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Effective input circuit protection is a critical aspect of electrical engineering design. The National Electrical Code (NEC) is an organization that ensures compliance in circuit design and sets nationwide standards for safe electrical design, installation and inspection, including the promotion of electrical system efficiency. Adhering to these regulations minimizes risks, enhances system efficiency and helps ensure smooth operations.
Understanding input circuit protection is vital for designing safe, reliable and compliant electrical systems. Learn how properly sizing your input circuit protection can keep equipment and systems safe and running longer.
What Is Input Circuit Protection?
Input circuit protection involves implementing a failsafe electrical design to prevent damage from harmful electrical conditions, such as voltage spikes caused by power surges, overcurrent, electrostatic discharge (ESD) and overheating. It's often designed in power supplies, DC-DC converters and other electronic equipment to protect against electrical failures and hazards. It ensures devices remain functional and in good condition to continue safe operations.
What Is Overcurrent Protection?
Overcurrent protection refers to electrical devices that limit and disable current flow when circuits exceed their maximum electric load. They function by tripping or opening a circuit to hinder a conductive pathway where current flows. Short circuits, overloads and ground faults can cause overcurrent, damage electrical devices and cause hazards. Fuses and circuit breakers are the two primary overcurrent protection devices.
Three Common Types of Input Circuit Protection
Input circuit protection devices vary across electronic systems for different types of equipment. The following section discusses some of the most common circuit protecters in various electrical equipment:
1. Fuses
Fuses protect against overcurrent when the current flow exceeds the device manufacturer's lo recommended ad. They use a low melting point wire that permits normal electrical load to pass through a circuit. Once an overcurrent occurs, the wire melts and safely disengages from the circuit. This opens a gap in the circuit, protecting the more important parts of the device from electrical failure.
Most standard fuses can only be used once and should be replaced once it has faulted. There are also fast-acting semiconductor fuses that safeguard sensitive devices from current spikes.
2. Circuit Breakers
Another overcurrent protection device, circuit breakers interrupt overcurrent flow with a branch or loop in the circuit to prevent damage. Unlike fuses, circuit breakers have key mechanisms that hinder overcurrent hazards. These basic mechanisms include a switch, a trip unit that activates when excess current flows and contact wires that disconnect from the circuit to stop the electricity. A circuit breaker switches off when too much current passes.
Circuit breakers are reusable and can be reset without replacement, which makes them common in many devices. There are many types of circuit breakers, one example being a molded-case circuit breaker that is ideal for handling higher currents in industrial environments. You can choose the right circuit breaker with respect to input characteristics, such as type, application, form factor, and regulatory standards.
3. Surge Protection Devices
Surge protection devices (SPDs) protect electronic equipment from surges caused by various factors, such as electrical grid issues and lightning strikes. SPDs work by suppressing overvoltage or transient overvoltage, which can damage devices like computers and equipment.
SPD usually contains a gas discharge tube (GDT) or a metal oxide varistor (MOV), which diffuses excess voltage. SPD resistance adjusts depending on the voltage passing a circuit, working like an electricity pressure relief valve. For example, when the voltage surges or spikes, the SPD resistance drops to stabilize the voltage, enabling the excess electric current to drain away safely.
Load Calculation for Input Circuit Protection of a Power Converter
The following section shows calculations for two types of inrush current: one caused by transformers and the other by capacitors. Inrush current is the first energy that flows when electrical power is applied to a device with circuits containing transformers or capacitors.
1. Inrush Current Related to Transformers
When transformers receive initial electrical power, they can generate a large current for a brief time. This inrush current can be calculated with the following formula:
Ipeak inrush = V _peak / R _primary
In this equation, V _peak is the peak voltage applied to the primary winding of the transformer. So, for an alternating current (AC) utility supply, this is:
Vrms √2
For example, let's say that the Root Mean Square (RMS) voltage is 230V. Using the formula, the V _peak value will be 325V. Now, if the resistance of the transformer's primary winding is 2 ohms — ohm being the standard unit of electrical resistance — the peak inrush current will be 162.6A, which is a large current. While the high current level is only for a short period, it rapidly magnetizes the transformer's core. When this occurs, the magnetic circuit that carries the flux forms an energy bridge between the primary and secondary sets of turns, promoting alternating electrical currents.
In a well-designed power converter, the transformer's core saturates for a while and usually does not damage its magnet. However, it can open fuses of trip undersized breakers. For this reason, power converters should have specified input circuit protection parameters, enabling the placement of appropriately sized fuses and breakers.
2. Inrush Current Due to Capacitors
Inrush current occurs in the presence of capacitors when they are initially charged. This happens when capacitors are connected across a bridge rectifier in a power supply circuit. If uncharged, the capacitors function like short circuits when power is first applied, leading to a current surge.
This formula calculates inrush current caused by capacitors:
Ipeak inrush = C. dV/dt
In this calculation, C is the capacitance and dV/dt is the rate at which voltage changes over time. For an AC source, this mathematically becomes:
Vac= Vpk. cos(ωt)
Ipeak inrush = C.Vpk.ω
For example, if we have a 230V AC source with a frequency of 50 Hz and a 220 μF capacitor, the I _peak voltage is calculated as:
Ipeak inrush = 220.10-6 * 230√2*(2*π*50) = 22.48A
In this example, the inrush current due to the charging of capacitors is 22.48A.
Based on these examples, inrush currents caused by transformers or capacitors can cause large initial power surges that trip fuses and circuit breakers. Knowing load calculations for input circuit protection will help users determine the proper size of breakers and fuses for power converters.
Thermal Protection in Circuits
Managing heat in electrical design helps maintain the dependability and safety of electronic systems. Effective thermal protection detects over-temperature conditions and disengages electric flow in the circuit, preventing damage to equipment. For example, excessive heat can prompt thermal runaway for battery-powered electronics, which may result in parts melting and equipment failure.
Power supplies can overheat due to environmental factors, such as ambient temperature, humidity, air pressure, pollution, and other causes. Poor airflow and warm weather make it harder for circuits to stay cool. These factors significantly affect power supply performance and longevity.
Different devices protect power supplies from overheating. Consider the following thermal protection devices:
Thermal Fuses
When over-temperature occurs, a thermal cut-off opens circuits when it detects excessive heat. Thermal fuses are designed to react to high temperatures, including high heat caused by overcurrent. You can choose between a non-resettable thermal fuse or a resettable one:
Non-resettable thermal fuse: This fuse can only be used once and should be replaced with a new link. It's made with a heat-sensitive fusible link that melts when over-temperature occurs.
Resettable thermal fuse: Made with a heat-sensitive bimetallic strip that curves away when there's high heat, the resettable thermal fuse eventually goes back in place when the temperature normalizes, allowing current to flow.
Positive Temperature Coefficient (PTC) Thermistor
A PTC thermistor is a temperature-sensitive resistor that increases resistance when the temperature rises. This is typically used as a self-regulating device that limits the current flowing in the circuit, preventing excess heat from increasing. PTC thermistors are usually made of ceramic material and can be mounted on aluminum heat sinks and grids.
A Negative Temperature Coefficient (NTC) Thermistor works, becoming less resistive as the temperature rises. It is commonly used as an inrush current limiter. When designed in a circuit, since it initially has a high resistance at cold start, it reduces the peak of a large current spike drawn by capacitors when they first energized.
Integrated Circuits (IC) With Thermal Protection
Many integrated circuits have built-in heating protection to prevent overheating, a feature commonly found in components like regulators and processors. IC thermal protection shuts down the circuit when the temperature exceeds safe levels, decreasing the flowing current. In effect, it cools down the circuit until it reaches safe levels. This helps prevent damage to the IC and the surrounding circuit, ensuring reliable performance and system longevity.
Consult With Astrodyne TDI for Custom Solutions
Astrodyne TDI provides custom solutions for power supplies, electromagnetic interference (EMI) filters, power distribution units and configurable multi-output power systems that meet unique specifications. Our team of experts leverages advanced tools and technology to ensure we meet rigorous compliance standards for medical facilities, military, aerospace applications and many other industries.
Your business may have challenging technical requirements, and our team can help you properly navigate complex regulations to meet your needs. Astrodyne TDI has over 60 years of experience as an industry power supply and EMI filter manufacturer, providing excellent engineering support to help your business thrive. Contact us online today for superior-quality, custom solutions that protect your equipment and enhance system reliability.
