Application Note. AN554 Datasheet
CHOICE OF PROTECTION IN
AUTOMOTIVE APPLICATIONS (CLASSICAL TOPOLOGY)
This paper describes a protection schematic based on discrete components, together with a general meth-
www.DataSheet4Uo.cdomof choosing the components to suppress the surge effects on automotive modules.
Figure 1. General Protection Topology
GENERAL PROTECTION SCHEMATIC
Positive impulsive overvoltages
This type of overvoltage is clamped by the protection component P at maximum voltage VCL. Resistance
RS limits the dissipated energy in the protection component without compromising the clamping function.
Negative impulsive overvoltages
There are two ways to limit these:
– Without diode D: the protection component operates as a rectifier diode and clamps the voltage at
the unit terminals to approximately 1V.
– With diode D: the diode is reverse-biased and therefore protects the unit.
One important thing to take into account is the peak reverse voltage limit of D. VRRM = 400V seems a good
compromise (see curve N° 6 of the ISO/TC22 standard).
Positive continuous overvoltages
During this phase, the protection component must be in the stand- by phase (very low current passing
through the component).
Negative continuous overvoltages
This protection is achieved by diode D which is reverse-biased.
Impulsive voltage drop
During this phase, the unit is fed by capacitor C while diode D prevents C from discharging into the battery
REV. D2A - 3584
AN554 APPLICATION NOTE
THE CHOICE OF COMPONENTS
The following parameters will constitute the selection criteria:
- The average current used by the electronic module.
- The maximum repetitive peak reverse voltage VRRM
- The maximum ambient temperature Tamb.
The following inequality must apply in all cases:
Tamb + Rth P < Tj max
P = VTO IF (AV) + rd I2F(RMS)
Rth = thermal resistance (Junction - ambient) for the device and mounting in use.
Its presence allows a "size" (and thus cost) reduction of the protection component.
Its value is a function of the following elements:
Vbat min: lowest battery voltage which is specified in the technical note issued by the manufacturer.
VCC min: minimum voltage needed for the electronic unit in operation.
ICC max: maximum supply current of the electronic module.
The maximum value of RS will be:
RS max = (Vbat min - VCC min)/ICC max
Its role is to make sure that the voltage at the terminals of the electronic unit is greater than or equal to
Vcc min while the starter circuit is active.
Its value depends on:
Vbat: voltage across the battery before the disturbance
VCC min: see “B: Battery voltage”.
T: length of the disturbance (130 ms: see application note 4.1, paragraph III.4)
The minimum value of C will be:
Cmin = (130 * 10-3/Req)/ln (VCC min/Vbat)
with Req = equivalent resistance of the electronic unit
Req = VCC min/ICC max
Protection component (P)
- How it works:
Figure 2. Transil Behaviour
AN554 APPLICATION NOTE
B: Voltage across the protection device
C: Current through the protection device
The role of the protection device is to suppress the destructive effects of the surge (see Figure 2a), the
most agressive being the load dump impulse.
To achieve this, the TRANSIL clamps the spike at a maximum value VCL (see Figure 2b). A surge current
flows through the suppressor during this phase (see Figure 2c).
THE CHOICE OF THE PROTECTION DEVICE
Parameters to take into account
To choose the TRANSIL we have to know the surge parameters and the application requirements.
Surge parameters. The surge is defined by the peak value Ip and the duration tp of the current wave flow-
ing through the protection device during the clamping.
As shown in the ISO/TC22 standard the most energetic impulsive disturbance is the load dump surge.
Most car manufacturers recommend the SCHAFFNER NSG 506 generator to synthesise this wave (see