Switched Mode Power Supply (SMPS) is an indispensable component of modern electronic equipment and is widely used in computers, communication equipment, household appliances and other fields. Many people may be curious: In such an efficient and complex power supply device, is it still necessary to use traditional fuses for protection? This article will explore this issue in depth, revealing the role, necessity, and selection and design of fuses in switching power supplies.

1. What role does the fuse play in switching power supplies?

Fuses, as an overcurrent protection element, have a history of more than a hundred years. So, what role does the fuse play in modern switching power supplies?

Basic principles of fuses

A fuse is a protective element that uses the heat of a conductor to melt under overcurrent conditions. When the current in the circuit exceeds its rated value, the fuse will quickly heat up and melt, cutting off the circuit to prevent excessive current from damaging other components in the circuit or causing a fire.

Application in switching power supplies

In switching power supplies, fuses are usually installed at the input end, which is the first line of defense for the mains to enter the switching power supply. Its main functions include:

Overcurrent protection: When a short circuit or overload occurs at the input end, the fuse can quickly blow to prevent excessive current from entering the circuit.
Prevent fault spread: If a fault occurs inside the switching power supply, the fuse can prevent the fault current from feeding back to the power grid, protecting the safety of upstream equipment and the power grid.
Safety isolation: In some cases, the fuse can also provide a certain degree of safety isolation to prevent the risk of electric shock caused by circuit faults.

Why are fuses still needed?

Although modern switching power supplies have integrated a variety of protection mechanisms, such as overvoltage protection, overcurrent protection, and overtemperature protection, fuses, as a simple, reliable, and passive protection element, still play an irreplaceable role in circuit protection.

First, the fuse has a very fast reaction speed and can blow in milliseconds or even microseconds to quickly cut off the circuit. Second, the fuse does not require external power or control signals and can provide protection in any situation. Other protection circuits may not work properly when the power supply fails or the control signal fails.

2. Why do switching power supplies need fuses?

Whether a switching power supply needs a fuse depends on many factors, such as its application environment, safety requirements, and cost. Next, we will explore the necessity of using fuses in switching power supplies.

Improving circuit safety

During operation, a switching mode power supply may encounter various abnormal conditions, such as a sudden increase in input voltage, a short circuit at the output end, and internal component failure. These conditions may cause the current to increase instantly, endangering circuit safety. As the most basic overcurrent protection method, the fuse can cut off the circuit in time when the current is too large to prevent equipment damage or safety accidents.

Compliance with safety standards and regulations

In many countries and regions, electrical equipment needs to comply with specific safety standards and regulations, such as IEC, UL, CCC, etc. These standards usually require an overcurrent protection device to be set at the power input end. Fuses, as a mature and reliable protection element, can meet the requirements of these standards.

Cost and reliability considerations

Compared with other protection circuits, fuses are low-cost, simple in structure, and highly reliable. Although modern switching power supplies can achieve overcurrent protection through electronic protection circuits, these circuits may increase design complexity and cost. The use of fuses can improve the safety of the circuit without significantly increasing the cost.

The necessity of double protection

Even if the switching power supply has electronic protection mechanisms inside, these mechanisms may fail in some cases. For example, in the case of extreme voltage surges or lightning strikes, the electronic protection circuit may not respond in time. Fuses, as passive components, are not affected by power and control signals, and can provide the last line of defense to ensure the safety of equipment and users.

3. How to select and design fuses in switching power supplies?

In switching power supplies, choosing the right fuse is crucial to protecting circuit safety. Factors to consider include rated current, rated voltage, breaking capacity, fusing characteristics, etc.

Determine rated current and rated voltage

The rated current of the fuse should be slightly higher than the maximum operating current of the circuit to avoid false fuses during normal operation. But it should not be too high, otherwise it will not provide effective protection under overload conditions.

Calculation example:

Assuming that the input voltage of the switching power supply is 220V and the maximum input power is 500W, the maximum input current is:

Imax= P/V= 500W/220V≈2.27A
Considering factors such as starting current and grid fluctuations, a fuse with a rated current of 3A can be selected.

The rated voltage should not be lower than the maximum operating voltage of the circuit. For a 220V power supply, a fuse with a rated voltage of 250V or higher should be selected.

Select the fusing characteristics

The fusing characteristics of the fuse are divided into fast fusing (F), medium fusing (M) and slow fusing (T). For switching power supplies, it is necessary to select the appropriate type according to the specific situation.

Fast fusing (F): Suitable for circuits that need to cut off current quickly, but may cause false fusing due to surge current at the moment of startup.

Slow fusing (T): Suitable for circuits with surge current, such as transformers, motors, etc. Able to withstand short-term overcurrent.
Since the switched-mode power supply may have a surge current when starting, a slow-blow (T) type fuse is usually selected to avoid false blows.

Consider the breaking capacity

The breaking capacity refers to the maximum current that the fuse can safely cut off. For a switching power supply powered by the mains, the breaking capacity of the fuse should meet the fault current requirements of the power grid. It is usually necessary to select a fuse with a high breaking capacity to ensure safe blows in extreme cases.

Environmental and temperature factors

The ambient temperature will affect the fuse's fusing characteristics. In a high temperature environment, the fuse's fusing current will decrease, and the selection of the rated current needs to be adjusted appropriately. In addition, the temperature resistance of the fuse must also be considered to ensure that it can operate reliably for a long time in the working environment.

Installation and physical dimensions

The physical dimensions of the fuse need to match the circuit board or fuse holder. Common fuse forms include glass tube fuses, ceramic fuses, chip fuses, etc. When selecting, space limitations and installation methods need to be considered.

4. What other protection measures are there in the switching power supply besides fuses?

In addition to fuses, a variety of protection mechanisms are integrated into the switching power supply to ensure safe and reliable operation of the equipment.

Overvoltage Protection (OVP)

When the output voltage exceeds the set value, the overvoltage protection circuit will start to prevent excessive voltage from damaging the load device. Usually a comparator or voltage regulator chip is used to detect the output voltage. When the voltage is too high, the switch circuit is turned off or the protection mechanism is triggered.

Overcurrent Protection (OCP)

Overcurrent protection is used to prevent the output current from exceeding the safe value and protect the power supply and load. Common overcurrent protection methods include current limiting and short-circuit protection. When the current is detected to be too large, the circuit will reduce the output current or turn off the output.

Overtemperature Protection (OTP)

Overtemperature protection detects the internal temperature of the switch mode power supply through a temperature sensor. When the temperature exceeds the safety threshold, the protection circuit will reduce the output power or turn off the power supply to prevent the components from being damaged due to overheating.

Undervoltage Protection (UVP)

When the input voltage is lower than the set value, the undervoltage protection will start to prevent the power supply from working under unstable input conditions, protecting the safety of the power supply and load.

Electromagnetic Interference Filtering

When the switching power supply works at high frequency, it will generate electromagnetic interference (EMI). By adding filters at the input and output ends, the electromagnetic interference can be reduced to meet the requirements of EMC (electromagnetic compatibility) standards.

Lightning and Surge Protection

In some applications, the power supply may be affected by lightning or grid surges. By adding components such as varistors (MOVs) and gas discharge tubes (GDTs) to the circuit, the power supply's protection against transient overvoltages can be improved.

Safety Isolation

For occasions where electrical isolation is required, the switching power supply usually uses a transformer to achieve primary and secondary isolation to prevent the high voltage on the input side from being transmitted to the output side, protecting the safety of users and equipment.

Conclusion

As an efficient and complex power supply device, the switching power supply integrates a variety of electronic protection mechanisms, but traditional fuses still play an important role in it. As the most basic and reliable overcurrent protection element, the fuse can provide the last line of defense in extreme situations to ensure the safety of equipment and users. When designing and selecting fuses in switching power supplies, factors such as rated current, rated voltage, fusing characteristics, and breaking capacity need to be considered comprehensively. In addition, the switching power supply also integrates multiple protection mechanisms such as overvoltage, overcurrent, and overtemperature, which together build a solid barrier for power supply safety.

By deeply understanding the fuses and other protection measures in the switching power supply, we can better design, use, and maintain power supply equipment and improve the safety and reliability of electronic products.