Schottky Diodes and Fast Switching
A Schottky diode uses a metal-semiconductor junction instead of a p-n junction. That construction gives it low forward voltage and extremely fast switching, but also higher reverse leakage and lower reverse-voltage ratings than many silicon p-n diodes.
Learning Objectives
By the end of this lesson, you should be able to explain why Schottky diodes switch quickly, estimate conduction loss, choose a reverse-voltage rating, apply Schottky clamps, and recognize leakage and temperature limits.
Metal-Semiconductor Junction
In a p-n diode, stored minority charge must be removed before the diode stops conducting. In a Schottky diode, conduction is mainly by majority carriers. With little stored charge, reverse recovery time is very small.
Important parameters:
- (V_F): forward voltage, often 0.2 V to 0.5 V at moderate current.
- (I_R): reverse leakage, strongly increasing with temperature.
- (V_{RRM}): repetitive peak reverse voltage.
- (I_F): average forward current.
- (C_J): junction capacitance, important at high speed.
Fast Rectifier Example
layout direction=LR gap=90
VIN: Connector_Generic:Conn_01x02 value="Switch node"
D1: Device:D_Schottky value="Schottky" rotate=270
COUT: Device:C value="Output cap" rotate=0
LOAD: Device:R value="Load" rotate=0
group SOURCE label="Pulsed input" direction=TB {
VIN
}
group RECT label="Fast rectifier" direction=LR {
D1
}
group OUT label="DC output" direction=TB {
COUT LOAD
}
VIN.1 --> D1.A color=#b91c1c
D1.K --> local:VOUT color=#b91c1c
COUT.1 --> local:VOUT color=#b91c1c
LOAD.1 --> local:VOUT color=#b91c1c
VIN.2 --> global:0V
COUT.2 --> global:0V
LOAD.2 --> global:0V
In a buck converter or polarity-protection path, low (V_F) reduces heat. In high-frequency rectification, low reverse recovery reduces switching loss and ringing.
Conduction Loss
Approximate diode conduction loss is:
[
P_D \approx V_F I_F
]
For a 0.35 V Schottky carrying 2 A:
[
P_D = 0.35 \times 2 = 0.7W
]
That heat must leave through the package, copper, and airflow. A small package may be electrically rated for the current but thermally unsuitable.
Reverse Leakage and Temperature
Reverse leakage is the main tradeoff. A Schottky that leaks 20 uA at room temperature can leak hundreds of microamps or milliamps at high temperature. Leakage matters in:
- battery-powered reverse protection;
- high-impedance sensor clamps;
- precision sample-and-hold circuits;
- high-temperature automotive or industrial products.
Always check the datasheet leakage at the real reverse voltage and maximum junction temperature.
Switching Waveform
This idealized waveform shows why reverse recovery matters. A p-n diode can conduct reverse current briefly after polarity changes; a Schottky stops much faster.
title "Illustrative reverse recovery comparison"
time start=0 end=100 unit=ns divisions=10
IF: square label="Forward current command" low=-1 high=1 duty=50 cycles=1 unit=A color=#2563eb
PN: pulse label="p-n reverse recovery" low=0 high=-0.8 at=50 width=18 unit=A color=#dc2626
SKY: pulse label="Schottky leakage spike" low=0 high=-0.08 at=50 width=3 unit=A color=#16a34a
marker COMM at=50 label="polarity changes" color=#475569
The waveform is explanatory, not a simulation of a specific diode.
Selection Checklist
Choose a Schottky diode by checking:
- maximum repetitive reverse voltage with transient margin;
- average and surge current;
- forward voltage at expected current and temperature;
- reverse leakage at maximum temperature;
- package thermal resistance and PCB copper area;
- capacitance if the node is high speed or high impedance.
Common Mistakes
- Selecting only by low forward voltage and missing reverse-voltage margin.
- Ignoring leakage in battery products.
- Assuming a 3 A diode in a small package can dissipate continuous heat without copper area.
- Using a Schottky clamp on a precision high-impedance node without checking leakage.
- Replacing a synchronous MOSFET with a Schottky without checking efficiency loss.
Summary
Schottky diodes are excellent for fast switching, low-voltage rectification, clamps, and reverse-polarity paths. Their strengths come with leakage, voltage-rating, capacitance, and thermal tradeoffs that must be checked from the datasheet.
Further Reading
- STMicroelectronics, "Schottky Rectifier Application Notes."
- ON Semiconductor, "Schottky Barrier Rectifier Characteristics."
- Texas Instruments, "Diode Selection for Switching Regulators."