07/08/2023Voltage surges pose a significant threat to many devices that rely on mains power supply. If left unaddressed, they can cause severe damage to the devices and their power supplies. In this article, we will delve into the fundamentals of surge protection by exploring the root causes of voltage surges, familiarizing ourselves with the surge test standards set by regulatory authorities, and examining various surge suppression designs and components.
If you’ve ever experienced a power surge, you know how frustrating it can be to have your electronic devices fail without warning. Power surges are a common occurrence that can happen for various reasons, including lightning strikes, reactive loads, and faults. These surges can damage your devices, and in some cases, render them useless. However, there are ways to protect your electronics from voltage surges.
One of the most common sources of external surge transients is lightning strikes. Lightning carries a far greater amount of current at much higher voltages than most electronic systems can handle. These voltage surges are typically significant enough to cause immediate device failure if the appropriate level of protection has not been applied.
Surges can also occur on the AC supply line when other devices on the same electrical circuit are turned on or off. Reactive loads, such as motors or capacitor banks, can resemble short circuits before establishing their electric and magnetic fields. When turned off, the energy stored in these fields can quickly be dumped back into the system. In both cases, the large and fast current transients can induce voltage spikes, leading to the failure of unprotected devices.
Faults can also be the source of surges and lead to excessive voltages being applied to the power supply input. The failure of system components and devices can result in transient voltages and currents in other parts of the system, caused by unintentional shorting or opening of circuits.
When it comes to power supplies, ensuring that they meet the necessary protection standards is crucial. The International Electrotechnical Commission (IEC) has developed various standards, with IEC 61000-4-5 being the most common.
IEC 61000-4-5 is a standard developed by the IEC to provide guidance on the necessary level of protection for electronic equipment against voltage surges caused by lightning or other electrical disturbances. This standard is referenced in many national immunity standards, such as EN 55035, which sets immunity requirements for multimedia equipment.
The IEC 61000-4-5 standard defines a standardized test method that uses simulated electrical transients to test the level of protection provided by electronic equipment. The test method involves inducing voltage surges into the power supply input and output lines, and monitoring the equipment’s behavior during and after the surge.
IEC 61000-4-5 also defines different levels of protection based on installation class and coupling methods. Installation classes refer to the location of the equipment and the level of exposure to electrical transients, with class 3 being the lowest level of exposure and class 5 being the highest. Coupling methods refer to how the voltage surge is coupled to the equipment, with coupling mode 1 being the most common and coupling mode 2 being less common.
When it comes to DC power supplies, they are typically concerned with installation classes 3-5, which have test requirements from 1kV to 4kV. This means that DC power supplies must be designed and tested to withstand the specified level of voltage surge to meet the IEC 61000-4-5 standard.
Meeting the IEC 61000-4-5 standard is crucial for power supplies because it ensures that they can withstand voltage surges caused by lightning or other electrical disturbances. This not only protects the power supply but also the electronic equipment it powers. In addition, compliance with this standard can give manufacturers a competitive edge by demonstrating the quality and reliability of their products.
To safeguard power supplies and their connected devices against surges, it is necessary to incorporate some kind of internal or external surge protection circuit. There are primarily two types of surge protection circuits – clamps and crowbars.
Voltage clamps work by preventing the voltage from surpassing the selected clamping voltage. When a surge event occurs, the voltage is held at the clamp voltage while the current is diverted through the clamp until the surge passes. Two commonly used devices for clamps are transient voltage suppression diodes (TVS) and metal-oxide varistors (MOV). The speed and energy handling capacities of each device are inversely proportional and can necessitate combining different types of clamps as per Table.
| Clamp Device | Surge Energy Handling | Speed |
| Transient Voltage Suppression Diodes (TVS) | Low | Fast |
| Metal Oxid Varistors (MOV) | Medium | Medium |
| Gas Disharge Tubes (GDT) | High | Slow |
TVS diodes are designed to absorb the excess energy of a voltage spike and clamp it. They may be either unidirectional or bidirectional. These diodes have a knee voltage similar to a Zener diode, which causes the voltage to be clamped at the knee voltage, and the excess energy to be diverted away from the power supply.
Fig. 1 – AC Transient Voltage Suppressor Circuit
Metal oxide varistors (MOV) are voltage-sensitive variable resistors that are bidirectional. MOVs have a high resistance at low voltage and low resistance at high voltage, resulting in a softer clamp voltage and slower reaction time than TVS diodes. MOVs tend to wear out and can only withstand a limited number of surge events. However, due to their affordability and surge handling capabilities, they are commonly used for surge protection in power supplies.
Fig. 2 – MOV construction
The crowbar is another type of surge protection circuit that functions differently from clamps. Crowbar devices short circuit the circuit nodes together, bringing the voltage close to zero instead of limiting it to a maximum value. Gas discharge tubes (GDT) are commonly used as crowbars. GDTs, similar to TVSs, behave as voltage-dependent switches. This device typically functions as an open circuit, but when its voltage threshold is exceeded, it behaves as a short circuit. GDTs can handle more current but also tend to react slowly compared to other surge protection devices. As a result, power supplies sometimes employ them in conjunction with other methods for a more robust solution.
Voltage surges are inherently unpredictable and can cause significant damage to electronic systems. However, it is possible to evaluate a system and determine the types of surges it may experience, thereby allowing for the selection of an appropriate level of surge protection.
The recommended level of surge protection can vary depending on the system in question. Some systems may experience mostly common and relatively easy-to-handle over-voltages, such as those caused by nearby equipment switching on and off. Conversely, other systems may be located in areas with high lightning activity, necessitating protection against more severe spikes.
Our experts can help evaluate the unique circumstances of a system and recommend the appropriate surge protection. They can also assist in selecting the right power supply for the application, taking into account factors such as the system’s power needs and potential surge risks.
