In the constant current mode, the working power supply needs to adopt the overvoltage protection function. Regardless of the load, the constant current power supply can produce a constant output current. If the load resistance increases, the output voltage of the power supply must also increase. This is how the power supply maintains a constant current output. If the power supply detects excessive load resistance or the load is disconnected, the output voltage can increase beyond the rated voltage range of the IC or other discrete circuit components. Constant current LED drivers can use multiple overvoltage protection methods. One method is to use a Zener diode in parallel with the LED. This method can limit the output voltage to the Zener breakdown voltage and the reference voltage of the power supply. Under overvoltage conditions, the output voltage will increase to the Zener breakdown point and begin to conduct. The output current passes through the Zener diode and is then grounded through the current sense resistor. The power supply can continuously produce a constant output current when the Zener diode limits the maximum output. A better approach to overvoltage protection is to monitor the output voltage and turn off the power when the overvoltage threshold is reached. In the event of a fault, shutting down the power supply during overvoltage conditions reduces power consumption and extends battery life.
Load disconnection One of the often overlooked functions in LED drive power is load disconnection. When the power fails, the load disconnect function can disconnect the LED from the power. This feature is critical in two situations, namely power down and PWM dimming. During power-down of the boost converter, the load is still connected to the input voltage through the inductor and capture diode. Since the input voltage is still connected to the LED, a small current will continue to be generated even if the power supply has failed. Even small leaks can dramatically reduce battery life during long idle periods. Load disconnection is also important during PWM dimming. During the PWM idle period, the power supply has failed, but the output capacitor is still connected to the LED. If there is no load disconnect function, the output capacitor will be discharged through the LED until the PWM pulse turns on the power again. Since the capacitor is partially discharged at the beginning of each PWM cycle, a power supply must charge the output capacitor at the beginning of each PWM cycle. Therefore, an inrush current pulse is generated at each PWM cycle. Inrush current reduces system efficiency and creates transient voltages on the input bus. If there is a load disconnect function, the LED will be disconnected from the circuit. In this way, there is no leakage current when the power fails, and the output capacitors are full between PWM dimming cycles. It is best to place a MOSFET between the LED and the current-sense resistor when implementing a load-disconnect circuit. Placing a MOSFET between the current-sense resistor and ground creates an additional voltage drop.