TVS diodes protect circuits from voltage spikes. Let's discover what they are and what makes them special!
TVS Diodes
Transient voltage suppressor (TVS) diodes are a type of diodes, that is specifically designed to protect circuits. To use it, connect the TVS diode reverse biased in parallel to the circuit.
The circuit seems to be identical to the one used for voltage regulation or voltage references using a zener diode. Technically, this is true. This is the circuit of a shunt regulator. However, it is used differently. To protect a 5 V circuit one chooses a TVS diode that starts to conduct slightly above 5 V. A TVS diode is intended as overvoltage protection and not as voltage regulator. As the name implies, it suppresses voltage transients by absorbing them.
What Are Transients?
A transient, voltage spike or power surge is a sudden, short time, release of electrical energy. For a short time the voltage can rise high above the usual operating voltage of the circuit. This has the potential to destroy a circuit. Typical, sources for voltage transients are electrostatic discharge (ESD), lightning strikes, the switching of inductive loads or the turn-on overshoot of switch mode power supplies.
TVS diodes are zener diodes, however, they are special zener diodes. A TVS diode can absorb huge amounts of power. Small zener diodes are usually rated for 0.5 W, TVS diodes can handle several hundred watts and often more than a kilowatt for a very short amount of time without getting damaged. For a short amount of time is the important keyword here. A short amount of time, is all that is needed to prevent a voltage transient from damaging a circuit. However, TVS diodes are not intended for permanent loads.
What is a short amount of time? That is specified in the datasheet of each diode. What is essential to understand is, that values like the peak pulse power rating of TVS diodes are measured according to industry-standards like the IEC61643-123 (10/1000 µs waveform). These standards make certain assumptions about the voltage transients. The 10/1000 µs waveform for example tries to resemble short, but high power, transients as they occur in case of lightning strikes or ESD. Voltage transients caused by inductive loads often last longer. In such a case, a TVS diode needs to be derated.
As TVS diode can absorb such high amounts of power, it is not required to add a current limiting resistor in series. The source and wire impedance is usually high enough. No power has to wasted by an additional current limiting resistor.
The schematic symbol for TVS diodes is identical to the one used for zener diodes. However, if used correctly, there are two type of TVS diodes to differentiate between: unidirectional and bidirectional TVS diodes.
Unidirectional TVS diodes are for DC circuits. If forward-biased they conduct at roughly 0.7 V like normal diodes. Their voltage rating only applies when they are reverse biased.
Bidirectional TVS diodes are for AC circuits where there is no fixed polarity. A bidirectional TVS diode consist of two anti-serial unidirectional TVS-diodes. It behaves the same, no matter, of the direction the current flows.
Properties of TVS Diodes
TVS diodes can be characterized by the following set of important properties:
- Stand-off or reverse maximum voltage (\(V_{RM}\)): Voltage until which the diode does not conduct expect for a small leakage current.
- Breakdown voltage (\(V_{BR}\)): Voltage at which the diode starts to conduct when reverse-biased.
- Peak pulse power dissipation (\(P_{PP}\)): Maximum power dissipation that the diode can handle in case of a surge according to the assumptions the used standard.
- Clamping voltage (\(V_{CL}\)) and peak pulse current (\(I_{PP}\)): Voltage when the diode conducts the peak pulse current. Often multiple value pairs or a graph is given for \(V_{CL}\) and \(I_{PP}\).
When selecting a TVS diode for a circuit, pick one where the stand-off voltage equals to the supply voltage of circuit. An additional, interesting value is the clamping voltage. Like almost all components, a TVS diode has an internal resistance (\(R_{DYN}\)). This dynamic resistance limits its ability to regulate the voltage down. The higher this dynamic resistance and the current that flows through the TVS diode, the higher the voltage the protected circuit is exposed to. The clamping voltage shows you what that voltage is going to be. Lower values provide better protection.
Where to Use TVS Diodes?
TVS diodes should be used wherever potentially damaging voltage transients are to be expected. This is typically the case for external connectors to a PCB and for inductive loads. For the later a flyback diode is often enough, for the former a TVS diode is the correct choice. What connectors should you protect, and how do you do it?
The first typical use case is to protect against transients caused by a power supply. This is especially recommended, when the power supply regularly plugged and unplugged or if alternators are used as this is the case in the automotive area. The TVS diode should be connected across the supply lines in proximity to the power connector of the PCB.
The second typical use case is to protect a circuit with external connectors against ESD. Again, this is especially important, if the connector is regularly are plugged and unplugged, like for example USB connectors. In this case, it makes sense to combine TVS and clamping diodes. Schottky diodes can derive the current flow of several data lines to a single TVS diode. All diodes need to be able to handle the high surge current, which is why not every schottky diode can be used for this protection circuit. Typically, a specialized IC, that combines all the required diodes, is used and installed in proximity to the connector.
