PCB Tech - PCB power supply design some experience

1. The power cord is an important approach of EMI in and out of the printed circuit board, through the power cord, interference can be introduced to the internal circuit of the outside world, influence the index of RF circuit, in order to reduce the electromagnetic radiation and coupling, require DC - DC module's side, secondary side and load side loop area, no matter how complex form of power supply circuit, the large current loop should be as small as possible, Power and ground cables should always be placed close together.

2. If a switching power supply is used in the circuit, the layout of the peripheral devices of the switching power supply should conform to the principle of short backflow path of each power. The filter capacitor should be close to the relevant pins of the switching power supply, and the common mode inductor should be close to the switching power supply module.

3. The veneer on the long-distance power cord can not close to or at the same time through the cascade amplifier (gain greater than 45 db) near the output and input, avoid the power cord to become the RF signal transmission way, may cause self-excited or lower sector isolation, long distance at both ends of the power cord need with high frequency filter capacitance, even in the middle and high frequency filter capacitance.

4. The power inlet of RF PCB is combined with three filter capacitors in parallel, and the advantages of these three capacitors are used to filter the low, medium and high frequencies on the power line respectively, such as 10uF, 0.1uF and 100PF, and they are close to the input pins of the power supply in descending order.

5. If the same group of power supply is used to feed the small signal cascade amplifier, it should start from the last stage and supply power to the front stage successively, so that the EMI generated by the last stage circuit has a small impact on the front stage, and each power supply filter has at least two capacitors: 0.1uF and 100pF. When the signal frequency is higher than 1GHz, 10pF filter capacitor should be added.

6. Commonly used to small power electronic filter, filter capacitor close to triode tube feet, more close to the pin high frequency filter capacitor, triode choose lower cut-off frequency, if both high frequency tube, the triode electronic filter is working in the district, peripheral device layout is unreasonable, the power output is easy to produce high frequency oscillation.

The linear voltage regulator module may also have the same problem, because there is a feedback loop in the chip, and the internal triode works in the amplification area, and the high-frequency filter capacitor is required to be close to the pin during the layout, so as to reduce the distributed inductance and destroy the oscillation condition.

7. The copper foil size of the POWER part of the PCB is in accordance with the current it flows through, and allowance is taken into account (generally 1A/mm line width for reference).

8. Input and output of power cables cannot be crossed.

9. Pay attention to power decoupling and filtering to prevent interference from different units through power lines. Power lines should be isolated from each other and from other strong interference lines (such as CLK).

10. The wiring of the power supply of the small signal amplifier needs to be isolated by ground copper skin and ground hole to avoid the intrusion of other EMI interference and the deterioration of the signal quality.

11. Different power layers should avoid overlapping in space, mainly in order to reduce the interference between different power sources, especially between some power sources with very different voltages. The overlapping problem of power supply planes must be avoided.

12.PCB layer allocation facilitates the subsequent wiring processing. For a four-layer PCB (commonly used in WLAN), in most applications, components and RF leads are placed on the top layer of the PCB, the second layer is systematically placed on the third layer, and any signal lines can be distributed on the fourth layer.

The second layer adopt continuous ground plane layout for it is necessary to establish impedance controlled RF signal path, it is also easy to get to the loop as short as possible, provide highly electrical isolation layer and layer 3, makes the coupling between the two layers, of course, can also use other board layer defined way, especially in the circuit board with different layers, But the structure above is a proven success story.

13. The power of large area Vcc wiring layer can be made easily, however, this structure is often a prelude to system performance deterioration, in a larger plane take all power fuses together will not be able to avoid the noise transmission between pins, on the other hand, if you use a star topology can reduce the coupling between the different power supply pin.

Good power supply decoupling technique and precise PCB layout, the Vcc (star topology), lead to lay a solid foundation for any of the RF system design, although the actual will reduce system performance index in the design of other factors, however, has a "no noise" of the power supply is essential to optimize system performance.