Selecting Fuses: Simple Procedures to Get the Right Overcurrent Protection for DC-DC Converters

Aug 1, 2010 12:00 PM
Nabil Sadiq, Senior Field Application Engineer Trina Noor, Field Applications/New Product Development Engineer, Cooper Bussmann

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Although features and functionality attract the most attention for new electronic products, whether consumer, industrial, or medical, their reliability depends on protecting their power systems from overcurrent events. Internal, external, and nuisance threats can affect circuit and system reliability. Through proper fuse selection, you can minimize risks and failures so that an electronic product retains its competitive edge.

Fuses are overcurrent devices that protect electrical and electronic devices by melting and opening a circuit to prevent excessive current from causing damage or starting fires. Fuses serve two main purposes:

1. To protect components, equipment and people from risk of fire and electric shock
2. To isolate sub systems from the main system.

The fusing action begins when the circuit current is high enough to heat the fuse element and starts it to melt. Once melting begins, a gap is created that the current will “arc” across. Melting continues and the gap grows wider until it is too wide to sustain the arc. At that point, current ceases to flow and the overcurrent event is “cleared,” opening and making the circuit safe.

1. There are two types of overcurrent events:
1) Overload - simply drawing excessive current beyond the designed capacity of the circuit,
2) Short-circuit, or fault current.

Regardless of the overcurrent event, fuses are designed and specified to be a circuit’s “weakest link.” These “thermally operated” devices typically employ a metal wire or strip element in their construction.

Fuse Types

Fast-acting fuses open very quickly when their current rating is exceeded. This action is needed when speed is important for sensitive electronics and for many dc power applications. They are generally used in resistive loads with low inrush current levels.

Time-delay fuses have a time-delay mechanism. They are designed to open only on an excessive current draw for a defined period of time and are typically used to protect inductive and capacitive loads that experience heavy current draws upon initial powering. The time delay action prevents the fuse from needlessly blowing during a temporary heavy current draw or surge. Time-delay fuses tolerate higher inrush currents than fast-acting fuses and are often ideal for dc-dc converter input protection, as most converters have an input capacitor that draws a large amount of current when initially charged.

Selecting the right fuse is critical in all electronic and electrical system designs. Catastrophic system failure can be prevented with the proper fuse on the dc-dc converter input. In the event the converter’s internal circuitry can no longer withstand an overload condition, the fuse will prevent fire or further damage to the board, the converter, or neighboring components. Most dc-dc converters are protected from short-circuits on their outputs by either circuit-sensing current limit and/or thermal overload circuits. Fuses are required to protect against a catastrophic component failure (e.g., MOSFET failure) or if a component failure creates a short-circuit on the input side of the dc-dc converter.

Proper selection of an input fuse for a dc-dc converter involves understanding and consideration of the following factors:
1. Voltage Rating
2. Current Rating
3. Interrupting Rating
4. Temperature Derating
5. Melting Integral (I²t)
6. Maximum Circuit Fault Current
7. Required Agency Approvals
8. Mechanical Considerations

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