Speaking of switching power supply difficulties, PCB layout is not a big problem, 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 alarmist) is the reason is that PCB layout board when considering many factors, such as: electrical performance, process routes, safety requirements, EMC impact, etc. The following is a list of twenty-two directions to share the PCB layout and EMC.
Share point: familiar with the circuit to be comfortable with PCB design EMI circuit
Of course, the magnetic ring I am talking about here has two levels of meaning, on the one hand, it is the filtering inductor at the input and output, using different materials of magnetic ring, different turns will have corresponding effects, and on the other hand, it means that the magnetic ring is directly set on the input and output line, sometimes it can play a wonderful use, but not in all occasions, at least it can still be used as a The blue and black lines above are the output lines;
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 has an unexpected effect. Share point fifteen: PCB alignment of the key signal
Attention.
- CS signal (sampling signal): from the sampling resistor R25, R26 pull out, pay attention to the IC ground to the sampling resistor as a reference, sampling resistor positive and negative differential alignment pull down the IC CS pin and IC GND pin.
- Drive signal from the driver circuit to pull down 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 role in shielding;
- 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;
- FB feedback network signal attention to check the alignment and a single point of connection to the IC;
- RCD absorption network should not be placed in the main circuit;
- VCC rectifier filter ground needs to be connected to the main power ground, secondary filtering can be connected to the IC ground;
- Y capacitor alignment is connected separately, not to be confused with the main power to avoid interference;
Share point: the main power and control part of the ground wiring schematic
Many people may see this figure, the clouds, a general introduction to the following.
- PFC driver and IC common ground connected to the PFC tube, more specific point is connected to the sampling resistor ground;
- DC-DC part of the drive and control ground connected to the DC switching tube part of the sampling ground;
- 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;
- 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 messed up, is one of the most important places in the alignment!!!! Share point 17: 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 reduce the electric field interference to play the purpose of attenuation; Figure 2: magnetic field shielding principle
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 is played 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 interference to achieve shielding purposes;
Sharing point 18: Switching devices and EMC
Which parameters of switching devices have an important impact on EMC, we often say that fast tubes, slow tubes are used as a reference to what? We all know that fast tube opening loss is small, in order to do high efficiency are like to use, but in order to EMC passed smoothly, have to give up efficiency, reduce the switching speed to weaken the switching radiation;
For MOS tube, 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 diagrams, are different models of MOS tubes, compare the input junction capacitance and output junction capacitance, 2400PF and 6800PF; 780PF and 2200PF; at a glance, you know that the first specification is a fast tube, the second is a slow tube, this time to decide the switching speed and drive resistance to match; conventional driving resistance in 10R-150R more, the selection of driving resistance and Junction capacitance related, for fast plate drive resistance can be increased, slow tube drive resistance can be 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 the loss is also the smallest, the only drawback is that the price is expensive, so rarely used; the next is the Schottky diode, the forward voltage is reduced, the reverse recovery time is short, in turn is fast Response and ordinary diodes; 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;
Share point 19: EMC filter
The choice of filter architecture 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 choose 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 crucial to the design of the filter inductor. The use of different initial permeability materials will work in different frequency bands, and choosing the wrong material will completely lose its effect;
Sharing point 20: EMC equivalent model analysis of flyback high frequency
First, the EMC is understood from the simplest model.
The path of EMC, of course, space radiation is related to the loop, the loop is also the path construction into; analyze the flyback high frequency equivalent model to help understand the mechanism of EMC formation; our test receiving equipment will receive the conduction from the L,N end, in order to reduce the received interference, it is necessary to let the interference through the ground loop circulation and not from the L,N port flow to the receiving equipment; 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 the circulation path;
Share point 21: the form of radiation and frequency distribution
This figure may be a bit abstract, but just 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 antenna emission; Therefore, it just confirms that the layout of the switch loop when we lay out the board and the alignment of the line do not take the acute angle, the conventional go 45 degrees, the best is a 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, the need to combine with other knowledge;
And many of the principles mentioned here things, in many EMC information is not seen, and not so focused, need to repeat experience!
As shown: the relationship between some frequency end and the switching power supply to produce parts, this is just a general rule, do not fully believe; both the law and can not be fully believe in why? 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;
Share point twenty-two.
EMC example
