JPH05153782A - Noise preventive device - Google Patents

Abstract

PURPOSE:To reduce a noise voltage to an allowable value or less even if a small noise filter is used by adding a noise preventive circuit for canceling a noise current flowing to a parasitic capacitance between a power converter circuit and a metal case. CONSTITUTION:In a noise preventive circuit 5, a compensating capacitor CC2 and a primary winding of a current transformer CT1 are connected in series between an output bus O and a power supply bus N, a current iCC2 responsive to a variation in an output voltage flows, and a noise preventing current iCC3 flows to a secondary side of the transformer CT1 through a compensating capacitor CC3 in a direction from a metal case E to the bus N. Here, the capacities of the capacitors CC2, CC3 and a current ratio of the transformer CT1 are suitably selected. A noise current iCS2 and the current iCC3 are substantially equalized to reduce a noise current cf3 flowing to a filter capacitor Cf3, thereby decreasing a noise voltage DELTAV3. A noise filter 2 can be reduced in size by adding the circuit 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention reduces noise generated inside a power converter utilizing transistor switching, such as a motor variable speed transistor inverter, a switching power supply, a high frequency induction / dielectric heating power supply, and the like. Regarding the device. In the following figures, the same reference numerals indicate the same or corresponding parts.

[0002]

2. Description of the Related Art FIG. 6 is a block diagram of a conventional power conversion circuit. A conventional technique will be described below with reference to FIG. 2 and FIG. In FIG. 6, 1 is a DC power supply, 2 is a noise filter, 31 is a high frequency inverter as the power conversion circuit 3, and 41 is a load. Also, Lf1,
Lf2 is a filter reactor forming the noise filter 2, and Cf1 and Cf2 are also filter capacitors. In the high frequency inverter 31, CD is a power supply smoothing capacitor, Q1 and Q2 are transistors as switching elements, and Cr1 and Cr2 are resonance capacitors that form a resonance circuit with the load 41.

E is a metal chassis or case for mounting or accommodating the noise filter 2 and the power conversion circuit 31 or parts thereof. The chassis and case are electrically coupled to each other and the circuits 2 and 31 are grounded. Is formed. Cs1 and Cs2 are the chassis, case E, and especially the transistor Q in the power conversion circuit 31.
It is a stray capacitance (parasitic capacitance) formed between the collectors of 1 and Q2. Cf3 is the chassis,
It is a filter capacitor attached between the case E and the negative power source bus N.

Next, FIG. 2 shows the operation waveforms of the main parts of FIG. 6 in comparison between the conventional case and the case of the present invention. Figure 2 (a)
Shows a conventional operation waveform. In FIG. 2A, V
Q2 is the output voltage of the transistor Q2, ics2, icf
3 is a noise current as a current of the stray capacitance Cs2 and the filter capacitor Cf3, and ΔV3 is a noise voltage as a voltage across the filter capacitor Cf3.

By the way, for example, in a high frequency inverter as shown in FIG.
The output voltage VQ2 shown by is converted into an AC output including a sharp potential change. In this example, the transistors Q1 and Q2 are alternately turned on and off, that is, switched to obtain an AC output. The transistors Q1 and Q2 are closely contacted with a cooling fin or a metal structure portion such as a chassis or case E in a large area for heat dissipation so as to be in close contact with each other, and particularly between the collector (C) portion of the transistors Q1 and Q2 and the case E. Is a tens to hundreds of pF (picofarad) stray capacitance (parasitic capacitance) Cs.
1 and Cs2 are created. The stray capacitance Cs1 generated in the collector of Q1 exists between the positive power source bus P and the case E and does not pose a problem.

On the other hand, the stray capacitance Cs2
Between the output bus O and the case E, a large noise current i due to a sudden change in the voltage VQ2 as shown in FIG.
cs2 will flow. If this is left, Case E
Since a voltage substantially equivalent to the reaction of VQ2 is generated between the power supply bus N and the power supply bus N, a filter capacitor Cf3 is attached,
This voltage (noise voltage) is reduced by one digit or more as indicated by ΔV3 (Vcf3). Here, ΔV3 is expressed by the following equation. ΔV3≈VQ2 × Cs2 / Cf3 Therefore, if the value of the filter capacitor Cf3 is large, ΔV3
Since 3 can be made small, but there is a limit, a large LC filter shown by the noise filter 2 is conventionally provided in addition to the filter capacitor Cf3.

[0007]

As described above, the stray capacitance Cs generated between the power conversion circuit and the case E as described above.
In order to cancel the noise voltage generated between the power source bus and the case E by 2, the conventional method is to use the filter capacitor Cf3, the noise filter 2, etc., so that the noise voltage ΔV3 is within the allowable value (legal regulation value or radio etc.). Therefore, a large part, especially the noise filter 2, was required in order to put it in a level where there is no real damage. Therefore, it is an object of the present invention to provide a noise prevention device that can keep the noise voltage ΔV3 within an allowable value by using a small noise filter 2.

[0008]

In order to solve the above-mentioned problems, a noise prevention device according to claim 1 comprises a metal case (E or the like) for holding or housing a power converter (3 or the like), and this power conversion device. The noise prevention current (ics2, etc.) that is approximately equal in magnitude to the noise current (ics2, etc.) generated in the stray capacitance (Cs2, etc.) between the portion in which the potential with respect to the case sharply changes (the collector of the transistor Q2, etc.) and has the opposite phase. (Icc3 or the like) is generated through a noise prevention current generating means separately provided in the power conversion circuit, and is caused to flow in the case,

According to the noise prevention device of claim 2,
In the noise prevention device described in (1), the noise prevention current generating means includes a capacitor (Cc2, Cc3, etc.) and a current transformer (CT1, etc.) (noise prevention circuit 51,
52).

According to the noise prevention device of claim 3,
In the noise prevention device described in (1), the noise prevention current generating means is configured without an transformer using electronic circuits such as an operational amplifier (OP1, OP2, etc.) (noise prevention circuits 54, 55, etc.).

According to another aspect of the noise prevention device of the present invention,
4. The noise prevention device according to claim 3, wherein the noise prevention current generating means has noises of substantially the same magnitude as the noise currents generated in the stray capacitances (Cs2, Cs4, Cs6, etc.) at a plurality of locations and having an opposite phase. Preventive currents are collectively superimposed and sent to the case (noise prevention circuit 53, etc.).

[0012]

The noise current flowing in the parasitic capacitance Cs2 and the like generated between the power conversion circuit and the case and chassis E cannot be stopped unless Cs2 and the like are removed. If it flows into a specific part such as a chassis or a case, the noise current can be effectively canceled if the same current as the noise current flows into the power conversion circuit part from the specific part such as the chassis or the case. become. Therefore, in the present invention, the parasitic capacitance C
A circuit for canceling the noise current flowing through s2 etc. is added.

[0013]

1 shows an embodiment in which a noise prevention circuit 51 according to the present invention is applied to a high frequency transistor inverter 31 for induction heating, and FIG. 2 (b) shows a case where the present invention in FIG. The operation waveforms of the main parts (when added) are shown. As described above, FIG. 2 (a) is an operation waveform before implementing the present invention in FIG. In FIG. 1, a large smoothing capacitor CD that absorbs a ripple current is connected between a positive electrode bus P and a negative electrode bus N of a DC power supply 1, and switching transistors Q1 and Q2 are connected in series from the bus P to N. . The output bus O is located at the midpoint between the transistors Q1 and Q2. By alternately turning on and off Q1 and Q2, an AC output voltage such as VQ2 shown in the upper part of FIG. 2A can be obtained. ing. The output section O is connected to the middle point of the resonance capacitors Cr1 and Cr2 through the load 41. The capacitors Cr1 and Cr2 are connected in series from P to N.

A parasitic capacitor C is provided between the collector (c) of the switching transistor Q2 of the high-frequency inverter which is connected and operates as described above, and the chassis and case E.
If there is s2, in the case of the conventional case shown in FIG. 6, as described above, the noise current ics2 as in the middle stage of FIG. a) The noise voltage ΔV3 shown in the lower stage is the noise current i indicated by the mark (↑)
It occurs due to the relationship between cf3 and the filter capacitor Cf3.
The noise filter 2 and the like are necessary in order to suppress the noise voltage ΔV3 to a predetermined allowable value or less.

By the way, the noise prevention circuit 5 according to the present invention
(51) is the current transformer CT1 and the primary winding of this current transformer, 2
Compensation capacitors Cc2, C connected in series to the next winding
This series circuit on the primary side of the current transformer CT1 is connected between the collector of the transistor Q2 and the power supply bus N, and the series circuit on the secondary side of the current transformer CT1 is also connected to the power supply bus N. It is connected to the case E. When such a noise prevention circuit 51 is added, the compensating capacitor Cc2 and the primary winding of the current transformer CT1 are connected in series between the bus lines O and N, so that the → mark corresponding to the change of the output voltage VQ2 is shown. The current icc2 flows from the secondary side of the current transformer CT1 through the compensation capacitor Cc3 in the direction from the case E to the bus line N to cancel the noise current indicated by the arrow (i.e., noise prevention current) icc3 shown in FIG. ) Flows like the upper row.

When the capacitances of the compensation capacitors Cc2 and Cc3 and the current transformer ratio K of the current transformer CT1 are properly selected, the noise current ics2 indicated by ↓ and the noise prevention current icc3 indicated by ↑ become substantially equal, and FIG. The current icf3 flowing through the filter capacitor Cf3 can be canceled out to a value smaller by one digit or more as shown in the middle part of FIG. 2, and the noise voltage ΔV3 can be significantly reduced as shown in the lower part of FIG. 2B.

The values of the capacitors Cc2 and Cc3 are practically determined as follows. That is, Cc2 × K≈Cs2 << Cc3≈Cf3 ≦ 0.1 μF For example, if the current change ratio K = 2 and Cs2 = 200 pF, Cc2 = Cs2 / K = 200 pF / 2 = 100 pF Cc3 is sufficiently larger than Cs2 = 200 pF and 2000
It becomes 10000 pF. The value of this Cc3 becomes substantially equal to the value of the filter capacitor Cf3. In a high-frequency inverter operating at several tens of KHz, several hundreds of volts, and several KW, the electric power handled by the noise prevention circuit 51 may be a small power capable of processing only a few VA of only the noise component, and can be composed of extremely small and lightweight parts.

If the noise prevention circuit 51 is not attached to the noise filter 2 as well, a large component equivalent to several KVA having an energizing capacity of several tens of amperes is required in the filter reactor of Lf1 and Lf2, and even an inductance of several mH is required. Although it is a large one having a weight of several hundred grams, the noise filter 2 is attached by the addition of the noise prevention circuit 51.
Can be significantly miniaturized.

FIG. 3 shows the configuration of a power conversion circuit according to a second embodiment of the present invention. In the figure, the collector of the switching transistor Q2 of the DC / DC converter 32 of the one-stone type as the power conversion circuit 3 ( It shows a case where the influence of the stray capacitance Cs2 in the section C) is removed by using the noise prevention circuit 5 (52). Thereby, the noise filter 2 can be downsized. The present invention can be similarly applied to a power converter that operates in a completely different manner as described above.

FIG. 4 shows the configuration of a noise prevention circuit as a third embodiment of the present invention. That is, in FIGS. 1 and 3, the noise prevention circuit 5 is composed of a capacitor and a passive component of a current transformer, and in FIG. 4, an active component such as an operational amplifier is used without a transformer. Here, in the noise prevention circuit 5 (54) in FIG. 4A, the operational amplifier OP1 outputs the output current icc3 (↑) proportional to the input current icc2 (→ mark).
Mark) is generated. In addition, the noise prevention circuit 5 of FIG.
In (55), the operational amplifier OP2 has three capacitors C
And a single resistor R are used to form a simple current inverting circuit. 4 (a) and 4 (b) provide substantially the same operation, and various other circuits are conceivable.

FIG. 5 shows the configuration of a power conversion circuit according to a fourth embodiment of the present invention. In the figure, a noise prevention circuit 5 (5
3) is applied. In this case, the negative power source bus N
The switching transistors connected to are Q2, Q4
Since there are three Q6s, there are a total of three collectors of the transistors Q2, Q4, and Q6 where the potential in question changes suddenly and there is stray capacitance, in other words, the stray capacitance in question is Cs2, Cs4.
There are three, Cs6.

As a noise prevention circuit in this case, a noise prevention circuit 51 as shown in FIG.
A total of three pieces may be provided in each of s2, Cs4, and Cs6, but here, an example is shown in which the noise prevention circuit 53 is used to collectively process the effects of stray capacitances at three locations. That is, in FIG. 5, compensating capacitors Cc2, Cc4, Cc6 are connected between the upper terminal of the primary winding of the current transformer CT1 and the collectors of the transistors Q2, Q4, Q6, respectively. The noise prevention current icc in which the currents of these three compensation capacitors are superposed on the secondary winding and the phase of the superposed current is inverted from the secondary winding of the current transformer CT1.
3 (↑↑↑ mark) is made to flow into the case E. As a result, the noise filter 2 can be made small or eliminated. In this way, it is possible to collectively cancel the noise currents at different points of the switching transistors which perform different operations.

[0023]

Although the stray capacitance between the semiconductor switch and the metal cooling body (and hence the case in which this cooling body is mounted) cannot be eliminated, the present invention aims at the noise current flowing through this stray capacitance. I added a noise prevention circuit to pass a noise prevention current to cancel this.
The noise prevention circuit composed of the combination of T can cancel the influence of the stray capacitance. Further, by configuring the noise prevention circuit with an electronic circuit such as an operational amplifier without a transformer, it is possible to make a smaller ic (semiconductor integrated circuit). The noise prevention circuit may be configured to collectively process the effects of the stray capacitances of a plurality of transistors that perform different operations as described in FIG. Due to these effects, the large noise filter 2 can be omitted or replaced with a small one.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a power conversion circuit as a first embodiment of the present invention.

FIG. 2 is a diagram showing a comparison of operation waveforms of main parts before and after application of the present invention in FIG.

FIG. 3 is a configuration diagram of a power conversion circuit as a second embodiment of the present invention.

FIG. 4 is a configuration diagram of a noise prevention circuit according to a third embodiment of the present invention.

FIG. 5 is a configuration diagram of a power conversion circuit according to a fourth embodiment of the present invention.

FIG. 6 is a configuration diagram of a conventional power converter.

[Explanation of symbols]

 1 power supply P positive power supply bus bar N negative power supply bus bar E chassis, case 2 noise filter Cf3 filter capacitor 3 (31, 32, 33) power conversion circuit 31 high frequency inverter 32 DC / DC converter 33 three-phase inverter Q1 switching transistor Q2 switching transistor Q3 switching transistor Q4 switching transistor Q5 switching transistor Q6 switching transistor Cs2 stray capacitance Cs4 stray capacitance Cs6 stray capacitance 41 load 42 load 43 load 5 (51-55) noise prevention circuit Cc2 compensation capacitor Cc3 compensation capacitor Cc4 compensation capacitor Cc6 compensation capacitor OP1 operation Magnifier OP2 Computational amplifier

Claims (4)

[Claims] 1. A noise current generated in a stray capacitance between a metal case for holding or housing a power converter and a portion in the power converter where the potential with respect to the case changes abruptly and substantially the same. A noise prevention device characterized in that a noise prevention current of a phase is generated through a noise prevention current generating means separately provided in a power conversion circuit and is made to flow to the case. 2. The noise prevention device according to claim 1, wherein the noise prevention current generating means includes a capacitor and a current transformer. 3. The noise prevention device according to claim 1, wherein the noise prevention current generating means is configured without a transformer by using an electronic circuit such as an operational amplifier. Prevention device. 4. The noise prevention device according to claim 1, wherein the noise prevention current generating means has a noise phase which is substantially equal in magnitude to each noise current generated in the stray capacitances at a plurality of locations and has a phase opposite thereto. A noise prevention device, characterized in that a prevention current is collectively superposed and supplied to the case.