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From 9 directions about the relationship between power supply PCB layout and EMC

Time:2021-06-22 15:25

When it comes to the difficulty of switching power supply, PCB layout is not a big difficulty, but if you want to lay out a good PCB board must be one of the difficulties of switching power supply (PCB design is not good, may lead to no matter how to debug parameters are debugging cloth out of the situation, so to say is not alarming) is the reason is that PCB layout is still a lot of factors to consider, such as: electrical performance, process route, safety requirements, EMC impact, etc.; among the factors to consider electrical is the most basic, but EMC is the most difficult to understand, many projects progress bottleneck is the EMC problem; the following is to share the following nine directions of PCB layout and EMC problems. EMC impact, and so on; among the factors considered electrical is the most basic, but EMC is the most difficult to understand, the progress of many projects bottleneck lies in the EMC problem; the following nine directions to share the PCB layout and EMC.
01
The EMI circuit of the PCB design can be done with ease only if you are familiar with the circuit
Some products EMC is difficult to deal with at the source, you can use magnetic ring filtering, of course, the magnetic ring I am talking about here has two levels of meaning, on the one hand, the input and output filter inductors, the use of different materials magnetic ring, different turns will have the corresponding effect, and on the other hand, the meaning is directly in the input and output line set of magnetic ring, sometimes can play a wonderful use, but not in all occasions can be used, at least still can be used as a basis for judgment.
 
The blue and black lines above are the positive and negative outputs, the top set of a magnetic ring to solve the output rectifier caused by the high-frequency end beyond; some times the port interference on the PCB board with a filter may not have an effect, put the magnetic ring on the output line will have an unexpected effect.
 
02
PCB alignment of the key signal   
Note that
1. CS signal (sampling signal): from the sampling resistor R25, R26 pull out, note that the IC ground to the sampling resistor as a reference, sampling resistor positive and negative differential alignment pull down the IC CS pin and the IC GND pin.
2. drive signal from the driver circuit to pull down the IC drive pins, pay attention not to interfere with the CS foot; as shown in the alignment of the three lines side by side, and the ground line to go in the middle of the drive first and CS line to play a certain shielding role;.
3. double panel best IC layer of pavement shielding, pavement network must be led from the IC GND, non-key signal GND can be directly punched through the hole, the key signal ground needs a single point of ground, directly connected to the IC;.
4. FB feedback network signal attention to check the alignment and a single point to IC.
5. RCD absorption network should not be placed in the main circuit.
6. VCC rectifier filter ground needs to be connected to the main power ground, secondary filtering can be connected to the IC ground.
7. Y capacitor alignment is connected separately, not to be confused with the main power to avoid interference.
 
03
Main power and control part of the ground wiring diagram
Many people may see this diagram, foggy, the general introduction of.
1. PFC driver and IC common ground connected to the PFC tube, more specific point is connected to the sampling resistor ground.
2. DC-DC part of the drive and control ground connected to the DC switching tube part of the sampling ground.
3. auxiliary source part of the control ground connected to the auxiliary source MOS tube sampling first, MOS tube ground and then connected to the main power ground.
4. The respective IC power supply ground through the auxiliary source EC filter connected to the IC ground, pay attention to the RC filter near the IC.
 
Summary: pay attention to the respective single point of grounding, the ground is not chaotic, is one of the most important places to go!
 
04
Electromagnetic field shielding mechanism analysis  
As the figure control: input and output electric field interference can be coupled through capacitive transmission, if you increase the shielding plate, it increases the size of C4, and C1 will also be reduced, the electric field interference to play the purpose of attenuation.     
As shown: the characteristics of the magnetic field shielding and magnetic field is not the same, the need for shell shielding, electric field only requires a flat shielding plate, so the heat sink shielding brings the electric field shielding, some use the shell closed power supply then play a certain magnetic field shielding.
 
Magnetic field shielding principle, the magnetic field through the shield will change the magnetic circuit, resulting in magnetic lines of force to the surrounding diffusion, the middle magnetic field interference to achieve shielding purposes.
 
05
Switching devices and EMC
 
Which parameters of switching devices have important influence on EMC, we often say fast tube, what is the slow tube as a reference? We all know that fast tubes have small turn-on loss, and they like to be used in order to do high efficiency, but in order to pass EMC smoothly, they have to give up the efficiency and reduce the switching speed to weaken the switching radiation.

for MOS tubes, the turn-on speed is determined by the drive resistance and input junction capacitance; turn-off speed is determined by the output junction capacitance and tube internal resistance.
 
Referring to the above two pictures, are different models of MOS tubes, compare the input junction capacitance and output junction capacitance, 2400PF and 6800PF; 780PF and 2200PF; at a glance, we know that the first specification is a fast tube, the second is a slow tube, this time to decide the switching speed but also to match the driving resistance; conventional situation driving resistance in 10R-150R more, the selection of driving resistance and junction capacitance related, for fast plate drive resistance can be appropriately increased, slow tube drive resistance can be appropriately reduced.
 
For the diode, there are Schottky diode, fast reply diode, ordinary diode, there is a less used SIC diode, switching speed SIC diode is almost zero, so there is no reverse recovery, switching radiation is the smallest, and also the smallest loss, the only disadvantage is that the price is expensive, so rarely used; followed by Schottky diode, the forward pressure is reduced, the reverse recovery time is short, in order is fast reply and ordinary diode; need to compromise between loss and EMC; generally can be taken to change the absorption and set of magnetic beads and other measures to rectify EMC.
 
06
EMC filter 
Filter architecture selection is very important to the impact of the filter, in different occasions, the filter is based on impedance matching to achieve the filtering effect, you can refer to the principles of this figure to select how to filter; for example, the most commonly used output rectifier bridge after the use of π-type filtering and the output side of the LC filter.
                     
 
 
The material of the filter is also critical to the design of the filter inductor, using different initial permeability of the material will work in different frequency bands, the wrong choice of material will completely lose the desired effect.
 
07
EMC of flyback high frequency equivalent model analysis
The EMC is first understood from the simplest model.
 
EMC path, of course, space radiation is related to the loop, the loop is also the path construction into; analysis of the flyback high-frequency equivalent model to help understand the mechanism of EMC formation; our test receiving equipment will receive from the L, N end of the conduction, in order to reduce the interference received, it is necessary to let the interference through the ground loop circulation and not from the L, N port flow to the receiving device; this time our EMI inductance and Y capacitance Through impedance matching can be achieved; In addition, the original side of the interference can be through the original vice side Y capacitance, transformer stray capacitance and earth coupling to the vice side, forming more loops; Of course, some junction capacitance parameters, such as MOS tube junction capacitance, heat sink junction capacitance can also constitute a circulation path.
 
08
The form of radiation and frequency distribution
This figure may be somewhat abstract, but it happens that EMC is very difficult to do specific, need to give us some insight, can be known: differential mode radiation is in the form of loops, and common mode radiation is in the form of antennas emitted; therefore, it just confirms what we said earlier when the layout of the switch loops and alignment of the board do not take the acute angle, the conventional go 45 degrees, preferably circular arc alignment, of course, the alignment will be less efficient.
 
These principles of good understanding of the basics, the actual handling of EMC work and the cloth board is very useful that, if not this awareness, may be useless, because it does not provide a direct method, need to be combined with other knowledge.
 
And many of the principle things mentioned here, in many EMC information is not seen, and not so focused, need to repeatedly experience!
This is just a general rule, don't believe in it completely; it is a rule and you can't believe in it. The law does not hold in all cases, different power supplies also vary greatly, so the principle is to help you analyze, rather than follow the method to rigid.
 
09
EMC example

According to the conduction example, the key to the distribution point of the frequency is the relationship between the specific data and the fundamental frequency, after this test, you need to speculate on the law of these values, may be able to find any clues; of course, for these frequencies how to solve the means through the filter has been said before.

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