What is Power Supply Bypassing and Why is it Important?

Power supply bypassing, also known as decoupling, is a critical aspect of printed circuit board (PCB) design. It involves strategically placing capacitors near integrated circuits (ICs) to provide a stable and clean power supply, minimizing noise and voltage fluctuations. Proper power supply bypassing ensures the reliable operation of electronic devices and prevents issues such as signal integrity problems, electromagnetic interference (EMI), and even device failure.

In a PCB, the power supply is distributed through copper traces, which have inherent resistance and inductance. When ICs switch states, they draw sudden bursts of current from the power supply, causing voltage drops across these traces. These voltage fluctuations, often referred to as power supply noise, can propagate throughout the PCB and affect the performance of other components.

Bypassing capacitors act as local energy reservoirs, providing a low-impedance path for high-frequency currents and effectively filtering out power supply noise. They are placed close to the power pins of ICs, minimizing the loop area and reducing the inductance of the current path.

Types of Bypassing Capacitors

There are several types of capacitors commonly used for power supply bypassing:

1. Ceramic capacitors
2. High-frequency response
3. Low equivalent series resistance (ESR)
4. Available in small packages

5. Prone to piezoelectric effect and microphonic noise

6. Tantalum capacitors

7. Higher capacitance density compared to ceramic capacitors
8. Good low-frequency response
9. Higher ESR than ceramic capacitors

10. Polarized and require correct orientation

11. Aluminum electrolytic capacitors

12. High capacitance values
13. Good for bulk energy storage
14. Higher ESR and lower frequency response compared to ceramic and tantalum capacitors

15. Polarized and require correct orientation

16. Film capacitors

17. Low ESR and high stability
18. Suitable for high-frequency and high-temperature applications
19. Larger size compared to ceramic and tantalum capacitors

The choice of bypassing capacitors depends on the specific requirements of the circuit, such as the frequency range, current demands, and available board space.

Bypassing Capacitor Placement and Layout

Proper placement and layout of bypassing capacitors are crucial for effective power supply bypassing. Here are some guidelines to follow:

1. Place bypassing capacitors as close to the IC power pins as possible to minimize the loop area and inductance.

2. Use short and wide traces to connect the capacitors to the power and ground planes to reduce trace impedance.

3. Minimize the distance between the capacitor pads and the via connecting to the power and ground planes.

4. Use multiple vias for each capacitor pad to reduce via inductance and improve current handling capacity.

5. Place capacitors on the same layer as the IC to avoid the use of vias, which can add inductance.

6. Consider the use of via-in-pad or micro-vias for dense layouts or high-speed designs.

7. Use a combination of capacitor values to target different frequency ranges and provide broadband bypassing.

Here’s an example of a good bypassing capacitor layout:

     Power Plane
         |
  +-----------------+
  |                 |
  |     IC          |
  |     +-+         |
  |  C1 |*| C2      |
  |     +-+         |
  |                 |
  +-----------------+
         |
     Ground Plane

In this layout, capacitors C1 and C2 are placed close to the IC power pin, minimizing the loop area. The capacitor pads are connected to the power and ground planes using short and wide traces and multiple vias (represented by *).

Selecting Bypassing Capacitor Values

Choosing the appropriate capacitor values for power supply bypassing is essential for effective noise suppression across the desired frequency range. A common practice is to use a combination of capacitor values to target different frequency bands.

1. Bulk capacitance (1 μF to 10 μF)
2. Provides low-frequency bypassing and energy storage
3. Placed near the power entry point of the PCB

4. Typically uses tantalum or aluminum electrolytic capacitors

5. Medium-frequency bypassing (0.1 μF to 1 μF)

6. Targets the mid-frequency range
7. Placed near ICs and distributed across the PCB

8. Typically uses ceramic capacitors

9. High-frequency bypassing (1 nF to 100 nF)

10. Suppresses high-frequency noise
11. Placed very close to IC power pins
12. Uses ceramic capacitors with low ESR

Here’s an example of a bypassing capacitor network for an IC:

Frequency Range	Capacitor Value
Bulk	10 μF
Medium	0.1 μF
High	10 nF

It’s important to consider the self-resonant frequency (SRF) of the capacitors when selecting values. The SRF is the frequency at which the capacitor’s impedance is the lowest. Choose capacitor values such that their SRFs are higher than the frequencies you want to bypass.

Power and Ground Plane Design

A well-designed power and ground plane system is essential for effective power supply bypassing. Here are some guidelines for designing power and ground planes:

1. Use dedicated layers for power and ground planes to provide low-impedance current paths and minimize voltage drops.

2. Keep the power and ground planes close together to increase the inter-plane capacitance and reduce the plane impedance.

3. Avoid splitting the ground plane, as it can create multiple current return paths and introduce noise.

4. Minimize the number of cuts and slots in the power and ground planes to maintain their integrity and reduce impedance.

5. Use solid fills for power and ground planes instead of gridded or hatched fills to maximize the current-carrying capacity.

6. Consider using power and ground planes on multiple layers for complex designs or high-current applications.

Here’s an example of a four-layer PCB Stackup with dedicated power and ground planes:

Layer	Description
Top	Signal Layer
L2	Ground Plane
L3	Power Plane
Bottom	Signal Layer

By placing the power and ground planes adjacent to each other, the inter-plane capacitance is increased, reducing the plane impedance and improving the bypassing performance.

Bypassing for Mixed-Signal and RF Circuits

Power supply bypassing for mixed-signal and RF circuits requires additional considerations due to their sensitivity to noise and the presence of both analog and digital components.

1. Separate analog and digital power supplies to prevent digital noise from coupling into analog circuits.

2. Use separate bypassing capacitors for analog and digital sections to avoid noise coupling through shared capacitors.

3. Place ferrite beads or inductors in series with the power supply lines to isolate high-frequency noise between analog and digital sections.

4. Use a star grounding scheme, where analog and digital grounds are connected at a single point to minimize ground loop currents.

5. Implement split power and ground planes for analog and digital sections, with a single connection point to the main power supply.

Here’s an example of a mixed-signal bypassing network:

             Ferrite Bead
     +-------[  ]-------+
     |                  |
 VCC |                  | VCC_ANALOG
     |                  |
     |       C1         |   C2
     |       ||         |   ||
     +-------||---------|---||------+
             ||         |   ||      |
             ||         |   ||      |
           GND_DIGITAL  | GND_ANALOG|
                        |           |
                        +-----------+

In this example, the ferrite bead isolates the analog and digital power supplies, while capacitors C1 and C2 provide separate bypassing for the digital and analog sections, respectively. The analog and digital grounds are connected at a single point to minimize ground loop currents.

Decoupling vs. Bypassing

The terms “decoupling” and “bypassing” are often used interchangeably, but they have slightly different meanings:

• Decoupling: Decoupling refers to the practice of isolating noise and preventing it from coupling between different sections of a circuit or system. It involves techniques such as using separate power supplies, splitting power and ground planes, and using filters or ferrite beads to block noise propagation.

• Bypassing: Bypassing specifically refers to the use of capacitors to provide a low-impedance path for high-frequency currents, shunting them away from sensitive components and reducing power supply noise. It is a subset of decoupling techniques.

In practice, the term “bypassing” is commonly used to describe the overall process of using capacitors for power supply noise suppression, while “decoupling” encompasses a broader range of techniques for noise isolation and reduction.

Frequently Asked Questions (FAQ)

1. What is the purpose of power supply bypassing?

2. Power supply bypassing is used to provide a stable and clean power supply to integrated circuits by reducing power supply noise and voltage fluctuations. It helps ensure reliable operation and prevents signal integrity issues.

3. What types of capacitors are commonly used for bypassing?

4. Ceramic, tantalum, aluminum electrolytic, and film capacitors are commonly used for power supply bypassing. The choice depends on factors such as frequency range, current requirements, and available board space.

5. Where should bypassing capacitors be placed on a PCB?

6. Bypassing capacitors should be placed as close to the IC power pins as possible to minimize the loop area and inductance. They should be connected to the power and ground planes using short and wide traces and multiple vias.

7. How do I select the appropriate capacitor values for bypassing?

8. Use a combination of capacitor values to target different frequency ranges. Bulk capacitance (1 μF to 10 μF) provides low-frequency bypassing, medium-frequency bypassing (0.1 μF to 1 μF) targets the mid-frequency range, and high-frequency bypassing (1 nF to 100 nF) suppresses high-frequency noise.

9. What additional considerations are required for bypassing in mixed-signal and RF circuits?

10. Mixed-signal and RF circuits require separate analog and digital power supplies, separate bypassing capacitors for analog and digital sections, the use of ferrite beads or inductors to isolate high-frequency noise, and a star grounding scheme to minimize ground loop currents.

Conclusion

Power supply bypassing is a critical aspect of PCB design that ensures the reliable operation of electronic devices. By strategically placing capacitors near integrated circuits, designers can suppress power supply noise, minimize voltage fluctuations, and prevent signal integrity issues.

Effective power supply bypassing involves selecting the appropriate types and values of capacitors, proper placement and layout, and well-designed power and ground plane systems. Additional considerations are required for mixed-signal and RF circuits to prevent noise coupling and maintain signal integrity.

By following best practices and guidelines for power supply bypassing, PCB designers can create robust and reliable electronic systems that perform optimally in various applications.