PCB Blog - Analysis and Design of PCB Board Power Supply System

Today, the design of high-speed PCB board electronic systems is difficult to succeed without a thorough grasp of the power supply system characteristics of the chip, the package structure, and the PCB board. In fact, in order to meet lower supply voltage, faster signal inversion speed, higher integration, and many more and more challenging requirements, many companies at the forefront of electronic design are in the product design process to ensure power supply. And signal integrity, a lot of money, manpower, and material resources are invested in the analysis of the power supply system. The analysis and design of power supply systems (PDS) is becoming more and more important in the field of high-speed circuit design, especially in the computer, semiconductor, communications, networking, and consumer electronics industries. With the inevitable further scaling of VLSI technology, the supply voltage of integrated circuits will continue to decrease. As more and more manufacturers move from 130nm technology to 90nm technology, it is foreseeable that the supply voltage will drop to 1.2V or even lower, while the current will also increase significantly. From the perspective of DC IR voltage drop to AC dynamic voltage fluctuation control, because the allowable noise range is getting smaller and smaller, this development trend has brought great challenges to the design of power supply systems.

PCB board power supply system design overview

Usually in AC analysis, the input impedance between the power supply ground is an important observation used to measure the characteristics of the power supply system. The determination of this observation evolves into the calculation of the IR drop in the DC analysis. Whether in the DC or AC analysis, the factors that affect the characteristics of the power supply system are: the layering of the PCB board, the shape of the power board layer plane, the layout of components, and the distribution of vias and pins, and so on. The concept of input impedance between power ground can be used in the simulation and analysis of the above factors. For example, a very broad application of power-to-ground input impedance is to evaluate the placement of decoupling capacitors on a board. With a certain number of decoupling capacitors placed on the board, the unique resonance of the circuit board itself can be suppressed, thereby reducing the generation of noise, and also reducing the edge radiation of the circuit board to alleviate electromagnetic compatibility problems. In order to improve the reliability of the power supply system and reduce the manufacturing cost of the system, system design engineers must often consider how to cost-effectively select the system layout of decoupling capacitors. The power supply system in the high-speed circuit system can usually be divided into three physical subsystems: chip, integrated circuit packaging structure, and PCB board. The power grid on the chip is composed of several layers of metal layers placed alternately. Each metal layer consists of metal strips in the X or Y direction to form a power or ground grid, and vias connect the metal strips of different layers. For some high-performance chips, many decoupling units are integrated into the power supply of the core or IO. The integrated circuit package structure, like a reduced PCB board, has several layers of power or ground planes with complex shapes. On the upper surface of the package structure, there are usually installation positions for decoupling capacitors. The PCB board usually contains a continuous large-area power and ground plane, as well as some large and small discrete decoupling capacitor components, and a power rectifier module (VRM). Bonding wires, C4 bumps, and solder balls connect the chip, package, and PCB together. The entire power supply system must ensure that each integrated circuit device is provided with a stable voltage within the normal range. However, switching currents and parasitic high-frequency effects in those power supply systems always introduce voltage noise. Its voltage variation can be calculated: where ΔV is the voltage fluctuation observed at the device and ΔI is the switching current. Z is the input impedance between the power supply and ground of the entire power supply system as observed at the device. To reduce voltage fluctuations, keep low resistance between power and ground. In the case of DC, since Z becomes a pure resistance, low resistance corresponds to a low power supply IR voltage drop. In the AC case, the low resistance also reduces the transient noise generated by the switching current. Of course, this requires Z to be kept small over a wide frequency band. Note that power and ground are often used as a signal return and reference planes, so there is a close relationship between the power supply system and the signal distribution system. However, due to space limitations, the noise phenomena and current loop control issues in power supply systems introduced by synchronous switching noise (IO SSO) will not be discussed here. The following sections will ignore the signal system and focus purely on the analysis of the power supply system.

DC IR drop

Since the feature size of the chip's Power Grid is very small (several microns or even smaller), the resistance loss in the chip is serious, so the IR voltage drop in the chip has been widely studied. In the following cases, the IR voltage drop on the PCB (in the range of tens to hundreds of millivolts) will also have a greater impact on high-speed system design. On the power board layer, the board plane is divided due to the Swiss-Chess structure, Neck-Down structure, and dynamic wiring (Figure 1); device pins, vias, solder balls, and C4 bumps through which current passes on the power board layer Insufficient number of power supply boards, the insufficient thickness of power supply plates, unbalanced current paths, etc.; system design requires low voltage, high current, and a tighter voltage floating range. For example, a device with high density and high pin count will often form the so-called Swiss-Chess structure effect on the chip package structure and the power distribution layer of the PCB board due to a large number of vias and anti-pads. The Swiss-Chess structure produces many tiny metal regions of high resistance. Depending on the power supply system, there is such a high resistance current path, that the voltage currently sent to the components on the PCB may be lower than the design requirements. Therefore, a good DC IR voltage drop simulation is the key to estimating the allowable voltage drop range of the power supply system. Provide design solutions or rules for pre and post-placement and routing through analysis of various possibilities. Layout engineers, system engineers, signal integrity engineers, and power design engineers can also incorporate IR drop analysis in the constraint manager as the final step in performing design rule checks on each power and ground netlist on the PCB. Inspection Tool (DRC). This design flow through automated software analysis can avoid layout and wiring problems on complex power supply system structures that cannot be found by visual inspection or even experience. Figure 2 shows that IR drop analysis can accurately pinpoint the distribution of critical voltages and currents in a power supply system on a high-performance PCB.

AC Power Ground Impedance Analysis

Many people know that a pair of metal plates constitute a plate capacitor, so they think that the characteristic of the power plate layer is to provide plate capacitance to ensure the stability of the power supply voltage. When the frequency is low and the signal wavelength is much larger than the size of the panel, the power board layer and the floor do form a capacitor. However, when the frequency increases, the characteristics of the power plane layer start to become complicated. More precisely, a pair of flat plates constitutes a flat plate transmission line system. The noise between the power supply and the ground, or the corresponding electromagnetic field, propagates between the boards following the transmission line principle. When the noise signal propagates to the edge of the panel, a portion of the high-frequency energy is radiated, but a larger portion is reflected back. Multiple reflections from different boundaries of the plate constitute the resonance phenomenon in the PCB board. In AC analysis, the resonance of the power-to-ground impedance of the PCB board is a unique phenomenon. For comparison, the impedance characteristics of a pure capacitor and a pure inductance are also plotted. The size of the board is 30cm×20cm, the spacing between the boards is 100um, and the filling medium is FR4 material. The power rectifier module on the board is replaced with a 3nH inductor. It is a 20nF capacitor that exhibits a purely capacitive impedance characteristic. It can be seen from the figure that when there is no power rectifier module on the board, in the frequency range of tens of megabytes, the impedance characteristics (red line) of the flat plate are the same as the capacitance (blue line). Above 100MHz, the impedance characteristics of the slab are inductive (along the green line). After reaching the frequency range of several hundred megabytes, the appearance of several resonance peaks shows the resonance characteristics of the plate, and the plate is no longer purely inductive. By now, it is clear that a low-resistance power supply system (from DC to AC) is the key to obtaining low voltage fluctuations: reducing inductive effects, increasing capacitive effects, and eliminating or reducing those resonant peaks are design goals.

To reduce the impedance of a power supply system, some design guidelines should be followed:

1) Reduce the distance between the power supply and the floor layer;

2) Increase the size of the plate;

3) Improve the dielectric constant of the filling medium;

4) Use multiple pairs of power and floor layers.

However, due to manufacturing or some other design considerations, design engineers also need to use some more flexible and effective methods to change the impedance of the power supply system. To reduce impedance and eliminate those resonance peaks, placing discrete decoupling capacitors on the PCB has become a common method.

The input impedance of the power supply system is calculated with Sigrity PowerSI:

a. There is no power rectifier module, and no decoupling capacitors are placed on the board.

b. The power rectifier module is simulated with a short circuit, and no decoupling capacitors are placed on the board.

c. The power rectifier module is simulated with a short circuit, and decoupling capacitors are placed on the board.

Placing discrete decoupling capacitors on the board gives designers the flexibility to adjust the impedance of the power supply system to achieve lower power-to-ground noise. However, how to choose where to place it, how many to choose, and what kind of decoupling capacitor to choose remains a series of design issues. Therefore, it is often necessary to seek decoupling solutions for a particular design and use appropriate design software and perform extensive simulations of the power supply system on PCB board.