How to detect whether the switching power supply transformer is good or bad?

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How to detect whether the switching power supply transformer is good or bad? [Copy link]

Switching power supply transformer

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

Function and classification of switching power supply transformers

The switching power supply transformer and the switching tube together form a self-excited (or externally 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, and releases it when the switch tube is turned off. In the forward circuit, when the switch 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 is used to transfer the energy to the load.

Classification:

The switching power supply transformer is divided into single-excitation switching power supply transformer and dual-excitation switching power supply transformer. The working principles and structures of the two switching power supply transformers are not the same. The input voltage of the single-excitation switching power supply transformer is a unipolar pulse, and it is also divided into forward and reverse voltage output; while the input voltage of the dual-excitation 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: The insulation capacity between the transformer windings and between the core.

Dielectric strength: The degree to which a transformer can withstand a specified voltage within 1 second or 1 minute.

Switching power supply transformer principle

For switching power supplies, the working principle of switching transformers is different from that of ordinary transformers. The positive and negative half-cycle waveforms of the AC voltage or current input by ordinary transformers are symmetrical, and the input voltage and current waveforms are generally continuous. Within one cycle, the average value of the input voltage and current is equal to 0, which is the basic characteristic of the working principle of ordinary transformers; while switching transformers generally work in a switching state, and their input voltage or current is generally not continuous, but intermittent, and the average value of the input voltage or current within a cycle is mostly not equal to 0. Therefore, switching transformers are also called pulse transformers, which is the biggest difference between switching transformers and ordinary transformers in terms of working principles.

By controlling the switch tube through PWM (pulse width modulation), the rectified DC voltage is turned on at high frequency, so that the high-frequency current flows into the primary side of the high-frequency transformer of the switching power supply, thereby generating an induced current on the secondary side of the transformer. After rectification, the required voltage or multiple voltages can be obtained.

Switching power supply transformer good or bad detection details

1. Check whether there are any obvious abnormalities by observing the appearance of the transformer:

For example, whether the coil lead is broken or desoldered, whether the insulating material has signs of burning, whether the core fastening screws are loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.

2. Insulation test:

Use the multimeter R&TImes;10k to measure the resistance between the core and the primary, the primary and each secondary, the core and each secondary, the electrostatic shielding layer and the secondary, and the resistance between each secondary winding. The multimeter pointer should point to the infinite position and not move. Otherwise, it means that the insulation performance of the transformer is poor.

3. Detection of coil on and off:

Set the multimeter to the R&TImes;1 position. During the test, if the resistance value of a winding is infinite, it means that the winding has a circuit breaker fault.

4. Identify 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 often marked with 220V, while the secondary winding is marked with the rated voltage value, such as 15V, 24V, 35V, etc. Then identify it based on these marks.

5. Detection of no-load current:

a. Direct measurement method: Open all secondary windings, set the multimeter to the AC current range (500mA, and connect the primary winding in series. When the primary winding plug is plugged into the 220V AC mains, the multimeter indicates the no-load current value.

This value should not be greater than 10% to 20% of the full load current of the transformer. The normal no-load current of the power transformer of common electronic equipment should be around 100mA. If it exceeds too much, it means that the transformer has a short circuit fault.

b. Indirect measurement method: Connect a 10/5W resistor in series in the primary winding of the transformer, and the secondary winding is still completely unloaded. Set the multimeter to the AC voltage range. After power is turned 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 Iempty, that is, Iempty = U/R. F? No-load voltage detection.

Connect the primary of the power transformer to 220V AC power, and use a multimeter to measure the no-load voltage of each winding (U21, U22, U23, U24) in turn. The no-load voltage value should meet the required value. 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 center taps should be ≤±2%.

6. Detection of the temperature range of the power transformer:

Generally, the allowable temperature rise of small-power power transformers is 40℃~50℃. If the quality of the insulating material used is good, the allowable temperature rise can be further increased.

7. Detect and identify the same-name ends of each winding:

When using a power transformer, sometimes two or more secondary windings can be connected in series to obtain the required secondary voltage. When using a power transformer in series, the same-name ends of the windings in series must be connected correctly and cannot be mistaken. Otherwise, the transformer cannot work properly.

8. Comprehensive detection and judgment of short-circuit fault of power transformer:

The main symptoms of a short-circuit fault in a power transformer are severe heating and abnormal secondary winding output voltage. Generally, the more short-circuit points there are between turns in the coil, the greater the short-circuit current and the more severe the transformer heating. A simple way to detect whether a power transformer has a short-circuit fault is to measure the no-load current.

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 heat up rapidly within tens of seconds after power is applied at no load, and the iron core will feel hot when touched by hand. At this time, it is not necessary to measure the no-load current to determine that the transformer has a short-circuit point.

What is the difference between switching power supplies and transformers in terms of use?

Switching power supply: A switching power supply can stably convert voltage within a certain range into very precise low voltage or high voltage (for example, with a 110V-250 input, the output voltage can be stably controlled within the required voltage within a deviation of 0.5V).

Transformer: The output voltage of the transformer changes continuously with the input voltage, that is, as the input voltage increases, the output voltage also increases, and as the input voltage decreases, the output voltage also decreases.

In summary, it can be concluded that the switching power supply first converts AC into DC, and then converts DC into higher frequency AC through the power switch tube, and then converts the voltage through the high-frequency transformer. Not only does the efficiency improve, but the volume is greatly reduced after the frequency is high, and the copper and iron loss is also saved. Because it is controlled by the power switch tube, the switch tube is turned on 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 to maintain the output voltage. Therefore, the output voltage of the switching power supply is stable, and it can be used as a high-precision instrument such as an LED display.

To sum up, the current application scope of switching power supplies is wider and more adapted to market demand. After understanding the differences between switching power supplies and transformers as above, we also hope that the manufacturers of switching power supplies can do a good job in quality control and not harm the interests of customers in pursuit of profits.

END

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Jacktang

Use the same-name terminals of each winding to identify

Mainly to facilitate the "enhancement" or "weakening" of the effect and to simplify the graphical representation, the method of marking the ends with the same name is used.