Protecting Sensitive Electronic Circuits

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Today, our use of technology is unprecedented even on an individual level. Many laborious activities have been automated to improve their efficiency and convenience, and new ways of entertaining ourselves have been conjured up altogether. Electronic circuits make automation possible and have advanced rapidly with each new call for “better, faster, and more innovative.”

Complex electronic circuitry as found in microprocessors is now commonplace. Appliances such as children’s toys, cell phones, computers, washing machines, and cars feature these circuits. Furthermore, on an industrial level, their applications are almost limitless.

A problem with such small-sized circuits is their sensitivity to unexpected and fast electrical transients which can damage their components. Depending on the application, such damage could prove catastrophic. As a result, circuit protection from transients is a priority. One such protective measure is to use a Transient Voltage Suppression Diode, or TVS diode, in the circuit. Understanding how a TVS diode fits in a designed circuit is critical in ensuring the circuit is protected.

How Transient Suppression Devices Work

Short duration electrical transients are inevitable during electrical distribution. Power outages, lightning strikes, and power transitions in larger equipment on the same power line are some examples of events that cause energy spikes (generally referred to as voltage spikes) in a circuit.

Various devices exist for suppressing transients. Based on their connection to the circuit load they are meant to protect as well as their subsequent response to the transient event, suppression devices can be broadly categorized into 3 groups:

  1. Low-pass filters placed in series with the load: These block high-frequency transients from propagating through the circuit while allowing the low-frequency signal to continue along. They do so by absorbing excess currents. An example is a mains filter extension cord.
  2. Voltage clamping devices placed parallel to the load: Examples are metal oxide varistors, Zener diodes, and TVS diodes. These devices exhibit a variable impedance depending on the current flowing through them (or the voltage across their terminals). During normal conditions, these devices have a high impedance and therefore appear “invisible” to the circuit. However, in the presence of a transient voltage, their impedance decreases and current flows through them to ground. That is, they divert the transient away from the load (shunting), thereby, putting a clamp on the residual voltage.
  3. Crowbar devices placed parallel to the load: Examples are thyristors and triacs. These also divert the transient away from the load, but through a switch-on, switch-off type action. Like voltage clamping devices, they also appear “invisible” under normal conditions due to a high impedance. In the presence of a transient, they switch on rapidly, thereby providing a very low impedance path for the current. A crowbar will continue to conduct current even after the overvoltage event has ended, until the power is removed entirely.

Of these groups of devices, crowbar and clamping devices are the most commonly used. They are available in a wide range of voltage ratings and energy absorbing properties to gain control over destructive voltage spikes.

The table below compares between parallel-placed transient suppression devices:

  MOV TVS diodes Thyristors
Protection time 10-20 ns 50 ps < 3 ns
Protection voltage >300 V 3 – 400 V 30 -400V
Power dissipation Nil Low Nil
Reliable performance No Yes Yes
Longevity Degrades (fusing required) Long Long

A Transient Voltage Suppressor Diode is Effective In Protecting Sensitive Electronics

TVS diodes make an attractive choice for transient voltage suppression because:

  • They respond almost instantaneously (picoseconds) to transients (faster than varistors).
  • TVS diodes have high impedance, so they act like open circuits during normal operating conditions (have low current leakage).
  • They do not degrade, provided they are operated within the limits for which they are designed.
  • They are manufactured in a variety of surface and through-hole circuit mounting options, making them perfect as board level protectors.
  • TVS diodes have low clamping voltages, making them ideal for sensitive circuits operating at low voltages (e.g., microprocessors).
  • They also protect circuits from Electrostatic Discharge (ESD)

A transient voltage suppressor diode is a p-n junction clamping device that relies on “the avalanche effect” to protect circuits from transient voltages. During the “avalanche,” the diode which previously was not conducting electricity (ideally it draws zero current, although there is some electric leakage) begins to conduct electric current in response to a voltage spike (transient) in the power supply. This large voltage knocks electrons in the TVS diode’s atoms free from their bonds. These fast-moving free electrons collide with other atoms, producing more electrons which leads to more collisions. Hence the avalanche of electrons and the flow of electric current (low impedance state). When normal conditions return, the TVS diode reverts to its high impedance state.

Put more clearly, the excess transient current is diverted from the protected circuit and flows through the transient voltage suppressor diode to ground. When the voltage across the circuit exceeds the Breakdown Voltage of the TVS diode, the diode clamps the excess voltage, allowing the protected circuit to operate safely.

The breakdown voltage of a TVS diode depends on its doping concentration, its cross-sectional junction area, and temperature of the p-n junction.  A large junction area can absorb high transient currents.

What Causes Transient Voltage Suppressor Diode Failure

If the TVS diode is exposed to voltages or conditions beyond those it was designed for, it will fail. For example: to protect a port operating within a 0 – 5 V range, selecting a TVS diode with a breakdown voltage of 5 V is a good choice. When a voltage spike occurs, the TVS diode clamps the input to 5 V and allows any excess current to flow through it, keeping the circuit protected. That is, all voltages above 5 V will be shunted to ground.

If a TVS diode with a breakdown voltage of, say, 2.5 V was used instead to protect the same circuit experiencing a voltage spike greater than 5 V, the TVS diode will fail, leaving the circuit unprotected and the diode itself destroyed in the process.

Important Parameters Guiding the Selection of a TVS Diode to Protect a Circuit

There are a number of parameters used to describe a transient voltage suppressor diode in a typical data sheet. A common problem circuit designers face is that corresponding information to describe a working circuit may not exist. The more that is known about the parameters describing both the TVS diode and the circuit to be protected, the easier it would be to select the right diode. In the absence of relevant information, testing the system to determine the worst case transients in the operating environment becomes a necessity.

TVS diodes are characterized by:

  • Reverse standoff voltage/ Working Voltage: Refers to the normal operating voltage of the diode. At this value (and below), the TVS conducts no electricity. It acts “invisible.”
  • Reverse breakdown voltage: At this voltage, the TVS diode begins to conduct electric current because of the avalanche effect. The breakdown voltage is measured at a specified test current (1mA or 10mA).
  • Peak pulse current: Above this current, the TVS diode will be damaged. Datasheets specify this current for a specific transient waveform (e.g., 8/20 ?s, 10/1000 ?s).
  • Clamping voltage: This is the maximum voltage drop across the TVS diode, corresponding to a given peak pulse current.

The difference between unidirectional and bidirectional TVS diodes

A TVS diode may either be unidirectional or bi-directional. The unidirectional diode acts as a rectifier in the forward direction. They are designed to handle large peak currents for a short time. In spite of their name, unidirectional TVS diodes can provide protection for both negative and positive transient events in DC power applications.

Bi-directional TVS diodes, on the other hand, have opposing junctions inside a single device, making them perfect for AC power applications. As a consequence of their construction, they have less capacitance than their unidirectional counterparts.

Selecting the Right TVS Diode Still Depends on the Designer

  1. Careful design should consider the environment in which the circuit will be operational. The potential effects of related components on the power supply line should also be taken into account.
  2. Unfortunately, information about the immunity ratings of an integrated circuit, for example, may not be provided in its datasheet. This is because the performance of a circuit depends on numerous factors. To determine the maximum voltage such a circuit will survive will require testing.
  3. In high-speed digital applications, the capacitance of a TVS diode will be important. To preserve the integrity of the signal, choose a diode with minimal capacitive loading while not sacrificing clamping performance.
  4. Multiple TVS diodes can be stacked in series to meet almost any requirements, as opposed to stacking them in parallel (clamping voltage of the individual devices will need to be matched carefully to prevent all the current going to the device with the lowest clamping voltage). This way, all the diodes will need to reach their clamping voltage to turn on their neighbor.
  5. The more that is known about the parameters describing both the TVS diode and the circuit to be protected, the easier it would be to select the right diode. However, trial and error remains a valid option.