JPH0731152A - Constant power factor control method for pwm converter - Google Patents

Abstract

PURPOSE:To provide a control method for a PWM converter in which the lowering of input power factor due to lowering of load can be suppressed. CONSTITUTION:The input circuit for a mixed bridge comprising FETs 16, 17 and diodes 18, 19 and 20, 21 built in the FETs is provided with a current detecting circuit 3 which feeds current signals sequentially to a current/voltage converting circuit 7, a voltage/frequency converting circuit 8, and a carrier generating circuit 9 thus modifying the switching frequency fs of carrier. Since the reduction of current signal causes increase of switching frequency fs, the lowering of input power factor due to decrease of the load is suppressed by the increase of the switching frequency. Alternatively, the output circuit is provided with a current detection circuit feeding a current signal to the current/ voltage converting circuit 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an input power factor control method for a PWM converter, and more particularly to a method for constantly controlling the input power factor.

[0002]

2. Description of the Related Art The circuit configuration of a conventional PWM converter is as shown in FIG. In FIG. 3, commercial AC power supply 10
1 is FETs 106 and 10 via the boosting reactor 102.
7, connected to a mixed bridge composed of FET built-in diodes 108 and 109 and diodes 110 and 111, and a capacitor 11 is provided on the DC output side of the mixed bridge.
2 and the load 115 are connected in parallel. Further, the detected current signal of the current detection circuit 116 provided in the AC input circuit is input to the control circuit 104. Voltage dividing resistors 113 and 114
The output voltage signal V ₀ detected from the comparison amplifier circuit 105
Is compared with the target voltage signal V _ref, and the difference signal is input to the control circuit 104 and becomes a signal wave (operation amount) u and is input from the control circuit 104 to the PWM drive circuit 103.
The carrier wave f _s input to the PWM drive circuit 103 is modulated by the signal wave u, and the pulse width of the drive pulse that controls the PWM converter is adjusted.

[0003]

Generally, it is known as an operating characteristic of a PWM converter that the input power factor decreases as the load decreases. This is because the fundamental wave component of the input current decreases as the load decreases, but the harmonic component of the input current generated by the switching operation hardly changes. Conventionally, the operating characteristics of the PWM converter are defined by the power factor at the time of rating, and the deterioration of the power factor at the time of low load operation has been ignored. However, the PWM converter is not always operated at the rated output, and it may be forced to operate at low load for a long time. Therefore, from the viewpoint of energy efficiency, the PWM converter is not affected by the change in load and is constantly operated. It is desirable to be able to operate at a high power factor. The present invention has been made to solve the above-described problems, and suppresses a decrease in input power factor due to a decrease in load by reducing a harmonic component of an input current, and a PWM that can always operate at a high power factor. It is intended to realize a converter control method.

[0004]

In order to achieve the above-mentioned object, a constant power factor control method in a PWM converter according to the present invention is a current signal detected from a current detection circuit provided in an input circuit or an output circuit of the PWM converter. Is converted into a frequency signal that changes in inverse proportion to the current signal by performing current / voltage conversion and voltage / frequency conversion, and the converted frequency signal is superimposed on a carrier wave preset in the carrier generation circuit to generate a carrier wave. By changing the frequency and inputting the changed carrier frequency to the PWM drive circuit, the drive pulse is adjusted and the decrease in the input power factor due to the load reduction is suppressed.

[0005]

When the load of the PWM converter decreases, the input current and output current also decrease. The decrease of the current signal detected by the current detection circuit provided in the AC input circuit or the output circuit becomes the increase of the frequency signal through the current / voltage conversion circuit and the voltage / frequency conversion circuit and is input to the carrier generation circuit. . As a result, the frequency of the carrier wave preset in the carrier generation circuit increases because the frequency signal input from the voltage / frequency conversion circuit is added. Since the input power factor is in proportion to the carrier frequency, it is possible to suppress the decrease of the input power factor due to the load reduction by increasing the carrier frequency.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a circuit configuration of a PWM converter according to the present invention, in which a boosting reactor 2 provided in an AC input circuit and an FE constituting a mixing bridge.
A comparator / amplifier circuit 6 for comparing the voltage signal V ₀ detected from the T16 and 17, FET built-in diodes 18 and 19, the diodes 20 and 21, and the voltage dividing resistors 23 and 24 with the target value V _ref and outputting the difference signal. , The difference signal is input and P
The control circuit 5 that outputs the signal wave (manipulation amount) u to the WM drive circuit 4 is the same as the PWM converter in the related art.

The current signal detected by the current detection circuit 3 provided in the input circuit is input to the control circuit 5 and the current / voltage conversion circuit 7 and converted into a voltage signal. When the load decreases, the input current i _i decreases, so the current signal detected by the current detection circuit 3 also decreases, and the voltage signal output from the current / voltage conversion circuit 7 also decreases accordingly. The voltage signal output from the current / voltage conversion circuit 7 is input to the voltage / frequency conversion circuit 8 and converted into a frequency signal, but since a frequency signal inversely proportional to the input voltage signal is output, the voltage signal decreases. Appears as an increase in the frequency signal. That is, the decrease in the input current due to the decrease in the load results in an increase in the frequency signal, which is input to the carrier generation circuit 9. The carrier generation circuit 9 has a preset frequency f _s.
The carrier frequency f _s is a frequency signal (switching frequency f ′ _s ) obtained by adding the frequency signals from the voltage / frequency conversion path 8. The frequency signal (switching frequency f ′ _s ) output from the carrier generation circuit 9 is input to the PWM drive circuit 4 together with the signal wave (manipulation amount) u output from the control circuit 5, and FETs 16 and 17 forming a mixing bridge. The pulse width of the drive pulse that controls the pulse width is modulated. Therefore, when a decrease in the input current due to the load decrease is detected and input to the carrier generation circuit 9 via the current / voltage conversion circuit 7 and the voltage / frequency conversion circuit 8, the frequency of the carrier wave _changes from f _s to f ′ _s . To rise. As explained separately, since the input power factor is proportional to the switching frequency, increasing the switching frequency will increase the input power factor and suppress the decrease in input power factor that occurs with load reduction. Will be done.

Next, the fact that the power factor of the input current in the PWM converter is proportional to the switching frequency will be described based on a theoretical formula. The power factor of the input current including the harmonic component can be expressed by the following equation.

[0009]

[Equation 1]

Where I ₁ is the fundamental wave component current, I _rms is the effective value of the input current, and θ is the phase difference between the input voltage and the current. Further, I _rms can be expressed by the following equation. I _rms ≈I _L ( ^1/2 + K _r ) ^1/2 (2) where I _L = I ₁ , K _r = K _r2 · K _r3 , and K _r2 = (T / L) ² (E ^{_{i / 2E O I L) 2}} /3 ...... (3) K r3 = (3E i 2/8) - (8E O E i / 3π) + (E O 2/2) ...... (4) E i is Peak value of input voltage, E _O is output voltage (DC), L
Is the inductance of the input circuit, T = 1 / f _s .
Substituting equation (2) into which equation (3) and equation (4) are substituted into equation (1),

[0011]

[Equation 2]

From the equation (5), it is understood that the power factor PF of the input current containing the harmonic component is substantially proportional to the inductance L of the boosting reactor in the input circuit and the switching frequency f _s . That is, the power factor PF∝L · f _s (6) It is not easy to change the inductance L of the input circuit, but the switching frequency f _s can be easily changed. By converting the detected current signal into a frequency signal that changes in inverse proportion to this current signal and increasing the switching frequency f _{s of the} carrier wave by this frequency signal, it is possible to suppress the decrease in the input power factor due to the load decrease. Becomes Although the above description relates to a single-phase PWM converter, it can be applied to a three-phase PWM converter.

FIG. 2 is a block diagram of a PWM converter showing another embodiment according to the present invention. In FIG. 2, the current signal detected by the current detector 3 provided in the AC input circuit is input only to the control circuit 5, and the current signal detected by the current detector 25 provided in the output circuit is converted into current / voltage. It is input to the circuit. Since the input current i _i and the output current I _L are in a proportional relationship, the current signal detected from the output current I _L that decreases with a load decrease is converted into a frequency signal that changes in inverse proportion to this current signal. When the switching frequency f _{s of the} carrier wave is increased by the signal, it is possible to suppress the decrease in the input power factor due to the load decrease.

[0014]

As described above, the P according to the present invention is
The constant power factor control method in the WM converter is based on a current / voltage conversion circuit, a voltage / current
The frequency signal is converted through a frequency conversion circuit into a frequency signal that changes in inverse proportion to the current signal, and this frequency signal is superimposed on the carrier frequency f _s preset in the carrier generation circuit to generate the switching frequency f ′ _s . . When the load decreases, the input current and output current decrease and the input power factor also decreases, but if the current signal detected from the input current or output current is converted to a frequency signal and input to the carrier generation circuit, the switching frequency rises. Therefore, the decrease of the input power factor can be suppressed, and the input power factor can be controlled to be constant.

[Brief description of drawings]

FIG. 1 is a block diagram showing a circuit configuration of a PWM converter showing an embodiment according to the present invention.

FIG. 2 is a PWM showing another embodiment according to the present invention.
Block diagram of the converter.

FIG. 3 is a block diagram showing a circuit configuration of a conventional PWM converter.

[Explanation of symbols]

 1 Commercial AC power supply 2 Reactor 3,25 Current detection circuit 4 PWM drive circuit 5 Control circuit 6 Comparison amplification circuit 7 Current / voltage conversion circuit 8 Voltage / frequency conversion circuit 9 Carrier generation circuit 16, 17 FET 18, 19 FET built-in diode 20 , 21 Diode 22 Capacitor 23, 24 Voltage dividing resistor 26 Load

Claims (2)

[Claims] 1. A PWM converter comprising a semiconductor switching element for converting commercial AC power input through a reactor into DC power and sending the DC power, wherein a current signal detected by a current detection circuit provided in an input circuit of the PWM converter is supplied. The PWM drive circuit is input to a control circuit that outputs a signal wave, and the current signal is input to a current / voltage conversion circuit to convert the current signal into a voltage signal proportional to the current signal. A voltage signal is input and converted into a frequency signal that is inversely proportional to this voltage signal, and this frequency signal is superimposed on a carrier wave that has been preset in the carrier generation circuit to change the carrier wave frequency. Adjust the drive pulse that controls the PWM converter by inputting it to the drive circuit, and reduce the load Power factor constant control method in the PWM converter, characterized in that to suppress the decrease of the input power factor caused. 2. A current signal detected by a current detection circuit provided in an input circuit of a PWM converter is input to a control circuit for outputting a signal wave to a PWM drive circuit, and the PWM is provided.
The constant power factor control method for a PWM converter according to claim 1, wherein a current signal detected by a current detection circuit provided in the output circuit of the converter is input to the current / voltage conversion circuit.