Principles and Design of Industrial and Automotive Surge Protection Circuits

Surge protection circuit is a circuit known as an AC power line voltage spike protector. However, there is no particular restriction in AC power lines. A surge protector or surge protection device is a device that provides surge suppression or voltage spike suppression so that sensitive equipment is not damaged.

Surge protectors can handle voltage spikes up to the several thousand volt range (depending on the type of surge protector). Others can only handle a few hundred volts, and so on. Although surge protectors are designed to withstand high voltage spikes for a short period of time, they are not designed to withstand longer durations.

What is a power surge?

Generally, a surge is a sudden increase in level or amplitude from a normal or standard value. In electricity, surge is often used to describe voltage transients, voltage surges, or voltage spikes. A voltage surge, spike, or transient is not a permanent event. It only occurs for a short period of time, but is enough to destroy equipment if there is no countermeasure.

Voltage surges exist not only in power lines but also in circuits with inductive properties. However, voltage surges in power lines are the most destructive because they can be as high as several thousand volts.

The following figure shows a power surge on an AC power line.

AC line transient surge protectors are often installed in houses, offices, and buildings to prevent damage to equipment or devices. They should be installed in the section where all equipment or devices get their source. This way, all devices will be protected from line surges and spikes. This approach is called universal surge protection. If all equipment or devices have local surge protection circuits, universal surge protectors may not be required.

There are two main categories of surge protection circuits used in power lines

1. Mains surge protector

A primary surge protector is installed at the point of entry of electrical wires into a house, office, or building. It will protect all devices or appliances connected to the line after the entry point. Usually, a primary surge protector is very powerful. However, it is large, bulky, and expensive.

2. Secondary surge protector

Secondary surge protectors are not as effective and powerful as primary surge protectors.

However, it is portable and convenient to use. Usually, this type of surge protector is easily plugged into an electrical outlet. It only provides protection to devices that draw power from the electrical outlet where the second surge protector is installed.

The following diagram shows how primary and secondary surge protectors are installed in a building.

Common Types of Secondary Surge Protection Circuits

There are few known secondary surge protection circuits. One is the so-called power strip. A power strip is easily plugged into an electrical outlet. Apart from that, it comes with multiple electrical outlets into which multiple devices and appliances can be plugged and protected from surges. The most important function of a power strip is the ability to terminate the power supply in case of a surge.

Another known type of secondary surge protector is the well-known UPS or Uninterruptible Power Supply. Some sophisticated UPS have built-in surge protectors that provide the same safety protection features as a power strip.

How do surge protectors work?

There is a surge protector

Once a surge occurs, the power can be cut off. This type of surge protector is complex, complicated and of course expensive. The basic components of this type are the voltage sensor, controller and latch/unlatch circuit. The voltage sensor will monitor the line voltage, and the controller will read the sensed voltage and decide when to signal the latch/unlatch circuit to terminate the voltage. The latch/unlatch circuit is a controllable power contactor or power switch that can connect or disconnect the line voltage.

There is also a type of surge protector that does not provide voltage shutdown, but simply clamps voltage transients and absorbs energy. This type of surge protector is often used as a built-in surge protector, such as in switch mode power supplies. This type of protection is effective at voltages up to several thousand volts. This type of surge protection is best described in a circuit as shown in the figure below.

Surge protector 1 on ACLINE 1 and AC2 is called differential mode surge suppression. Whereas surge protectors 2 and 3 are both called common mode surge suppression. A differential mode surge suppressor clamps any voltage spikes on ACLINE 1 and AC line 2. It is called differential mode because it is installed on both hot wires. On the other hand, common mode is the term used for surge protectors 2 and 3 because both clamp voltage transients on a single hot wire relative to earth ground. In less stringent surge requirements, surge protector 1 is already sufficient to pass the standard. However, for very stringent requirements such as higher surge voltages, surge protectors 2 and 3 are added.

Causes of voltage surges

Voltage surges occur for many reasons. It can be caused by lightning, power system switching (such as capacitor banks), resonant circuits with switching devices, wiring errors, and sudden turning on and off of switches, motors, and other highly inductive appliances and devices. AC line voltage surges are present anywhere in the world. Therefore, it is recommended to protect devices and equipment from this destructive event.

Some common surge media

These are common paths by which a power surge or voltage spike can enter the device or equipment that uses it.

Power Lines – This is the number one medium for surges because all electrical and electronic devices use AC power. AC line surges are common around the world.

RF lines – including antennas. Antennas are susceptible to lightning strikes. Lightning can produce very high voltage spikes for a short period of time. When lightning strikes the antenna, it will penetrate the RF receiver.

Automotive Alternator – In automotive electronics, voltage surge is also defined. This is because the alternator is capable of producing high voltage spikes during load dump.

Inductive Circuits/Loads – Any inductive circuit or load will always introduce a surge voltage. Typically, this surge is called inductive kickback.

Surge standards defined by IEC

IEC61000-4-5 defines the standards for AC power line surges. The table below provides specific information about the categories and voltage levels. The table is taken from the following link

According to the standard, the maximum transient voltage that equipment should withstand and pass under Level 4 is 4kV (although there is Level 5, it is still called Level 4).

The transient voltage defined by IEC61000-4-5 is shown in the figure below. It has a rise time of 1.2us and a pulse width of 50us. The table is taken from Shanghai Leimao Electronics Link

IEC61000-4-5 also defines the short-circuit current shape, as shown in the figure below. It has a rising edge of 8us and a pulse width of 20us. The table is taken from the Shanghai Leimao Electronics website.

The following table shows the surge current or short-circuit current level corresponding to each category. The worst value is 2000A.

What is the short-circuit current specified in IEC61000-4-5?

To answer this question, I will first say that all devices connected to the power line must have surge protection. Surge protection works by clamping transient voltages to a safer level. Once the surge protection circuit clamps, a short circuit path from the power source to the protection device and then to power ground will appear.

How to Design a Surge Protection Circuit

Designing a surge protection device is not difficult. In fact, the built-in surge protection of some electronic devices can be just one device. This can be an MOV or a metal oxide varistor or a transient voltage suppressor TVS. In the figure below, surge protectors 1 to 3 can be either MOVs or TVSs.

Sometimes, a surge protection device between the AC lines is sufficient to pass IEC standards. In a few cases, a surge protection circuit is required across the line and ground. This is especially true for higher surge voltage requirements (4kV and above).

Using MOVs as Surge Protection Devices

Basic properties

MOV stands for Metal Oxide Varistor; a common surge protector used in power lines

MOV is a voltage dependent resistor

The MOV operates like a diode, with nonlinear and non-ohmic current and voltage characteristics, but is bidirectional

Its operation can also be compared with a bidirectional transient voltage suppressor TVS

When the clamping voltage is not reached, it opens

Below is the voltage-current curve of an MOV. As you can see, it has an almost constant voltage on quadrants 1 and 3, which makes it a bidirectional device. ZnO and SiC stand for zinc oxide and silicon carbide, respectively. These are two common materials that MOVs are made of.

Model Selection

A typical MOV is rated at 300Vrms for a common 90-264Vac line. 300Vrms is the RMS or continuously applied voltage that the MOV can withstand. This is not yet the clamping voltage. For example, we will use the 14D471KJ from leiditech, which has an AC rating of 300Vac, but a clamping voltage of 775V at 50A peak current, according to the datasheet.

The next thing to verify is that the surge current rating of the MOV is capable of handling the levels specified in Table 2 above (considering the maximum levels). Based on the selected MOV datasheet below, at 2000A and 20us pulse duration, the MOV is capable of handling more than 15 strikes but less than 100 strikes. The 2000A is estimated with a dotted line on the device graph.

Although the clamping voltage is specified in the datasheet, it may no longer be valid at 2000A. The figure below shows the corresponding clamping voltage at 2000A when using the selected MOV. The intersection of the yellow lines is the clamping voltage. Note that it is already over 1000V. Make sure that all devices used in the device can withstand this voltage level. If not, consider another MOV with a lower clamping voltage.