Diode Types and Selection: Schottky, Zener, TVS and Rectifier
A diode is a basic semiconductor that passes current in only one direction; but "diode" isn't a single component — it's a family, each optimized for a different job. From rectification to voltage regulation, from overvoltage protection to high-speed switching, there's a different diode type for every task. Choosing the wrong type causes problems ranging from efficiency loss to circuit failure. This guide covers diode types and their selection criteria.
Diode types
- Rectifier: used to convert AC to DC; carries high reverse voltage and current but is slow.
- Schottky: very low forward voltage drop (Vf) and high speed; brings efficiency in power supplies and at high frequency. Downside: high reverse leakage current and limited reverse voltage.
- Zener: conducts in the reverse direction at a specific voltage; used for a voltage reference and regulation.
- TVS (transient voltage suppressor): protects the circuit by clamping sudden voltage spikes (ESD, lightning, load dump).
- Signal / switching: low current and fast; used in logic and signal circuits.
- LED: a diode that produces light in the forward direction; in indication and lighting.
Critical parameters
The following datasheet values are decisive in diode selection:
- Forward voltage drop (Vf): the loss in conduction; a low Vf means high efficiency (the Schottky advantage).
- Peak reverse voltage (VR / PIV): the maximum reverse voltage the diode can withstand; it should be chosen safely above the circuit voltage.
- Forward current (IF): the current it can carry continuously.
- Reverse recovery time (trr): determines the switching speed; a low trr (or Schottky) is needed at high frequency.
- Reverse leakage current (IR): rises with temperature, especially in Schottky; important in low-power circuits.
Diode types comparison table
| Type | Vf | Speed | Typical use |
|---|---|---|---|
| Rectifier | ~0.7 V | Low | AC-DC rectification |
| Schottky | ~0.2–0.4 V | High | SMPS, high frequency |
| Zener | — | — | Voltage reference/regulation |
| TVS | — | Very fast | Overvoltage/ESD protection |
| Signal/switching | ~0.7 V | High | Logic/signal |
Common mistakes
- Leaving an insufficient reverse-voltage margin: the PIV must be safely above the peak reverse voltage in the circuit.
- Using a slow rectifier at high frequency: a high trr increases switching losses and heating; a Schottky or fast diode is needed.
- Ignoring the Schottky's leakage current: IR rising with temperature causes problems in low-power circuits.
- Choosing a TVS with the wrong clamping voltage: the clamping voltage should be above the operating voltage but below the protected circuit's limit.
Frequently asked questions
Why is a Schottky preferred?
Thanks to its low forward voltage drop and high speed, it raises efficiency in power supplies and heats up less. But the reverse voltage and leakage current limits must be watched.
What's the difference between a Zener and a TVS?
A Zener is designed for a continuous voltage reference/regulation, while a TVS is designed to suppress short, sudden voltage spikes. Their jobs are different.
Is a rectifier diode used at high frequency?
Generally no; because of its slow reverse recovery, a Schottky or fast rectifier is preferred in high-frequency switching.
Practical example: PIV selection for a rectifier
Suppose you design a bridge rectifier powered from 230 V AC mains (peak value ≈ 325 V). The reverse peak voltage each diode can see is about 325 V. With a safety margin, you need to choose a diode with at least 1.5–2× that, i.e. a PIV of 600 V or higher. Choosing a diode at the edge without a margin causes it to break down and fail permanently during mains fluctuations or transient spikes.
Freewheeling diode
When driving inductive loads like a relay, motor or coil, a high reverse voltage spike appears across the coil when the load is cut. To discharge this spike safely, a freewheeling (flyback) diode is used in reverse parallel with the load. This diode protects the driving transistor from the sudden voltage; if the switching speed is high, a Schottky or fast rectifier is preferred.
The series resistor relationship for an LED
An LED is a current device; connected directly to a voltage it draws uncontrolled current and burns out. That's why the current is always limited with a series resistor. The resistor value is found with R = (Vsource − VLED) / ILED. For example, for a 5 V source, a 2 V LED and a 10 mA target current, R = (5 − 2) / 0.01 = 300 Ω.
Reverse recovery and switching losses
When a diode goes from conduction to cutoff, it continues to conduct current in the reverse direction for a short time; this period is called the reverse recovery time (trr). In high-frequency switch-mode power supplies, if this time is long, extra loss and heating occur every cycle. Because Schottky diodes show almost no reverse recovery, they have an advantage over standard rectifiers in these applications. But the Schottky's low reverse-voltage limit and its leakage current rising with temperature can make it unsuitable for high-voltage rectification.
Choosing the right diode
In short, first determine the type by the task (rectifier, Schottky, Zener, TVS, signal); then choose Vf, PIV, IF and trr with a margin according to your application. For a candidate part, you can compare price and stock with the comparison tool and find equivalent alternatives with the cross-reference tool; you can start searching here.