Methods for preventing electromagnetic interference in switching power supply

When verifying DCDC power products, they often encounter electromagnetic interference (EMI) problems, which sometimes take a lot of time to deal with. Many engineers are more experienced than theoretical when dealing with electromagnetic interference, and they know which frequency band to deal with which components, but they lack theoretical analysis. I have been engaged in the design of switching power supply for many years, and hope to make a sort of arrangement based on the experience of previous countermeasures and relevant theoretical basis, so that the personnel who are engaged in or want to engage in the design of switching power supply can have a preliminary understanding of electromagnetic interference prevention technology.

The electromagnetic interference test of specially customized switching power supply can be divided into conduction test and radiation test. Generally, the conduction test frequency band of customized non-standard switching power supply refers to 150K~30MHz, while the frequency band of radiation interference refers to 30M~300MHz. The frequency band after 300MHz is generally not generated by the power supply, so most of them can be ignored.

The following describes the conduction testing regulations, testing and measuring methods, basic concepts, filter design to inhibit conduction interference, wiring and transformer design, etc.

2 Regulations for conduction testing

According to different products, the applicable provisions of transmission regulations are also different. Generally, EN-55022 of Europe or FCC part15 of the United States is used to define the limit line, which can be divided into CLASS A and CLASS B standards. CLASS A refers to the product used in commercial and industrial areas. CLASS B refers to the product used in residential and family areas. The product designed by the author is a household power supply of 3C, and the conduction test frequency band is 150K~30MHz. Please confirm the safety rules applied before product test, and different safety rules and grades will have different standard lines.

For example, the limit diagram of EN-55022CLASS B shows that the red line is the limit line of Quasi peak (QP, Quasi peak), and the pink line is the limit line of AV, the ultimate goal of conduction test is that the test machine can be completely below its limit line, no matter QP value or AV value. In general, when applying for safety regulations, although only under the limit can apply, most of them will be less than 2dB error to prevent the difference caused by different test sites, and sometimes the client will require that it must be less than 4~6dB to prevent the error caused by mass production of products.

As an example after measurement, peak value measurement will be used first in general measurement, because peak value measurement is the simplest and fastest method. The measuring instrument takes 9KHz as a unit and holds the maximum hold between 150K and 30MHz to obtain the peak reading value of conduction. It is used to confirm the maximum peak value of the power supply and then capture the actual QP and AV values of the highest peak to reduce the scanning time. The blue curve in Figure 2 shows the peak measurement results of the quasi-peak value. Generally, after the peak measurement, the peak value (QP) and the average value (AV) of the six higher frequency points will be measured accurately, as indicated in Figure 2.

The difference between peak value and quasi-peak value is that the peak value measurement means that the signals that appear frequently or occasionally are placed in the receiver's reading value in the way of maximum value. The quasi-peak value measurement means that the pulse signals of this frequency band are taken several times in a time. If the signal of a certain frequency has a high recurrence rate in a period of time, the higher measurement value will be obtained. The average is the average of the amplitude of this band. A typical spectrum analyzer can set the bandwidth around 30Hz to get the most realistic average signal.

The detection values of QP and AV are necessarily lower than the peaks. If the initial peak measurement has sufficient redundancy, it is unnecessary to do single point QP and AV measurement.

Now IC in order to prevent EMI conduction, in the operation frequency will do the function of shaking frequency, such as IC main frequency of 65KHz, but in the operation will be 65KHz plus or minus 6K changes, so as to disperse the differential mode frequency doubled signal, will not concentrate on a single root frequency, if there is no shaking function, Differential mode interference at the frequency doubling of the main frequency will present a single very solid QP and AV, as 157KHz, the instrument sees a high peak, but still read more than 9dB redundancy.

3. Testing and measuring methods of conduction

As a reference figure for testing conduction, this is the figure of conduction equipment inside Tongjia. The object to be measured is placed on the table after receiving simulated load. LISN(linear impedance stable network) is connected to the object to be measured through an input cable (AC cable), and then the LISN signal is connected to the receiver. The load of the object to be measured shall be 10 cm away from the object to be measured. If the surrounding area needs to be connected to a power supply, its power supply shall be connected to an independent power supply and shall not use the same power supply as the object to be measured. If the power supply is a 2PIN input, the output load shall be grounded to simulate the ground of the system.

After the input power source enters the LISN from the left, the low-frequency noise of the power supply is filtered through LF and CF, and the coupling capacitor CC and detection resistor RSL/RSN are used to obtain the high-frequency signal Vsn, and then the signal is passed through the receiver or spectrum analyzer to get its amplitude (dBuV).

Please remember that the input wire should not contact with the ground, the author has input wire and ground contact or not, the reading value difference of about 10dB experience; In addition, the power supply of peripheral instruments should be clean and isolated from the main power supply, otherwise it is easy to cause common mode interference due to common ground. Many test sites will directly take an extension cord to use external power supply to the peripheral, but this way may still be interfered due to common ground. If it can connect to another LISN, it is a better way. LF and CF in LISN can be used for signal isolation.

4. Basic concepts of EMI conduction

4.1 Differential mode signals and Common mode signals

The conduction measurement connection method is made by the receiver measuring the frequency and amplitude between L/N/GND. The signal that exists between L and N is called differential mode signal, while the signal that exists between L and FG or N and FG is called common mode signal. It can also be said that the signal that forms a loop with FG is called common mode signal.

The input sources of the general power supply are all from L and N, so in the design of the electromagnetic interference of the power supply, the suppression of the differential mode components is very important, especially the first frequency band 150K~1MHz is mostly the differential mode interference from the main frequency and double frequency of the switching power supply.

Is the conduction test result before the game, the front end is formed by the frequency doubling difference mode interference caused by the operating frequency of IC plus the common mode interference of the body, it can be seen from the graph that the frequency between each peak is 100KHz, the operating frequency of the IC can be judged to be 100KHz, and the measured reading value is presented by the linear attenuation of the frequency doubling of IC 100KHz. Therefore, there is a differential mode interference signal caused by IC operating frequency every 100KHz. It can also be said that in the former frequency band, the common-mode signal presents a linear plane decline, while the differential mode signal is superimposed on the top of the common mode.

The same machine after countermeasure test results, after the worst have more than 6dB redundancy, has been able to meet most customer requirements.

In general, L and N must be tested in the test. Generally, the reading values of L and N will not differ much. If the difference is large, it is generally caused by the strong common mode energy of a certain item.

The input voltage of the test is to see the application of safety rules to decide, generally with 110V and 230V to do high and low voltage measurement; In addition, the products need to be burned for a long time when confirming the conduction test. Sometimes the magnetic components will overheat after burning, resulting in abnormal inductance and EMI deterioration.

4.2 Electromagnetic interference, electric field interference and magnetic field interference

Electrical Magnetic Interference (EMI) can be divided into electric field interference and magnetic field interference. Electric field and magnetic field are two different properties, but the energy between them will affect each other. Electric field changing with time will produce magnetic field, while magnetic field changing with time will also produce electric field. These electric and magnetic fields constantly oscillating in phase together form electromagnetic interference (electromagnetic wave).

In general, electric fields can be explained by the charge formula and the capacitance formula,

Simply put, any conductor under the electric field can be equivalent to a charged capacitor, the capacity of which varies with the distance/surface area/medium between the conductor and the surrounding; According to the capacitance formula, the capacitance decreases with the distance between the two conductors and increases with the cross-sectional area between the two conductors. The medium (dielectric coefficient) between the two conductors also affects the capacitance.

When there is a voltage difference in the potential of the two ends of the capacitor within the time, a current will be generated according to the charge formula (voltage/time change), and any generated current must return to its starting position through another path to form a current loop, the current loop caused by the voltage change will cause electric field interference.

Therefore, the way to improve the electric field interference is to reduce the loop current. According to the above two formulas, we can reduce the capacitance effect by reducing the contact area between two conductors/increasing the distance/changing the medium in between, or reduce the voltage difference or time change rate to reduce the electric field induction.