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Advances in automotive power-load control

Posted: 28 Dec 2015     Print Version  Bookmark and Share

Keywords:relay  power MOSFETs  electromagnetic compatibility  EMC  pre-driver 

The control of the MOSFET and the collection of load-fault information are realised with the help of the pre-driver's GATx and DRNx pins, and then fed back to the microcontroller through the SPI communication port and fault-flag (FLTB) output. The pre-driver is able to detect short to battery and short to ground faults for each of the loads. The device also monitors the battery supply for abnormal levels to protect the loads from being operated at the restricted levels. There are additional functionalities such as auto-retry and fast charge which can be enabled via SPI communication to tailor the external load needs. These are some examples of the diagnostic functions and protection functions that the pre-driver solution can offer that are not possible using a conventional relay.

To maximise the advantages of this combination of pre-driver and MOSFETs, designers must pay attention to a number of aspects, such as configuration of fault protection and diagnostic parameters, selection of the appropriate MOSFET according to actual load conditions, and calculating the heat dissipated by the MOSFET in order to ensure adequate thermal management.

Fault detection and capture
Each channel of the NCV7518 has independent fault diagnostics, and is capable of detecting shorted-load faults when the channel is on, and short-to-ground or open-load faults when the channel is off. This allows the driver to be used with different types of loads, such as inductive or resistive loads, and to meet various national standards relating to environmental tests for automotive electronics.

Each fault type is uniquely encoded as three-bit per channel fault data. This three-bit encoding allows the faults to be prioritized so that the most severe fault data is available at the next SPI read. Accordingly, shorted-load fault data has top priority, followed by short-to-ground and open-load fault data. The DRNx feedback input for each channel compares the voltage at the drain of the channel's external MOSFET against several internal reference voltages. Shorted-load detection thresholds are programmed via the SPI, and separate detection references are used to distinguish the three fault types. Blanking and filter timers are used respectively to allow output-state transitions to settle and for glitch suppression.

A shorted-load fault is detected when a channel's DRNx feedback is greater than its selected fault reference after either the turn-on blanking or the filter has timed out. An auto-retry mode enables the pre-driver to recover automatically after a shorted-load fault. In this mode, the GATx output of the affected channel is turned off for the duration of the programmed fault-refresh time. The output is turned back on after the refresh time ends, and DRNx is sampled once more after the turn-on blanking time. If the fault is still present, the channel is automatically turned off. This feature is desirable for inrush current conditions and for intermitted faults.

MOSFET selection
As far as selection of the MOSFET is concerned, the maximum drain-source voltage (VDSS) of the chosen device must be higher than the maximum allowed fly-back voltage generated by inductive loads, since the MOSFET is configured in a low side driver. For inductive loads, external clamping diodes are used to protect the MOSFETs. For a 12V system, the fly-back voltage can be clamped to a maximum of 36V, thus a VDSS of over 40V is acceptable. Note that VDSS of 60V would be more appropriate when the fly-back clamps used have a 40V to 50V breakdown range. The load power is the next most important factor governing MOSFET selection. In principle, a higher load current calls for a MOSFET of lower on-resistance.

Save weight, increase reliability
Semiconductor switches and smart pre-drivers can help save weight, reduce size and enhance the reliability of automotive systems traditionally controlled using electromechanical relays. Correct device selection and configuration of pre-driver diagnostics will accelerate the trend towards more and better automotive electronic systems.

About the author
Nixon Mathew is a product marketing engineer with ON Semiconductor.

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