“Conducted interference mainly evaluates the interference noise flowing on the input and output lines. The EUT of the equipment to be tested is connected to a clean AC power source through an impedance matching network LISN.
A Conducted Disturbance Concept
Conducted interference mainly evaluates the interference noise flowing on the input and output lines. The EUT of the equipment to be tested is connected to a clean AC power source through an impedance matching network LISN.
(1) The role of LISN is as follows
1. Isolate the EUT of the device under test and the AC input power supply, and filter out the noise and interference introduced by the input power line.
2. The interference noise generated by the EUT passes through the high-pass filter and 50 Ω resistor inside the LISN in turn, and the corresponding signal value is obtained on the 50 Ω resistor and sent to the receiver for analysis.
(2) Analysis of test principle
Conducted interference comes from differential mode current noise and common mode current noise. These two types of noise interference are shown in the following figure:
FIGURE 2: DIFFERENTIAL-MODE CURRENT AND COMMON-MODE CURRENT
1. The differential mode current flows in opposite directions between the two input power lines, and the two form a current loop with each other, that is, one is the source line of the differential mode current, and the other is the return line of the differential mode current.
2. The common mode current flows in the same direction on the two input power lines, and they respectively form a current loop with the ground, that is, they simultaneously serve as the source line or return line of the common mode current.
2. Causes of noise and solutions
(1) Common mode current
1. Causes of common mode current
The common mode current flows between the input and output lines and the ground, and its generation is mainly the transient change of the voltage generated by the high frequency operation of the power device. The generation of common mode current mainly includes the following parts:
① The capacitance Cde from the MOSFET source to the ground. If the design of the IC is improved, such as for a single-chip power chip, the MOSFET source is connected to the chip body for heat dissipation instead of the drain for heat dissipation, which can reduce the parasitic capacitance of the drain to the ground. Reducing the copper area of the drain region during PCB wiring can reduce the parasitic capacitance of the drain to the ground, but care should be taken to ensure that the temperature of the chip meets the design requirements.
②Generate common mode current through Cm and Cme.
③ A common mode current is generated through Ca and Cme.
④ Generate common mode current through Ct and Coe.
⑤ The common mode current is generated through Cs and Coe, and this part is dominant in the common mode current. Decreasing the amplitude and rate of change of the drain voltage can reduce the common-mode current, such as reducing the reflected voltage and increasing the drain-source capacitance, but this will cause the MOSFET to bear a large current stress, its temperature will increase, and at the same time increase The drain-to-source capacitance produces a larger magnetic field emission.
FIGURE 3: COMMON MODE CURRENT GENERATION
(1) Increase the Y capacitor
FIGURE 4: Y CAPACITOR EFFECT
Voltage If a Y capacitor is added to the system, as shown in Figure 4, most of the common mode current through Cs is bypassed by the Y capacitor and returns to the primary ground, because the value of the Y capacitor is greater than Coe.
The Y capacitor must be connected directly and with the shortest possible straight line to the primary and secondary cold spots. As a rule, if the dV/dt of the turn-on leaf MOSFET is greater than the turn-off value, connect the Y capacitor to ground on the primary. Connect to Vin instead.
Emphasis: The point where the voltage does not change is called the static point or cold point, and the point where the voltage changes is called the moving point or hot spot. The primary ground and Vin are both cold spots. For the auxiliary and output windings, the cold spot can be adjusted by the position of the diodes. In Fig. 18, A, B and Vin are cold spots, and F, D, B and C are hot spots; while in Fig. 5, A, Vcc, Vin and Vo are cold spots, and D, F and G are hot spots.
Figure 5: Cold Spot Location
(2) Change the structure of the transformer
Removing the Y capacitor cannot effectively bypass the common mode current, which leads to too much noise of the common mode current and cannot pass the test standard. The design method is to improve the structure of the transformer. Common Fagari shielding methods do not allow devices to pass EMI testing without Y capacitors. Due to the large voltage change amplitude at the drain terminal of the MOSFET, it is mainly designed for this part. Always note that voltage variations are the main cause of differential and common mode currents, and parasitic capacitance is the path through which they flow.
As mentioned earlier, Cm and Cme and Cme and Ca also generate common-mode currents. Part of the current in the primary interlayer capacitance forms differential-mode currents, and part of them also forms common-mode currents. This also shows that differential-mode and common-mode currents can interact with each other. convert.
If the cold spots (blue dots) and windings are arranged according to the structure in Figure 7, when there is no Y capacitor, the current flow direction of the primary winding and the secondary winding and the capacitance between the auxiliary winding and the secondary winding can be obtained based on the direction of the voltage change. , both the primary and auxiliary winding currents flow into the secondary winding.
After adjusting the cold spot, as shown in Figure 8, it can be seen that the currents of the primary winding and the secondary winding and the capacitance between the auxiliary winding and the secondary winding flow in the same direction, which can cancel out part of the common mode current flowing into the secondary winding. Thereby reducing the size of the overall common mode current.
The rectifier diodes of the auxiliary winding and the secondary winding are placed at the lower end to change the direction of the voltage change. At the same time, pay attention to the cold spot as close as possible, so that there is no voltage change between the two, so no common mode current will be generated.
Further, if a copper sheet is placed between the inner layer and the primary winding and the secondary winding, the width of the copper sheet is less than or equal to the width of the primary winding, and the midpoint of the copper sheet is led from the wire to the cold spot, as shown in Figure 9. The skin is a cold spot, and the slew rate of the voltage between the winding and the copper skin in contact with it is reduced, thereby reducing the common mode current, and at the same time, the common mode current is introduced to the cold spot from the copper skin bypass. Note that the lap joints of the copper skins cannot be short-circuited, separated by insulating tape, and the directions of the inner and outer copper skins should be the same.
The common mode currents of the auxiliary and secondary windings can be compensated by:
① Add auxiliary shield winding
The winding direction of the auxiliary shield winding is consistent with the winding direction of the secondary winding. The auxiliary shield winding is connected to the same name end of the secondary winding and connected to the cold spot, and the other end of the auxiliary shield winding is floating. Since their voltage changes in the same direction, no current flows between the two.
②Add the auxiliary shielding copper of the outer layer
The midpoint of the auxiliary shield copper is connected to the midpoint of the auxiliary winding. Similarly, analyzing the flow direction of the current based on the change direction of the voltage, it can be seen that the current between the two forms a circulating current, which compensates and cancels each other, thereby reducing the common-mode current.
For the conducted emission of the power adapter, this article analyzes the causes and solutions, hoping to help everyone in the design and rectification, the design is considered in advance, and the solution can be multiplied with half the effort!
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