PCB News - What exactly is the power distribution system on the PCB

In general, PDS refers to the subsystem that distributes the Power of the Power Source to the devices and components in the system. Power distribution systems exist in all electrical systems, such as the lighting system of a building, an oscilloscope, a PCB board, a package, a chip, its internal power distribution system.

Power distribution system on PCB board

In a typical product, the power distribution system consists of all interconnections from the voltage regulator module (VRM) to the PCB, packaging, and then on the chip. It can be divided into four sections:

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

Bulk capacitance on PCB, high frequency decoupling capacitance, interconnect lines, through holes, power/ground plane -- power distribution system on PCB;

Packaged pins, bonding wires, interconnects and embedded capacitors -- packaged power distribution systems;

On - chip interconnection and capacitance - on - chip power distribution system.

This article focuses on part 2, the power distribution system on PCB. The rest is outside the scope of this article.

The Power distribution system on PCB refers to the system in which PCB distributes the Power of Power Source to various chips and devices that need Power supply. This article focuses on the power distribution system on PCB, so we agree that the power distribution system or PDS mentioned below refers to the power distribution system on PCB.

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

Power integrity

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

Generally speaking, the components that need power supply on PCB have certain requirements for the working power supply. Taking the chip as an example, it is usually expressed as three parameters:

Ultimate power supply voltage: refers to the ultimate power supply voltage that the chip's power supply pin can bear. The power supply voltage of the chip cannot exceed the required range; otherwise, the chip may be damaged. Within this range, the function of the chip is not guaranteed; If the chip is in the limit value of this parameter for a certain time, the long-term stability of the chip will be affected.

Recommended operating voltage: refers to the voltage range that the chip power supply pin must meet in order to make the chip work normally and reliably. It is usually represented by "V± X %", where V is the typical operating voltage of the chip power supply pin, x% is the allowable voltage fluctuation range, and the common X is 5 or 3.

Power supply noise: The ripple noise allowed at the power supply pin voltage of the chip for the chip to work reliably and normally represented by its peak-to-peak value.

The "limit supply voltage" and "recommended operating voltage" requirements are usually provided for the chip, but "power noise" may not be provided separately, which may be included in the parameter "recommended operating voltage". The "power noise" is the focus of this paper and will be discussed separately later.

To illustrate the above examples, the problem of power supply integrity is to discuss how the system power supply meets the requirements of "limit power supply voltage", "recommended operating voltage" and "power supply noise" at different power supply pins of the chip after passing through the power distribution system.

Three characteristics of a power distribution system

The physical media of the Power distribution system are diverse, including Connector, cable, Trace, Power Plane, GND Plane, Via, solder, Pad, chip pin, etc. They differ in physical properties (material, shape, size, etc.). Since the purpose of the power distribution system is to supply the power of the system power supply to the device that needs power supply, to provide stable voltage and complete current loop, so 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 representing the hindrance effect of conductor on DC current, usually represented by R. Its main physical feature is that when current I flows through, electric energy is converted into heat energy (I2R), and dc voltage drop (IR) is generated at both ends.

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

-- Resistivity of conductor

-- Length of conductor

-- The cross-sectional area of the conductor

Among them

A physical property of a conductor and related to temperature. The resistivity of a metal generally increases with temperature.

Resistance exists everywhere in the power distribution system: dc resistance and contact resistance exist in cables and connectors, distributed resistance exists in copper wire, power layer, stratum and through hole, dc resistance exists in solder, pad and chip pin and contact resistance exists between them.

These resistors, when current flows through them, 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, thus reducing the voltage of the port of the device requiring power supply, resulting in power supply integrity problems.

Thermal Power Dissipation: Thermal Power Dissipation converts Power from Power supplies to heat and increases system temperature, compromising system stability and reliability.

Equivalent the resistance and load of the power distribution system to the circuit as shown in Figure 1.1:

Where, Vsource represents the power supply voltage, Voutput represents the output voltage, RS represents the power supply resistance, R1 represents the distributed resistance on the power supply path, R2 represents the distributed resistance on the return path. Assuming the loop current is I, the power supply voltage of the load is shown in Equation 1.2:

The voltage drop IRS on RS reduces the output voltage Voutput of the power supply, the voltage drop IR1 on the power supply path decreases 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 drop of the resistors RS, R1 and R2 above will reduce the supply voltage vCC-GND of the load, resulting in power supply integrity problems.

The heat loss on the resistance of the power distribution system will cause the power of the power supply to be converted into heat and dissipated, thus reducing the efficiency of the system. At the same time, heating will cause the temperature rise of the system, reduce the life of some devices (such as electrolytic capacitors), thus affecting the stability and reliability of the system. Excessive current density in some areas will also cause the local temperature 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 influence is proportional to the size of the resistance value, so reducing the resistance characteristics of the power distribution system is one of our design goals.

Inductance characteristic

Inductance is a physical quantity that characterizes the resistance of a conductor to alternating current. When the current flows through the conductor, a magnetic field will be formed around the conductor. When the current changes, the magnetic field will also change, and the changing magnetic field will form an induced voltage at both ends of the conductor. The polarity of the voltage will make the induced current obstruct the change of the original current. When a change in the magnetic field around a conductor is caused by a change in the current in other conductors, an induced voltage will also be generated in the conductor, and the polarity of the voltage will cause the induced current to obstruct the change in the original current. The effect of this conductor against the change of current is called inductance, the former called self-inductance L, the latter 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 common to the two conductors;

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