JPH11113256A - Three-phase power factor improved converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a three-phase power factor improved converter which can suppress the occurrence of ripple currents and higher harmonics and can accomplish current balance in each phase, by connecting three sets of single- phase converters having higher harmonic suppressing function among the phases of a three-phase three-wire AC power source. SOLUTION: In a three-phase power factor improved converter in which three sets of single-phase converters having higher harmonic suppressing functions are connected among the phases of a three-phase three-wire AC power source, high harmonic suppressing circuits 1, 17, and 18 and insulated DC-DC converters 8, 19, and 20 are respectively connected to the single-phase converters. The circuits 1, 17, and 18 are respectively composed of rectifier circuits and boosting chopper circuits which boost the outputs of the rectifier circuits to high DC voltages. The outputs of the insulated DC-DC converters 8, 19, and 20 which input the outputs of the boosting chopper circuits are connected in parallel with each other. Each insulated DC-DC converter 8, 19, and 20 has a current mode control circuit 23 composed of an output voltage detecting error amplifier 22 which outputs control signals commonly to the converters 8, 19, and 19.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a three-phase power factor improving converter having a harmonic current suppressing function in a three-phase three-wire AC power supply.

[0002]

2. Description of the Related Art In a three-phase three-wire AC power supply, three input / output insulated converters each having a single-phase harmonic suppression function are connected between each phase, and their outputs are connected in parallel to form a power supply circuit. Methods of construction are known.

When each converter is provided with a control circuit individually in this circuit configuration, the current balance of each phase, that is,
The load current shared by each converter is not always the same. Therefore, a function for balancing current is required. (2) The main switch of the DC-DC converter of each phase is driven by the same control signal, and the current of each phase is balanced by regulating the output voltage of the step-up chopper.
−１120358.

FIG. 4 shows an example of this circuit. FIG.
Numerals 1, 17, and 18 denote step-up type harmonic suppression circuits whose inputs are connected between the respective phases of a three-phase three-wire system. The outputs are each connected to the input of an isolated DC-DC converter. Each DC
The outputs of the DC converters 8, 19 and 20 are connected in parallel to supply DC power to the load 16; The harmonic suppression circuit sections 1, 17, and 18 are step-up harmonic suppression circuits each including a rectifier diode bridge 2, a choke coil 3, a switching element 4, a diode 5, a smoothing capacitor 6, and a control circuit 7.

The DC-DC converters 8, 19 and 20 are respectively composed of main switches 9 and 10, which are simultaneously turned on and off, a transformer 11, a rectifier diode 12, a flywheel diode 13, a smoothing choke coil 14, and a smoothing capacitor 15. Forward converter. An output voltage detection error amplifier 22 compares the output voltage with a reference value and amplifies the error. The output signal of 22 is sent to the width control circuit 25 and determines the pulse width of the converter necessary to keep the output voltage constant. The current transformer detects the switching current and controls the pulse width to provide overcurrent protection.

The three DC-DC converters 8, 19, 2
Since 0 is driven by the same pulse, if the input voltage of the DC-DC converter, that is, the output voltage of the harmonic suppression circuit is constant and the characteristics of each component are the same, each DC-DC converter 8, 19, The current of 20 is balanced, and the currents of the harmonic suppression circuits 1, 17, and 18 as input power sources are also balanced. (3)

Actually, the characteristics of the components vary, but the output voltage of the harmonic suppression circuit has a negative fluctuation characteristic with respect to the output current.
The input voltage of the C-DC converter decreases to reduce the current, and thus acts to maintain the balance. As described above, according to this method, it is possible to suppress the harmonic current at the input of each phase and satisfy the current balance with a very simple circuit.

However, such a conventional technique has the following problems. The input current that has been subjected to full-wave rectification is smoothed by the smoothing capacitor 6, but a ripple voltage is generated which is determined by the input current, the capacitance and characteristics of the capacitor. Although the output of the step-up chopper, that is, the input of the DC-DC converter, has the same average value in one cycle of the commercial power supply in each phase, the ripple voltage corresponds to the phase difference of the AC input voltage of each phase. Considering this, there is a difference corresponding to the phase between each phase.

Therefore, when the main switch of the DC-DC converter of each phase is driven by the same control signal, the current of the DC-DC converter of each phase changes in a positive relationship with the input voltage. In-phase ripple currents are superimposed.

[0010] The ripple current increases the effective value of the current in each part of the DC-DC converter, resulting in a large power loss. Further, in order to reduce the ripple current, it is necessary to increase the capacity of the output smoothing capacitor 6 of the harmonic suppression circuit.

Also, the output current of the three-phase power factor correction type converter (load current as viewed from the parallel connection point of the three DC-DC converters) is detected, and a current command is sent from the current sharing device to the boost chopper or the DC-DC converter. A method has also been proposed in which a current is supplied to a converter to balance the current of each phase. (4)

FIG. 5 shows an example of this method. The same reference numerals indicate the same circuit parts and components as those in FIG. Reference numeral 28 denotes a DC current detector, and reference numeral 27 denotes a shared signal generator, which amplifies the voltage of the current detector and generates a command voltage of the shared current. 26 is a current control unit which detects the current of the unit from the current transformer 21 and
The reference voltage for controlling the output voltage is controlled and sent to the pulse width control unit 25 so as to eliminate the error from the signal 27. 25
Then, the reference voltage is compared with the output of the output voltage detection error amplifier 22 to determine the pulse width.

The drive circuit 24 drives the main switch with this pulse width. Therefore, the current of the DC-DC converter is always balanced, and the current of the harmonic suppression circuit using this as a load is also balanced. Further, the ripple current can be suppressed. However, the circuit configuration and each control unit 2
8, 27 and 26 are complicated and cost increase is inevitable. In addition, an increase in the number of components requires more mounting space, which is an obstacle to miniaturization. As described above, the conventional technology has a drawback that the ripple current causes the effective current value of each part to increase, and the power loss to increase. To suppress this ripple current, a large-capacity electrolytic capacitor is required if a smoothing capacitor is used, and a current detector and a load-sharing control device are required if an electronic circuit is used. Had the disadvantage of becoming larger.

[0014]

SUMMARY OF THE INVENTION The present invention uses a current mode control circuit to control a DC-DC converter, thereby suppressing ripple current with a simple circuit configuration, suppressing harmonics, and balancing the current of each phase. Is to achieve. (5)

[0015]

SUMMARY OF THE INVENTION In a three-phase power factor correction type converter in which three sets of single-phase converters having a harmonic suppression function are connected between each phase of a three-phase three-wire AC power supply, each single-phase converter has a The wave suppression circuit and the isolated DC-DC converter are connected one by one. The harmonic suppression circuit includes a rectifier circuit and a boost chopper circuit that boosts the output of the rectifier circuit to a high DC voltage. The outputs of the isolated DC-DC converter that receives the output of the boost chopper circuit as inputs are connected in parallel. The insulated DC-DC converter has a current mode control circuit, and the insulated DC-DC converter is commonly controlled by an output of an output voltage detection error amplifier.

[0016]

FIG. 1 shows an embodiment of the present invention, in which circuit portions and components having the same reference numerals as those in FIGS. FIG. 2 is a block diagram of the current mode control, and the same reference numerals as those in FIG. FIG. 3 shows the operation of FIG.
The signs of the waveforms correspond to the signs indicating the respective parts in FIG.

First, the current mode control will be described with reference to FIGS. In FIG. 2, reference numeral 23 is a block diagram showing an example of the current mode control circuit. The flip-flop 54 is set by the clock pulse (D in FIG. 3) of the clock pulse generator 53. When set, the flip-flop 54 sends a signal (E in FIG. 3) for turning on the main switch to the drive circuit 24 until the flip-flop 54 is reset.
10 are turned on simultaneously. When the main switches 9 and 10 are turned on, the input voltage (voltage of the capacitor 6) Vc of the DC-DC converter is applied to the primary coil Np of the transformer 11.
The voltage Vc × Ns / Np is generated in the secondary coil Ns of the transformer 11, the diode 12 turns on, and the choke coil 1
4. The DC power is output via the capacitor 15. While the main transistors 9 and 10 are on, the difference between the transformer secondary voltage and the output voltage is applied to the choke coil 14, and the choke current increases with the current gradient determined by the following equation (6). di / dt = (Vc × Ns / Np−Vo) / L (1) where L is the inductance of the choke coil 14.

While the main switches 9 and 10 are on, the current waveform of the choke 14 and the transformer secondary current waveform are the same, and the transformer primary current waveform is similar. The transformer primary current waveform is detected by the CT 21 and converted into a voltage waveform by the current waveform detector 52. The signal (B in FIG. 3) of the current waveform detector 52 and the output (A in FIG. 3) of the output voltage error amplifier are compared by the comparator 51. When B> A, the reset signal of the flip-flop 54 (C in FIG. 3). Occurs.

The flip-flop 54 sends a signal E to the drive circuit 24 so that the main switches 9 and 10 are turned off until the flip-flop 54 is reset and set again. While the main switches 9 and 10 are off, the current of the choke coil 14 decreases. The flip-flop 54 is set by the next clock pulse of the clock pulse generator 53, and enters the next cycle.

Thus, the pulse width is controlled by the output of the output voltage error amplifier, and the output is controlled to a constant voltage. What is important here is the process of controlling the pulse width.
The signal (B in FIG. 3) of the current waveform detector 52 and the output (A in FIG. 3) of the output voltage error amplifier are compared by the comparator 51, and B> A
At this time, the reset signal (C in FIG. 3) of the flip-flop 54 is generated, and the main switches 9 and 10 are turned off.

That is, when the output of the output voltage error amplifier is DC
The current of the DC converter is controlled. FIG.
Means that the output voltage is kept constant when the load changes.
FIG. 3 shows how the pulse current changes in response to a change in the output A of the output voltage detection error amplifier so that the output voltage becomes constant when the load changes. (7) When the load current is large, the voltage of the output signal A of the error amplifier 22 is high so that the peak value of the current waveform is high, and the width of the on-pulse (E in FIG. 3) is wide. When the load current is small, the voltage of the output signal A of the error amplifier 22 is low so that the peak value of the current waveform is low, and the width of the on-pulse (E in FIG. 3) is narrow.

As described above, the peak value of the output waveform of the current waveform detector 52 is controlled to be the same as the voltage of the output signal A of the output voltage detection error amplifier 22. Since the output waveform of the current waveform detector 52 is similar to the pulse current waveform,
The peak value of the pulse current waveform is proportional to A. Since the signal A is sent to each DC-DC converter in common, each DC-D
The peak value of the pulse current waveform of the C converter is the same. Here, n: turns ratio of the D / D transformer 11, T: oscillation cycle of D / D Ip: current peak value of D / D output smoothing choke Vp: PFC output voltage, Wp: output power of PFC ΔI: smoothing choke (Where D / D is a DC-DC converter and PFC is a harmonic suppression circuit), the output current of Id: D / D is simply expressed by equation (2). . Id ≒ Ip− (ΔI / 2) × (Vo + ΔVo / (n × (Vp + ΔVp))) (2) From this, the output current Id of the DC-DC converter is obtained by subtracting the second term of the expression (2) from Ip. Value. ΔVo and ΔVp are the sum of the drop voltage of each part such as the forward voltage of the diode. Even if they vary, the change in the second term is sufficiently smaller than Ip. Further, since Ip is substantially proportional to the peak value of the pulse current, the output current Id is substantially proportional to the voltage of the signal A. If Wi: input power, Vi: input voltage, and Ii: input current, equation (3) is obtained. Wi = Vi × Ii ≒ Wp = Vo × Id (3) (8) From the equations (2) and (3), Ii is also proportional to the voltage of the signal A, and the input current of each phase is balanced. . Further, even if Vp changes with the ripple voltage of the PFC output in the equation (2), the change of Id is sufficiently small. For example, the ripple voltage is 10% of Vp, ΔI
Is 20% of Ip, the second term of the expression (2) becomes 2% of Ip, and the ripple current of the DC-DC converter due to the ripple voltage of the output of the PFC is sufficiently reduced.

From the above, it is apparent that the current of each phase can be balanced by the output of the common output voltage error amplifier, and the ripple current of each DC-DC converter can be reduced.

[0024]

As described above, according to the present invention, three input / output insulated converters each having a single-phase harmonic suppression function are connected between each phase, and their outputs are connected in parallel.
In a phase power factor improved converter, the input current can be balanced between the phases by simple control, and the ripple current of the DC-DC converter due to the ripple voltage of the output of the harmonic suppression circuit can be sufficiently reduced, reducing the power loss. Is peeled off. As a result, a three-phase power factor correction type converter can be realized at low cost and small size.
It is effective for miniaturization.

[Brief description of the drawings]

FIG. 1 shows an embodiment of the present invention.

FIG. 2 is a block diagram of current mode control.

FIG. 3 is an operation explanatory diagram of FIG. 2;

FIG. 4 is an explanatory diagram of a conventional method.

FIG. 5 is an explanatory diagram of a conventional system. (9)

[Explanation of symbols]

 1, 17 and 18 are boost type harmonic suppression circuits 2 are rectifier diode bridges 3 are choke coils 4 are switching elements 5 are diodes 6 are smoothing capacitors 7 are control circuits 8, 19 and 20 are DC-DC converters 9 10 is a main switch 11 is a transformer 12 is a rectifier diode 13 is a flywheel diode 14 is a smoothing choke coil 15 is a smoothing capacitor 16 is a load 21 is a current transformer 22 is an output voltage detection error amplifier 23 is a current mode control circuit 24 Is a drive circuit 25 is a pulse width control unit 26 is a current control unit 27 is a shared signal generator 28 is a DC current detector 51 is a comparator (10) 52 is a current waveform detector 53 is a clock pulse generator 54 is a flip-flop

Claims (2)

[Claims] 1. A three-phase three-wire AC power supply, wherein three sets of single-phase converters having a function of suppressing harmonics are connected between each phase. A boost chopper circuit that boosts the output of the rectifier circuit to a DC high voltage, and an isolated DC-DC converter that receives the output of the boost chopper circuit as an input, and connects each of the isolated DC-DC converters. A three-phase power factor improving converter, wherein outputs are connected in parallel, and the isolated DC-DC converter is controlled by a common output voltage detection error signal. 2. The isolated DC-DC converter is a forward converter having a current mode control circuit in which a peak value of a switching current waveform is controlled by the output voltage detection error signal. 3. The three-phase power factor improving converter according to 1.