Abstract: A self-startup circuit for a DC fan includes a voltage sampling device receiving a first control signal, a comparator, and a self-startup device. The comparator includes two input terminals and an output terminal. One input terminal is connected to an output terminal of the voltage sampling device. The self-startup device has a switching device and a diode connected in parallel. The self-startup device has an input terminal connected to the output terminal of the comparator, and outputs a second control signal for keeping the DC fan rotating even when the first control signal has a very low duty cycle.

Abstract: A start-up circuit which decreases a start-up current and stabilizes running of a DC fan. The start-up circuit includes a digital-analog converter for convert a digital control signal from a control chip to an analog control signal, a voltage sampling device connected to an output terminal of the digital-analog converter, a comparator, a switching device for controlling start-up of the DC fan, and a feedback device adjusting current passing through the switching device. The comparator includes two input terminals and an output terminal. One input terminal is connected to an output terminal of the voltage sampling device. The switching device is connected to the output terminal of the comparator. An output signal of the switching device is inputted to the other input terminal of the comparator via the feedback device.

Abstract: A power supply switching circuit includes an input terminal (2) for receiving a signal from a motherboard, an output terminal (4) to output a control signal, a first Bipolar Junction Transistor (BJT) (5), and a second BJT (7). A base of the first BJT is connected to the input terminal via a base bleeder circuit. A collector of the first BJT is connected to a stand-by power supply terminal (10) via a collector resistor. A base of the second BJT is connected to the collector of the first BJT. A collector of the second BJT is connected to a system power supply terminal (12) via a collector resistor. The collector of the second BJT is connected to the output terminal. Emitters of the first and second BJTs are grounded.