Circuits in action: LED driver using flyback topology

The voltage on C1 in the figure is the voltage after bridge rectification. Rs1 samples the current flowing through the MOS tube and performs cycle-by-cycle current limiting control to prevent the current peak of the MOS tube from being too large, ensuring that the transformer does not experience magnetic saturation when the load is short-circuited. The auxiliary winding is used to complete the current zero-crossing detection of the primary winding of the transformer (APFC transformer demagnetization), and the power switch tube Q2 is controlled to restart the next switch conduction working cycle. The resistance value of the resistor R4 connected to the Idet pin of FAN7527B is within the range of tens of kilo-ohms, so that the circuit works in the "quasi-zero voltage conduction" working mode.

　　The resistance value of R4 is related to the inductance of the primary winding of the transformer and the output capacitance of the power switch MOSFET. The specific resistance value can be determined through experiments. In this circuit, the value is 33K. Rs2 samples the load LED current signal, and R7 and R8 form a voltage divider network to sample the voltage on the LED.

　　Rs2 samples the current on the LED and compares it with the reference signal CVin on TM101. The output is controlled by the error amplifier for constant current. The brightness of the LED is basically proportional to the current flowing through the LED. The brightness of the LED can be adjusted by controlling the current flowing through the LED.

　　R7 and R8 sample the voltage on the LED and compare it with the reference signal CVin on TM101. The output voltage is controlled by the error amplifier. The two signals sent to TM101 are "ANDed" and sent to the error amplifier of the control chip FAN7527B through the optical coupler and enter the multiplier.

　　The other path of the multiplier is to sample the full-wave rectified mains signal through R13, R19, R23 and R27. The product of these two signals is the multiplier output. This output signal makes the inductor current track the output waveform signal of the multiplier. The generated PWM pulse controls the switch of the MOS tube Q1 to control the load current and input current, and complete the constant current and voltage limiting control of the LED and the correction of the input power factor. The LED driver power supply circuit using the flyback topology: