JP2003134835A - Noise reducer of power converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a highly efficient small noise reducer by reducing the loss. SOLUTION: A power converter comprising a rectifier circuit 2 connected with an AC power supply 1 and a smoothing capacitor C0 and an inverter circuit 3 connected with the DC output of the rectifier circuit 2 is provided with a current detector 5 for detecting a leakage current flowing from the power converter to the earth, a current supply circuit 6 having an amplifier for amplifying a current depending on the output from the current detector 5, a series circuit of resistors R3 , R4 and capacitors C4 , C5 , and low frequency cut-off circuits 7a and 7b comprising auxiliary MOSFET3 , MOSFET4 , and the like. The amplification MOSFET in the current supply circuit 6 does not respond to a low frequency current of several tens kHz and thereby an amplification MOSFET of low current capacity can be used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a noise reduction device in a power conversion device such as an inverter device.

[0002]

2. Description of the Related Art FIG. 2 shows a conventional example of this type.

[0003] That is, the AC input power source 1 to the rectifier 2, a capacitor C ₀ through the input of the current detector 5 to the output of the rectifier 2 is, the semiconductor switches Q ₁ ~ Q in the capacitor C ₀
An inverter 3 composed of ₆ is connected to a motor 4 at the output of the inverter 3 and a series circuit of capacitors C ₂ and C ₃ is connected to the AC power source 1. In addition, N-channel MOSFET (metal oxide film type field effect transistor)
FET ₁ and FET ₂ is connected in series in parallel with the capacitor C _0, the output of the current detector 5 via the FET ₁ and the resistors between the gate and source of the _{FET 2 R 1, R 2,} FET 1
And the drain of the FET ₂ are connected to the ground G through the capacitor C ₁ and the connection point between the capacitors C ₂ and C ₃ is also connected to the ground G, respectively.

The FETs ₁ and ₂ are used as amplifiers here, and the leakage current (leakage current) detected by the current detector 5 by the FETs ₁ and ₂ and the capacitor C _1.
Accordingly, a current supply circuit 6 for supplying a current of opposite phase to this to the power line in which the leakage current of the power conversion device is flowing is formed.

The operation of FIG. 2 will be described.

Switches Q ₁ to Q of the three-phase inverter circuit 3
₆ is ON / OFF controlled by a PWM (pulse width modulation) pulse, and the motor 4 is driven by the output voltage of the inverter circuit 3. There is a capacitance C between the motor 4 and the ground G. Therefore, every time a pulsed voltage is applied from the inverter circuit 3, the leakage current i passes through the capacitance C.
_c (= C · dv / dt) flows. The current detector 5 detects a leak current flowing through the motor 4 and a capacitance C (not shown), and drives the FET ₁ or the FET ₂ . When the output voltage v _G1 (v _G2 ) of the current detector 5 is applied to the gate of the FET ₁ (FET ₂ ), the current i _c1 corresponding to this voltage v _G1 (v _G2 )
Flows through FET ₁ (FET ₂ ). FIG. 3A shows each current waveform at this time.

For example, when the leakage current i _c flows in the direction of the arrow in FIG. 2, i _c ′ flows in the primary winding 5a of the current detector 5, and the secondary winding 5b of the current detector 5 The voltages v _G1 and v _G2 are generated at 5c. Then, the FET ₂ is turned on and i _c1 flows through the capacitor C ₁ . As a result,
Most of the leakage current i _c flows as i _c1 and i _E (= i _c
-Ic1 ) is reduced, and the noise voltage (noise terminal voltage) is also reduced.

When the direction of the leakage current i _c is opposite to the above, the voltages v _G1 and v _G2 generated in the secondary windings 5b and 5c of the current detector 5 are reversed and the FET ₁ is turned on to turn on the capacitor. A current i _{c1 in the} opposite direction to the above flows through C ₁ . Also in this case, most of the leakage current i _c flows as i _c1 and i
_E (= _{ic- ic1} ) is reduced, and the noise voltage (noise terminal voltage) is also reduced.

[0009]

In the circuit configuration of FIG. 2,
The current of the common mode component is output to the output side of the current detector regardless of the frequency band. Therefore, the MOSFET is driven by the output of this detector, and a current flows between the drain and the source. Generally, the frequency component of the leakage current is in the range of several hundred kHz to several MHz, and in order to reduce the leakage current, it is sufficient to detect only the current in the above frequency band. However, for example, 3
If a component such as a phase unbalanced current is detected and output, the extra current will flow into the noise current reduction circuit, resulting in a large MOSFET loss and the use of an element with a large allowable loss.

Further, when a ground fault occurs at the output part of the inverter, a large current flows at a low frequency. If a current corresponding to this current flows in the noise current reduction circuit, the loss of the MOSFET becomes excessively large, and in the worst case, damage may occur. There is a problem of everywhere.

Therefore, an object of the present invention is to reduce the capacity of a current amplification element used in a current supply circuit of a noise reduction device, and to cope with a large current common mode current at a low frequency such as at the time of a ground fault. To be able to protect.

[0012]

In order to solve such problems, in the invention of claim 1, a smoothing circuit and an inverter circuit are connected to a rectifier circuit connected to an AC power source and the DC output of this rectifier circuit. For the power converter configured as, between the AC power supply and the output of the smoothing circuit, a current detector for detecting a leakage current flowing from the power converter to the ground is provided,
A current supply circuit that has an amplifier that amplifies a current according to the output of the current detector and supplies a current having a phase opposite to the detected leakage current to the power line in which the leakage current of the power conversion device is flowing is the smoothing circuit. In a noise reduction device for a power converter provided between DC outputs of a circuit, a low-frequency cutoff circuit for cutting low-frequency current is provided between the current detector and the current supply circuit.

In the invention of claim 1, the low-frequency cutoff circuit includes a series circuit of a resistor and a capacitor provided on the output side of the current detector, and a drain between which the gate and source terminals are connected to the capacitor. The MOSFET may have a terminal connected to a signal voltage input terminal of the amplifier circuit (the invention of claim 2).

That is, in the case of a low frequency component of several tens of kHz level, in the series circuit of the resistor and the capacitor connected to the output side of the current detector, the impedance of the capacitor increases and the capacitor voltage rises. Since the auxiliary MOSFET is turned on and the gate and source of the MOSFET forming the noise current reduction circuit are short-circuited, this MOSFET is turned off and does not operate. Therefore, since an unnecessary low frequency current is not passed through the noise current reduction circuit, the effective current value is reduced and a small capacity MOSFET can be applied. Further, the MOSFET constituting the noise current reduction circuit is inoperative even with respect to a low frequency and large current common mode current such as a ground fault current, so that the circuit is protected.

[0015]

1 is a circuit diagram showing an embodiment of the present invention.

The difference from FIG. 2 is that the secondary winding 5b of the current detector 5 has a series circuit of a resistor R ₃ and a capacitor C ₄ and an N-channel MOSFE driven by the voltage of the capacitor C _4.
A low-frequency cutoff circuit 7b composed of a FET _{3 of} T, a series circuit of a resistor R ₄ and a capacitor C _{5 in} the secondary winding 5c of the current detector 5, and an N-channel MOSFET driven by the voltage of the capacitor C _5. The low-frequency cutoff circuit 7c composed of the FET ₄ is added.

Even in the case of the above configuration, the noise reduction effect itself is the same as that of the case of FIG. 2, and the difference is that the FET ₁ and FET ₂ are different as shown in FIG. , It will not operate at low frequency currents below several tens of kHz. So for this circuit,
Since FET ₁ and FET ₂ compensate only for high frequency noise current of 150 kHz or more, MOSF with small current capacity
ET will do. Since high-frequency noise of 150 kHz or higher can be reduced, the noise reduction effect itself is the same as in the conventional case.

Although a series circuit of a resistor and a capacitor is connected to the output part of the current detector in FIG. 1, any structure may be used as long as it has a function of integrating the output value of the current detector. In addition, the low frequency cutoff circuit
Although T is used, any element may be used as long as it is a voltage-driven element driven by the integrated output value from the current detector.

[0019]

According to the present invention, since a current amplification element having a smaller capacity than that of a conventional noise reduction apparatus can be used, the noise current reduction apparatus can be applied to a larger capacity apparatus. When applied to the same device, the generated loss is reduced, the conversion efficiency is increased, and the device is downsized. Further, since the MOSFET becomes inoperable with respect to a low-frequency, large-current common mode current that occurs when a ground fault occurs on the output side of the inverter,
It can prevent damage.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a conventional example.

FIG. 3 is an operation explanatory diagram of FIGS. 1 and 2;

[Explanation of symbols]

1 ... AC power supply, 2 ... rectifier, 3 ... inverter circuit, 4 ...
Motor, 5 ... Current detector, 5a ... Primary winding, 5b, 5c
... secondary winding, 6 ... current supply circuit, 7b, 7c ... low-frequency cut-off circuit, FET _1, FET ₂ ... N-channel MOSFET
(Metal oxide film type field effect transistor), FET ₃ , F
ET ₄ ... N-channel MOSFET for blocking at low frequency, C ₀
-C ₅ ... capacitors, R ₁ ~R ₄ ... resistor.

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Claims (2)

[Claims] 1. A power converter comprising a rectifier circuit connected to an AC power supply and a DC output of the rectifier circuit connected to a smoothing circuit and an inverter circuit. A current detector for detecting a leak current flowing from the power converter to the ground is provided between the power converter and an amplifier for amplifying the current in accordance with the output of the current detector, and the detected leak current and the opposite phase current In a noise reduction device for a power conversion device, wherein a current supply circuit that supplies a power line in which a leakage current of the power conversion device is flowing is provided between the DC outputs of the smoothing circuits, the noise reduction device between the current detector and the current supply circuit. A noise reduction device for a power conversion device, characterized in that a low-frequency cutoff circuit for cutting off low-frequency current is provided in the. 2. The low-frequency cutoff circuit includes a series circuit of a resistor and a capacitor provided on the output side of a current detector, a gate-source terminal of which is connected to the capacitor, and a drain terminal of which is a signal of the amplifier circuit. The noise reduction device for a power conversion device according to claim 1, comprising a MOSFET connected to a voltage input terminal.