Diodes Inc: Self-Protecting MOSFETs Deliver Improved Reliability in the Harsh Environment of Automotive Applications

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By: Ian Moulding, Automotive Marketing Manager

While it’s been said many times before, the automotive electrical environment is tough! As demonstrated in Figure 1, the nominal battery voltage of an automobile can vary from -12VDC, under reverse battery condition, to +125VDC due to load transients and inductive field decay. Factor in wide variations in operating temperature, numerous interconnections and an open environment that is subject to possible ESD damage from human interactions, and you have an operating environment that is far more challenging than, for example, that of the consumer market segment.

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Figure 1. Causes of automotive battery voltage variations

The automotive industry demands cost-effective and fully reliable solutions but this potentially destructive environment poses a huge challenge to the power semiconductor devices needed for the myriad of control functions that are now commonplace in modern automobiles.

Power semiconductors such as standard MOSFETs have been proven to be insufficiently rugged for many automotive applications. Inductive spikes and load dumps are transients that require either larger MOSFETs or external clamps to absorb the energy that would otherwise destroy the MOSFET. Both of these options add to the cost and complexity of discrete designs.

Self-protected MOSFETs, as developed by Diodes Incorporated, address this issue with monolithic circuit topologies that incorporate clamping and other protection features to provide a more reliable and lower cost/smaller size solution for driving relays, LEDs and other inductive loads.

Lamp Driving
To help cope further with transients, self-protected MOSFETs, such as the ZXMS6004FFQ from Diodes Incorporated, utilize a fully protected topology that incorporates over-temperature and overcurrent protection circuits. As can be seen in the block diagram in Figure 2, this is in addition to over-voltage and ESD input protection. This device leads the industry by using a small form factor SOT-23 package, 6x smaller than comparable SOT223-packaged parts.

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Figure 2. The self-protection features of Diodes’ ZXMS6004FFQ MOSFET

This self-protected MOSFET uses a temperature sensor and thermal shutdown circuit to protect against over-temperature. This circuit is active when the MOSFET is on and is triggered once a threshold temperature, typically 175°C, is exceeded. This turns off the MOSFET, interrupting the current flow to limit further heat dissipation. In-built hysteresis allows the output to automatically turn back on once the device has cooled by around 10°C.

An incandescent lamp has a low resistance when off, which rapidly increases when the lamp is switched on and heats up. Overcurrent protection, effected with a current limit circuit, not only protects against fault conditions but also avoids the high in-rush current associated with the lamp’s low turn-on resistance. The current limit circuit detects the substantial increase in MOSFET drain-source voltage (VDS) resulting from an excessive load current and reacts by reducing the internal gate drive and restricting the drain current (ID). This functionality protects the MOSFET and prolongs the life of the lamp and its behavior is illustrated in Figure 3.

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Figure 3. Typical output characteristic showing current limit function

While these protection circuits are implemented independently, they nevertheless normally function in combination. For example, overcurrent regulation can operate for some time but may not prevent the temperature eventually reaching the threshold where over-emperature cycling will kick in.

With their built-in protection features, self-protected MOSFETs provide a cost effective solution for switching loads in a wide variety of automotive applications. Their intrinsic features increase system reliability while the small size of the SOT-23 packaged devices from Diodes Incorporated offers significant space and cost savings when compared to competitive devices.

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