Three different jobs, not three grades of the same thing
All three are diodes and all three appear on protection circuits, which is why they get swapped. But the underlying design intent differs:
- Zener — holds a defined voltage by operating continuously in reverse breakdown. It clamps a steady or slowly varying overvoltage, and it is built to dissipate that power continuously.
- Transient voltage suppressor (TVS) — clamps a brief, high-energy event. It is specified for how much surge energy it can absorb in a single pulse, and for how fast it responds, not for continuous operation.
- Schottky — a rectifier optimised for low forward voltage drop and fast switching. It is a conduction device, not a clamp.
Choosing the wrong one of the three is not a marginal decision. A Zener used as a surge clamp will be asked to absorb energy it was never rated for; a TVS used as a voltage reference will be operating outside its purpose; a Schottky in a clamping position will simply conduct.
Schottky: efficiency, with a temperature caveat
The Schottky's advantage is a low forward drop — roughly a third of a volt against around seven tenths for a conventional silicon rectifier, with the exact figures depending on current. That difference matters in efficiency, in heat, and at low output voltages where a conventional diode's drop is a significant fraction of the rail.
Its characteristic weakness is reverse leakage, which rises steeply with temperature. A Schottky that leaks acceptably at room temperature may leak orders of magnitude more when hot, which can matter in battery-powered circuits, in high-temperature environments, and anywhere a small standing current is unacceptable. Typical uses are rectification, freewheeling across an inductive load, reverse-polarity protection and simple OR-ing of supplies.
Zener: a reference, not a surge absorber
A Zener is specified by its breakdown voltage, its tolerance on that voltage, and its power dissipation. Because the breakdown voltage shifts slightly with current, the tolerance and the test current both matter when the voltage has to be accurate.
Two limits to keep in mind. First, its power rating is a continuous rating, and it is derated as ambient temperature rises — a device that handles its rated power at 25 °C may handle only a fraction of that in a hot enclosure. Second, the response is not designed for the nanosecond-scale edge of an electrostatic or lightning-induced transient. A Zener will eventually conduct during a surge, but it is not what you want carrying that energy.
TVS: the four numbers that decide it
A TVS is specified by a set of parameters that must all be satisfied together — selecting on one alone is where most protection circuits go wrong.
- Standoff voltage (VRWM). The highest voltage the device can sustain continuously in its non-conducting state. It must be above the highest voltage that legitimately appears on the line, including supply tolerance, ripple and any normal transient. Set it too low and the TVS conducts during normal operation.
- Breakdown voltage. Where the device begins to conduct meaningfully, specified as a range with a test current. It sits above the standoff voltage.
- Clamping voltage (VC). The voltage across the device at a specified peak pulse current. This is the number that actually protects your circuit, and it must be below the absolute maximum rating of the device being protected. A TVS is often selected on standoff voltage and then let down by a clamping voltage that is too high at the real surge current.
- Peak pulse power. The surge energy the device can absorb, quoted for a defined pulse waveform and duration. It is derated for ambient temperature, and it is a single-pulse or limited-pulse figure — a TVS protects against transients, not against sustained overvoltage. A TVS will not survive a permanently over-voltage rail.
The parameter people forget: capacitance
A TVS is a semiconductor junction and therefore has capacitance, typically far higher than a small signal diode's. On a power line this is irrelevant. On a high-speed data line it is not: the added capacitance loads the line, rounds off fast edges and can degrade the signal enough to cause errors.
Two consequences follow. For signal-line protection, either choose a low-capacitance TVS intended for the purpose, or accept a deliberately small capacitance against the data rate. And for lines that swing both positive and negative, a bidirectional device is needed — a unidirectional part will conduct on the normal negative half of the waveform.
Selection errors that only appear later
- Standoff voltage set below the maximum operating voltage, so the device conducts in normal use and eventually fails.
- Clamping voltage never checked against the protected device's limits — the circuit looks protected and is not.
- Peak pulse power taken at face value, with no derating for the ambient temperature inside the enclosure.
- Leakage ignored in a hot Schottky application.
- Capacitance ignored on a data line.
- A Zener expected to handle a surge, or a TVS expected to survive a sustained fault.
- Unidirectional device on a bidirectional signal.
- Layout treated as an afterthought. A suppressor's value depends heavily on the return path: long, thin traces add inductance that lets the transient past before the clamp responds. Placement close to the connector or the protected pin matters as much as the part number.
Sourcing the protection device
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