Protection Devices

There are a variety of products on the market to provide protection to electronic systems and components. Each of the products has its own characteristics as outlined in Table 1.

Table 1 Strengths and Weaknesses of Various Protection Elements

Type		Polarity	Clamp or Crowbar	Protection Mechanism
Gas Discharge Tube	GDT	Bidirectional	Crowbar	Breakdown of a gas at high voltage
Thyristor Surge Protection Devices	TSPD	Unidirectional or Bidirectional	Crowbar	Turn on of coupled bipolar transistors
Polymer ESD Devices	PESD	Bidirectional	Crowbar	Arcing between particles in polymer
Metal Oxide Varistor	MOV	Bidirectional	Clamp	Non-linear resistance of ceramic of zinc oxide grains
Transient Voltage Suppressor	TVS	Bidirectional or Unidirectional	Clamp	Forward bias and reverse bias diode conduction

Unidirectional and Bidirectional Protection

The protection requirements for electrical nodes with only positive or only negative voltage differ from nodes whose voltage extends above and below zero volts. Unidirectional and bidirectional protection elements address this issue and can be easily explained using TVS devices as an example.

Figure 1 shows examples of unidirectional and bidirectional TVS protection of an input. The I-V curve for a unidirectional TVS device is similar to that of a standard diode. If a unidirectional TVS is inserted into the circuit as shown in Figure 1 the signal voltage will be undistorted if it remains between 0V and the reverse breakdown voltage. For negative voltage stresses the unidirectional TVS protects by forward bias operation of the TVS. For positive stress on the signal line the TVS protects by reverse bias breakdown. If the signal ranges between 0V and a negative voltage, the unidirectional TVS device can be inserted with the opposite polarity.

Figure 1 I-V curves and symbols for unidirectional and bidirectional TVS devices

A bidirectional TVS behaves as two anti-parallel diodes in series. If a bidirectional TVS device is used as the protection device in Figure 1 the input voltage can range over positive and negative values. Protection is provided by reverse bias breakdown in series with a forward bias diode in both polarities.

Clamp and Crowbar Devices

Another classification of protection devices is clamp versus crowbar. TVS devices are clamp devices, since they try to clamp the voltage at a defined voltage during a stress event. A crowbar device attempts to create a short circuit when a trigger voltage is reached. It is like putting a metal crowbar across the high voltage to provide a short. Both cases are illustrated in Figure 2.

Figure 2 I-V characteristics of a bidirectional crowbar device (black) and a unidirectional clamping device (red).

Crowbar devices are very attractive for protection. Crowbar devices with low on state voltage can keep voltage levels well below the critical values for sensitive electronic elements and carry considerable current without damage to themselves due to low power dissipation. Care must be taken in the use of crowbar devices. The lowest current and voltage point that can sustain the on state of the crowbar device is an important parameter and is often called the holding point, see Figure 2 . If the electrical node being protected can supply the voltage and current levels of the holding point, a crowbar device may not turn off after the electrical stress has been removed. Careful selection of the crowbar device is needed or other precautions must be taken to insure the protection turns off when the stress is removed and does not turn on during normal operation.

Voltage clamp devices do not have the problem of not turning off after a stress, but they also must be selected with care. Clamp devices protecting in the reverse bias direction dissipate considerable power which they must dissipate internally. Clamp devices need to have very low dynamic resistance in the on state to insure that while carrying large currents the voltage does not exceed the allowed levels for sensitive circuit elements.

Protection Device Technologies

Gas Discharge Tubes rely on the breakdown of a gas in a sealed container and are inherently a bidirectional, crowbar protection element. They are limited to relatively high voltage turn on but can carry significant current and are often used for primary protection.

The term Varistor is a combination of variable and resistor. At low currents and voltages they have a high resistance but at higher voltages and currents the resistance drops dramatically. The resistance elements are made from ceramic of zinc oxide grains. Varistors are always bidirectional, voltage clamping devices.

Polymer ESD devices consist of a polymer embedded with conducting particles. At high voltage arcs between the particles create a low resistance path. Polymer Devices are bidirectional crowbar devices.

Thyristors are multilevel sandwiches of n and p doped silicon which form intertwined bipolar transistors. When the bipolar transistors are triggered they can enter into a self sustaining low resistance state. Thyristors are inherently unidirectional crowbar devices. Modifications of the basic Thyristor have produced a variety of bidirectional and unidirectional options.

Transient Voltage Suppressors are based on Avalanche and Zener diodes designed to handle the high current requirements for surge suppression. Single diodes are inherently unidirectional clamping devices. TVS devices are however manufactured in a manor similar to integrated circuits, making it possible to produce a wide variety of unidirectional, bidirectional and multi-pin protection elements. TVS devices can often protect multiple signal lines with a single small protection device.

Table 2 Protection Device Strengths and Weaknesses

Type		Speed	Voltgae Accuracy	Current Capability	Size on Board	Lowest Trigger Voltage	Wearout
High Power Surge Events - 8x20 µs, 10x1000 µs, ect.
Gas Discharge Tube	GDT	Slow	Fair	High	Large	75V	No
Thyristor Surge Protection Devices	TSPD	Fair	Good	Medium	Small	80V	No
Transient Voltage Suppressor	TVS	Fast	Good	Low	Small	NA	No
Metal Oxide Varistor	MOV	Fair	Poor	Medium	Small	NA	Yes
Very Fast Surge Events – ESD (IEC61000-4-2)
Polymer Device	PESD	Fast	Poor	Low	Small	~100V	Yes
Metal Oxide Varistor	MOV	Fair	Poor	Medium	Small	NA	Yes
Silicon ESD Suppressor	TVS	Fast	Good	Medium	Small	NA	No

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