Selection and Layout of Peripheral Components for Switching Power Supply (DCDC) Design

Here’s a detailed, professional guide on selection and layout of peripheral components for a DC–DC switching power supply, covering design principles, key components, and PCB layout considerations.

1. Introduction

A DC–DC switching power supply converts one DC voltage to another efficiently using high-frequency switching, energy storage components, and control circuits. While the controller (IC or MCU) handles regulation, peripheral components such as inductors, capacitors, resistors, diodes, and MOSFETs determine efficiency, stability, ripple, and EMI performance. Proper selection and layout of these peripherals is crucial to achieving a reliable, efficient design.

2. Key Peripheral Components

2.1 Inductors

• Function: Store energy and filter current; determine ripple and dynamic response.

• Selection criteria:

  • Saturation current (Isat): Must exceed peak inductor current.

  • DC resistance (DCR): Lower DCR reduces conduction loss but may increase size.

  • Core type: Ferrite for high-frequency operation; powdered iron for low EMI.

  • Inductance value: Balances ripple current and transient response.

• Practical tip: Ensure inductors are rated for continuous current above max load with margin (typically 20–30%).

2.2 Capacitors

• Function: Filter voltage ripple, store energy, and provide dynamic response.

• Types and placement:

  • Input capacitors: Low ESR ceramic (X7R, C0G) to reduce input ripple and EMI.

  • Output capacitors: Mix of ceramics and electrolytics/tantalum to balance ESR for stability.

  • Bulk capacitors: For load transient support; usually electrolytic or polymer.

• Selection criteria:

  • Capacitance & voltage rating: Ensure ripple voltage is acceptable.

  • ESR & ESL: Low ESR for fast response, low ESL to reduce switching spikes.

2.3 Diodes

• Function: Rectification in non-synchronous designs; freewheeling paths in synchronous ones.

• Selection criteria:

  • Reverse recovery time (trr): Low trr for fast switching to reduce losses.

  • Forward voltage drop: Minimize conduction loss.

  • Current rating: Exceed peak current with safety margin.

• Common choices: Schottky diodes for low-voltage, fast-switching; Si diodes or synchronous MOSFETs for higher voltage stages.

2.4 MOSFETs / Switching Devices

• Function: High-speed switching of input voltage to control output.

• Selection criteria:

  • Rds(on): Low to reduce conduction loss.

  • Gate charge (Qg): Low for fast switching with minimal driver loss.

  • Voltage and current rating: Must exceed maximum operating conditions with margin.

  • Thermal rating: Consider heat dissipation and cooling options.

• Tip: For high-efficiency designs, synchronous rectification with MOSFETs is preferred over diodes.

2.5 Resistors

• Function: Voltage sensing, feedback, soft-start, and current limiting.

• Selection criteria:

  • Power rating: Must handle expected current and power dissipation.

  • Tolerance: Tight tolerance (1% or better) for voltage dividers and feedback networks.

  • Temperature coefficient: Stable for accurate feedback in varying temperatures.

3. Layout Guidelines for Peripheral Components

3.1 General Principles

• Keep high-current loops as short and wide as possible.

• Place decoupling capacitors close to IC power pins.

• Separate switching node traces from sensitive analog nodes (feedback, error amplifier).

• Avoid long loops for MOSFET switching to reduce EMI.

3.2 Power Loop Layout

• Input loop: VIN → MOSFET → diode/synchronous MOSFET → capacitor → ground. Keep this loop minimal in area.

• Output loop: Inductor → capacitor → load. Place output capacitors close to the load if possible.

• Use wide copper planes or thick traces for high-current paths.

3.3 Grounding

• Star ground: Connect analog and power grounds at a single point (usually near IC) to avoid noise coupling.

• Separate planes: Analog and power grounds should meet only at the star point.

• Return paths: High-frequency return currents should flow directly under the switching trace.

3.4 Placement of Sensitive Components

• Feedback resistors & compensation network: Place near the controller IC.

• Voltage sense traces: Keep short and away from high dV/dt switching nodes.

• Gate resistors: Place close to MOSFET gate to control ringing and EMI.

3.5 Thermal Considerations

• Provide copper pours or thermal vias under MOSFETs, diodes, and power resistors.

• Use heat sinks or plane spreads to distribute heat evenly.

• Keep sensitive analog components away from high-heat sources.

4. EMC and Noise Reduction

• Place input/output filtering capacitors close to connectors.

• Use common-mode chokes if EMI is high.

• Minimize loop areas of fast-switching nodes.

• Shield analog traces and avoid crossing high-speed switching paths.

5. Practical Example: Buck Converter Layout

1. High-current path: VIN → high-side MOSFET → switching node → inductor → output capacitor → GND.

2. Controller IC: Close to MOSFET gate/drain with decoupling capacitor.

3. Feedback network: Connect directly to IC’s feedback pin, away from switching node.

4. Ground plane: Full-plane copper with star grounding for analog reference.

5. Thermal vias: Under MOSFET and diode for heat dissipation.

6. Summary

Proper selection and layout of peripheral components in DC–DC switching power supplies are critical for:

• Efficiency

• Stability and transient response

• EMI compliance

• Thermal reliability

Key tips: choose components rated for maximum currents and voltages, minimize high-current loop areas, separate sensitive analog nodes, and carefully manage thermal paths.