How to use diodes in the charging and discharging protection of lithium battery packs?

一, The compatibility between diode technology characteristics and lithium battery protection
1. Unidirectional conductivity: Building basic protective barriers
The core characteristic of a diode lies in the unidirectional conductivity of its PN junction, which only allows current to flow from the anode (A) to the cathode (K), with reverse cutoff. This characteristic forms a triple protection mechanism in lithium battery protection:

Anti reverse connection protection: Schottky diodes (such as MBR1045CT) are connected in series in the charging interface or circuit design. When the power polarity is reversed, the diode automatically cuts off to prevent current backflow from damaging the battery management system (BMS). According to test data from a new energy vehicle manufacturer, the BMS failure rate caused by misoperation decreased by 92% after adopting this solution.
Polarity isolation: In a multi cell series system, the physical isolation between the charging circuit and the discharging circuit is achieved through a diode array. For example, a certain energy storage power station adopts a reverse diode design, which decouples the charging end from the rectifier inverter module, ensuring continuous conduction of the discharge circuit and increasing system availability by 40%.
Anti backflow protection: Connect a TVS diode (such as SMAJ5.0A) in parallel at the output end of the DC/DC converter. When the voltage at the load end rises abnormally, the diode quickly conducts to form a discharge path, protecting the lithium battery from reverse voltage impact.
2. Fast switching characteristics: efficiency revolution in high-frequency scenarios
Schottky diodes, with their metal semiconductor junction structure, achieve near zero reverse recovery time (Trr<10ns), demonstrating significant advantages in high-frequency switching power supplies

Continuous current protection: In the Buck/Boost topology, Schottky diodes (such as SS34) serve as continuous current components, and their low conduction voltage drop (VF ≈ 0.3V) reduces switching losses by more than 60%. The actual test of a drone battery management system shows that after adopting this scheme, the DC/DC conversion efficiency has increased from 88% to 94%.
Synchronous rectification replacement: In low voltage and high current scenarios (such as 48V energy storage systems), Schottky diodes can replace the body diodes in traditional MOSFET synchronous rectification schemes, eliminating oscillations caused by reverse recovery charges (Qrr) and reducing system EMI noise by 15dB.
3. Avalanche breakdown characteristics: the ultimate defense against transient overvoltage
TVS diodes clamp transient high voltage to a safe level in picosecond time through avalanche breakdown effect, with key parameters including:

Clamp voltage (VC): It should be lower than the absolute maximum rated voltage of the BMS chip (e.g. for STM32G4 series, VC should be<36V)
Peak Pulse Power (PPP): According to the IEC 61000-4-5 standard, it is required to withstand a surge current of at least 100A under an 8/20 μ s waveform
After using SMBJ15CA TVS diodes in a certain photovoltaic energy storage system, it successfully withstood the transient high voltage of 3000V generated by lightning strikes, and the equipment failure interval time (MTBF) was extended to 120000 hours.
二, Typical application scenarios and engineering practices
1. Design of Charging Interface Protection Circuit
At the input end of the new energy vehicle OBC (on-board charger), a typical protection circuit adopts a three-level protection architecture:

First level protection: Series Schottky diodes (such as CBRD1045-40) are used to prevent reverse connection, and their 40V withstand voltage covers the requirements of 12V/24V systems
Second level protection: Parallel TVS diodes (such as P6KE36CA) suppress surge voltage, and their 36V clamp voltage matches the BMS input range
Third level protection: using self recovering fuses (PPTC) to achieve overcurrent protection, forming complementary protection with diodes
According to actual testing data from a leading car company, this solution reduces the charging interface failure rate from 0.8% to 0.12% and reduces annual maintenance costs by 23 million yuan.
2. Innovation in Balanced Protection at the Cell Level
In the Tesla 4680 battery module, a passive balancing circuit combined with Schottky diodes (such as BAT54S) is used to achieve:

Balanced current control: By adjusting the diode conduction voltage drop (VF ≈ 0.2V) and balancing resistance (R=10 Ω), the balanced current is limited to within 200mA
Thermal runaway suppression: When the voltage of a certain battery cell rises abnormally, the corresponding balance circuit diode will preferentially conduct, forming a bypass current to prevent thermal diffusion
This design increases the cycle life of the battery pack by 35% and reduces the capacity decay rate from 0.8% per month to 0.5%.
3. EMI optimization of wireless charging system
In the Xiaomi 80W wireless charging module, the high-frequency noise problem is solved through the following diode combination solution:

Rectification process: SiC Schottky diodes (such as C3D02060A) are used instead of traditional silicon-based devices, resulting in an 80% reduction in Qc value
Filtering process: Connect small signal diodes (such as BAS70-04) in parallel at both ends of the transmitting/receiving coil to form an RC absorption network, which suppresses switch noise by 40dB
Protection step: Use ESD protection diodes (such as ESD5Z5.0T1) to protect against electrostatic discharge, with a response time of<1ns
Actual testing has shown that this solution improves the system transmission efficiency from 82% to 89%, and shortens the Qi2.0 certification testing cycle by 60%.
三, Industry development trends and technological challenges
1. Material innovation drives performance breakthroughs
GaN Schottky diode: eGaN FET device launched by EPC company, with VF reduced to below 0.1V and reverse recovery charge reduced by 90%, has been applied to the 800V high-voltage platform of BMW iX
SiC hybrid module: ROHM Semiconductor integrates SiC MOSFET with Schottky diode, enabling charging module power density to exceed 3kW/in ³
2. Upgrade of Intelligent Protection Requirements
Digital control diode: TPD2E007 launched by TI company realizes programmable clamp voltage and dynamically adjusts protection threshold through I2C interface
Self diagnostic function integration: Ansenmei NSD1624 diode has a built-in temperature sensor, which automatically triggers protection action when the junction temperature exceeds 150 ℃
3. Standardization and Reliability Challenges
Vehicle level certification: AEC-Q101 standard requires diodes to have a VF drift of<5mV/℃ within the temperature range of -40 ℃~150 ℃
Life test specification: IEC 60747-1 standard adds 100000 switch cycle tests, requiring Trr change rate<20%