Power distribution system and power supply integrity

A Power distribution System (PDS) is a subsystem that allocates power from power source to each device and device that needs power in the system. In all electrical systems there are power distribution systems, such as the lighting system for a building, an oscilloscope, a PCB, a package, a chip, and a power distribution system inside.

1. Power distribution system on the PCB

   In a typical product, the power distribution system consists of a voltage regulator module (VRM) to the PCB board, the package, and all the interconnects in the chip. Can be divided into four sections:

• The voltage Regulator module (VRM) includes its filter capacitance-the power supply;
• Bulk capacitance on PCB board, High frequency decoupling capacitance, interconnect, over hole, power/ground plane --PCB power distribution system;
• Package pins, bonding wires, interconnects and embedded capacitors-the power distribution system on the package;
• In-chip interconnection and capacitance, etc.-in-chip power distribution system.

This paper mainly discusses the 2nd part, that is, the power distribution system on the PCB, the rest of the content is not within the scope of this article.

A power distribution system on a PCB is a system that distributes power (power source) power to a variety of chips and devices that require power. This article focuses on the power distribution system on the PCB, so we agreed 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 the correct and stable voltage, which means that the voltage in all locations on the PCB can be maintained correctly and stably under any load. To study the relevant content of the power distribution system to work correctly and stably, we call it the power integrity problem.

1. Power integrity

   The so-called power integrity refers to the system power supply after the power distribution system after the device needs to supply the port relative to the device port to the operating power requirements of the degree of compliance.

   In general, the PCB needs to power the device for the work of the power supply has a certain requirements, in the case of chips, usually shown as three parameters:

• Ultimate Supply voltage : refers to the voltage at which the chip's power supply pin can withstand the limit. The supply voltage of the chip can not exceed the requirement range of this parameter, otherwise it may cause permanent damage to the chip, in this range, the function of the chip is not guaranteed; the chip is in the limit value of this parameter for a certain time, it will affect the long-term stability of the chip;
• recommended operating voltage : To make the chip to work reliably, the voltage of the chip power supply PIN must be guaranteed to meet the range, usually with "v±x%" to indicate, wherein V is the chip power supply pin typical operating voltage, X-percent is the allowable voltage fluctuation range, the common × is 5 or 3;
• power supply Noise : refers to the chip to allow the normal and reliable operation of the chip supply pin voltage allowable ripple noise, usually with its peak-to-peak to characterize.

The datasheet of the chip typically provides requirements for "limit supply voltage" and "Recommended operating voltage", which may be included in the parameter "recommended operating voltage" for "power Noise", which is not necessarily provided separately. The "Power Noise" is the focus of this paper and will be discussed separately.

The above example shows that the problem of power integrity is to discuss the system power supply after the power distribution system in the chip at different power supply pin relative to the chip pin to the power supply of the "limit supply voltage", "Recommended operating voltage" and "power supply noise" and other requirements of the degree of compliance.

1. Three characteristics of the power distribution system

   Power distribution system has a variety of physical media, including connectors (Connector), cables, transmission lines (Trace), Power layer (power Plane), formations (GND Plane), Over-hole (Via), solder, pad (PAD), Chip pin , and so on, their physical characteristics (material, shape, size, etc.) are different. Since the purpose of the power distribution system is to supply the power of the system to a device that needs power, providing a 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 . The

2. resistance characteristic

   Resistor is a physical quantity that characterizes the resistance of a conductor to a DC current, usually denoted by R, whose main physical feature is the conversion of electrical energy into thermal energy (I2R) when current I flows, and a DC voltage drop (IR) at both ends. The

   Resistor is the characteristic of the conductor itself, which is related to the temperature, material, length and cross-sectional area of the conductor, and is determined by the formula 1.1:

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   1.1)

   --resistivity of conductors

   -The length of the conductor

   -the cross-sectional area of the conductor

   Where the physical properties of the conductor and temperature are concerned, the resistivity of the metal generally increases with the increase of temperature.

   Resistors exist everywhere in the power distribution system: wires and connector have DC resistance and contact resistance, copper wire, power layer, formation, vias are distributed resistors, solder, pads, chip pins have DC resistance and there is a contact resistance between them.

   These resistors have two effects when current flows:

• DC Voltage drop (IR drop): This effect causes the power supply voltage to gradually decrease along the power distribution network, or cause the reference voltage to rise, thereby reducing the voltage required to supply the device port, causing power integrity problems;
• Thermal loss (Thermal power dissipation): This effect converts the power supply's power to heat, while causing the system temperature to rise, endangering the stability and reliability of the system.

The resistor and load of the power distribution system is equivalent to the circuit shown in 1.1:

Figure 1.1 equivalent circuit diagram of resistance and load for power distribution systems

Wherein, Vsource represents the supply voltage, voutput represents the output voltage, RS represents the power supply resistance, R1 represents the distribution resistor on the power path, R2 represents the distribution resistor on the return path, assuming that the loop current is I, the load supply voltage as shown in Formula 1.2:

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1.2)

The voltage drop on the RS will reduce the output voltage of the power supply voutput, the voltage drop on the power path IR1 reduce the load supply voltage VCC, and the voltage drop on the return path IR2 to raise the GND level of the load. The voltage drop of the above resistors RS, R1 and R2 will cause the power supply voltage VCC-GND of the load to be reduced, causing the problem of the integrity of the mains.

The heat loss generated by the resistor on the power distribution system causes the power to be converted to heat and dissipated in vain, thereby reducing the efficiency of the system. At the same time, heating will cause the system temperature rise, reduce the life of some devices (such as electrolytic capacitors) to affect the stability and reliability of the system, some areas of excessive current density will cause local temperature to continue to rise or even burn.

As can be seen from the above analysis, these two effects are harmful to the system, and their influence is proportional to the size of the resistance value, so it is one of our design goals to reduce the resistance characteristic of the power distribution system.

1. Inductance characteristics

   Inductance is the physical quantity that characterizes the conductor's blocking effect on alternating current. As the conductor flows through the current, a magnetic field is formed around the conductor, and when the current changes, the magnetic field changes, and the changing magnetic field will form an inductive voltage at both ends of the conductor, and the polarity of the voltage will cause the resulting induced current to obstruct the change of the original current, and the magnetic field around the conductor changes , an inductive voltage is also generated in the conductor, and the polarity of the voltage causes the resulting induced current to obstruct the change in the original current. The effect of this conductor blocking the current change is called inductance, the former is called self-inductance L, the latter is called mutual inductance m. Here we directly give the 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, that is, the mutual inductance is equal to two conductors common;
• Mutual inductance is less than self-inductance: the mutual inductance of any two conductors is less than that of either conductor.

The values of the induced voltages generated by these current changes are determined by the formula 1.3 and 1.4:

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1.3)

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1.4)

This induced voltage caused by current changes is significant in signal integrity (including power integrity), which can cause transmission line effects, mutations, crosstalk (Crosstalk), synchronous switching Noise (SSN), orbital collapse (Rail Collapse), ground bombs (Ground Bounce) and most of the electromagnetic interference (EMI).

In the power distribution system, inductors are ubiquitous, plug-ins, cables, copper wires, power layers, formations, vias, pads, chip pins, and so on, there are inductance, and mutual inductance between the conductors close to each other.

For ease of analysis, consider the current loop shown in 1.2, parallel branch A with branch B and a short retrace to form the complete current loop. This structure is very common, the branch A can represent the signal path or the power path, the branch B represents its return path, such as the chip package on the adjacent power pin and return pin (ground pin), the decoupling capacitor to the chip pin of the power supply through the hole and return to the hole (through hole), The adjacent power plane and return plane (ground plane) on the PCB.

Figure 1.2 Two-branch current loop: initial current and return current

Assuming that the local self-inductance of the branch A is LA, the local self-inductance of the branch B is lb, the local mutual inductance between the two branches is M, the current in the loop is I. Since the two branches are parallel and flow through the opposite direction of the current, so they produce the opposite direction of the magnetic field, assuming I increases, to the branch A, la generated by the polarity of the induced voltage will hinder the increase of I in branch A, and m generated by the polarity of the induced voltage will help the branch a increase in I. Therefore, the total inductance of the branch A is the self-inductance of the branch A and the difference of the two-branch mutual inductance, the total inductance of the branch B is likewise available, as shown in the formula 1.5 and 1.6:

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1.5)

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1.6)

In combination with 1.3 and 1.4, when the loop current I changes, the induced voltage in the branch A and branch B is:

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1.7)

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1.8)

If Branch a represents the power path and the path is returned by Branch B, the VA represents the power noise on the Power path (track collapse/power Bounce), and VB represents the track collapse/ground bounce noise on the return path. Both of these noise can cause the supply voltage instability, causing the problem of power integrity, so one of our design purposes is to minimize the above two types of voltage, there are two ways:

• Minimizing the rate of change in the loop current: This means reducing the mutation speed of the load-absorbing current, limiting the number of power ports that share the power supply path and return path;
• Minimizing the total inductance of the branch: This means reducing the local self-inductance of the branch and increasing the local mutual inductance between the two branches, reducing the local self-inductance of the branch to mean the shortest possible and widest possible power path and return path, and increasing the local mutual inductance means that the two branches should be approached as closely as possible in parallel and reverse direction.

From the above analysis, it can be seen that inductance induced by inductor voltage is the source of many problems in power supply integrity , so reducing the power distribution system above the inductive voltage is one of our design goals.

The
1. capacitance characteristic

   Capacitance is a measure of the capacity of two conductors to store charge at a certain voltage. If two conductors are given positive and negative charges respectively, the voltage between the two conductors will be present. The electrical capacity of the conductor is the ratio of the amount of charge stored on a single conductor to the voltage between the conductors:

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   1.9)

   --Represents the capacitance, Unit is Faraday (F)

   -represents the number of charges, in Coulomb (C)

   -represents the voltage between conductors, in volts (V)

   When the voltage between the two conductors changes, the current flows between the two conductors, which can be expressed as Formula 1.10:

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   1.10)

   When dv/ When the DT remains constant, the larger the capacitance C, the greater the current flowing through the capacitor, which means that the capacitor can supply the current at the cost of the voltage change, as long as the capacitance C is large enough to provide a large enough current as long as a small voltage change. The

   has a capacitance between the power path and the return path in the power distribution system, as shown in equivalent circuit 1.3:

   figure 1.3 Span style= "font-family: Arial" > equivalent circuit diagram of capacitance between power path and return path

• When the load current is constant, its current is provided by the regulated power supply part, namely is in the figure, at this time the capacitor ends voltage and the load are consistent, ic=0;
• When the load transient current changes, it is necessary to provide sufficient current for the load chip in a very short period of time. However, the power supply cannot quickly respond to changes in load currents, i.e. current is does not immediately meet load transient current requirements, and the load voltage will be reduced. However, since the capacitance voltage is the same as the load voltage, there is a voltage change at both ends. For the capacitor voltage changes will inevitably generate current, when the capacitance to the load discharge, the current IC is no longer 0, to provide current for 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, hinder the voltage transient change , for the load power supply port integrity is beneficial, so enhance the capacitance characteristics of the power distribution system is one of our design goals.

1. Summary

   The Power distribution system is the main object of this paper, and the related content of its work is the problem of power integrity. The power distribution system has the characteristics of resistance, inductance and capacitance, respectively, and the resistance and inductance are harmful to the integrity of the power supply, and the capacitance characteristics are useful for power integrity. Our design objective is to reduce or even eliminate the effects of resistance and inductance characteristics, and to enhance the impact of capacitance characteristics.

Power distribution system and power supply integrity