Schottky Diode Selection Guide for Switching Circuits: Vf, VRRM, IF Explained

Key Parameters Explained

A PN diode has a forward voltage drop around 0.7V. In comparison, a Schottky diode drops 0.3 to 0.5V. This unique feature results in lower power loss, reduced heat generation, and negligible reverse recovery, making Schottky diodes an excellent choice for power switching circuits. So excellent though this feature is, pickingasuitable Schottky diode is far more than just looking at Vf.

Three NumbersYou Should Never Ignore

Besides Vf, three other key parameters deserve attention: reverse voltage (VRRM), forward current (IF), and leakage current (IR). These parameters determine whether your Schottky diode works reliably or not.

Schottky diodes have lower reverse voltage ratingscompared with PN diodesof similar size. A 40V or 60V Schottky is common. If your switching circuit has ringing or spikes above the rating, the diode goes into avalanche and that damages the diode.For this reason, it would be necessary to set at least 20% margin above the maximum voltage the diode mayexperience.

Although forward current seems straightforward, there is still another factor that needs to be considered: temperature derating.Forexample, a diode rated at 3A at 25°C might only handle 1.5A at 100°C, which is worthy to be noticed when the Schottky is used inan environmentwhere temperature might change.So, to check the derating curve in the datasheet is significant. Don'tjust readthefirst page specs only.

Leakage Current: A Hidden Problem

Schottky diodes leak more than PN diodes at high temperatures.For example,a 40 V Schottky diode might leak 0.5 mA at 125°C. In a low-power circuit, that leakage canbeseenas a load.While inthe battery-powered equipment, it drains the battery when the circuit is off.

If your switching circuit runsat a hightemperature or has a high-impedance off-state,you had better pay attention to the leakage at your operating temperature, not justthe leakage current specified at 25°C. Diodes Inc. and onsemi usually publish these curves, so do ST and NXP. Therefore, readingthese curves in advance helps avoidthese failures.

The Core Advantage ofSchottkyDiodes:Reverse Recovery

Thecore advantage of choosing Schottky for a switching circuit isits reverse recovery time. A PN diode takes tens to hundreds of nanoseconds to stop conducting after the voltage reverses.ButSchottky has negligible recovery time because it has no stored charge. It just stops.

Thisfeature is quite important in DC-DC converters operating above 100 kHz. The recovery loss in a standard diode at 500 kHzcan account for a significant portion of the total diode loss. A Schottkylargely eliminates the loss.For this great advantage,Schottkys are the default choice for the output rectifier of flyback converters, boost converters, and buck converters.

Common Part Numbers for Switching Circuits

The 1N5819 (40V, 1A) is the classic through-hole Schottky for low-power switching. Surface-mount equivalent: SS14 (40V, 1A in SMA).

For higher current, the SS34 (40V, 3A in SMC) and the 1N5822 (40V, 3A through-hole) are common.

For 60 V applications, the SS16 (60 V, 1 A) and SS36 (60 V, 3 A) cover many common requirements.

There are higher voltage Schottkys (like 100 V, 150 V) as well, butasthe Vfincreases, and efficiency advantage shrinks.