Fully describe relationship between power PCB layout and nine-way EMC.

The so-called RC circuit is a voltage divider circuit consisting of a resistor R and a capacitor C. As shown in Figure 1 below, input voltage is applied to both ends of series RC circuit, and output voltage is taken from resistor R or capacitor C Due to special properties of capacitors, different output voltages in (a) and (b) below show different frequency responses.

Speaking of difficulty of switching power supply, problem of PCB layout is not a big difficulty, but if you want to place a good PCB, it should be one of difficulties of switching power supply (if PCB design is not good, it may cause that no matter how to debug It is not alarmist to say that all parameters are debugged and laid out The reason is that there are still many factors to be considered in PCB layout, such as: electrical characteristics, workflow Route, requirements safety, EMC impact, etc. Among factors considered, electrical are most basic, but EMC is most difficult to understand.The bottleneck of many projects is EMC problem, below we will share with you PCB layout and EMC from nine directions.

01

Those who are familiar with circuit can easily design PCB EMI circuit

The EMC of some products is difficult to solve at source, and magnetic ring filtering can be used. Of course, magnetic ring I mentioned here has two meanings. On one hand, filter inductors at input and output terminals use different materials. A different number of turns will have corresponding effects. On other hand, this means that magnetic ring is directly placed on input and output lines, which can sometimes play a magical role, but not in all cases, at least it can be used as a basis for judgment;

The blue and black lines in image above represent positive and negative terminals of output, and a magnetic ring is installed on them, which eliminates high frequency end caused by output rectifier; sometimes port interference may be ineffective if a filter is added to PCB. Installing a magnetic ring on output line has an unexpected effect.

02

PCB Routing Key Signal

Note.

1. CS signal (sampling signal): is drawn from sampling resistors R25 and R26. Note that ground wire of IC is based on sampling resistor, and positive and negative differential wire of sampling resistor pulls down chip's CS pin and chip's GND pin.

2. The excitation signal pulls IC control pin out of excitation circuit, and be careful not to interfere with CS pin; as shown in figure, three wires are laid side by side, and ground wire is placed between driver and CS line to play a certain shielding role;

3. It is best to shield IC on first layer of double-sided board. The ground network must be drawn from IC GND. The non-critical GND signal can be drilled directly. Need key signal ground to be grounded at one point and directly connected to IC;

4.Network FB feedback signal, pay attention to check routing and single point connection to IC;

5. The RCD absorption network should not be placed in main circuit;

6. The VCC rectification and filtering ground needs to be connected to main power ground, and secondary filter can be connected to IC ground;

7. Capacitor Y wiring is connected separately and should not be confused with main power to avoid interference.

03

Scheme for connecting power section and control unit

Perhaps many people see this picture, cloudy and foggy, here is a general introduction:

1. The KKM driver and IC are connected to KKM tube, or rather to ground of selective resistor;

2. The drive ground and control ground of DC-DC part are connected to sampling ground of DC switch tube part;

3. The control ground of auxiliary supply part is connected to auxiliary source MOS sampling tube, and MOS tube ground is connected to main power supply ground;

4. The power supply ground of each IC is connected to IC ground through auxiliary power supply EC filter. Note that RC filter is located close to IC.

Conclusion: pay attention to single point grounding of everyone, and grounding line is not tangled, which is one of most important places for wiring! ! !

04

Analysis of electromagnetic field shielding mechanism

Compared to picture: Input and output electric field interference can be coupled through capacitive transmission. If a shield plate is added, size of C4 will be increased and C1 will also be reduced to reduce electric field interference.

As shown in figure: The magnetic field shielding characteristics are different from magnetic field characteristics, which require shell shielding, and electric field only requires a flat shielding plate, so heat sink shielding provides electric field shielding, and some enclosed power supplies with cases play a certain role in shielding magnetic field;

Magnetic field shielding principle: The magnetic field passing through protective cover changes magnetic circuit, causing magnetic field lines to propagate, and intermediate magnetic field interference achieves purpose of shielding.

05

Device switching and electromagnetic compatibility

What parameters of switching devices are important for EMC, we often talk about fast lamps, what is benchmark for slow lamps? We all know that turn-on loss of a fast lamp is small, and we like to use it for high efficiency, but in order to pass EMC smoothly, we must give up efficiency and reduce switching speed to reduce switching. radiation;

For MOS lamps, turn-on speed is determined by drive resistance and capacitance of input junction; turn-off speed is determined by capacitance of output junction and internal resistance of lamp;

See two pictures above are different types of MOS lamps, compare input junction capacitance and output capacitance, 2400PF and 6800PF; 780PF and 2200PF; at a glance, you can see that first specification is a fast lamp. , and second is slow. At this time, switching speed must be matched with drive resistance, generally, drive resistance is more than 10-150 ohms, and choice of drive resistance is related to junction capacitance. fast board can be increased accordingly, and resistance to movement of slow camera can be correspondingly reduced;

For diodes, there are Schottky diodes, fast recovery diodes, conventional diodes, and a relatively small SIC diode. The switching rate of SIC diodes is almost zero, which means there is no reverse recovery, and switching radiation is minimal, and losses are also smallest, and only minus - a little expensive, so it is rarely used; second is Schottky diode, which has low forward voltage and low reverse recovery time, followed by fast recovery and ordinary diodes, this must be exchanged between losses and electromagnetic compatibility Medium, generally measures such as changing absorption and installing magnetic beads, can be taken to eliminate electromagnetic compatibility.

06

EMC filter

The choice of filter architecture is very important for a filter. In different cases, filter is based on impedance matching to achieve filtering effect. You can refer to how to filter according to principle of this figure, such as most commonly used output rectifier bridge Then use a π-type filter and an LC filter on output;

The filter material is also very important to design of filter inductor. Materials with different initial permeability will work in different frequency ranges. If wrong material is selected, desired effect will be completely lost.

07

Analysis of high-frequency EMC flyback equivalent model

First, understand EMC on a simple model:

The EMC path, of course, cosmic radiation is connected to loop, and loop is also formed by path; analyze high frequency flyback model to help understand mechanism of EMC formation; our test receiving equipment will start from L, N terminal of receiving conduction, in order to reduce receiving interference, it is necessary that interference pass through ground loop, and not from L and N ports to receiving device; at this time, our EMI inductor and capacitor Y can be realized by impedance matching; in addition, noise on primary side can be coupled to secondary side through original secondary side capacitance Y, parasitic capacitance of transformer, and ground to form more loops; of course, some junction capacitance parameters, such as junction capacitance of MOS tube and junction capacitance of heatsink, can also shape flow path.

08

Shape and frequency distribution of radiation

This picture might be a bit abstract, but it's hard to be specific about EMC. We need to give us some clarification. It can be seen that differential radiation exists as a loop, while common-mode radiation is emitted as an antenna; therefore, this just confirms previous statement that when we lay out circuit breaker stub diagram and do not take sharp corners when wiring, usual wiring is 45 degrees, and it is best to use circular arc routing. Of course, routing efficiency will be relatively low.

The basic knowledge of these principles is well understood, which is very useful for actually doing job and EMC layout. If there is no such understanding, it may be useless as it cannot provide a direct method and must be combined with other knowledge.

Moreover, many of principles and things mentioned here are not seen in many EMC materials, and they are not so concentrated that they need to be experienced repeatedly!

As shown in figure: relationship between some frequency terminals and generating parts of switching power supply, this is just a general rule, don't believe it completely, why can't you believe even if it is rule? The law is not fulfilled in all cases, and difference between different power supplies is also great, so principle is to help you analyze, and not strictly follow methodology.

09

EMC Instance

According to conductance example, key to frequency distribution point is relationship between specific data and fundamental frequency. of course, how these frequencies pass through filter Ways to solve this were mentioned earlier.

I'll add seemingly mysterious EMC to everyone. How did it come about?