Introduction to the principle of power electronic transformer

2019-04-23 15:10:05 JuKe Technology (DongGuan) Co., Ltd. Read

Introduction to the principle of power electronic transformer

Electrical Transformers

The electronic transformer has an electronic device that converts the alternating voltage of the commercial power into a direct current and then forms a high-frequency alternating current voltage output through the semiconductor switching device and the electronic component and the high-frequency transformer winding, and is also described in the electronic theory. A crossover inverter circuit. Simply put, it is mainly composed of a high-frequency transformer core (core) and two or more coils, which do not change position from each other, from one or two electric circuits, through AC power Magnetic induction, converted into AC voltage and current. At the output of the high-frequency transformer, one or two or more power circuits are supplied with high-frequency alternating current or direct current of different voltage levels.
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Electronic transformer principle:

Power electronic transformer is a power transmission and distribution device that combines power electronic converters (rectifiers, inverters) and high-frequency transformers to realize the basic functions of electric power conversion, energy transmission and system isolation of traditional power transformers. Since power devices currently applied to power systems are lower in terms of capacity and withstand voltage than transmission systems, it is expected that power electronic transformers should be first implemented in the power distribution field in power system applications.
The power electronic converter (rectifier, inverter) of the power electronic transformer should consist of two parts: the main circuit and the control circuit. For the transformer of the power distribution system, in order to be consistent with the conventional power transformer, the power electronic converter connected to the power supply side and the corresponding winding of the corresponding high frequency transformer are defined as the primary side; the power electronic converter connected to the load side And the corresponding winding of the corresponding high frequency transformer is defined as the secondary side. The two are connected by a high frequency transformer. [2]
The working principle is: on the primary side, the high frequency of the power frequency busbar becomes a high frequency alternating square wave through the action of the power electronic converter, that is, the frequency of the voltage is increased on the primary side to achieve the effect of frequency up. Since the volume of the transformer is related to the saturation magnetic flux density of the core material and the maximum allowable temperature rise of the winding, the transformer having a large saturation magnetic flux density is also large. The saturation flux density of the core material is inversely proportional to the operating frequency of the transformer. Therefore, the up-regulation of the primary-side power electronic converter can improve the utilization of the core material, thereby reducing the volume of the transformer and saving the transformer. space. This is also a major advantage of power electronic transformers compared to conventional power transformers.
Electronic transformer, the input is AC220V, the output is AC12V, and the power can reach 50W-300W. It is mainly a transformer circuit developed on the basis of high-frequency electronic ballast circuit. Its performance is stable, small in size and large in power, thus overcoming the shortcomings of traditional silicon steel sheet transformers, such as large size, heavy weight and high price.
An electronic transformer is an unregulated switching power supply, which is actually an inverter. First, the alternating current is rectified into direct current. Then, a high-frequency oscillator is formed by electronic components to convert the direct current into high-frequency alternating current, and the required voltage is output through the switching transformer and then rectified for use by the electric appliance. Switching power supplies have the advantages of small size, light weight and low price, so they are widely used in various electrical appliances.
According to the driving method of the high-frequency switch tube, it can be divided into a self-excited oscillation type and a other excitation type.

 The circuit is shown in the figure. The principle of the electronic transformer is similar to that of the switching power supply. The diodes VD1 to VD4 form a rectifier bridge to convert the mains into direct current. The high-frequency oscillation circuit consisting of the oscillating transformer T1 and the triodes VT1 and VT2 converts the pulsating DC into a high-frequency current. The high frequency and high voltage pulses are stepped down by the ferrite output transformer T2 to obtain the required voltage and power. R1 is a current limiting resistor. The resistor R2, the capacitor C1 and the bidirectional trigger diode VD5 constitute a start trigger circuit. The triode VT1 and VT2 are selected as S13005, and the B is 15 to 20 times. High-power triodes such as C3093 and other BUceo>=35OV can also be used. The trigger diode VD5 selects DB3 or VR60 of about 32V. The oscillating transformer can be self-made and wound on the magnetic ring of the H7 X 10 X 6 with an audio cable. TIa, T1b around 3 turns, Tc around 1 turn. The ferrite output transformer T2 also needs to be self-made, and the core is an EI type ferrite having a side length of 27 mm, a width of 20 mm, and a thickness of 10 mm. T2a is wound with a diameter of 0.45 mm high-strength enamelled wire 100 匝, and T2b is wound with a diameter of 1.25 mm high-strength enameled wire 8 匝. Diodes VD1 ~ VD4 select IN4007 type, bidirectional trigger diodes use DB3 type, capacitors C1 ~ C3 use polypropylene polypropylene polyester capacitors, withstand voltage 250V.
When the circuit is working, the working voltage at point A is about 12V; the point B is about 25V; the point C is about 105V; the point D is about 10V. If the voltage does not meet the above values, or the circuit does not oscillate, check the circuit for mis-welding, missing soldering or soldering. Then check if VT1 and VT2 are good, and whether the phases of T1a and T1b are correct. After the entire circuit is successfully adjusted, it can be placed in a small box made of metal material, which is beneficial for shielding and heat dissipation, but must pay attention to the insulation between the circuit and the outer casing. In addition, changing the number of turns of the T2 a and b coils can change the high frequency voltage of the output.
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After the power frequency mains supply is turned on, the bridge rectifier divides the current flowing through the Rs into the startup current on the IC pin VCC, and most of the remaining current charges the capacitor CVCC1. When the voltage on the IC pin VCC reaches the startup threshold (11.8 V), the IC starts operating. Once the IC is enabled, a charge pump circuit consisting of CSNUB, DCP1, and DCP2 feeds current to the IC pin VCC. The bootstrap diode DB and capacitor CB power the IC high side driver circuit. The Zener diode DZ is used as a shunt IC excess current to prevent IC damage.
The halogen filament resistance has a positive temperature coefficient, and the "cold resistance" at room temperature is much smaller than the "thermal resistance" at lamp operation. When the lamp is turned on, a large inrush current is generated, which affects the lamp life. However, the IR2161 provides a soft-start operation to avoid inrush current generation. The IR2161 outputs a high frequency of 125 kHz during lamp start-up. Because the primary leakage inductance of the output high-frequency transformer T1 in the system is fixed, it exhibits higher impedance at higher frequencies and lower voltage on the primary winding. As a result, the transformer output voltage is lower, the lamp current is smaller, and the protection circuit is prevented from being triggered. The circuit operates at a lower frequency for approximately 1 s. During this process, the voltage on the external capacitor CSD of IC pin 3 is increased from OV to 5V.
When unloaded, VCSD = OV and the oscillator frequency is approximately 60 kHz. At maximum load, VCSD = 5V and the oscillator frequency is approximately 30kHz. When the output is shorted, a large current flows through the half bridge and is sensed by the RCS. As long as the threshold level of the voltage on IC pin 4 (CS) exceeds 1V for more than 50ms, the system will shut down. If the load exceeds 50% of the maximum load, the voltage on IC pin 4 will exceed the lower threshold voltage of O 5 V, and after 0.5S, the system will shut down. Whether it is short circuit protection or overload protection, it can be automatically reset. The IR2161 also provides a thermal shutdown feature. When the junction temperature of the chip exceeds the over temperature limit of 135 ° C, the half bridge switch will stop working to avoid MOSFET burnout.