Secondary protection fuse

21A (Small) SFJ Series

Suitable for lithium-ion batteries in smartphones, laptops, etc.

  • Product name
    SFJ-0421A
  • Features
    • The world's thinnest *1, with a low profile of 0.58mm, is suitable for devices that require a compact and thin design.
    • Protects Li-ion batteries from overcharging (overvoltage) and overcurrent.
    • When overcharging occurs, the fuse element melts to shut off the circuit, and simultaneously the power to the heater is also cut off.
    • Placing a heater directly under the fuse element breaks the circuit quickly.
    • This is a Pb-free *2product.
      *1 World's thinnest Secondary protection fuse for lithium batteries (as of January 5, 2026, according to our research)
      *2 The European RoHS Directive, which restricts the use of hazardous substances, defines a product as lead-free if it contains 1,000 ppm or less of lead.

Specifications

Product name SFJ-0421A
Applicable cells in series 1 cell
Rated current (A) 21
Rated voltage (VDC)*1 10
Rated breaking capacity (A) 50
Operating voltage (V) 3.8–4.8
Heater resistance (Ω) 0.68 to 1.00
Fuse resistance (mΩ) 1.1
Dimension (mm)*2 4.0 x 3.0 x 0.58 (see diagram below)
  • *1 Maximum voltage at which cutoff of the fuse is possible. It is not the operating voltage for the heater.
    UL certification: File No. E167588
    TUV certification: Certificate No. J9650637
    RoHS directive: Conforming
  • *2 Values without tolerance indications are nominal values.
fuse15-chart-sfj0815.gif

<Recommended land size>
circuitboard_SFJ.png

These are recommended sizes and we can't guarantee the quality of product when mounted.
We recommend our customer to verify if our products could work with your design guidelines.

Application Example

Suitable for lithium-ion batteries in smartphones, laptops, etc.

caution

Note on the characteristic data given - Data on the characteristics of the products described in this page based on the results of evaluations carried out by the company. This does not guarantee that the characteristics of the product conform with your usage environment. Before use, review the usage conditions based on evaluation data obtained from the equipment and substrates actually used.

Product information

Optical Semiconductor

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