The Basics of Power Surge Protection
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The Basics of Power Surge Protection

Posted Date: 2024-01-24

Power surge issues

Voltage surges are a problem faced by many mains-powered equipment. If not properly designed for the intended environment, surges can damage power supplies and the equipment they power. This article discusses the basics of surge protection by identifying the causes of voltage surges, familiarizing you with regulatory standards for surge testing, and demonstrating surge suppression designs and components.

There are three main causes of surges:

lightning strike

load transient

Fault

Lightning strikes are a common source of external surge transients, producing currents and voltages well above the ratings of most electronic systems. These voltage surges are often large enough to cause immediate failure of electronic equipment if the appropriate level of protection is not applied.

Surges on AC power lines can also be caused when other equipment in the circuit is turned on or off. A reactive load such as a motor or capacitor bank resembles a short circuit until it builds up its electric and magnetic fields. When turned off, the energy stored in these fields also flows rapidly into the system. In both cases, high and fast transient currents can cause voltage spikes and cause unprotected equipment to malfunction.

A fault may also create a surge, causing excessive voltage to be applied to the power input. Failure of system components and equipment can cause transient voltages and currents to occur in other parts of the system due to accidental shorts or opens in circuits.

The intensity and magnitude of surge voltages at the power input depends on many factors, including location, wiring, and the level of surge protection applied at the power input (whether internal or external).

International standards define protection levels

Standards have been developed to classify and provide guidance on the level of protection required. The most commonly used power supply standard is IEC 61000-4-5 developed by the International Electrotechnical Commission. It is referenced in anti-interference standards in many countries, such as EN 55035, which specifies anti-interference requirements for multimedia equipment.

The IEC61000-4-5 standard defines standardized test methods and different levels of protection based on installation category and coupling method. DC power supplies are typically associated with installation categories 3-5 and have test requirements of 1kV to 4kV (Table 1).

Table 1: IEC 61000-4-5 surge test voltages for AC and DC power supplies connected to the mains

Surge suppression circuits and devices

To protect a power supply and its load from surges, some form of internal or external surge protection circuitry is usually required. Surge protection circuits are mainly divided into two categories:

clamp

arc suppression

Surge clamping

A voltage clamp circuit prevents the voltage from exceeding a selected clamping voltage. During a surge, the voltage will be held at the clamp voltage, shunting the current through the clamp circuit until the surge passes. Two devices commonly used as clamping circuits are transient voltage suppression diodes (TVS) and metal oxide varistor (MOV). The speed and energy handling capabilities of each device are inversely related, and as shown in Table 2, combinations of clamping circuit types may be required.

Table 2: Typical surge protection components and characteristics

TVS Diode

TVS are diodes designed to absorb excess energy from voltage spikes, thus clamping the voltage. They can be one-way or two-way. These diodes have a knee voltage similar to that of a Zener diode, above which the diode will begin to conduct. This will cause the voltage to be clamped at the knee voltage and shunt excess energy away from the power supply.

Rheostat clamp

Bidirectional semiconducting metal oxide varistor (MOV) is a pressure sensitive varistor. MOVs have high resistance at low voltages and low resistance at high voltages. It provides a softer clamping voltage and slower response than TVS diodes. MOVs also wear out and can only handle a limited number of surge events. However, due to their low cost and surge handling capabilities, they are often used for surge protection in power supplies.

Figure 2: MOV schematic symbol (left) and voltage-current relationship (right)

crowbar circuit

A crowbar circuit is a different type of surge protection circuit. Instead of limiting the voltage to a maximum value, the crowbar device shorts circuit nodes, bringing the voltage close to zero. Gas discharge tubes (GDTs) are often used as crowbar devices. GDT is similar to TVS and acts as a voltage dependent switch. This device is normally open circuit and will short circuit when the voltage threshold is exceeded. GDTs can handle larger currents, but tend to be the slowest responding surge protection devices. Power sources sometimes combine this with other methods for a more powerful solution.

Figure 3: Example of GDT used with TVS and MOV to implement a powerful surge suppression circuit

Surge immunity of off-the-shelf power supplies

Off-the-shelf power supplies may or may not include internal surge protection. Board-mounted power supplies offer options ranging from no internal protection to the highest level of protection. Typically, manufacturers provide reference designs to improve inherent performance levels. Designers need to pay attention to the datasheet to see if the manufacturer's external circuitry meets the appropriate performance level for the application.

Figure 4: Recommended EMC circuits to meet higher surge requirements

Surge prediction and system assessment

The nature of voltage surges is that any single surge is completely unpredictable. That said, it is still possible to evaluate a system, predict the types of surges it may be subject to, and recommend an appropriate level of surge protection. The corresponding level of protection may vary. It can be expected that some systems will experience the most common and relatively easy to deal with overvoltages. For example, other nearby devices turn on and off. Another extreme scenario could be that the system is located in an area with a lot of lightning activity. In this case, protective measures may be recommended to deal with more severe spikes.

Review Editor: Tang Zihong


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