Noise countermeasures for LED bulbs (top): There are two modes of noise current

Differential mode noise current flows in the differential direction

The countermeasures for conducted noise are implemented in three cases: 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 types of noise are relatively large.

First, we will introduce the countermeasures when the differential mode noise is large and the common mode noise is small. The current of the differential mode noise flows in the differential direction on the AC power line. Therefore, it is not possible to attenuate on a common common mode choke. This is because the common mode choke coil generates inductance for the current in the same phase (common mode), but hardly generates inductance for the current in the differential direction (differential mode).

Therefore, as a countermeasure against differential mode noise, a differential mode choke coil and a capacitor connected to both ends of the AC power supply line (hereinafter referred to as "X capacitor") are generally used. Through these two components, a path for returning the differential mode noise current flowing through the AC power source to the noise source is formed in the object to be measured (Fig. 7(a)).

Figure 7: Using the differential mode choke and X capacitor to suppress electromagnetic noise

To suppress differential mode noise, a differential mode choke and an X capacitor are used to form a path (a) for returning the differential mode noise current flowing through the AC power line back to the noise source within the product. If it is common mode noise, the Y capacitor is generally used to suppress noise, but in the power supply circuit of the lighting product, the effect is insufficient. Therefore, the common mode noise (b) is suppressed by adding a common mode choke coil to the Y capacitor or by using only the common mode choke coil.

The differential mode choke can increase the impedance of the AC power line, making the noise current difficult to flow. Then, based on this, the X capacitor is used to reduce the impedance between the AC power lines, so that the noise current returns to the noise source. This method prevents electromagnetic noise from being conducted outside the product.

Choke countermeasure

Next, the noise suppression method when the common mode noise is large and the differential mode noise is small is introduced. In common mode noise, since the noise current flows in the same phase (common mode) on the AC power line, even if the X capacitor is connected to both ends of the AC power line, it has no effect. When noise is suppressed by a capacitor, a capacitor that directs a noise current to the ground (hereinafter referred to as "Y capacitor", FIG. 7(b)) is used.

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

Use mixed chokes to suppress noise

3 When the differential mode noise and the common mode noise are relatively large, it is necessary to take countermeasures for each type of noise, which leads to an increase in required components, which is a factor that causes cost increase and hinders miniaturization.

In this case, a "mixed choke" with both common mode chokes and differential mode chokes is most effective.

The hybrid choke has the same degree of common mode impedance and a higher differential mode impedance as the common mode choke of the same size (Figure 8). The hybrid choke is also available in a flat shape and can be selected according to the product size.

Figure 8: The hybrid choke has a high differential mode impedance

The hybrid choke not only has the same level of common mode impedance as the common mode choke of the same size, but also has a higher differential mode impedance.

The key to electromagnetic noise countermeasures for LED lighting equipment lies in the configuration of electronic components.

The above is an outline of a method of suppressing conducted noise originating from a power supply unit. Next, an example in which the power supply unit of the LED lighting fixture suppresses noise will be described.

In the power supply unit of the LED lighting fixture, there are basically three places where noise countermeasures are required, which are before and after rectification of the primary side of the power supply, and on the secondary side of the power supply.

This article will introduce the countermeasures for the primary part of the power supply side rectification for the noise mode that is the easiest to highlight the effect of the component. This part is equivalent to the above AC power line.

The first one to introduce is the conductive noise countermeasures of LED chandeliers. Conducted noise must be measured correctly before exploring countermeasures.

First, the conduction noise of the LED chandelier was measured using a V-type artificial power supply network using only the X capacitor as a countermeasure component. It was confirmed by measurement that noise was generated in a large frequency range of 150k to 10MHz (Fig. 9).

Figure 9: Example of countermeasures for conducted noise of LED chandeliers

This figure is an example of the countermeasure against conductive noise of LED chandeliers. As can be seen from the figure, the type and configuration of the components are different, and the frequency characteristics of the generated conducted noise also vary. Therefore, the use of X capacitors and hybrid chokes is the most effective and the least number of components.

Next, the Δ-type artificial power network is used to measure each noise mode. Common mode noise occurs in a large frequency range, and differential mode noise occurs in a low frequency band around 1 MHz. That is to say, the conducted noise of the LED chandelier is mixed with two noise modes.

As a countermeasure against common mode noise with a high noise level, 1 a standard common mode choke (3 mH) is installed. In this way, although the common mode noise is greatly attenuated, the differential mode noise is not attenuated, so the electromagnetic noise in the low frequency band range is still higher than the specified value.