UPS systems and backup power equipment are vital in data centers, industry, hospitals, renewable energy, and EV charging. With rising power densities and still-increasing operating voltages, electrical protection is a paramount design consideration for such products. Of all protection components electrical fuses remain one of the most ideal for protecting UPS and backup power equipment.
This article explains where fuse solutions are implemented in today’s UPS and backup systems, the primary criteria for choosing fuse types, and how they complement the silicon carbide (SiC) power electronics used in today’s high-efficiency designs.Why Electrical Fuses Are Necessary for UPS SystemsUPS systems must immediately do something in response to electrical abnormalities. Short circuits, overloads, component short-circuits, faulty batteries, or other inexplicable events can all put expensive components at risk of destruction if not immediately isolated. Fuses can provide fast and predictable electrical interruption to protect the system from catastrophic failure as well as minimize the risk of fire, should that be a concern.
While faster-acting than circuit breakers, fuses achieve:
Faster fault clearingHigher current-interrupting capacityCompact high-power form-factorsStable and predictable performance with minimal maintenanceFuses are used on both low voltage and high voltage UPS architectures, at the input, AC/DC links, inverter, battery, and output.
Fuse Applications in UPS and Backup Power Equipment1 AC InputProtection of the rectifiers and power modules on the utility or generator input side from short circuits and inrush currents is achieved using fuses. High-breaking-capacity fuses are typically used, capturing the fault energy and maintaining acceptable selectivity, that is a choice of which circuit to trip to isolate a fault.DC Bus and Inverter Protection
More modern UPS solutions tend to use a mixture of SiC MOSFETs and SiC diodes to implement higher switching frequency and lower losses. Fast overcurrent and short circuit events can damage these components, and semiconductor fuses tend to be the most appropriate solution. DC fuses in this stage must:
Be matching DC Voltage Ratings
Withstand High Fault Currents
Coordinate with the operation of SiC power modules
Lower total I²t, protecting semiconductors
Battery Protection
Strings of Batteries are inarguably the most important and vulnerable section. Lithium-ion battery and lead-acid systems can fault and present very high currents for a short time. Battery fuses provide:
Disconnection of Faulty battery strings
Protection of internal short circuits
Protection in accordance with electrical safety standards
Output and Load Protection
On the outboard side, the fuse also protects loads below it and allows fault isolation from it without having to shut the UPS system down. This can be important in mission-critical applications where uptimes need to be preserved.
Criteria For Selecting The Right Fuse In UPS Applications
The selection of the right fuse for UPS and other backup power applications involves careful consideration of these factors:
Rated Voltage and Current
Choosing a fuse with a voltage rating higher than the maximum nominal operating voltage of the system, especially in DC systems. Current rating must be based on continuous load current, allowing sufficient margin for other effects.Interrupting Capacity
UPS systems are capable of producing very high short-circuit currents; the fuse must be capable of safe interruption of the maximum prospective fault current without failure during service.
Time-Current Characteristics
High-speed acting fuses are a normal selection for semiconductor protection and time delay fuses may be fitted for transformer or input protection to cope with inrush.
Coordination and Selectivity
Fuse selection is critical to ensuring that only the faulted section of a UPS is isolated, failing to achieve this can make the entire system unusable. This is particularly true of multi-level UPS architectures.
Compatibility with SiC Power Electronics
The new generation of power electronic systems based on silicon carbide operate at higher temperatures and in faster switching cycles than traditional silicon devices. It is imperative that fuse technology continues to perform well in these environments.
The Application of Silicon Carbide to Our Modern UPS Designs
The introduction of silicon carbide semiconductors into our UPS offers significant benefits including:
Increased efficiency
Higher power density
More compact systems
Better thermal performance
However, these features put even greater demand on fast and accurate electrical protection requiring precise and matched fuse solutions in order to avoid nuisance device overstress on fault events. For manufacturers and system integrators of UPS systems, selecting fuse solutions proven in SiC based power electronics has rapidly become a point of differentiation from their competitors.
Compliance and Safety Standards
A UPS that is equipped with fuse solutions tailored to this new requirement must perform in terms of conformity to important international standards including:
General IEC and UL fuse specs.
UPS safety and performance standards
Battery safety regulations
Industrial electrical systems
Using certified fuse components helps to provide regulatory compliance and gives the customer greater confidence in the long-term reliability of the entire system.
Choosing a Fuse Supplier
For vendors of UPS and backup power, working with a supplier of electrical components who can deliver the right fuse specification is as important as making the right selection of fuse specification. Important features include:
Trusted consistency
Transparent technical documentation
Ability to adjust design for special or specific applications
Regular supply from a stable source, attractive price
Many designers of UPS sourcing their electrical protection from China, focus on suppliers with a strong bias to industrial power equipment and SiC related specifications.
Final Reflections
Electrical fuse solutions still form the bedrock for our UPS and backup power systems. As power electronics increase in demand for higher voltage, higher efficiency and a wider range of SiC, the selection of the correct fuse technology, is critical to safety and reliability.
