Method for implementing USB port on smart charging of NIMH lithium batteries

　　Author ：Iflowpower – Portable Power Station Supplier

Overview Universal Serial Bus (USB) Port is a two-way data port with power and ground. USB can connect all types of peripherals, including external drives, storage devices, keyboards, mouse, wireless interface, camera, camera, MP3 player, and number of unique electronic devices. These devices have many battery powered, some with built-in batteries.

With regard to battery charging design, the extensive USB has brought both opportunities and challenges. This article describes how to interface a simple battery charger with the USB power supply. The article reviews the characteristics of the USB power bus, including voltage, current limit, surge current, connector, and cable connection problem.

At the same time, NiMH and lithium-ion battery technology, charging method, and charging termination technique are introduced. A complete sample circuit is given to realize the USB port to charge the NIMH battery, and the charging data is given. USB feature USB bus can supply power for low-power electronic devices.

Bus power supply is isolated from grid and has good stability. However, there is a limited number of current current, and there is potential interoperability between load and host or power. USB port consists of 90ohm; bidirectional differential shield twisted pair, VBus (+ 5V power) and.

This 4 line is shielded by the inner shield of the aluminum foil and the weaving online shield. The latest USB specification standard is version 2.0, you can get free from USB organization.

To achieve the standard standard, you must implement two-way communication between devices and hosts through a functional controller. Normally mean 1 unit load is 100mA (maximum). Any device allows for maximum current to be 5 units.

The USB port can be divided into two types of low power ports and high power ports, and low power ports can supply 1 unit load current, high-power ports can supply up to 5 unit loads. When the device is just connected to the USB port, the enumeration process identifies the device and determines its load requirements. During this process, only the device is allowed to take up to 1 unit load from the host.

After the enumeration process is completed, if the power management software of the host allows, the high power device can learn more current. Some host systems (including downstream USB hubs) supply current limiting function through fuses or active current detectors. If the USB device does not take the enumeration process, the large current (more than 1 unit load) is absorbed from the USB port, and the main chance detects the overcurrent state and closes one or more USB ports being used.

Many USB devices supplied on the market, including independent battery chargers, there is no functional controller to handle enumeration processes, but the currents of the absorbed exceed 100mA. These devices may cause problems in this inappropriate conditions. For example, if a device in which a 500mA current is inserted into the bus-powered USB hub, and the correct enumeration process may cause the hub port and the host port to overload.

The host operating system is more complicated when using advanced power management, especially laptops, it always wants to be as low as possible. In some power saving modes, the computer issues a pending command to the USB device, and then it is considered that the device entered the low power mode. The device contains a functional controller that can communicate with the host is always a better approach, even if it is about low power equipment.

The USB 2.0 specification is very comprehensive, the quality of the power supply, the connector structure, cable material, allowed voltage drop and the surge current, etc. Low current and large current ports have different power indicators.

This is important to determine the voltage drop in the connector between the host and the load and the voltage dropped, and includes the voltage dropped on the hub supplied by USB. Including a computer or a host, a host, a host, a large current port, a current of the maximum 500mA. Passive, bus powered USB hub has low current ports.

Table 1 lists the voltage tolerance allowed by USB large current and low current port upstream (power) pins. Table 1.USB2.

0 Specifications Power Quality Standard ParameterRequirementDcVoltage, High-PowerPort * 4.75VTO5.25VDCVOLtage, Low-PowerPort * 4.

40VTO5.25VMaximumQuiescentCurrent (LowPower, SuspendMode) 500µAmaximumquiescentcurrent (HighPower, Suspendmode) 2500µAmaximumAllowableInputCapacitance (Loadside) 10µFMinimumRequiredOutputPutcapacitance (Hostside) 120µFly±20% MaximumallowableinRushchargeIntoload50µC * These indicators apply to the connector pins of the upstream host or hub port. The IXR drop on the cable and connector needs to be considered.

In the host in accordance with USB2.0 specification, the upstream end of the high power port has 120µF, low ESR capacitance. The input capacitance of the connected USB device is limited to 10µF, within the initial load connection phase, the maximum amount of charge obtained from the host (or self-powered hub) is allowed to be 50µC.

In this way, when the new device is connected to the USB port, the transient voltage of the upstream port is less than 0.5V. If the load should be more input when the load is working properly, the surge current limiter must be supplied to ensure that the current does not exceed 100mA when charging the greater capacitance.

When the USB port is powered by a bus power supply, the Hub is connected to the low power device, and the DC voltage drown on the USB port is shown in Figure 1. When the high-power load is connected to the bus, the voltage drop will exceed the indicator given in Figure 1, and will cause the bus overload. Figure 1.

The voltage drop of the host to the low power load is larger, which causes the bus overload when the allowed DC voltage given in the figure. Battery charging requires a single-cell lithium ion and lithium polymer battery Today's lithium ion battery is charged to the maximum rated capacity, and its voltage is usually between 4.1V to 4.

2V. The current market is selling, updated, larger battery, with a voltage range between 4.3V to 4.

4V. Typical prismatic lithium ions (Li +) and lithium polymers (li-poly) battery capacity is 600mAh to 1400mAh. For li + and li-poly batteries, the preferred charging curve begins with constant current charging, continuous to the battery voltage to reach the rated voltage.

The charger is then adjusted on the voltage at both ends of the battery. These two adjustment methods constitute a constant current (CC) constant voltage (CV) charging method. Therefore, this type of charger is often referred to as CCCV charger.

After the CCCV charger enters the CV mode, the charging current of the battery begins to fall. If the typical charging rate of 0.5c to 1.

5c is charged, the charger is converted to CV mode when the battery reaches 80% to 90% of its full capacity. Once the charger enters the CV charging mode, the battery current is monitored; when the current reaches the minimum threshold (several milliamps or tens of milliamps), the charger terminates charging. A typical charging curve of a lithium-ion battery is shown in Figure 2.

Figure 2. Typical curve when charging using a CCCV charger, from the USB voltage drop index shown in Figure 1, the downstream low power port voltage of the port power supply hub does not have sufficient margin, it is difficult to charge the battery To 4.2V.

The small amount of additional resistance present on the charging path will hinder normal charging. LI + and Li-Poly batteries should be charged at a suitable temperature. The highest charge temperature recommended by the manufacturer is usually +45°C to +55°Between C, the maximum discharge temperature allowed can be higher than 10°Class C left.

These batteries use materials, chemical properties are very lively, if the battery temperature exceeds +70°C, combustion will occur. The lithium-ion battery charger should have a thermal shutdown circuit that monitors the battery temperature. If the battery temperature exceeds the maximum charge temperature recommended by the manufacturer, the charging is terminated.

NiMH Battery (NIMH) NIMH battery is more important than lithium ion batteries, and its energy density is also lower than lithium ion batteries. The NIMH battery has been cheap than the lithium-ion battery, but the price difference of both recently is shrinking. The NIMH battery has standard size, which can replace alkaline batteries in most applications.

The nominal voltage of each battery is 1.2V, which will reach 1.5V after it is full.

Usually charges NIMH batteries in constant current sources. When it is fully filled, the thermal chemical reaction occurs, causing the battery temperature rise, and the battery-side voltage is lowered. The battery temperature rising rate or negative voltage change rate is detected, and the charging is used to terminate.

These charging termination methods are called DT / DT and -V, respectively. When the charging rate is very low, the DT / DT and -V are not obvious, it is difficult to detect accurately. When the battery begins to enter the overcharge state, the DT / DT and -V responses begin to appear.

At this point, if you continue to charge, the battery will be damaged. Termination detection is much easier when the charging rate is greater than C / 3. The temperature rise rate is approximately 1°C / minute, the -V response is more pronounced than the low charge rate.

After the fast charge is over, it is recommended to take a less time to completely fully plenty of batteries (charges charge) with a smaller current. After the completion of the charging phase, the self-discharge effect is compensated for the self-discharge effect using C / 20 or C / 30, so that the battery is maintained in full state. Figure 3 shows a battery voltage curve that charges the NIMH battery (a portion of the electricity in advance) using a DS2712NIMH charger.

In this figure, the data of the above curve is obtained when the charging current is in the battery, and the data of the curve below is measured at the time of cutting the current. In DS2712, the voltage difference is used to distinguish NIMH batteries and alkaline batteries. If an alkaline battery is detected, the DS2712 does not charge it.

Figure 3. Charge the NIMH battery using the DS2712 charging controller.