JP2726712B2 - DC / AC inverter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Overview] Regarding a DC / AC inverter that outputs an AC voltage to a rectifier load circuit of a capacitor input type in an uninterruptible power supply, the apparent capacity is improved to almost the rated capacity by improving the power factor. A DC / AC inverter that converts a DC voltage into an AC voltage with a pulse width modulation circuit and outputs the converted voltage to a capacitor input type rectifier load circuit via a transformer. A current detection unit for detecting a current flowing in the circuit, a control circuit for outputting a control pulse signal whose pulse width distribution changes according to the magnitude of an output signal of the current detection unit, and a control output from the control circuit A driving circuit for driving the pulse width modulation circuit according to a signal is provided, and the power factor is improved by changing the distribution of the pulse width from a sine wave to a rectangular wave as the load increases. The configuration is as follows.

[Field of Industrial Application] The present invention relates to a DC / AC inverter used in an uninterruptible power supply, and more particularly, to a DC / AC inverter capable of improving a power factor.

Uninterruptible power supplies are widely used in various fields, such as exchanges where temporary power outages are not allowed.

For example, in an exchange, when supplying an AC voltage to an AC load in which a temporary power failure is not permitted, the DC voltage output from the storage battery and the DC voltage obtained by rectifying and smoothing the output voltage of the AC power supply are supplied to a common DC / AC inverter. In addition, an AC voltage output from the DC / AC inverter is supplied to an AC load.

[Prior Art] FIG. 8 is a block diagram showing an example of a conventional DC / AC inverter. In the figure, a DC voltage is converted to a pulse width modulation signal by a pulse width modulation circuit 1 and then applied to a capacitor input type rectification load circuit 3 via a transformer 2.

By the way, as shown in FIG. 9, the pulse width modulation circuit 1 in such a conventional DC / AC inverter outputs a pulse width signal whose pulse width distribution always becomes a sine wave v regardless of the magnitude of the load current. Had been driven to be.

On the other hand, paying attention to the output current Ii flowing through the rectifier load circuit 3, in order to charge up the capacitor, FIG.
As indicated by Ii, the peak current flows out of phase with respect to the sine wave output voltage Vi applied to the rectifier load circuit 3. The broken line indicates the equivalent sine wave current of the peak current Ii, and Vd indicates the rectified output voltage.

[Problems to be Solved by the Invention] However, since the phases of the voltage Vi and the current Ii are not equal, the power factor decreases to about 0.5.

That is, for example, when an inverter having an output capacity of 1 KVA is considered, the capacity that can be taken out as an output when a pure resistance load with a power factor of 1 is connected is 1 KVA × 1 = 1 KW. When a rectifying load circuit 3 of the capacitor input type is connected, 1 KVA × 0.5 = 500 W, which is apparently a half capacity.

The present invention has been made in view of such a problem, and an object of the present invention is to provide a DC / AC inverter in which an apparent capacity can be made almost a rated capacity by improving a power factor.

[Means for Solving the Problems] FIG. 1 is a principle block diagram of a DC / AC inverter of the present invention, and the same components as those in FIG. 8 are denoted by the same reference numerals. In the figure, a load current flowing through a rectifier load circuit 3 of a capacitor input type is detected by a current detection unit 4, and an output signal thereof is applied to a control circuit 5. The control circuit 5
The drive circuit 6 of the pulse width modulation circuit 1 has a pulse width distribution of a pulse width signal output from the pulse width modulation circuit 1 according to the value of the detection signal, that is, the magnitude of the load current, so that the distribution of the pulse width becomes a predetermined distribution. Control.

[Operation] The distribution of the pulse width of the pulse width signal output from the pulse width modulation circuit 1 becomes a sine wave similar to the conventional one in a light load state where the load current flowing through the rectification load circuit 3 is small, and the load becomes heavy. , And is controlled so as to finally become a rectangular wave as shown in FIG.

When the distribution of the pulse width of the pulse width signal output from the pulse width modulation circuit 1 becomes a rectangular wave as shown in FIG. 2, the phase of the load current Ii flowing through the rectification load circuit 3 becomes as shown in FIG. The power factor is improved by being equal to the phase of the rectangular wave voltage v, and the output capacity of the inverter can be used to the maximum.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 4 is a block diagram showing one embodiment of the present invention.
1 are denoted by the same reference numerals. In the figure, a pulse width modulation circuit 1 is composed of four transistors Q1 to Q4. That is, the transistor
The collectors of Q1 and Q2 are connected to the positive power supply line + L, the emitters of the transistors Q3 and Q4 are connected to the negative power supply line -L, and the emitter of the transistor Q2 and the collector of the transistor Q3 are connected to the primary winding n1 of the transformer 2. The emitter of the transistor Q1 and the collector of the transistor Q4 are connected to the other end of the primary winding n1 of the transformer 2. A drive signal DR is individually applied from the drive circuit 6 to the bases of the transistors Q1 to Q4.

The control circuit 5 includes an output current detector 7 that outputs a reference voltage RV according to the magnitude of an output signal of the current detection unit 4 and a sawtooth wave generation circuit 8 that outputs a constant sawtooth wave SW regardless of the size of the load. And a comparator 9 to which a reference voltage RV output from the output current detector 7 is applied to one input terminal and a sawtooth wave SW output from the sawtooth wave generation circuit 8 is applied to the other input terminal. I have. The output signal CO of the comparator 9 is
Has been added to

5 to 7 are waveform diagrams for explaining the operation of FIG. 4. FIG. 5 shows an operation waveform of a lightly loaded state, and FIG. 6 shows an operation waveform of a slightly heavily loaded state. FIG. 7 shows operation waveforms in the maximum load state.

That is, as shown in FIG. 5, when the load is light,
The reference voltage RV output from the output current detector 7 changes in a relatively large triangular waveform. As a result, the pulse width of the output signal of the comparator 9 becomes narrow when the reference voltage RV is large, and becomes wide when the reference voltage RV is small. Accordingly, the distribution of the pulse width of the pulse width signal output from the pulse width modulation circuit 1 is sinusoidal, and the output voltage Vi is also sinusoidal.

Next, as shown in FIG. 6, when the load is slightly heavy,
The change in the reference voltage RV output from the output current detector 7 in a triangular waveform becomes small. As a result, the change in the pulse width of the output signal of the comparator 9 is reduced, the distribution of the pulse width of the pulse width signal output from the pulse width modulation circuit 1 is close to a rectangular wave, and the output voltage Vi is also close to a rectangular wave. Become. Therefore, the power factor is improved as compared with the state shown in FIG.

Then, as shown in FIG. 7, when the maximum load state is reached, the reference voltage RV output from the output current detector 7 does not change and becomes constant. As a result, the pulse width of the output signal of the comparator 9 also becomes constant, the pulse width distribution of the pulse width signal output from the pulse width modulation circuit 1 becomes a rectangular wave, and the output voltage Vi also becomes a rectangular wave. Therefore, the power factor is improved to the maximum state.

[Effects of the Invention] As described in detail above, according to the present invention, when the load becomes heavy, the output voltage can be made a rectangular wave to greatly improve the power factor, and the apparent capacity can be substantially reduced to the rated capacity. A DC / AC inverter that can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram of the principle of the present invention, FIGS. 2 and 3 are explanatory diagrams of the operation at the time of maximum load, FIG. 4 is a block diagram showing an embodiment of the present invention, and FIGS. 8 is a block diagram showing an example of a conventional DC / AC inverter, and FIGS. 9 and 10 are waveform diagrams respectively explaining the operation of FIG. . In FIGS. 1 and 4, 1 is a pulse width modulation circuit, 2 is a transformer, 3 is a rectifier load circuit of a capacitor input type, 4 is a current detector, 5 is a control circuit, and 6 is a drive circuit.

Claims (1)

(57) [Claims] 1. A DC / AC inverter for converting a DC voltage into an AC voltage by a pulse width modulation circuit (1) and outputting to a capacitor input type rectification load circuit (3) via a transformer (2). A current detection unit (4) for detecting a current flowing through the load circuit (3); and a control circuit for outputting a control pulse signal whose pulse width distribution changes according to the magnitude of the output signal of the current detection unit (4). And (5) a driving circuit (6) for driving the pulse width modulation circuit (1) in accordance with a control signal output from the control circuit (5), and the pulse width distribution increases as the load increases. A DC / AC inverter characterized by improving the power factor by changing from a sine wave to a square wave.