PCB News - Power supply noise countermeasures in PCB design

Distributed noise caused by the inherent impedance of the power supply. In high-frequency PCB, power supply noise has a greater impact on high-frequency signals. Therefore, a low-noise power supply is first required. A clean ground is just as important as a clean power supply; common-mode field interference. Refers to the noise between the power supply and the ground. It is the interference caused by the common mode voltage caused by the loop formed by the interfered circuit and the common reference surface of a certain power supply. Its value depends on the relative electric field and magnetic field. The strength depends on the strength.

In high-frequency PCB boards, a more important type of interference is power supply noise. Through systematic analysis of the characteristics and causes of power noise on high-frequency PCB boards, combined with engineering applications, some very effective and simple solutions are proposed.

Analysis of power supply noise

Power supply noise refers to the noise generated by the power supply itself or induced by disturbance. 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 greater impact on high-frequency signals. Therefore, a low-noise power supply is first required. A clean ground is as important as a clean power source.

Ideally, the power supply has no impedance, so there is no noise. However, the actual power supply has a certain impedance, and the impedance is distributed on the entire power supply. Therefore, noise will also be superimposed on the power supply. Therefore, the impedance of the power supply should be reduced as much as possible, and it is best to have a dedicated power layer and ground layer. In high-frequency circuit design, it is generally better to design the power supply in the form of a layer than in the form of a bus, so that the loop can always follow the path with the least impedance. In addition, the power board must also provide a signal loop for all generated and received signals on the PCB board, so that the signal loop can be minimized, thereby reducing noise.

2) Common mode field interference. Refers to the noise between the power supply and the ground. It is the interference caused by the common mode voltage caused by the loop formed by the interfered circuit and the common reference surface of a certain power supply. Its value depends on the relative electric field and magnetic field. The strength depends on the strength.

On this channel, a drop in Ic will cause a common-mode voltage in the series current loop, which will affect the receiving part. If the magnetic field is dominant, the value of the common mode voltage generated in the series ground loop is:

Vcm = - (△B/△t) * S (1)

In formula (1), ΔB is the change in magnetic flux density, Wb/m2; S is the area, m2.

If it is an electromagnetic field, when its electric field value is known, its induced voltage is:

Vcm = (L*h*F*E/48) (2)

Equation (2) generally applies to L=150/F or less, where F is the frequency of electromagnetic waves in MHz.

If this limit is exceeded, the calculation of the maximum induced voltage can be simplified to:

Vcm = 2*h*E (3)

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 is not necessary to discuss it 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, the interfered circuit will appear:

a. The voltage coupled through the capacitive impedance is

Vcm = Rv*C1-2*△Vc/△t (4)

In formula (4), Rv is the parallel value of the near-end resistance and the far-end resistance of the interfered circuit.

b. Series resistance through inductive coupling

V = M1-2*△Ic/△t (5)

If there is common mode noise in the interference source, the line-to-line interference generally takes the form of common mode and differential mode.

5) Power line coupling. It refers to the phenomenon that after the AC or DC power cord is subjected to electromagnetic interference, the power cord transmits the interference to other devices. This is the indirect interference of power supply noise to the high-frequency circuit. It should be noted that the noise of the power supply is not necessarily generated by itself, but may also be the noise induced by external interference, and then superimpose this noise with the noise generated by itself (radiation or conduction) to interfere with other circuits or devices.

Countermeasures to eliminate power supply noise interference

In view of the different manifestations and causes of power supply noise interference analyzed above, the conditions under which they occur can be destroyed in a targeted manner, and the interference of power supply noise can be effectively suppressed. The solutions are:

1) Pay attention to the through holes on the board. The through hole requires an opening on the power layer to be etched to leave space for the through hole to pass through. If the opening of the power layer is too large, it will inevitably affect the signal loop, the signal will be forced to bypass, the loop area will increase, and the noise will increase. At the same time, if some signal lines are concentrated near the opening and share this loop, the common impedance will cause crosstalk.

2) Sufficient ground wires are required for the connection wires. Each signal needs to have its own dedicated signal loop, and the loop area of the signal and loop is as small as possible, that is to say, the signal and loop must be parallel.

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. And it is a two-way radio frequency filter, which can not only filter the noise interference introduced from 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 interfere with the serial mode common mode. Both have an inhibitory effect.

4) Power isolation transformer. Separate the power loop or the common mode ground loop of the signal cable, it can effectively isolate the common mode loop current generated in the high frequency.

5) Power supply 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 equipment.

7) Separate analog and digital power supplies. High-frequency devices are generally very sensitive to digital noise, so the two should be separated and connected together at the entrance of the power supply. If the signal needs to span both analog and digital parts, a loop can be placed at the signal span to reduce the loop area. As shown in Figure 4.

8) Avoid overlapping of separate power supplies between different layers. Stagger them as much as possible, otherwise the power supply noise is easily coupled through parasitic capacitance.

9) Isolate sensitive components. Some components, such as phase-locked loops (PLL), are very sensitive to power supply noise. Keep them as far away from the power supply as possible.

10) Place the power cord. In order to reduce the signal loop, the noise can be reduced by placing the power line on the edge of the signal line.

11) In order to prevent the power supply noise from interfering with the circuit board and the accumulated noise caused by the external interference on the power supply, a bypass capacitor can be connected to the ground in the interference path (except for radiation), so that the noise can be bypassed to the ground to avoid Interfere with other equipment and devices.

in conclusion

Power supply noise is directly or indirectly generated from the power supply and interferes with the circuit. When suppressing its impact on the circuit, a general principle should be followed. On the one hand, the power supply noise should be prevented as much as possible. The influence of the circuit, on the other hand, should also minimize the influence of the outside or the circuit on the power supply, so as not to worsen the noise of the power supply.