To test the quality of SMPS transformers, here are some common items that should be examined:
1.View the surfaces and accessories of SMPS transformers
Good switching power supply transformers should have no abnormalities on their surface. They should also have a fully connected coil lead. The switching power supply transformer is defective if the insulating material above is scorched, the fastening screw is loose, or the silicon steel sheet is corroded, etc.
2.Test the insulation of SMPS transformer
Measure the resistance values between the iron core and primary winding and the secondary winding and the iron core with the secondary winding. The pointer of the multimeter should point to the infinite position. Otherwise, the transformer’s insulation performance is inadequate.
3.Identify and detect the end of each winding with the same name
The secondary voltage of a power transformer can be obtained by connecting two or more secondary windings in series. If the power transformer is used in series, the terminals with the same names of the windings must be connected correctly and cannot be misconnected. The power transformer cannot function normally if this is not the case.
4.Test no-load currents
It is possible to test the no-load current in two different ways:
a.Measuring directly
Connect the multimeter in series to the primary winding and open all secondary windings. Multimeters indicate the no-load current value when the primary winding plug is connected to 220V AC mains. The transformer’s full load current should not exceed 10% to 20% of this value. Normally, the no-load current of a power transformer should be around 100mA. The transformer has a short-circuit if it exceeds too much.
b.Measuring indirectly
A 10Ω/5W resistor is connected in series with transformer’s primary winding, and its secondary is completely unloaded. Set the multimeter to AC voltage. When the 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 I empty.
5.Detection of power transformer short circuits comprehensively
Short-circuit faults of SMPS transformer are characterized by extreme heat generation and abnormal output voltage. The more inter-turn short-circuit points inside the coil, the greater the short-circuit current and the higher the transformer’s heat.
6.Check the SMPS transformer’s temperature range
For small SMPS transformer, the allowable temperature rise is generally 40°C to 50°C. This can be increased if the insulating material is of better quality.
7.On/off detection coil
During the test, if the resistance value of a certain winding is infinite during the test, it means the winding has an open circuit fault.
8.Identifying primary and secondary coils correctly
SMPS transformersusually have primary and secondary pins drawn from both sides, and the primary winding is usually marked with 220V, while the secondary winding is marked with the rated voltage value, such as 15V, 24V, 35V, etc. The primary and secondary coils can be identified by their markings.
What causes the SMPS transformer to emit howling sounds?
1.The SMPS transformer itself has problems
Noise will be produced if the core of the SMPS transformer is not fully dipped and dried. SMPS transformer with uncontrolled air gaps will cause noise and unstable working conditions. The SMPS transformer will produce noise if the wire package is not tightly wound.
2.Circuit setup problem
When wiring SMPS transformer, optocouplers and 431 must be paired with them. If there is a deviation, the circuit will work in an unstable state, causing oscillations and noise.
3.Layout of the board is incorrect
The wiring of the SMPS transformer is of paramount importance, and it must be carried out strictly in accordance with the regulations. Incorrect wiring will result in interference, which will naturally cause oscillations and noise.
4.Deficiency of craftsmanship
Noise will occur if the SMPS transformer is not manufactured well.
