DC-DC Converter Basics: Efficiency, Ripple, and Layout Tips

DC-DC Converter Basics: Efficiency, Ripple, and Layout Tips

DC-DC converters (switching regulators) are the standard way to power modern electronics efficiently. But they’re also one of the easiest circuits to mess up. Most problems come from layout, not from “wrong math.” When a DC-DC converter is wrong, you’ll see: unstable output, random MCU resets, high ripple, overheating, EMI failures, or audible whining.

This guide explains the basics of DC-DC converters and gives practical, real-world tips for efficiency, ripple control, and PCB layout.

⸻

What is a DC-DC converter?

A DC-DC converter converts one DC voltage to another using high-frequency switching plus energy storage.

Common types:
• Buck: step down (12V → 5V)
• Boost: step up (3.7V → 5V)
• Buck-boost: step up or step down (battery range → fixed output)

Core parts you’ll almost always see:
• Switching IC (controller/regulator)
• Inductor
• Input capacitor
• Output capacitor
• Sometimes a diode (or synchronous MOSFETs inside the IC)

⸻

Efficiency: what actually determines it?

Efficiency isn’t a fixed number. It changes with input voltage, load current, and component choices.

Main sources of loss
1. Conduction loss

• MOSFET on-resistance (internal or external)
• Inductor DCR (copper loss)
• PCB trace resistance

These losses grow with current:
• roughly I² × R

2. Switching loss

• MOSFET gate charge and switching transitions
• higher at higher switching frequency
• increases with voltage and frequency

3. Diode loss (non-synchronous designs)

• power loss ≈ Vf × I
Schottky helps, but diode loss can still be significant at high current.

4. Capacitor loss

• ESR in capacitors causes ripple heating (especially output cap)

⸻

Ripple: what it is and why it matters

Ripple is the “wiggle” on your output voltage caused by switching and load pulses.

Too much ripple can cause:
• ADC measurement noise
• RF module instability
• MCU brown-out resets
• EMI problems

Ripple comes from two main things:
• Inductor ripple current
• Output capacitor ESR and capacitance

Good output ripple usually needs:
• correct inductor value (not too small)
• adequate output capacitance
• low ESR caps in the right places
• clean, tight layout

⸻

Component selection: practical rules that avoid pain

1) Inductor selection (most important)

Key specs:
• Inductance (µH): use the regulator datasheet recommended range
• Saturation current (Isat): must be above peak current
• RMS/continuous current rating: must cover your load
• DCR: lower DCR = higher efficiency

Common mistake:
Choosing an inductor with enough “average current” rating but too low saturation current. Under load, inductance collapses and ripple spikes.

Quick rule:
• Choose Isat with comfortable headroom above peak current (not just average).

⸻

2) Input capacitor selection

The input cap handles sharp pulses from the switch. If it’s wrong or placed poorly:
• input voltage dips
• EMI increases
• regulator becomes unstable

Best practice:
• Place a ceramic MLCC (X7R) very close to VIN and GND pins
• Add bulk electrolytic/polymer if supply is far away or cable is long

Common mistake:
Only using a bulk electrolytic far away. You still need a close ceramic.

⸻

3) Output capacitor selection

The output cap smooths the inductor current into a stable voltage.

Best practice:
• Use the exact capacitor type and value recommended by the datasheet as your starting point
• Often you’ll use multiple ceramics in parallel (lower ESR and ESL)
• Sometimes you’ll add electrolytic/polymer for better bulk stability

Common mistake:
Picking “any 10µF cap” without checking voltage rating and DC bias. Some 10µF ceramics become much less in real operation.

⸻

4) Switching frequency choice

Higher frequency:
• smaller inductor and caps possible
• but more switching loss and EMI risk

Lower frequency:
• better efficiency often
• but larger magnetics and output ripple can increase if design isn’t tuned

If you’re not sure:
• follow the regulator’s default design and reference layout first

⸻

PCB layout: the #1 reason converters fail

Even with perfect parts, bad layout can destroy performance.

The most important concept: “hot loop”

There is a high-current switching loop (the “hot loop”) that must be as small as possible.

In a buck converter, the hot loop usually includes:
• input capacitor
• high-side switch
• low-side switch (or diode)
• ground return back to the input capacitor

If this loop is big, you get:
• huge EMI
• voltage spikes
• unstable switching
• heat

⸻

Practical layout rules (do these every time)

1) Put the input ceramic capacitor closest to the IC
• The input cap ground should connect to the IC ground with the shortest path.
• This is non-negotiable.

2) Keep the switch node short and isolated

The “SW” node is noisy. Keep it:
• short
• away from sensitive signals (ADC, RF, clocks)
• not under signal traces if possible

3) Place the inductor close to the switch node

Short path from SW pin → inductor → output capacitor.

4) Put output capacitors close to the inductor and feedback point

Output caps should sit close to:
• inductor output
• regulator ground reference

5) Feedback routing must be clean

The feedback pin is sensitive. Route it:
• away from the switch node
• from the quiet output voltage point (after the output cap)
• with a clean ground reference

6) Use solid ground and correct return paths
• A good ground plane is your friend.
• Avoid splitting ground unless you truly know why.
• Ensure current returns don’t pass through sensitive analog ground areas.

⸻

Troubleshooting symptoms (fast diagnosis)

Output ripple is high

Check:
• output capacitor value/type (DC bias)
• ESR and placement
• inductor saturation
• loop area and grounding

Converter runs hot

Check:
• Rds(on) / diode loss
• inductor DCR and saturation
• switching frequency too high
• airflow and copper area

MCU resets when load turns on

Check:
• input voltage droop (need better input bulk + close ceramic)
• soft-start/inrush
• ground bounce from poor layout

Audible whining

Often caused by:
• inductor mechanical vibration
• operating in discontinuous mode at light load
• poor filtering causing current ripple

Fix may include:
• different inductor type
• forced PWM mode (if supported)
• adding output capacitance or changing compensation (advanced)

⸻

Simple “safe design process” (recommended)
1. Start with the regulator datasheet “typical application”
2. Copy the reference PCB layout as closely as possible
3. Use the recommended inductor and caps first
4. Only optimize cost/size after it works reliably
5. Validate under worst-case:
• max input voltage
• max load current
• hot ambient temperature
• fast load steps

⸻

Quick checklist before you finalize a DC-DC BOM
• Inductor Isat and current rating are safe
• Input ceramic cap is correct value and placed very close
• Output caps match recommended value/type
• Feedback routing is clean and quiet
• Switch node is short and isolated
• Thermal dissipation is acceptable on your PCB
• Parts are available and second-source is possible