LED power protection and load characteristics

In steady current mode, the LED display manufacturer's driver components must provide overvoltage protection. The LED is disconnected from the power supply, so that when the power fails, there is no leakage current, and the output capacitor is full during the cycle of PWM brightness adjustment. No matter how much load there is, a steady current can be generated. However, if the load resistance increases, the corresponding output voltage of the power supply must also increase. When the power supply detects the load resistance, or the load is disconnected, the output voltage may exceed the operating voltage range of the IC or other components.

　　Overvoltage protection must be provided in the driver. Load disconnection is a commonly overlooked function in LED driver power supplies, because when the power supply fails, the load disconnection can be used to disconnect the LED from the power supply. This function is very serious in the following two situations, namely power failure and PWM dimming. For example: when the boost converter is powered off, the load simulation is still connected to the input voltage through the inductor and diode. For example: a Zener diode can be used in parallel with the LED, because when the PWM is not operating, the power supply is ineffective, but the output capacitor simulation is still connected to the LED. If there is no load disconnection, the output capacitor simulation will still provide power to the LED until the PWM turns on the power supply again. Because the capacitor will discharge at the beginning of each PWM cycle, the power supply must charge the output capacitor at the beginning of each PWM cycle, so a surge pulse will occur at the beginning of each PWM cycle. The inrush current will cause system efficiency to decrease and a transient voltage will appear on the input bus. If there is a load disconnect function, such a system can limit the output voltage to the Zener diode breakdown voltage and the power supply. When an overvoltage occurs, the output voltage will increase to the Zener diode breakdown point and pass through the Zener diode and then to the grounded current detection resistor, so the output current can be stably supplied with the Zener diode in parallel with the LED.

　　It is also possible to monitor the output voltage and shut down the power supply before reaching overvoltage. When overvoltage occurs, the LED driver can reduce power consumption and extend battery life. Portable LCD backlight applications require limited brightness adjustment. Two dimming methods can be used in this part, namely analog or PWM. Using analog dimming, as everyone is familiar with, adding 50% current to the LED, so that the brightness can be increased by 50%. But this method has disadvantages, that is, LED color shift will occur, and analog control signals need to be used. This mode is generally not used much.