Switched-Mode Power Supply Design

Understanding Switched-Mode Power Supply Fundamentals

Switched-mode power supplies (SMPS) have become the cornerstone of modern electronic systems, replacing traditional linear regulators in applications ranging from consumer electronics to industrial automation equipment. For engineering firms across Atlantic Canada, understanding SMPS design principles is essential as regional industries—from offshore energy operations to advanced manufacturing—increasingly demand efficient, reliable power conversion solutions.

Unlike linear power supplies that regulate voltage by dissipating excess energy as heat, switched-mode power supplies operate by rapidly switching a power transistor on and off at frequencies typically ranging from 50 kHz to several megahertz. This switching action, combined with energy storage elements such as inductors and capacitors, enables power conversion efficiencies of 85% to 95% or higher—a critical advantage for battery-powered devices and energy-conscious industrial applications throughout Nova Scotia's growing technology sector.

The fundamental principle behind SMPS operation involves storing energy in an inductor or transformer during the "on" period of the switching cycle, then releasing that energy to the load during the "off" period. The duty cycle—the ratio of on-time to total switching period—directly controls the output voltage, allowing precise regulation through pulse-width modulation (PWM) techniques.

Common SMPS Topologies and Their Applications

Selecting the appropriate topology is one of the most critical decisions in switched-mode power supply design. Each configuration offers distinct advantages depending on input/output voltage relationships, power levels, isolation requirements, and efficiency targets.

Buck (Step-Down) Converters

Buck converters reduce a higher input voltage to a lower output voltage, making them ideal for powering microprocessors, memory chips, and other low-voltage components from standard 12V or 24V rails. Modern synchronous buck converters achieve efficiencies exceeding 95% at full load, with typical switching frequencies between 200 kHz and 2 MHz. These converters are commonly found in telecommunications equipment deployed across Maritime provinces, where power efficiency directly impacts operating costs and thermal management requirements.

Boost (Step-Up) Converters

Boost converters increase voltage from a lower input to a higher output, essential for applications such as LED backlighting, battery-powered systems requiring stable voltage as battery charge depletes, and power factor correction (PFC) front-end stages. A typical boost converter for LED lighting applications might convert a 12V input to a regulated 48V output at currents up to 2A, with efficiency ratings around 92% to 94%.

Buck-Boost and SEPIC Converters

When input voltage may be higher or lower than the required output, buck-boost or Single-Ended Primary-Inductor Converter (SEPIC) topologies provide flexibility. SEPIC converters offer the additional advantage of non-inverted output polarity and input-to-output isolation through the coupling capacitor, making them popular in automotive and portable equipment applications.

Flyback Converters

Flyback converters dominate the isolated low-power market, typically serving applications from 5W to 150W. Their simplicity—requiring only a single magnetic component that functions as both transformer and inductor—makes them cost-effective for consumer electronics, auxiliary power supplies, and industrial control systems. Many of the embedded systems used in Nova Scotia's ocean technology sector rely on flyback converters for isolated power rails.

Forward, Half-Bridge, and Full-Bridge Converters

Higher power applications demand more sophisticated topologies. Forward converters serve the 100W to 500W range efficiently, while half-bridge and full-bridge configurations handle power levels from 500W to several kilowatts. Full-bridge converters with phase-shifted PWM control achieve soft-switching operation, reducing switching losses and electromagnetic interference (EMI) while enabling switching frequencies above 100 kHz even at multi-kilowatt power levels.

Critical Component Selection for Reliable SMPS Design

Component selection profoundly impacts SMPS performance, reliability, and longevity. Engineers must carefully analyse the operating conditions and select components rated appropriately for the application's environmental and electrical stresses.

Power Semiconductors

Modern SMPS designs typically employ MOSFETs for switching frequencies up to several megahertz, with gallium nitride (GaN) devices enabling frequencies beyond 10 MHz for high-density applications. Key parameters include on-resistance (RDS(on)), gate charge (Qg), and drain-source voltage rating. For a typical 100W flyback converter operating from universal AC input (85-265VAC), a 650V or 800V MOSFET with RDS(on) below 1Ω provides adequate margin for voltage spikes while minimising conduction losses.

Silicon carbide (SiC) MOSFETs and diodes have gained prominence in high-voltage, high-power applications, offering superior switching performance and reduced losses compared to silicon devices. These components are increasingly specified for electric vehicle charging infrastructure being deployed across Atlantic Canada.

Magnetic Components

Transformers and inductors represent perhaps the most challenging aspect of SMPS design. Core material selection—whether ferrite, powdered iron, or amorphous metal—depends on switching frequency, power level, and efficiency requirements. For a typical 65W laptop adapter operating at 100 kHz, an EE25 or EE30 ferrite core with an effective permeability around 2000 provides adequate inductance while maintaining core losses below acceptable limits.

Proper winding techniques are essential to minimise leakage inductance and AC resistance. Interleaved primary and secondary windings can reduce leakage inductance by 50% or more compared to simple layer-wound constructions, directly improving efficiency and reducing voltage spikes that stress semiconductor devices.

Capacitors

Output capacitor selection balances equivalent series resistance (ESR), ripple current rating, and capacitance value. Modern SMPS designs often combine electrolytic capacitors for bulk energy storage with ceramic capacitors for high-frequency filtering. For a 12V/5A output, a typical specification might include 2 x 470µF/25V electrolytic capacitors with ESR below 50mΩ, paralleled with 4 x 22µF ceramic capacitors to achieve acceptable output ripple below 50mV peak-to-peak.

Electromagnetic Compatibility and Regulatory Compliance

Switched-mode power supplies inherently generate electromagnetic interference due to their high-frequency switching operation. Meeting regulatory requirements—including Industry Canada ICES-003 for Canadian markets and FCC Part 15 for products sold in the United States—requires careful attention to EMI mitigation throughout the design process.

Conducted EMI Mitigation

Conducted emissions, measured from 150 kHz to 30 MHz, typically present the greatest compliance challenges. Input filtering using common-mode and differential-mode filter stages attenuates noise before it reaches the AC mains. A typical EMI filter for a 100W SMPS might include a 10mH common-mode choke, 0.1µF Y-capacitors rated for 250VAC, and 0.47µF X-capacitors, achieving 40-50 dB of attenuation at the fundamental switching frequency.

Radiated EMI Considerations

Radiated emissions, measured from 30 MHz to 1 GHz, often result from high-frequency ringing on switching nodes and inadequate shielding. Proper PCB layout—including minimised high-current loop areas, continuous ground planes, and appropriate trace routing—reduces radiated emissions at the source. Snubber circuits across rectifier diodes and switching transistors damp parasitic oscillations that contribute to high-frequency radiation.

Safety Certifications

Products intended for the Canadian market must comply with CSA C22.2 safety standards, with specific requirements depending on the application. Medical power supplies must meet IEC 60601-1 requirements, including reinforced isolation with minimum 4mm creepage distances and 8mm clearance between primary and secondary circuits. Industrial equipment typically references IEC 62368-1, which replaced the legacy IEC 60950-1 and IEC 60065 standards.

Thermal Management and Reliability Engineering

Effective thermal management directly impacts SMPS reliability and operational lifespan. For electronics deployed in Atlantic Canada's climate—where ambient temperatures may range from -35°C in winter to 35°C in summer—robust thermal design ensures consistent performance across all operating conditions.

Power Loss Analysis

Comprehensive power loss budgets identify thermal challenges early in the design process. For a typical 150W SMPS operating at 92% efficiency, approximately 13W of heat must be dissipated. Major loss contributors include switching and conduction losses in power semiconductors, core and copper losses in magnetic components, and ESR losses in capacitors. Detailed loss calculations enable targeted thermal management efforts where they provide maximum benefit.

Thermal Interface Materials and Heat Sinking

Power semiconductors often require thermal interface materials and heat sinks to maintain junction temperatures within safe limits. For a MOSFET dissipating 3W with a maximum junction temperature of 150°C and ambient temperature of 50°C, the total thermal resistance from junction to ambient must remain below 33°C/W. This typically necessitates a heat sink with thermal resistance below 10°C/W when accounting for junction-to-case and interface material thermal resistances.

Reliability Prediction and Testing

Mean time between failures (MTBF) calculations using standards such as MIL-HDBK-217F or Telcordia SR-332 provide reliability estimates for design comparison and system-level planning. Highly accelerated life testing (HALT) subjects prototypes to combined thermal cycling and vibration stresses, revealing design weaknesses before production. For mission-critical applications in maritime or offshore environments, these reliability engineering practices prove invaluable.

Advanced Control Techniques and Digital Power Management

Modern SMPS designs increasingly incorporate digital control, enabling sophisticated power management features impossible with analogue controllers alone.

Digital PWM Controllers

Digital signal processors (DSPs) and microcontrollers with high-resolution PWM peripherals enable advanced control algorithms, including adaptive dead-time control, digital PID compensation with automatic tuning, and non-linear control techniques for improved transient response. Texas Instruments' C2000 series and Microchip's dsPIC families provide dedicated peripherals for digital power control with PWM resolutions below 1 nanosecond.

Power Factor Correction

Regulatory requirements, including IEC 61000-3-2, mandate power factor correction for equipment drawing more than 75W from AC mains. Active PFC circuits—typically boost converters operating in continuous conduction mode—achieve power factors above 0.99 and total harmonic distortion below 5%. Combined PFC and DC-DC stages using single-stage topologies or integrated controllers reduce component count and improve efficiency.

Synchronous Rectification and Soft Switching

Replacing output diodes with synchronous MOSFETs reduces rectification losses, particularly in low-voltage, high-current applications. Secondary-side synchronous rectification in a 5V/20A output stage might reduce rectifier losses from 15W (using Schottky diodes) to under 3W, significantly improving efficiency. Zero-voltage switching (ZVS) and zero-current switching (ZCS) techniques further reduce switching losses, enabling higher switching frequencies and smaller magnetic components.

Partner with Sangster Engineering Ltd. for Your Power Electronics Projects

Designing high-performance switched-mode power supplies requires expertise spanning multiple engineering disciplines—from power electronics and magnetics design to thermal management and EMC compliance. Whether you're developing new products, upgrading existing systems, or troubleshooting field failures, having an experienced engineering partner makes the difference between success and costly delays.

Sangster Engineering Ltd., based in Amherst, Nova Scotia, provides comprehensive electronics engineering services to clients throughout Atlantic Canada and beyond. Our team brings decades of experience in power supply design, helping clients navigate the complexities of topology selection, component specification, regulatory compliance, and production optimisation.

From initial concept development through prototype validation and production support, we work alongside your team to deliver power conversion solutions that meet your performance, reliability, and cost objectives. Our central location in the Maritimes enables us to serve clients across Nova Scotia, New Brunswick, Prince Edward Island, and Newfoundland while maintaining strong partnerships with manufacturers and suppliers nationwide.

Contact Sangster Engineering Ltd. today to discuss your switched-mode power supply design requirements. Let our expertise in power electronics engineering help bring your next project from concept to successful production.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.