GREATER NOIDA, INDIA: Imagine you bought the latest electronic device and it got damaged just by the touch of your finger to its USB port or your laptop getting damaged by electrical surge in LAN cable. The discharge of electrical charges from the human body or a lightning surge in a power cable can create enough current and voltage to damage the silicon of any electronic chip. As the devices are getting faster and smaller they become more and more susceptibility to electrical surges.
This unpleasant situation can be avoided if circuits are properly protected against these surges by using specific silicon devices called protection ICs, which can absorb these surges and not let them reach the functional ICs. This article emphasizes the need to protect ICs in electronic devices for better reliability, user loyalty, and operating cost reduction due to the cost of repairs.
Key apps needing protection ICs
Mobile phones: Audio lines, USB interfaces, bottom connectors and keypads, touch screens, Video Serial Interface, HDMI, MIPI (Mobile Industry Processor Interface), memory cards, SIM cards, battery and charger ports.
Consumer goods: USB2.0, USB3.0, HDMI1.3, HDMI1.4, Display port, LNB for STB
Networking and telecom Infrastructure: SLIC (line cards, voice band), Digital lines (xDSL, ADSH2+, CDSL, VDSL2 modems, Ethernet), SLVU on RJ45 side, power over Ethernet
Industrial applications: MOSFET protection in Inverter/UPS or other applications that use MOSFETs along with transformers.
Types of protection
Electro static discharge [ESD] protection: The human body can pick up an electrical charge from the environment or from interaction with another charged object. You may have experienced this - when you rub a comb with cloth and bring it close to your hair, the hair tends to rise. This is because the charges on our hair attract the opposite charges of the comb.
Sometimes, we can even experience an electrical shock by touching a non-electrical device. Any of these sudden discharges of current can generate enough of an ESD pulse to damage the silicon chip of our electronic devices.
The various standards relating to IC ESD are listed below.
• MIL STD 883E-Method 3015-7 (human body model, HBM): This simulates an ESD event when a person is charged either to a positive or negative potential and touches an IC at another potential.
• JESD22-A115-A (machine model, MM): This simulates an ESD event that occurs when part of the equipment or tool comes into contact with a device at a different potential.
• ESD STM5.1- 2007 (charged device model, CDM): This simulates an ESD event when a device discharges from a certain potential to equipment at another potential.
Most of the silicon chips today have one of the above built-in ESD protection standards to prevent the chip getting damaged in the production line. But this protection is not enough to protect the IC when the device is actually used by the end user. When we touch a port of a device, the induction of the PCB trace can amplify the surge discharge. So the surge which reaches the IC is much higher. The ESD standard needed to protect the device at the consumer’s end should be more stringent than HBM.
IEC 61000-4-2 (electromagnetic compatibility) simulates the whole environment where equipment may be subjected to ESD. This standard is considered as a reference in the wireless environment because this simulates more of a real-case stress. IEC61000-4-2 ensures protection at the system level whereas HBM defines protection at IC level.
It is difficult to ensure IEC61000-4-2 protection embedded in the functional IC. So specialized protection ICs should be used to comply with this standard. In fact as the ICs become more complex, faster, and smaller, the gate oxide thickness is also being reduced.
Hence, implementing protection is becoming increasingly difficult as protection is occupying a major portion of the silicon landscape (see fig 1). Moreover, better control of ESD risks in the factories (ionizers, grounded straps, grounded boots on conductive floors) ensures that the ICs don’t get damaged due to ESD failure. The Industry Council on ESD target levels propose a reduction for HBM (Human Body Model) levels from 2 kV to 1 kV. But the user environment remains the same, so the need for external ESD protection is even more necessary.
Fig. 1: Proportion of protection circuits in various technologies
Source: STMucroelectronics.Electrical over stress [EOS] protection: Telecom lines are exposed to lightning in three main ways:
• Direct lightning strike on the equipment. This is not common as there is usually a lightning conductor. However, in this case, the surge current can reach hundreds of thousands of amperes, so it is clear that the equipment will be destroyed.
• Lightning induces radiation in the air and causes surges through coupling with the line.
• Lightning induces a potential change in the ground which disturbs the equipment.
Lightning can damage central office equipment, gateways, phones, and other electronic devices. In addition to these surges, a telecommunication line may also be disrupted by the power line. Two scenarios are possible:
• Power induction, where a current is induced in the telecommunication line. This is generally a low current surge.
• Power contact, involving a direct contact between the telecommunication line and the power line. This is generally quite a high current over several minutes.
Depending on the country, these surges have been modeled in standards:
• Telcordia GR-1089 core and TIA-968-A (formerly FCC part 68) for America
• ITU-T K series for the rest of the world
• China is using YDT series, but this is based on ITU-T K standards
Basics of protection ICs
Protection basically uses the diode technology for its functioning. A rectifier uses the 1st quadrant and ensures that no negative pulse passes through it. A protection IC works in 3rd quadrant and does not allow voltage beyond a specified value to pass through it. A rectifier is put in series with the circuit whereas the protection is put in parallel to the circuit.
Protection ICs are classified in broadly two categories – transils and trisils. The two categories are explained below:
a) Transils: A Transil diode is a solid state, monolithic PN junction device. Transils are used in sensitive semiconductors for parallel protection against EOS or ESD. A Transil clamps any over-voltage above its Breakdown Voltage (VBR). Transils can be unidirectional or bidirectional.
b) Trisils: In Transils the Ipp is proportional to Vpp. In telecommunication systems, surges are high in terms of energy, so a Transil will blow up. For this reason, Trisils based on Thyristor/Triac technology have been developed: The electrical characteristics of a Trisil are similar to those of a Triac, except that the Trisil has only two terminals.
Why silicon-based protection ICs?
The two main alternatives of silicon-based protection ICs are a) gas tubes and b) Metal Oxide Varistors (MOV). Gas tubes and MOVs have their advantages in certain applications which require higher surge capabilities. But there are distinct capabilities of silicon devices which make them unique with regards to non–silicon-based solutions.
Performance degradation in gas tubes and MOVs
The performance of gas tubes and MOVs degrades with every surge, whereas silicon ICs do not degrade with surges. The breakdown voltage of MOVs and gas tubes is unstable and also increases with every pulse, whereas the breakdown voltage follows a predefined path in silicon ICs with little deviation from stated value.
Silicon devices also have fast response time, Vbo independent of dv/dt, high reliability with no ageing, and smaller footprints. These advantages make silicon protection ICs the favorite for applications where surge capabilities are not very high.
Key parameters
Protection ICs of several makes are available in the market. At first glance, they look similar - with compatible pin configuration and functionality - but it is important to closely observe their parameters. An incorrectly selected device can interfere not only in the normal functionality of the system but may also be worthless in providing adequate protection.
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Here are some of the critical parameters:
Signal integrity: Transparency of protection device is an important criterion. Protection ICs should not interfere with the normal functioning of system. Critical parameters which impact transparency are low leakage current, low capacitance, and high bandwidth.
STMicroelectronics’ high speed interfaces come with ULC Technology. This technology provides the widest bandwidth, even for 15 KV contact ESDs. As is evident in the below description, the ULC technology devices do not attenuate the signals even at gigahertz frequencies.
As a result, they provide an excellent diagram even at 3.2 GHz. Also, low capacitance means that the original signal is not impacted. The capacitance of the device is so low that it does not impact the original signal or the differential impedance of the system.
Robustness: Since a protection IC is the insurance of an electrical circuit, it should protect the circuit even in adverse conditions. The device should have high surge capabilities and should not degrade with changes in temperature.
The graphs depict the surge capabilities of equivalent devices. Both devices are similar but their degradation with regards to temperature follow different paths. ST’s SMCJ protection series offers full surge capabilities even at 110oC, whereas similar ICs from other makers degrade from 100 percent at 25 degrees C to 0 percent at 150 degrees C.
Package flexibility: Package flexibility is another important feature of protection devices. The new protection ICs come with pass-through rails which means the rails don’t have to be cut to embed protection ICs in the design. This flexibility allows the development and validation of the functionality of the system without the protection IC on board.
The protection can then be added during the surge tests and production stage. The pass-through rail feature also saves PCB space and provides the option of adding protection to already developed products.
Cost of non-protection: Investment in protection ICs is not a waste of money. In fact, it can prove to be a money-saver. This can be understood through a typical industry scenario.
Suppose a firm is producing 1 Million boards per month, and it gets 1% as field returns. Out of the total returns, 45 percent have been detected to be caused by ESD damage that could have been protected. So, 4.5 K boards have to be repaired. If the cost of field service - from shipping damaged boards from retail to service centre, and then providing a repaired box from service centre to the customer - comes out to be $100, the total cost incurred by the company is $450K.
This scenario could have been avoided by using protection ICs – typically 3 ICs per board at $ 0.1 each. Thus, the cost of protection ICs for 1 Million boards comes out to be $300K.
Typically, protection ICs not only save money for the manufacturer but also help to avoid the high risk of brand image destruction.
Conclusion
Protection ICs have made it possible to make electronics devices rugged. These ICs can protect costly electronic devices from damage because of ESD or EOS. At the same time, continuous innovation has made it possible to make them transparent to the normal function of devices.
Investment in protection ICs may add to BOM cost at the beginning, but the advantages - avoiding returns and saving brand image - will have dramatic effect. While selecting a protection IC, it is important to look closely at various parameters from different vendors as seemingly similar looking devices may offer varied degrees of protection.
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