WO2015177746A1

WO2015177746A1 - Active emi differential mode line filter

Info

Publication number: WO2015177746A1
Application number: PCT/IB2015/053734
Authority: WO
Inventors: Erdinc YALVAC; Yunus SENER
Original assignee: Aselsan Elektronik Sanayi Ve Ticaret Anonim Şirketi
Priority date: 2014-05-21
Filing date: 2015-05-21
Publication date: 2015-11-26

Abstract

The present invention relates to a differential mode line filter (1) which enables to eliminate unwanted noise signals by injecting a current flowing in same magnitude and opposite direction to the differential mode current, the suppression value of which can be improved by its serial use, and essentially comprises at least one passive filter (2) which enables to filter high frequency noises and the common mode noise, at least one current detection circuit (3) which enables the noise to be detected, at least one wave generator circuit (4) which enables the voltage obtained via the current detection circuit (3) to come to the same waveform with the noise, at least one supply circuit (5) which generates the power required for an active filter, at least one current amplifier circuit (6) which transfers a signal generated by an active filter and having the same waveform with the noise to the line in the input of the current detection circuit (3).

Description

DESCRIPTION ACTIVE EMI DIFFERENTIAL MODE LINE FILTER

Field of the Invention

The present invention relates to an active EMI (Electromagnetic Interference) differential mode line filter which enables the differential mode noise originating from the current on DC power lines to be filtered.

Background of the Invention

The most common technique in eliminating propagation problems by transmission is to use passive filter. Especially the unwanted signs can be filtered by using filters comprised of passive materials in input lines of power sources.

The insertion loss of the filter, cut-off frequency, the load being capacitive of inductive, requirement for common mode or differential mode, and load current are important elements for filter design. According to these qualities, the filters are designed that can be obtained by serial/parallel use of main passive circuit elements.

The suppression values of passive filters used for filtering unwanted differential mode noises generated by the electronic devices vary according to the frequency. The suppression value of the filters decreases generally upon the noise generated by the power sources drop to frequency band below 200 kHz. Inductor and capacitor values should be increased in order to improve suppression value. Using materials with higher inductance or capacitance causes increase in cost and size of the device, and also it may not be possible to provide material with high value. Using coil with higher value causes the discharged heat to increase and the device to overheat due to the increase in electrical resistance.

United States Patent Document no US2004008527, an application known in the state of the art, an active EMI filter which senses noise in a ground line or a power transmission line without using a current transformer. A circuit comprising common mode and differential mode filters enable to decrease unwanted noise with the said technique. The filter operating in single phase and three phase line voltage detects both common mode noise and differential mode noise, and then processes noise signals and transmits to ground line. Therefore, the filter should be connected to the ground line.

United States Patent Document US2004264220 discloses an EMI filter. The said technique enables to reduce noise generated by a circuit including a common mode filter and a differential mode filter. Common mode filter comprises an input for a common mode signal detection between the power source and the circuit, an output, and an active circuit for differential mode signal detection and generating changes between its input-output. A transistor is used as a variable resistance in the differential mode filter circuit and the noise is damped on the transistor.

Summary of the Invention

The objective of the present invention is to provide a differential mode line filter which enables to eliminate noise signals generated in 50-500 kHz frequency interval by injecting to the line a current in same magnitude and opposite direction with the differential mode current occurring on the line.

Another objective of the present invention is to provide a differential mode line filter wherein higher level and broader band of suppression can be provided with materials with smaller sizes by improving current amplifier circuit design. Suppression value can be increased by serial use of differential mode line filter. In high current DC power applications wherein differential mode noise is dominant, there is no need for using power coils with large sizes required for filtering especially noises the frequency of which is below 200 kHz in differential mode line filter. By means of not using materials with high capacity, the heat discharged through the inventive filter is less than the passive filters.

Detailed Description of the Invention

A differential mode line filter developed to fulfill the objectives of the present invention is illustrated in the accompanying figures, in which:

Figure 1 is the schematic view of the inventive differential mode line filter.

Figure 2 is the circuit diagram of the passive filter.

Figure 3 is the circuit diagram of the current detection circuit.

Figure 4 is the circuit diagram of the wave generator of the active filter.

Figure 5 is the circuit diagram of the current amplifier.

Figure 6 is the circuit diagram of the supply circuit.

Figure 7 is the graphical view of the difference between the magnitudes of the current injected to the circuit by the active filter and the noise current, and the phase difference in the frequency axis.

Figure 8 is the graphical view of the suppression curve of the passive and passive+active filters while the voltage of the power source is 24V.

Figure 9 is the graphical view of the noise spectrum of the DC-DC converter in 10 kHz- 10 MHz frequency interval.

Figure 10 is the graphical view of the noise spectrum generated when passive filter is attached to the input of the DC-DC converter.

Figure 11 is the graphical view of the noise spectrum generated while both passive and active filters are active in the input of the DC-DC converter. The components shown in the figures are each given reference numbers as follows:

1. Differential mode line filter

2. Passive filter

3. Current detection filter

4. Wave generator circuit

5. Supply circuit

6. Current amplifier circuit

P. Power source

N. Noise source

CI. First capacitor

C2. Second capacitor

C3. Third capacitor

C4. Fourth capacitor

C5. Fifth capacitor

C6. Sixth capacitor

D. Diode

Rl. First resistance

R2. Second resistance

R3. Third resistance

R4. Fourth resistance

R5. Fifth resistance

R6. Sixth resistance

R7. Seventh resistance

R8. Eighth resistance

L. Current transformer

01. First oeprational amplifier

02. Second operational amplifier

IC. Lineai- regulator

TR. Transistor Detection+ Positive current detection point

Detection- Negative current detection point

REF. Reference voltage Differential mode line filter (1), which enables to eliminate unwanted noise signals by being injected the current flowing in the same magnitude with the differential mode current and in opposite direction, essentially comprises

at least one passive filter (2) which enables to filter high frequency noises and the common mode noise,

- at least one current detection circuit (3) which enables to detect the noise, at least one wave generator circuit (4) which enables the voltage obtained via the current detection circuit (3) to come to the same waveform with the noise,

at least one supply circuit (5) which generates the power required for an active filter,

at least one current amplifier circuit (6) which transfers a signal generated by an active filter and having the same waveform with the noise to the line in the input of the current detection circuit (3). The main circuit structure of the inventive differential mode line filter (1) (in other words the active filter) is given in Figure 1. Differential mode current flows in same magnitude and opposite directions from positive and negative lines. For this reason, by means of the differential mode line filter (1), a current flowing in same magnitude but in opposite direction of the differential mode current moving on the positive line is injected to the positive line and thus the unwanted noise signals can be eliminated. Suppressing noise with the differential mode line filter (1) is provided first by detecting the unwanted noise signals and then processing them.

Detecting the noise is provided with a current transformer (L) located in the current detection circuit (3). The view of the current transformer (L) in the circuit is given in Figure 3. The current transformers (L) operate in the frequency band between 50 kHz and 500 kHz such that they will generate the best result. For this reason the frequency of the noise current targeted to be suppressed is also in this interval. The current in the primary part in the current transformer (L) shown in Figure 3 induces the secondary part by decreasing in ratio of the winding number of the current transformer (L). The induced current passes through the first resistance (1) and creates a voltage difference between the positive current detection point (detection+) and the negative current detection point (detection-). The generated voltage has the same phase with the noise. The voltage obtained via the current transformer (L) enters to the wave generator circuit (4) part of the active filter shown in Figure 4 so that it comes to the same waveform with the noise. The current generator circuit (4) of the active filter generates the signal having the same waveform with the noise and injects to the line in the input of the current detection circuit (3) through the current amplifier circuit (6).

The supply voltage of the active filter is in 5V level. The reference voltage (REF) generated besides the supply voltage in 5V level generated by the supply circuit (5) is in 2.5V level. The source of the supply circuit (5) shown in Figure 6 is VCC point.

Processing the noise detected by means of the current detection circuit (3) is enabled with the operational amplifiers. The first operational amplifier (01) given in Figure 4 is used for amplifying the voltage between the positive current detection point (detection+) and the negative current detection point (detection-). The ratios between the seventh resistance (R7) and the eighth resistance (R8), and the fifth resistance (R5) and the sixth resistance (R6) determine the amplification value of the new signal wanted to be created. The reference voltage (REF) is in 2.5V, it comprises the reference voltage of the first operational amplifier (01) and the second operational amplifier (02). The AC signal oscillates on this reference voltage (REF). The first operational amplifier (01) and the second operational amplifier (02) are present inside single integrated circuit, and they are fed from single place. The sixth capacitor (C6) performs compensation function in order to prevent phase shift between the noise and the signal wanted to be generated. The signal generated by the first operational amplifier (01) and the second operational amplifier (02) is in the same phase with the noise. The generated signal drives the current amplifier circuit (6) given in Figure 5. The current passing through the fourth resistance (R4) drives the PNP BJT type transistor (TR). The transistor (TR) operates as a current amplifier since it amplifies the current passing through the fourth resistance (R4) in ratio of a certain coefficient. The current passing through the transistor (TR) has AC and DC components. The AC component is in same magnitude and the phase with the noise. The current passing through the third resistance (R3) is DC current. The current passing through the second resistance (R2) and the fifth capacitor (C5) is AC current and does not comprise DC current. The sum of the currents passing through the fifth capacitor (C5) and the third resistance (R3) equals to the current passing through the transistor (TR). In this way, the current wanted to be injected to the circuit from the VCC point is in opposite direction with the detected current, but has the same magnitude. Filtering is performed by obtaining the sum of the current injected to the circuit by the active filter and the noise current in a lower level than the noise current. In the graphic given in Figure 7, in frequency interval between 50 kHz and 500 Khz the difference of these two currents' magnitude is maximum +0.5dB, the phase difference is maximum +8°. The operating frequency interval of the active filter is related with the detection capacity of the current transformer (L) and the signal processing accuracy of the operational amplifier.

On the power source (P) part of the circuit, there is a passive filter (2) in addition to the active filter circuit, for filtering the noise in high frequencies and the common mode noise. The passive part of the filter is given in Figure 2. In the circuit which is tested to see the suppression value of the active filter more clearly and not to increase the size of the circuit, the first capacitor (CI), the second capacitor (C2), the third capacitor (C3) and the fourth capacitor (C4) are not mounted and the circuit compresses the metal chassis. The second capacitor (C2) and the third capacitor (C3) are used for filtering the noise and they can be attached if needed. The first capacitor (CI) and the fourth capacitor (4) can be used to make improvement according to noise level and frequency. The diode (D) is TVS (Transient Voltage Suppressor) diode which is used to prevent the filter from getting damaged from the sudden voltage amplifications.

While the voltage of the power source (P) is 24V, the suppression curves are given in Figure 8 when only the passive filter (2) is active (the bottom curve) and the passive filter (2)+active filter are active (top curve). While the active filter is active, the suppression value of the filter at 60 kHz is improved 24dB. This improvement is made without using differential mode coil. By looking at the noise of the device to be filtered, the first capacitor (CI), the second capacitor (C2), the third capacitor (C3), and the fourth capacitor (4) which are optional can be added for noises above 1 MHz frequency.

In Figure 9, the noise spectrum of a DC-DC converter the switching frequency of which is 150 kHz in 10 kHz- 10 MHz is given. In Figure 10, the noise spectrum created when the passive filter (2) is attached to the input of the converter is given. In Figure 11, the noise spectrum created when both passive filter (2) and the active filter are active in input of the converter is given. The suppression of passive filter (2) in 150 kHz is lOdB, it increases to 40dB when the active filter is added.

The suppression value can be increased by serial using of the differential mode line filter (1).

Claims

A differential mode line filter (1), which enables to eliminate unwanted noise signals by being injected to a current flowing in the same magnitude with the differential mode current and in opposite direction, essentially comprising at least one passive filter (2) which enables to filter high frequency noises and the common mode noise,

and characterized by

at least one current detection circuit (3) which enables to detect the noise, at least one wave generator circuit (4) of an active filter which enables the voltage obtained via the current detection circuit (3) to come to the same waveform with the noise,

at least one current amplifier circuit (6) which transfers a signal generated by an active filter and having the same waveform with the noise to the line in the input of the current detection circuit (3).

A differential mod line filter (l)according to claim 1, characterized by current detection circuit (3) which comprises a current transformer (L) enabling to detect the noise.

A differential mod line filter (1) according to claim 1, characterized by current detection circuit (3) which enables the noise preferably in 50 kHz to 500 kHz frequency band to be detected.

A differential mod line filter (1) according to claim 1, characterized by supply circuit (5) which creates the supply voltage of the active filter.

A differential mod line filter (1) according to claim 1, characterized by wave generator circuit (4) which comprises a capacitor performing compensation process in order to prevent the phase shift between the noise signal and the signal generated by the active filter.

A differential mod line filter (1) according to claim 1 or 5, characterized by wave generator circuit (4) which comprises operational amplifiers enabling to process the noise detected by means of the current detection circuit (3).

A differential mod line filter (1) according to claim 1 or 6, characterized by wave generator circuit (4) which comprises operational amplifiers creating signal in the same phase with the noise.

A differential mod line filter (1) according to claim 1 or 7, characterized by current amplifier circuit (6) which is driven with the signal generated by the wave generator circuit (4).