Section 4 - Power Transformer Design

Nov 2, 2026

Uploaded by Jacob

Section 4 focuses on the design of power transformers used in buck-derived switch-mode power supply topologies, covering forward, bridge, half-bridge, and full-wave center-tap configurations. It explains transformer functions such as turns-ratio selection, multi-output secondary design, and primary-to-secondary isolation. The section analyzes energy storage in leakage and mutual (magnetizing) inductance, undesirable effects including switching delays and voltage spikes causing EMI and component stress, and the role of snubbers and clamps. It further discusses transformer losses, hot-spot temperature rise limits, thermal resistance components, and implications for core sizing and efficiency.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages13

Reading Time33 min

AccessFree to download

Functions of a Transformer

Energy Storage in a Transformer

Leakage Inductance

Mutual (Magnetizing) Inductance

Undesirable Effects of Energy Storage

Losses and Temperature Rise

Temperature Rise Limit

Thermal Resistance

Section 4 – Power Transformer Design Power Transformer Design This Section covers the design of power transformers used in buck-derived topologies: forward converter, bridge, half-bridge, and full-wave centertap. Flyback transformers (actually coupled inductors) are covered in a later Section. For more specialized applications, the principles discussed herein will generally apply. Functions of a Transformer The purpose of a power transformer in SwitchMode Power Supplies is to transfer power efficiently and instantaneously from an external electrical source to an external load. In doing so, the transformer also provides important additional capabilities: • The primary to secondary turns ratio can be established to efficiently accommodate widely different input/output voltage levels. • Multiple secondaries with different numbers of turns can be used to achieve multiple outputs at different voltage levels. • Separate primary and secondary windings facilitate high voltage input/output isolation, especially important for safety in off-line applications. Energy Storage in a Transformer Ideally, a transformer stores no energy–all energy is transferred instantaneously from input to output. In practice, all transformers do store some undesired energy: • Leakage inductance represents energy stored in the non-magnetic regions between windings, caused by imperfect flux coupling. In the equivalent electrical circuit, leakage inductance isin series with the windings, and the stored energy is proportional to load current squared. • Mutual inductance (magnetizing inductance) represents energy stored in the finite permeability of the magnetic core and in small gaps where the core halves come together. In the equivalent circuit, mutual inductance appears in parallel with the windings. The energy stored is a function of the volt-seconds per turn applied to the windingsand is independent of load current. Undesirable Effects of Energy Storage Leakage inductance delays the transfer of current between switches and rectifiers during switching transitions. These delays, proportional to load current, are the main cause of regulation and cross regulation problems. Reference (R4) included in this manual explains this in detail. Mutual inductance and leakage inductance energy causes voltage spikes during switching transitions resulting in EMI and damage or destruction of switches and rectifiers. Protective snubbers and clamps are required. The stored energy then ends up as loss in the snubbers or clamps. If the loss is excessive, non-dissipative snubber circuits (more complex) must be used in order to reclaim most of this energy. Leakage and mutual inductance energy is sometimes put to good use in zero voltage transition (ZVT) circuits. This requires caution–leakage inductance energy disappears at light load, and mutual inductance energy is often unpredictable, depending on factors like how well the core halves are mated together. Losses and Temperature Rise Transformer loss is sometimes limited directly by the need to achieve a required overall power supply efficiency. More often, transformer losses are limited by a maximum “hot spot” temperature rise at the core surface inside the center of the windings. Temperature rise (°C) equals thermal resistance (°C/Watt) times power loss (Watts) . ∆ T = RT × PL Ultimately, the appropriate core size for the application is the smallest core that will handle the required power with losses that are acceptable in terms of transformer temperature rise or power supply efficiency. Temperature Rise Limit In consumer or industrial applications, a transformer temperature rise of 40-50°C may be acceptable, resulting in a maximum internal temperature of 100°C. However, it may be wiser to use the next size larger core to obtain reduced temperature rise and reduced losses for better power supply efficiency. Losses Losses are difficult to predict with accuracy. Core loss data from

FAQs

What is the main purpose of a power transformer in switch-mode power supplies?

It transfers power efficiently and instantaneously from an external electrical source to a load. It also enables turns-ratio selection, multiple outputs, and electrical isolation between primary and secondary windings.

How do leakage inductance and mutual inductance affect transformer performance during switching?

Leakage inductance delays current transfer between switches and rectifiers, which can cause regulation and cross-regulation issues. Mutual and leakage energy can create voltage spikes during switching transitions, leading to EMI and potential damage unless protective snubbers/clamps are used.

How is temperature rise determined and how does it relate to transformer losses?

Temperature rise equals thermal resistance multiplied by power loss, expressed as ΔT = RT × PL. Core sizing is guided by the smallest core that can handle the required power without unacceptable temperature rise or efficiency loss.

Nov 2, 2026

Uploaded by Jacob