What is a power distribution system and what exactly does power integrity mean?

Generally speaking, the power distribution system (PDS) refers to the subsystem that distributes the power of the power source to various devices and devices in the system that require power supply. There is a power distribution system in all electrical systems, such as the lighting system of a building, an oscilloscope, a PCB board, a package, and a chip. There is a power distribution system inside them.

Power distribution system on PCB

In general products, the power distribution system includes everything from the voltage regulation module (VRM) to the PCB board, packaging, and all interconnections within the chip. Can be divided into four sections:

The voltage regulation module (VRM) includes its filter capacitor - power supply;

Bulk capacitors, high-frequency decoupling capacitors, interconnect lines, vias, power/ground planes on the PCB - the power distribution system on the PCB;

Package pins, bond wires, interconnects and embedded capacitors - power distribution system on the package;

In-chip interconnects and capacitors, etc. - the power distribution system within the chip.

This article mainly discusses part 2, the power distribution system on the PCB, and the rest of the content is outside the scope of this article.

The so-called power distribution system on the PCB refers to the system on the PCB that distributes the power of the power source to various chips and devices that require power supply. This article mainly focuses on the power distribution system on the PCB, so we agree that the power distribution system or PDS mentioned below refers to the power distribution system on the PCB.

The function of the power distribution system is to transmit correct and stable voltage, which means that the voltage at all locations on the PCB can remain correct and stable under any load. The study of content related to the correct and stable operation of power distribution systems is called power integrity issues.

power integrity

The so-called power integrity refers to the degree to which the system power supply meets the working power requirements of the device port after passing through the power distribution system at the device port that needs power supply.

Generally speaking, the devices that need to be powered on the PCB have certain requirements for the working power supply. Taking the chip as an example, it usually displays three parameters:

Limit power supply voltage: refers to the limit power supply voltage that the chip's power supply pin can withstand. The power supply voltage of the chip cannot exceed the required range of this parameter, otherwise it may cause permanent damage to the chip; within this range, the function of the chip is not guaranteed; if the chip is at the extreme value of this parameter for a certain period of time, it will affect Long-term stability of the chip;

Recommended working voltage: refers to the range that the voltage of the chip power supply pin must meet to make the chip work normally and reliably. It is usually expressed as "V±x%", where V is the typical working voltage of the chip power supply pin, x% is the allowable voltage fluctuation range, common x is 5 or 3;

Power supply noise: refers to the allowable ripple noise on the chip power supply pin voltage to make the chip work normally and reliably. It is usually characterized by its peak-peak value.

The chip's Datasheet usually provides requirements for "limit supply voltage" and "recommended operating voltage". "Power supply noise" may not be provided separately. In this case, it may be included in the parameter "recommended operating voltage". "Power supply noise" is the focus of this article and will be discussed separately later.

Using the above example to illustrate, the issue of power integrity is to discuss the "limit supply voltage" and "recommended operating voltage" of the system power supply at different power supply pins of the chip relative to the chip pins after the system power supply passes through the power distribution system. " and "power supply noise" requirements.

Three characteristics of power distribution systems

There are various physical media in the power distribution system, including connectors, cables, transmission lines (Trace), power plane (Power Plane), ground plane (GND Plane), vias (Via), solder, pads (Pad) ), chip pins, etc., their physical properties (material, shape, size, etc.) are different. Since the purpose of the power distribution system is to provide the power of the system power supply to the devices that need power, providing stable voltage and a complete current loop, we only focus on three electrical characteristics of the power distribution system: resistance characteristics, inductance characteristics and capacitance characteristics. .

Resistance characteristics

Resistance is a physical quantity that represents the resistance of a conductor to DC current, usually represented by R. Its main physical characteristic is that when a current I flows through, it converts electrical energy into heat energy (I2R) and produces a DC voltage drop (IR) at both ends. ).

Resistance is a property of the conductor itself, which is related to the temperature, material, length and cross-sectional area of ​​the conductor and is determined by Equation 1.1:

——Resistivity of conductor

——The length of the conductor

——Cross-sectional area of ​​conductor

in

It is a physical property of a conductor and is related to temperature. The resistivity of metal generally increases as the temperature increases.

Resistance exists everywhere in the power distribution system: DC resistance and contact resistance exist in cables and connectors, distributed resistance exists in copper wires, power layers, ground layers, and vias, and DC resistance exists in solders, pads, and chip pins. There is contact resistance between them.

When current flows through these resistors, they produce two effects:

DC voltage drop (IR Drop): This effect will cause the power supply voltage to gradually decrease along the power distribution network, or cause the voltage of the reference ground to increase, thereby reducing the voltage of the port of the device that needs to be powered, causing power integrity problems;

Thermal Power Dissipation: This effect converts the power of the power supply into heat, causing the system temperature to rise and endangering the stability and reliability of the system.

The resistance and load of the power distribution system are equivalent to the circuit shown in Figure 1.1:

Figure 1.1 Equivalent circuit diagram of resistance and load of power distribution system

Among them, Vsource represents the power supply voltage, Voutput represents the output voltage, RS represents the internal resistance of the power supply, R1 represents the distributed resistance on the power path, and R2 represents the distributed resistance on the return path. Assuming that the loop current is I, the supply voltage of the load is as follows: Equation 1.2 Shown:

The voltage drop IRS on RS will reduce the output voltage Voutput of the power supply, the voltage drop IR1 on the power path reduces the supply voltage Vcc of the load, and the voltage drop IR2 on the return path increases the GND level of the load. The voltage drops of the above-mentioned resistors RS, R1, and R2 will cause the load's supply voltage VCC-GND to decrease, causing power supply integrity problems.

The heat loss generated by the resistance of the power distribution system will cause the power of the power supply to be converted into heat and dissipated in vain, thus reducing the efficiency of the system. At the same time, heat will cause the system temperature to rise, reducing the life of some components (such as electrolytic capacitors) and affecting the stability and reliability of the system. Excessive current density in some areas will also cause local temperatures to continue to rise or even burn out.

It can be seen from the above analysis that these two effects are harmful to the system, and their impact is proportional to the resistance value of the resistor. Therefore, reducing the resistance characteristics of the power distribution system is one of our design goals.

Inductance characteristics

Inductance is a physical quantity that represents the resistance of a conductor to alternating current. When a current flows through a conductor, a magnetic field will be formed around the conductor. When the current changes, the magnetic field will also change. The changing magnetic field will form an induced voltage at both ends of the conductor. The polarity of this voltage will cause the resulting induction The current hinders the change of the original current; when the current changes in other conductors around the conductor cause the magnetic field around the conductor to change, an induced voltage will also be generated in the conductor. The polarity of the voltage will cause the induced current to hinder the original current. The change. The effect of this conductor on hindering the change of current is called inductance, the former is called self-inductance L, and the latter is called mutual inductance M. Here we directly give two characteristics of mutual inductance:

Symmetry: Two conductors a and b, regardless of size, shape and relative position, the mutual inductance of conductor a to conductor b is equal to the mutual inductance of conductor b to conductor a, that is, the mutual inductance is equally shared by the two conductors;

Mutual inductance is less than self-inductance: The mutual inductance of any two conductors is less than the self-inductance of either conductor.

The value of the induced voltage generated by the above current change is determined by equations 1.3 and 1.4:

This induced voltage caused by current changes is of great significance in signal integrity (including power integrity). It can cause transmission line effects, mutations, crosstalk, synchronous switching noise (SSN), rail collapse, Ground Bounce and most electromagnetic interference (EMI).

In the power distribution system, inductance is ubiquitous. Inductance exists in connectors, cables, copper wires, power layers, ground layers, vias, pads, chip pins, etc. At the same time, there is mutual inductance between conductors that are close to each other.

For the convenience of analysis, consider the current loop as shown in Figure 1.2. Parallel branch a and branch b and a short retracement form a complete current loop. This structure is very common. Branch a can represent the signal path or power path, and branch b represents its return path, such as the adjacent power pins and return pins (ground pins) on the chip package, decoupling The power vias and return vias (ground vias) from the capacitor to the chip pins, and the adjacent power plane and return plane (ground plane) on the PCB.

Figure 1.2 Current loop of two branches: initial current and return current

Assume that the local self-inductance of branch a is La, the local self-inductance of branch b is Lb, the local mutual inductance between the two branches is M, and the current in the loop is I. Since the two branches are parallel and the currents flowing in them are in opposite directions, the magnetic fields they generate are in opposite directions. Assuming that I increases, for branch a, the polarity of the induced voltage generated by La will hinder the flow of I in branch a. increases, but the polarity of the induced voltage generated by M will help the increase of I in branch a. Therefore, the total inductance of branch a is the difference between the self-inductance of branch a and the mutual inductance of the two branches. The total inductance of branch b can be obtained in the same way, as shown in Equations 1.5 and 1.6:

Combining Equations 1.3 and 1.4, when the loop current I changes, the induced voltages caused in branch a and branch b are respectively:

If branch a represents the power path and branch b represents the return path, Va represents the power supply noise (rail collapse/power bounce) on the power path, and Vb represents the rail collapse/ground bounce noise on the return path. Both types of noise will cause instability in the supply voltage and cause power integrity problems. Therefore, one of our design goals is to minimize the above two voltages. There are two ways:

Reduce the rate of change of the loop current as much as possible: This means reducing the sudden change rate of the load current draw and limiting the number of power supply ports that share the power path and return path;

Reduce the total inductance of the branch as much as possible: This means that the local self-inductance of the branch needs to be reduced and the local mutual inductance between the two branches needs to be increased. Possibly wide power paths and return paths, and increasing local mutual inductance means that the two branches need to be as close as possible under the premise of being parallel and opposite.

It can be seen from the above analysis that the induced voltage caused by the inductance when the current changes is the source of many problems in power supply integrity. Therefore, reducing the above-mentioned induced voltage of the power distribution system is one of our design goals.

Capacitance characteristics

Capacitance is a measure of the ability of two conductors to store electric charge at a certain voltage. If you add a positive charge and a negative charge to two conductors, a voltage will exist between the two conductors. The capacitance of this pair of conductors is the ratio of the amount of charge stored on a single conductor to the voltage between the conductors:

——Indicates capacitance, unit is Farad (F)

——Indicates the number of charges, the unit is Coulomb (C)

——Indicates the voltage between conductors, in volts (V)

When the voltage between two conductors changes, a current will flow between the two conductors. The flowing current can be expressed as Equation 1.10:

When dV/dt remains constant, the larger the capacitance C, the greater the current flowing through the capacitor. That is to say, the capacitor can provide current to the outside at the expense of voltage change. As long as the capacitance C is large enough, as long as the voltage is small, changes can provide a large enough current.

In the power distribution system, there is a capacitor between the power path and the return path. The equivalent circuit is shown in Figure 1.3:

Figure 1.3 Equivalent circuit diagram of capacitance between power path and return path

When the load current remains unchanged, its current is provided by the regulated power supply part, which is IS in the figure. At this time, the voltage across the capacitor is consistent with that across the load, IC=0;

When the load transient current changes, sufficient current must be provided to the load chip in a very short time. However, the power supply cannot respond quickly to changes in load current, that is, the current IS will not immediately meet the load's transient current requirements, and the load's voltage will decrease. But since the capacitor voltage is the same as the load voltage, there is a voltage change across the capacitor. For capacitors, voltage changes will inevitably generate current. At this time, the capacitor discharges the load, and the current IC is no longer 0, providing current to the load chip.

From the above analysis, it can be seen that the capacitance of the power distribution system can provide transient current to the load and hinder transient changes in voltage. It is beneficial to the power integrity of the power supply port of the load, so it is our goal to enhance the capacitance characteristics of the power distribution system. One of the design goals.

summary

The power distribution system is the main discussion object of this article, and the relevant content to study its operation is the issue of power integrity. The power distribution system has resistance characteristics, inductance characteristics and capacitance characteristics respectively. The resistance characteristics and inductance characteristics are harmful to the power integrity, and the capacitance characteristics are beneficial to the power integrity. Our design goal is to reduce or even eliminate the effects of resistive and inductive characteristics and enhance the effects of capacitive characteristics.