JPH11308879A - Neutral point type inverter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a neutral point type inverter from which a constant voltage can be obtained, without changing its circuit constant even when its input voltage is raised. SOLUTION: A neutral point type inverter is constituted, in such a way that commercial power supply is connected to a rectifier DP through a low-pass filter LPF and a serial circuit of a smoothing capacitor Cs and voltage dividing capacitors C1 and C2 and another serial circuit of switching elements Q1 and Q2 which alternately make turning on/off operations are connected in parallel with the output of the rectifier DB. Then diodes D1 and D2 are respectively connected in anti-parallel for the switching elements Q1 and Q2 and a drive circuit DR is connected to the elements Q1 and Q2. In addition, the serial circuit of a capacitor Cx and an inductor Lo is connected between the junctions between the voltage-dividing capacitors C1 and C2 and between the switching elements Q1 and Q2, and a load RL is connected in parallel with an inductor Lo. A constant output voltage is obtained from an inverter, by reducing the boosting effect of the inverter as a boosting inverter through causing the capacitor Cx absorb the voltage the back electromotive force of the inductor Lo.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter device for rectifying and smoothing an AC voltage, temporarily converting the AC voltage to a DC voltage, and further converting the AC voltage to a high-frequency voltage to supply high-frequency power to a load. Neutral point inverter type ballast suitable for neutral point inverter device or lighting equipment (hereinafter, both are simply referred to as "neutral point type inverter")
It is about.

[0002]

2. Description of the Related Art Recently, small-sized home appliances and office automation equipment are equipped with a high-frequency inverter device for high performance and high efficiency.

[0003] Further, in a fluorescent lamp device for home use or a fluorescent lamp device for facilities, a circuit system for lighting a fluorescent lamp is conventionally called a copper-iron type ballast such as a choke current limiting type or a leakage transformer type. However, due to limitations in shape, weight, and efficiency, today's fluorescent lighting fixtures use a high-frequency lighting type ballast (inverter type ballast). It is also used for HID lamp appliances such as mercury lamps and metal halide lamps, and bulb-type fluorescent lamps.

[0004] The inverter type ballast has the advantages of being efficient and capable of saving power, reducing the flicker of lamps and noise of the ballast, and achieving weight reduction. The use of inverters is rapidly increasing.

However, the high-frequency inverter device and the inverter type ballast (hereinafter, these are simply referred to as an “inverter”) are generally rectifiers (diodes).
The capacitor smoothing method of full-wave rectification, which uses a smoothing with an electrolytic capacitor, is widely used.Distorted current caused by the nonlinearity of the diode flows to the commercial power supply, and harmonics are added to the input current on the commercial power supply side. The current starts to flow,
The problem of harmonic interference caused by this harmonic current has become significant.

For this reason, circuit technologies for suppressing harmonic currents have been studied. For example, an AC reactor insertion system, a partial smoothing system, an active smoothing filter system (inverter fluorescent lamp; electronic technology, Vol. 32, No. 3, (See pp.113-119)
・ Dither rectification method (High power factor switching regulator using dither effect; Proceedings of the National Meeting of the Institute of Electrical Engineers of Japan, N
o.546, pp.5-137).

Further, as an electronic ballast for a fluorescent lamp, a neutral point type electronic ballast circuit capable of reducing a harmonic component of an input current on the commercial power supply side only by an inverter for lighting a fluorescent lamp similarly to the dither rectification method. (Neutral point inverter type ballast)
(One method of a simple harmonic reduction circuit;
Author: Yoshito Kato, Journal of the Institute of Electrical Installations, Vol.12, No.10, pp.90
2-904). The theoretical analysis of this neutral point inverter type ballast is also considered (Development of low current distortion type electronic ballast using neutral point inverter; Author: Yoshito Kato, Journal of the Illuminating Engineering Institute, Vol. 79 , No. 2, pp. 14-20).

FIG. 11 is a basic circuit diagram of a neutral point type inverter. This inverter includes a full-wave rectifier DB (rectifying diodes are simply referred to as 1 to 4 in a circuit diagram, and DB1 to DB4 in the specification) for rectifying a commercial power supply Vi to a DC voltage Ed via a low-pass filter LPF. A smoothing capacitor Cs for smoothing the output of the full-wave rectifier DB, and a voltage dividing capacitor C1 connected in parallel with the smoothing capacitor Cs to divide the DC voltage Ed.
A series circuit consisting of C2, a series circuit consisting of switching elements Q1 and Q2 connected in parallel with the smoothing capacitor Cs,
Load connected between the connection point (neutral point) of voltage dividing capacitors C1 and C2 and the connection point (SW point) of switching elements Q1 and Q2
RL. The neutral point is connected to one end of the commercial power supply Vi.

The inverter operates by converting the ripple voltage contained in the output of the full-wave rectifier DB into a DC voltage Ed by using a smoothing capacitor Cs, and then turning on and off the switching elements Q1 and Q2 to make the neutral point a reference. A closed circuit is configured to charge the voltage dividing capacitor C1 or C2 from the smoothing capacitor Cs. This charging current becomes a load current flowing to the load RL, and a current in the reverse direction is secured in a section where no load current flows.
When the switching elements Q1 and Q2 are alternately turned on and off (inverter operation) at a high frequency, a voltage VL obtained by superimposing a high frequency on a commercial frequency is applied to the load RL. Diode DB
Since the currents 1 to DB4 have a triangular waveform of a high frequency with a pause period proportional to the load, a current waveform of a pseudo sine waveform is obtained by passing through a low-pass filter LPF. As a result, the neutral point inverter device can reduce the harmonic components of the input current on the commercial power supply side.

[0010] Such a neutral point inverter has the following features. (1) By inserting a low-pass filter LPF on the commercial power supply side, it is possible to reduce the harmonic component contained in the input current in the same manner as in the active smoothing filter system by turning on the fluorescent lamp. (2) It is not necessary to use a new circuit as in the dither rectification method, and it can be applied to the improvement of existing half-bridge type ballasts. (3) Harmonic components of input current Is I
EC standard (IEC1000-3-2) or less, (4) High power factor of 97% or more in input power factor, (5) Simple circuit configuration and fluorescent lamp as load Has many advantages, such as a small decrease in the luminous efficiency, and is being used as a suitable circuit for preventing harmonic interference of inverter equipment.

Further, as shown in FIG. 12, the present applicant connects an inductor Lo between a neutral point and a SW point, and
By connecting a load RL in parallel with Lo, a neutral point inverter capable of obtaining a higher output and a stable output voltage has been proposed (see Japanese Patent Application No. Hei 9-104563).

[0012]

As is well known, the input voltage of a commercial power supply is, for example, 100 V to 120 V (100 V system) as in Japan and the United States, and 220 V to 240 V (200 V system) as in Europe. )). Therefore, for example, if a neutral point type inverter device for an input voltage of 100 V manufactured in Japan or the United States is used in Europe as it is, the output voltage will be approximately doubled, and conversely, the inverter is manufactured in Europe. If a neutral point type inverter device for 200V system is used in Japan or the United States as it is, the output voltage will be reduced to about half, so that 100V system country and 200V system can be used to obtain a constant output voltage. It is necessary to change the design such as changing the constant according to the country.

However, changing the constants to correspond to the destination requires management of the parts and the destination of the apparatus, which is very troublesome. In order to supply the same type of neutral point inverter worldwide, it is desirable that the device be a universal device that does not require a design change such as a constant change.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances.
It is an object of the present invention to provide a device which can obtain a constant output voltage by a simple switching operation even when an input voltage is switched between a system and a 200V system.

[0015]

A neutral point inverter according to the present invention comprises a low-pass filter for passing a fundamental frequency of an alternating current and blocking a harmonic signal, and an alternating-current voltage passing through the low-pass filter. A rectifier for rectification, a series circuit of first and second capacitors connected in parallel to the output of the rectifier, a third capacitor for smoothing the output of the rectifier, and a first capacitor connected in parallel to the output of the rectifier. And a series circuit of the second switching element, and the first and second switching elements which are connected in parallel in the DC direction in reverse to the first and second switching elements, respectively (such a connection is referred to as “inverse parallel connection”). And a drive circuit for driving the first and second switching elements, and one end of the AC input of the rectifier and a connection point of the first and second capacitors. A load circuit connected between a point and a connection point (SW point) of the first and second switching elements and supplied with AC power by alternately turning on and off the first and second switching elements. Wherein the load circuit comprises a series circuit including an inductor and a fourth capacitor, and a load to which AC power is supplied by a voltage generated in the inductor. It is characterized by the following.

[0016]

According to the neutral point inverter of the present invention, a series circuit of an inductor and a capacitor is connected between the neutral point and the SW point, and the load is connected in parallel to the inductor. Unlike the conventional neutral point type inverter which connected the inductor between the point and the SW point, connected the load in parallel to the inductor, and operated the boost inverter to obtain a high output voltage, it was based on the back electromotive force of the inductor. Since the voltage can be absorbed by the capacitor, the operation as a boost inverter can be reduced and the boost effect can be reduced. For example, even if the input voltage is doubled, the capacitor and inductor are connected in series without changing the constant. By doing so, it is possible to obtain almost the same output voltage as in the case of the conventional neutral point inverter without connecting a capacitor. Therefore, if switching of connection / non-connection of the capacitor is performed, it is possible to easily cope with the switching of the input voltage without changing the constant. A neutral point type inverter can be provided, which is extremely effective.

[0017]

Embodiments of the present invention will be described below in detail with reference to the drawings. First, an inductor is connected between the neutral point and the SW point, a load RL is connected in parallel with the inductor, and a capacitor (a fourth capacitor in the claims) is not connected in series to the inductor. A neutral point type inverter according to a conventional form (see Japanese Patent Application No. 9-104563) will be outlined, and then a neutral point type inverter according to the present invention in which the fourth capacitor is connected in series to an inductor will be described.

FIG. 1 shows a circuit configuration of a neutral point inverter 50 according to a conventional embodiment. This neutral point inverter 50 uses a fluorescent lamp LT as a load and turns on the fluorescent lamp LT. It constitutes a neutral point inverter type ballast.

A commercial power supply is composed of an inductor Lf and a capacitor Cf.
Are connected to the AC input terminal of the full-wave rectifier DB via a low-pass filter LPF composed of In the figure,
The constituent diodes of the full-wave rectifier DB are indicated as 1, 2, 3, and 4. A DC output terminal of the full-wave rectifier DB has a series circuit of two small-capacity capacitors C1 and C2, and first and second switching elements (transistors) Q1 and Q1 that are alternately turned on and off by a drive circuit DR. A series circuit of Q2 is connected in parallel with a smoothing capacitor Cs for smoothing the output of the rectifier DB. The switching element Q1 has a first diode D1
However, a second diode D2 is also connected in anti-parallel to Q2. One end of the AC input of the rectifier DB is connected to the connection point (neutral point) of the capacitors C1 and C2.

In the neutral point inverter 50, transistors are used as the switching elements Q1 and Q2. However, as long as the means can be switched on and off, the present invention is not limited to this. For example, FETs and IGBTs can be used.
And so on. When these elements are used, the diodes included in these elements can be used, so that the diodes D connected in anti-parallel with the switching elements Q1 and Q2 can be used.
1, D2 can be omitted, and the circuit becomes simpler.

The drive circuit DR may be a so-called separately-excited type or a so-called self-excited type obtained by feeding back the self-resonant frequency of the load circuit. Further, an IC (integrated circuit) provided for a well-known inverter device, for example, an IC for a modified half-bridge circuit or the like can be used for the drive circuit DR, so that the device can be downsized. Specifically, “International Rectifier Fareast Co, Ltd”
It is preferable to use IR2151, IR2155, IR51H420 and the like. When the IR51H420 is used, since the first and second switching elements are included in the IC, a smaller device can be obtained.

The load circuit R comprises an inductor Lo and a fluorescent lamp LT. An inductor Lo is connected between the neutral point and the connection point (SW point) of the switching elements Q1 and Q2. A series circuit of a fluorescent lamp LT and an inductor La functioning as a choke coil is connected in parallel to the inductor Lo, and a resonance capacitor C4 can be connected in parallel to the fluorescent lamp LT if necessary. When the resonance capacitor C4 is connected, the inductor Lo and the resonance capacitor C4 form a parallel resonance circuit, and a resonance voltage generated at both ends of the resonance capacitor C4 is applied to the fluorescent lamp LT. When a load other than the fluorescent lamp LT is used, the load can be directly connected in parallel with the inductor Lo without connecting the inductor La.

The operation of the neutral point inverter 50 having the above configuration will be described below. The charging / discharging current flows through the voltage dividing capacitors C1 and C2 and the smoothing capacitor Cs by the on / off operation of the switching elements Q1 and Q2. When one switching element is on and the other switching element is off, a current flows through the inductor Lo and energy is stored.

Next, when both the switching elements Q1 and Q2 are turned off, the energy accumulated in the inductor Lo up to that point is stored in the smoothing capacitor Cs via the diodes D1 and D2 connected in antiparallel to the switching elements Q1 and Q2. Charged, the smoothing capacitor Cs is boosted to about 2 Vm (Vm is the peak value of the input sine wave voltage Vi).

As a result, the voltage waveforms of the voltage dividing capacitors C1 and C2 have waveforms as shown in FIGS. 2 and 3, respectively. However, the waveform slightly changes depending on the values of the inductor Lo, the voltage dividing capacitors C1, C2, and the like.

The above description is for the case where the voltage Ed of the smoothing capacitor Cs is a complete direct current. However, as shown in FIG. 4 corresponding to the input voltage Vi, the voltage Ed of the smoothing capacitor Cs actually has a ripple voltage. For this reason, all the diodes constituting the rectifier DB are not turned on, and the diodes DB3 and DB4 are turned on only in a section near 1/4 T of the input voltage Vi of 0 V, and currents (Id3 and Id4) flow through the diodes. The rest is off and there is a section where no current flows through the diode. Further, as is apparent from FIG. 4, the on-period of the diodes DB3 and DB4 constituting the rectifier DB hardly changes depending on the magnitude of the ripple voltage. That is, the AC component of the ripple voltage crosses at the same time regardless of the capacity of the smoothing capacitor Cs. At this time, the frequency of the ripple voltage is approximately twice the frequency of the input Vi.

The feature of the neutral point inverter device is that there is a section where DB3 and DB4 are turned on.
This is different from a simple voltage doubler rectifier circuit. DB3, DB4
Is on, this is the same as the above description, but when DB3 and DB4 are off, this is equivalent to a configuration in which this is removed. In this case, a voltage doubler rectifier circuit is configured at first glance.

At this time, due to the voltage drop of the voltage dividing capacitors C1 and C2, the voltage Ed of the smoothing capacitor Cs rises and falls due to the capacitance of the voltage dividing capacitors C1 and C2. Therefore, the voltage waveform V _C1 of the voltage dividing capacitors C1, _C2, V _C2 becomes a waveform as shown in FIG. 5, both ends of the voltage VR i.e. inductor Lo of the load circuit R is a waveform as shown in FIG. 6 Become. In FIG. 5, the voltages V _{C1 and} V _C2 are shown superimposed on the zero AC line.

The ratio of the capacitance value of the smoothing capacitor Cs to the capacitance value of the voltage dividing capacitors C1 and C2 is desirably about 1: 1/10000.
For example, when the capacitance value of the smoothing capacitor Cs is 7 to 10 to 100 μF
The capacitance value of the voltage dividing capacitors C1 and C2 is 0.004 to 0.01 μ
It is about F. This is considered as follows. If the capacity of the voltage dividing capacitors C1 and C2 increases, a pause period of the input current occurs near the AC of the input voltage, resulting in a low power factor current waveform (a so-called capacitor input type current waveform). On the other hand, if the capacitances of the voltage dividing capacitors C1 and C2 are small, a high voltage is obtained in the smoothing capacitor Cs, but it becomes unstable. Considering this point, the above ratio is appropriate.

As described above, according to the neutral point inverter 50, even when the input voltage Vi has an effective value of 100 V (Vm is about 140 V), an output voltage of about 270 to 300 V is applied across the inductor Lo. Will be obtained.

That is, as described above, the inductor
When a high-frequency alternating current flows through Lo alternately, a high-frequency voltage is generated in the inductor Lo. This voltage resonates by the resonance capacitor C4 connected in parallel with the fluorescent lamp LT to generate a high voltage, and the fluorescent lamp LT is turned on by this resonance voltage together with the preheating of the filament of the fluorescent lamp. After the fluorescent lamp LT is turned on, the fluorescent lamp current is controlled to a constant current by the function of the inductor La as a choke coil to stably light. Note that, depending on the type of the fluorescent lamp, lighting can be started in a cold cathode state, and in this case, the resonance capacitor C4 may not be used.

Further, regarding the harmonic current of the commercial power supply,
Since this is reduced by inserting a low-pass filter LPF into the input, it can sufficiently cope with the harmonic problem of commercial power. Rather, it is better because no peak current flows. Further, since this peak current does not flow, there is a degree of freedom in selecting the smoothing capacitor Cs, and various constants can be selected in consideration of the ripple voltage and the like. Thus, a stable high-frequency voltage having a small difference between the maximum peak Vmax and the minimum peak Vmin can be obtained (see FIG. 6).

Since the above neutral point type inverter device does not employ a completely smoothing method, desired characteristics can be obtained with a small-capacity smoothing capacitor Cs. However, a large-capacity capacitor reduces the ripple voltage. Needless to say. In the active smoothing filter system and the dither rectification system as described above, several hundred μF is usually required as the smoothing capacitor Cs, whereas in the neutral point inverter device 50, a smoothing capacitor Cs of 7 to 10 μF is desired. Therefore, a small-sized smoothing capacitor Cs can be used, which is suitable for downsizing the device.

Next, a neutral point type inverter 52 according to the present invention in which a capacitor is connected in series to an inductor will be described with reference to FIG.

This neutral point type inverter 52 has a configuration in which the inductor Lo of the load circuit of the inverter 50 shown in FIG. 1 is replaced with a series circuit consisting of an inductor Lo and a capacitor Cx. This is the same as the inverter 50 described above.

By connecting the capacitor Cx in series with the inductor Lo in this manner, the voltage due to the back electromotive force of the inductor Lo can be absorbed by the capacitor Cx, and the operation of the neutral point inverter 50 as a boost inverter can be reduced. Since the output voltage can be suppressed low by reducing the boosting effect described above, a voltage approximately corresponding to the peak of the input sine wave can be obtained, and the input voltage Vi can be obtained.
Is doubled, that is, 2Vi, it is possible to obtain, at both ends of the inductor Lo, almost the same output voltage as when the input voltage is set to Vi in the inverter 50.

Thus, for example, when the input voltage is 100 V, the capacitor Cx is not attached unlike the inverter 50, and when the input voltage is 200 V, the inverter 52 is not connected.
The same output voltage can be obtained in both cases if the capacitor Cx is attached as shown in the figure.Thus, it is necessary to change the withstand voltage of the capacitor Cf. , The input voltage can be easily switched. this is,
For example, as in Japan and the United States, the input voltage is 100V ~ 1
The device used in the country of the 20V zone has the configuration shown in FIG. 1, and the input voltage is 220
The same output voltage can be obtained by using the configuration shown in FIG. 7 in the devices used in the countries of the V-240V area, so that the neutral point common to the world except for the connection of the capacitor Cx is obtained. Type inverter can be provided, which is extremely effective.

Next, a neutral point inverter in which the primary winding of the transformer is replaced with the above-described inductor will be described with reference to FIGS. 8 and 9, the same elements as those in FIGS. 1 and 7 are denoted by the same reference numerals, and description thereof will be omitted unless otherwise required. First, the case where the capacitor Cx is not connected will be outlined with reference to FIG.

As shown in FIG. 8, the neutral point inverter 54 has a primary winding of a transformer T1 connected between the neutral point and the SW point, and a fluorescent lamp between the secondary winding of the transformer T1. LT is connected in parallel, and fluorescent lamp LT and resonance capacitor C4 are connected in parallel. Secondary winding of transformer T1 and resonance capacitor C4
Constitutes a parallel resonance circuit, and a resonance voltage generated at both ends of the resonance capacitor C4 is applied to the fluorescent lamp LT to stably light. Note that taps may be provided at both ends of the secondary winding of the transformer T1 for preheating the fluorescent lamp LT, and the filament may be turned on after the filament of the fluorescent lamp LT is preheated, thereby alleviating the rapid lighting of the fluorescent lamp LT.

The primary winding of the transformer T1 is connected to the primary inductor Le.
Is equivalent to the connection of the inductor Le between the neutral point and the SW point.
It can be seen that the same operation as the inverter 50 shown in FIG. Since the inverter 54 uses the transformer T1, the primary winding (primary inductor) Le of the transformer T1 is used.
Lamp that is connected to the secondary side effectively using the power supplied to the
It can be transmitted to LT.

In the inverter 54, the transformer T1
A high frequency voltage is supplied to the fluorescent lamp LT by the secondary winding of the fluorescent lamp LT, so that a desired high frequency voltage can be freely obtained according to the type of the fluorescent lamp LT without depending on the design of the primary side. it can. That is, the high-frequency alternating current alternately flows through the primary side of the transformer T1, and the
A high-frequency voltage corresponding to the winding ratio is generated on the secondary side. Fluorescent lamp LT
The secondary side voltage resonates by the resonance capacitor C4 connected in parallel with the fluorescent lamp LT, and the fluorescent lamp LT is turned on by the resonance voltage (high voltage) together with the preheating of the filament of the fluorescent lamp LT. Transformer T1 is a leakage type transformer that controls the fluorescent lamp current to a constant current and stably lights. Further, since power is supplied to the fluorescent lamp LT via the transformer T1, the fluorescent lamp LT is insulated from the primary side of the commercial power supply, and is excellent in safety. When a load having a negative resistance with respect to an applied voltage is used as in the fluorescent lamp LT, a leakage type transformer is used as the transformer T1 as described above, and a load having a positive characteristic is used. If used, a normal transformer may be used.

Next, a neutral point type inverter 56 according to the present invention in which a capacitor is connected in series to the primary winding of the transformer.
Will be described with reference to FIG.

The inverter 56 is connected to the connection point between the capacitors C1 and C2 of the primary winding of the transformer T1, that is, to the neutral point side.
A capacitor Cx is added so as to be connected in series with the primary winding (primary inductor). In this way, it is possible to set the winding ratio according to the high frequency voltage while effectively transmitting the power supplied to the primary inductor to the secondary side, and to insulate the secondary winding from the primary side. In addition to the above-described configuration, the inverter 52 shown in FIG.
Similarly to the above, by switching the connection / non-connection of the capacitor Cx, it is possible to easily cope with the switching of the input voltage.

As described above, the basic configuration of the present invention in which a capacitor is connected in series to an inductor and an inductor and a load are connected in parallel is not limited to those shown in FIGS. As shown in the basic circuit diagram of FIG. 10, the present invention can be applied to any conventional neutral point inverter having a load connected in parallel to the inductor shown in FIG.

For example, in the configuration shown in FIG. 9, an inductor may be further connected in series to the primary winding of the transformer T1. By adding this inductor, the voltage generated between the primary windings of the transformer T1 can be reduced by the voltage generated in the inductor, and the secondary winding of the transformer T1 can be changed without changing the turns ratio of the transformer T1. Can be stepped down to a predetermined voltage according to the load connected to the
The heat generation of T1 can also be prevented.

Further, the transformer T1 may be changed to an automatic transformer, and a load such as a fluorescent lamp LT may be connected between one end of the automatic transformer and the tap output. In this case, the same operation as that in which the inductor is connected in series to the primary winding of the transformer T1 is performed.

Further, a zero-cross detection circuit for detecting a crossing point of AC zero of the fluorescent lamp current (AC current) may be provided, and the driving circuit DR may be controlled based on the output of the zero-cross detection circuit. Generally, the secondary winding of the transformer T1 is used when the fluorescent lamp LT starts lighting and when the fluorescent lamp LT continues lighting.
The resonance frequency of the resonance circuit consisting of the fluorescent lamp LT and the resonance capacitor C4 fluctuates.If the zero-crossing detection circuit detects the intersection of the fluorescent lamp current with AC zero and detects this frequency fluctuation, the fluorescent light will always be in the optimum state. The drive circuit can be controlled so that the high frequency voltage is applied to the lamp LT.

In the neutral point inverter according to the present invention, it is desirable to detect an abnormal state of the load circuit and stop the operation of the drive circuit DR when an abnormality occurs to improve the reliability of the inverter.

Specifically, for example, an abnormal voltage is generated in the fluorescent lamp, an abnormal current is flowing, the filament is broken, the fluorescent lamp is leaking, and a fluorescent lamp of an inappropriate size is connected. If the device detects an abnormal state, such as the presence of a fluorescent lamp or no fluorescent lamp, and determines that the device is in an abnormal state, the operation of the drive circuit is stopped to stop the drive of the switching element, and the switching element and the load are stopped. It is desirable to prevent deterioration and destruction of each element such as a circuit. As a method of detecting an abnormal state of the load circuit, for example, a method of detecting a voltage generated on the secondary side of the transformer or a current flowing on the secondary side, or connecting a snubber circuit including a resistor and a capacitor in parallel to the switching element. Then, there is a method of detecting an abnormal state by a voltage generated in the snubber circuit.

When the operation of the drive circuit is stopped, it is preferable to stop the voltage supply to the drive circuit by operating the thyristor when an abnormal state occurs, for example, by using a thyristor. Alternatively, a startup circuit is provided, when power is supplied to the neutral point inverter, power is supplied from the startup circuit to the drive circuit, and after a predetermined time has elapsed, power is supplied from its own output voltage. Power supply from the output voltage may not be performed.

In the above description, a load using a fluorescent lamp has been described as a load. However, the present invention is not limited to this, and may be applied to various loads as long as the load is operated by supplying AC power. it can. In that case, the resonance capacitor connected in parallel with the load is not always necessary.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a neutral point inverter in which an inductor and a fluorescent lamp (load) are connected in parallel.

FIG. 2 is a voltage waveform diagram of a first capacitor of the neutral point inverter.

FIG. 3 is a voltage waveform diagram of a second capacitor of the neutral point inverter.

FIG. 4 is a voltage waveform diagram of a smoothing capacitor of the neutral point inverter.

FIG. 5 is a voltage waveform diagram (actual) of the first and second capacitors of the neutral point inverter.

FIG. 6 is a voltage waveform diagram of a load of the neutral point inverter.

FIG. 7 is a circuit diagram of a neutral point inverter according to the present invention in which a capacitor and an inductor are connected in series.

FIG. 8 is a circuit diagram of a neutral point inverter in which an inductor is replaced by a primary winding of a transformer.

FIG. 9 is a circuit diagram of a neutral point inverter according to the present invention in which a capacitor and a primary winding of a transformer are connected in series.

FIG. 10 is a basic circuit diagram of a neutral point inverter according to the present invention.

FIG. 11 is a basic circuit diagram of a neutral point inverter.

FIG. 12 is a basic circuit diagram of a neutral point inverter in which an inductor and a load are connected in parallel;

[Explanation of symbols]

Vi AC power supply LPF Low pass filter DB rectifier Q1, Q2 Switching element D1, D2 diode C1, C2 Voltage dividing capacitor (first and second capacitors) Cs Smoothing capacitor (third capacitor) C4 Resonant capacitor DR drive circuit Lo inductor R Load circuit LT fluorescent lamp (load)

Claims (1)

[Claims] 1. A low-pass filter that passes a fundamental frequency of an alternating current and cuts off a harmonic signal, a rectifier that rectifies an AC voltage that has passed through the low-pass filter, and is connected to an output of the rectifier. A series circuit of first and second capacitors; a third capacitor for smoothing the output of the rectifier; a series circuit of first and second switching elements connected to the output of the rectifier; A first and a second diode connected in parallel to the second switching element so as to be in the opposite direction in the DC direction, a drive circuit for driving the first and the second switching element, and an AC input of the rectifier Is connected to a connection point of the first and second capacitors, and the first and second capacitors are connected between the connection point and a connection point of the first and second switching elements. And a load circuit to which AC power is supplied by alternately turning on and off the two switching elements, wherein the load circuit comprises: a series circuit including an inductor and a fourth capacitor; A neutral point inverter comprising a load connected in parallel with the inductor and supplied with the AC power by a voltage generated in the inductor.