Teach you how to simply judge whether the transformer is good or bad

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Teach you how to simply judge whether the transformer is good or bad [Copy link]

Source: Switching Power Supply Analysis Author: lk95818171

Recently I saw a friend leave a message saying that there was a problem with the power supply. He repaired it and replaced many components. He also measured the continuity of the transformer pins and found that there was no problem. However, he finally found that it was a problem with the transformer. So he wanted to ask how to determine whether there is a problem with the transformer.

To determine whether there is a problem with the transformer, we must first understand several parameters of the transformer.

Classification of transformers:

We generally divide the switching power supply transformer 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.

Characteristics of transformer:

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

2. DC resistance: also known as copper resistance.

　 3. Efficiency: Output power/Input power*[100%]

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

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

Transformer composition:

The main materials of the switching power supply transformer: magnetic materials, wire materials and insulation materials are the core of the switching transformer.

　　Magnetic material: The magnetic material used in the switching transformer is soft ferrite, which can be divided into two categories according to its composition and application frequency: MnZn and NiZn. The former has high magnetic permeability and high saturation magnetic induction, and has low loss in the medium and low frequency ranges. There are many shapes of magnetic cores, such as EI type, E type, EC type, etc.

　　Conductor material - enameled wire: The enameled wire generally used for winding small electronic transformers includes high-strength polyester enameled wire (QZ) and polyurethane enameled wire (QA). According to the thickness of the paint layer, it is divided into type 1 (thin paint type) and type 2 (thick paint type). The former has a polyester paint for insulation, which has excellent heat resistance and insulation dielectric strength of up to 60kv/mm; the latter has a polyurethane paint for insulation, which has strong self-adhesiveness and self-welding performance (380℃), and can be directly welded without removing the paint film.

　　Pressure-sensitive tape: Insulating tape has high electrical strength, is easy to use and has good mechanical properties. It is widely used in interlayer, intergroup insulation and outer insulation of switch transformer coils. It must meet the following requirements: good adhesion, peeling resistance, certain tensile strength, good insulation performance, good pressure resistance, flame retardant and high temperature resistance

　　Skeleton material: The switch transformer skeleton is different from the general transformer skeleton. In addition to being the insulation and support material of the coil, it also undertakes the installation, fixation and positioning of the entire transformer. Therefore, the material used to make the skeleton should not only meet the insulation requirements, but also have considerable tensile strength. At the same time, in order to withstand the welding heat of the pins, the thermal deformation temperature of the skeleton material is required to be higher than 200°C. The material must be flame retardant and have good processability and be easy to process into various shapes.

Working principle of transformer:

The switching power supply transformer and the switch 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 a role in 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 flow to the load. Convert the input DC voltage into various required low voltages.

Here are 2 pieces of information about transformers from Internet experts:

The figure above shows a voltage absorption network connected in parallel at both ends of the primary winding N1 of the switching transformer. (a), (b), and (c) are common three-way circuit modes, which are used to provide a reverse current path for the switch tube, suppress the amplitude of the reverse voltage between the drain/source (or collector/emitter) during the cut-off period of the switch tube, protect the safety of the switch tube and avoid the accumulation of magnetic potential. When (a) C29 in the circuit leaks; (b) Z1~Z3 in the circuit breaks down or leaks; (c) Z101 in the C circuit breaks down or leaks, the switch transformer is overloaded and the induced voltage of its secondary winding is reduced. At this time, for the switch tube Q1/T103, although it will not cause it to be overloaded (it is in the cut-off period), due to the reduction of the induced voltage of the secondary winding (see the circuit in Figure 2), when the induced voltage of the N2 winding is low (such as lower than the undervoltage action threshold of 10V/PC1), the internal oscillation circuit stops working and an intermittent oscillation fault (manifested as melting) occurs. Note that this voltage intermittent oscillation phenomenon is caused by the undervoltage action of PC1, rather than the conventional overload protection caused by overload of the secondary load circuit. At this time, if you check the load circuit, there will certainly be no overload fault.

When C29 in the circuit (a) of Figure 1 has been damaged by leakage, but its leakage resistance is thousands of ohms; when Z1~Z3 in the circuit (b) of Figure 1 is broken down or leaking, but its breakdown voltage is several volts or more (exceeding the range of the diode block of the digital multimeter, or the breakdown voltage is more than 9V, exceeding the voltage value of the battery inside the pointer multimeter) or its leakage resistance is also thousands of ohms; when the bidirectional breakdown diode Z101 in the circuit (c) of Figure 1 is broken down or leaking, whether it is a pointer multimeter or a digital multimeter, even if we patiently and carefully measure it many times, we may not be able to draw the accurate conclusion that C29, Z1, and Z101 are broken!

　　For convenience, taking the circuit with N1 connected in parallel at both ends in Figure 2 as an example, when the leakage resistance of C4 reaches several thousand ohms, if the diode block of a digital multimeter is used to measure (put the test leads on both ends of C4) and measure twice in forward and reverse directions, it is obvious that one of the measurement results is the forward conduction voltage drop of D2, and one measurement shows an infinite "1", and it is impossible to get an accurate measurement result that C4 has leaked electricity; if the resistance block of a pointer multimeter is used for measurement, the measured value is the total parallel resistance value of the leakage resistance of C4 and R8 and the associated parallel external circuit, so the value is large, and it is not easy to judge that C4 has been damaged by leakage.

　　For example, in the circuit (b) of Figure 1, although Z1 is damaged, its breakdown value is much higher than the battery voltage in the multimeter, and what is measured is only the forward resistance value (or forward conduction voltage drop) of the diode, and its reverse resistance value is also extremely large. For the circuit (c) of Figure 1, if its fault breakdown value is much higher than the internal resistance voltage of the multimeter, then its forward and reverse conduction voltage drops or forward and reverse resistance values are extremely large, and it is impossible to determine that it is broken!

　　It should be known that the capacitor leakage or diode breakdown state will only show the fault state of the component when the voltage applied to the two ends of the component is higher than a certain threshold. When the multimeter is tested under low voltage conditions, the faulty component may sometimes "behave normally". This is why electricians use insulation testers instead of multimeters when testing the insulation between cables or windings.

　　In summary, when the components C29, Z1, Z101 in the figure are damaged, in fact, we have measured the component many times, but are still deceived by the measurement results, and the measurement judgment of other components is also very obvious (no problem), then a fault judgment will pop up in the mind, maybe the switch transformer is broken (internal turn short circuit)? Some repairers may take further measures, such as replacing 3844 and all peripheral circuits with an oscillating board (the voltage absorption circuit connected in parallel at both ends of the N1 winding is not touched), and the result after substitution is still the same. This seems to confirm the suspicion of the switch transformer failure. If the switch transformer of the same model can be replaced for testing, the faulty machine that should be easily repaired may sleep in a corner from now on.

　　It can be imagined that the probability of a switching transformer breaking down is extremely low. The "difficult fault" manifested by intermittent oscillation is therefore misjudged as a switch transformer failure, which shows that there are still limitations in our fault detection method.