Power Supply EMI Filter Design Method with Certification FCN Determination

Product Specification

Switching power supply EMI filter test method

Power supply EMI filter design method:

1. General method for determining fcn:

The choke cutoff frequency fen should be determined according to the electromagnetic compatibility design requirements. For the disturbance source, it is required to reduce the disturbance level to the specified range; for the receiver, its reception quality is reflected in the requirements for noise tolerance. The cutoff frequency of the first-order low-pass filter can be determined by the following formula:

Disturbance source: fcn=kT×(low disturbance frequency in the system); Receiver: fcn=kRX(low disturbance frequency in the electromagnetic environment).

In the formula, kT and kR are determined according to the electromagnetic compatibility requirements, and generally take 1/3 or 1/5. For example: the cutoff frequency of the power supply noise choke or power supply output filter is fen=20~30kHz (when the switching power supply frequency f is 100kHz); the cutoff frequency of the signal noise choke is fcn=10~30MHz (for information technology equipment with a transmission rate of 100Mbps).

In addition, for devices with special input current waveforms, such as power input circuits connected to direct rectification and capacitor filtering (this is usually the case for switching power supplies and electronic ballasts without power factor correction (PFC)), the noise choke cutoff frequency fcn may be lower to filter out the 2~40th harmonic conduction interference of current. For example, the Federal Communications Commission (FCC) of the United States stipulates that the starting frequency of electromagnetic interference is 300kHz; the International Special Committee on Radio Interference (CISPR) stipulates that it is 150kHz; and the US military standard stipulates that it is 10kHz.

2. Noise filter circuit

When the choke is inserted into the circuit, the noise suppression effect it provides depends not only on the size of the choke impedance ZF, but also on the impedance before and after the circuit where the choke is located (i.e., source impedance and load impedance). Network analysis points out that within the operating frequency range, the input and output impedance of the transmission line are matched, which can maximize the transmission of signal power; for noise, we naturally think of inserting a noise filter to make its input and output impedance mismatched within the noise frequency range to minimize noise suppression.

Therefore, the selection of noise filter structure and components depends on the source impedance and load impedance of the circuit where the noise filter is located. In this sense, the anti-EMI filter is actually a noise mismatch filter. Here, we specifically propose the concept of noise mismatch to facilitate the analysis of the interaction between noise and noise filters (see the application principle section below).

Figure 1 Basic circuit of noise filter

Noise filter circuits usually use x-shaped, T-shaped, L-shaped circuit structures and their combinations to make low-pass filters. The basic circuit structure is shown in Figure 1. Generally speaking, for high-frequency noise, the n-shaped structure can provide low input and output impedance, which is suitable for occasions where the source impedance and load impedance of the circuit are high; the T-shaped structure can provide high input and output impedance, which is suitable for occasions where the source impedance and load impedance of the circuit are low; the L-shaped structure can provide high input impedance and low output impedance (or vice versa), which is suitable for occasions where the source impedance and load impedance of the circuit are low (or vice versa). The determination of the L and C values ​​of the filter components must meet the circuit's requirements for insertion loss at the noise frequency, and can be approximately calculated as follows:

L=Z/(2I×fc), C=1/(2n×fe×Z)

Z is the noise choke impedance, filter input or output impedance. It should be pointed out that the calculation of L and C values ​​can only be approximate. Because for frequencies as high as 100kHz and its harmonics, the circuit distributed parameters can no longer be ignored, and the noise suppression effect of the noise filter is often determined by experiments. To facilitate design calculations, the impedance frequency characteristics of an actual capacitor and the calculation method of lead inductance are given below. Considering the influence of capacitor loss and lead inductance, the actual capacitor equivalent circuit and impedance frequency characteristics are shown in Figure 2.

The lead inductance is calculated by the following formula:

L=0.002/[ln(4l/d)-1]

Where d is the wire diameter (cm), 1 is the wire length (cm), and L is the inductance (uH).

For example, a 0.31mm wire with a length of 1=1cm, L=0.0077uH, when the frequency is 1MHz, Z=0.0499; when the frequency is 100MHz, Z=4.99. When 1=2cm, L=0.0182uH, when the frequency is 100MHz, Z=11.44 ohms.

3. Noise filter application principle

The method or procedure for selecting and using noise filters according to electromagnetic compatibility requirements is not unique. This should be solved as part of the electromagnetic compatibility design process in electrical design, production, and debugging. Nevertheless, before designing and using noise filters, it is beneficial to understand the electromagnetic disturbance propagation mode, noise frequency range, and electromagnetic environment of the inserted circuit.

There are roughly two ways of propagation of electromagnetic disturbance:

One is conducted interference and the other is radiated interference. The board-mounted noise filter used to improve the circuit noise tolerance can be designed to work in a certain frequency band within the frequency range of 9kHz~1780MHz (according to the relevant electromagnetic compatibility standards). Generally speaking, it can be considered that: the low frequency segment of noise is manifested as conducted interference (harassment), and the noise filter mainly relies on the inductive reactance of the choke to provide noise suppression; at the high end of the noise frequency, the conducted noise power is absorbed by the equivalent resistance of the choke and bypassed by the distributed capacitance. At this time, radiated disturbance becomes the main form of interference.

Radiated disturbance induces noise current on nearby components and leads, and in severe cases, it can cause circuit self-excitation, which becomes more prominent in the case of small and high-density circuit component assembly. Most anti-EMI devices are inserted into the circuit as low-pass filters to suppress or absorb noise interference. According to the noise frequency to be suppressed, the filter cutoff frequency fcn can be designed or selected. As mentioned above, the noise filter is inserted into the circuit as a noise mismatch. Its function is to severely mismatch the noise higher than the signal frequency. Using the concept of noise mismatch, the function of the filter can be understood as follows: through the noise filter, the noise may reduce the noise output level due to voltage division (attenuation); or absorb the noise power due to multiple reflections; or destroy the parasitic oscillation conditions due to channel phase changes, thereby improving the noise tolerance of the circuit.

In addition, the following issues should be noted when designing and using anti-EMI devices:

(1) Understand the electromagnetic environment and select the frequency range reasonably;

(2) Whether there is DC or strong AC in the circuit where the noise filter is located to prevent the device core from saturation failure;

(3) Understand the impedance size and properties before and after the insertion circuit to achieve noise mismatch. The impedance of the choke is generally 30~5009, and it is suitable to be used under low source impedance and load impedance;

(4) Pay attention to the inductive x interference generated by the distributed capacitance and adjacent components and wires;

(5) Control the temperature rise of the device, generally not exceeding 60℃.