No circuit operates without power; current can only flow through a closed loop, and some energy is necessary to power the electronics. Power supply design and the power distribution network (PDN) are critical to circuits, yet less thought goes into what happens to the PDN when circuit conditions fluctuate. Rarely does a circuit operate solely at a set power draw for its entire uptime. When sub-circuits increase power draw from the source, that power is unavailable elsewhere in the circuit, which could lead to runtime issues. Bypass capacitors (alternatively, decoupling capacitors) act as local power storage throughout the circuit, delivering the missing power at times of high overall supply draw to ensure reliable performance.
| The “Difference” Between Bypass Capacitors and Decoupling Capacitors | ||
|---|---|---|
| Performance Characteristic | Analysis Mode | |
| Bypass | Signal integrity/noise | AC |
| Decoupling | Power integrity/stability | DC |
Bypass Capacitor? Decoupling Capacitor? What’s the Difference?
Although bypass and decoupling capacitors have slightly different terminology, they are synonymous: both act as local charge reservoirs to meet immediate, local power demands when current traveling from the power source is still en route due to timing/inductance delays. Traditionally, “decoupling” indicated the capacitor’s role in preventing electrical energy from transferring from one sub-circuit to another. In other words, decoupling capacitors promote signal integrity. However, power stability is another way of looking at signal integrity (at least in the context of power electronics): having the ability to source extra energy/voltage when the draw on the supply increases ensures that signal parameters meet the power requirements of the components.
Bypass capacitors provide an additional hint about usage patterns: they intend to bypass high-impedance pathways at high frequencies by offering an AC shunting to ground. Because the flow of current follows the path of least impedance in AC systems, bypass capacitor selection must focus on low impedance at high frequencies, and since impedance is frequency dependent, capacitor selection should prioritize the lowest impedance response at the targeted frequency rather than the lowest frequency response overall. Bypass capacitors do not require the bulk storage capacity of larger capacitors (e.g., electrolytic capacitors), so ceramics are the materials of choice for their superior high-frequency response.
When selecting bypass or decoupling capacitors, consider the following performance attributes:
- Parasitics – The nonideal reality of circuit performance that deviates from ideal models. The equivalent series resistance (ESR) characterizes the total real energy losses of the capacitor owing to the dielectric and metal constituting it. This value is typically on the magnitude of milliohms.
- Quality (Q) factor – The quality indicates the resonance response – a low Q means low selectivity (i.e., a larger bandwidth response) and low amplitudes. In contrast, a high Q means higher selectivity (shorter bandwidth) and large amplitudes. Generally, a higher Q factor is desirable due to its inherent ability to tune out more frequencies around the targeted frequency and “sharpen” the system response. Mathematically, Q is the absolute difference between the inductive and capacitive reactance over the ESR.
- Series resonant frequency (FSR) – The series resonant frequency is the value most important to circuit designers, as it indicates the frequency of the lowest impedance for the capacitor
. Mathematically, where LS is the series inductance of the capacitor, and C is the capacitance.
. Mathematically, where LS is the series inductance of the capacitor, and C is the capacitance.- Parallel resonant frequency (FPR) – The FPR occurs at approximately double the series resonant frequency and represents the highest possible impedance for the circuit; for bypassing, this is the worst possible outcome, but rejection circuits can leverage the FPR.
Bypass Capacitor Layout Applications
Every IC in a design will likely receive a bypass or decoupling capacitor per power net to provide a low-impedance pathway to the respective supply. As such, bypass capacitors are often the most populated element of a BOM/assembly. During placement, layout designers will want to consider proximity from the power pins to the capacitor; capacitor charge/discharge time is proportional to the total capacitance, so designers should prioritize placing the smallest capacitance capacitors closest to the pins in question for rapid response. Larger capacitance bypass capacitors can sit further away from the same-net power pins.
BGAs merit special consideration due to the large number of power/ground pins required for the various BGA applications, so decoupling capacitors is necessary. Decoupling capacitors act as the lowest impedance pathway to power (for smoothing, adequate power distribution under load) and ground (general EMC/signal integrity); a direct connection from BGA pins to capacitor pads with via-in-pad offers a superb low-impedance pathway. However, not all BGA power/ground pins are located conveniently toward the perimeter of the pinout. For nested BGA power/ground pins, the closest capacitor placement may be directly underneath the BGA; via-in-pad can drop down directly from pin to pad (analogous to the in-plane via-in-pad routing), or the designer can offset the capacitor on the reverse side slightly to avoid overlapping capacitor pads and breakout via.
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Bypass capacitors (or decoupling capacitors) provide essential power integrity performance for circuits when the power draw on the supply increases and the current to individual components sags. Choosing the correct capacitance for the series resonant frequency to match the target bandwidth ensures impedance is at its lowest possible value. Selecting components to match the design – especially when considering price, availability, and packaging – may be difficult, but VSE is here to help. Our engineers at VSE are committed to building electronics for our customers, including a thorough BOM review to optimize cost and lead times. We’ve been realizing life-changing and life-saving designs for over forty years.
