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Design Guidelines for Schottky Rectifiers
www.vishay.com For technical questions within your region, please contact: Document Number: 88840
1468 PDD-Americas@vishay.com
, PDD-Asia@vishay.com, PDD-Europe@vishay.com Revision: 26-Aug-08
Application Note
Vishay General Semiconductor
Known limitations of Schottky rectifiers
- including limited high temperature operation, high leakage and limited voltage range -
can be measured and controlled, allowing wide application on switch mode power supplies.
By Jon Schleisner, Senior Technical Marketing Manager
Schottky rectifiers have been used in the power supply
industry for approximately 15 years. During this time,
significant fiction as well as fact has been associated with
this type of rectifier. The primary assets of Schottky devices
are switching speeds approaching zero-time and very low
forward voltage drop (V
F
). This combination makes Schottky
barrier rectifiers ideal for the output stages of switching
power supplies. On the negative side, Schottky devices are
also known for limited high-temperature operation, high
leakage and limited voltage range B
VR
. Though these
limitations exist, they are quantifiable and controllable,
allowing wide application of these devices in switch mode
power supplies.
High leakage, when associated with standard P-N junction
rectifiers, usually indicates “badness,” implying poor
reliability. In a Schottky device, leakage at high temperature
(75 °C and greater) is often on the order to several milliamps,
depending on chip size. In the case of Schottky barrier
rectifiers, high-temperature leakage and forward voltage
drop are controlled by two primary factors: the size of the
chip’s active area and the barrier height (φB).
Design of a Schottky rectifier can be viewed as a tradeoff. A
high barrier height device exhibits low leakage at high
temperature, however, the forward voltage drop increases.
These parameters are also controlled by the die size and
resistivity of the starting material. A larger die will lower the
VF but raise the leakage if all other parameters are held
constant. The resistivity of the starting material must be
chosen in a range where the breakdown voltage (B
VR
) is not
degraded at the low end and the forward end of the resistivity
range. Since a larger chip size is obviously more expensive,
this is not the primary method for controlling these
parameters. Chip size is usually set to a dimension where the
current density through the die is kept at a safe level.
BARRIER HEIGHT (φB), A FACTOR
Vishay General Semiconductor produces two product lines
of Schottky barrier rectifiers. One line is referred to as the
“MBR” series, a high-temperature, low-leakage, relatively
high VF type of Schottky device with a high barrier height
(φB). The second line is the “SBL” series, designed to
operate at lower temperature (125 °C or less); however,
while leakage current is higher, forward voltage drop (V
F
) is
significantly lower and they are designed with a low-φB
barrier height. The low- φB-line SBL series uses a nichrome
barrier metal with a barrier height of φB = 0.64 eV. The
high-φB MBR series uses a nichrome-platinum barrier metal
to achieve barrier height (φB = 0.71 eV). Both series are
guard-ring protected against excessive transient voltages.
Figure 1.
Both the low and high-barrier-height Schottky devices are
valuable in a variety of applications.When the true operating
temperature of the Schottky rectifier exceeds 125 °C, the
high-barrier-height series must be used to avoid thermal
runaway.
This occurs when excessive self-heating of the rectifier
causes large leakage currents, resulting in additional
selfheating. The process becomes a form of positive thermal
feedback and may lead to damage in the rectifier or
inappropriate functioning of the circuit utilizing the device.
Using a high-barrier-height (MBR) component prevents this
anomaly, but sacrifices higher forward voltage. Operating the
low barrier height (SBL) series at a junction temperature of
125 °C, a decision on the use of a low- or high-barrier-height
Schottky device must be made.
The following procedure has been developed to provide an
analytical method of selecting the most efficient Schottky
barrier device for a given application.
4030 6050
0.001
0.01
0.1
1
Voltage (V)
A/cm
2
150 °C
02010
125 °C
100 °C
75 °C
