MOSFETs provide higher reliability in the harsh environments of automotive applications

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I have mentioned many times before that the automotive electronic environment is very strict! Due to load transients and inductive field decay, the car's rated battery voltage can vary to 125V DC at -12V DC (in reverse battery state). Coupled with changes in operating temperatures, interconnects and open environments, ESD damage from interactions with humans is at risk, and your operating environment is far more challenging than the consumer market.

The automotive industry needs cost-effective and completely reliable solutions, but this potentially damaging environment poses a huge challenge to the power semiconductor devices required for the large number of control functions common in modern automobiles.

Power semiconductors such as standard MOSFETs have proven to be less robust than many automotive applications. Inductive surges and load dumps require larger MOSFETs or external clamps to absorb energy transients that would otherwise destroy the MOSFET. Both options add cost and complexity to the independent design.

Developed by Diodes and others, the self-protecting MOSFET solves this problem with a monolithic topology that combines clamping and other protection functions to provide more reliable, lower cost, and smaller drive relays, LEDs, and other inductive loads. Size solution.

Relay drive

Diodes' DMN61D8LQ is a clamp-topology self-protecting MOSFET in a SOT23 package that has been optimized to meet the cost and performance requirements of driving automotive relays. It has ESD protection in the input section and active bucking in the output section. The latter is particularly useful when switching relays due to its inductive nature, because large transients are generated when the relay is deactivated, and these transients can damage unprotected MOSFETs.

The back-to-back Zener stack shown in Figure 2 is located between the gate and the drain of the MOSFET and is the primary component of this low-side, active clamp configuration. The clamp voltage is set by the Zener stack voltage and is designed to be less than the collapse voltage of the MOSFET's drain-to-source junction and is high enough not to be triggered during normal operation.

This means that when the MOSFET is turned off, that is, the input to the device is grounded, the voltage at the drain pin will rise above the Zener stack voltage, and current will flow to ground through the Zener and input resistors. Then, as the final voltage generated by the MOSFET gate approaches the threshold, the MOSFET will begin to conduct and consume the load current.

This ensures that the inductive energy generated by the deactivation relay can be absorbed by the power MOSFET operating in the normal active region, rather than dissipating more energy in the local machine in the reverse collapse mode. At the same time, since the clamp voltage is lower than the sag voltage, the MOSFET consumes less power in the clamp mode than the sag mode, thus providing higher energy handling capability.

Lamp driver

To further respond to transients, self-protecting MOSFETs (such as Diodes' ZXMS6004FFQ) use a fully protected topology, including overtemperature protection and overcurrent protection circuitry. As shown in the block diagram of Figure 3, overvoltage and ESD input protection have been added. The device is available in a small SOT23 package that is six times smaller than the same SOT223 package.

This self-protecting MOSFET is protected by a temperature sensor and thermal shutdown circuitry to avoid overheating. This circuit is active when the MOSFET is turned on and will trigger when the critical temperature is exceeded (typically 175 °C). The MOSFET is turned off and the current is interrupted to limit further heat dissipation. Built-in hysteresis allows the output to automatically recover after the unit has cooled down to approximately 10 °C.

When the incandescent lamp is turned off, the resistance is low. When the lamp is turned on, the resistance will increase rapidly and the temperature will rise. The overcurrent protection provided by the current limiting circuit not only provides protection against faults, but also avoids high surge currents associated with low on-resistance of the luminaire. The current-limit circuit detects a large increase in the MOSFET source voltage (VDS) due to the overload current and responds by reducing the internal gate drive and limiting the drain current (ID). This feature protects the MOSFET and extends the life of the luminaire.

Although the above protection circuits are implemented independently, they can also be combined and function properly. For example, overcurrent regulation can operate for a period of time, but may not prevent the temperature from eventually reaching the threshold of entering the superheat cycle.

With its built-in protection, self-protecting MOSFETs provide a cost-effective solution for switching loads in a variety of automotive applications. Its internal features increase system reliability, and the compact size of Diodes' SOT23 package saves space and cost compared to competing devices.