Introduction of High Frequency Switching Power Transformer

- Jan 29, 2020-

High frequency switching power transformer is a power transformer added with switching tube. In addition to the voltage conversion function of ordinary transformers in the circuit, it also has insulation and power transmission functions. It is generally used in switching power supplies and other high frequency circuits.

The switching power supply transformer and the switching tube together form a self-excited intermittent oscillator, thereby modulating the input DC voltage into a high-frequency pulse voltage. It plays the role of energy transfer and conversion. In the flyback circuit, when the switch tube is turned on, the transformer converts the electrical energy into magnetic field energy and stores it. When the switch tube is turned off, it is released. 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 continues to flow to the load. The input DC voltage is converted into various low voltages as required.

Switching power supply transformers are divided into single-excited switching power supply transformers and double-excited switching power supply transformers. The working principles and structures 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, and it also divides forward and flyback voltage output; while the input voltage of a dual-excited switching power supply transformer is a bipolar pulse, which is generally a bipolar pulse voltage output.

1. Check whether there is obvious abnormal phenomenon by observing the appearance of the transformer. Such as whether the coil leads are broken, de-soldering, whether the insulation material has scorch marks, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, and whether the winding coil is exposed.

2. Insulation test. Use the multimeter R × 10k gear to measure the resistance between the core and the primary, the primary and the secondary, the core and the secondary, the electrostatic shield and the 衩 secondary, and the windings of the secondary. The pointer of the multimeter should be at the infinite position. move. Otherwise, it indicates that the transformer has poor insulation performance.

3. Detection of coil continuity. Put the multimeter in R × 1 gear. In the test, if the resistance value of a winding is infinite, it means that this winding has an open circuit fault.

4. Identify the primary and secondary coils. The primary and secondary pins of a power transformer are generally drawn from both sides, and the primary winding is usually marked with 220V, and the secondary winding is marked with a rated voltage value, such as 15V, 24V, 35V, etc. These tags are used for identification.

5. Detection of no-load current.

A direct measurement method.

Open all secondary windings and place the multimeter in the AC current block (500mA, string it into the primary winding. When the plug of the primary winding is plugged into 220V AC mains, the multimeter indicates the no-load current value. This value should not More than 10% to 20% of the transformer full load current. The normal no-load current of common electronic equipment power transformers should be around 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 in the primary winding of the transformer, and the secondary is still completely unloaded. Set the multimeter to AC voltage. After power-on, use two test leads to measure the voltage drop U across the resistor R, and then use Ohm's law to calculate the no-load current I empty, that is, I empty = U / R. F? No-load voltage detection. Connect the primary of the power transformer to 220V mains, and use the AC voltage of the multimeter to measure the no-load voltage values of each winding (U21, U22, U23, U24) in sequence. The allowable error range is generally: high voltage winding ≤ ± 10 %, Low voltage winding ≤ ± 5%, voltage difference between two sets of symmetrical windings with center tap should be ≤ ± 2%.

6. The temperature rise of general low-power power transformers is 40 ℃ ~ 50 ℃. If the insulation material used is of good quality, the temperature rise can be increased.

7. Detect and judge the ends with the same name of each winding. When using a power transformer, sometimes in order to obtain the required secondary voltage, two or more secondary windings can be used in series. When using the series method to use a power transformer, the same-named ends of the windings participating in the series must be connected correctly and must not be mistaken. Otherwise, the transformer will not work properly.

8. Comprehensive detection and discrimination of short-circuit faults of power transformers. The main symptoms after a short-circuit fault of the power transformer are severe heat generation and abnormal output voltage of the secondary winding. Generally, the more inter-turn short-circuit points inside the coil, the greater the short-circuit current, and the more severe the transformer heats up. The simple way to detect whether the power transformer has a short-circuit fault is to measure the no-load current (the test method has been described earlier). With a short-circuit fault, the no-load current value will be much greater than 10% of the full-load current. When the short circuit is serious, the transformer will quickly heat up within tens of seconds after no-load power-on. Touching the iron core with your hands will feel hot. At this time, it is not necessary to measure the no-load current to determine that the transformer has a short-circuit point.