JPH0442779A - Power unit - Google Patents

Abstract

PURPOSE:To make use capacitor capacity for auxiliary power source effectively by the voltage rise action of a reactor so as to lengthen power stoppage compensating time by turning on an auxiliary switch when the AC voltage of an AC power terminal becomes zero or a lower value or when the voltage on the output side of a capacitor for smoothing drops. CONSTITUTION:At the output stage of a main switch 9, the total voltage of the sine wave rectified output voltages of input terminals 1 and 2 and the voltage based on the energy accumulated in a reactor 8 can be gotten. Moreover, a capacitor 19 for auxiliary power supply is charged through resistance 21. If the voltages of the AC power terminals 1 and 2 become zero or values lower than the specified values due to the power stoppage, or the like, an auxiliary control circuit 22 detects it and turns on an auxiliary switch 20, and starts the power supply from the capacitor 19 for auxiliary power supply. Even if the voltage of the capacitor for auxiliary power supply drops, the voltage is stepped up by the step-up action of the reactor 8, and high output voltage can be gotten.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a power supply device having an auxiliary power supply consisting of a storage battery or a capacitor.

[Prior art and problems to be solved by the invention] A step-up converter in which a rectifier circuit is connected to an AC power supply, the output stage of this rectifier circuit is connected via a reactor, and a smoothing circuit is connected to the output stage of a switch. is publicly known.

In addition, a method in which a rectifier circuit and a reactor are connected between an AC power supply and an inverter, and a conversion switch of the inverter is used to intermittently short-circuit the DC lines to store energy in the reactor is disclosed in Japanese Patent Publication No. 1986-1.90
It is disclosed in Japanese Patent Application No. 557 (Japanese Patent Application No. 19453/1982).

When the switch in this type of converter or the conversion switch of an inverter is controlled on/off at high frequency,
The sinusoidal voltage is interrupted, resulting in an approximately sinusoidal input current. Further, the power factor becomes approximately 1.

This type of converter does not include a smoothing capacitor for smoothing the rectified output obtained from the rectifier circuit. Furthermore, a storage battery charged by the output of the rectifier circuit cannot be connected as a backup power source.

Therefore, when there is a power outage or a voltage drop in the AC power supply, power supply is continued for a short period of time only by the output smoothing capacitor.

Furthermore, in conventional general converters, a large capacity capacitor or storage battery may be provided as a backup power source for the DC power source. However, when the voltage of the capacitor or storage battery decreases, it becomes impossible to obtain a desired output voltage, and as a result, the capacity of the capacitor or storage battery cannot be effectively utilized.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a power supply device that can effectively utilize the capacity of a capacitor or a storage battery.

[Means for Solving the Problem] The present invention for achieving the above object includes an AC power supply terminal,
A rectifier circuit connected to the AC power terminal, a main switch connected between one output terminal and the other output terminal of the rectifier circuit, and a power line between the rectifier circuit and the main switch. A reactor connected in series, a smoothing capacitor connected in parallel to the main switch via a rectifier diode, and controlling the main switch on and off so that the voltage of the smoothing capacitor becomes a constant value. a main switch control circuit, and a capacitor connected via an auxiliary switch between one output terminal and the other output terminal of the rectifier circuit or between a pair of input terminals of the rectifier circuit on the input side of the reactor. or an auxiliary power source consisting of a storage battery, a charging circuit for charging the auxiliary power source, and when the AC voltage at the AC power supply terminal becomes zero or a low value, or when the voltage on the output side of the smoothing capacitor decreases; The present invention relates to a power supply device characterized by comprising an auxiliary switch control circuit that controls turning on of the auxiliary switch.

In addition, as shown in claim 2, it is desirable to control the main switch so that the voltage of the smoothing capacitor (output voltage) is set to a predetermined value, and the current of the AC power supply terminal is approximated to an input waveform or a sine wave. .

Further, as shown in claim 3, it is desirable to replace the main switch of claim 1 with an inverter.

Moreover, as shown in claim 4, a rectifying and smoothing circuit can be connected to the output stage of the inverter.

Moreover, as shown in claim 5, it is desirable to control the inverter so that the current at the AC power supply terminal approximates an input waveform or a sine wave and obtains a desired output voltage.

Furthermore, as shown in claims 6 and 7, the rectifier circuits of claims 1 and 3 can be replaced with a DC power supply that supplies a flat voltage.

[Function] According to each claimed invention, the voltage of the auxiliary power source is level-converted (controlled) by the reactor and the main switch.

Therefore, even if the voltage of the auxiliary power supply decreases, it can be increased by the boosting action of the reactor. Therefore, a desired output voltage can be obtained in a wide range from a high value to a low value of the voltage of the auxiliary power supply. In short, the capacity of the auxiliary power source can be used effectively.

[First Embodiment] Next, a power supply device according to an embodiment of the present invention will be described based on FIGS. 1 to 3. A full-wave rectifier circuit 3 consisting of four bridge-connected diodes D1, D2, D3, and D4 is connected to a power supply terminal 1.2 in FIG. 1 that supplies a commercial AC voltage of, for example, 50 Hz.

A main switch 9 is connected via a reactor 8 between a pair of power supply lines 6.7 connected to a pair of output terminals 4.5 of the rectifier circuit 3. The main switch 9 consists of an N-channel insulated gate field effect transistor whose source is connected to the substrate, and has a built-in diode.

A smoothing capacitor 11 is connected in parallel to the main switch 9 via a rectifier diode 10 . A load]4 is connected between a pair of output terminals 12 and 13 connected to the smoothing capacitor 11.

A main switch control circuit 15 connected to a control terminal (gate) of the main switch 9 supplies an on/off control signal to the main switch 9. This control circuit 15 includes an output voltage detection circuit 16 connected to an output terminal 12.13,
A current detector 17 connected to the input line of the rectifier circuit 3 and an input voltage detection circuit 1 connected to the AC power supply terminal 1.2.
8 are connected to each other.

19 is a capacitor for auxiliary power supply, and auxiliary switch 2
0 between the output power lines 6 and 7 of the rectifier circuit 3. As a charging circuit for the auxiliary power supply capacitor 19, a charging resistor 21 is connected between the output terminal 12 and one end of the auxiliary power supply capacitor 19.

The auxiliary switch 20 consists of a thyristor, and this gate (
An auxiliary switch control circuit 22 is connected to the control terminal (control terminal). The auxiliary switch control circuit 22 consists of a power outage or voltage drop detection circuit connected to the power supply terminal 1.2, and turns on the auxiliary switch 20 when a power outage or voltage drop is detected.

FIG. 2 shows the main switch control circuit 15 in detail.

The output line of the current detector 17 is connected to the first full-wave rectifier circuit 23
It is connected to one input terminal (inverting input terminal) of the first error amplifier 24 via. The output line of the input voltage detection circuit 18 is connected to the other input terminal (non-inverting input terminal) of the first error amplifier 24 via a second full-wave rectifier circuit 25 and a coefficient circuit, ie, a multiplier 26. . The first error amplifier 24 generates an output corresponding to the difference between the current including the ripple component and the sinusoidal voltage.

In order to control the main switch 9 to keep the output voltage constant, the output line of the output voltage detection circuit 16 is connected to one input terminal (inverting input) of the second error amplifier 27,
A reference voltage source 28 is connected to the other input terminal (inverting input) of the error amplifier 27. This second error amplifier 27 generates an output voltage corresponding to the difference between the detection voltage and the reference voltage, and sends it to the multiplier 26 . The multiplier 26 adds a second amplitude to the reference sine wave waveform provided from the second full-wave rectifier circuit 25.
A value obtained by multiplying the output of the error amplifier 27 by the output of the error amplifier 27 is applied to the non-inverting input terminal of the first error amplifier 24.

One input terminal of the voltage comparator 29 is connected to the low pass filter 3.
0 to the output terminal of the first error amplifier 24, and the other input terminal is connected to the sawtooth wave generation circuit 31. The output terminal of comparator 29 is connected to main switch 9.

[Work] Next, the operation of the circuit shown in FIG. 1 will be explained.

In the circuit shown in FIG. 1, no capacitor is connected between the output lines 6 and 7 of the rectifier circuit 3 for smoothing the full-wave rectified waveform of the 50 Hz AC power supply voltage.

The output stage of the main switch 9 has a sine wave rectified output voltage corresponding to the sine wave AC power supply voltage of the input terminal 1.2 and a reactor 8.
It is possible to obtain the sum voltage based on the stored energy of .

When the main switch 9 is controlled on/off, the currents on the input side and the output side of the rectifier circuit 3 change accordingly. The on/off frequency of the main switch 9 is, for example, 5QHz, which is sufficiently higher than the power supply frequency (50Hz) of the input terminal 1.2, so even if the sine wave voltage is interrupted by the main switch 9, it will not pass through the reactor 8. The current flowing through the current and the corresponding AC input current become an approximate sine wave including ripples without interruption. That is, as shown in FIG. 3(A), the error signal AI obtained through the low-pass filter 30 and the sawtooth signal A2 are compared by the comparator 29, and the result is determined by the output of the comparator 29 shown in FIG. 3(B). When the main switch 9 is controlled on and off, a current detection signal F1 of an approximate sine wave including ripples as shown in FIG. 3(C) is obtained.

The current detection signal F1 is input to one input terminal of the error amplifier 24, and the current detection signal F1 is input to the other input terminal from the multiplier 26 as shown in FIG. 3(C).
When the reference sine wave F2 shown in FIG. Comparison of signal A1 and sawtooth wave A2 The width of the output pulse changes as the output voltage changes. Control to keep the output voltage constant is thus achieved.

Note that the main switch 9 is turned on during periods tl-t2, t3-t4, and t5-t6, and the current detection signal F1 gradually increases. t2~t3, t4~ when the main switch 9 turns off
During the t5 period, the current detection signal Fl gradually decreases.

The smoothing capacitor 11 is charged to a value higher than the peak value of the output voltage of the rectifier circuit 3 by the voltage of the reactor 8 . Further, the auxiliary power supply capacitor 19 is also charged via the resistor 21, and similarly to the smoothing capacitor 11, it is charged to a value higher than the peak value of the rectified voltage.

When the voltage at the AC power supply terminal 1.2 becomes zero or a value lower than a predetermined value due to a power outage or the like, this is detected by the auxiliary switch control circuit 22, and the auxiliary switch 20 is turned on. As a result, the auxiliary power supply capacitor 1 replaces the rectifier circuit 3.
Power supply starts from 9. Auxiliary power supply capacitor 1
Even if the voltage of the auxiliary power supply capacitor 19 decreases, it is possible to increase the voltage by the boosting action of the reactor 8 and obtain the output voltage, so the energy of the auxiliary power supply capacitor 19 can be effectively used and the power supply time during a power outage can be extended. .

[Second Example] Next, a power supply device of a second example shown in FIG. 4 will be described. However, in FIG. 4 and FIGS. 5 to 13, which will be described later, parts that are substantially the same as those in FIGS. 1 to 3 are designated by the same reference numerals, and their explanations will be omitted.

The circuit of FIG. 4 is the circuit of FIG. 1 with a charging diode 41 and a peak value holding capacitor 42 added. Note that the at of the diode 41 is connected to the output end of the reactor 8. Therefore, the voltage before smoothing is rectified by the diode 41, and its peak value is first rectified by the capacitor 42.
After that, the charge of the capacitor 42 is transferred to one capacitor via the resistor 21.

Even with this configuration, the same effects as in the embodiment can be obtained.

[Third Embodiment] In a third embodiment shown in FIG. 5, an inverter 51 is connected to the rectifier circuit 3 via a reactor 8 instead of the main switch 9 of the first and second embodiments. The inverter 51 includes first, second, third, and fourth conversion switches Ql, Q2, Q3, and Q4 connected in a bridge manner, and an output transformer 52.
It consists of The primary winding 53 of the output transformer 52 is connected between the connection point between the first and second conversion switches Q1 and Q2 and the connection point between the third and fourth conversion switches Q3 and Q4. The secondary winding 54 is formed with a center tap,
It is connected to a rectifying and smoothing circuit 58 consisting of diodes 55, 56 and a capacitor 57.

The inverter control circuit 15a is the same in most respects as the main switch control circuit 15 of FIG. 2, and is configured as shown in FIG. 6. As is clear from a comparison between FIG. 2 and FIG. 6, a control signal forming circuit 59 for the first to fourth conversion switches Q1 to Q4 is added.

The control signal forming circuit 59 includes, for example, as shown in FIG. 8, a square wave generating circuit 62, a NOT circuit 61, a trigger pulse generating circuit 62, and a trigger type flip-flop 63.
It consists of The square wave generating circuit 60 generates a fixed square wave pulse for controlling the on/off of the first switch Q1 shown in FIG. 7(B). The NOT circuit 61 is connected to the square wave generation circuit 60 and generates a square wave that controls the third switch Q3 shown in FIG. 7(C). Square wave generation circuit 6
0 is also connected to the sawtooth wave generating circuit 31. The sawtooth wave generation circuit 31 generates the sawtooth wave A2 shown in FIG. 7(A) in synchronization with the square wave shown in FIG. 7(B). That is, the sawtooth wave generating circuit 31 generates the sawtooth wave A2 in response to the leading edge and trailing edge of the pulse shown in FIG. 7(B).

The comparator 29 generates the comparison output shown in FIG. 9(A) based on the comparison between the signal AI shown in FIG. 7(A) and the sawtooth wave A2. A trigger pulse generation circuit 62 connected to the comparator 29 generates the trigger pulse shown in FIG. 9(B) in response to the leading edge of the comparison output pulse shown in FIG. 9(A).

Each time the trigger pulse shown in FIG. 9(B) is input from the trigger pulse generation circuit 62 to the trigger input terminal T of the trigger type flip-flop 63, the state of the output terminal changes.
The switch control signal shown in FIG. 9(C) is applied from the non-inverting output terminal to the second switch Q2, and the switch control signal shown in FIG. 9(D) is applied from the inverting output terminal to the fourth switch Q4.

The pulses in FIGS. 9C and 9D are the same as the pulses in FIGS. 7D and 7E.

Note that the control signal forming circuit 59 is not limited to that shown in FIG. 8, and can be modified in various ways, for example, it can be configured with a logic gate circuit.

As shown in FIG. 7(A), the comparator 29 compares the signal A1 with the sawtooth wave A2, and based on this, the signal shown in FIG. 7'(B) is
When the switch control signals of ~(E) are formed and the conversion switches Q1~Q4 are controlled on/off, tO~t1°, t
In the periods 2 to t3 and t4 to t5, the power supply lines on the output side of the reactor 8, that is, the input terminals of the inverter are short-circuited. Therefore, energy is accumulated in the reactor 8 during this period. During periods tl-t2, t3-t4, etc., the inverter operates normally, and as shown in FIG. 7(G), an AC voltage with a frequency sufficiently higher than the commercial voltage can be obtained from the transformer 52.

The current detection signal F1 is compared with a reference sine wave signal F2 as shown in FIG. 7(F), and controlled to follow this. Therefore, the current at the power supply terminal 1.2 becomes an approximate sine wave, and the power factor becomes approximately 1.

The principle of output voltage control in the inverter 51 is shown in Example 1.
is the same as

In this embodiment, an input terminal of an inverter 51 is connected to a peak charging capacitor 42 via a diode 41. Therefore, the capacitor 42 is connected to the reactor 8
is charged to the peak value of the boosted voltage, and this charge is transferred to the auxiliary power supply capacitor 19. As a result, the second embodiment provides the same effects as the first embodiment.

[Fourth Example] Next, a power supply device according to a fourth example will be described with reference to FIG. 10. In this embodiment, a parallel inverter 51a is connected between the output line of the reactor 8 and the power supply line 7. The inverter 51a is the first and second conversion switch Ql.

Q2 and a center tap type transformer 52.

The center tap of the primary winding 53 of the transformer 52 is connected to the output terminal of the reactor 8, and first and second conversion switches Ql are connected between both ends of the primary winding 53 and the power supply line 7.
, Q2 are connected to each other.

When operating this power supply device, first, the control circuit 15
The first and second conversion switches Ql and Q
Turn on 2 at the same time. As a result, energy is accumulated in the reactor 8 as in FIG. 1. Next, turn on the first conversion switch Ql, and turn on the second conversion switch Q2.
Turn off. As a result, a voltage obtained by adding the voltage of the reactor 8 to the power supply voltage is applied to the upper half of the primary winding 53, and a corresponding voltage is obtained in the secondary winding 54. next,
The first and second conversion switches Q1 and Q2 are turned on simultaneously again to store energy in the reactor 8. Next, the second conversion switch Q2 is turned on and the first conversion switch Q1 is turned off. As a result, a voltage obtained by adding the voltage of the reactor 8 to the power supply voltage is applied to the lower half of the primary winding 53, and a voltage in the opposite direction to the previous one is obtained in the secondary winding 54.

The principles of controlling the output voltage and improving the input current waveform are as follows:
This is the same as the third embodiment. Therefore, the fourth embodiment can also provide the same effects as the first to third embodiments.

[Fifth Embodiment] In the power supply device of the fifth embodiment shown in FIG. 11, an inverter 51b includes a conversion main switch Q, a capacitor C, and a transformer 52. During the period when the main switch Q is controlled to be turned on by the control circuit 15c, the reactor 8
At the same time that energy is stored in the capacitor C, a discharge circuit of the capacitor C is formed, and a current flows through the primary winding 53 from the bottom to the top. While the main switch Q is off, a current flows from top to bottom through the primary winding 53 in a circuit consisting of the reactor 8, the capacitor C, and the primary winding 53.

The capacitor 42 is connected to the tertiary winding 7 provided in the transformer 52.
0 via a diode 41. Since a constant voltage is obtained in the transformer 52, the capacitors 42 and 19 are charged with a constant voltage.

The fifth embodiment also provides the same effects as the first to fourth embodiments.

[Sixth Embodiment] The power supply device of FIG. 12 showing the sixth embodiment is obtained by omitting the input current waveform improvement part from the circuit of FIG. 1. In this case, since the input waveform is not improved, a smoothing capacitor Cr is connected to the output stage of the rectifier circuit 3.

Therefore, the DC voltage smoothed by the main switch 9 is switched on and off. The auxiliary power supply capacitor 19 of this embodiment has the same effects as those of the first to fifth embodiments. Note that the charge in the smoothing capacitor Cr is also used during a power outage.

[Seventh Embodiment] The power supply device of the seventh embodiment shown in FIG. 13 is obtained by removing the input current waveform improvement portion from the circuit shown in FIG. 5. In this embodiment, since the smoothing capacitor Cf is connected between the output lines 6 and 7 of the rectifier circuit 3, the inverter 51 is driven with a smoothed voltage. This embodiment also has the same effect as that shown in FIG.

[Modifications] The present invention is not limited to the above-described embodiments, and, for example, the following modifications are possible.

(1) In each embodiment, the control circuit 22 may detect a power outage or voltage drop based on the output voltage of the rectifier circuit 3, the voltage on the output side of the smoothing capacitor 11.57, or the output voltage of the inverter.

(2) The current detector 17 may be provided on the output line 6 or 7 of the rectifier circuit 3. Furthermore, in the case of FIG. 5, the input current may be detected indirectly based on the currents of the switches Q1 to Q4.

(2) A diode may be connected in series with the resistor 21 in FIG.

(3) In FIGS. 5, 10, and 13, the transformer 52 may be provided with a tertiary winding, and the capacitor 19.42 may be charged with the voltage of this tertiary winding.

(4) Capacitor 19 can be replaced with a storage battery.

(5) The chopper circuit or inverter circuit after the reactor 8 can be modified in various ways.

(6) A high frequency removal filter can be connected between the power supply terminal 1.2 and the rectifier circuit 3.

(7) A capacitor Cf can be connected between a pair of input terminals of the rectifier circuit 3.

[Effects of the Invention] As is clear from the above, according to the invention of each claim, the capacity of the auxiliary power supply capacitor or storage battery can be effectively utilized by the boosting action of the reactor, and the power outage compensation time can be extended. can.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a power supply device according to a first embodiment of the present invention, Fig. 2 is a block diagram showing the main switch control circuit of Fig. 1, and Fig. 3 shows the state of each part in Fig. 2. 4 is a circuit diagram showing a power supply device according to a second embodiment of the present invention, FIG. 5 is a circuit diagram showing a power supply device according to a third embodiment of the present invention, and FIG. 6 is a circuit diagram showing a power supply device according to a third embodiment of the present invention. 7 is a waveform diagram showing the states of each part in FIGS. 5 and 6. FIG. 8 is a block diagram showing the control signal forming circuit in FIG. 6. FIG. 9 is a block diagram showing the control signal forming circuit in FIG. 6. A waveform diagram showing the state of each part in Fig. 8, Fig. 10,
FIGS. 11, 12, and 13 are circuit diagrams showing the fourth, fifth, sixth, and seventh embodiments, respectively. 1.2... Power supply terminal, 3... Rectifier circuit, 8... Reactor, 9... Main switch, 15... Main switch control circuit, 19... Auxiliary power supply capacitor, 20...
- Auxiliary switch, 22... Auxiliary switch control circuit. Agent: Nori Takano Figure 1, Figure 2...Archie - Prefectural Hamaguri Criminal Shinkan (Figure 8, Figure 9 CD)] - 1 - 1-cho-11

Claims (1)

[Scope of Claims] [1] An AC power terminal; a rectifier circuit connected to the AC power terminal; and a main switch connected between one output terminal and the other output terminal of the rectifier circuit; a reactor connected in series to a power supply line between the rectifier circuit and the main switch; a smoothing capacitor connected in parallel to the main switch via a rectifier diode; and a voltage across the smoothing capacitor. a main switch control circuit that controls on/off the main switch to maintain a constant value; and a pair of the rectifier circuits between one output terminal and the other output terminal of the rectifier circuit on the input side of the reactor. an auxiliary power source consisting of a capacitor or storage battery connected between the input terminals of the auxiliary power source via an auxiliary switch; a charging circuit for charging the auxiliary power source; Alternatively, a power supply device comprising: an auxiliary switch control circuit that turns on the auxiliary switch when the voltage on the output side of the smoothing capacitor decreases. [2] The main switch control circuit controls the main switch so that the voltage of the smoothing capacitor reaches a desired value and the current of the AC power supply terminal approximates an input waveform or a sine wave. The power supply device according to claim 1, which is a circuit that turns on and off in a sufficiently short cycle. [3] An AC power supply terminal, a rectifier circuit connected to the AC power supply terminal, a reactor connected in series to the output line of the rectification circuit, and a reactor connected to the rectification circuit via the reactor and turned on/off. an inverter including a conversion switch that is turned off; an inverter control circuit that controls the conversion switch of the inverter so that the output voltage of the inverter is a constant value; and an inverter control circuit that is located on the input side of the reactor. an auxiliary power source consisting of a capacitor or a storage battery connected via an auxiliary switch between one output terminal and the other output terminal of the rectifying circuit or between a pair of input terminals of the rectifying circuit; and a charging device for charging the auxiliary power source. and an auxiliary switch control circuit that controls the auxiliary switch to turn on when the AC voltage at the AC power supply terminal becomes zero or a low value or when the voltage at the output side of the inverter decreases. [4] The power supply device according to claim 3, further comprising a rectifying and smoothing circuit connected to the inverter. [5] The inverter control circuit intermittently shorts the DC input lines of the pair of inverters using the conversion switch so that the current flowing through the AC power supply terminal approximates an input voltage waveform or a sine wave. 5. The power supply device according to claim 4, further comprising a circuit that controls the conversion switch so that the output voltage of the rectifying and smoothing circuit becomes constant. [6] A DC power supply; a main switch connected between one end and the other end of the DC power supply; a reactor connected in series to a power line between the DC power supply and the main switch; A smoothing capacitor connected in parallel to the switch via a rectifying diode, a main switch control circuit that controls on/off the main switch so that the voltage of the smoothing capacitor is constant, and from the reactor. An auxiliary power source consisting of a capacitor or a storage battery connected between the power supply lines on the input side through an auxiliary switch, a charging circuit for charging the auxiliary power source, and a voltage of the DC power source that becomes zero or a low value. and an auxiliary switch control circuit that controls the auxiliary switch to turn on when the voltage on the output side of the smoothing capacitor decreases. [7] A DC power source; an inverter connected to the DC power source via a reactor; an inverter control circuit that controls a conversion switch of the inverter so that the output voltage of the inverter is constant; An auxiliary power source consisting of a capacitor or a storage battery connected between the power supply lines on the input side via an auxiliary switch, a charging circuit for charging the auxiliary power source, and when the voltage of the DC power source becomes zero or a low value. Alternatively, a power supply device comprising an auxiliary switch control circuit that turns on the auxiliary switch when the voltage on the output side of the inverter decreases.