Common Transformer Types in Switching Power Supplies
From lianglite • Electronics Classroom
Transformers used in switching power supplies are fundamentally different from traditional mains-frequency transformers, as they must transfer energy under high-frequency pulse conditions. Based on topology, function, and material properties, lianglite summarizes the mainstream types and their typical applications.
I. Classified by Topology (Core Classification)
Flyback Transformer
- Working principle: Single-switch topology. Energy is stored in the primary during conduction and released to the secondary when the switch turns off — energy transfer is opposite to the switching state.
- Application: Low-power devices (<100W), e.g., phone chargers, set-top box power supplies.
- Advantage: Simple circuit, low cost, no secondary freewheeling inductor needed.
Forward Transformer
- Working principle: Energy transfers directly to the secondary while the switch is on (synchronous with switching). A reset winding or magnetic reset circuit is required to avoid core saturation.
- Application: Medium–low power supplies (50W–300W), e.g., laptop adapters, industrial control power.
Push–Pull Transformer
- Working principle: Center-tapped primary with two switches alternating conduction. The core operates on both positive and negative halves of the hysteresis loop, offering high utilization.
- Application: Medium–high power (200W–1000W), e.g., server power, telecom power supplies.
- Note: Core bias must be suppressed to prevent switch damage.
Half-Bridge Transformer
- Working principle: Two capacitors divide input voltage; two switches alternate. Input voltage utilization is half that of push–pull, with lower voltage stress on switches.
- Application: Medium–high power (500W–2000W), e.g., industrial power supplies, EV chargers.
Full-Bridge Transformer
- Working principle: Four switches form a bridge for alternating conduction. High voltage utilization and low voltage stress, ideal for high-power conditions.
- Application: High-power supplies (>1000W), e.g., welding machines, data center power.
II. Classified by Function & Structure
Isolated Transformer
Electrically isolated between primary and secondary windings for safety and voltage separation. Flyback, forward, push–pull, half-bridge, and full-bridge types all fall into this category, widely used in AC-DC, home appliances, and industrial equipment.
Non-Isolated Transformer (Inductor Type)
Shared magnetic core without electrical isolation, equivalent to an energy-storage inductor. Commonly used in Buck, Boost, and other DC-DC converters, e.g., low-voltage power on mobile phone motherboards.
Planar Transformer
Uses PCB or thin copper sheets as windings with a flat integrated core. Features small size, low leakage inductance, and excellent heat dissipation. Used in high-density power supplies such as laptops and medical equipment.
III. Classified by Core Material
Ferrite Core Transformer
Low high-frequency loss (suitable for 10kHz–1MHz), cost-effective. MnZn ferrite is the mainstream choice for switching power supplies.
Nanocrystalline / Amorphous Core Transformer
High permeability and superior high-frequency performance, ideal for >1MHz or high-power applications, at a relatively higher cost.
lianglite Tip: Choose transformers based on power level, operating frequency, and isolation requirements. Low-power chargers typically use flyback ferrite transformers, while high-power industrial power supplies prefer full-bridge or push–pull topologies.