Early planning of the system's EMI compliance is critical to the success of the project. Well-designed design should use the correct filter, low EMI, PMIC and low EMI power modules, coupled with good PCB layout and shielding technology, this will be able to ensure a one-time success of a large probability.
EMC (EMI) Compliance testing should be done at the end of the product development cycle. Failure to pass EMI testing is a nightmare for system engineers. This is not only a huge obstacle to the product delivery plan, but also means that the redesign of the power supply is costly. Fortunately, designing a power scheme that meets EMI standards is not a matter of success. Well-designed design should use the correct filter, low EMI components, low EMI power regulator IC and/or low EMI power modules, coupled with good PCB layout and shielding technology, which will be able to ensure a high probability of one-time success.
What is EMI noise? Why do you have to be cautious?
EMI interference occurs when an electronic device is connected to or near another electronic device that generates EMI, or when it is shared with a power supply. EMI can be conductive or radioactive. EMI problems can prevent electronic devices from working with neighboring devices. Common examples of EMI that we may encounter in our daily life are:
1. A low-flying aircraft interferes with the audio/video signal of a radio or television.
2. The transmitter causes local television stations to not display their images.
At worst, the screen disappears, or it shows some kind of image pattern.
3. The mobile phone communicates with the tower to handle the disturbance caused by the call (so the flight requires the passenger to turn off the phone during flight).
4. The interference from the microwave will affect the nearby WiFi signal.
With the increase of electronic equipment consumption, EMC has become an important issue. As a result, standardization organizations have been created to ensure that electronic devices have normal performance even in an EMI environment. Now, with modern electronic devices, it is almost impossible to make normal use of mobile phones and other wireless devices near any electronic device, with little or no impact. In order to achieve the above goal, it is necessary to ensure that the equipment does not emit harmful radiation, but also make the equipment not susceptible to RF radiation.
EMI Design Requirements:
CISPR 22 (commonly referred to as the EN55022) EMI specification divides equipment, devices, and appliances into two categories:
Class B: equipment, devices and appliances designed to meet the CISPR22 standard B-Class launch requirements for the home environment. Category A: equipment, devices, and appliances that do not meet the requirements of the CISPR22 standard B launch but meet the requirements of a more relaxed CISPR22 standard A-class launch. Class A equipment should have the following warning: "This product is a class A equipment." In a home environment, this product may cause radio interference, at which point the user may need to take appropriate action. ”
EMI testing consists of two parts: conduction and radiation. The conduction emission test is carried out at 150kHz to 30MHz frequency range. This is the AC current that is transmitted to the power supply and is measured by two methods: quasi-peak and average, each with its own limit value. The radiation emission test is performed at a high 30MHz to 1GHz RF range. This is the radiated magnetic field from the device being tested (DUT). The upper limit of the test range 1GHz is applicable to DUT with the highest oscillation frequency of 108MHz. The upper limit extends to 2GHz when the internal oscillator reaches 500MHz, and the internal oscillator is extended to 5GHz as high as 1GHz, and the internal oscillator is extended to 1GHz when the frequency is as high as 6GHz.
The following is an illustration of the CISPR 22 specification: The Y axis is the size of the test EMI and the unit is DBUV. The X axis is the test frequency and the unit is Hz.
Figure 1: CISPR 22 Standard B class conduction EMI specification.
Figure 2: CISPR 22 Standard A-class conduction EMI specification.
Figure 3:CISPR 22 Standard B class radiation EMI specification.
Figure 4:CISPR 22 Standard A-Class radiation EMI specification.
EMI noise source in switching power supply
The switching power supply will generate electromagnetic energy and noise, and also be affected by the electromagnetic noise of external interference sources. The noise generated by switching power supply can be divided into two types: conduction and radiation. The form of conduction emission can be voltage or current, which can be further divided into common or differential mode. In addition, the finite impedance of the connecting line causes the voltage conduction, which causes the current conduction, and vice versa, the differential mode conduction causes common mode conduction, and vice versa.
We then discuss the noise sources in the switching power supply. This is a buck regulator schematic diagram and its working circuit waveform:
Figure 5: Buck Regulator schematic diagram and its working waveform conduction EMI.
As shown in Fig. 5, the input current of the buck Adjuster is the pulse waveform, which is the main conduction source, and is the reverse recharge power VS differential EMI. The conduction emission is mainly affected by the fast changing waveform of the converter input (DI/DT). The value of conduction emission is the voltage versus measurement at the input of the converter, and the Line Impedance Stabilization Network (LISN) is used. The function of the input capacitor CI is to remove the AC (pulse) component. Network current is the difference between II and ICI. We want the is to be DC or as smooth as possible. If CI is an ideal capacitor with infinite capacitance, it will keep the VI constant and effectively filter out all the AC components of the I1, retain the constant (DC) current from the power supply VS, and ensure the DC pressure drop on the source impedance RS to a constant value. In this case, the conduction EMI will be zero due to the DC current.
In practice, we use a π filter between the input source and the converter to make the conduction EMI within the range of the adjustment limits. The problems caused by conduction emission to the fixed system are usually larger than the portable system. Because the portable device is working with a battery, its load and source are not externally connected for conducting launch.
Radiation EMI is a rapidly changing magnetic field with a high frequency component of 30MHz and above. The magnetic field is generated by the circuit's current loop. If the change in the magnetic field is not properly filtered or shielded, the change is coupled to other adjacent circuits and/or devices, causing a radiation EMI effect.
Figure 6: Buck Regulator schematic diagram and its fast di/dt current loop.
Figure 6 Shows the buck converter and its fast di/dt circuit loop I1 and I2. The current loop I1 conducts conduction during conduction, the S1 conduction, the S2 turn-off, the current loop I2 conducts conduction during the turn-off, the S1 shuts off, and the S2 leads through. The fluctuation properties of current loop I1 and I2 cause magnetic field change, and the field strength is proportional to the current amplitude and the area of conduction loop. The fast di/dt current generates high-frequency harmonic EMI and is within the prescribed radiation range. So that the area of the current loop as small as possible, will be able to reduce the maximum field strength. Slowing down these signals will reduce the high-frequency harmonic components of the switch regulator, but slower jumps will affect the regulator efficiency because of the waste of energy. We then discuss ways to minimize EMI radiation without affecting efficiency.
Figure 7: The magnetic field generated by the current loop.
Voltage node LX (some manufacturers call SW or other name) for rectangular wave (temporarily ignoring parasitic oscillation), connected to inductance. LX's fast DV/DT voltage discontinuous signal is coupled with the high frequency current to the CO and the load by the parasitic capacitance of the output inductance, which in turn generates EMI noise. It is very important to minimize the parasitic capacitance of the output inductance to mitigate the noise coupling problem. LX also has high frequency parasitic oscillations.
Using an RC buffer network from LX to GND helps reduce this oscillation. The principle of EMI noise source mentioned above also applies to other switch converter structures. But the degree of noise severity depends on the current and voltage waveform of the concrete structure.
For example, the input current of the boost converter working in continuous conduction mode is more continuous than the input current of buck converter, so the conduction EMI at its input is smaller. It is difficult, time-consuming and costly to correct the EMI problem in the power system by taking the time to mend. Pre-design and planning of EMI compliance is critical for project success. The common EMI suppression technology has power filter, power design, correct PCB layout and shielding.
Filter design of EMI power supply
To reduce the conduction emission from the power converter, a π filter is used between the input source and the power converter. The design steps for selecting the filter element are as follows:
1. Determine the input impedance RIN: At worst, the closed-loop input impedance of buck converter is RIN = ro/d2 at all frequencies, where RO is the output load and D is the duty-duty ratio. The input impedance is minimal when the converter works at the minimum input voltage.
Example: Taking Maxim Himalaya SiP Power Module MAXM17575 As an example, the device uses 4.5-60vin, 0.9-12vout to provide maximum 1.5A current. Take the MAXM17575 evaluation Board for example, the minimum input voltage is 7.5V. Output load is RO = Vo/io = 5v/1.5a =3.3Ω. Max work duty ratio is D = Vo/vinmin = 5v/7.5v = 0.66. Therefore, the lowest possible input impedance for RIN = RO/D2 = 3.3Ω/0.662 = 7.6Ω.
2. Design EMI filters In accordance with the output impedance smaller than the RIN 10db or smaller: increasing the input filter will affect the performance of the DC converter. To minimize this effect, the impedance of the filter must be less than the input impedance of the power converter at all frequencies of the maximum converter crossing frequency.
Figure 8. The conduction EMI input filter is inserted between the input and the Power module.
LC filter at resonant frequency of the output impedance (maximum value) is:
In our design, we consider that the effective impedance of the filter is smaller than the input impedance of buck converter 10dB, approximately equal to one-third of the input impedance. In the case of MAXM17575, Zo is required to RIN/3 = 7.6/3 = 2.5Ω, which is applicable to all frequencies below the MAXM17575 circuit crossing frequency (45kHz).
Best practice for PCB layout
PCB layout is very important to EMI compliance. Poor PCB layout will completely destroy the design of the perfect power converter. The following is a good PCB layout practice, using the same buck converter in the above example to minimize EMI noise Source:
1. Minimizes the high di/dt current loop: The LO, CO, and S2 are properly arranged together to minimize I2 current loops. Then, the group of components is close to S1 and C1, minimizing the I1 current loop. When using the Buck regulator IC (that is, the buck controller of the set success Switch S1 and S2), it is important to choose an IC that has a good pin alignment to support this minimization. The same principle applies to the use of power modules.
Figure 9: buck Converter's high di/dt current loop.
1. Using Faraday Shield: Faraday Shield (or Faraday cage), named after British scientist Michael Faraday, is a shell used to obstruct electromagnetic fields. There are usually two ways to implement Faraday shielding in a power system:
A. A cage made of conductive material (e.g. copper) that surrounds the entire power supply system or equipment. The electromagnetic field remains inside the cage.This method is generally costly because of the material cost of the cages and the additional assembly labor.
B. The top and bottom layers of the PCB are provided with shielded grounding zones, which are connected by an over hole to simulate Faraday cages. All high di/dt loops are arranged in the inner layer of the PCB, so Faraday cages can shield electromagnetic fields from outward radiation. The method is low cost and is usually sufficient to inhibit EMI. Fig. 10 is a schematic diagram of the technique.
Figure 10: Faraday Shield for multilayer PCB board.
The best practices for implementing these PCB layouts provide a reasonable approach to EMI compliance and do not sacrifice power converter efficiency because of the need to slow down the switching signal along.
Now, we take the Maxim Himalaya MAX17502 wide input range IC For example, the device works in 4.5-60vin, 0.9-54vout, providing 1 a load current. The following is the layout of the PCB for the MAX17502 EMI Evaluation board, using Faraday shielding technology (b). Figure 11a shows the top and bottom, using the method Ladi; Figure 11b shows the inner second and third layers for wiring. The second layer here is used as an extra shield and can also be used for wiring. In this layout, the high di/dt current loop I1 and I2 are arranged in the third layer, which is completely enveloped in the Ladi of the law.
Fig. 11a: The top and bottom layer of the Ladi by means of practice.
Figure 11b: The second and third (inner) layer, the high DI/DT loop is arranged on the third floor.
The following is the EMI test results of the MAX17502 EMI Evaluation Board, which has passed the CISPR 22 standard class B requirements with very good margin.
Figure 12: max17502 EMI Evaluation Board conducted EMI test results. Left: quasi-peak; right: average.
Figure 13:max17502 EMI Evaluation Board radiated EMI test results.
Low EMI Power Components
The magnetic field of the output inductor also generates radiation, causing EMI problems. The use of low EMI inductance can reduce radiation EMI. It is recommended to use shielded inductors. The inductor is shielded by a magnetic field and is restrained within the inductance structure. Avoid the use of magnetic energy free radiation inductance type. The use of shielding inductance and the implementation of good PCB layout practice of the Power module will present good EMI performance.
Low EMI power supply regulators and modules
Maxim's Himalaya voltage regulator and power module family adopt low EMI Power inductors and good PCB layout practices to provide inherently low EMI power solutions. Using the Himalaya scheme means you don't have to worry about compliance, in stark contrast to other simple switches on the market. Maxim IC, module, and sample reference layouts have completed all the work and you are able to pass the CISPR (EN 55022) standard at the optimal cost. The following are the EMI test results for the MAXM17575 example and the input EMI filter information:
EMI Filter Configuration-conducting EMI test
Figure 14: EMI Filter configuration for MAXM17575 Evaluation Board for conducting EMI tests.
Fig. 15:maxm17575 Evaluation Board conducting EMI test results. Blue: quasi-peak; Green: Average
EMI Filter Configuration-radiation EMI test
MAXM17575 essentially has a very low radiation EMI. For radiation testing, the input filters shown in the conduction EMI test are not required or used. The use of input filters can provide a greater margin for radiation testing.
Figure 17:maxm17575 Evaluation Board radiated EMI test results.
Early planning of the system's EMI compliance is critical to the success of the project. This article discusses the most common technology to reduce EMI, but also provides a guide to the Power filter design, good PCB layout, shielding practice and practice examples. Well-designed design should use the correct filter, low EMI pmic/components and/or low EMI power modules, coupled with good PCB layout and shielding technology, which will ensure a one-time success of a large probability.
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