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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