JP2000197351A - Power supply having improved power factor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power supply having improved power factor in which element having low current capacity can be employed by detecting the peak level of a current flowing through a switching FET and turning the switching FET off when the peak current reaches a specified level thereby limiting the peak level of a current flowing through a choke coil, the switching FET and a commutation diode. SOLUTION: A current detecting circuit 10 detects a drain current flowing through a switching FET Q1. When a current higher than a specified level flows through the switching FET Q1, output of a comparator 8 goes low and the switching FET Q1 is turned off. Consequently, a current higher than a specified level does not flow through the switching FET Q1 and the peak level of current is limited in the vicinity of peak level of commercial AC input power supply voltage. Since the current flowing through a choke coil L1, the switching FET Q1 and a commutation diode D1 can be limited below a specified peak level, saturation current of the choke coil L1 can be lowered.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a constant voltage power supply for producing a stable DC voltage from a commercial AC power supply, and more particularly to a power factor improving power supply using an active filter comprising a rectifying and smoothing circuit and a step-down chopper circuit. It is.

[0002]

2. Description of the Related Art In a rectifying / smoothing circuit of a switching power supply, an input current flows only during a charging period of a smoothing capacitor, that is, a conduction angle period, and the input current has a peak value several times as large as an effective value. The resulting pulsed current flows, and the power factor remains at about 0.6. The harmonic noise generated when the power factor is reduced has recently become a problem.

[0003] Since a pulse-like current waveform contains many harmonic components, when a device having a power supply device employing a rectifier circuit of a capacitor input type is connected to the same AC line, the greater the number thereof, the higher the power The line waveform may be distorted, resulting in malfunction of other equipment, or the peak of the AC voltage waveform may become trapezoidal due to the peak current and the line impedance, and still need to be used to supply constant power. Problems such as an increase in AC line current and an increasingly sharp pulse waveform, which further increases a voltage drop and causes a vicious cycle, have occurred.

In recent years, various methods have been proposed to solve these problems of power factor and harmonic noise. Among them, high-speed switching of active elements such as transistors and FETs has been proposed to reduce the filter effect. Active filters that improve and, at the same time, achieve miniaturization are attracting attention.

FIG. 4 shows the configuration of a conventional power factor improving type power supply device using a general step-down chopper type active filter, and its configuration will be described. Reference numerals 1 and 2 denote AC power supply input terminals, and reference numerals 3 and 4 denote output terminals from which a predetermined DC voltage is output. Although not shown, the output terminals 3 and 4 are connected to a load such as a DC / DC converter.

[0006] DB1 indicates a diode bridge for full-wave rectification of the AC power supply voltage, Q1 indicates a switching FET, and D1 indicates a commutation diode. L1 denotes a choke coil, and its inductance value is selected according to the power required by the load connected between the output terminals 3 and 4. In the description, it is assumed that the inductance value is set such that the current flowing through the choke coil L1 is always discontinuous in the range of power required by the load connected between the output terminals 3 and 4.

C1 denotes an electrolytic capacitor having a relatively large capacity, reference numeral 5 denotes a filter for cutting off the switching component of the current flowing through the switching FET Q1 from the AC part, and reference numeral 6 denotes a filter between both ends of the output terminals 3 and 4. Reference numeral 7 denotes an oscillator, and reference numeral 8 denotes an output of the oscillator 7. The output of the oscillator 7 is compared with the output of the output voltage detection circuit 6. Reference numeral 9 denotes a comparator which outputs a high level only during a period when the output is large. When the output of the comparator 8 is at a high level, the switching FET Q1 is turned on. When the output of the comparator 8 is at a low level, the switching FET Q1 is turned off. 2 shows a driving circuit for generating a driving pulse for the following.

Next, the operation of the circuit of the power supply device shown in FIG. 4 will be described. A commercial AC power is supplied from the input terminals 1 and 2, and an output voltage detection circuit 6 is
When power is supplied to the oscillator 7, the comparator 8, and the drive circuit 9, the output voltage detection circuit 6 detects a potential difference between the output terminals 3 and 4, and outputs an output voltage detection signal proportional to the potential difference. An output voltage detection signal that is an output of the output voltage detection circuit 6 is input to a negative input terminal of the comparator 8. On the other hand, the oscillator 7
Is input to the + input terminal of the comparator 8. The comparator 8 compares the output voltage detection signal with the triangular wave. The output of the comparator 8 becomes high only during a period in which the level of the triangular wave is higher than the level of the output voltage detection signal. Switching FET Q
1 as a result, a current flows through the circuit via the electrolytic capacitor C1, the choke coil L1, and the switching FET Q1. Then, when the output voltage detection signal and the level of the triangular wave are inverted, the comparator 8 outputs a low level, and the drive circuit 9 turns off the switching FET Q1 based on the low level.

That is, when the output voltage between the output terminals 3 and 4 decreases, the level of the output voltage detection signal output from the output voltage detection circuit 6 decreases, and as a result, the duty of the output of the comparator 8 increases and the output increases. The voltage rises. Conversely,
When the output voltage increases, the level of the output voltage detection signal output from the output voltage detection circuit 6 increases, the duty of the output of the comparator 8 decreases, and the output voltage decreases.

As a result, the on-duty of the switching FET Q1 is controlled so that the potential difference between the output terminals 3 and 4 is always kept constant.

The ON period of the switching FET Q1 is as follows:
A current flows through the switching FET Q1 via the electrolytic capacitor C1 and the choke coil L1, energy is stored in the choke coil L1, and energy is also supplied to a load such as a DC / DC converter connected to the output terminals 3, 4. .

The energy stored in the choke coil during the OFF period of the switching FET Q1 is applied to the DC / D connected to the output terminals 3 and 4 via the diode D1.
Energy is also supplied to a load such as a C converter.

As a result, as shown in FIG. 5, assuming that the DC output voltage between reference numerals 3 and 4 is Vout, the AC input voltage Va
With respect to the sine wave of c (a), the current IL (b ′) flowing through the choke coil L1 becomes a pulsating current as shown in FIG. In addition,
As shown in FIG. 5, the AC input current Lac (c ′) is such that the instantaneous voltage of the AC input voltage
The current flows through the circuit only for a period longer than out, and the conduction angle becomes very wide as compared with a general capacitor input type switching power supply.

However, the current IL flowing through the choke coil L1 has a very large peak value with respect to the average value, and a load connected between the output terminals 3 and 4 requires the maximum power. It is necessary to select a choke coil having a large current capacity that can maintain a predetermined inductance value without saturation even at the peak value of the current IL flowing through the choke coil.

[0015]

In the case of the circuit shown in the conventional example, the current IL flowing through the choke coil L1 has a very large peak value with respect to the average value. A choke coil having a large current capacity capable of maintaining a predetermined inductance value without saturating even at the peak value of the current IL flowing through the choke coil when the load connected between the four requires a maximum power, that is, an average current. On the other hand, a choke coil which allows a very large peak current had to be used. The current flowing through the choke coil is the switching F
Since the current flows through the ET and the commutation diode, it is necessary to use an element having a large allowable peak current for each element.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a power factor improving type power supply which does not require the use of various components having a large capacity by suppressing the peak value of the current flowing through the choke coil.

[0017]

A peak value of a current flowing through a switching FET is detected, and when the current reaches a predetermined value, the switching FET is turned off to limit a peak value of a current flowing through a choke coil, a switching FET, and a commutation diode. This makes it possible to use an element having a lower current capacity than in the past.

[0018]

(Embodiment 1) FIG. 1 is a block diagram showing a main circuit of a power factor improving type power supply device according to Embodiment 1 of the present invention. The configuration of the power factor improving power supply device of FIG. 1 will be described. Reference numerals 1 and 2 indicate AC power supply input terminals, and reference numerals 3 and 4 indicate output terminals, from which a predetermined DC voltage is output. Although not shown, a load such as a DC / DC converter is connected to the output terminals 3 and 4.

DB1 indicates a diode bridge for full-wave rectification of the AC power supply voltage, Q1 indicates a switching FET, and D1 indicates a commutation diode. L1 denotes a choke coil, and the inductance value is selected according to the power required by the load connected between the output terminals 3 and 4.
In this description, the choke coil L is always used in the range of power required by the load connected between the output terminals 3 and 4.
The description will be made on the assumption that the inductance value is set so that the current flowing through 1 is discontinuous.

C1 denotes an electrolytic capacitor having a relatively large capacity, reference numeral 5 denotes a filter for cutting off the switching component of the current flowing through the switching FET Q1 from the AC section, and reference numeral 6 denotes a filter between both ends of the output terminals 3 and 4. Reference numeral 7 denotes an oscillator, reference numeral 10 denotes a current detection circuit for detecting a current flowing through the circuit, and reference numeral 11 denotes a detection value detected by the current detection circuit 10 and a reference value. Reference numeral 8 denotes a comparator that outputs a high level only when the output of the oscillator 7 is larger than the output of the output voltage detection circuit 6 and the output of the error amplifier 11, respectively. Reference numeral 9 denotes a switching FET when the output of the comparator 8 is at a high level.
A drive circuit for generating a drive pulse for turning off the switching FET Q1 when Q1 is turned on and the output of the comparator 8 is at a low level is shown.

Next, the operation of the power supply circuit shown in FIG. 1 will be described. A commercial AC power is supplied from the input terminals 1 and 2, and an output voltage detection circuit 6
When power is supplied to the oscillator 7, the comparator 8, the drive circuit 9, and the error amplifier 11, the output voltage detection circuit 6
4 and outputs an output voltage detection signal proportional to the potential difference. An output voltage detection signal that is an output of the output voltage detection circuit 6 is input to a negative input terminal of the comparator 8.

On the other hand, the triangular wave output from the oscillator 7 is input to the + input terminal of the comparator 8. Then, the comparator 8 compares the output voltage detection signal with the triangular wave, and the output of the comparator 8 becomes high level only during the period when the level of the triangular wave is higher than the level of the output voltage detection signal. The switching FET Q1 is turned on, and the electrolytic capacitor C1, the choke coil L1, the switching FET Q
A current flows through the circuit through the circuit 1.

When the levels of the output voltage detection signal and the triangular wave are inverted, the comparator 8 outputs a low level, and based on this, the drive circuit 9 turns off the switching FET Q1.

That is, when the output voltage decreases, the level of the output voltage detection signal output from the output voltage detection circuit 6 decreases, so that the output duty of the comparator 8 increases, and when the output voltage increases, the output voltage of the output voltage detection circuit 6 increases. The on-duty of the switching FET Q1 is controlled so that the level of the output voltage detection signal, which is the output, rises and the output duty of the comparator 8 is reduced, and as a result, the potential difference between the output terminals 3 and 4 is always kept constant. .

During the ON period of the switching FET Q1, a current flows through the switching FET Q1 via the electrolytic capacitor C1 and the choke coil L1, energy is stored in the choke coil L1, and a DC / DC converter connected to the output terminals 3, 4 is provided. Energy is also supplied to loads such as.

During the off period of the switching FET Q1, the energy stored in the choke coil during the on period is also supplied to a load such as a DC / DC converter connected to the output terminals 3 and 4 via the diode D1. You.

The current detection circuit 10 includes a switching FET Q
1 is detected, and a current detection signal having a voltage waveform similar to a low-frequency component obtained by removing a high-frequency component which is a frequency component of the oscillator 7 from the drain current is output to the error amplifier 11. In the error amplifier 11, an overcurrent detection signal which is a signal obtained by non-inverting amplification of a potential difference between the current detection signal and the reference voltage Vref <b> 2 is input to the comparator 8.

At the time of a low load in which the drain current flowing through the switching FET Q1 does not reach the reference value, the switching FET Q is compared with the output voltage detection signal output from the output voltage detection circuit 6 and the triangular wave output from the oscillator 7.
When the on-duty of 1 is determined and the load is such that the drain current reaches the limit value, the output voltage detection signal and the triangular wave are the same as during the steady state until the instantaneous value of the commercial AC input power supply reaches the limit value. , The on-duty of the switching FET Q1 is determined. As the instantaneous value of the commercial AC input power increases, the switching FET Q1
The peak value of the drain current of the current detection circuit 10 rises.
Is higher than the reference voltage Vref2, the error amplifier 1
1 is inverted from the level of the output voltage detection signal output from the output voltage detection circuit 6, and a current equal to or greater than a predetermined value is switched.
When the current flows through the ETQ1, the output of the comparator 8 becomes low level, which acts to turn off the switching FET Q1. Therefore, a current of a predetermined value or more does not flow through the switching FET Q1, and a current having a limited peak current flows near the peak of the commercial AC input power supply voltage.

Next, FIG. 2 shows the waveform of each part. In FIG. 2, when the output voltage between the output terminals 3 and 4 is Vout, the current IL (see FIG. 2B) flowing through the choke coil L1 with respect to the sine wave of the AC input voltage Vac (a) is similar to that in FIG. It becomes a current waveform. The AC input current lac
In FIG. 1C, as shown in FIG. 1, the instantaneous voltage of the AC input voltage Vac flows through the circuit only during a period longer than the DC output voltage Vout between the output terminals 3 and 4.

Therefore, the current flowing through the choke coil L1, the switching FET Q1, and the commutation diode D1 can be limited by a predetermined peak value (ILpeak), so that the saturation current of the choke coil can be reduced. It is possible to use an element having a small peak current rating also in the diode D1.

In the step-down chopper type power factor improving type power supply described with reference to FIG. 1, the switching element is inserted in the GND line of the output terminals 3 and 4, but Vout
The same effect can be obtained even when inserted on the + side.

(Embodiment 2) FIG. 3 is a block diagram showing a main part circuit of a power factor correction type power supply unit according to Embodiment 2 of the present invention. In FIG. 3, components using the same reference numerals as those used in the first embodiment of FIG. 1 have similar functions or configurations, and the description thereof will be omitted.

FIG. 3 differs from the first embodiment shown in FIG. 1 in that the choke coil L1 '(the choke coil L1 in FIG.
And a choke coil L1 ′.
When the current flowing through the choke coil L1 'reaches a predetermined value, the switching FET Q1 is turned off, and the choke coil L1'
Q1 is that the peak value of the current flowing through the commutating diode D1 is limited, and the same effect as that of the first embodiment in FIG. 1 can be obtained.

In the step-down chopper type power factor improving power supply described with reference to FIG. 3, the switching element is inserted into the GND lines of the output terminals 3 and 4, which are the output terminals.
The same effect can be obtained even when inserted on the Vout + side.

[0035]

As described above, according to the present invention,
In a step-down chopper type power factor correction type power supply, the peak value of the current flowing through the choke coil, switching FET, and commutation diode can be limited to a predetermined value, so that a choke coil with a low saturation current and a peak current rating It is possible to use a switching FET and a commutation diode having a low power consumption.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram of a power supply device according to a first embodiment of the present invention.

FIG. 2 is a waveform diagram of a main part of a circuit block diagram of the power supply device according to the first embodiment of the present invention.

FIG. 3 is a circuit block diagram of a power supply device according to a second embodiment of the present invention.

FIG. 4 is a circuit block diagram of a conventional power supply device.

FIG. 5 is a waveform diagram of a main part of a circuit block diagram of a conventional power supply device.

[Explanation of symbols]

DB1 Diode bridge D1 Commutation diode C1 Electrolytic capacitor Q1 Switching FET L1 Choke coil 1, 2, Switching power supply input terminal 3, 4 Output terminal (DC / DC converter input terminal) 5 Filter 6 Output voltage detection circuit 7 Oscillator 8 Comparator 9 Drive Circuit 10 Current detection circuit 11 Error amplifier

Claims (3)

[Claims] 1. A rectifier which receives a commercial AC power supply as an input and rectifies the commercial AC power supply in a full-wave manner, and a buck chopper circuit connected to the rectifier for shaping an AC input current waveform into a sine wave and outputting a DC voltage. An output voltage detecting means for detecting the DC output constant voltage and outputting an output voltage signal, and a current for detecting a pulsating current flowing through the step-down chopper circuit and outputting an overcurrent detection signal. A detection unit, a comparison unit that receives an output voltage signal from the output voltage detection unit and an overcurrent detection signal from the current detection unit, compares them, and generates an ON / OFF signal;
A driving circuit for driving a switching element of the step-down chopper circuit based on the ON / OFF signal from the comparing means, for forming a PWM signal for controlling the DC constant voltage to a predetermined constant value; And a control circuit for turning off the switch element when the peak value of the pulsating current reaches a predetermined value. 2. The power factor improving power supply device according to claim 1, wherein said current detecting means detects a pulsating current flowing through the switch element. 3. The power factor improving type power supply device according to claim 1, wherein said current detecting means detects a pulsating current flowing through a choke coil of said chopper circuit.