Switching power supply transformer quality inspection details

The role and classification of switching power supply transformers
The switching power supply transformer and the switching tube together form a self-excited (or other) intermittent oscillator, thereby modulating the input DC voltage into a high-frequency pulse voltage. 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 for storage, and releases it when the switch tube is turned off. In the forward circuit, when the switch 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 is turned off, the energy storage inductor will continue to flow to the load. Classification switching power supply transformers are divided into single excitation switching power supply transformers and double excitation switching power supply transformers. The working principle and structure of the two switching power supply transformers 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. Switching power supply transformer characteristic parameters
Voltage ratio: refers to the ratio of the primary voltage to the secondary voltage of the transformer. DC resistance: copper resistance. Efficiency: output power/input power*100[%] Insulation resistance: insulation between the transformer windings and between the core Capability. Dielectric strength: the degree to which the transformer can withstand the specified voltage within 1 second or 1 minute.
The principle of switching power supply transformer
For switching power supplies, the working principle of a switching transformer is different from that of an ordinary transformer. The positive and negative half-cycle waveforms of the AC voltage or current input by an ordinary transformer are symmetrical, and the input voltage and current waveforms are generally continuous. Within a cycle, the average value of the input voltage and current is equal to 0, which is common The basic characteristics of the working principle of the transformer; and the switching transformer generally works in the switching state, and its input voltage or current is generally not continuous, but intermittent. The average value of the input voltage or current within a cycle is mostly It is not equal to 0, therefore, switching transformers are also called pulse transformers, which is the difference in working principle between switching transformers and ordinary transformers. The switching tube is controlled by PWM (Pulse Width Modulation) to conduct high-frequency switching on the rectified DC voltage, so that high-frequency current flows into the primary side of the high-frequency transformer of the switching power supply, so that induced current is generated on the secondary side of the transformer. After rectification, the high-frequency current flows into the primary side of the high-frequency transformer of the switching power supply. You can get the required voltage or multiple voltages.

1. Check whether there are obvious abnormal phenomena by observing the appearance of the transformer: For example, 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 corroded, and whether the winding coil There are exposed and so on.
2. Insulation test: Use a multimeter R&TImes; 10k block to measure the resistance values between the core and the primary, the primary and the secondary, the core and the secondary, the electrostatic shielding and the secondary, and the secondary windings. The pointers of the multimeter should be Refers to not moving at infinity. Otherwise, the insulation performance of the transformer is poor.
3. Detection of coil continuity: Place the multimeter in R&TImes;1 gear. During the test, if the resistance value of a certain winding is infinite, it means that this winding has an open-circuit fault.
4. Distinguish the primary and secondary coils: The primary and secondary pins of the power transformer are generally drawn from both sides respectively, and the primary winding is mostly marked with 220V, and the secondary winding is marked with the rated voltage value, such as 15V , 24V, 35V, etc. Then identify based on these marks.
5. Detection of no-load current: a. Open all secondary windings by direct measurement method, 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 is The indication is the no-load current value.
This value should not be greater than 10% to 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. The indirect measurement method connects a 10/5W resistor 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 power-on, use two meter pens to measure the voltage drop U across the resistor R, and then use Ohm's law to calculate the no-load current I null, that is, I null = U/R.
Connect the primary of the power transformer to the 220V mains, and use a multimeter to measure the no-load voltage of each winding (U21, U22, U23, U24) in turn, and the allowable error range is generally: high voltage winding ≤±10 %, low-voltage winding ≤±5%, and the voltage difference between the two sets of symmetrical windings with center taps should be ≤±2%.
6. Detect the temperature range of the power transformer: Generally, the allowable temperature rise of the low-power power transformer is 40℃～50℃. If the insulating material used is of good quality, the allowable temperature rise can be increased.
7. Detect and distinguish the ends of each winding with the same name: 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 the power transformer is used in series, the ends of the windings with the same name in the series must be connected correctly, and no mistakes can be made. Otherwise, the transformer cannot work normally.
8. Comprehensive detection and discrimination of short-circuit faults of power transformers: The main symptoms of short-circuit faults of power transformers are severe fever and abnormal secondary winding output voltage. Generally, the more short-circuit points between turns inside the coil, the greater the short-circuit current, and the more serious the transformer heats up. The simple way to detect and judge whether the power transformer has a short-circuit fault is to measure the no-load current.
The no-load current value of the transformer with short-circuit fault 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 the no-load power-on, and it will feel hot to touch the iron core with your hands. At this time, it can be concluded that there is a short-circuit point in the transformer without measuring the no-load current.
What is the difference between the use of switching power supply and transformer?
Switching power supply: The switching power supply can stably convert the voltage within a certain range to very low or high voltage (for example, 110V-250 input, the output voltage can be stably controlled at the required voltage plus or minus 0.5v). Transformer: The output voltage of the transformer changes continuously with the input voltage, that is, the input voltage increases and the output voltage increases, and the input voltage decreases and the output voltage decreases. In summary, it can be concluded that the switching power supply first converts alternating current into direct current, and the direct current is transformed into a higher frequency alternating current through the power switch tube. The voltage conversion through the high-frequency transformer not only improves the efficiency, but also greatly reduces the frequency after the high frequency. The volume is reduced, and copper and iron losses are also saved. Because it is controlled by the power switch tube, the switch tube conducts for a short time when the current is small, and the output voltage can be maintained. When the load is large, the switch tube keeps working continuously to maintain the output voltage. Therefore, the switching power supply has stable output voltage and can be used as a high-precision instrument such as an LED display.