How to select power line polarity protection diodes

July 03, 2012 //By Soo Man (Sweetman) Kim
How to select power line polarity protection diodes
Soo Man (Sweetman) Kim of Vishay reviews the major parameters that need considering when selecting a power line polarity protection diode for an automotive application.

A major challenge in automotive design is protecting electronics, such as control units, sensors, and entertainment systems, against damaging reverse voltages, voltage transients, electrostatic discharge (ESD), and noise that are present on the power line. Diode rectifiers are ideal solutions for automotive electronic power line protection and have several important parameters for these applications, including: Forward current, repetitive reverse voltage, forward surge current, and fusing rate.

Parameters in automotive electronic equipment test conditions and applications

Basic circuits for polarity protection are shown below. Circuit (A) offers polarity protection only, while Circuit (B) features polarity protection with load dump suppression.

Following are definitions for major parameters to consider when selecting a power line polarity protection diode for an automotive application.

Maximum repetitive reverse voltage (V RRM)
The maximum repetitive reverse voltage is the maximum voltage that the diode can withstand in reverse bias mode. In reverse bias mode, leakage current through the diode can generate heat in the diode junction and lead to thermal runaway. Tests that simulate this condition include the U.S.’s ISO-7637-2 pulse 1 and 3a, and Japan’s JASO D001-94, standard type B and E. Each peak voltage for these tests is specified in the following table.

According to the above test conditions, the V RRM of a diode for power line protection should be 300 to 400V for a 12V power train and 600V for a 24V power train.

Forward current (I F(AV))
The specification for forward current in datasheets usually means the maximum average forward current the diode can handle in the forward bias state—given the thermal limitations of the package. This parameter is related to the current usage of the circuit in operation.

The maximum forward current derating curve of a rectifier on a 5 mm x 5 mm Cu pad with a FR-4 PCB drops off above a given temperature limit.

The forward current capability varies by the temperature of the diode’s junction,

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