How to detect switching power supply transformer good or bad?

Switching Power Supply Transformer

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

The role of switching power supply transformer and classification

Switching power supply transformer and switching tube together constitute a self-excited (or other excitation) type of intermittent oscillator, so that the input DC voltage modulation into a high-frequency pulse voltage.

Play the role of energy transfer and conversion. In flyback circuits, when the switch is energized, the transformer converts the electrical energy into magnetic energy and stores it, and releases it when the switch is turned off. In a flyback circuit, when the switch is on, the input voltage is supplied directly to the load and the energy is stored in the energy storage inductor. When the switch is turned off, the energy is transferred to the load by the storage inductor.

Classification:

Switching power supply transformer is divided into single excitation switching power supply transformer and double excitation switching power supply transformer, two kinds of switching power supply transformer working principle and structure is not the same. The input voltage of single excitation switching power supply transformer is unipolar pulse, and it is also divided into forward and reverse excitation voltage output; while the input voltage of double excitation switching power supply transformer is bipolar pulse, generally bipolar pulse voltage output.

Switching power supply transformer characteristic parameters

Voltage ratio: the ratio of the primary voltage to the secondary voltage of the transformer.

DC resistance: i.e. copper resistance.

Efficiency: that is, the output power / input power * 100 [%].

Insulation resistance: transformer between the windings and the insulation capacity between the core.

Resistance: the transformer in 1 second or 1 minute can withstand the degree of the specified voltage.

Switching Power Supply Transformer Principle

For switching power supply, the working principle of switching transformer is different from that of ordinary transformer. Ordinary transformer input AC voltage or current positive and negative half-waveforms 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 characteristics of the principle of operation of the ordinary transformer; switching transformer is generally working in the switching state, the input voltage or current is generally not continuous, but intermittent, and the input voltage or current within a cycle most are not equal to 0, therefore, switching transformer principle is different from the ordinary transformer. Input voltage or current within a cycle of the average value of most are not equal to 0, therefore, the switching transformer is also known as the pulse transformer, which is the switching transformer and ordinary transformers in the principle of operation of the biggest difference.

Through the PWM (Pulse Width Modulation) control switching tube, the rectified DC voltage for high-frequency switching on, so that the high-frequency current flows into the switching power supply high-frequency transformer primary side, so that the transformer secondary side of the induced current, after rectification can be derived from the required voltage or multiple voltages.

Switching power supply transformer good or bad detection details

1, by observing the external appearance of the transformer to check whether it has obvious abnormal phenomena:

Such as the coil lead is broken, desoldering, insulation material whether there are traces of scorching, the core fastening screw whether there is loose, silicon steel sheet with or without corrosion, winding coil whether there is exposed and so on.

2, insulation test:

With a multimeter R & TImes; 10k block were measured with the core and primary, primary and secondary, core and secondary, electrostatic shielding layer and secondary, secondary windings between the resistance value, the multimeter pointer should be pointed at infinity position does not move. Otherwise, the transformer insulation performance is poor.

3, coil pass detection:

The multimeter will be placed in the R & TImes; 1 block, test, if a winding resistance value of infinity, it shows that this winding has a circuit-breaking fault.

4, identify the primary and secondary coils:

Power transformer primary and secondary pins are generally from both sides of the lead, and the primary winding more marked with 220V, the secondary winding is marked with the rated voltage value, such as 15V, 24V, 35V, etc.. Then identify according to these marks.

5, no-load current detection:

▲ direct measurement method all secondary windings open circuit, the multimeter placed in the AC current block (500mA, series into the primary winding.). When the primary winding plug into the 220V AC mains, the multimeter is indicated by the no-load current value.

This value should not be greater than 10% to 20% of the full load current of the transformer. General common electronic equipment power transformer should be in the normal no-load current of about 100mA. If it exceeds too much, then the transformer has a short-circuit failure.

▲ indirect measurement method in the primary winding of the transformer in series with a 10/5W resistor, secondary still all no-load. Dial the multimeter to AC voltage block. After charging, use the two pens 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. F. Detection of no-load voltage.

The primary power transformer connected to the 220V mains, with a multimeter AC voltage connected to the no-load voltage value of the winding measured in turn (U21, U22, U23, U24) should be in line with the requirements of the value of the permissible range of error is generally: high-voltage windings ≤ ± 10%, low-voltage windings ≤ ± 5%, with the center of the two symmetrical windings should be the center of the tap of the difference between the voltages of the ≤ ± 2%.

6, test the temperature range of the power transformer:

General small power power transformer allows temperature rise of 40 ℃ ~ 50 ℃, if the insulation material used is of good quality, the allowable temperature rise can be increased.

7, testing and judging the winding of the same name end:

In the use of power transformers, sometimes in order to get the desired secondary voltage, two or more secondary windings can be used in series. Using series connection method when using power transformers, participate in the series connection of the winding of the same name end must be correctly connected, can not be mistaken. Otherwise, the transformer can not work properly.

8, power transformer short-circuit fault detection and discrimination:

Power transformer short-circuit failure occurs after the main symptom is a serious heat and secondary winding output voltage abnormality. Usually, the more the coil internal turn-to-turn short-circuit point, the greater the short-circuit current, and the transformer heating is more serious. Detection to determine whether the power transformer has a short-circuit fault is a simple way to measure the no-load current.

There is a short-circuit fault transformer, the value of its no-load current will be much larger 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 charging, touching the iron core will have a hot feeling. At this time, without measuring the no-load current can be concluded that the transformer has a short-circuit point exists.

Switching power supply and transformer in the use of what is the difference?

Switching power supply: switching power supply can be very stable within a certain range of voltage into a very precise low voltage or high voltage (for example, 110V-250 input, the output voltage can be stably controlled in the required voltage plus or minus no difference of 0.5v).

Transformer: the output voltage of the transformer is constantly changing with the input voltage, that is, the input voltage increases the output voltage also increases, the input voltage decreases the output voltage also decreases.

In summary, it can be concluded that the switching power supply is the first AC into DC, DC through the power switching tube and then into a higher frequency AC through the high-frequency transformer voltage conversion not only improves the efficiency and high frequency after a significant reduction in size, but also saves copper and iron losses. Because through the power switching tube control so in small current switching tube conduction time is short, to maintain the output voltage can be. When the load is large, the switching tube works constantly to maintain the output voltage. Therefore, the output voltage of the switching power supply is stable, and it can be selected as a high-precision instrument such as LED display.