Design principle for obtaining efficient dual-rail power from USB

Overview:

When designing a low-power USB circuit with a power supply other than 5V, you must determine whether to use a stand-alone battery or a small power supply from the host. The problem is even more complicated if the circuit requires a dual-rail power supply greater than 5V (such as an op amp-based instrumentation amplifier) ​​or if it must be used on a portable computer such as a laptop. The USB2.0 standard specifies the power requirements for connected devices, that is, the maximum power consumption is 100mA, which is regarded as low power; when the power consumption is 500mA, it is considered as high power. The circuit described in this article was originally designed for a thermoluminescent (TL) instrument design. The microcontroller, USB interface controller, and 10 op amps were designed as low-power devices to obtain full power from a USB port. The operation of the equipment requires high performance, low noise pickup, making the system RF radiation as low as possible. Before the circuit was built, it was simulated and verified, and then used in the TL system. The appeal of this design is that because it uses common components, it improves repeatability while reducing costs. The circuit operation principle is based on the flyback concept (Fig. 1). During operation, a small transformer is driven by a pulse-modulated 555 unstable circuit operating at 115 kHz to 300 kHz. The high operating frequency allows the overall size of the circuit to be small, while providing relatively high power output and good regulation, making output filtering easier to achieve low ripple. A MOSFET is used in the actual circuit to implement the switch. In Figure 1, the diode's positive VOUT appears as a positive bias. Reverse the polarity of the diode and a transformer winding to obtain a negative VOUT. The circuit operates in three different phases. At phase one, the switch is closed and the energy is stored as a magnetic field as current flows through the primary of the transformer. The diode is reverse biased and no current flows through the secondary. In phase two, the switch opens and the diode becomes forward biased, and energy is transferred from the magnetic field to capacitor C. At phase three, the dump of energy is completed and any remaining charge stored in the switched drain capacitor is completely released. Then repeat this loop. (Please read the PDF for details)