PCB Tech - Power supply noise of high frequency PCB in PCB design

Abstract: This article systematically analyzes the various forms and causes of power noise interference in high-frequency PCB boards, and through formula derivation, combined with engineering experience, puts forward corresponding countermeasures, and finally summarizes the general principles that should be followed in suppression. Power supply noise. In high frequency PCB boards, the more important interference is power supply noise.

On the basis of systematic analysis of the characteristics and causes of high-frequency PCB board power noise, combined with engineering applications, some very effective and simple solutions are proposed.

Power supply noise analysis Power supply noise refers to the noise generated by the power supply itself or the noise interfered by interference. The interference is manifested in the following aspects: 1) Distributed noise caused by the inherent impedance of the power supply itself. In high-frequency circuits, power supply noise has a great influence on high-frequency signals. Therefore, a low-noise power supply is first required.

Clean ground is as important as clean electricity. Ideally, the power supply has no impedance, so there is no noise. However, in practice, the power supply has a certain impedance, and the impedance is distributed throughout the power supply, so noise will also be superimposed on the power supply. Therefore, the impedance of the power supply should be minimized, and a special power supply layer and ground layer are preferably used.

In high-frequency circuit design, the power supply in the form of layer design is usually better than the form of bus design, so the circuit always follows the path of least impedance.

In addition, the power board also provides a signal loop for all generated and received signals on the PCB, thereby minimizing signal loops and reducing noise. 2) Common mode field interference.

Refers to the noise between the power supply and the ground. This is because the power supply is caused by the common mode voltage interference caused by the loop formed by the interference circuit and the common reference plane. Its value depends on the relative strength of the electric field and magnetic field to be set. On this channel, the reduction of Ic will result in a common-mode voltage in the series current loop, thereby affecting the receiving part.

If the magnetic field occupies the main position, the common mode voltage generated in the series loop is:

The ΔB in the formula (1) is the change of the magnetic induction intensity, Wb / m2; s is the area and m2.

In the case of an electromagnetic field, when the electric field value is known, the induced voltage is

Equation (2) usually applies below L = 150 / F, and F is used for electromagnetic frequencies in MHz.

The experience of PCB designers is that if this limit is exceeded, the calculation of the maximum induced voltage can be reduced to: 3) Differential mode field interference. Refers to the interference between the power supply and the input and output power lines.

In the actual PCB design, the author found that its proportion in the power supply noise is very small, so it will not be discussed here. 4) Inter-line interference. Refers to interference between power lines.

When there are mutual capacitance C and mutual inductance M1-2 between two different parallel circuits, if there are voltage VC and current IC in the interference source circuit, an interference circuit will appear:

A. The voltage of capacitive impedance coupling is

Type (4) RV is the parallel value of the near-end resistance and the far-end resistance of the interference circuit. B. Series resistor via inductive coupling

If there is common mode noise in the interference source, the line-to-line interference usually represents two forms of common mode and differential mode. 5) Power line coupling. Refers to the phenomenon in which an AC or DC power cord is subjected to electromagnetic interference, and the power cord transmits these interferences to other devices. This is the power supply noise indirectly interferes with the high-frequency circuit.

It should be noted that the noise of the power supply is not necessarily generated by itself, it may also be the noise of external interference sensing, and then the noise is superimposed with the noise (radiation or conduction) generated by itself to interfere with other circuits or equipment.

The countermeasures to eliminate power supply noise interference can target the different manifestations and causes of power supply noise interference, which can target the conditions under which it occurs, and effectively suppress the interference of power supply noise.

The solution is:

1) Pay attention to the through holes on the board. The through hole makes it necessary to etch the opening on the power layer to allow space to pass through the through hole.

If the power layer opening is too large, it will inevitably affect the signal loop, the signal is forced to bypass, the loop area increases, and the noise increases. If some signal lines are concentrated near the opening, share this part of the circuit, and the common impedance will trigger Crosstalk.

2) The cable needs enough ground wire.

Each signal needs to have its own proprietary signal loop. The signal and loop area of the loop is as small as possible, which means that the signal is parallel to the loop.

3) Place a power supply noise filter. It can effectively suppress the noise inside the power supply and improve the anti-interference and safety of the system. It is a two-way radio frequency filter, which can not only filter the noise interference introduced by the power line (to prevent interference from other equipment), but also filter out the noise generated by itself (to avoid interference with other equipment), and the serial mode common mode interference can be suppressed effect.

4) Power isolation transformer. The common mode ground loop of the power circuit or signal cable is separated, which can effectively isolate the common mode loop current generated by high frequency.

5) Power regulator.

Regaining a cleaner power supply can greatly reduce the noise level of the power supply.

6) Wiring.

The input and output lines of the power supply should not be laid on the edge of the dielectric board, otherwise it is easy to generate radiation and interfere with other circuits or devices.

7) Separate analog and digital power supplies. High-frequency equipment is usually very sensitive to digital noise, so the two should be separated and connected at the power inlet. If the signal spans the analog and digital parts, a loop can be placed on the signal to reduce the loop area.

8) Avoid separate power supplies that overlap between different layers.

Stagger it as much as possible, otherwise the power supply noise is easily coupled through the parasitic capacitance in the past.

9) Isolate sensitive PCB components.