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the battery. As a result of this disconnect, the full charge current from the alternator is placed on the power rail, which raises the rail voltage to very high (>100V) levels for hundreds of milliseconds. Communications applications can have a number of possible surge causes, ranging from hot swapping communication cards to outdoor installations that can be exposed to lightning strikes. Inductive voltage spikes are also possible with long cables used in large facilities.


Ultimately, the environment in which the device must operate must be understood along with meeting published specifications. This helps the designer to put together an optimal protection mechanism that is both robust and unobtrusive, but allows downstream electronics to operate within safe voltage levels with minimal interruption.


Traditional protection circuitry With so many different types of electrical events to consider, what should be in an electronics engineer’s arsenal to protect the sensitive downstream electronics? A traditional protection implementation relies on several devices rather than just one - for example, a transient voltage suppressor (TVS) for overvoltage protection, an in-line fuse for overcurrent protection, a series diode for reverse battery/supply protection, and a mix of capacitors and inductors to filter out lower energy spikes. While discrete setups can meet published specs - protecting downstream circuits - they result in complex implementations, requiring multiple selection iterations to correctly size the filtering.


Let us take a closer look at each of these devices, touching on the advantages and disadvantages of this implementation.


TVS - Transient voltage suppressor This is a relatively simple device that helps to protect downstream circuitry from high voltage spikes on the power supply. It can be broken into several different types, which have a wide range of characteristics. Although these feature a range of constructions and characteristics, they all operate in a similar manner: shunting the excess current when the voltage exceeds the device threshold. A TVS clamps the voltage at the output to the rated level within a very short period of time. A TVS diode, for example, can respond in as low as picoseconds in time, while a GDT can take a few microseconds to respond but can handle much larger surges.


Figure 3 shows the simple implementation of a TVS diode to protect a downstream circuit. Under normal operating conditions, the TVS is high impedance and the input voltage simply passes to the output. When an overvoltage condition occurs at the input,


In-line fuse


Figure 2: Traditional protection devices


the TVS becomes conductive and responds by shunting the excess energy to ground (GND), clamping the voltage seen by the downstream load. The rail voltage rises above the typical operation value but is clamped to a value at a safe level for any downstream circuitry.


Figure 3:


Protecting against voltage surges with a traditional TVS solution


Overcurrent protection can be implemented using the ubiquitous in-line fuse with a fuse blow rating at some margin above nominal - for example, 20% higher than the max rated current (the percentage will depend on the type of circuit as well as the typical operational loads expected). The biggest problem with fuses, of course, is that they must be replaced once blown. Time and cost savings resulting from fuses’ simple design can be negated as additional costs can be incurred later because of relatively complex maintenance, especially if the application is physically hard to reach. Maintenance requirements can be reduced with alternate fuses, such as resettable fuses, which utilise a positive temperature coefficient to open the circuit when a larger than normal current passes through the device (the increased current level increases the temperature, resulting in a sharp increase in resistance). Maintenance issues aside, one of the biggest problems with fuses is their reaction time, which can vary widely depending on the type of fuse selected. Fast blow fuses are available, but clearing time (time to open the circuit) can still range from hundreds of microseconds to milliseconds, so the circuit designer must consider the energy released over these extended times to ensure that downstream electronics can survive.


Series diode Figure 4:


Adding a series diode protects against reverse polarity, but the voltage drop of the diode can be a problem in high current systems


Although TVS devices are effective in suppressing very high voltage excursions, they are not immune to damage when faced with sustained overvoltage events, resulting in a requirement for regular device monitoring or replacement. Another concern is that a TVS can fail short and thus crowbar the input supply. Also, depending on the energy involved, they can be physically large to match with margin, increasing the solution size. Even when a TVS has been correctly sized, the downstream circuitry must be capable of handling the clamped voltage, resulting in increased voltage rating requirements downstream.


In some environments, circuits are exposed to supply disconnection and reconnection - for example, in a battery-operated environment. In such instances, correct polarity is not guaranteed in reconnection of the supply. Polarity protection can be achieved by adding a series diode on the positive supply line of the circuit. While this simple addition is effective in protecting against reverse polarity, the voltage drop of the series diode results in commensurate power dissipation. In relatively low current circuits, the trade-off is minimal, but for many modern high current rails, an alternative solution is required. Figure 4 shows an update to Figure 3, showing both the TVS and the added series diode to protect against the reverse polarity connection.


Filters using inductors and capacitors


The passive solutions discussed so far all limit the amplitude of the events passed through but generally capture larger events while leaving some smaller spikes to pass. These smaller transients can still cause damage to downstream circuitry, so additional passive filters are required to clean the line. This is achievable using discrete inductors and capacitors, which must be sized to attenuate the voltage at the unwanted frequencies. Filter design requires test and measurement before design to ascertain the size and


MAY 2021 | ELECTRONICS TODAY 15


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