What Is an SMPS Transformer?
A SMPS transformer incorporates switching transistors and serves multiple roles within circuits, beyond the typical voltage conversion of standard transformers. It also provides insulation and power transfer capabilities, making it suitable for high-frequency applications, such as switching power supplies. Using switching modulation technology, it converts the input voltage to the required output voltage, and is commonly found in computer power supplies, chargers, and various electronic devices.
SMPS Transformer Components
The main materials used in switching power supply transformers include magnetic materials, wire materials, and insulation materials.
Magnetic Materials
The magnetic materials used in switching transformers are soft ferrites, which can be categorized into two main types: MnZn and NiZn, based on their composition and application frequency. MnZn ferrites have high permeability and saturation magnetic induction, resulting in lower losses in the mid and low-frequency ranges. Various core shapes are available, such as EI, E, and EC types.
Wire Materials
Enamelled wire is typically used for winding small electronic transformers, with high-strength polyester enamelled wire (QZ) and polyurethane enamelled wire (QA) being the two main types. They are categorized by enamel thickness into Type 1 (thin enamel) and Type 2 (thick enamel). The former features a polyester enamel coating, offering excellent heat resistance and an insulation breakdown strength of up to 60 kV/mm. The latter uses a polyurethane coating that exhibits strong self-adhesion and soldering capability (at 380°C), allowing for direct soldering without the need to remove the enamel.
Pressure-sensitive Adhesive Tape
Insulation tape is characterized by high dielectric strength, convenience, and good mechanical properties, making it widely used for interlayer, inter-group, and external insulation in switching transformer coils. It must meet several requirements, including good adhesion, resistance to peeling, a certain tensile strength, excellent insulation properties, voltage resistance, flame retardance, and high-temperature resistance.
Framework Materials
The framework of a switching transformer differs from that of standard transformers, as it not only serves as an insulating and supporting material for the coils but also facilitates the installation and positioning of the entire transformer. The thermal deformation temperature of the framework material must exceed 200°C, and the material should be flame retardant while also being easy to process into various shapes.
Function of SMPS Transformer
The switching power supply transformer, along with the switching transistor, forms a self-oscillating (or externally triggered) intermittent oscillator that modulates the input DC voltage into a high-frequency pulse voltage. This process plays a crucial role in energy transfer and conversion.
In a flyback circuit, when the switching transistor is turned on, the transformer converts electrical energy into magnetic energy, which is stored. When the switching transistor turns off, this stored energy is released. In a forward converter circuit, when the switching transistor is on, the input voltage is directly supplied to the load, and energy is stored in the energy storage inductor. When the transistor turns off, the energy stored in the inductor continues to flow to the load, ensuring a steady output.
How to Test an SMPS Transformer?
To test an SMPS transformer, start by visually inspecting it for any obvious issues. Look for broken or detached wires, signs of burning on insulation materials, loose screws on the core, rust on the silicon steel plates, or exposed winding wires. If any of these issues are present, it may indicate that the transformer is compromised.
Next, test the insulation by using a multimeter set to R×10k. Measure the resistance between the core and the primary, between the primary and each secondary winding, between the core and each secondary, and between any static shielding layer and each secondary. The readings should indicate infinite resistance; otherwise, the insulation is likely deteriorated.
Finally, test the continuity of the windings. Set the multimeter to R×1, and check each winding; if a winding reads as open (infinite resistance), it may have a break. To further diagnose, check the no-load current by measuring the primary with the secondary open-circuited. The no-load current should be around 10%-20% of the transformer’s full-load current. Excessive no-load current often signals a short-circuit fault.
FAQs
How does an SMPS transformer work?
The SMPS transformer uses a switching circuit that rapidly turns the input voltage on and off, creating a high-frequency pulse. This pulse is then transformed into a different voltage level through electromagnetic induction within the transformer, allowing for efficient power conversion.
What are the advantages of using an SMPS transformer?
The advantages include higher efficiency compared to traditional transformers, smaller size due to high-frequency operation, and the ability to provide multiple output voltages from a single input source.
What are the common issues with SMPS transformers?
Common issues include overheating, insulation failure, core saturation, and winding short circuits, which can lead to reduced performance or complete failure of the power supply.
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