistor and a junction transistor. A self-excited oscillation circuit configured to provide positive feedback to
It can be set independently of the frequency of the WM signal.

[0026]

According to the driving circuit for a DC high voltage power supply according to the first aspect, the circuit is driven by the DC power supply, has an input winding, an output winding, and a base winding, and outputs an AC high voltage from the output winding. Step-up transformer, a high-voltage rectifying / smoothing circuit including a rectifier and a capacitor connected to the output winding, a power transistor for turning on / off a voltage applied to an input winding of the step-up transformer, and a base of the step-up transformer The base current of the power transistor is limited by current control means interposed between the winding and the base of the power transistor.
Since it is configured to include a synchronous drive circuit that matches off timing and a control circuit that controls the current control unit,
According to the duty ratio of the externally applied PWM signal, the current limit value of the current control means can be linearly varied, and as a result, the output voltage of the DC high voltage can be linearly changed, The variable range of the duty ratio of the PWM signal can be expanded, and the current flowing through the input winding can be prevented from becoming an overcurrent.

According to a second aspect of the present invention, as a current control means for limiting a base current of a power transistor, a junction type transistor, a collector of the junction type transistor and a base winding of a step-up transformer are provided. , The base current of the power transistor can be easily limited.

According to the DC high voltage power supply drive circuit of the third aspect, the current limit value of the current control means corresponding to the duty ratio of the PWM signal is determined by the serial DA conversion circuit using a primary low-pass filter including a resistor and a capacitor. The magnitude of the output voltage value of the DC high-voltage power supply, which is set by controlling the saturation value of the collector current of the junction transistor by using an analog signal voltage corresponding to the duty ratio of the generated PWM signal, It can be easily set by changing the duty ratio.

According to the drive circuit of the DC high voltage power supply of the fourth aspect, the synchronous drive circuit converts the electromotive force generated in the base winding of the step-up transformer through a capacitor, a resistor and a junction type transistor into the power supply. By using a self-excited oscillation circuit that positively feeds back to the base of the transistor, the resonance frequency of the booster circuit and the on / off timing of the power transistor match, so-called zero volt switching (ZV
S) Operation becomes possible, as a result, the occurrence of parasitic vibration and noise can be prevented, and unstable operation due to interference between the resonance frequency and the PWM signal frequency can be eliminated. Since no spike voltage is generated, a snubber circuit for removing the spike voltage is not required, and the performance can be improved and the size can be reduced, which is very economical.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which a DC high voltage power supply drive circuit of the present invention is used in a copying machine will be described below. Portions having the same structure as a conventional DC high-voltage power supply drive circuit will be described with the same reference numerals. 8 In FIG. 1, the step-up transformer T is inserted into the input winding T _I in series, shows the DC power source for supplying electric power to the step-up transformer T, 15 the resistance r ₃ which is inserted in parallel to the DC power source 8,
r ₄ , r ₅ , r ₆ and the transistor 16, and the resistor r
₆ and resistor r ₇ formed by connecting the other hand the end to the connection point x of the transistor 16, a capacitor C, wherein connected to the other end of the resistor r ₇ whereas the end, was connected to the other end to the negative electrode side of the DC power supply
A control circuit constituted by _3, 9 is a pulse width modulation circuit connected to the output terminal to the connection point of the resistor r ₃ and r ₄ of the control circuit 15.

Reference numeral 17 denotes a base winding T of the step-up transformer T.
And _B, whereas the capacitor C ₄ to connect the ends to the base winding T _B, a resistor r ₈ formed by connecting the one end to the other end of the capacitor C _4, the collector to the other end of the resistor r ₈ Reference numeral 19 denotes a synchronous drive circuit including a junction transistor 18 connected to the base.
A power transistor formed by connecting the other end of the input winding T _I of.

[0032] 20 input winding T _I, base winding T _B, is connected to the output terminal of the step-up transformer T consisting output winding T _O, capacitor C ₅ for smoothing the output voltage and current of the step-up transformer T, This is a high voltage rectifying / smoothing circuit of a voltage doubler type including C ₆ , rectifiers D ₃ and D _4, and a resistor r ₉ . Further, R is a control object to be driven and controlled by the output voltage of the high-voltage rectifying / smoothing circuit 20, that is, a load of an exposure device or the like constituting an image forming unit of a copying machine, as in the conventional example shown in FIG. .

Next, the operation of the driving circuit for the DC high voltage power supply shown in FIG. 1 will be described. First, a PWM signal having "H" and "L" voltage values adjusted to a predetermined duty ratio from the pulse width modulation circuit 9 shown in FIG. 3 is applied to the base of the transistor 16 of the control circuit 15 shown in FIG. When input, the transistor 16 turns on when the PWM signal is "H" and turns off when the PWM signal is "L". When the transistor 16 is turned on, the collector current I _C ″ flows through the collector. At this time, the potential at the connection point x between the resistors r ₆ and r ₇ has a value very close to 0 [V]. voltage across the series circuit of r ₇ and capacitor C ₃ becomes extremely small, so that the charge stored in the capacitor C ₃ is small.

When the PWM signal input to the base of the transistor 16 is "L", the transistor 1
6 is turned off, no current flows through the collector of the transistor 16 and the connection point x between the resistors r ₆ and r ₇
Potentials higher value than the 0 [V], in particular become the same voltage value and the voltage value of the DC power source 8, as a result, increases also voltage across the series circuit of the resistor r ₇ and capacitor C ₃ to, a current flowing through the resistor r ₇ is increased, as a result, the greater the amount of蓄Wae is charged in the capacitor C _3. That is, the voltage waveform of the PWM signal input from the pulse width modulation circuit 9 to the base of the transistor 16 changes from the waveform shown in FIG. 4A to the off time (t ₂ −) as shown in FIG. As t ₁ ) becomes longer, the control circuit 15
The amount of charge stored in the capacitor C ₃ also increases as shown in FIG from a small amount of that shown in (b) of FIG. 4 (d).

[0035] Then, to control the base current I _{B ',} and the collector current I _C of the junction type transistor _18' of the amount of charge stored in the capacitor C ₃ is intact junction transistor 18 the maximum value of (saturation value). That is, the control circuit 15 linearly controls the saturation value of the collector current of the junction transistor 18 according to the duty ratio of the PWM signal.

Next, the initial operation of the power transistor 19 will be described. Initially, the step-up transformer T
Since no current flows in, the electromotive force does not occur in the base winding T _B of the step-up transformer T, therefore, the power transistor 19 starts to turn on by the base current I _{B 'of} the junction transistor 18. Specifically, the PW
The base current I _B of the junction transistor 18 that flows in response to the M signal _', the base current I _{B =} I B of the power transistor 19 via an _emitter' becomes, thereby the power transistor 19 becomes slightly turned on.

[0037] By this small on-state, current I _C flows from the DC power supply 8 to the input winding T _I of the step-up transformer T,
Whereby an electromotive force is generated in the base winding T _B of the step-up transformer T. Then, through a junction transistor 18 which is between the ON state receives a base current I _{B 'as} the collector current of the electromotive force generated in the base winding T _B resistance r ₈ and junction transistor 18 The current determined by the saturation value is input to the base of the power transistor 19.

[0038] Then, the power transistor 19 is the base current I _{B (=} I _E ') is turned further on state when supplied with a power transistor 1 the state is accelerated
9 quickly reaches the on state. As a result, the collector of the power transistor 19 with a slope current I _C is determined by the inductance and the load R of the input winding T _I of the step-up transformer T, increases as shown by the described in (a) of FIG. 5 (b) I will do it. At this time, the collector-emitter voltage V _CE of the power transistor 19 descends to the vicinity of the 0 [V] as shown in (b) of FIG. 5 (b), also the voltage across the input winding T _I is It has a constant value as shown in (c) shown in (c) of FIG. Here, the base current I _B of the power transistor 19, junction transistor is controlled linearly with respect to the duty ratio of the PWM signal 18
Since the determined by the saturation value of the collector current I _C, eventually the power transistor 19 enters the saturation operating region, the collector current I _C will not increase. That is, I _C = I _B × hf
e: When becomes (hfe power current amplification factor of the transistor) Collector current I _C will not change.

[0039] Then, the voltage value of the base winding T _B to be excited by a current I _C flowing through the input winding T _I is to abruptly falls as shown by FIG. 5 (d) (d), accordingly Te, the collector current I _{C 'also} decreases the junction transistor 18, thereby gradually be reduced base current I _B of the power transistor 19. When the base current I _B of the power transistor 19 is decreased, of course the power transistor 1
Since also decreases the saturation value of the collector current I _C of 9, the collector current I _C of the power transistor 19 will approach zero as shown in (a) of FIG. 5 (e), as a result, the input winding voltage across T _I is also shown in FIG. 5 (c) is lowered as the (f).

[0040] As a result, further the voltage across the base winding T _B Iki rapidly lowered, the collector current I _C of the power transistor 19 is reduced, by subsequently repeating the operation, finally, the base take voltage across the line T _B will take a negative value. Then, the base current I _B which has been supplied to the base of the junction transistor 18 from the control circuit 15 _'is drawn through the collector from the base of the junction transistor 18 by the negative voltage generated at the base winding T _B Therefore, the junction type transistor 18
Emitter is no longer the current flows, the base current I _B of the power transistor 19 becomes zero, the power transistor 19 rapidly turned off. That is,
The off-timing of the power transistor 19 is initiated when the value of the current I _C which flows through the input winding T _I of the step-up transformer T has reached the collector current saturation value of the power transistor 19, quickly reaching the OFF state.

[0041] The off-state, stored in the self-inductance L of the step-up transformer T off just before the electromagnetic energy ^{(LI 2/2) (I} : current _{I C} flowing through the input winding _{T I} effective current) to the discharge of with a result of the negative electromotive force is generated in the base winding T _B of the step-up transformer T is reduced, the resistance r
For a negative _{current 8} through the flow into the base from the collector of the junction type transistor 18, the PWM signal to be increased current I _{B 'to} flow into the base of the junction transistor 18 from resistor r ₇ in response, positive polarity base current I _B is maintained until begins to flow to the base of the power transistor 19.

[0042] When the base of the power transistor 19 begins to flow positive polarity base current I _B, becomes the same state and the on state of the power transistor 19 during the above-mentioned initial power transistor repeats ON / OFF operation or less, the self Excitation oscillation operation is performed.

[0043] The law of the series of operations transformer ampere turns in the following equation, the collector current I _C of the junction transistor 18 which is controlled base current I _B of the power transistor 19 linearly with respect to the duty ratio of the PWM signal 'By controlling the saturation value of
This is to control the maximum value of the current I _O flowing through the output winding T _O.

[0044]

(Equation 2)

Here, N _O : the number of turns of the output winding T _O I _O : the current flowing through the output winding T _O N _I : the number of turns of the input winding T _I , respectively.

Therefore, the drive circuit for the DC high voltage power supply of the present invention shown in FIG. 1 controls the output voltage value of the DC high voltage power supply according to the duty ratio of the external PWM signal.
Also, the ON / OFF timing of the power transistor is
By configuring the self-oscillation circuit to determine the electromotive force generated in the base winding by positive feedback to the base of the power transistor, the output voltage value linearly follows the duty ratio of the PWM signal and In addition, unstable operation due to interference between the resonance frequency of the booster circuit and the PWM signal frequency can be eliminated.

[0047]

As described above, the present invention has the following effects. According to the driving circuit for a DC high-voltage power supply according to claim 1, the resonance frequency of the boosting circuit including a high-voltage rectifying and smoothing circuit including a boosting transformer, a rectifier, and a capacitor, and the voltage applied to the input winding are turned on / off. The current control means for matching the on / off timing of the power transistor to be turned off and controlling the base current of the power transistor is provided with an external P
By controlling by a control circuit driven by the WM signal, the current limit value of the current control element is linearly varied according to the duty ratio of the PWM signal, and the output voltage of the DC high voltage is linearly changed. Can be
Thereby, the variable range of the duty ratio of the PWM signal can be expanded, and the current value flowing through the input winding does not become an overcurrent.

According to the drive circuit of the DC high voltage power supply of the present invention, the current control means is interposed between the junction type transistor and the collector of the junction type transistor and the base winding of the step-up transformer. Since the power transistor is constituted by the resistor and the capacitor, the base current of the power transistor can be easily limited.

According to the drive circuit of the DC high voltage power supply of the third aspect, the current limit value of the current control means corresponding to the duty ratio of the PWM signal is determined by the serial DA conversion circuit using the primary low-pass filter including the resistor and the capacitor. The magnitude of the output voltage value of the DC high-voltage power supply, which is set by controlling the saturation value of the collector current of the junction transistor by using an analog signal voltage corresponding to the duty ratio of the generated PWM signal, It can be easily set by changing the duty ratio.

According to the drive circuit of the DC high voltage power supply of the fourth aspect, the synchronous drive circuit converts the electromotive force generated in the base winding of the step-up transformer through the capacitor, the resistor and the junction type transistor into the power supply. By using a self-excited oscillation circuit that positively feeds back to the base of the transistor, the resonance frequency of the booster circuit and the on / off timing of the power transistor match, so-called zero volt switching (ZV
S) Operation becomes possible, as a result, the occurrence of parasitic vibration and noise can be prevented, and unstable operation due to interference between the resonance frequency and the PWM signal frequency can be eliminated. Since no spike voltage is generated, a snubber circuit for removing the spike voltage is not required, and the performance can be improved and the size can be reduced, which is very economical.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a drive circuit for a DC high-voltage power supply according to the present invention.

FIG. 2 is a configuration diagram schematically showing an image forming unit such as a copying machine;

FIG. 3 is a diagram illustrating a pulse width modulation circuit.

FIG. 4 is a waveform diagram showing the relationship between the on / off timing of a PWM signal and the amount of charge stored in a capacitor.

5 is a waveform control diagram across both ends of the base winding T _B of the collector current and the collector-emitter voltage and the input winding T _I of the power transistor.

FIG. 6 is a circuit diagram showing an example of a conventional driving circuit for a DC high-voltage power supply.

[Explanation of symbols]

 Reference Signs List 8 DC power supply 9 Pulse width modulation circuit 10, 19 Power transistor 15 Control circuit 17 Synchronous drive circuit 18 Junction transistor 11, 20 High voltage rectification smoothing circuit T Step-up transformer R Load

Claims (4)

[Claims] 1. A step-up switching DC high voltage power supply driving circuit which is driven by a DC power supply and outputs a DC high voltage from its output, comprising an input winding, an output winding and a base winding, A step-up transformer that outputs an AC high voltage from a winding; a high-voltage rectifying and smoothing circuit that includes a rectifier and a capacitor connected to the output winding; and a power that turns on / off a voltage applied to an input winding of the step-up transformer. A booster circuit comprising a transistor, a current control means inserted between a base winding of the booster transformer and a base of the power transistor to limit a base current of the power transistor, and comprising the booster transformer and a high-voltage rectifying / smoothing circuit; A synchronous drive circuit configured to match the resonance frequency of the power transistor with the on / off timing of the power transistor; A control circuit for controlling the control means, wherein the control circuit linearly varies a current limit value of the current control means in accordance with a duty ratio of a PWM signal supplied from the outside, so that an output voltage of the DC high voltage is changed. A drive circuit for a DC high-voltage power supply, wherein the drive circuit is configured to change linearly. 2. The current control means for limiting a base current of the power transistor includes a junction transistor, and a resistor and a capacitor inserted between a collector of the junction transistor and a base winding of a step-up transformer. 2. The driving circuit for a DC high-voltage power supply according to claim 1, wherein: 3. A current limit value of a current control means corresponding to a duty ratio of the PWM signal is an analog signal voltage corresponding to a duty ratio of a PWM signal generated by a serial D / A conversion circuit using a primary low-pass filter including a resistor and a capacitor. 3. The driving circuit for a DC high-voltage power supply according to claim 1, wherein the setting is made by controlling a saturation value of a collector current of the junction transistor. 4. The self-excited synchronous drive circuit is configured to positively feedback an electromotive force generated in a base winding of the step-up transformer to a base of the power transistor via a capacitor, a resistor, and a junction transistor. 4. The driving circuit for a DC high-voltage power supply according to claim 1, wherein the driving circuit comprises an oscillation circuit and can be set independently of the frequency of the PWM signal.