Key points of power supply design for the hottest

2022-08-23
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Key points of power supply design of RF circuit

(1) power line is an important way of EMI in and out circuit. Through the power line, external interference can be transmitted to the internal circuit, affecting the RF circuit indicators. In order to reduce electromagnetic radiation and coupling, the primary side, secondary side and load side loop area of DC-DC module is required to be minimum. No matter how complex the form of the power circuit is, its large current loop should be as small as possible. The power cord and ground wire 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 comply with the principle of the shortest return path of each power. The filter capacitor should be funded in time, close to the relevant pins of the switching power supply. Use common mode inductors close to the switching power supply module

(3) the long-distance power line on the single board cannot approach or pass near the output and input of the cascade amplifier (gain greater than 45dB) at the same time. Avoid the power line becoming the RF signal transmission path, which may cause self excitation or reduce the sector isolation. High frequency filter capacitors are required at both ends of long-distance power lines, and even in the middle

(4) three filter capacitors are combined and connected in parallel at the power inlet of rfpcb, and the low, medium and high frequencies on the power line are filtered by using the respective advantages of these three capacitors. For example: 10uF, 0.1uF, 100pF. And close to the input pin of the power supply in order from large to small

(5) when feeding the small signal cascade amplifier with the same group of power supplies, it should first supply power to the front stage from the last stage, so that the EMI generated by the last stage circuit has little impact on the front stage. And each level of power filter has at least two capacitors: 0.1uF, 100pF. When the signal frequency is higher than 1GHz, 10PF filter capacitor should be added

(6) it is commonly used in low-power electronic filters. The filter capacitor should be close to the triode pin, and the high-frequency filter capacitor should be closer to the pin. The triode has a low cut-off frequency. If the triode in the electronic filter is a high-frequency tube, which works in the amplification area, and the layout of peripheral devices is unreasonable, it is easy to produce high-frequency oscillation at the output end of the power supply. The same problem may exist in the linear regulator module, because there is a feedback loop in the chip and the internal triode works in the amplification area. In the layout, the high-frequency filter capacitor is required to be close to the pin to reduce the distributed inductance and destroy the oscillation conditions

(7) the copper foil size of the power part of the PCB conforms to the maximum current flowing through it, and the allowance is considered (generally referred to as 1a/mm line width)

(8) the input and output of the power line 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 during power wiring. The power line is isolated from other strong interference lines (such as CLK) with ground wire

(10) the power wiring of the small signal amplifier needs to be isolated from the ground copper sheet and the grounding via to avoid other EMI interference, which will deteriorate the signal quality of this level

(11) different power layers should avoid overlap in space. It is mainly to reduce the interference between different power supplies, especially between some power supplies with large voltage differences. The overlapping of power supply planes must be avoided. If it is difficult to avoid, the interlayer can be considered

(12) PCB layer distribution is convenient to simplify the subsequent wiring processing. For a four layer PCB (the circuit board commonly used in WLAN), in most applications, the top layer of the circuit board is used to place components and RF leads, the second layer is used as the system ground, the power part is placed in the third layer, and any signal lines can be distributed in the fourth layer

the continuous ground plane layout of the second floor is very necessary to establish an impedance controlled RF signal path. It is also convenient to obtain the shortest possible ground loop, provide a high degree of electrical isolation for the first and third floors, and minimize the coupling between the two layers. Of course, other board layer definitions can also be used (especially when the circuit board has different layers), but the above structure is a proven successful example

(13) large area power supply layer can make VCC wiring easier, but this structure is often the fuse that leads to the deterioration of system performance. Connecting all power leads together on a large plane will not avoid the noise transmission between pins. On the contrary, if the star topology is used, the coupling between different power supply pins will be reduced

the figure above shows the VCC wiring scheme of star connection, which is taken from max2826ieee802. This customer is a high-tech enterprise specializing in the production of cold-rolled sheet and downstream hot-dip galvanized coil Evaluation board of 11a/g transceiver. In the figure, a main VCC node is established, from which the power lines of different branches are led to supply power to the power pins of RFIC. Each power supply pin uses independent leads to provide spatial isolation between the pins, which is conducive to reducing the coupling between them. In addition, each lead also has a certain parasitic inductance, which is one of the most important markets of aoshengde, which is exactly what we hope. It helps to filter out the high-frequency noise on the power line

when using star topology VCC leads, it is also necessary to take appropriate power decoupling, and the decoupling capacitor has a certain parasitic inductance. In fact, the capacitor equivalent to the overseas wood pulp production layout has become an important cost advantage. The advantage is a series RLC circuit, and the capacitor plays a leading role in the low frequency band, but after the self-excited oscillation frequency (SRF):

, the impedance of the capacitor will show inductance. It can be seen that the capacitor has decoupling effect only when the frequency is close to or lower than its SRF, and the capacitance shows low resistance at these frequency points

typical S11 parameters under different capacitance values are given. From these curves, we can clearly see the SRF. It can also be seen that the larger the capacitance, the better the decoupling performance provided at lower frequencies (the lower the impedance presented)

it is best to place a high-capacity capacitor at the main node of VCC star topology, such as 2.2 μ F。 The capacitor has a low SRF, which is very effective for eliminating low-frequency noise and establishing a stable DC voltage. Each power pin of the IC needs a low capacity capacitor (such as 10nf) to filter out the high-frequency noise that may be coupled to the power line. For those power pins that supply power to noise sensitive circuits, two bypass capacitors may be connected externally. For example, using a 10PF capacitor in parallel with a 10nf capacitor to provide bypass can provide decoupling in a wider frequency range and try to eliminate the impact of noise on the power supply voltage. Each power supply pin needs to be carefully checked to determine how much decoupling capacitance is needed and at which frequency the actual circuit is vulnerable to noise interference

good power decoupling technology combined with rigorous PCB layout and VCC leads (star topology) can lay a solid foundation for any RF system design. Although there are other factors that reduce the system performance index in the actual design, having a "noise free" power supply is the basic element to optimize the system performance

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