The DC into high-frequency AC power provided to the transformer transformer, resulting in the required group or groups of voltage! Re-transmission to high-frequency AC because high-frequency AC transformer transformer circuit in the efficiency of more than 50HZ A lot, so the switching transformer can be done very small, and the work is not very hot, the cost is very low, if you do not change the high-frequency 50HZ switching power supply that does not mean
Switching power supply workflow is:
Power → input filter → full-bridge rectifier → DC filter → switch (oscillation inverter) → switch transformer → output rectifier and filter.
The AC power input is rectified and filtered into DC
Through the high-frequency PWM (pulse width modulation) signal control switch, the DC will be added to the primary switch transformer
Switching transformer secondary induction of high-frequency voltage, the rectifier filter supply to the load
Output part of the circuit through a certain feedback to the control circuit, control PWM duty cycle, in order to achieve the purpose of stable output
AC power input generally go through the Eau circle of a class of things, filter out the interference on the grid, but also filter out the power supply interference on the grid;
At the same power, the higher the switching frequency, the smaller the size of the switching transformer, but the higher the requirements of the switch;
The secondary of a switching transformer can have multiple windings or multiple windings per winding to obtain the desired output;
General should also increase some protection circuit, such as no-load, short circuit protection, or it may burn the switching power supply.
Mainly used in industry and some household appliances, such as televisions, computers and so on
Switching Power Schematic Analysis
1, forward circuit
The working process of the circuit:
a> When the switch S is turned on, the voltage across the transformer winding N1 is positive and negative, and the voltage across the N2 winding coupled with it is also positive and negative, so VD1 is in the on state, VD2 is in the off state, and the current in the inductor L is gradually increasing ;
b> S off, the inductor L through VD2 freewheeling, VD1 off .S off after the transformer excitation current through the N3 winding and VD3 back to power, so S off after the withstand voltage.
c> Transformer core reset: After the switch S is turned on, the excitation current of the transformer starts from zero and increases linearly with time until S turns off.To prevent the transformer from magnetizing the excitation inductance, we must try to make the excitation current in S Turn off to the next reopening of a period of time back to zero, the process known as the transformer core reset.
Idealized waveform of the forward circuit:
Transformer core reset time:
Tist = N3 * Ton / N1
Output voltage: output filter inductor current continuous case:
Uo / Ui = N2 * Ton / N1 * T
Core reset process:
2, flyback circuit
Flyback circuit schematic
The transformer in the flyback circuit plays the role of energy storage element, which can be seen as a pair of coupled inductors.
work process:
After the opening of S, VD is in the off state, the current of N1 winding linearly increases, and the energy storage of the inductor increases.
S turns off, the current of the N1 winding is cut off, and the magnetic field energy in the transformer is released to the output through the N2 winding and the VD. The voltage after the S is turned off is: us = Ui + N1 * Uo / N2
Flyback circuit mode of operation:
Current Continuous Mode: When S is on, the current in the N2 winding has not dropped to zero.
Output voltage relationship: Uo / Ui = N2 * ton / N1 * toff
Current Intermittent Mode: The current in N2 winding has dropped to zero before S turns on.
The output voltage is higher than the calculated value of the above equation and increases as the load decreases