电子产品设计之五--旁路和耦合

Electronic Product Design 52405S 2.3.2001 TS © Rev. 1.0
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Concepts
Bypassing and decoupling refers to preventing energy transference from one circuit to another in addition to enhancing the quality of the power distribution system. Three circuit areas are of primary concern: power and ground planes, components, and internal power connections. Decoupling is a means of overcoming physical and time constraints caused by digital circuitry switching logic states. Digital logic usually involves two possible states, “0” or “1”. The setting and detection of these states is achieved with switches internal to the component that determines whether the device is to be at logic LOW or HIGH. There is a finite time period for the device to make this determination. Within this window, a margin of protection (i.e., noise margin) is provided to guarantee against false triggering. Moving the logic state near the trigger level creates a degree of uncertainty. If we add high-frequency noise, the degree of uncertainty increases and false triggering may occur. Decoupling is also required to provide sufficient dynamic voltage and current for proper operation of components during clock or data transitions when all component signal pins switch simultaneously under maximum capacitive load. Decoupling is accomplished by ensuring a low-impedance power source is present in both circuits traces and power planes. Because decoupling capacitors have an increasingly low impedance at high frequencies up to the point of self-resonance, high-frequency noise is effectively diverted from the signal traces, while the low-frequency RF energy remains relatively unaffected. Optimal implementation is achieved bulk, bypass and decoupling capacitors. All capacitor values must be calculated for a specific function and selected having proper dielectric material.
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Noise: PART 5: Bypassing and Decoupling
Teuvo Suntio
Professor of Applied Electronics at University of Oulu
Suntio©
Electronic Product Design 52405S 2.3.2001 TS © Rev. 1.0
Electronic Product Design 52405S 2.3.2001 TS © Rev. 1.0
Slide 4/9
Non-ideal capacitor
An equivalent circuit of a capacitor is shown in Fig. 1. The parallel resistor Rp is not affecting the behavior of a capacitor used as a single element or in parallel with other capacitors, and is, therefore, omitted (i.e., Rp = ∞). It should be considered when a capacitor is used in series e.g. in high-voltage applications. The impedance |Z | can be, therefore, presented as follows.
Electronic Product Design 52405S
The impedance Z exhibits series resonance. At this self-resonant point fo, the capacitive and inductive portion of the impedance are equal, from which it follows that 1
Ls
Rs
C
Rp
Fig. 1. Equivalent circuit
Z = Rs2 + (2π fLs −
where
Z = impedance (Ω)
1 2 ) 2π fC
Rs = Equivalent Series Resistance – ESR (Ω) Ls = Equivalent Series Inductance – ESL (H) C = capacitance (F) f = frequency (Hz)
Electronic Product Design 52405S 2.3.2001 TS © Rev. 1.0
Electronic Product Design 52405S 2.3.2001 TS © Rev. 1.0
Slide 3/9
Concepts
Three common uses of capacitors follow. Decoupling: Removes RF energy injected into the power distribution network from high-frequency components consuming power at the speed the device is switching at. Decoupling capacitors also provides a localized source of DC power for devices and components, and is particularly useful in reducing peak current surges propagated across the board. Bypassing: Diverts unwanted common-mode RF energy from components or cables. This is essential in creating a AC shunt to remove undesired energy from entering susceptible areas in addition to providing other functions of filtering. Bulk: Used to maintain constant DC voltage and current to components when all signal pins switch simultaneously under maximum capacitive load. It also prevents power dropout due to dI/dt current surges generated by components. An ideal capacitor has no losses in its conductive plates and dielectric. An ideal capacitor does not exist, but inductance and resistance is always present due to physical structure and connection to other circuitry.
2.3.2001 TS © RZ min = Rs
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Planar capacitor
For an ideal planar capacitor where current uniformly enters from one side and exits from from an other side, inductance will be zero. For those cases, Z will approach Rs at high frequencies and will not exhibit an inherent resonance, which is exactly what a power and ground plane structure within a PCB does. This difference is best described as a bode diagram of Z (Fig. 2).