 Hello, and welcome to this online seminar that provides an introduction to protection against surges. We will review what a surge is, explain its effect on circuits, the root causes, and describe the standards used to define them. We will also cover how a device can be protected, giving a simple example. In order to understand what a surge is, let's consider a simple circuit. A voltage applied across a device, that is the device under test, or DUT, causes a current, I, to flow through it. When a surge occurs, the current, as well as the voltage, can increase so much that it can damage the DUT. Many events can generate surges, each of them leading to different types and levels of surges. Switching transients occur when lightning strikes near a power line or a data line. Switching transients occur in the circuit when electrical loads are turned on and off. Electrostatic discharges, or ESDs, occur with the release of static electricity in the circuit generated by the human contact, for example. The standards related to each type of surge all have the same prefix, IEC 61004. The standard for lighting and switching is IEC 61004.5, also known as the EOS standard for electrical over-stress. It can be a high-power surge. For electrostatic discharges, the standard is IEC 61004.2, ESD is a low-power surge. We will now look at these standards in more detail, starting with electrostatic discharge. Let's consider this basic circuit with V being the voltage across the device under test. IEC 61004.2 is defined with a single electrical parameter and a single environmental parameter. The electrical parameter is the peak voltage of the surge. The peak is shown in the graph. The higher this peak, the stronger the surge is. The environmental parameter is key to describing the ESD, since the electrostatic discharge behavior depends on the distance between the circuit and what causes the surge. This difference in distance represents two cases. In the first, the surge generator is already in contact when the overvoltage occurs. When in contact, the system must be able to manage a peak of 8 kilovolts according to the standard. In the second, the generator is already charged and is brought close to the circuit. The charge flows through the air. In the case of air discharge, the system must be able to manage 15 kilovolts. Applications, including USB, HDMI, or Ethernet, where users can touch the circuit through electrical connectors, must be compliant with this standard. Now let's look at IEC 610045. This type of surge is known as electrical overstress or EOS. In the case of lightning, the main mechanisms leading to surge voltages are direct lightning strikes on outdoor circuits, which inject high currents and produce overvoltages, indirect lightning strikes that induce voltages and currents on the conductors outside and inside a building, and direct-to-earth discharges, which can induce electromagnetic disturbances in adjacent equipment. The purpose of the IEC 610045 standard is to provide a model to simulate these surges and to be able to check if the equipment is able to survive them. The generator that models the surge is defined by two setups, open circuit and short circuit. For open circuits, the surge generated must reach its peak voltage in 1.2 microseconds and fall down to half this peak value in 50 microseconds. The peak voltage to be considered depends on the location of the circuit compared to the lightning. If the circuit is in a protected environment, the peak is 25 volts, whereas if the circuit is outdoors and connected to the grid, the peak voltage can be 6 kilovolts or even higher. For short circuits, the surge generated must reach its current peak in 8 microseconds and fall to half its peak value in 20 microseconds. The peak current depends on the resistance placed between the generator and the circuit. A table must therefore be used to account for this. The peak current is defined by a resistance and a peak voltage. The resistance depends on how the DUT to be protected is connected. When the surge happens between two lines not connected to the ground, the serial resistance is two ohms. When the surge happens between two lines connected to the ground, the serial resistance is 12 ohms. When the surge happens between two communication lines where there is data exchange, the serial resistance is 42 ohms. To summarize what we have covered so far, the surge is specified with a peak voltage, which depends on DUT proximity from the lightning surge and the level of primary protection upstream in the circuit. Its duration is also specified, and this does not vary, being 8 over 20 microseconds for the current and 1.2 over 50 microseconds for the voltage. A resistance is specified according to how the protected DUT is connected. We now know how to describe electrostatic discharges and electrical overstress. So let's explore two of the most important families of protection devices. Transils that are transient voltage suppressors, or TVS, and trisels that are crowbar diodes. Both transil and trisel are trademark devices from ST Microelectronics. Let's start with the transils or transient voltage suppressors. In order to protect the DUT, the transil should be connected in parallel. Step by step, let's look at how the circuit with the transil is going to behave. When the voltage surges, the voltage across the protection and across the DUT increases. At some point, the voltage reaches a value called the breakdown voltage, or VBR. At this moment, the current flows through the protection because of the avalanche effect in the diode. The increase in current in the transil up to the peak current, IPP, causes the voltage to increase slightly and reach a value called the clamping voltage, or VCL. Once the peak value is passed, the current decreases and the breakdown voltage is reached. The current then stops flowing through the transil and the voltage can decrease down to its operating value. A transil thus clamps the voltage to a value between its breakdown voltage and its clamping voltage. Protection devices with a transil-like behavior are used to protect against ESD and against low power EOS. Now let's look at protection with a trisel. The trisel is connected in the same way as a TVS, in parallel with the device. The initial behavior of a trisel is equivalent to the transils. The voltage increases up to the breakdown voltage. The current then starts flowing through the trisel and causes its voltage to increase to a value called the breakover voltage, or VBO. The trisel is said to be triggered and acts as a closed switch, which short circuits the lines. The voltage decreases to the on value. The switch is closed and conducts the current. In order to reopen it, the current must decrease to a value called the holding current, or IH. As soon as the current reaches this value, the current stops flowing in the trisel and the system goes back to its initial state. The advantage of a trisel over a transil is that the trisel can support higher currents. The drawback is that before adding a trisel in a circuit, designers must make sure that the current, in normal conditions, is lower than the holding current. Why is IH a key parameter? And why must designers make sure that the current is lower than the holding current? The reason is that if the current can never get back to the holding current, the trisel, triggered during the surge, never gets back to its initial state, and so always short circuits the lines. So now let's summarize all we've looked at about protection. In this seminar we have described the two main standards, IEC 610042 and 45. We have described their key parameters and which standard you should consider for an application. In addition, we have described two reliable types of protection available from ST. Transils as transient voltage suppressors clamp the voltage, while trisels as crowbar diodes short circuit the lines. You can find more information on our protection web page at www.st.com slash protection, where data sheets, application notes, piece by piece models, product selector, and sample ordering information are available. Thank you for your attention.