Noise countermeasures for LED bulbs (I): Noise current has two modes

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

In general, the EMC (electromagnetic compatibility) standard defines two types of electromagnetic noise measurements, namely "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 observing the measured object from the artificial power network was calculated (a). According to the results, the larger the shape, the greater the distributed capacitance between the shielded room reference plane and the measured object, 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 Figure 6 lists the common mode impedance at 1MHz and the value 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.