Detailed explanation of noise countermeasures for LED lighting fixtures

The revised Japanese Electrical Appliances and Materials Safety Law will be implemented from July 2012. After this revision, LED bulbs have also become the applicable objects of this regulation. Among the several restrictions, the restrictions on electromagnetic noise intensity are particularly noteworthy. LED lighting devices with higher electromagnetic noise than incandescent bulbs and bulb-type fluorescent lamps are currently on the market. With the implementation of the revised version of the Electrical Appliances and Materials Safety Law, strict noise countermeasures must be taken. This article will analyze the types of noise, evaluation methods and countermeasures of LED lighting based on the current status of LED lighting.

The power shortage problem caused by the Great East Japan Earthquake has rapidly increased people's awareness of energy conservation, and energy-saving products such as LED lighting fixtures and LCD TVs using LED backlights are gradually becoming the mainstream of the market. As for LED lighting fixtures, bulb-type, fluorescent tube-type, pendant lamps and ceiling lamps have already been put on the market.

As for LED bulbs, not only well-known lighting manufacturers but also new entrants have begun to sell their products at low prices in home improvement stores, and the LED bulb market is expanding rapidly.

At the same time, an environment is being established to popularize LED bulbs, such as standardization and introduction of regulations. Previously, LED bulbs were not covered by the Electrical Appliances and Materials Safety Law. Therefore, some LED lighting products have high electromagnetic noise. As a result, if street lamps are replaced with LED bulbs from mercury lamps, it will cause problems with TV and radio reception.

Incandescent bulbs are resistive loads with no internal power circuit, so this type of electromagnetic noise problem does not exist. However, the problem becomes prominent after switching to LED bulbs. If the popularization of LED lighting is promoted as it is, multiple noise sources will appear in the home.

Therefore, overseas, LED lighting fixtures, like ordinary lighting fixtures, must comply with the international standard CISPR15 (Limits and Methods of Measurement of Radio Disturbance Characteristics of Electrical Lighting and Similar Equipment), and all countries have introduced restrictions based on this standard.

Japan will also begin to implement such restrictions. From July 2012, LED bulbs will become subject to the Electrical Appliances and Materials Safety Law. This also includes regulations on noise intensity (the frequency band of the noise terminal voltage is 526.5kHz to 30MHz, and the frequency band of the noise power is 30MHz to 300MHz).

Regardless of whether the Electrical Appliance Safety Law is implemented, as the LED lighting market expands, the problem of mutual interference with other electronic products is inevitable.

The source of electromagnetic noise in LED bulbs is their power supply circuit. Since the size of the power supply section of LED bulbs is strictly limited, it is necessary to implement electromagnetic noise countermeasures with the least number of components. The selection of noise countermeasure components is particularly important. Therefore, this article will focus on the types of electromagnetic noise leaked from LED lighting power supply circuits, their measurement methods, and the selection method of components that can effectively suppress electromagnetic noise.

Noise current has two modes

Generally speaking, the EMC (electromagnetic compatibility) standard defines two types of electromagnetic noise measurements: "radiated noise" radiated into the air and "conducted noise (noise terminal voltage)" flowing through the power line (Figure 1). There are two types of noise components in the noise current: "differential mode" and "common mode". Differential mode noise is the noise generated between the signal line and the ground line. Common mode noise is the noise generated between the earth and the signal line and between the earth and the ground line. The noise type between the signal line and the ground line and the earth is the same, that is, it has the same phase and the same amplitude.

Figure 1: Example of electromagnetic noise observed in an LED light bulb

EMC regulations define the measurement of two types of electromagnetic noise: radiated noise and conducted noise, and LED bulbs are no exception. The noise of some LED bulb products exceeds the specified values ​​of CISPR15 (quasi-peak value: QP and average value: AV).

The main component of radiated noise is common mode noise (Figure 2(a)). This is because the current loop area of ​​this noise is much larger than that of differential mode noise.

Figure 2: Electromagnetic noise exists in two modes

Electromagnetic noise has two modes: differential mode and common mode. Radiated noise is mainly common mode (a). In conducted noise, differential mode and common mode components are often mixed and propagated (b)

In the case of conducted noise, two components can be observed: differential mode and common mode (Figure 2(b)). If it is conducted noise, it is necessary to understand the characteristics of the noise components and take countermeasures based on their characteristics. First, let's introduce methods for suppressing conducted noise.

Differentiating the Noise Modes of a Power Supply

Conducted noise is generally measured using a V-shaped artificial power network to measure the quasi-peak value* (QP value) and average value (AV value, Figure 3 (a)) of the electromagnetic noise of power line 1 (L1) and power line 2 (L2). Although the V-shaped artificial power network can measure the noise voltage between each power line and the earth, it is unclear which noise mode is the main one because differential mode noise and common mode noise are combined.

Figure 3: Measurement using V-type and Δ-type artificial power networks

In the measurement of conducted noise, the quasi-peak value and average value of the electromagnetic noise of power line 1 (L1) and power line 2 (L2) are generally measured using a V-type artificial power network (a). In this measurement, differential mode noise and common mode noise are combined, making it difficult to distinguish which noise mode is the main one. However, if a Δ-type artificial power network is used, it is easier to distinguish the type of noise mode (b). This power network can measure the frequency characteristics of the noise mode (Sym: differential mode, ASym: common mode).

*Quasi-peak value: When detecting electromagnetic noise, etc., the value measured by the detection method with the detector time constant enlarged. It is a value between the maximum value and the average value. When the quasi-peak value of electromagnetic noise is large, it is easy to cause radio reception problems. The correlation with the same receiving sensitivity is stronger than the peak value.

However, the type of noise mode can be determined by using a "Δ-type artificial power network" (Figure 3(b)). This circuit network can measure the frequency characteristics of each noise mode in the conducted noise.

This frequency characteristic varies by product type. For example, the frequency characteristics of electromagnetic noise are different between LED bulbs, pendant lamps, and large-size LCD TVs (Figure 4). LED bulbs are dominated by differential mode noise, while LED pendant lamps are a mixture of differential mode noise and common mode noise. Large-size LCD TVs are dominated by common mode noise.

Figure 4: Noise content varies by product

As the types of electronic products change, the composition of noise components will also change. For example, LED bulbs are mainly differential mode noise, while LED pendant lights have a mixture of differential mode noise and common mode noise (a, b). Large-size LCD TVs are mainly common mode noise (c).

So why do the conducted noise components of different products have specific tendencies? By analyzing this tendency using electromagnetic field analysis simulation, we can understand the reason.

Noise pattern depends on size

Conducted noise is measured in a shielded room. The measurement conditions are specified by standards such as "CISPR16-2" or "ANSI63-4". Both standards specify that the distance between the reference plane of the shielded room and the object to be measured should be maintained at 0.4m, the length of the wire connecting the artificial power network and the object to be measured should be 0.8m, and the object to be measured should be placed on a table 0.8m high (Figure 5).

Figure 5: Conducted noise measurements are performed in a shielded room

This figure shows the measurement of conducted noise. The measurement is performed in a shielded room. The specific measurement conditions are specified in standards such as "CISPR16-2" or "ANSI63-4".

At this time, the common mode noise flows out through the distributed capacitance between the inner wall (metal) of the shielded room and the object under test. We modeled this situation and analyzed the relationship between the size of the object under test and the ease of common mode noise outflow (common mode impedance) using electromagnetic field simulation.

We used electromagnetic field simulation to analyze four objects of different sizes (5×5×5cm3, 10×10×10cm3, 20×20×20cm3, and 100×80×20cm3) and calculated the impedance when observing the objects through an artificial power network (Figure 6).

Figure 6: Noise pattern depending on product size

Electromagnetic field analysis simulation was performed using four objects of different sizes, and the common mode impedance when the object under test is observed from the artificial power network was calculated (a). The results show that the larger the shape, the greater the distributed capacitance between the shielding room reference plane and the object under test, and the lower the impedance of the common mode path (b). In addition, the higher the frequency, the lower the common mode impedance (c).

The table in FIG. 6 lists the common mode impedance at 1 MHz and the value of the impedance converted into distributed capacitance.

From the results of analyzing the four types of objects using electromagnetic field simulation, it can be seen that the larger the shape, the greater the distributed capacitance between the inner wall of the shielding room and the object being measured. In other words, the larger the product size, the lower the impedance of the common mode path, the easier it is for the common mode noise current to flow, and the greater the noise component.

The next article will introduce countermeasures based on the characteristics of the above-mentioned conducted EMI noise.

Differential mode noise current flows in the differential direction

Countermeasures for conducted noise are implemented in three situations: ① When the differential mode noise is large and the common mode noise is small; ② When the common mode noise is large and the differential mode noise is small; ③ When both noises are large.

First, let's introduce the countermeasures when ① differential mode noise is large and common mode noise is small. The current of differential mode noise flows in the differential direction on the AC power line. Therefore, it cannot be attenuated by ordinary common mode choke coils. This is because the common mode choke coil generates inductance for the current in the same phase direction (common mode), but generates almost no inductance for the current in the differential direction (differential mode).

Therefore, as a countermeasure for differential mode noise, differential mode choke coils and capacitors connected to both ends of the AC power line (hereinafter referred to as "X capacitors") are generally used. These two components form a path inside the object under test that allows the differential mode noise current flowing through the AC power line to return to the noise source (Figure 7(a)).

Figure 7: Using differential mode chokes and X capacitors to suppress electromagnetic noise

To suppress differential mode noise, a differential mode choke and X capacitor are used to form a path in the product that allows the differential mode noise current flowing through the AC power line to return to the noise source (a). In the case of common mode noise, Y capacitors are generally used to suppress the noise, but this effect is not sufficient in the power supply circuit of lighting products. Therefore, common mode noise is suppressed by adding a common mode choke to the Y capacitor or by using only the common mode choke (b).

Using a differential mode choke coil can increase the impedance of the AC power line, making it difficult for noise current to flow. Then, based on this, using an X capacitor to reduce the impedance between AC power lines, the noise current returns to the noise source. This method can prevent electromagnetic noise from being transmitted outside the product.

Choke Countermeasures

Next, we will introduce the noise suppression method when the common mode noise is large and the differential mode noise is small. In common mode noise, since the noise current flows in the same phase (common mode) on the AC power line, it will not work even if X capacitors are connected to both ends of the AC power line. When using capacitors to suppress noise, capacitors that guide the noise current to the ground (hereinafter referred to as "Y capacitors", Figure 7 (b)) are used.

However, the effect of using Y capacitors to reduce common mode noise is generally not significant. Therefore, it is necessary to use chokes effectively. In order to increase the impedance of the AC power line and reduce the common mode noise current, a common mode choke or differential mode choke with a high inductance value is connected to the primary side of the power supply. The common mode choke can obtain a large impedance for the noise current flowing in the same phase direction, so it is suitable for common mode noise countermeasures.

Noise Suppression Using Hybrid Chokes

③ When both differential mode noise and common mode noise are large, it is necessary to take measures for each type of noise separately. This will increase the number of components required, which will increase costs and hinder miniaturization.

In this case, a "hybrid choke" that has the functions of both a common mode choke and a differential mode choke is most effective.

Hybrid chokes have the same common mode impedance as common mode chokes of the same size, but higher differential mode impedance (Figure 8). Hybrid chokes are also available in flat shapes, so you can choose according to the product size.

Figure 8: Hybrid chokes have higher differential mode impedance

Hybrid chokes have common-mode impedance similar to that of common-mode chokes of the same size, but also have higher differential-mode impedance.

The key to countering electromagnetic noise in LED lighting equipment lies in the configuration of electronic components

The above is an overview of the methods for suppressing conducted noise from the power supply section. Next, we will introduce an example of noise suppression in the power supply section of an LED lighting fixture.

In the power supply section of LED lighting equipment, there are roughly three parts where noise countermeasures need to be taken: the primary side of the power supply before and after rectification, and the secondary side of the power supply.

This article will introduce measures to address the noise pattern that is most likely to affect the components on the primary power supply side before rectification. This section is equivalent to the AC power line mentioned above.

The first thing to be introduced is the countermeasures for conducted noise in LED ceiling lights. Before exploring countermeasures, it is necessary to accurately measure conducted noise.

First, we measured the conduction noise of the LED pendant light using a V-type artificial power network with only the X capacitor as the countermeasure element. The measurement confirmed that noise was generated in a wide frequency band from 150k to 10MHz (Figure 9).

Figure 9: Example of conduction noise countermeasures for LED pendant lights

This figure shows an example of a countermeasure for conducted noise in an LED pendant light. As can be seen from the figure, the frequency characteristics of the generated conducted noise vary depending on the type and arrangement of components. Therefore, using X capacitors and hybrid chokes is the most effective countermeasure with the least number of components.

Next, we used a Δ-type artificial power network to measure each noise mode. Common-mode noise occurred in a wide frequency range, while differential-mode noise occurred in a low-frequency band around 1 MHz. In other words, the conducted noise of the LED chandelier was a mixture of the two noise modes.

As a countermeasure against common mode noise with a high noise level, ① a standard common mode choke coil (3mH) was installed. This greatly attenuated the common mode noise, but the differential mode noise was not attenuated, so the electromagnetic noise in the low frequency band was still higher than the specified value.

Pay attention to the interaction of countermeasure components

To suppress electromagnetic noise in the low-frequency band, ② added a differential mode choke (2.2mH). This reduced the differential mode noise, but there was a difference in the noise level between L1 and L2. Since the differential mode choke was only added to the L1 side, only the noise on the L1 side was reduced.

To correct this imbalance, we tried to change the position of the X capacitor in ③. As a result, the noise of L1 increased, and the noise levels of L1 and L2 became almost the same. However, this did not solve the problem. Therefore, as another method to eliminate the imbalance, we restored the position of the X capacitor in ④ and added a differential mode choke (2.2mH) to L2. In other words, differential mode chokes were installed in L1 and L2 respectively. This time, not only did the noise levels of L1 and L2 become almost the same, but they were all greatly attenuated. However, a new problem arose, that is, the common mode noise around 1MHz increased.

The estimated reason is that the series resonance between the inductance of the differential mode choke and the distributed capacitance of the common mode choke itself causes the common mode impedance near 1MHz to decrease.

As mentioned above, the interaction between components sometimes leads to an increase in electromagnetic noise. As a countermeasure, there is a method of using a hybrid choke coil.

By using a hybrid choke, you can achieve the same effect as using one common mode choke and two differential mode chokes. In addition, you can reduce the number of components while suppressing the effects of interaction between components.

Differential mode noise accounts for more than half

Next, let's introduce an example of a countermeasure for an LED light bulb. As with LED pendant lights, let's first check the noise components of conducted noise before looking for countermeasures (Figure 10). As mentioned above, LED light bulbs are small in size, so common mode noise is rarely generated, and differential mode noise is the main noise generated.

Figure 10: Example of conduction noise countermeasures for LED bulbs

This figure shows an example of a countermeasure for conducted noise in an LED light bulb. By placing an X capacitor outside the differential mode choke coil, the conducted noise of L1 and L2 is reduced and their magnitudes are almost the same.

As with the LED chandelier, let's verify the noise suppression effect of each component. First, ① add a differential mode choke coil (3mH) to L1 and place the X capacitor outside the coil, so that the noise of L1 and L2 is reduced. The noise level of L1 and L2 is also basically the same.

This is an effective countermeasure for LED bulbs. However, we tried ② to equip L1 and L2 with differential mode chokes. In this way, the common mode noise of 1MHz frequency increased, just like the LED chandelier. It is estimated that after installing two differential mode chokes, the inductance of the common mode path and the common mode capacitance of the noise current flow path resonate in series.

As described above, the effect of noise suppression varies greatly depending on the interaction between the capacitance and inductance of the path through which the noise current flows and the noise suppression element.

Chip Bead Countermeasures

LED bulbs will be subject to the Electrical Appliances and Materials Safety Law from July 2012, so this article will also introduce countermeasures for radiated noise. This time, the radiated noise of LED bulbs was first measured when an X capacitor was configured outside the differential mode choke (Figure 11). The results showed that radiated noise was generated in a wide frequency range, especially in the 100M to 200MHz band, which far exceeded the specified value of CISPR15.

Figure 11: Example of countermeasures for radiation noise in LED light bulbs

Chip ferrite beads are used to counteract radiation noise in LED bulbs. The effect of suppressing radiation noise varies depending on the position of the beads.

As for the radiation noise countermeasure, chip ferrite beads were used to suppress common mode radiation noise. Since installing the beads only on the L1 side did not reduce the radiation noise, ① ferrite chip beads were installed on both power supply lines L1 and L2. Although this reduced the radiation noise, it was still greater than the specified value.

Therefore, we changed the configuration of the chip ferrite beads. Specifically, ② we moved the beads installed outside the X capacitor to the inside of the X capacitor. This further reduced the radiation noise, and this time it was reduced to below the specified value. In other words, the effect of suppressing radiation noise varies depending on the position of the X capacitor and the chip ferrite beads.

Optimal configuration varies by lamp

The best component configuration for suppressing radiation noise varies from one LED bulb to another. In other LED bulbs, although the configuration of ② in Figure 11, which achieved excellent results, was adopted, that is, the chip ferrite bead was configured inside the X capacitor, it did not achieve the same excellent effect. So the chip ferrite bead was installed outside the X capacitor, and the radiation noise was reduced to below the specified value (Figure 12). In other words, for each LED bulb, even if the same noise countermeasure component is used, the effect is different.

Figure 12: Changing component configuration based on LED bulbs

Different LED bulbs have different optimal component configurations for suppressing radiated noise. For example, the position relationship between the X capacitor and the chip ferrite bead must be adjusted.

Therefore, the selection and installation position of noise countermeasure components must be changed according to the positional relationship between noise countermeasure components and other components and the performance parameters of LED bulbs. The effect of noise countermeasure components needs to be repeatedly confirmed at the development site, and then the type and configuration of components are changed according to the confirmation results. Therefore, in order to increase the freedom of component configuration, considering pattern design in advance is also a way to reduce the EMC countermeasure load.

This time, we introduced the countermeasures for conducted noise and radiated noise of lighting fixtures, along with the basics of EMC. The methods introduced in this article are only part of the solution. In the future, we will continue to develop various countermeasure components and propose countermeasure methods to achieve even better electromagnetic noise countermeasures.