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

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SMD aluminum electrolytic capacitor

Introduction

The Universal Serial Bus (USB) port is a two-way data port that also provides power and ground. USB allows connectivity with various peripherals like external drives, storage devices, keyboards, mice, wireless interfaces, cameras, MP3 players, and countless other electronic devices. Many of these devices are battery-powered, some with built-in batteries. Designing a battery charger to work with USB presents both opportunities and challenges. This article explains how to interface a simple battery charger with a USB power source. It covers the key aspects of the USB power bus, including voltage, current limits, inrush current, connectors, and cable connections. Additionally, it introduces nickel-hydrogen (NiMH) and lithium battery technologies, charging methods, and termination techniques. A complete example circuit is provided for intelligent charging of a NiMH battery through the USB port, along with relevant charging data.

1. Features of USB

The USB bus supplies power to low-power electronic devices. The power is isolated from the grid and maintains excellent stability. However, the available current is limited, and there may be 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 supply), and ground. These four wires are shielded with an inner foil shield and a braided outer shield. The latest USB specification is version 2.0, which can be accessed for free from the USB organization. To fully comply with the specification, a two-way communication between the device and the host is necessary using a function controller. The specification defines a unit load of 100 mA (maximum), with the maximum current any device is allowed to draw being 5 unit loads.

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

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 shut down one or more of the active USB ports. Many commercially available USB devices, such as standalone battery chargers, lack a function controller to manage the enumeration process and often draw more than 100 mA. Under these circumstances, these devices can cause issues with the host. For instance, if a device drawing 500 mA is connected to a bus-powered USB hub without the correct enumeration process, the hub and host ports could become overloaded simultaneously.

The situation becomes even more complex when the host operating system employs advanced power management, particularly in laptops, which 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 into 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 comprehensive, detailing the quality of the power supply, connector construction, cable materials, allowable voltage drops, and inrush currents. Low-current and high-current ports have different power supply standards, primarily due to the voltage drop across the connector and cable between the host and the load, including voltage drops on powered USB hubs. Hosts, such as computers or self-powered USB hubs, feature high-current ports capable of providing up to 500 mA. Passive, bus-powered USB hubs have low-current ports. Table 1 lists the allowable voltage tolerances for the upstream (power) pins of USB high-current and low-current ports.

Table 1. USB 2.0 Specification Power Quality Standards Parameter Requirements

* These metrics apply to the connector pins of the upstream host or hub port. Voltage drops on cables and connectors require additional consideration.

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