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Hi,
can you please help me to simulate this circuit ? and explain it to me,
i tried to use LT spice to simulate the circuit, but every time i failed to do that
S is IRF9540, L-L1, D-D2, C-C1, R-D1 on your drawing. It remains to determine what controls the key S.
R1 is current sensor for R(D1). If U(R1)> 0.65V -> Q2 turn on, Q1 and IRF9540 turn off. After some time, determined by the energy stored in the inductance L current through R1 begins to decrease. If U(R1)< 0.65V -> Q2 turn off, Q1 and IRF9540 turn on, current through R1 begins to increase, the process is repeated.
Note that the circuit is very sensitive to the parameters of inductance L1. For it small values or low quality of the generation breaks down and scheme degenerates into a linear current regulator.
Here is my simulation of a basic buck converter, showing waveforms.
The coil is at the heart of the action. It will help if you put a scope trace on it, to show the current through it.
The duty cycle is determined by the proportion of output V to supply V. (I guessed 3V for the led, 12V for the supply.)
The operating frequency is largely determined by the coil's value (130uH in your schematic).
I determined the frequency by clicking the switch to turn on the transistor. I tried to maintain 300 mA through the high-brightness led. Eventually I found a constant tempo. The scope trace showed 41 kHz or so.
Notice how the waveforms soar for a few cycles after startup. Then they settle down at an even pattern.
To get a grasp on the concept of operation, it helps to have an interactive animated simulation (I used Falstad's). Click the link below to open the website falstad.com/circuit, load my schematic in his simulator, and run it on your computer.
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