1. The correct PCB layout is crucial for a successful power supply design. The non-isolated power section serves as the foundational element of the power system. Grasping the current flow direction and understanding the construction of the high-frequency loop are arguably the most critical steps in PCB design.
2. This article explores power supply design techniques for buck, boost, and single-ended primary inductance converter (SEPIC) power sections.
3. Step-down converter
4. We begin with a buck converter, which provides an output voltage that is lower than its input voltage. Figure 1 presents the schematic and PCB layout for this buck converter.
5. This simplified schematic includes input and output capacitors, inductors, switching transistors, and blocking diodes.
6. During the on period of the pulse width modulator (PWM), current flows along the path indicated by the green arrow, from the input capacitor to the inductor through the switching transistor. In the PWM off period, current continues to flow through the inductor, following the route marked by the pink arrow. This indicates that the output maintains a continuous current flow.
1. The input high-frequency current is activated and deactivated once in each cycle. The most critical aspect of this power section layout is minimizing high-frequency loops. The blue arrow at the top illustrates this loop. When the transistor conducts, the current briefly flows through diode D1 to ground. If the input capacitors are not closely positioned, this significant current surge may lead to design issues.
2. Ensure that the power trace or plane is sufficiently wide to handle the power current. Generally, the power plane should be maximized except around the switch node. A substantial dV/dt signal exists at the switch node, which can couple into other areas of the PCB layout. Thus, reducing its surface area is essential for effective design. Utilize multiple vias to connect power planes across different layers. A helpful guideline is that each via (with a 10mil drill) should not exceed 1A. Creating a continuous ground plane as large as the PCB will help diminish noise and high-frequency loops.
3. Boost converter: The boost converter is employed to produce a higher output voltage from a lower input voltage. You can apply the same approach in a boost converter as in a buck converter to pinpoint critical paths and loops. During the PWM on period, current flows from the input terminal to the switching transistor (indicated by the green arrow) through the inductor. During this time, energy builds up in the inductor and is then transferred to the output when the PWM turns off. The current then flows along the pink arrow, from input to output, indicating that input current is continuous while output current exhibits high-frequency switching. To minimize high-frequency noise, the loop depicted in blue must be kept as short as possible.
4. While the transistor conducts, the current flows briefly from the output to ground via the diode. If this current is not effectively shunted by the output capacitor, it can create issues in power supply design. General layout techniques used for buck converters are also applicable here. Minimize the switch node area and use multiple vias to connect to the ground plane.
5. SEPIC converter: The SEPIC converter is suitable when the input voltage is either higher or lower than the output voltage. This type of power converter can act as a boost when the input voltage is lower and as a step-down when it is higher. This circuit utilizes two inductors or a single coupling inductor.
6. With two inductors, there are two current paths for each switching cycle. During the PWM on period, current flows along the green arrow, accumulating energy in the inductor. When PWM turns off, energy is delivered to the output via the pink current path. In this SEPIC design, the input current remains continuous, while the output experiences high-frequency switching currents, necessitating minimized loops. It’s advisable to place a via near the output capacitor for connecting to the ground plane. The ground plane provides a low-impedance route between all PCB components, thus reducing noise.
7. Concluding remarks: Designing power layouts is a complex endeavor. The initial step is to ascertain the current flow in the power supply and subsequently identify and minimize high-frequency loops. Then, employ the ground plane and power plane to interconnect PCB components with low impedance. Ensure the plane is wide enough to accommodate the designed current. Keep the high-frequency switching node as compact as possible to reduce noise coupling risks to other signals. Using multiple vias to connect extensive, continuous ground planes across various devices can also enhance design quality.
2. This article explores power supply design techniques for buck, boost, and single-ended primary inductance converter (SEPIC) power sections.
3. Step-down converter
4. We begin with a buck converter, which provides an output voltage that is lower than its input voltage. Figure 1 presents the schematic and PCB layout for this buck converter.
5. This simplified schematic includes input and output capacitors, inductors, switching transistors, and blocking diodes.
6. During the on period of the pulse width modulator (PWM), current flows along the path indicated by the green arrow, from the input capacitor to the inductor through the switching transistor. In the PWM off period, current continues to flow through the inductor, following the route marked by the pink arrow. This indicates that the output maintains a continuous current flow.
1. The input high-frequency current is activated and deactivated once in each cycle. The most critical aspect of this power section layout is minimizing high-frequency loops. The blue arrow at the top illustrates this loop. When the transistor conducts, the current briefly flows through diode D1 to ground. If the input capacitors are not closely positioned, this significant current surge may lead to design issues.
2. Ensure that the power trace or plane is sufficiently wide to handle the power current. Generally, the power plane should be maximized except around the switch node. A substantial dV/dt signal exists at the switch node, which can couple into other areas of the PCB layout. Thus, reducing its surface area is essential for effective design. Utilize multiple vias to connect power planes across different layers. A helpful guideline is that each via (with a 10mil drill) should not exceed 1A. Creating a continuous ground plane as large as the PCB will help diminish noise and high-frequency loops.
3. Boost converter: The boost converter is employed to produce a higher output voltage from a lower input voltage. You can apply the same approach in a boost converter as in a buck converter to pinpoint critical paths and loops. During the PWM on period, current flows from the input terminal to the switching transistor (indicated by the green arrow) through the inductor. During this time, energy builds up in the inductor and is then transferred to the output when the PWM turns off. The current then flows along the pink arrow, from input to output, indicating that input current is continuous while output current exhibits high-frequency switching. To minimize high-frequency noise, the loop depicted in blue must be kept as short as possible.
4. While the transistor conducts, the current flows briefly from the output to ground via the diode. If this current is not effectively shunted by the output capacitor, it can create issues in power supply design. General layout techniques used for buck converters are also applicable here. Minimize the switch node area and use multiple vias to connect to the ground plane.
5. SEPIC converter: The SEPIC converter is suitable when the input voltage is either higher or lower than the output voltage. This type of power converter can act as a boost when the input voltage is lower and as a step-down when it is higher. This circuit utilizes two inductors or a single coupling inductor.
6. With two inductors, there are two current paths for each switching cycle. During the PWM on period, current flows along the green arrow, accumulating energy in the inductor. When PWM turns off, energy is delivered to the output via the pink current path. In this SEPIC design, the input current remains continuous, while the output experiences high-frequency switching currents, necessitating minimized loops. It’s advisable to place a via near the output capacitor for connecting to the ground plane. The ground plane provides a low-impedance route between all PCB components, thus reducing noise.
7. Concluding remarks: Designing power layouts is a complex endeavor. The initial step is to ascertain the current flow in the power supply and subsequently identify and minimize high-frequency loops. Then, employ the ground plane and power plane to interconnect PCB components with low impedance. Ensure the plane is wide enough to accommodate the designed current. Keep the high-frequency switching node as compact as possible to reduce noise coupling risks to other signals. Using multiple vias to connect extensive, continuous ground planes across various devices can also enhance design quality.
