High frequency switching power supply transformer
High frequency switching power supply transformer
The high-frequency switching power supply transformer is a power supply transformer that incorporates a switching tube. In addition to the voltage conversion function of the ordinary transformer, the circuit also has the functions of insulation isolation and power transmission, which are generally used in switching power supplies and the like involving high-frequency circuits.
The switching power supply transformer and the switching tube together form a self-excited intermittent oscillator that modulates the input DC voltage into a high frequency pulse voltage. In the flyback circuit, when the switch is turned on, the transformer converts the electric energy into a magnetic field and can be stored. When the switch is turned off, it is released. In the forward circuit, when the switch When the 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 switch tube is turned off, the energy storage inductor performs a freewheeling flow to the load. Convert the input DC voltage to the various low voltages required.
The switching power supply transformer is divided into a single-excitation switching power supply transformer and a double-excited switching power supply transformer. The working principle and structure of the two switching power supply transformers are not the same. The input voltage of a single-excited switching power supply transformer is a unipolar pulse, which is also divided into a positive flyback voltage output; and the input voltage of a double-excited switching power supply transformer is a bipolar pulse, which is generally a bipolar pulse voltage output.
1. Check if there is any obvious abnormality by observing the appearance of the transformer. If the coil lead is broken, de-soldering, whether the insulating material has burnt marks, whether the iron-tightening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed or the like.
2. Insulation test. Use the multimeter R×10k block to measure the resistance between the core and the primary, the primary and the secondary, the core and each secondary, the electrostatic shielding layer and the secondary and secondary windings. The multimeter pointer should refer to the infinity position. move. Otherwise, the transformer insulation performance is poor.
3. Detection of coil on and off. Place the multimeter in the R × 1 block. During the test, if the resistance value of a winding is infinite, the winding has a faulty fault.
4. Discriminate between the primary and secondary coils. The primary and secondary pins of the power transformer are generally led out from both sides, and the primary winding is marked with 220V, and the secondary winding is labeled with rated voltage, such as 15V, 24V, 35V. Then identify them based on these markers.
5. Detection of no-load current.
a, direct measurement method.
Open all the secondary windings and place the multimeter in the AC current block (500mA, stringed into the primary winding. When the plug of the primary winding is inserted into 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 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. Turn the multimeter to the AC voltage block. After power-on, the voltage drop U across the resistor R is measured with two test leads, and then the no-load current I is calculated by Ohm's law, that is, I null = U/R. F? Detection of no-load voltage. Connect the primary of the power transformer to 220V mains, and use the multimeter AC voltage to measure the no-load voltage value (U21, U22, U23, U24) of each winding in order to meet the required value. The allowable error range is generally: high voltage winding ≤±10 %, low voltage winding ≤ ± 5%, the voltage difference between two sets of symmetrical windings with center tap should be ≤ ± 2%.
6. Generally, the low-power Power Transformer allows the temperature rise to be 40 ° C ~ 50 ° C. If the quality of the insulating material used is good, the temperature rise can be increased.
7. Detect and identify the same name end of each winding. When using a power transformer, sometimes two or more secondary windings can be used in series in order to obtain the required secondary voltage. When the power transformer is used in series, the same name of each winding participating in the series must be correctly connected, and no mistake can be made. Otherwise, the transformer will not work properly.
8. Comprehensive detection and identification of short-circuit faults of power transformers. The main symptoms after a short-circuit fault in the power transformer are severe heat generation and abnormal secondary winding output voltage. Generally, the more short-circuit points between turns in the coil, the greater the short-circuit current, and the more severe the transformer heats up. A simple way to detect if a power transformer has a short-circuit fault is to measure the no-load current (tested earlier in the test method). A transformer with a short-circuit fault will have a no-load current value that is much greater than 10% of the full-load current. When the short circuit is severe, the transformer will heat up quickly within a few tens of seconds after the no-load power-on, and the iron core will feel hot when touched by the hand. At this time, it is not necessary to measure the no-load current to conclude that the transformer has a short-circuit point.