Analysis of the principle of switching power transformer

The magnetic material used in switching transformers is soft magnetic ferrite, which can be divided into MnZn series and NiZn series according to its composition and application frequency. The former has high permeability and high saturation magnetic induction, and has low losses in the mid and low frequency ranges. The magnetic core has many shapes, such as E-type, E-type, EC type, etc
Generally, there are two types of enameled wire used for winding small electronic transformers: high-strength polyester enameled wire (QZ) and polyurethane enameled wire (QA). According to the thickness of the paint layer, there are two types: type 1 (thin paint type) and type 2 (thick paint type). The insulation coating of the former is polyester paint, which has excellent heat resistance, and the insulation and electrical strength can reach 60 kv/mm; The latter insulation layer is polyurethane paint, with strong self-adhesiveness and self soldering performance (380 ° C), allowing direct welding of pressure sensitive adhesive tapes without removing the paint film
Insulating tape has high electrical strength, convenient use, and good mechanical properties. It is widely used for interlayer, group, and outer insulation of switch transformer coils. The following requirements must be met: good adhesion, peeling resistance, certain tensile strength, good insulation performance, good pressure resistance, flame retardant and high temperature resistance
Skeleton material
The switch transformer skeleton is different from the general transformer skeleton in that it serves not only as the insulation and support material for the coil, but also for the installation, fixation, and positioning of the entire transformer. Therefore, in addition to meeting the insulation requirements, the material used to make the skeleton should also have a considerable tensile strength. In addition to withstanding the welding heat of the bow and leg, the thermal deformation temperature of the skeleton material is required to be higher than 200 ° C, and the material must meet flame retardant requirements, It should also have good processability and be easy to process into various shapes.
Workflow of switching power supply
Power Input Filter Full Bridge Rectification Constant Current Filtering - Switching Transistor (Oscillating Inverter) - Switching Transformer Output Rectification and Filtering.
The AC power input is rectified and filtered into DC
The switching tube is controlled by a high-frequency PWM (pulse width modulation) signal. The DC is applied to the primary switch transformer and the secondary switch transformer induces a high-frequency voltage. After rectification and filtering, the load output part is fed back to the control circuit through a certain circuit to control the PWM duty ratio, so as to achieve stable output. When AC power is input, it generally passes through an eddy current loop or something to filter out interference on the power grid, At the same time, it also filters out the interference of the power supply to the power grid;
When the power is the same, the higher the switching frequency, the smaller the volume of the switching transformer, but the higher the requirement for the switching tube;
The secondary of a switching transformer can have multiple windings or a winding can have multiple taps to obtain the desired output;
Generally, some protection circuits should also be added, such as no-load and short circuit protection, otherwise the switching power supply may be burned out.
Mainly used in industry and some household appliances, such as televisions, computers, etc
Principle of switching power transformer
A switching power transformer and a switching tube together form a self-excited (or separately excited) intermittent oscillator to modulate the input DC voltage into a high-frequency pulse voltage. In a flyback circuit, when the switch is turned on, the voltage converter converts electrical energy into magnetic field energy for storage, and when the switch is turned off, it is released. In a forward circuit, when the switch is turned on, the input voltage is directly supplied to the load and energy is stored in the energy storage inductor. When the switch tube is turned off, the energy storage inductor carries out continuous flow load transfer.
A switching power transformer is a power transformer with a switching transistor added. In addition to the voltage conversion function of a common transformer, it also has insulation isolation and power transmission functions. It is generally used in switching power supplies and other occasions involving high-frequency circuits.