Application Note

A u t o m o t i v e D o o r L o c k s

occur, such as physical damage to the door or short-circuiting due to water leakage into the door panel orlow temperatures causing freezing of mechanicalmechanisms. Current designs include wiring from thecontrol module in the cockpit of the car to the doorlocks via the fuse box under the dash. Using standardfusing, once the fuse is blown, the door locks remainin a non-operative state until the fuse is replaced.

Agency certifications such as those issued by Under-writers Laboratory (UL), are concerned strictly withsafety standards. Automotive standards far exceed the testing required by agencies such as UL. TheAutomotive Electrical Council (AEC) sets forth thestandards for passive electrical components in theQEC 200 test standards. These tests are concernedwith reliability as well as safety. Companies such asVisteon, Delphi, and Daimler/Chrysler are currentlytesting to QEC 200 requirements. (For a copy ofAEC’s QEC 200, please contact your local Bournssales representative.)

The Application:

In a typical door lock design, the lock motor is ener-gized through a relay by a control processor via aninverter IC (See Figure 1). A pulse is sent to theprocessor from the contact lock/unlock switch, keylock, motion sensor, door ajar circuit or remote con-trol. The coils of the relay have a positive voltagefrom V(Batt) at pin 5 to prevent relay chatter fromnoise or nuisance tripping of the relays (See Figure 1).V(Batt) is typically 12 volts, but will soon be upgradedfor the 42 volt automotive platform. The signal fromthe logic controller is sent to an inverter to bias thecoil for current flow through pins 6 or 7 depending onthe required workload of unlocking or locking.

Figure 1

The Design Challenge:

Two considerations in automotive engineering areweight and reliability. As cars become equipped withmore options and safety requirements, the weight ofthe car increases. As the weight increases, performancecharacteristics decrease and fuel efficiency decreases.Standard fusing requires wiring through the fuse panelto the circuit being protected. This adds to a vehicle’sweight, while the added wiring increases the opportu-nity for malfunctions. A standard blown fuse must bemanually changed, and the circuit is inoperative whilethe fuse is in the non-functional state. The fuse boxmust be accessible in order to service fuses, limitingthe locations the fuse box can be placed.

Automotive door lock circuits require overcurrent protection. If a door lock is jammed or forced into aposition so the lock cannot move in the ordered direc-tion, the IC chips controlling the system can be damaged. A multitude of hazardous situations can

RG11000180T

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If the coil from pin 5 to pin 7 is energized, the arma-ture from pin 4 will change state to pin 3 conductingcurrent through the door lock motor in a direction tolock the door (See Figure 2). The path of the currentwill continue from the door lock motor to pin 2 of therelay. At pin 2 the second armature will not beengaged and will continue contacting pin 1, giving aground through the PPTC. If a short or fault condi-tion were to occur, the excess current flow fromV(Batt) through the door lock motor and relay wouldcause I2R heating in the PPTC. As the PPTC heatsup, the resistance of the PPTC goes exponentiallyhigh, isolating the ground and negating circuit flow.Once the overcurrent fault condition is removed, thepolymer will return to its semi-crystalline structurewith the carbon chains reconnected, and current willonce again flow through the PPTC.

Figure 2

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The unlocking portion is similar. The coil from pin 5to pin 6 of the relay will be energized by the inverterand controller circuit. The armature at pin 1 willchange state to pin 3. When this happens, theV(Batt) will let current flow through the contacts atpins 3 and 2 (See Figure 3) through the motor(changing lock position) back through pin 4 to pin 1to the PPTC and ground, thus changing the state ofthe lock position.

Figure 3

Consult you local Bourns, Inc., representative foradditional automotive applications such as:

• Window lifts • Modules• Electric seats • Underhood applications• Wiring harnesses • Accessory outlets• Lighting • Moon/sun roofs

INVERTERS

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