The small class mentioned the content of PCB in the previous sharing, so what is the connection between switching power supply PCB wiring and ripple? Let’s share the relationship between the PCB wiring and ripple of the switching power supply, and the reasons for the ripple of the switching power supply.

Damaged power plug

1. The relationship between PCB wiring and ripple

Arrangement: The pulse voltage connection line should be as short as possible, in which the input switch tube is connected to the transformer, and the output transformer is connected to the rectifier tube. The pulse current loop should be as small as possible, such as the positive pole of the input filter capacitor to the negative pole of the return capacitor of the transformer switch tube. From the output of the transformer to the rectifier, to the output inductance and then to the output capacitor and back to the transformer circuit, the X capacitor should be as close as possible to the input of the switching power supply, and the input line should not be parallel to other circuits. The Y capacitor should be placed on the shell ground terminal or FG connection terminal. In order to avoid magnetic coupling, a certain distance should be kept between the common inductance and the transformer. If the handling is inconvenient, you can add a shielding layer between the male inductor and the transformer of the switching power supply. The above will have a great impact on the EMC performance of the switching power supply.

Generally, two output capacitors can be used, one near the rectifier tube and the other near the output terminal, which will affect the output ripple index of the power supply. The parallel effect of two small-capacity capacitors is better than a large-capacity capacitor. The heating device should keep a certain distance from the electrolytic capacitor to extend the service life of the whole machine. Electrolytic capacitor is the bottleneck of the life of switching power supply. For example, transformers, power tubes and high-power resistors should be kept away from electrolysis. There must be a space for heat dissipation between the electrolysis, and it can be placed in the air inlet if possible.

Wired switching power supply

Generally speaking, these three methods are to improve the current carrying capacity of the PCB. I think the first thing to do is to shorten the length of the wiring, which is the most effective way to reduce the parasitic inductance on the board.

If the length is determined, tin lining is the most ideal: tin lining is easy for the designer to achieve, as long as the window is opened on the solder mask, there is no need to contact the PCB manufacturer in any special way, and no additional cost . The default anti-oxidation process of PCB is hot air leveling, that is, tin spraying and hot air blowing. The disadvantage of tin lining is that it is easy to short-circuit when exposed.

Increasing the thickness of the copper foil requires the use of a special metal plate, usually a 1oz plate. If thickening is needed, copper plating twice is generally used, which has low strength and is easy to fall off. Need to explain to the PCB manufacturer, which may increase the cost.

Use switch

Large area copper plating is feasible for networks with less frequent voltage changes, such as ground wires or regulated output terminals. Large area copper plating is not suitable for large voltage fluctuations during switching operations. The changed electric field will generate a magnetic field. The magnetic field generates an electric field. Yes, it is electromagnetic radiation. That is spatial interference, and it also affects performance.

Switching power supply ripple generation

Our ultimate goal is to reduce the output ripple to a tolerable level. The most fundamental solution to this goal is to avoid the generation of ripple as much as possible. First of all, we must know the type and reason of the switching power supply ripple.

When the switch is switched, the current in the inductor L fluctuates with the root mean square value of the output current. Therefore, the same ripple as the switching frequency is generated at the output. Generally speaking, ripple refers to this. It is related to output capacitance and ESR. The frequency of the ripple is the same as that of the switching power supply, ranging from tens to hundreds of kilohertz.

Socket in black and white

In addition, the switch generally uses bipolar transistors or MOSFETs. No matter which one, the switch will have a rise time and a fall time when it is opened and closed. At this time, there will be noise or odd multiples of the same frequency as the switch rise and fall times, generally tens of MHz. At the same time, the equivalent circuit of the diode D is a series connection of resistance, capacitance, and inductance, which will cause resonance and produce a noise frequency of tens of MHz. These two kinds of noise are generally called high-frequency noise, and their amplitude is usually much larger than the ripple.

If it is an AC/DC converter, in addition to the above two types of ripple (noise), there is also AC noise. The frequency is the frequency of the input AC power, about 50-60Hz. There is also a kind of common mode noise, which is caused by the equivalent capacitance of many power devices using the shell as a heat sink.

The above is the sharing of Minrong’s small classroom. It was mentioned in the small classroom at the beginning, and there was also sharing about PCB before. And this sharing is interpreted from another angle, they are different and connected. The same is that it can give everyone a certain harvest. Friends who have gained something, don’t forget to follow, like and forward. If you have any questions, you can leave a message or privately write to the small class.