Robust automotive supply protection for ISO 7637-2 and ISO 16750-2 compliance
03 April 2017
Automotive power supplies produce formidable transients that can readily destroy exposed onboard electronics. Over time, as electronics have proliferated in vehicles, automotive manufacturers have duly noted failures, compiling a rogues’ gallery of the responsible power supply transients.
Manufacturers have independently created standards and test procedures in an effort to prevent sensitive electronics from falling prey to these events. Recently, though, automotive manufacturers have combined efforts with ISO to develop the ISO 76372 and ISO 167502 standards, which describe the possible transients and specify test methods to simulate them.
ISO 7637-2 and ISO 167502 standards
ISO 7637 is entitled “Road vehicles—Electrical disturbances from conduction and coupling” and is an electromagnetic compatibility (EMC) specification. This article addresses the second of the three parts of this document, ISO 76372 “Part 2: Electrical transient conduction along supply lines only.”
Although ISO 7637 is primarily an EMC specification, prior to 2011 it also included transients related to power supply quality. In 2011, those portions related to power supply quality and not EMC were moved to ISO 16750, “Road vehicles — Environmental conditions and testing for electrical and electronic equipment” in the second of five parts, “Part 2: Electrical Loads.”
While most manufacturers still maintain their own specifications and requirements rather than adopt ISO 76372 and ISO 167502 verbatim, there is a trend toward more closely conforming to the ISO standards, with manufacturer specifications following the international standards with minor variations.
ISO 76372 and ISO 167502 provide specifications for both 12V and 24V systems. For simplicity, this article only describes 12V specifications.
Load dump is the most challenging of the power supply transients because of the substantial energy in the event. It occurs when the alternator is charging a battery, and the battery connection is lost.
Alternators without internal voltage clamps
Originally, alternators in cars were unclamped and could produce extraordinarily large voltages during load dump, about 100V for 12V systems. Newer alternators are clamped internally to limit the maximum voltage to a lower value during load dump. Because older alternators, and some modern alternators, do not include internal clamps, the load dump specification in ISO 167502 is split into “Test A—without centralised load dump suppression” and “Test B—with centralised load dump suppression.”
Figure 1 shows a schematic of an alternator’s 3phase stator windings and the 6diode rectifier that converts the stator’s AC output to the DC that charges the battery. When the battery connection is lost, the resulting current flow is as shown in Figure 2. Without the battery to absorb the stator’s current, the output voltage surges to the very high voltages seen during unclamped load dump, as shown in Figure 3 from the ISO 167502 specification. This corresponds to the unclamped alternator scenario in “Test A—without centralised load dump suppression.”
Alternators with internal voltage clamps
Newer alternators use avalanche diodes that have well specified reverse breakdown voltages which limit the maximum voltage during load dump. Also, be aware that when load dump was part of ISO 76372, only one pulse was specified, but when the load dump specification moved to ISO 167502 in 2011, the minimum test requirements increased to include multiple pulses with a one minute interval between pulses.
TVS protection problems
The internal resistance, Ri, of the alternator in both Test A and Test B is specified to be between 0.5O and 4O in ISO 167502. This limits the maximum energy that is delivered to protection circuits.
Nevertheless, one fact is frequently overlooked by those implementing protection from the ISO 167502 load dump transient: the internal resistance, Ri, does not appear in series with the 35V clamped voltage. Ri actually appears before the avalanche diode, as shown in Figure 6.
If the onboard electronics are locally protected by a shunt device such as a TVS (transient voltage suppressor) diode with a breakdown voltage less than 35V, the TVS may be forced to absorb the alternator’s energy.
Sometimes a series resistor is placed in front of the electronics and the TVS diode, but unfortunately this introduces a voltage drop and extra power dissipation in the resistor even during normal operation.
Active protection with a surge stopper
A better solution is to use a series active protection device, such as the LTC4380 low quiescent current surge stopper. By its very nature, a surge stopper protects the downstream electronics from load dump as well as the other conditions in ISO 167502 and ISO 76372 without relying on the internal resistance of the alternator. The surge stopper solution shown in Figure 8 provides uninterrupted power while operating from a clamped alternator. Furthermore, if it is subjected to load dump from an unclamped alternator, it will not be damaged. In the unclamped scenario, it may shut off to protect itself and then automatically reapply power to the load after a cooldown period. It is important to note that power is only shut off in the presence of multiple simultaneous faults.
Surge stopper protection solution
The design in Figure 8 protects downstream electronics from ISO 167502 and ISO 76372 transients while providing up to 4A of output current. At the same time, it protects the upstream system from overcurrent events caused by conditions such as shortcircuit faults in the downstream electronics. As it does this, it consumes a miserly 35µA of quiescent current, an important consideration in modern automobiles featuring countless batterydraining loads while the vehicle is not running.
This protection solution is based on the LTC4380 low supply current surge stopper, limiting the output voltage to 22.7V from input voltages as high as 100V at the input—sufficient protection against an ISO167502 load dump as well as ISO 76372 pulses 1, 2a, 2b, 3a, and 3b. It also prevents current flow during reverse battery conditions, and provides continuous power during the ISO 167502 superimposed alternating voltage test at severity level 1 where the peaktopeak AC voltage is 1V. (It may temporarily shut off power in the presence of larger AC voltages.) Continuous power is provided to the load when the input voltage drops as low as 4V to satisfy the minimum supply voltage requirements of ISO 167502.
The MOSFETs in this circuit are protected by limiting the time spent in high power dissipation conditions, such as when the input voltage surges high during load dump or when the output is shorted to ground. If a fault exceeds the conditions specified in ISO 167502 and ISO 76372, MOSFET M2 shuts off to protect the circuit, reapplying power after an appropriate delay.
Load dump and overvoltage protection
To understand the operation of the circuit in Figure 8, consider a simplified description of the LTC4380. During normal operation, the LTC4380’s internal charge pump drives the GATE pin to enhance M2. The voltage at GATE is clamped to a maximum of 35V above ground (when the SEL = 0V), thereby limiting the output voltage at M2’s source to less than 35V.
The circuit in Figure 8 further improves on that voltage limit by adding a 22V avalanche diode D3, in combination with R6, R7, R8, and Q2 to regulate the output voltage to a maximum of the avalanche diode voltage, 22V, plus the baseemitter voltage of Q2, roughly 0.7V. When the output voltage exceeds 22V + 0.7V = 22.7V, Q2 weakly pulls down on M2’s GATE to regulate M2’s source and the output voltage at 22.7V.
MOSFET M1, in conjunction with D1, D2, R1, R3, R4, and Q1, protects the circuit from reverse voltage conditions. When the input falls below ground, Q1 pulls M1’s gate down to the negative input voltage, keeping the MOSFET off. This prevents reverse current flow when the battery is connected backward and protects the output from the negative input voltages.
D2 and R3 allow the LTC4380’s internal charge pump to enhance M1 during normal operation when the input is positive so that M1 is effectively a simple passthrough device, dissipating less than I2R = (4A)2 • 4.1mO = 66mW of power in the NXP PSMN4R8100BSE.
When the input voltage is high, the output voltage of this circuit is limited to a safe level by controlling MOSFET M2. This results in significant power dissipation as voltage is dropped across M2 while current is delivered to the load at the output.
If the input is subjected to a sustained overvoltage condition, or an overcurrent fault condition occurs in the onboard electronics at the circuit’s output, M2 is protected by shutting off after a duration configured by the timer network made up of R13, R14, R15, C4, C5, C6, and C14. The output current at the LTC4380’s TMR pin is proportional to the voltage across MOSFET M2 while M2 is in current limit.
Effectively, the TMR current is proportional to the power dissipated in MOSFET M2. The resistor/capacitor network at the TMR pin is similar to an electrical model of the MOSFET’s transient thermal impedance. This serves to limit the maximum temperature rise of the MOSFET to keep it within its rated safe operating area.
Because allowable MOSFET SOA current falls off at high draintosource voltages, the 20V avalanche diode D6, in conjunction with R9, R11, and Q3 provides extra current into the timer network when the INtoOUT voltage exceeds 20V plus Q3’s baseemitter voltage. The 4.7V avalanche diode D7 works with Q4, R12, and C3 to prevent this extra current from pulling the TMR pin above its maximum rated voltage of 5V.
The resistor/capacitor network on the LTC4380’s TMR pin protects against events that are faster than about one second. For slower events, the case temperature of M2 is limited by the circuit connected to the LTC4380’s ON pin.
The thermistor, RPTC, is a small surface mount 0402size component with a resistance of 4.7k at 115°C. Above 115°C, its resistance rises exponentially with temperature. To prevent the timer network from falsely integrating offsets in the power multiplier, the LTC4380 does not generate timer current at the TMR pin until M2’s draintosource voltage reaches 0.7V. With 4A and 0.7V, the MOSFET could dissipate 0.7V • 4A = 2.8W continuously without the TMR network detecting the MOSFET’s temperature rise. The PTC resistor, RPTC, in conjunction with resistors R17–R21 and transistors Q5A, Q5B, Q6A, Q7A, and Q7B shuts down the circuit if MOSFET M2’s case temperature exceeds 115°C.
Do not be dismayed by the number of components in the thermal protection circuit. The overall solution is relatively easy to implement and consists of small components that consume little board area. It is a selfbiased circuit that is balanced when RPTC equals R20’s 4.75kO value. When the temperature of RPTC, which is placed in close proximity to M2, exceeds 115°C, its resistance grows and causes more current to flow through Q5B than Q5A. Because that results in more current through R17 than R18, Q8A’s base voltage rises and Q8A’s collector pulls the ON pin of the LTC4380 low, turning off M2. At lower temperatures, Q5A’s current is greater than Q5B’s, and Q8A remains off, allowing the ON pin’s internal pullup to keep the ON pin high. Note that the ON pin current is used as the startup current of this selfbiased circuit through the diodeconnected device Q8B.
Figure 1. A standard alternator’s 3phase stator windings and 6diode rectifier produce a DC output voltage.
Figure 2. Unclamped load dump: If the battery connection is lost during charging, the alternator’s output voltage can surge to 100V.
Figure 3. Unclamped load dump pulse shape as described in ISO 167502 specification (“Test A…”)
Figure 4. Clamped load dump: An internally clamped alternator has diodes with well specified reverse breakdown voltages that limit the output voltage to 35V during load dump.
Figure 5. Clamped alternator load dump pulse shape
Figure 6. If the onboard electronics are protected by TVS diodes that break down at a lower voltage than the alternator’s clamped voltage, the TVS diodes will be forced to absorb all of the alternator’s energy.
Figure 7. Block diagram of the LTC4380 surge stopper
Figure 8. An LTC4380based circuit protects downstream electronics from ISO 167502 and ISO 76372 transients while providing up to 4A of output current.