LED power driver chip with optocoupler

The optocoupler of the switching power supply is mainly used for isolation, feedback signal and switching. The power supply of the optocoupler in the switching power supply circuit is provided by the secondary voltage of the high-frequency transformer. When the output voltage is lower than the voltage of the voltage regulator tube, the signal optocoupler is turned on, the duty cycle is increased, and the output voltage is increased; otherwise, the optocoupler is turned off to reduce the duty cycle, so that the output voltage is reduced. Once the secondary load of the high-frequency transformer is overloaded or the switching circuit fails, there is no optocoupler power supply, and the optocoupler controls the switching circuit to prevent oscillation, thereby protecting the switching tube from being broken down and burned.

Usually the optocoupler is used together with TL431 . Below is a partial circuit of the LED power driver chip (switching power chip). The two resistors are connected in series and sampled to the 431R end and compared with the internal comparator. Then the resistance of the 431K end (the end where the anode is connected to the optocoupler) to the ground is controlled according to the compared signal, and then the brightness of the light-emitting diode inside the optocoupler is controlled. (There is a light-emitting diode on one side and a photosensitive transistor on the other side inside the optocoupler) through the intensity of light emission. Controlling the resistance of the CE end of the other end of the transistor is to change the current of the detection pin of the LED power driver chip (switching power chip) (pin 1: voltage feedback pin, the duty ratio is adjusted by connecting the optocoupler to the ground). According to the size of the current, the LED power driver chip (switching power chip) will automatically adjust the duty ratio of the output signal to achieve the purpose of voltage stabilization.

The above chip is a highly integrated, high-performance PWM + MOSFET two-in-one current-type offline switching power supply controller. It is suitable for various types of low-power switching power supplies such as chargers, power adapters, LED driver power supplies, etc. It uses DIP8 package and can output 0-36W power without adding a heat sink (adding heat dissipation can make it larger). The circuit structure is simple and the cost is low. It has complete protection functions, including overvoltage, undervoltage, overtemperature, overload and short circuit protection. Fixed oscillation frequency and frequency jitter function can reduce EMI. The standby power is low, and it enters the skip cycle mode when in standby mode, which meets the "Energy Star" standby power consumption standard requirements.