Increased safety and longer battery life for standalone medical devices

The range of battery-powered devices extends from powerful hand tools for craftsmen, to sensors for room surveillance all the way through to life-saving medical aids. The safety of such devices presents quite a challenge. To ensure the safe operation of standalone devices, Schurter has recently introduced two SMD fuses with low power dissipation and tight tolerances for tripping time.

Many billions of lithium-ion batteries were manufactured in 2010. At the same time, the number of application areas for these products has also increased. These days, battery-powered devices can be found everywhere. Although these handy assistants are very useful, they also present a source of danger. Any such device can catch fire, or an overcurrent condition can damage electronic circuits. In some cases, for example in medical technology, even a brief malfunction could place a patient’s health at risk.

Safe overcurrent protection

Many types of damage can be avoided with effective overcurrent protection. In principle, a fuse is sufficient for this purpose. Although unimposing, a fuse must meet a wide range of challenging requirements, if it is to provide for the efficient and effective safe operation of insulin pumps, hearing aids or blood glucose meters.

When it comes to safety technology, a short and precisely defined reaction time in the event of faults is important along with reliability across a wide temperature range. Both requirements have to be assured during the device’s overall service life. Furthermore, the typical design parameters of standalone devices must be considered:

The focus here is on low power consumption, small form factor and an attractive price/performance ratio. In addition, there are further desirable characteristics such as those relating to recycling.

In principle, overcurrent protection can also be implemented with an active protection circuitry. Newly released SMD fuses by Schurter aim to replace such an approach with some advantages.

USF 0402 and USFF 1206 SMD fuses

A fuse is clearly more reliable compared to an active protection circuitry. In addition, it requires little space at low cost. In terms of power dissipation and tripping time, there is a clear difference among the various fuses available on the market. As for all these aspects, both model USF 0402 and USFF 1206 SMD fuses provide values that make them suitable for overcurrent protection in battery-driven devices.

Both fuses have been developed for the overcurrent protection of secondary circuits. The rated currents for the USF 0402 range from 375mA to 5A, while it has a rated voltage of 32Vdc up to 4A and 24Vdc up to 5A. The breaking capacity is specified at 35A at rated voltage. The USFF 1206 was developed for smaller rated currents of from 50 to 250mA; its rated voltage is 63Vdc and its breaking capacity is 100A.

Low power dissipation

The first characteristic that differentiates these units from standard commercial products is its low power dissipation. For example, the 200mA version of the USFF 1206 provides a voltage drop of 87mV at rated current. This value is much lower than for any comparable product in the market. As for the USF 0402, the voltage drop of the 1A version is 65mV at rated current. This is made possible by a unique design. The melting wire is wire-bonded and it is just a few micrometers thick. It is made of a special metal alloy. In the case of the USF 0402, the melting wire is encapsulated in a special epoxy resin; on the USFF 1206, which is somewhat larger, it is surrounded by air. Both technologies inhibit the dissipation of heat and thus drop power dissipation.

Narrow window of tripping time

The second differentiating characteristic is their tight tolerance in tripping time. For instance, the USFF 1206 is designed for a tripping time between 0.1 and 1 millisecond at tenfold rated current according to the component standard UL 248-14. But the tripping-time window of the USFF 1206 is considerably narrower: With a nominal tripping time of 0.5 milliseconds, for instance, the fuse trips within a tolerance band between 0.4 and 0.6 milliseconds.

The USF 0402 and USFF 1206 are currently the only fuses in this dimension that trip with such high precision.

*See FIGURE 6.

Fig. 6: Precise tripping-time window at tenfold rated current

Fuses with clear advantages

This precise tripping time increases safety for users, the environment and the device itself. Both SMD fuses make it possible to design standalone devices without active protection circuitry, which has a positive impact on the cost, size and battery lifetime of the corresponding device.

The low power dissipation and tight tolerance in tripping time are valid over the entire specified temperature range. In addition, the fuses feature a high service lifetime. In particular, they do not change their performance characteristics when handling pulsed loads such as in devices with electronically commutated motors.

Continual advances in development

In past years, SCHULTER has continually enhanced its know-how and applied it to both of these unique fuses. The company is delighted to be able to support its customers with this knowledge. Furthermore, custom-designed solutions are also possible such as versions of both fuses with current ratings outside the E Series.