Advances in automotive power-load control

To make vehicles more attractive, safer and easier to use, manufacturers integrate a huge variety of electric accessories and electronic safety aids in cars. In addition, traditional hydraulic systems such as power steering and automatic transmission are being replaced with electrically operated equivalents to help reduce overall weight and improve fuel economy.

As the trend towards greater electrification continues, the traditional electromechanical relay is still widely used for load switching. Relays help isolate vehicle occupants safely from high-power circuitry, and minimise the demand for expensive, bulky high-current wiring. Control logic is relatively simple, while form factors and pin-outs have become written into ISO standards that help simplify system design as well as supply-chain and inventory management.

Superseding traditional relay
There are some drawbacks, however. Although the standardised relay sizes and terminal patterns now include miniaturized formats such as Mini 280 and Micro 280, the Mini size has a 1-inch cube body while the Micro is 1" x 1" x 1/2". In an era where designers are under relentless pressure to build extra circuitry into smaller ECUs, a more compact load-switching solution is needed. Relay reliability is also relatively weak: although mechanical lifetime can be significantly greater than one million operations, the electrical lifetime is typically only around 100,000 operations depending on the load and operating conditions.

A conventional relay can also generate appreciable electromagnetic switching noise. A voltage spike occurs as the magnetic field in the coil of the relay's control circuit collapses at turn-off. A built-in resistor or clamping diode can prevent damage to nearby circuitry, but additional suppression or shielding may be needed to prevent electromagnetic interference.

Finally, there is demand for improved diagnostics throughout automotive electrical infrastructures, to enable enhanced information and safety systems and to assist service and repair. Conventional relays cannot support self-diagnostic functionality and load protection without additional circuitry.

Designers are turning to automotive-qualified power MOSFETs to meet future goals in terms of reduced size and weight, improved reliability, better electromagnetic compatibility (EMC), and enhanced intelligence and diagnostics. MOSFETs with suitable current and voltage rating have on-resistance of just a few milliohms, which helps simplify thermal management.

It is recommended to control the MOSFET gate using a pre-driver, since MOSFETs have relatively weak self protection and can be permanently damaged by over-voltage or over-current spikes. The ON Semiconductor NCV7518 six-channel pre-driver provides suitable protection, and also integrates fault-detection and diagnostic circuitry. Designers are free to scale their applications through choice of external MOSFETs.

The figure shows a sample application circuit comprising the NCV7518 with six MOSFETs to control various loads including a lamp, heater and simple motor. The pre-driver is housed in a 5mm x 5mm x 0.9mm QFN32 package. A suitable MOSFET type is the NID9N05CL, which is a 9.0A 52V device featuring logic-level switching and integrated clamping diode and ESD protection. A small number of resistors and capacitors are required, resulting in significantly smaller size and lower height than a comparable system built using relays. In addition, relay noise is eliminated. With adequate heatsinking, the junction temperature of the MOSFETs can be managed to ensure reliability for at least the intended life of the vehicle.