Power transformer is a ring transformer with a switching tube added. In the power supply circuit, in addition to the working voltage conversion function of the general transformer, it also takes into account the insulation layer protection and output power transmission function. It is generally used in the power transformer and other places involving high-frequency circuits.
The role and classification of switching power transformers.
The switching power transformer and the switching tube together form a self-excited (or individually excited) block oscillator to modulate the input DC voltage to a high-frequency pulse voltage. Play the role of energy transfer and conversion. In flyback circuits, the transformer converts electrical energy into a magnetic field as the switch tube passes through and releases it when the switch tube expires. In a forward circuit, when the switching tube is on, the input voltage is supplied directly to the load and the energy is stored in the energy-storage inductor. When the switch tube is cut, the storage inductor is loaded continuously. Classification switching power transformers are divided into single excitation switching power transformers and double excitation switching power transformers. The working principle and structure of the two switching power transformers are not the same. The input voltage of single-excitation switching power transformer is monopole pulse, which is also divided into forward and reverse excitation voltage output; However, the input voltage of a dual-excitation switching power transformer is a bipolar pulse, usually the bipolar pulse voltage output. Characteristic parameters of switching power supply transformer.
Voltage ratio: the ratio of primary voltage to secondary voltage of transformer. DC resistance: copper resistance. Efficiency: output power/input power *100[%]. Insulation resistance: The insulation ability between windings and cores of a transformer. Electrical strength: The degree to which a transformer can withstand a specified voltage for one second or one minute.
Principle of switching power supply transformer.
For power transformer, the principle of switch transformer is different from that of general transformer. The positive and negative half-cycle waveforms of ac voltage or current input in a normal transformer are symmetric. And the input operating voltage and current wave patterns are generally all continuous. Within a cycle time, the mean value of input working voltage and current is equal to 0, which is the basic characteristic of general transformer principle. The switching transformer is generally in the work of the power switch, its input working voltage or current is generally not continuous, but intermittently, input working voltage or current in the period of time most of the mean is not equal to 0, therefore, the switching transformer is also called the pulse transformer, It is a big difference between switching transformer and general transformer in principle. Based on PWM (pulse width allocate) control switch tube, after the rectifier ac voltage of the high frequency switch on and off, prompting high frequency current flow into the power of the power transformer transformer primary side, which is caused is vice edge induction electromotive force, after rectifier can get must work voltage or multi-channel working voltage.
Power transformer transformer quality inspection details
1. Carefully observe the appearance of the transformer to check whether there are significant abnormalities: such as whether the electromagnetic coil wire is broken, open welding, whether the insulating layer material is burned mark, whether the iron core is loosened when tightening the extruder screw, whether the ferrite core is rusty, whether the winding electromagnetic coil is exposed, etc.
2. Insulation test: use a multimeter R & TImes10k to measure the resistance values between the iron core and primary, primary and secondary, iron core and secondary, electrostatic shielding and secondary and secondary winding respectively. The multimeter pointer should be fixed at infinity. Otherwise, the transformer insulation performance is poor.
3, electromagnetic coil conduction test: the digital multimeter placed R&TImes; 1 gear, in the detection, if the resistance value of a winding is infinite, it indicates that the winding has a common fault of short circuit.
4. Distinguish one and two coils: the one and two pins of the power transformer are generally drawn from both sides. The primary winding is marked with 220V, and the secondary winding is marked with rated voltage value, such as 15V, 24V, 35V, etc. And then identify them based on those markers.
5, full load current test: a, immediate measurement method will be all secondary coil winding all leads, the digital multimeter placed ac circuit block (1000mA, series into the primary and intermediate winding resistance. When the intermediate winding power plug was inserted into the 380V ac voltage, the digital multimeter marked the full load current.
The value should not be greater than 10% to 20% of the transformer's full load current. The normal no-load current of a power transformer for ordinary electronic equipment should be about 100 mA. Too much, transformer short circuit failure.
B, indirect measurement of 10/5W resistance in series in the primary winding of the transformer, secondary or all empty load. Switch the multimeter to the AC voltage switch. After charging, use two meters to measure the voltage drop U at both ends of resistance R, and calculate the no-load current I empty by ohmic rule, that is, I empty =U/R.
The primary power transformer is connected to 220V mains, and the no-load voltage value (U21, U22, U23, U24) of the windings should be measured in sequence through the multimeter AC voltage connection. The error range is generally ≤±10% for the high-voltage windings, ≤±5% for the low-voltage windings, and the voltage difference between the two groups of symmetric windings with a center tap is ≤±2%
6. Test the temperature range of the power transformer: generally, the allowable temperature of the low-power power transformer is 40℃ ~ 50℃. The allowable temperature rise can be increased if the insulation used is of good quality.
7, check the identification of the winding resistance of the computer: in the application of the ring transformer, sometimes in order to better obtain the required secondary coil working voltage, can be 2 or several secondary winding series application. When using ring transformer in series method, the computer which signed up for the winding resistance in series must be properly connected and can not be mistaken. Otherwise, the transformer can not work properly.
8. Comprehensive detection and discrimination of power transformer short circuit fault: the main symptoms of power transformer short circuit fault are serious heating and abnormal output voltage of secondary winding. The more short-circuit points between turns in the general coil, the greater the short-circuit current, the more serious the transformer heat. A simple method to detect and judge whether a power transformer has a short circuit fault is to measure the no-load current.
The transformer with short circuit fault has a no-load current value much greater than 10% of the full load current. When the short circuit is serious, the transformer is heated rapidly in tens of seconds after no-load power, and the iron core will feel scalded when touched by hand. At this point, there is no need to measure the no-load current, and the transformer can be judged to have a short circuit point.
What is the difference between the main purpose of a power transformer and a transformer?
Power transformer: power transformer can be very stable within a certain range of working voltage into very accurate bottom voltage or high voltage (such as 110V-250 input, output voltage can be stable operation in the necessary working voltage positive or negative very low 0.5V). Transformer: the output voltage of the transformer is continuously changing with the input working voltage, that is, the output voltage is improved when the input working voltage is increased, and the output voltage is reduced when the input working voltage is reduced. To sum up, the conclusion is that the switching power supply first changes the alternating current into direct current, and the direct current becomes the alternating current of higher frequency through the power switch tube. The voltage conversion through the high-frequency transformer not only improves the efficiency, but also greatly reduces the volume and saves the loss of copper and iron after the increase of frequency. Due to the control of power switch tube, the switching time is short when the current is small, and the output voltage can be maintained. When the load is heavy, the switch tube is kept working to maintain the output voltage. Therefore, switching power supply output voltage stability, can be used as LED display and other high-precision instruments.