JPH04308458A - power supply - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply device such as an AC power supply for developing bias in electrophotographic printers and copying machines.

0002

2. Description of the Related Art Conventionally, an AC power source for developing bias often uses a low-frequency step-up transformer. However, in recent years, the duty has been changed to 3:7 or 4: rather than square waves.
It has been found that a rectangular wave with a biased duty of 6 has better developing characteristics. Therefore, in order to avoid biased magnetism in the transformer and downsize the transformer, the applicant proposed a method to obtain a rectangular wave of arbitrary duty by charging the load capacitance with a high frequency converter and discharging it with a high voltage transistor. , has been put into practical use.

0003

[Problems to be Solved by the Invention] Conventionally, a high-frequency DC-DC converter is used to charge a load capacitance, and a high-voltage switching element is used to discharge the load capacitance. Obtains high voltage pulse output. The most important component in this system is the switching element, which requires high withstand voltage, large current, and high-speed switching characteristics. At present, high-voltage transistors are the optimal element, but the allowable value of collector current in the high-voltage region is severely limited, and in order to obtain the desired fall speed, expensive high-voltage transistors must be connected in parallel. It is necessary to use multiple pieces. Furthermore, considering the variation in hfe of the transistor, it is necessary to take a sufficient margin from the maximum allowable range of the collector current, and the ability of the transistor cannot be fully utilized.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a power supply device for a capacitive load that can obtain an output that falls at a high speed without using a large number of high-voltage switching elements. This is the purpose.

[0005]

[Means for Solving the Problems] In order to achieve the above object, the present invention controls the current of a high voltage switching element,
In addition, the switching period is time-divided, and immediately after the start of discharge,
In other words, when the collector voltage is high, the collector current is decreased, and as the load capacitance discharges and the collector voltage decreases, the collector current is increased to increase the falling speed. (1
), (2).

(1) A converter transformer, a rectifier that rectifies the output of the converter transformer and supplies it to an output terminal to which a capacitive load is connected, a switching element connected between the output terminal and ground, and the switching element. and control means for controlling the current so that the current gradually increases as the voltage between the output terminal and the ground decreases.

(2) The power supply device according to (1) above, wherein the control means performs control according to data stored in advance in the storage means.

(3) The method according to (1) above, wherein voltage detection means for detecting the voltage applied to the switching element is provided, and the control means controls the switching element in accordance with the output of the voltage detection means. power supply.

[0009]

[Operation] With the configurations (1) and (2) above, the current of the switching element increases as the voltage between the output terminal and the ground decreases, so the margin for the maximum allowable current of the switching element can be reduced. .

0010

[Examples] The present invention will be explained in detail below using examples. (Embodiment 1) FIG. 1 is a block diagram of a "developing bias power source" according to Embodiment 1, and FIG. 2 is a voltage waveform diagram of each part thereof. In Figure 1, 1 is a microcomputer (hereinafter referred to as CP).
It also includes peripheral devices such as memory (storage means). The CPU 1 is connected to a sequence controller (not shown) of a main body of a printer (copying machine) through a bus line or a communication port, and exchanges sequence signals. The first counter 6 gradually decrements the reference clock of the CPU 1 to a repetition frequency f1 (100 KHz). The second counter 7 further calculates the repetition frequency f2 (2KHz
) to generate a low frequency signal with a duty of 7:3.

A switching element Q1 on the primary side of the high frequency converter transformer T1 is controlled by a PWM circuit 2. The output of the PWM circuit 2 has a repetition frequency f1, and its pulse width is controlled by the CPU 1.

After being rectified by a rectifier D1, the boosted output from the secondary side of the converter transformer T1 charges the load capacitor connected to the output terminal P1, as shown in FIG. 2(C).

The output of the output terminal P1 is divided into voltages at a predetermined ratio by the voltage detection circuit 4 and input to the comparator 3. The comparator 3 compares the output of the voltage detection circuit 4 and the output of the reference voltage generation circuit 5 and outputs the comparison result to the CPU 1. When the output of output terminal P1 reaches a predetermined level, PWM
The output of circuit 2 is stopped. When the output of the PWM circuit 2 stops and the output of the output terminal P1 becomes below the threshold value due to discharge, the output of the comparator 3 is inverted and the PWM circuit 2 starts outputting again.

The high voltage transistor Q2 is shown in (c) in FIG.
, (d), and (e), they are turned on and off in synchronization with the output of the second counter 7. High voltage transistor Q
When 2 is on, the output of the PWM circuit 2 is stopped. When the high voltage transistor Q2 is turned on, the D/A converter 8 changes the current of the high voltage transistor Q2 to increase as the voltage between the output terminal 1 and the ground decreases according to the programming of the CPU 1, as shown in (d). let That is, when the output voltage of the D/A converter 8 is Vd, the peak value Ip of the collector current of the high-voltage transistor Q2 is approximately suppressed to Ip=Vd/R4, and the margin for the maximum allowable current can be reduced. Therefore, as shown in (c), the output that was obtained in the conventional example by connecting two or three high-voltage transistors in parallel can be obtained with a single transistor with a faster fall rate. .

[0015] Frequencies f1, f2, output duty, PW
The pulse width of the output of the M circuit 2, the control sequence of the D/A converter 8, etc. are determined based on data stored in the memory in the CPU 1 in advance. Note that the above-mentioned determination can also be partially performed by hardware.

(Embodiment 2) FIG. 3 is a block diagram of Embodiment 2, and FIG. 4 is an explanatory diagram of the same embodiment. In this embodiment, the reference voltage in the first embodiment is controlled by the CPU 1. The output of the CPU 1 is converted into analog by the D/A converter 21 and inputted to the comparator 3 and the base of the high voltage transistor Q2.

By controlling the reference voltage with the CPU 1,
As shown in FIG. 4, it becomes possible to easily switch the threshold value of the comparator 3.

The PWM circuit 2 is constructed by providing a plurality of threshold values in FIG.
An example of controlling the output pulse width of is shown below. When the output of the output terminal P1 reaches the threshold value B in (a) of FIG. 4, the pulse width of the PWM circuit 2 is shortened by a predetermined amount. Therefore, the charging curve of the load capacity becomes gentle. When the threshold value A is reached, the PWM output is stopped. In (b), three threshold values are provided and the output pulse width of the PWM circuit 2 is switched to three levels including zero. The D/A output to the base of the high voltage transistor Q2 is controlled in the same manner as shown in FIG.

(Embodiment 3) FIG. 5 is an explanatory diagram of Embodiment 3. The circuit configuration of this embodiment is exactly the same as that in FIG.

In this embodiment, the collector current of the high voltage transistor Q2 is controlled in accordance with the output voltage. When the high voltage transistor Q2 turns on, the threshold value of the comparator 3 is switched to level C as shown in (a). When the output of the voltage detection circuit 4 passes level C, the threshold value of the comparator 3 is switched to D and the D/A output to the high voltage transistor Q2 is increased by one step. Similarly, when the voltage output exceeds the D level, the threshold value is switched to F and the D/A output to Q2 is further increased by one step. When the voltage output exceeds the F level, the threshold value is switched to 0, and the output of the D/A converter 21 to Q2 is switched to the maximum level. The switching element Q1 on the primary side of the high frequency converter transformer T1 is
As shown in FIGS. 5A and 5B, control is performed in the same manner as in the second embodiment.

[0021] The above embodiments are explained in the following a, b, c.
It can be transformed as shown.

a. In Examples 1 to 3, a load current detection circuit is provided on the secondary side of the converter transformer, and its detection output is compared with a second reference voltage and a second comparator to determine the PWM for converter primary side control. Limit or stop the output of circuit 2.

b. In the above a, instead of using the second reference voltage or the second comparator, the voltage detection output and the current detection output are switched in a time division manner using an analog switch and inputted to the comparator 3, and the data of the reference voltage is also switched at the same time. It is input to the D/A converter 21.

c. In a, the comparator 3 compares the load current detection output with the threshold value during the ON timing of the switching element Q1, and compares the voltage detection output with the comparator 3 at other timings.

[0025]

[Effects of the Invention] As explained above, according to the present invention,
The current of the switching element that shorts the output terminal is connected to the output terminal -
Since the voltage increases as the ground-to-ground voltage decreases, the margin for the maximum allowable current of the switching element can be reduced, there is no need to use a large number of switching elements, and an output that falls at high speed can be obtained.

According to the third aspect of the invention, since the voltage applied to the switching element is detected and the current of the switching element is controlled, the margin can be secured more accurately.

[Brief explanation of drawings]

[Figure 1] Block diagram of Example 1

[Figure 2] Waveform diagram of each part of Example 1

[Figure 3] Block diagram of Example 2

[Figure 4] Explanatory diagram of Example 2

[Figure 5] Explanatory diagram of Example 3

[Explanation of symbols] 1 Microcomputer D1 Rectifier Q2 High voltage transistor

Claims (3)

[Claims] 1. A converter transformer, a rectifier that rectifies the output of the converter transformer and supplies it to an output end to which a capacitive load is connected, a switching element connected between the output end and ground, and a switching element connected to the output end. 1. A power supply device comprising: a control means for controlling the current to gradually increase as the voltage between the output terminal and the ground decreases. 2. The power supply device according to claim 1, wherein the control means performs control according to data stored in advance in the storage means. 3. A voltage detection means for detecting a voltage applied to a switching element is provided, and a control means controls the switching element according to an output of the voltage detection means. 1. The power supply device according to 1.