Battery charging new method - USB - Power Circuit - Circuit Diagram

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SMD aluminum electrolytic capacitor

Introduction:

The Universal Serial Bus (USB) port serves as a dual-directional data port that also provides power and ground. It can connect to various peripherals such as external drives, storage devices, keyboards, mice, wireless interfaces, cameras, MP3 players, and countless other electronic gadgets. Many of these devices are battery-operated, some featuring built-in batteries. For battery charging designs, the widespread adoption of USB presents both opportunities and challenges. This article explains how to interface a basic battery charger with a USB power source. It reviews key aspects of the USB power bus, including voltage levels, current limits, inrush currents, connectors, and cable configurations. Additionally, it covers nickel-metal hydride (NiMH) and lithium battery technologies, charging methods, and termination techniques. A complete example circuit is provided for intelligently charging a NiMH battery via the USB port, along with relevant charging data.

1. Features of USB:

The USB bus supplies power to low-power electronics. The bus power is isolated from the mains and maintains excellent stability. However, the available current is restricted, and there are potential compatibility issues between the load and the host or power supply.

The USB port comprises a 90Ω bidirectional differential shielded twisted-pair cable, VBUS (+5V power), and ground. These four wires are shielded by an inner foil shield and an outer braided shield. The latest USB specification is version 2.0, which is freely accessible from the USB organization. To fully adhere to the specification, a two-way communication between the device and the host must occur through a function controller. The specification defines a unit load as 100 mA (maximum). The maximum current any device can draw is five unit loads.

USB ports can be categorized into two types: low-power and high-power. A low-power port can supply one unit load of current, while a high-power port can provide up to five unit loads. When a device connects to the USB port, the enumeration process identifies the device and determines its current requirements. During this process, the device is allowed to draw up to one unit load of current from the host. Once enumeration is complete, the high-power device can draw more current if the host's power management software permits it.

Some host systems, including downstream USB hubs, implement current limiting through fuses or active current detectors. If a USB device draws a large current (more than one unit load) without proper enumeration, the host will detect an overcurrent condition and may shut down one or more active USB ports. Many commercially available USB devices, like standalone battery chargers, lack a function controller to manage the enumeration process and often draw more than 100 mA. Under such improper conditions, these devices might cause issues with the host. For instance, plugging a device that draws 500 mA into a bus-powered USB hub without proper enumeration could overload both the hub and the host ports simultaneously.

The complexity increases further when the host operating system employs advanced power management, particularly in laptops, which typically aim to keep port currents as low as possible. In some power-saving modes, the computer sends a suspend command to the USB device, putting it in a low-power state. Even for low-power devices, it’s always advisable to include a function controller capable of communicating with the host.

The USB 2.0 specification is thorough, detailing the quality of the power supply, connector construction, cable materials, allowable voltage drops, and inrush currents. Low-current and high-current ports follow different power supply standards. This is primarily due to the voltage drop across the connectors and cables between the host and the load, including those on USB-powered hubs. Hosts, such as computers or self-powered USB hubs, offer high-current ports that can provide up to 500 mA. Passive, bus-powered USB hubs have low-current ports. Table 1 outlines the allowable voltage tolerances for the upstream (power) pins of the USB high-current and low-current ports.

Table 1. USB 2.0 Specification Power Quality Standards

* These metrics apply to the upstream connector pins of the host or hub port. Additional considerations include voltage drops on cables and connectors.

In a compliant USB 2.0 host, the upstream end of the high-power port has a 120 µF, low ESR capacitor. The input capacitance of the connected USB device is limited to 10 µF. During the initial load connection phase, the maximum charge the load is allowed to draw from the host (or self-powered hub) is 50 µC. Consequently, when a new device connects to the USB port, the transient voltage drop at the upstream port remains below 0.5V. If a larger input capacitor is necessary for normal load operation, an inrush current limiter must be added to ensure that the current does not exceed 100 mA during capacitor charging.