How to Detect the Good and Bad of Switching Power Transformer

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

The switching power supply transformer and the switch tube form a self-excited (or other excited) intermittent oscillator to modulate 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 is on, the transformer converts the electric energy into magnetic energy for storage, and releases it when the switch is off. In the forward circuit, when the switch is on, the input voltage is directly supplied to the load and the energy is stored in the energy storage inductor. When the switch is cut off, the energy storage inductor is used for continuous flow to the load.

Switching power transformer is divided into single excitation switching power transformer and double excitation switching power transformer. The working principle and structure of the two switching power 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 positive flyback voltage output; The input voltage of double excitation switching power supply transformer is bipolar pulse, which is generally bipolar pulse voltage output.Principle of switching power transformer

For switching power supply, the working principle of switching transformer is different from that of ordinary transformer. The positive and negative half cycle waveforms of the input AC voltage or current of the 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 the basic feature of the working principle of the ordinary transformer; Switching transformers generally work in the switching state, and their input voltage or current is generally not continuous, but intermittent. Most of the average value of input voltage or current in a cycle is not equal to 0. Therefore, switching transformers are also called pulse transformers, which is the biggest difference between switching transformers and ordinary transformers in working principle.

PWM (pulse width modulation) controls the switch tube to turn on the rectified DC voltage, so that the high-frequency current flows into the primary side of the high-frequency transformer of the switching power supply, and the induced current is generated at the secondary side of the transformer. After rectification, the required voltage or multi-channel voltage can be obtained.Test details of switching power transformer1. Check whether there are obvious abnormalities by observing the appearance of the transformer: such as whether the coil lead is broken, desoldered, whether the insulating material has scorched traces, 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: use multimeter R & times; 10K gear shall measure the resistance between iron core and primary, primary and secondary, iron core and secondary, electrostatic shielding layer and secondary and secondary windings respectively. The pointer of multimeter shall point to the infinity position. Otherwise, the insulation performance of the transformer is poor.3. Coil on-off detection: place the multimeter in R & times; Gear 1: in the test, if the resistance value of a winding is infinite, it indicates that the winding has open circuit fault.4. Discrimination of primary and secondary coils: the primary pin and secondary pin of power transformer are generally led out from both sides, and the primary winding is mostly marked with 220V, while the secondary winding is marked with rated voltage, such as 15V, 24V, 35V, etc. Then identify according to these marks.

5. Detection of no-load current: A. open all secondary windings by direct measurement method, and place the multimeter in AC current gear (500mA, connected in series into the primary winding. When the plug of the primary winding is plugged into 220V AC mains power, the multimeter indicates the no-load current value. This value shall not be 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 shall be about 100mA. If it exceeds too much, it indicates that the transformer has short-circuit fault. B. the indirect measurement method is used in voltage transformation A 10 / 5W resistor is connected in series in the primary winding of the power transformer, and the secondary is still no-load. Turn the multimeter to the AC voltage gear. After power on, use two probes to measure the voltage drop u at both ends of the resistor R, and then use Ohm's law to calculate the no-load current I empty, i.e. I empty = u / R. connect the primary stage of the power transformer to 220V mains power, and use the AC voltage of the multimeter to measure the no-load voltage of each winding in turn (U21, u22, U23 and u24) shall meet the required values, and the allowable error range is generally: high voltage winding 10%, low voltage winding 5%, and the voltage difference between two groups of symmetrical windings with center tap shall be 2%.

6. Test the temperature range of power transformer: generally, the allowable temperature rise of low-power power transformer is 40 50 . If the insulating material used is of good quality, the allowable temperature rise can be improved.7. Detect and judge the same name end of each winding: when using the power transformer, sometimes in order to obtain the required secondary voltage, two or more secondary windings can be used in series. When using the power transformer by series method, the same name end of each winding in series must be connected correctly without mistake. Otherwise, the transformer cannot work normally.

8. Comprehensive detection and discrimination of short-circuit fault of power transformer: the main symptoms of short-circuit fault of power transformer are serious heating and abnormal output voltage of secondary winding. Generally, the more inter turn short-circuit points in the coil, the greater the short-circuit current, and the more serious the heating of transformer. The simple method to detect and judge whether there is short-circuit fault of power transformer is to measure no-load power Current. For the transformer with short-circuit fault, its no-load current value will be much greater than 10% of the full load current. When the short-circuit is serious, the transformer will heat rapidly within tens of seconds after no-load power on, and it will feel hot to touch the iron core by hand. 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 switching power supply and transformer in use?Switching power supply: the switching power supply can stably turn the voltage within a certain range into a very accurate low voltage or high voltage (for example, 110v-250 input, the output voltage can be stably controlled at the required voltage positive and negative difference of 0.5V).Transformer: the output voltage of the transformer changes with the input voltage, that is, the input voltage increases, the output voltage increases, the input voltage decreases, and the output voltage decreases.

To sum up, it can be concluded that the switching power supply first changes the AC into DC, and then changes the DC into AC with higher frequency through the power switch tube. The voltage conversion through the high-frequency power transformer not only improves the efficiency, but also greatly reduces the volume after the frequency is high, and saves the copper and iron loss. Because it is controlled by the power switch tube, the switch tube is conductive at low current The power on time is short and the output voltage can be maintained. When the load is large, the switch tube works continuously to maintain the output voltage. Therefore, the output voltage of the switching power supply is stable and can be selected as high-precision instruments such as LED display screen.

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