button battery cr2032.Design and product selection of mobile power protection circuit

　　Nowadays, smartphones consume more and more power, and most smartphone batteries are not removable. A large-capacity and easy-to-carry mobile power supply has become a must-have electronic product for people to travel. However, the recent frequent safety incidents of mobile power supplies have forced consumers and engineers to re-examine the design and development of mobile power supplies. And how much do you know about the internal structure of mobile power supplies?

　　A mobile power supply usually consists of a casing, a battery cell and a circuit board. The shell is mainly used to encapsulate the product and achieve beautiful appearance, protection and other functions. It is commonly made of plastic and metal. Some well-known products often use plastic as a fireproof material. The circuit board is mainly used to implement voltage, current control, input and output control, and various other functions. Batteries are the most expensive component of mobile power supplies, and 18650 batteries and polymer batteries are the two most common. In addition to the battery core, the circuit board in the mobile power supply is also very important. For rechargeable batteries, the specifications include safe charging cut-off voltage and safe discharge cut-off voltage, as well as a calibrated rated maximum operating current. When designing a mobile power supply, the polymer battery must be charged safely first, because the cost of the battery is relatively high, and in order for the system to work safely and reliably, a charging management system is required. When a portable device needs to be charged, the polymer battery is discharged to the outside. Because the input voltage of portable devices is generally 5V, there is a 5V boost system. Whether it is a charging management system or a boosting system, it requires a circuit board to provide it. Therefore, the design of the internal circuit board of the mobile power supply determines whether the product is smart or not.

　　1. Overcharge protection

　　Lithium battery overcharge protection is implemented by using the power management chip to detect the voltage. The internal chip is in a baseline setting state (mobile phone lithium batteries are generally 3.5V). When the reference slowly rises, when it rises to the VSS-VDD design value, the voltage at this time is the protection overcharge shutdown voltage, and the external control circuit is controlled through a logic relationship to output a low or high level to achieve overcharge protection. When the voltage drops slowly, set the VSS-VDD voltage value to take the reference value. When the reference detects that it is below the setting, this is a logical relationship to release the overcharge protection.

　　2. Over-discharge protection

　　Over-discharge protection voltage refers to the lowest voltage that protects the battery during discharge transition. When the voltage reaches this voltage point, the protection circuit cuts off the circuit to protect the battery. According to the relationship between battery life and discharge depth, the relationship between battery voltage, discharge rate and discharge depth, combined with the actual load of the equipment, determine the battery discharge termination voltage and design the battery discharge protection circuit.

　　3. Short circuit protection

　　The loop current caused by a short circuit is generally more than 10 times the rated operating current, and overcurrent protection requires a delay of about tens of milliseconds. Dozens of times the rated current caused by a direct short circuit will also affect the performance of the battery pack within tens of milliseconds. Existing protection methods include the PPTC method, which cuts off the circuit thermally through current, which also requires millisecond-level response time and increases the impedance in the circuit. There are also short-circuit integrated chips dedicated to battery packs. This chip has a narrow application range and high cost.

　　4.PTC introduction

　　PTC positive temperature coefficient thermistor is also called a polyswitch, and a polymer resettable fuse is composed of a polymer matrix and carbon black particles that make it conductive. Because polymer resettable fuses are conductors, current will flow through them. When an overcurrent passes through a polymer resettable fuse, the heat generated (I2R) will cause it to expand. As a result, the carbon black particles will separate and the resistance of the polymer resettable fuse will increase. This will cause the polymer resettable fuse to generate heat faster and expand more, further increasing the resistance. When the temperature reaches 125°C, the resistance changes significantly, causing the current to decrease significantly. The small current flowing through the polymer resettable fuse at this time is enough to keep it at this temperature and in a high resistance state. When the fault is cleared, the polymer resettable fuse shrinks back to its original shape and reconnects the carbon black particles, thereby reducing resistance to a level with a specified holding current.

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