Last revised: December 11, 2024

Reviewed by: Scott Orlosky, consulting engineer

Switch mode transformers (switching transformers) are used mainly in switching power supplies and DC-DC converters. They provide a storage element for transferring energy from input to output in discrete packets as required in switching power supplies, regulators and converters. Switch mode transformers have several basic topologies: push-pull, flyback, boost, forward, buck and bridge.

Push-pull transformers are used in push-pull configuration circuits such as power supplies.

Flyback transformers use the flyback or kickback of an inductor to convert an input voltage to a desired output voltage. The input voltage or charging cycle produces energy which is stored in the inductor. The input energy or discharge cycle is then transferred to the output.

Boost or step-up switch mode transformers convert a lower DC input voltage to a higher DC output voltage of the same polarity.

Buck or step-down converters are used to convert a higher DC input voltage to a lower DC output voltage of the same polarity.

Forward converters are similar to buck-boost converters, but use a transformer to store energy and provide isolation between the input and output. The difference between flyback and forward transformers is in the way the energy transfer takes place.

Switch mode transformers with a half-bridge and full-bridge output configuration are also available. 

Mounting Configurations

Switch mode transformers differ in terms of mounting configurations.

Chassis-mounted devices use screw-down tabs.

Disk-mounted devices use a simple rubber washer, a metal disc with a hole in the middle, and a sturdy clamping screw.

H-frame mounting is used in applications with relatively high levels of shock or vibration.

Modular jack designs ensure high common mode noise immunity while maintaining signal integrity.

Switch mode transformers that mount on printed circuit boards (PCBs) use several mounting styles.

Surface mount technology (SMT) adds components to a PCB by soldering component leads or terminals to the top surface of the board. Through hole technology (THT) mounts components by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board.

Certifications and Approval Agencies

There are many certifications and approval agencies for switch mode transformers.

North America

North American organizations include the American National Standards Institute (ANSI), the Canadian Standards Association (CSA), and Underwriters Laboratories (UL).

Europe

European organizations include TÜV Rhineland Berlin-Brandenburg and the VDE Testing and Certification Institute.

Europe — Hazardous Substance

Restriction of Hazardous Substances (RoHS) is a European Union (EU) directive that requires all manufacturers of electronic and electrical equipment sold in Europe to demonstrate that their products contain only minimal levels of the following hazardous substances: lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyl and polybrominated diphenyl ether. RoHS hast been effective since July 1, 2006 and is in its third revision

Europe — Waste Disposal

The Waste Electrical and Electronics Equipment Regulations (WEEE Regulations) is a European Parliament Directive that is designed to encourage the reuse, recycling and recovery of electrical and electronic equipment, and to improve the environmental impact and performance of this equipment.

Other

Other standards organizations for switch mode transformers include the International Electrotechnical Commission (IEC). Suppliers that meet U.S. military standards (MIL-STD) are also available.

Switch Mode Transformers FAQs

What are the key differences between flyback and forward switch mode transformers in terms of efficiency and application?

Forward Transformers: Modern forward supplies tend to be more efficient than flyback transformers. This is due to the active clamp behavior of the forward's primary FET, which results in the need for lower voltage rating and lower RDS(on) FETs. Additionally, the forward topology allows for the optimization of two individual components, which helps in distributing power losses over two components instead of one, thereby increasing efficiency. This is particularly beneficial for high current, low output voltage supplies.

Flyback Transformers: Flyback transformers may have a slight efficiency edge in some cases, but generally, the forward topology is preferred when the requirement exists for the absolute best in efficiency.

Forward Transformers: The forward topology is often considered when high efficiency is a critical requirement. It is suitable for applications where the distribution of power losses and the management of transformer leakage inductance are important considerations.

Flyback Transformers: Flyback transformers are typically used in applications where simplicity and cost-effectiveness are more critical than achieving the highest possible efficiency. They are often used in lower power applications and where the design constraints allow for a single transformer component.

What is active clamp behavior in forward transformers?

The active clamp behavior in forward transformers is a technique used to manage transformer leakage inductance and improve efficiency.

The forward topology utilizes a clamp capacitor, denoted as C{CLAMP}, and a clamp FET, denoted as Q{CLAMP}.These components work together to handle the energy associated with transformer leakage inductance. When the primary FET, Q{PRI} turns off, the leakage current is redirected into the clamp capacitor C{CLAMP} by the clamp FET Q_{CLAMP} This controlled redirection helps in managing the energy that would otherwise cause voltage spikes. The active clamp circuit allows for the use of lower voltage rating and lower R_{DS(on)} FETs, which contributes to higher efficiency. By distributing power losses over two components instead of one, the forward topology can achieve better thermal management and efficiency, especially in high current, low output voltage applications.

Unlike flyback transformers, which may experience spiking due to parasitic capacitance, the forward topology with active clamp behavior minimizes such issues, making it a preferred choice when high efficiency is a critical requirement.

What is the role of transformer leakage inductance in flyback transformers?

In flyback transformers, leakage inductance does not participate in the primary to secondary energy transfer like the magnetizing inductance does. This energy is lost and generates a voltage spike at the beginning of the turn-off time at the drain terminal of the MOSFET switching element. The magnitude of this voltage spike is determined by the leakage inductance, winding capacitance, peak current magnitude, and the output capacitance of the switch.

The voltage spike caused by leakage inductance delays the transfer of power from the primary to the secondary side, which limits the maximum switching frequency of the converter. This can affect the overall efficiency and performance of the flyback transformer.

The primary leakage inductance, along with the primary winding capacitance and the output capacitance of the MOSFET, forms a parasitic LC network. This network influences the peak voltage at the switch node, which is a critical factor in the design and operation of flyback transformers.

How are voltage spikes mitigated in flyback transformers?

To mitigate voltage spikes in flyback transformers, several strategies can be employed. Snubber circuits are often used to dampen voltage spikes. They typically consist of resistors and capacitors that absorb the excess energy generated by the leakage inductance, thereby reducing the peak voltage at the switch node.

Similar to the active clamp behavior in forward transformers, clamp circuits can be used in flyback transformers to redirect the energy from leakage inductance into a controlled path, minimizing voltage spikes.

Optimizing the transformer design to reduce leakage inductance can help mitigate voltage spikes. This might involve adjusting the winding techniques or using different core materials.

Choosing MOSFETs with appropriate voltage ratings and output capacitance can help manage the effects of voltage spikes.

What is the role of snubber circuits in flyback transformers?

Snubber circuits primary role is to mitigate voltage spikes that occur due to transformer leakage inductance.

In flyback transformers, leakage inductance does not contribute to the primary-to-secondary energy transfer. Instead, it generates a voltage spike at the beginning of the turn-off time at the drain terminal of the MOSFET switching element. Snubber circuits are used to dampen these voltage spikes, which can otherwise lead to inefficiencies and potential damage to the components.

Snubber circuits typically consist of resistors and capacitors. These components absorb the excess energy generated by the leakage inductance, thereby reducing the peak voltage at the switch node. This helps in protecting the MOSFET and other components from high voltage stress.

The voltage spike caused by leakage inductance can delay the transfer of power from the primary to the secondary side, limiting the maximum switching frequency of the converter. By effectively managing these spikes, snubber circuits help maintain the desired switching frequency and improve the overall performance of the flyback transformer.

The primary leakage inductance, along with the primary winding capacitance and the output capacitance of the MOSFET, forms a parasitic LC network. Snubber circuits help in managing the effects of this network, ensuring stable operation of the flyback transformer.

What is the role of transformer design in improving flyback transformer performance?

Leakage Inductance Management

Transformer design can significantly impact the amount of leakage inductance. Reducing leakage inductance does not contribute to energy transfer and instead generates voltage spikes, which can affect efficiency and component stress.

Winding Techniques

The way the transformer windings are arranged can influence both leakage inductance and parasitic capacitance. Optimizing winding techniques can help minimize these unwanted effects, thereby improving performance.

Core Material and Geometry

Selecting appropriate core materials and geometries can enhance magnetic performance and reduce losses. This can lead to better efficiency and thermal management in flyback transformers.

Parasitic LC Network

The primary leakage inductance, along with the primary winding capacitance and the output capacitance of the MOSFET, forms a parasitic LC network. Transformer design can help manage the effects of this network, ensuring stable operation.

What is the role of winding techniques in transformer design?

The arrangement of windings can significantly impact the amount of leakage inductance. By optimizing winding techniques, designers can reduce leakage inductance, which is important because it does not contribute to energy transfer and can generate voltage spikes.

Winding techniques also influence parasitic capacitance. Proper winding arrangements can help minimize parasitic capacitance, which, along with leakage inductance, forms a parasitic LC network that affects the peak voltage at the switch node.

Effective winding techniques can improve thermal management by distributing heat more evenly across the transformer, which can enhance performance and reliability.

The way windings are arranged can affect how well the core is utilized, impacting the magnetic performance and efficiency of the transformer.

Switch Mode Transformers Media Gallery

References

GlobalSpec—White Paper: Verification Methods Of Snubber Circuits in Flyback Converters

GlobalSpec—Flyback Transformers Information