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 (VRRM)
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 VRRM
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 (IF(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, as shown above. Other related parameters include thermal resistance. Forward surge current (IFSM)
The specified forward surge current in a datasheet is the maximum peak current the diode can handle in the forward bias state within specified time and pulse conditions. This rating is limited by the diode’s thermal capacity.
The forward surge current specification is related to two major operations and is simulated in the ISO-16750-2 and JASO D001-94 automotive standards. The first operation is protecting circuitry against the high currents that occur during the load dump condition. The second operation is simulated by ISO-7637-2 test pulse 2a and 3b, consisting of 50 ms and 100 ms pulse widths and 2? and 50? line impedance, respectively. This is a relatively small amount of energy when compared to the forward surge current at the load dump test condition.
Load dump suppression (figure above) is simulated by tests such as ISO-16750-2 test A and B, JASO standard type A and D, and others.
In this situation, high surge current is passing through the polarity protection diode, and it requires a high enough forward surge capability to avoid failure. Estimating the surge current value in load dump suppression tests can be accomplished with the equation:
Ipeak = ( Vpeak – VFd - Vclamping) / (Ri + Rzd)
Vpeak: Surge voltage
Vclamping: Clamping voltage
VFd: Forward voltage drop of polarity protection diode
Ri: Line impedance
Rzd: Resistance of clamping device
With the applied condition of 101V Us, 12V UB, and 1.5? line impedance, as specified by ISO-16750-2 test A, the peak current is 51.3A and the actual clamped current is 50.3A.(For a detailed explanation of load dump protection, please refer to http://www.vishay.com/docs/88490/tvs.pdf.)