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

2019-05-10 14:31:32 JuKe Technology (DongGuan) Co., Ltd. Read

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

"Switching transformer" generally refers to the transformer used in the "switching power supply". When working in a pulse state of a frequency of ten to several tens of kilohertz or even hundreds of kilohertz, the iron core is generally made of ferrite material. The switching power supply transformer is a power transformer with a switching tube. In addition to the voltage conversion function of the ordinary transformer, the circuit also has the functions of insulation isolation and power transmission, which are generally used in switching power supplies and the like involving high frequency circuits.
How to judge whether the switching power supply transformer is good or bad.png

 Switching power supply transformer

The switching power supply transformer and the switching tube together form a self-excited (or other) intermittent oscillator that modulates the input DC voltage into a high frequency pulse voltage.

It plays the role of energy transfer and conversion. In a flyback circuit, when the switch is turned on, the transformer converts the electrical energy into a magnetic field that can be stored and released when the switch is turned off. In a forward circuit, when the switch is turned 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 storage inductor performs a freewheeling flow to the load.

Convert the input DC voltage to the various low voltages required.
Switching 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 the ordinary transformer are all 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. This is common. The basic characteristics of the working principle of the transformer; while the switching transformer generally works in the switching state, its input voltage or current is generally not continuous, but intermittent, the average value of the input voltage or current within a period is mostly Not equal to 0, therefore, the switching transformer is also called pulse transformer, which is the biggest difference between the working principle of the switching transformer and the ordinary transformer.

The switching tube is controlled by PWM (pulse width modulation), and the rectified DC voltage is turned on by the high frequency switch, so that the high frequency current flows into the primary side of the high frequency transformer of the switching power supply, so that the secondary side of the transformer generates an induced current, and after rectification It is possible to derive the required voltage or multiple voltages.
Switching transformer good or bad detection

1. Check if there is any obvious abnormality by observing the appearance of the transformer:

If the coil lead is broken, de-soldering, whether the insulating material has burnt marks, whether the iron-tightening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed or the like.

2. Insulation test:

Use the multimeter R&TImes; 10k block 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 secondary windings. The multimeter pointer should refer to the infinity position. move. Otherwise, the transformer insulation performance is poor.

3. Detection of coil continuity:

Place the multimeter in R&TImes; 1st gear. During the test, if the resistance value of a winding is infinite, the winding has a faulty fault.

4. Discriminate between 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 marked with 220V, and the secondary winding is labeled with rated voltage, such as 15V, 24V, 35V. Then identify them based on these markers.

5. Detection of no-load current:

a. Direct measurement method will open all the secondary windings, put the multimeter in the AC current block (500mA, string into the primary winding. When the plug of the primary winding is inserted into 220V AC mains, the multimeter indicates the no-load current value. This value should not be greater than 10% to 20% of the transformer's full load current. The normal no-load current of the common electronic equipment power transformer should be around 100mA. If it exceeds too much, 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 empty. Turn the multimeter to the AC voltage block. After power-on, the voltage drop U across the resistor R is measured with two test leads, and then the no-load current I is calculated by Ohm's law, that is, I null = U/R. F? Detection of no-load voltage. Connect the primary of the power transformer to 220V mains, and use the multimeter AC voltage to measure the no-load voltage value (U21, U22, U23, U24) of each winding in order to meet the required value. The allowable error range is generally: high voltage winding ≤±10 %, low voltage winding ≤ ± 5%, the voltage difference between two sets of symmetrical windings with center tap should be ≤ ± 2%.

6. Detect the temperature range of the power transformer:

Generally, the low-power power transformer allows the temperature rise to be 40 ° C ~ 50 ° C. If the quality of the insulating material used is good, the temperature rise can be increased.

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

When using a power transformer, sometimes two or more secondary windings can be used in series in order to obtain the required secondary voltage. When the power transformer is used in series, the same name of each winding participating in the series must be correctly connected, and no mistake can be made. Otherwise, the transformer will not work properly.

8. Comprehensive detection and identification of short-circuit faults of Power Transformers:

The main symptoms after a short-circuit fault in the power transformer are severe heat generation and abnormal secondary winding output voltage. Generally, the more short-circuit points between turns in the coil, the greater the short-circuit current, and the more severe the transformer heats up. A simple way to detect if a power transformer has a short-circuit fault is to measure the no-load current. A transformer with a short-circuit fault will have a no-load current value that is much greater than 10% of the full-load current. When the short circuit is severe, the transformer will heat up quickly within a few tens of seconds after the no-load power-on, 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 conclude that the transformer has a short-circuit point.