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A surge protection circuit, also known as an AC line voltage peak protector, is designed to suppress voltage spikes. While commonly associated with AC power lines, its application is not strictly limited to them. A surge protector, or surge protection device (SPD), prevents damage to sensitive electronics by suppressing voltage surges and spikes.
Depending on their type, surge protectors can handle voltage spikes ranging from a few hundred volts to several kilovolts. Although they are designed to withstand high-voltage events for a very short duration, they cannot handle sustained overvoltages.
What is a Surge?
A surge is a sudden increase in voltage or amplitude above its normal level. In electrical systems, the terms voltage transient, voltage surge, or voltage spike are often used to describe this phenomenon. A surge is a temporary event, lasting only for a brief period, but it can be destructive to equipment without proper countermeasures. Voltage surges occur not only on power lines but also in any circuit with inductive properties. However, power line surges are often the most destructive, as they can reach levels of several kilovolts.
AC line transient surge protectors are commonly installed in homes, offices, and buildings to prevent equipment damage. For comprehensive protection, they should be installed at the service entrance, ensuring all devices connected to the mains are shielded from line surges and spikes. This approach is known as whole-house or service entrance surge protection. This may not be necessary if individual devices are equipped with their own local surge protection circuits.
Two Main Categories of Power Line Surge Protectors
- Primary Surge Protectors: These devices are installed where the main electrical service enters a house, office, or building. They protect all appliances and equipment connected to the lines downstream from the entry point. Primary surge protectors are typically very robust but are also large, bulky, and expensive.
- Secondary Surge Protectors: These protectors are less powerful and effective than primary units but are portable and convenient. A common example is a plug-in power strip. It only protects the devices that are plugged into its outlets.
Common Types of Secondary Surge Protectors
A few common types of secondary surge protectors are widely used. One is the power strip, which plugs directly into a wall outlet and provides multiple protected outlets for various devices. The most critical function of a protective power strip is its ability to disconnect power during a surge event. Another well-known type is the Uninterruptible Power Supply (UPS). Many advanced UPS units have built-in surge protection that offers a similar level of safety as a dedicated power strip.
How Do Surge Protectors Work?
One type of surge protector works by completely cutting off the power supply when a surge is detected. These are complex and expensive, typically consisting of a voltage sensor, a controller, and a latching/unlatching circuit. The sensor monitors the line voltage, and the controller decides when to signal the latching circuit—a controllable power contactor or switch—to disconnect the line voltage.
A more common type of surge protector does not disconnect the voltage but instead clamps the transient voltage and absorbs its energy. This type is often used as built-in protection in devices like switch-mode power supplies and is effective for voltages below a few thousand volts. This protection scheme is best described with a circuit diagram.
Causes of Voltage Surges
Voltage surges have numerous causes, including lightning strikes, power system switching (e.g., capacitor banks), resonant circuits with switching devices, wiring errors, and the sudden switching of motors and other highly inductive loads.
Common Surge Paths
- Power Lines: This is the primary path for surges, as nearly all electrical and electronic devices connect to the AC mains.
- RF Lines: This includes antennas, which are susceptible to lightning strikes. A lightning strike can induce an extremely high voltage spike that can travel into an RF receiver.
- Automotive Alternators: In automotive electronics, the alternator can generate high-voltage spikes during a load dump condition.
- Inductive Circuits/Loads: Any inductive circuit or load can generate a surge voltage, often called inductive kickback.
What is the Short-Circuit Current in IEC 61000-4-5?
The standard for AC power line surges is defined by IEC 61000-4-5. All devices connected to the power line must have surge protection. This protection works by clamping the transient voltage to a safe level. When the surge protection circuit clamps, it creates a temporary short-circuit path from the power source through the protection device to ground.
How to Design a Surge Protection Circuit
Designing a surge protection circuit can be straightforward. For some electronic devices, built-in surge protection can consist of a single component, such as a Metal-Oxide Varistor (MOV) or a Transient Voltage Suppressor (TVS) diode.
Often, a single surge protection device between the AC lines is sufficient to meet IEC standards. In some cases, especially for higher surge voltage requirements (4 kV and above), additional protection circuits are needed across the lines and from line to ground.
Using an MOV for Surge Protection
Basic Properties
An MOV (Metal-Oxide Varistor) is a commonly used surge protector for power lines. It is a voltage-dependent resistor with a non-linear, non-ohmic, and bidirectional current-voltage characteristic, similar to two back-to-back Zener diodes or a bidirectional TVS diode. It acts as an open circuit until its clamping voltage is reached. Its V-I curve shows a nearly constant voltage in quadrants 1 and 3, confirming its bidirectional nature. MOVs are commonly made from zinc oxide (ZnO) or silicon carbide (SiC).
Component Selection
For a universal 90-264 Vac line, a common MOV voltage rating is 300 Vrms. This is the continuous RMS voltage the MOV can withstand, not its clamping voltage. For example, the Leidi Tech 14D471KJ has a 300 Vac rating but a clamping voltage of 775 V at a 50 A peak current, according to its datasheet. The next step is to verify that the MOV's surge current rating can handle the levels specified in the standards (e.g., 2000 A). According to the datasheet for this MOV, it can handle more than 15 but fewer than 100 strikes at 2000 A with a 20 μs pulse duration.
Although the datasheet specifies a clamping voltage, it may not be valid at 2000 A. The corresponding clamping voltage at 2000 A for the selected MOV might exceed 1000 V. It is crucial to ensure that all components in the device can withstand this voltage level. If not, an MOV with a lower clamping voltage should be considered.
Ideal MOV Placement in a Circuit
An MOV used for surge protection should be placed immediately after the main fuse. With this configuration, if the surge current is too large for the MOV to handle, the fuse will blow, disconnecting the circuit and preventing a potentially catastrophic failure.
Surge Suppression in Automotive Systems
Surges are not limited to AC power lines; they are also common in automotive systems. A typical automotive system uses a 12 V lead-acid battery, with a fully charged voltage of around 12.9 V. While the steady-state voltage is not destructive, a "load dump" event can be. A load dump occurs when the battery is abruptly disconnected while the alternator is charging. For a 12 V system, this can cause a voltage spike of up to 120 V, which is sufficient to destroy electronic components.
To counteract this, surge protection circuits using TVS diodes and varistors are employed. The automotive load dump waveform is defined by ISO 7637.
