Addressing Voltage Discrepancies with a USB Device
One common issue with AC mains adapters powering USB devices is the mismatch in voltage, often not aligning with the USB standard’s 5 V specification. This circuit tackles this problem effectively. It accepts input voltages ranging from 4 V to 9 V and converts them to a stable 6 V output. This output is then regulated to a clean 5 V level using a series regulator. The key to this conversion lies in the implementation of a boost/buck converter based on the SEPIC principle. A variation of the Cuk converter without the drawback of generating a negative output voltage.
SEPIC Principle for Efficient Voltage Conversion
This circuit relies on the SEPIC principle, combining a boost converter with a buck converter, to efficiently regulate the output voltage. The SEPIC converter, a modified version of the standard boost (step-up) converter, incorporates an additional capacitor (C2 in this case) and a second inductor (the secondary winding of transformer L1 in this setup). This configuration allows for precise voltage regulation without the complexities associated with negative output voltages, enhancing the overall efficiency and reliability of the circuit.
MAX668 Controller and Flexible Configuration
At the heart of this circuit is the MAX668, originally designed as a controller for boost converters. Its versatility allows for seamless integration into this SEPIC-based design. Notably, the circuit’s flexibility is highlighted by the ability to revert to a standard boost converter. By replacing capacitor C2 with a wire bridge and leaving the secondary winding of L1 open. The circuit transforms into a conventional boost converter, showcasing the adaptability of this versatile design.
Continuous Current Flow and Voltage Regulation in SEPIC Converter
In this configuration, a continuous current path is maintained from the input to the output through L1 and D1, even when IC1 doesn’t drive the FET. Consequently, the output voltage remains above the input voltage, accounting for the voltage drop across the diode. The SEPIC converter’s functionality can be simplified: C2 prevents any DC voltage from the input affecting the output, enabling easy adjustment of the output voltage below the input voltage.
Role of the Second Coil and Transformer Efficiency
The presence of the second coil ensures a specific voltage at D1’s anode. While it’s possible to replace the transformer with two uncoupled coils. The circuit’s efficiency benefits from coupled coils, as demonstrated here. The value of resistor R4 is meticulously selected to limit the maximum current to 500 mA, aligning with the USB bus specifications. R1 and R2 work in tandem, regulating the voltage across C3 and C7 to approximately 6 V. Utilizing a low-drop regulator (LM2940). A stable 5 V is derived from the 6 V output, albeit with a ripple voltage. The circuit’s efficiency is expected to range between 60% and 80%. Making it a balanced choice for practical applications.