Nowadays, smartphones consume more and more power, and most of the smartphone batteries are not removable. A mobile power source with large capacity and convenient carrying capacity has become an essential electronic product for people to travel. However, the recent mobile power safety accidents have caused consumers and engineers to re-examine the design and development of mobile power. How much do you know about the internal structure of mobile power?
Electronic enthusiasts and Huaqiang Electronic Network jointly created the “Finding the Best Mobile Power Source – Mobile Power PCBA Solution Promotion†campaign to provide engineers and consumers with the most valuable mobile power design information.
A mobile power source usually consists of a housing, a battery, and a circuit board. The outer casing is mainly product packaging, as well as the effect of beautiful appearance, protection, etc., which are commonly used for plastics and metals. Some good products are often fireproof materials for plastics. The board is primarily used for voltage, current control, input and output control, and more. Cell is the most costly component of mobile power, with 18650 cells and polymer cells being the two most common.
In addition to the battery core, the board in the mobile power supply is also very important. For rechargeable batteries, the specification has a safe charge cut-off voltage and a safe discharge cut-off voltage, with a nominal rated maximum operating current. The design of the mobile power supply must first securely charge the polymer battery because the battery cost is relatively high, and a charging management system is required for the safe and reliable operation of the system. When it is necessary to charge a portable device, the polymer battery is discharged to the outside. Since the portable device generally has a input voltage of 5V, there is a system that boosts 5V. Both the charging management system and the boosting system require a circuit board to provide, so the design of the internal circuit board of the mobile power supply determines whether the product is smart or not.
After a large number of security incidents have been exposed by the media, the security performance of mobile power has become an important factor for people to consider. So engineers should spend more time on the protection module when designing mobile power products. Which specific protection module is included in the product?
Overcharge protection
Lithium battery overcharge protection is realized by the power management chip to detect the voltage. The internal state of the chip is in the reference setting state (the mobile phone lithium battery is generally 3.5V). When the reference rises slowly, 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 by the logic output 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 the logical relationship to cancel the overcharge protection.
2. Over discharge protection
The over-discharge protection voltage refers to the lowest voltage that protects the battery during the discharge transition. When discharging to this voltage point, the protection circuit cuts off the circuit to achieve the purpose of protecting the battery. According to the relationship between battery life and discharge depth and the relationship between battery voltage and discharge rate and discharge depth, combined with the actual load of the equipment, the battery discharge termination voltage is determined, and the battery discharge protection circuit is designed.
3. Short circuit protection
The loop current caused by the short circuit is generally more than 10 times of the rated operating current, while the overcurrent protection needs to be delayed by about several tens of milliseconds. The tens of times of the rated current caused by the direct short circuit also affects the performance of the battery pack within several tens of milliseconds. The existing protection method is the PPTC method, which is a thermal cut-off circuit generated by current, which also requires a reaction time of a millisecond level, and increases the impedance in the loop. There is also a short-circuit integrated chip dedicated to the battery pack, which has a narrow application range and high cost.
4.PTC Introduction
PTC positive temperature coefficient thermistor, also known as polyswitch, polymer resettable fuse polymer self-resetting fuse consists of a polymer matrix and carbon black particles that make it conductive. Since the polymer self-resetting fuse is a conductor, current flows therethrough. When an overcurrent is passed through the polymer self-resetting 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 self-resetting fuse will rise. This will cause the polymer self-resetting fuse to generate heat more quickly, expand more, and further increase the resistance. When the temperature reaches 125 ° C, the resistance changes significantly, so that the current is significantly reduced. The small current flowing through the polymer self-resetting fuse at this point is sufficient to keep it at this temperature and in a high resistance state. When the fault is removed, the polymer self-resetting fuse shrinks to its original shape and rejoins the carbon black particles, thereby reducing the resistance to a level that has a specified holding current.
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