TVS diodes-Parasitic Capacitance&Steps to Avoid it
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The rapid evolution of electronics has led to increased density in circuits as well as higher operating frequencies in order to achieve higher performance, decreased size and lower power consumption. With the reduction of dimensions and spacing between components these are also becoming increasingly sensitive to any power anomalies in the circuit such as transient voltages. High density integrated circuits operate at very precise voltages, modern microprocessors for example requiring decimal level precision, so any disturbances must be properly handled to avoid errors, interruptions or even physical failure of these components. In order to prevent transient voltages from interfering with the operation of sensitive components transient voltage suppression devices, such as TVS diodes are used.
A Transient Voltage Suppression diode, or TVS diode, is a solid state electronic component that is used in electronic circuits to protect sensitive semiconductors against transient currents that occur in the circuit or from electrostatic discharges. Various types of diodes such as Zener diodes can also be used to suppress transient voltages but unlike other types transient voltage suppression diodes are designed and tested to deal with very large peak currents in excess of 1KW for limited durations thus providing a significantly wider range of protection for sensitive circuits. Efficient protection of circuits from transient currents also demands a fast response time from TVS diodes and in order to achieve this they are built to take advantage of avalanche breakdown effect. Avalanche breakdown occurs when the free electrons and holes in a p-n semiconductor are accelerated to sufficiently high speeds that they are able to free other electrons and thus increasing the number of charge carriers in the diode which in turn free even more electrons thus leading to a phenomenon similar to an avalanche. Due to the high current formed during breakdown in when a diode reverse biased regular diodes are at a significant risk of being permanently destroyed. TVS diodes are constructed so that the breakdown occurs uniformly in order to prevent current crowding. This makes them far more resilient and as long as their operating limits are respected no degradation of the electrical specifications will occur.
As with various other components in complex circuits, transient voltage suppression diodes also add various undesired effects, such as parasitic capacitance, which has to be taken into account and steps must be taken when designing the circuit to reduce it as much as possible. This parasitic capacitance is a result of the mobile electrons and holes on each side of the p-n junction and the depletion layer. These elements form a structure similar to a capacitor which has an insulating dielectric layer of silicon between two parallel plates. The capacitance is reduced in higher voltage TVS avalanche diodes as their dopant levels are lower and their depletion regions are wider but low voltage diodes require a higher level of doping and a narrower depletion region which results in a higher capacitance.
To understand why this parasitic capacitance appears in transient voltage suppression diodes we need to understand how a normal capacitor works. A capacitor is a passive two-terminal electrical device that consists of two conductors separated by a non-conductive region. When voltage is applied to a capacitor’s conducting wires, charge carriers start flowing towards the electrodes of the capacitor where they encounter a strong opposition from the dielectric or insulating material. However, even though the charge carriers cannot pass though the separation zone the charge on one side of the dielectric exerts a force on the charge carriers on the other side attracting those of opposite polarity. This results in trapping a significant amount of the charge carriers near the capacitors electrodes as they cannot move between the plates. Due to these trapped charge carriers around the plates the dielectric develops an electric field which in turn causes a net positive charge on one plate and a net negative charge on the other plate and through this process a flow of charge appears in the source circuit. In a basic capacitor, the capacitance is directly proportional to the size of electrodes or plates and inversely proportional to the distance between two plates.
When a p-n junction diode has a reverse biased voltage applied to its terminals it has all the required elements to create a capacitor. It’s P and N layers have low resistance and act as electrodes or plates and the depletion region, having positive and negative ions formed between them is the dielectric or insulating layer. Thus it is able to store an electric charge in its depletion region just as a capacitor would.
The capacitance of a p-n diode varies proportionally to the reverse biased voltage applied. As the reverse biased voltage applied to the p-n junction is increased a large number of electrons from the n side and a large number of holes from the p-side are pushed away from the p-n junction proportionally to the voltage. This causes the widening of the depletion region and the contraction of the plates (the p-type and n-type layers).
In p-n junction diodes with a narrow depletion width and large p-type and n-type regions we observe a greater amount of electric charge stored whereas in p-n junction diodes with a wide depletion region width such as TVS diodes we observe the opposite, a decreased ability to store electric charge.
While normal p-n junction diodes are designed to allow current to flow in the forward direction, TVS or avalanche diodes allow current to flow in both directions but are designed in such a way to purposefully operate in reverse biased mode when a set of required and specific conditions are met. In order to create this effect TVS diodes are built with a lightly doped and very wide junction region so that the required breakdown voltage is higher than in regular p-n diodes. This higher breakdown voltage is precisely controlled and set by varying the level of doping and the width of the junction region during construction. Wider depletion zones increase the breakdown voltage while a narrower and more doped depletion region decreases the breakdown voltage.
As TVS diodes have a wider junction region compared to some other types of diodes, they already have a decreased ability to store an electric charge and so their capacitance is naturally lower. But depending on their operating voltage requirements the width of the TVS diodes varies and so does capacitance. In devices that operate at lower voltages the junction width is lower than in higher voltage TVS diodes. The capacitance in transient suppression diodes is usually between 500pF and 10nF. As this intrinsic capacitance manifested by transient voltage suppression diodes is undesirable it is commonly referred to as parasitic capacitance or stray capacitance.
Low frequency circuits are not significantly affected by the small parasitic capacitance of TVS diodes but high frequency integrated circuits that often also operate at lower DC voltages require low voltage TVS diodes. These low voltage TVS diodes naturally tend to induce a slightly higher level of parasitic capacitance to which these sensitive circuits they are intended to protect are most susceptible. Amplifier circuits are also affected by parasitic capacitance as when this occurs between their input and output it can act as a feedback path causing high frequency oscillations called parasitic oscillations.
Since the TVS diode’s capacitance is proportional to the physical size of the diode a practical solution is to decrease the overall size of the TVS diode which can effectively mitigate the performance degradation in the circuit due to parasitic capacitance. However the size cannot be decreased unlimitedly as the TVS diode has to remain sufficiently robust to handle the required level of transient currents.
Another solution for compensating and reducing the parasitic capacitance effect on the circuit is to package TVS diodes together with other components in various layouts, a process through which their capacitance is reduced significantly.
One method of creating a low capacitance transient voltage suppressing devices is to combine the TVS avalanche diode in series with a low capacitance, high-speed rectifier in opposite polarity to the TVS diode. Placing a TVS avalanche diode in series with a forward biased switching diode will effectively reduce the total shunt capacitance. When the transient voltage rises above the characteristic threshold voltage of the TVS diode the electric charge around the dielectric of the avalanche diode increases and thus the parasitic capacitance begins to accumulate. As soon as the parasitic capacitance of the TVS diode is completely charged the switching current will become non-conducting and the protection circuit will cease to draw any current. As a result the use of switching diodes in series with transient voltage suppressing diodes prevents current flow when the TVS diode is forward biased.
Depending on the application and requirements of the circuit such as unidirectional or bidirectional protection different arrangements can be employed by using multiple TVS diodes, switching diodes and other components but special consideration must be used with discrete components as some layouts can cause parasitic inductance.