Electromagnetic compatibility of the hottest power

2022-10-20
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Electromagnetic compatibility (EMC) of power converters has become a household name in the past decade. In the mid-1990s, Europe demanded to reduce the radiation and conducted emission levels of products sold to the region. Since then, many products have begun to introduce EMC testing in their design phase. This trend has continued to the current product development

a frequently asked question is: what is EMC? In fact, EMC is the ability of components, products or systems to work normally in the predetermined electromagnetic environment (existing in electromagnetic interference EMI), and at the same time, it will not degrade itself and become an interference source. To design such a function, we must follow EMC standards, which are formulated by groups such as IEC and CISPR. This article will discuss EMC's regulations on radiation and conduction, including common mode and differential mode emission, and discuss how to design power line filters to reduce input and output noise. Finally, it will provide some PCB design skills that can reduce noise

1 EMC regulations

in order to obtain a reliable EMC design, you must understand the requirements of EMC. These requirements are not only for the module power supply, but also for the common system level standards in Europe and North America

iec (International Electrotechnical Commission) is responsible for formulating European specifications, while CISPR (International Special Committee on radio frequency interference) is responsible for EMC tests using CISPR 22, which defines the most stringent limits of conducted emissions. These limitations (conducted emissions) are now described in product standards en55022 (Figure 1) and en55011 (Figure 2). Class A and class B requirements in Figure 1 and Figure 2 refer to industrial standards and domestic standards respectively. According to the different antennas used to test noise, the European standard has two restrictions. The higher limit is for quasi peak antennas and the lower limit is for general antennas, but both limits must be met so that the equipment can pass the requirements. The FCC standard specifications used in North America are similar to the en requirements in Europe. Please refer to figure 2. When testing the power supply, two European standards are used: en55011 and en55022. In North America, radiated EMI is usually measured in the frequency range of 30MHz to 10GHz (according to FCC regulations), while conducted EMI is generally measured in the frequency range of several to 30MHz (according to FCC regulations)

the purpose here is to develop a system that can meet all or part of the above requirements related to launch, which can be an independent device or a system integrated into a larger system

2Common mode and differential mode noise

common mode and differential mode are two main noise sources. The shared mode noise comes from the shared mode current. Shared mode energy coexists on two power lines of a single-phase system and is transmitted in the same direction between all conductors and grounding and on all power lines or conductors. Since the two wires have the same level at the same time, the equipment between the wires will not attenuate this

the shared mode noise from the shared mode current always exists on the cable entering the device. One way to reduce this current is to test the cable on the original model as early as possible (so that the designer can make all necessary changes before the final delivery of the design to production), and before EMC compliance testing. In many cases, if the device cannot pass the shared mode current test, it will not pass the radiated emission test. The shared mode current can be measured simply through a current probe with high frequency clamping and a spectrum analyzer. The current probe with a response range of up to 250MHz has already introduced two types of 3D printing polymer materials in the aerospace field

differential mode noise is the opposite of shared mode noise. Differential mode noise is produced by the refraction of current from another conductor after flowing through a charged or neutral conductor. This generates a noisy voltage between the live and neutral conductors

3 AC power line main filter

is an example of a single-phase AC power filter. This type of filter is often used to reduce the differential mode and shared mode noise of input and output power supplies

4.1 part a

inductor l1/l2 and capacitor C1 form a differential filter to cope with all the noise trying to achieve sterile and clean air supply into the power supply. Differential mode noise is produced by the refraction of current from another conductor after flowing through a charged or neutral conductor. The combination of L1 and C1 or L2 and C1 forms a voltage divider. According to the frequency of noise, capacitor C1 presents a small impedance (larger load) to the signal, thus reducing the noise on the power line. For example, at a specific frequency, if the equivalent impedance of L1 is 10K and the equivalent impedance of C1 is 1K, the noise passing through the filter is one tenth of its original intensity, or the noise is reduced by 20dB

4.2 Part B

capacitors C2 and C3 form a shared mode filter with ground reference. Common mode noise becomes apparent when the current is in phase with the current in live and neutral conductors and returns through a safe ground. This generates a noisy voltage between the live/neutral conductor and ground. C2, C3, C4 and C5 are all equal, and all shared mode noise on these lines will be shunted to ground. It should be noted that due to leakage current, Part B cannot be used for medical equipment

4.3 Part C

Zorro inductor without reference (shared mode choke). Selecting the direction of each winding to produce the opposite current can eliminate all noise. The magnetic flux caused by the shared mode current will gather and produce impedance, so it can reduce the noise on the power line. Since the current of the differential mode flows in different directions, the magnetic flux generated by the current of the differential mode will cancel each other, so there will be no impedance and the noise of the differential mode cannot be reduced

capacitors C1 and C16 are class X capacitors, which are used to reduce differential noise and need to be able to withstand power supply voltage. Class X capacitors are usually in the range of 0.01uF to 2uF. Capacitors C2 to C5 are Class Y capacitors for shared mode noise, which need to be able to ensure that they will not fail in case of short circuit (more expensive than class X capacitors). The capacity value of Class Y capacitor is small, usually between 0.002uf and 0.1uF

5 design guide for reducing internal and external noise of power converter

ac to DC power supply has three areas of noise generation:

(1) noise already existing in AC power supply enters the power supply device (common mode/differential mode)

(2) caused by the switching frequency of the power supply (common mode)

(3) fast switching edge and ringing (common mode) caused by it when MOSFET is turned off

5.1 AC power supply

if there is a noisy power main line, an AC power line filter can be used. When using an AC power line filter, make sure it is installed as close as possible to the AC power line entering the circuit board (PCB),. The grounding connection of the filter should also be as short as possible to connect with the primary grounding plate of the power supply

in order to reduce shared mode and differential mode noise from entering and leaving devices, AC power line filters should be used. See AC power line main filter

5.2 switching frequency of power supply

like the system using system clock, many power supplies use pulse width modulation (PWM) components, which work at a certain frequency to control the output voltage. Therefore, the system clock needs to be carefully laid out on the circuit board, and so does the PWM controller

for transformer design using flyback, forward or other topologies, it is very important to make the lead as wide and as short as possible in the design between the primary winding and the drain of the switching MOSFET,. This shortens the inductive path and keeps ringing to a minimum. It is best to connect the MOSFET and PWM controller to the ground plate at the same time to minimize the amount of holes on the ground plate (instead of looking like Swiss cheese). There should be a grounding wire laid in parallel with the current return lead (if there is no stray capacitance problem). If the noise problem still exists, remove the grounding plate under the lead and minimize the capacitance from the drain lead to the transformer. MOSFET switch structure has parasitic capacitance, which will inject current between the component and the ground. If the ground plate under the "green line part" trace is not removed, additional current will enter the ground plate, causing greater shared mode conducted noise

The source of the

switch MOSFET must be reliably connected to the ground plate of the primary power supply. Therefore, a large pad should be made for the grounding terminal so that an appropriate number of jumpers (depending on the absorbed current) can be used to reliably connect with the grounding plate

5.3 PWM switching edge and concurrent ringing

are resistance capacitance diode (RCD) circuits (R1, C1 and D1), which have two functions. First, C1 can slow down the rise time of collector voltage when Q1 is turned off (smooth and reduce radiated EMI); Secondly, it maintains the input voltage at 2vcc, which does not exceed the breakdown voltage of the switching MOSFET. When C1 is large enough, the rising collector voltage and the falling collector current intersect at a very low position, so the power consumption of the transistor can be greatly reduced

The ringing circuit of

c2 and R2 is also important to reduce the ringing of the transformer primary, which is caused when the MOSFET releases the power supply of the input voltage

as the first pilot, the following is a method to determine the values of C2 and R2:

(1) determine the frequency of the ringing waveform and calculate the period

(2) multiply the period determined in the first step by 5

(3) set the value of resistance (usually less than 100r)

(4) divide the value obtained in the second step by the resistance determined in the third step

the advantage of using resistance R2 and capacitor C2 is to reduce ringing, but the disadvantage is that the high-frequency ripple through capacitor C2 will be thermally dissipated on resistance R2. If reducing noise is more important than efficiency, it can be adopted, otherwise it will reduce efficiency

6 printed circuit board design guide

(1) properly place and determine the direction of components

(2) if the radiator is used, be sure to ground it

(3) component shielding may be required

(4) the ESR value of shared mode capacitor should be small, and the length of grounding lead should be shortened

(5) if the buffer circuit is connected across the transformer to slow down the rise time of MOSFET switch off, please remember to shorten the trace length of the drain and the lead ends of the two source level transformers. If possible, set the buffer circuit between the two primary lead ends

(6) avoid using slots in the ground plate and power board (if used)

(7) under 50MHz (the harmonic of PWM controller should be considered), the traditional decoupling method is effective. One or two decoupling capacitors (generally 0.1 or 0.01uF) can be used near the IC power supply and ground lead ends. Consider the loop area formed between IC and decoupling capacitor, and place capacitor to minimize the loop area

(8) make the grounding wire as short and thick as possible

(9) avoid sharp corners on the trace

(10) when shielding is required, all noise components should be concentrated in the same area as far as possible

(11) if possible, use multilayer printed circuit boards

7 security of medical devices

for application sensitive devices such as the medical field, shared mode noise is indeed a problem. If the device is connected to the patient

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