Self-protected MOSFETs boost reliability of vehicle electronics

By IAN MOULDING,
Automotive Marketing Manager,
Diodes
www.diodes.com

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

Automotive electrical equipment can be subject to extreme variations in nominal battery voltage, as illustrated by Fig. 1 . This can vary from –12 Vdc, arising from a reversed connection, to 125 Vdc due to load transients and inductive field decay. Add to this the extremes of operating temperature and numerous interconnections, which allow the possibility of ESD damage from human interactions, and the result is a more challenging environment than, for example, that found in the consumer market.

Standard MOSFETs have proven to be insufficiently rugged to meet the normal power semiconductor requirements of many automotive applications. Overcoming the transients caused by inductive spikes and load dumps, which would otherwise destroy the MOSFET, either requires a significantly larger and thus more costly MOSFET or the use of external clamps to absorb the energy, which clearly add to the complexity of a discrete power circuit design.

Fig. 1: Sources of automotive battery voltage variations.

This issue is addressed by self-protected MOSFETs, which incorporate clamping and other protection features in their monolithic circuit topology to provide a more reliable and lower-cost/smaller solution for driving relays, LEDs, and other inductive loads.    

Relay driving

Active drain clamping across the output of a MOSFET is particularly useful when switching relays because, due to their inductive nature, large transients can be created when deactivating the relay, and these transients have the potential to destroy unprotected MOSFETS. Fig. 2 illustrates how a self-protected MOSFET, which also features ESD protection on its input, solves this problem.

The back-to-back Zener stack, between the MOSFET’s gate and drain connections, is the key component in this low-side, active clamp configuration. The clamp voltage is set by the Zener stack voltage and is designed to be less than the avalanche breakdown voltage of the MOSFET’s drain-to-source junction but high enough not to be triggered in normal operation.

Fig. 2: Equivalent circuit of a low-side active over-voltage clamp.    

This means that when the MOSFET is switched off — i.e., the input to the device is grounded — and the voltage of the drain pin rises above the Zener stack voltage, current will flow via the Zener and input resistor to ground. Then, as the resulting voltage generated at the gate of the MOSFET nears its threshold, the MOSFET will start to turn on and draw load current.

This ensures that the inductive energy generated by a deactivating relay is absorbed by the power MOSFET operating in its normal active region rather than dissipating the energy more locally in a reverse avalanche mode. Because the clamp voltage is lower than the avalanche voltage, the MOSFET dissipates less instantaneous power in clamp mode than avalanche mode, providing a greater energy-handling capability.    

Lamp driving

To help cope further with transients, some self-protected MOSFETs utilize a fully protected topology that incorporates over-temperature and over-current protection circuits. As can be seen in the block diagram in Fig. 3 , this is in addition to over-voltage and ESD input protection.

Fig. 3: More integrated self-protected MOSFETs provide further protection against over-current and over-temperature.     

A self-protected MOSFET like this uses a temperature sensor and thermal shutdown circuit to protect against over-temperature. The 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. Built-in hysteresis will usually allow 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. Over-current 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 (V_DS ) resulting from an excessive load current and reacts by reducing the internal gate drive and restricting the drain current (I_D ). This functionality protects the MOSFET and prolongs the life of the lamp. Its behavior is illustrated in Fig. 4 .

Fig. 4: Typical output characteristic showing current limit function.    

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

With their built-in protection features, self-protected MOSFETs — such as the DMN61D8LQ from Diodes or the more fully featured ZXMS6004FFQ — 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 SOT23 packaged devices from Diodes offers space and cost savings when compared to alternative solutions.