A "switching transformer" generally refers to the transformer used in a "switching power supply ." It operates under pulse conditions with frequencies ranging from tens to hundreds of kilohertz, and its core is typically made of ferrite material. A switching power supply transformer is a power transformer with an added switching transistor. In addition to the voltage transformation function of a regular transformer, it also provides insulation and power transmission functions. It is generally used in applications involving high-frequency circuits, such as switching power supplies.
Switching power supply transformer effect
The switching power supply transformer and the switching transistor together form a self-excited (or externally excited) intermittent oscillator, thereby modulating the input DC voltage into a high-frequency pulse voltage.
They serve the functions of energy transfer and conversion. In a flyback converter, when the switching transistor is on, the transformer converts electrical energy into magnetic field energy and stores it; when the switching transistor is off, it releases the energy. In a forward converter, when the switching transistor is on, the input voltage is directly supplied to the load and the energy is stored in the energy storage inductor. When the switching transistor is off, the energy storage inductor then provides freewheeling current to the load.
It converts the input DC voltage into the required low voltage.
Switching transformer principle
The operating principle of a switching transformer differs from that of a general transformer in switching power supplies. In a general transformer, the positive and negative half-cycles of the input AC voltage or current are 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 the fundamental characteristic of the operating principle of a general transformer. In contrast, a switching transformer typically operates in a switching state. Its input voltage or current is generally not continuous but discontinuous. The average value of the input voltage or current within one cycle is mostly not equal to 0. Therefore, a switching transformer is also called a pulse transformer. This is the biggest difference between switching transformers and general transformers in terms of their operating principle.
By controlling the switching transistor with PWM (Pulse Width Modulation), the rectified DC voltage is switched on at high frequency, causing high-frequency current to flow into the primary side of the high-frequency transformer of the switching power supply. This induces current on the secondary side of the transformer, and after rectification, the required voltage or multiple voltages can be obtained.
Testing the condition of a switching transformer
1. Inspect the transformer's appearance to check for any significant abnormalities:
For example, check if the coil leads are cracked or detached from the solder joint, if the insulation material has burn marks, if the core fastening screws are loose, if the silicon steel sheets are corroded, and if the winding coils are exposed.
2. Insulation test:
Using a multimeter set to the R×10k range, measure the resistance between the core and the primary winding, between the primary and each secondary winding, between the core and each secondary winding, between the electrostatic shielding layer and the secondary winding, and between each secondary winding. The multimeter pointer should remain at infinity in all cases. Otherwise, it indicates poor insulation of the transformer.
3. Detection of coil continuity:
Set the multimeter to the R×1 range. During the test, if the resistance of a certain winding is infinite, it indicates that the winding has an open circuit fault.
4. Identify the primary and secondary coils:
The primary and secondary leads of a power transformer are usually led out from two separate points. The primary winding is often marked with "220V", while the secondary winding is marked with an additional voltage value, such as 15V, 24V, 35V, etc. These markings are then used for identification.
5. No-load current detection:
a. Direct Measurement Method: Open all secondary windings and set the multimeter to AC current range (500mA), connecting it in series with the primary winding. When the primary winding plug is connected to 220V AC mains power, the multimeter reading is the no-load current value. This value should not exceed 10% to 20% of the transformer's full-load current. The normal no-load current of a typical electronic equipment power transformer should be around 100mA. If it exceeds this value significantly, it indicates a short-circuit fault in the transformer.
b. Direct Measurement Method: Connect a 10/5W resistor in series with the primary winding of the transformer, leaving the secondary winding completely unloaded. Set the multimeter to AC voltage mode. After powering on, use the two probes to measure the voltage drop U across the resistor R. Then, use Ohm's law to calculate the no-load current I<sub>no-load</sub>, i.e., I<sub>no-load</sub> = U/R. F? No-load voltage test: Connect the primary winding of the power transformer to 220V AC power. Use a multimeter connected to AC voltage mode to sequentially measure the no-load voltage values of each winding (U21, U22, U23, U24). The values should meet the requirements, with the allowable error range generally as follows: high-voltage winding ≤ ±10%, low-voltage winding ≤ ±5%, and the voltage difference between two symmetrical windings with a center tap should be ≤ ±2%.
6. Detect the temperature range of the power transformer:
Generally, the allowable temperature rise of a small power transformer is 40℃～50℃. If the insulation material used is of good quality, the allowable temperature rise can be increased.
7. Detect and identify the corresponding terminals of each winding:
When using a power transformer, sometimes two or more secondary windings can be connected in series to obtain the required secondary voltage. When using a power transformer in series, the terminals of the windings connected in series must be correctly connected; otherwise, the transformer will not function properly.
8. Comprehensive detection and diagnosis of short-circuit faults in power transformers:
The main symptoms of a short-circuit fault in a power transformer are severe overheating and abnormal output voltage in the secondary winding. Generally, the more inter-turn short-circuit points within the coil, the larger the short-circuit current, and the more severe the transformer overheating. A simple way to detect and determine 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 far greater than 10% of the full-load current. When the short circuit is severe, the transformer will heat up rapidly within tens of seconds after being energized under no-load conditions; the core will feel hot to the touch. At this point, it is possible to determine that a short circuit exists in the transformer without measuring the no-load current.
Disclaimer: This article is a reprint. If it involves copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
