Power Electronics: Buck, Boost and Thermal Design

Why switch?

A linear regulator wastes the difference between the input and output voltage as heat: dropping 12 V to 5 V at 2 A burns (12 − 5) × 2 = 14 W. A switching converter turns a transistor on and off hundreds of thousands of times a second and uses an inductor and a capacitor to store and release energy, so almost all of the power is delivered: the same job loses less than 1 W. The price is complexity: ripple, noise and layout sensitivity. This page explains how the common converters work and gives worked designs. The basics of regulators and batteries are in Batteries and Power Supplies.

	Linear (LDO)	Switching
Efficiency	Vout ÷ Vin: poor when the drop is large	85–97 percent, nearly independent of the drop
Output noise	Very low	Ripple and spikes at the switching frequency
Parts and cost	One chip and two capacitors	Controller, inductor, diode or MOSFET, capacitors
Voltage range	Output below input only	Step down, step up or both
Layout	Forgiving	Critical: loop area, ground, feedback routing
Best for	Low power, small drop, quiet analog rails	Higher power, battery life, large voltage change

The buck converter

The buck converter steps voltage down. When the switch is on, the input voltage drives the inductor and the current rises, storing energy in its magnetic field. When the switch turns off, the inductor keeps the current flowing through the diode (or a second, synchronous MOSFET) into the output, and the current falls. The capacitor smooths the output. The average output voltage is the input voltage times the duty cycle D (the fraction of the time the switch is on): V_out = D × V_in.

Quantity	Formula (continuous conduction)	Meaning
Duty cycle	D = Vout ÷ Vin	Fraction of the period that the switch is on
Inductor ripple current	ΔI = (Vin − Vout) × D ÷ (f × L)	The up-and-down swing of the inductor current
Peak inductor current	Ipk = Iout + ΔI ÷ 2	What the inductor and switch must withstand
Output ripple voltage	ΔV ≈ ΔI ÷ (8 × f × C)	Ideal capacitor: ESR adds ΔI × ESR
Input capacitor ripple current	IRMS = Iout × √(D × (1 − D))	Largest at D = 50 percent: 0.5 × Iout

A converter in continuous conduction mode (CCM) never lets the inductor current reach zero; below a certain load it enters discontinuous mode and many chips switch to a pulse-skipping or burst mode (PFM) to keep the light-load efficiency high.

Designing a buck: inductor and capacitors

Switching frequency	Smaller inductor and capacitors	Higher switching losses and EMI	Typical use
100–300 kHz	Larger parts	Low losses; easy layout	High-power converters; wide-input designs
400 kHz–1 MHz	Compact	Moderate	Most point-of-load and battery regulators
1–3 MHz and more	Very small	Higher losses; careful layout needed; avoids the AM radio band	Space-constrained and automotive designs

Losses and efficiency

Efficiency η = P_out ÷ (P_out + losses). The main losses are the conduction loss in the switches (I² × R_DS(on)), the voltage drop of the diode, the switching loss while the transistor changes state, the inductor’s winding and core losses, and the controller’s own supply current.

• Light-load efficiency is set by the controller’s quiescent current and the switching loss: use a chip with PFM or burst mode for battery products.
• Efficiency curves in the datasheet show the real figures: at light load they can fall to 60–70 percent.
• Heat is what limits the output current, not the electrical rating of the parts: see thermal design below.

Feedback divider and the controller

A controller chip compares a fraction of the output with an internal reference V_ref (often 0.6–1.25 V) and adjusts the duty cycle to make them equal. A resistor divider R1 (top) and R2 (bottom) sets the output: V_out = V_ref × (1 + R1 ÷ R2).

• Voltage mode, current mode and constant on-time controllers differ in speed and stability; current-mode chips are the easiest to use.
• Compensation keeps the loop stable: many chips have it built in, others need a resistor and capacitor chosen from the datasheet.
• Integrated modules and simple switchers (LM2596, MP1584, TPS54xxx, module regulators with the inductor included) reduce the design to a few parts.
• Enable, power-good and soft-start pins sequence the supply rails.

Boost, buck-boost and other topologies

Topology	Output	How it works	Typical use
Buck	Below the input	Switch in series, inductor, freewheel diode	Reducing 12–48 V to 5 V, 3.3 V, 1.8 V
Boost	Above the input	The inductor charges from the input when the switch is on; when it opens, the inductor voltage adds to the input	One lithium cell to 5 V; LED strings
Buck-boost	Inverted (or above or below)	One inductor, switch and diode; output polarity reversed in the basic form	Negative rails; battery that falls above and below the output
SEPIC, Ćuk, four-switch buck-boost	Above or below, not inverted	Two inductors or capacitors, or four switches	A battery that crosses the output voltage (a lithium cell to 3.3 V)
Flyback	Any, isolated	A coupled inductor (transformer) stores energy and releases it to the secondary	Mains adaptors, isolated supplies up to 100 W
Forward, half- and full-bridge, LLC	Isolated, higher power	Transformer-coupled topologies	Server and industrial supplies
Charge pump	Multiple of the input, low current	Capacitors switched in stages; no inductor	Small negative rails, 3.3 V to 5 V at low current

Thermal design

Heat in a part raises its temperature above the surroundings by the dissipated power times the thermal resistance: T_j = T_a + P × θ_JA, where θ_JA (junction to ambient, °C/W) depends on the package and the copper area under it. Keep the junction below the maximum (usually 125 °C) with margin, and remember that ambient inside an enclosure is higher than in the room.

• Spread the heat: use a large copper pour under the thermal pad, with vias to inner layers or the other side. A good pad can reduce θ_JA from 80 to below 30 °C/W.
• Derate with temperature: the output current rating falls at high ambient temperature (read the derating curve).
• Look at the inductor, too: its winding loss and core loss heat it; a saturated inductor is a short.
• Measure. A thermal camera or a thermocouple on the chip case under full load tells more than any calculation.

Layout and noise

A switching converter makes sharp edges: the current in the input loop changes from zero to the full load in a few nanoseconds. The stray inductance of the wiring then produces a voltage spike V = L × di/dt that radiates and rings.

• Keep the hot loop tiny: the input capacitor, high-side switch and low-side switch (or diode) must be adjacent, with the shortest, widest traces.
• One solid ground plane under the converter, with the input and output grounds returning to it close to the chip.
• Keep the switch node small: it has fast voltage edges and is the main source of radiated noise. Do not route sensitive signals under it.
• Route the feedback trace away from the switch node and the inductor, on a quiet layer, from the output capacitor to the divider.
• Follow the datasheet layout example exactly if there is one.
• Add input filtering (a ferrite bead and capacitor, or an LC filter) when the converter shares a supply with analog or radio circuits.
• Shield and bond the enclosure and the cable shields as described in Cabling, Grounding and Noise.

Protection and start-up

Feature	What it does	Notes
Under-voltage lockout (UVLO)	Keeps the converter off until the input is high enough	Prevents chattering from a sagging battery; set with a divider on the enable pin
Over-current protection	Limits the peak (or average) switch current	Hiccup mode retries after a short; cycle-by-cycle limits protect the inductor
Thermal shutdown	Turns off above about 150 °C	A last resort, not a design aim
Soft start	Ramps the output slowly	Limits the inrush current into the output capacitors
Over-voltage protection	Clamps or shuts down on a high output	Protects the load from a feedback fault
Input protection	A fuse, a TVS diode, reverse-polarity protection	See Switches, Connectors and Protection
Input inrush	The input capacitors charge at the plug-in moment	A hot-plug onto a long cable can ring to twice the supply: use a bulk electrolytic or a TVS

Choosing a regulator

Need	Choice	Why
Quiet analog rail, small drop, under 100 mA	LDO	Low noise, no inductor
12–24 V to 3.3 V or 5 V, 0.5 A or more	Buck converter or module	Low heat
One lithium cell to 3.3 V	Buck-boost or low-dropout buck	The cell crosses 3.3 V
Cell to 5 V	Boost	Output above input
Mains to low voltage	Buy a certified adapter or module	Safety and approvals
Noise-sensitive load on a high-input rail	Buck followed by an LDO	Efficiency and a clean output
Prototype in a hurry	A ready-made buck module (MP1584, LM2596)	Tested layout; check the specifications

Common faults

Symptom	Likely cause	Try
No output	Enable low, UVLO not satisfied, wrong feedback divider, blown input	Check the enable pin and the input voltage at the chip
Output too high or too low	Wrong feedback resistors, a floating feedback pin	Check the divider values and the connection
Excessive ripple or noise	Too small or too high-ESR output capacitor, long hot loop, an inductor that saturates	Add ceramic capacitors; shorten loops; check the inductor’s saturation current
Converter hot or in thermal shutdown	Overload, a diode instead of a synchronous switch, poor copper, a high ambient	Measure the current; add copper; improve airflow
Hiccups or squeals under light load	Pulse skipping or burst mode, an audible ceramic capacitor	Force PWM mode, or change the capacitor type
Output droops when the load steps	Not enough output capacitance, slow loop	Increase the capacitance or use a faster controller
Chip destroyed at power-up	Input spike above the rating (hot-plug ringing), reversed supply	Add a TVS or a bulk capacitor; check the polarity protection
Radio or sensor interference	Switch-node noise, a long cable acting as an antenna	Layout, a snubber, a filter, a different frequency
Battery life shorter than expected	Quiescent current, light-load inefficiency, feedback divider drain	Measure the no-load current; pick a low-quiescent part

Related

• Power and Motor Electronics: the overview of this guide.
• Batteries and Power Supplies: cells, linear regulators and supply basics.
• Inductors, Transformers and Relays and Capacitors: the parts a converter is built from.
• Diodes and LEDs and Transistors: the switch and the rectifier.
• Driving Motors and Actuators: MOSFET switching and flyback in motor drives.
• PCB Design and Layout: trace width, copper area and layout.
• Resistor Network and Standard Value Calculator.

FAQ

How do I choose between a linear and a switching regulator?

If the voltage drop is small, the current is low or the rail must be very quiet, use an LDO. If the drop is large or the current is above a few hundred milliamperes, use a switcher. The loss of a linear regulator is (V_in − V_out) × I.

What inductor value should I use?

Choose it for a ripple current of 20–40 percent of the load current: L = (V_in − V_out) × D ÷ (f × ΔI). Then check the saturation current is above the peak and the heating current above the load current.

Why do ceramic capacitors lose capacitance?

High-permittivity ceramics (X5R, X7R) lose a large part of their capacitance when a DC voltage is applied, and more in small packages. Use the datasheet’s DC-bias curve and allow for the loss.

Do I need a synchronous converter?

At low output voltages and high currents the diode loss is large, so synchronous is worthwhile (and most modern chips are). At light load a diode converter can be more efficient than a synchronous one in forced-PWM mode.

Can I power a microcontroller straight from a buck converter?

Yes, with a clean layout and output capacitors. For an analog circuit or ADC reference, filter the output with an LC filter or follow the converter with an LDO.

Is it safe to design a mains converter?

Not without the safety standards, creepage and clearance rules, and approvals. Use a certified module or adaptor; do not work on live mains equipment.

References

• Erickson, R. W. and Maksimovic, D., Fundamentals of Power Electronics, 3rd ed., Springer.
• Texas Instruments, Basic Calculation of a Buck Converter’s Power Stage (SLVA477) and Thermal Design by Insight, Not Hindsight (SNVA419).
• Analog Devices and onsemi application notes on buck and boost design, layout and EMI; controller datasheets (TPS5430, LM2596, MP1584, TPS61xxx).
• Horowitz, P. and Hill, W., The Art of Electronics, 3rd ed., Cambridge University Press: chapter 9, power supplies.

Last reviewed: October 2026. Values are typical: check the datasheets of the parts you use. Do not work on live mains equipment unless you are qualified.