Consideration of power signal integrity in the design of electric meter pcb board

2021-01-05 11:17:56 hongling

Consideration of power signal integrity in the design of electric meter pcb board

In the electric meter pcb board, we usually care about the quality of the signal very much, but sometimes we are often limited to the study of signal lines, power and ideals. Although this can simplify the problem, in high-speed design, simplification is no longer feasible. Although the direct result of the circuit design is reflected in the signal integrity, we cannot ignore the power integrity design. Since the integrity of the power supply directly affects the signal integrity of the final PCB board. Power integrity and signal integrity are closely related. In many cases, the main cause of signal distortion is the power supply system. For example, the ground rebound noise is too large, the design of the decoupling capacitor is inappropriate, the circuit has a serious impact, the multi-power/ground plane is not well divided, the layer design is unreasonable, and the current is uneven.

1) Decoupling capacitor

We all know that adding some capacitance between the power supply and the ground can reduce the noise of the system, but how much capacitance does it add? What is the inherent capacity of each capacitor? The location of each capacitor is better? We are not serious Consider it carefully, but it is based on the designer’s experience and sometimes even less abilities. In high-speed design, we must consider parasitic capacitance parameters, quantitatively calculate the number of decoupling capacitors and the capacity value of each capacitor and the specific location of the placement, to ensure the range of impedance control of the system, a basic principle is the decoupling capacitor Need, one cannot be less, excess capacity, I don't like it.

2) rebound

When the edge of a high-speed device is less than 0.5 ns, the data exchange rate of the large-capacity data bus is so fast that when it is in the power supply layer, it is enough to affect the signal ripple and cause unstable power problems. When the current changes, the rate of change of the current increases, and the voltage on the ground increases with the voltage of the circuit. At this point, the ground plane (ground) is not an ideal zero level, and the power supply is not an ideal DC. As the gate circuit of the switch increases, the bounce of the ground becomes more serious. For a 128-bit bus, there may be 50_100 I/O lines switching on the same clock. At this point, the power of the I/O driver and the inductance of the ground circuit must be as low as possible. Otherwise, the voltage brush will appear on the same ground. Ground reflections can be seen everywhere, such as chips, packages, connectors or circuit boards, and these circuit boards can bounce back, causing problems with the integrity of the power system.

From a technical point of view, the increase in equipment will only decrease, and the width of the bus will only increase. The only way to maintain acceptable bounce is to reduce power supply and distribution inductance. For chips, it means moving to an array chip, placing as many power sources as possible and as short as possible to reduce inductance. For packaging, it means that the moving layers are encapsulated to make the power ground plane closer, just like in BGA packaging. For connectors, it means using more ground pins or redesigning the connector to have internal power and ground levels, such as a link-based ribbon line. For the circuit board, it means keeping adjacent power supplies as close as possible to the ground plane. When the inductance is proportional to the length, the connection between the power supply and the ground will be minimized.

3) Power distribution system

Abstract Power integrity design is a very complicated issue, but how to control the impedance between the power supply system (power supply and ground plane) is the key to the design. In theory, the lower the impedance of the power supply system, the lower the impedance, the lower the noise amplitude, and the lower the voltage loss. In the actual design, we can use the maximum voltage and power range to determine whether we want to achieve the impedance target, and then adjust the relevant factors to make the target impedance of each part of the circuit impedance of the power supply system (related to frequency) Close.