JP2002101666A - Ac-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To miniaturize the choke coil of an AC-DC converter. SOLUTION: Current in the choke coil is limited to prescribed value or less, the load on the choke coil is relieved, and the choke coil is miniaturized and made lightweight.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an AC power supply, and more particularly to an AC-DC converter for converting AC of a commercial AC power supply to DC.

[0002]

2. Description of the Related Art AC-DC converters for obtaining DC from a commercial AC power supply, such as a switching power supply, are used in power supply units of various electronic devices. FIG. 4 is a circuit diagram of an AC-DC converter widely used conventionally. The AC input from the AC power supply 1 is full-wave rectified by the full-wave rectifier circuit 2. The DC current output from the rectifier circuit 2 passes through the choke coil 4 and the diode 7 to charge the capacitor 8 having a sufficiently large capacitance and to be supplied to the load 10. A switch 5 and a resistor 6 of a semiconductor element are provided between a connection point of the choke coil 4 and the diode 7 and the circuit ground Gr.
Are connected in series. The resistor 6 functions as a current detecting element for detecting a current flowing through the switch 5. The gate G of the switch 5 is a control signal output terminal 21 of the control circuit 21.
A, and ON / OFF control of the switch 5 is performed by a control signal. The control signal is higher than the commercial AC frequency,
For example, it is a signal of 50 kHz to 500 kHz.

A control circuit 21 has a reference voltage source 11 and applies a DC reference voltage to one input terminal of an amplifier 12.
A divided voltage obtained by dividing the terminal voltage of the capacitor 8 (the output voltage of the AC-DC converter) by the second voltage divider 9 is applied to the other input terminal of the amplifier 12. The amplifier 12 amplifies and outputs a difference voltage between the reference voltage and the divided voltage. The output voltage of the amplifier 12 becomes a positive predetermined voltage when the divided voltage is equal to the reference voltage. When the divided voltage becomes lower than the reference voltage, the output voltage of the amplifier 12 becomes higher than the predetermined voltage. Conversely, when the divided voltage becomes higher than the reference voltage, the amplifier 12
Is lower than the predetermined voltage. Amplifier 12
Is input to one input terminal of the multiplier 13.
A divided voltage obtained by dividing the output voltage of the rectifier circuit 2 by the first voltage divider 3 is applied to the other input terminal of the multiplier 13, and a voltage obtained by multiplying the output voltage of the amplifier 12 by the divided voltage is output. You. The output voltage of the voltage divider 3 is a pulsating current having the same cycle as the rectified voltage, and the output voltage of the multiplier 13 is also a pulsating current. The output voltage of the multiplier 13 is applied to one input terminal of the comparator 14. The terminal voltage of the resistor 6 is applied to the other input terminal of the comparator 14. The comparator 14 compares the two input voltages and inputs a comparison output to the drive control circuit 15. The drive control circuit 15 has an oscillator for a high frequency signal of 50 kHz to 500 kHz, and applies a high frequency signal to the gate G of the switch 5 according to the output of the comparator 14.

Next, the operation of the conventional AC-DC converter will be described. When switch 5 is on, rectifier circuit 2
Then, a current flows through the choke coil 4 and the switch 5 to the circuit ground Gr, and the choke coil 4 is excited and the excitation energy is accumulated. The current flowing through the switch 5 is detected by the resistor 6, and the terminal voltage of the resistor 6, that is, the detected voltage is compared with the output voltage of the multiplier 13 by the comparator 14. When the detection voltage reaches the output voltage of the multiplier 13, the output of the comparator 14 is inverted, and the drive control circuit 15 turns off the switch 5 based on the output voltage of the comparator 14. When the switch 5 is turned off, the excitation energy of the choke coil 4 flows into the capacitor 8 via the diode 7 and is discharged.

In the above operation, the current flowing through the switch 5 is proportional to the output voltage of the rectifier circuit 2, and its magnitude becomes smaller than a predetermined value when the divided voltage of the voltage divider 9 is larger than the reference voltage, and becomes smaller than the reference voltage. If it is smaller, it is controlled to be larger than a predetermined value. The output voltage of the AC-DC converter is stabilized by the above operation. The current of the choke coil 4 has a high-frequency waveform having an envelope similar to the output voltage waveform of the rectifier circuit 2 as shown in FIG. Generally, since a filter for preventing noise is provided between the AC power supply 1 and the rectifier circuit 2, the high-frequency current waveform shown in FIG. 5 is smoothed, and the input voltage waveform and the input current waveform of the AC-DC converter are smoothed. Have the same period waveform.

[0006]

When a current exceeding the maximum current determined at the time of design is passed through the choke coil,
Causes magnetic saturation. Therefore, the size of the choke coil, that is, the cross-sectional area of the iron core and the number of turns of the coil are determined in consideration of the required maximum current. In the conventional AC-DC converter, a large current flows through the choke coil 4 only during a short period before and after the output voltage of the rectifier circuit 2 reaches a maximum value. The specifications of the choke coil, such as the cross-sectional area of the iron core of the choke coil, must be determined based on the current value of the choke coil 4 when the output voltage of the rectifier circuit 2 is maximized. The choke coil is the largest and heaviest component among the components of the AC-DC converter,
If this is reduced in size and weight, the size and weight of the AC-DC converter can be reduced. However, since the above-described specification is determined, the size and weight of the choke coil cannot be reduced. An object of the present invention is to reduce the size and weight of a choke coil used for an AC-DC converter.

[0007]

An AC-DC converter according to the present invention is connected to an AC power supply and rectifies an AC input, a first voltage divider connected to an output terminal of the rectifier circuit, A voltage limiter connected to a voltage dividing point of the first voltage divider to limit a divided voltage to a predetermined value or less; a choke coil having one end connected to an output terminal of the rectifier circuit; the other end of the choke coil and a circuit ground A switch connected between the other end of the choke coil and the circuit ground, a capacitor connected via a diode, and a second component connected between both terminals of the capacitor. A voltage divider, a first divided voltage of the first voltage divider, and a second divided voltage of the second voltage divider;
And a control circuit for controlling on / off of the switch means so as to limit the current flowing through the choke coil to a predetermined range based on the second divided voltage. According to the above configuration, by limiting the divided voltage applied to the control circuit by the first voltage divider to the predetermined range by the voltage limiting unit, the control circuit can control the current of the choke coil to the predetermined range. On / off of the switch unit is controlled so as to be restricted.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a circuit diagram of an AC-DC converter according to the present invention. The circuit of FIG. 1 is the same as the circuit of FIG. 4 except that a diode 16 serving as a voltage limiter is connected between the voltage dividing point of the first voltage divider 3 and a circuit ground Gr. In FIG. 1, the same components as those in FIG. 4 of the conventional example are denoted by the same reference numerals. The AC input from the AC power supply 1 is full-wave rectified by the full-wave rectifier circuit 2. The DC output current of the rectifier circuit 2 passes through the choke coil 24 and the diode 7 to charge the capacitor 8 having a sufficiently large capacitance and to be supplied to the load 10. A series connection of a switch 5 and a resistor 6 of a semiconductor element is connected between a connection point of the choke coil 24 and the diode 7 and the circuit ground Gr. The resistor 6 functions as a current detecting element for detecting a current flowing through the switch 5. Gate G of switch 5
Is connected to a control signal output terminal 21A of the control circuit 21,
On / off control of the switch 5 is performed by the control signal.
The control signal is higher than the frequency of the commercial AC, for example, 50 kHz.
This is a signal of z to 500 kHz.

The control circuit 21 has a reference voltage source 11 and applies a DC reference voltage to one input terminal of the amplifier 12.
A voltage applied to the load 10, which is the output voltage of the AC-DC converter, that is, a divided voltage obtained by dividing the terminal voltage of the capacitor 8 by the second voltage divider 9 is applied to the other input terminal of the amplifier 12. . The amplifier 12 amplifies and outputs a difference voltage between the reference voltage and the divided voltage. The amplifier 12 has a predetermined voltage when the divided voltage is higher than the reference voltage. When the divided voltage becomes lower than the reference voltage, the output voltage of the amplifier 12 becomes higher than the predetermined voltage. Conversely, when the divided voltage becomes higher than the reference voltage, the output voltage of the amplifier 12 becomes lower than the predetermined voltage. The output voltage of the amplifier 12 is input to one input terminal of the multiplier 13. A divided voltage obtained by dividing the output voltage of the rectifier circuit 2 by the first voltage divider 3 is applied to the other input terminal of the multiplier 13, and a voltage obtained by multiplying the output voltage of the amplifier 12 by the divided voltage is output. You. The output voltage of the voltage divider 3 is a pulsating current having the same cycle as the rectified voltage, and the output voltage of the multiplier 13 is also a pulsating current. The output voltage of the multiplier 13 is applied to one input terminal of the comparator 14. The terminal voltage of the resistor 6 is applied to the other input terminal of the comparator 14. The comparator 14 compares the two input voltages and inputs a comparison output to the drive control circuit 15. The drive control circuit 15 is 50 kHz to 500 k
It has an oscillator for a high frequency signal of 1 Hz, and applies a high frequency signal to the gate G of the switch 5 according to the output of the comparator 14.
The voltage limiter 16 is connected between the output terminal 3A of the voltage divider 3 and the ground Gr.

The AC-D of the present invention configured as described above
The operation of the C converter will be described below. The AC input from the AC power supply 1 is full-wave rectified by the rectifier circuit 2. The DC output of the rectifier circuit 2 is applied to the diode 7 and the switch 5 via the choke coil 24. Switch 5
Is repeatedly turned on and off at a predetermined frequency between 50 kHz and 500 kHz sufficiently higher than the AC frequency by the drive control circuit 15 of the control circuit 20. When the switch 5 is turned on, the full-wave rectified DC current is supplied to the choke coil 2.
4 flows through the path of the switch 5, the resistor 6, and the ground Gr. This current excites the choke coil 24 and stores the excitation energy. When the switch 5 is turned off, the excitation energy stored in the choke coil 24 becomes a current flowing through the diode 7 and charges the capacitor 8.

A voltage divider 3 for dividing the output voltage of the rectifier circuit 2
Functions as an input voltage detector. The forward diode 16 as a voltage limiting unit limits the voltage of the output terminal 3A that changes according to the output voltage of the rectifier circuit 2 to a range lower than the rising voltage of the diode 16. A Zener diode may be used instead of the diode 16. The voltage at the output terminal 3A of the voltage divider 3 is applied to the multiplier 13. As shown in FIG. 2, the output voltage of the output terminal 3A is limited by a voltage limiting unit and is kept at a fixed voltage V or lower. Capacitor 8
, Ie, the output voltage of the AC-DC converter, is divided by the voltage divider 9, and the divided voltage is applied to the amplifier 12. The amplifier 12 amplifies and outputs the difference voltage between the reference voltage of the reference voltage source 11 and the divided voltage of the voltage divider 9. Since the capacitance of the capacitor 8 is sufficiently large, the divided voltage is DC. Therefore, the output voltage of the amplifier 12 also becomes DC. When the output of the AC-DC converter (voltage applied to the load 10) increases, the output voltage of the amplifier 12 decreases, and when the output of the AC-DC converter decreases, the output voltage of the amplifier 12 increases. The output voltage of the amplifier 12 is applied to one input terminal of the multiplier 13. The multiplier 13 multiplies the output voltage of the voltage divider 3 by the output voltage of the amplifier 12 and outputs the result. The change in the output voltage of the multiplier 13 is almost the same as the change in the output voltage of the voltage divider 3 shown in FIG. The output voltage of the multiplier 13 decreases as the output voltage of the AC-DC converter increases, and increases as the output voltage of the AC-DC converter decreases. The voltage generated in the resistor 6 by the current flowing through the switch 5 is compared in the comparator 14 with the output voltage of the multiplier 13. The output of the comparator 14 is inverted when the voltage of the resistor 6 reaches the output voltage of the multiplier 13, and turns off the switch 5 via the drive circuit 15.

By the operation described above, the current flowing through the choke coil 24 decreases when the output voltage of the AC-DC converter (the voltage applied to the load 10) increases, and when the output voltage decreases, the choke coil 24 decreases.
The current flowing through becomes larger. This negative feedback action causes AC
-The power supplied to the DC converter is controlled to a substantially constant value.
The current flowing through the switch 5 has a substantially trapezoidal waveform that does not exceed a certain current value I as shown in FIG. Accordingly, the current flowing through the choke coil 24 has a substantially trapezoidal waveform shown in FIG. 3 regardless of the sine wave input voltage of the AC power supply. Therefore, the maximum value of the current flowing through the choke coil 24 is suppressed.
In the specification of the choke coil 24 such as the cross-sectional area of the iron core,
The maximum current may be designed as the current I in FIG. 3, and the cross-sectional area of the iron core, the number of turns of the coil, and the like can be reduced as compared with the conventional one that handles the same power.

[0013]

As described in detail in the above embodiment,
According to the present invention, the current flowing through the choke coil is limited by limiting the input voltage of the multiplier by the voltage limiting unit. Therefore, the cross-sectional area of the iron core of the choke coil can be reduced as compared with that of the conventional AC-DC converter that handles the same power, and the size and weight of the AC-DC converter can be reduced and the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an AC-DC converter according to an embodiment of the present invention.

FIG. 2 is a waveform diagram of an output voltage of a first voltage divider in the AC-DC converter of the present invention.

FIG. 3 is a waveform diagram of a current of a choke coil in the AC-DC converter of the present invention.

FIG. 4 is a circuit diagram of a conventional AC-DC converter.

FIG. 5 is a waveform diagram of a current of a choke coil in a conventional AC-DC converter.

[Explanation of symbols]

 Reference Signs List 1 AC power supply 2 Rectifier circuit 3 First voltage divider 4, 24 Choke coil 5 Switch 6 Resistance 7 Diode 8 Capacitor 9 Second voltage divider 10 Load 11 Reference voltage source 12 Amplifier 13 Multiplier 14 Comparator 15 Drive control circuit 16 Voltage limiter 20 Adder 21 Control circuit Gr Circuit ground

Claims (3)

[Claims] A rectifier circuit connected to an AC power supply for rectifying an AC input; a first voltage divider connected to an output terminal of the rectifier circuit; a voltage divider connected to a voltage dividing point of the first voltage divider; A voltage limiter for limiting the voltage to a predetermined value or less; a choke coil having one end connected to the output terminal of the rectifier circuit; and a resistor connected between the other end of the choke coil and circuit ground for detecting a current. A switch unit, a capacitor connected between the other end of the choke coil and circuit ground via a diode, a second voltage divider connected between both terminals of the capacitor,
And a first divided voltage of the first voltage divider and a second divided voltage of the second voltage divider are input, and the first and second divided voltages are input.
An AC-DC converter having a control circuit for controlling the on / off of the switch means so as to limit the current flowing through the choke coil to a predetermined range based on the divided voltage of the AC-DC converter. 2. The AC-DC converter according to claim 1, wherein said voltage limiting section is a diode. 3. The AC-DC converter according to claim 1, wherein said voltage limiter is a Zener diode.