Controller and method for a constant-current switch-mode power converter. For example, a controller for a constant-current switch-mode power converter includes: a constant-current controller configured to receive an input voltage and generate a modulation signal based at least in part on the input voltage; and a driver configured to receive the modulation signal, generate a drive signal based at least in part on the modulation signal, and output the drive signal to a transistor coupled to an inductive coil and a resistor; wherein: the drive signal corresponds to at least one switching cycle; the switching cycle includes an on-time during which the drive signal is at a first logic level and an off-time during which the drive signal is at a second logic level; and the off-time includes a demagnetization period during which the inductive coil undergoes a demagnetization process.