Function
The switching power supply transformer and the switch tube together form a self-excited (or other) intermittent oscillator, thereby modulating the input DC voltage into a high-frequency pulse voltage.< /p>
plays the role of energy transfer and conversion. In the flyback circuit, when the switch tube is turned on, the transformer converts the electric energy into magnetic field energy and stores it, and releases it when the switch tube is turned off. In the circuit, when the switching tube is turned on, the input voltage is directly supplied to the load and the energy is stored in the energy storage inductor. When the switching tube is turned off, the energy storage inductor will continue to flow to the load.
Convert the input DC voltage into various low voltages.
Classification
Switching power supply transformers are divided into single-excited switching power transformers and double-excited switching power transformers. The working principle and structure of the switching power supply transformer are not the same. The input voltage of the single-excited switching power supply transformer is a unipolar pulse, and it is also divided into forward and reverse excitation voltage output; while the input voltage of the double-excited switching power supply transformer is a bipolar pulse, which is generally a bipolar pulse voltage output.
Characteristic parameters
Voltage ratio: refers to the ratio of the primary voltage to the secondary voltage of the transformer.
DC resistance: copper resistance.
< p>Efficiency: output power/input power*100[%]Insulation resistance: the insulation capacity between the windings of the transformer and between the cores.
Electric strength: The degree to which the transformer can withstand the specified voltage within 1 second or 1 minute.
Composition
The main materials of switching power supply transformers: magnetic materials, wire materials and insulating materials are switching transformers Core.
Magnetic material: The magnetic material used in switching transformers is soft ferrite, which can be divided into two categories: MnZn series and NiZn series according to its composition and application frequency. The former has high permeability and high The saturation magnetic induction has low loss in the middle and low frequency ranges. There are many magnetic core shapes, such as EI type, E type, EC type, etc.
Wire material-enameled wire: generally used to wind small electronic transformers The enameled wire has two types: high-strength polyester enameled wire (QZ) and polyurethane enameled wire (QA). According to the thickness of the paint layer, it is divided into two types: type 1 (thin paint type) and type 2 (thick paint type). The insulating coating of the former is Polyester paint, with superior heat resistance, insulation and electric strength up to 60kv/mm; the latter insulating layer is polyurethane paint, which has strong self-adhesiveness and self-weldability (380℃), and it can be used without removing the paint film. Can be directly welded
Pressure-sensitive tape: Insulating tape has high electrical strength, easy to use and good mechanical properties. It is widely used in the interlayer, inter-group insulation and outsourcing insulation of switching 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
Frame material: The switch transformer skeleton is different from the general transformer skeleton, except as a coil In addition to the insulation and support materials, it also assumes the role of installation, fixing and positioning of the entire transformer. Therefore, in addition to meeting the insulation requirements, the material used to make the skeleton should also have considerable tensile strength. At the same time, in order to withstand the soldering heat of the pins, It is required that the heat distortion temperature of the framework material is higher than 200℃, the material must be flame-retardant, and it should also have good workability and be easy to process into various shapes.
Testing method
1 Check whether there is any obvious abnormality by observing the appearance of the transformer. Such as whether the coil lead is broken, de-soldering, whether the insulating material has burnt marks, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.
2, insulation test. Measure the resistance between the core and the primary, the primary and the secondary, the core and the secondary, the electrostatic shielding layer and the sub-secondary, and the secondary windings with a multimeter R×10k block. Move. Otherwise, the insulation performance of the transformer is poor.
3. Detection of on-off of the coil. Put the multimeter in the R×1 gear. During the test, if the resistance value of a certain winding is infinite, it means that the winding has an open-circuit fault.
4. Identify the primary and secondary coils. The primary and secondary pins of the power transformer are generally drawn from both sides, and the primary windings are mostly marked with 220V, and the secondary windings are marked with rated voltage values, such as 15V, 24V, 35V, etc. Then identify based on these marks.
5. Detection of no-load current.
a. Direct measurement method. Open all secondary windings and place the multimeter in the AC current block (500mA, serially connected to the primary winding. When the plug of the primary winding is plugged into the 220V AC mains, the multimeter indicates the no-load current value. This value should not be It is greater than 10%-20% of the full load current of the transformer. Generally, the normal no-load current of the power transformer of common electronic equipment should be about 100mA. If it exceeds too much, it means that the transformer has a short-circuit fault.
b. Indirect Measurement method. A 10?/5W resistor is connected in series with the primary winding of the transformer, and the secondary is still completely empty. Set the multimeter to the AC voltage block. After powering up, use two meter pens to measure the voltage drop U at both ends of the resistor R. Then use Ohm's law to calculate the no-load current I null, that is, I null = U/R. F? No-load voltage detection. Connect the primary of the power transformer to 220V mains, and use a multimeter to measure the no-load of each winding in turn. The voltage value (U21, U22, U23, U24) should meet the required value, and the allowable error range is generally: high-voltage winding ≤±10%, low-voltage winding ≤±5%, and the voltage difference between two sets of symmetrical windings with a center tap should be ≤± 2%.
6. Generally, the allowable temperature rise of low-power power transformers is 40℃～50℃. If the insulating material used is of good quality, the allowable temperature rise can be increased.
7 , Detection and identification of the same-named end of each winding. When using a power transformer, sometimes in order to obtain the required secondary voltage, two or more secondary windings can be connected in series. When the power transformer is used in series, participate in the series connection. The ends of the windings with the same name must be connected correctly and cannot be mistaken. Otherwise, the transformer will not work normally.
8. Comprehensive detection and identification of short-circuit faults of power transformers. The main symptoms after short-circuit faults of power transformers are Severe heating and abnormal output voltage of the secondary winding. Generally, the more short-circuit points inside the coil, the greater the short-circuit current, and the more serious the transformer heating. The simple method to detect whether the power transformer has a short-circuit fault is to measure the no-load Current (the test method has been introduced above). The no-load current value of a transformer with a short-circuit fault will be much greater than 10% of the full-load current. When the short-circuit is serious, the transformer will quickly be quickly within tens of seconds after the no-load power-up Heating, touching the iron core with your hands will feel hot. At this time, it can be concluded that there is a short-circuit point in the transformer without measuring the no-load current.