Explore the Applications of SMD Fuses

I. Composition of a Single-Cell Lithium Battery

Lithium-ion batteries are primarily composed of cells and battery protection boards.

Lithium-ion Battery Protection Board Main Functions: (1) Overcharge Protection, (2) Overdischarge Protection, (3) Overcurrent/Short Circuit Protection

Primary Components of Lithium Battery Protection Board: 1. Protection IC, 2. MOSFET, 3. SMD Fuse

Section 2: Selection of Fuses on Single-Cell Lithium Battery Protection Boards

Encapsulation: The trial size is determined based on the customer's PCB, typically using 1206/0603 dimensions.

Rated Voltage: Above 24V

3. Rated Current: Generally, 5A is used (considering that the fuse is a secondary protection on the lithium battery protection board, so...)

Consider the protection delay time of the IC to determine the fuse's melting time.

4. Cold resistance: Below 15 mΩ

Safety Regulations: UL Certification

Compliant with RoHS and Halogen-Free requirements

Example 1: The circuit diagram of a single lithium battery protection board is shown in Figure 3, with the protection IC using SII (Jinsheng) S8261ACJ, basic parameters as shown in Figure 4. The MOSFETs used are BF9028DNJ/BF9028DNJ-A, with some parameters depicted in Figure 5.

Application of SMD Fuses on Single-Cell Lithium Batteries

The circuit protection is functioning properly.

The IC Overcurrent Protection Principle: When the discharge current exceeds the normal level or a short circuit occurs, the protection IC activates overcurrent (short circuit) protection. At this point, the detection of overcurrent uses the Rds(on) of the PowerMOS as an inductive impedance to monitor the voltage drop. If the voltage drop exceeds the predetermined overcurrent detection voltage, the discharge is stopped.

Based on the formula 'V - (Overcurrent Detection Voltage) = I (Discharge Current) * Rds(on) * 2', the discharge current at which the protection IC begins to activate can be calculated.

1: Short-circuit Protection Status:

When the overcurrent detection voltage is 0.9V and the MOSFET on-resistance is set to a high value, the overcurrent can be detected at a low level. Calculating at this point, the overcurrent value is 15A, and the protection IC's delay time is 320us.

When the overcurrent detection voltage is 1.5V and the MOSFET's on-resistance is set to a low value, the overcurrent detection will be for a higher value.

The calculated overcurrent value is 46.875A, and the protection IC's delay time is 380us at this moment.