Ian Moulding - Automotive Marketing Manager, Diodes
I’ve mentioned numerous times that the automotive electronics environment is incredibly stringent! As depicted in Figure 1, the car’s rated battery voltage can fluctuate up to 125V DC under conditions like -12V DC (reverse battery state), due to load transients and inductive field decay. Combined with varying operating temperatures, interconnects, and open environments, there’s always a risk of ESD damage from human interaction. Your operating environment is far more demanding than the consumer market.
The automotive industry demands cost-effective yet fully reliable solutions. However, this harsh environment presents a significant challenge to the power semiconductor devices used for the multitude of control functions in modern vehicles.
Standard MOSFETs have proven insufficiently robust for many automotive applications. Inductive surges and load dumps necessitate larger MOSFETs or external clamps to absorb transient energy that could otherwise destroy the MOSFET. Both options add cost and complexity to the design.
Developed by Diodes and others, the self-protecting MOSFET addresses this issue with a monolithic topology that integrates clamping and other protection functions. This results in a more reliable, lower-cost, and compact solution for driving relays, LEDs, and other inductive loads.
### Relay Drive
Diodes’ DMN61D8LQ is a clamp-topology self-protecting MOSFET housed in a SOT23 package, optimized for cost and performance in automotive relay driving. It features ESD protection in the input section and active clamping in the output section. The latter is especially beneficial when switching relays due to their inductive nature. Large transients occur when relays deactivate, which can damage unprotected MOSFETs.
As shown in Figure 2, a back-to-back Zener stack resides between the gate and drain of the MOSFET and serves as the main component of this low-side, active clamp configuration. The clamp voltage is determined by the Zener stack voltage, designed to be below the collapse voltage of the MOSFET's drain-to-source junction while remaining high enough to avoid triggering during normal operation.
When the MOSFET turns off—meaning the input is grounded—the voltage at the drain rises above the Zener stack voltage. Current then flows to ground via the Zener and input resistors. Once the final voltage at the MOSFET gate approaches its threshold, the MOSFET begins to conduct, consuming the load current.
This ensures that inductive energy from deactivating relays is absorbed by the power MOSFET operating in the normal active region, rather than dissipating excessive energy in the reverse collapse mode. Additionally, since the clamp voltage is lower than the sag voltage, the MOSFET consumes less power in clamp mode than in sag mode, offering higher energy-handling capability.
### Lamp Driver
For even greater resilience against transients, self-protecting MOSFETs (like Diodes’ ZXMS6004FFQ) adopt a fully protected topology, including overtemperature and overcurrent protection circuits. As illustrated in the block diagram of Figure 3, overvoltage and ESD input protection have been integrated. Available in a small SOT23 package, it is six times smaller than the equivalent SOT223 package.
This self-protecting MOSFET includes a temperature sensor and thermal shutdown circuitry to prevent overheating. When the MOSFET is turned on, the circuit becomes active and triggers when the critical temperature (usually 175°C) is exceeded. The MOSFET turns off and current is interrupted to limit further heat dissipation. A built-in hysteresis allows automatic recovery once the unit cools to approximately 10°C.
When an incandescent lamp is switched off, its resistance is low. Upon turning on, the resistance increases rapidly, causing the temperature to rise. The overcurrent protection provided by the current-limiting circuit not only safeguards against faults but also prevents high surge currents associated with the low on-resistance of the luminaire. The current-limiting circuit detects a substantial increase in the MOSFET source voltage (VDS) due to overload current and responds by reducing the internal gate drive and limiting the drain current (ID). This feature protects the MOSFET and extends the lifespan of the lamp. Its characteristics are shown in Figure 4.
Although these protection circuits operate independently, they can also work together seamlessly. For instance, overcurrent regulation may function for some time but might not prevent the temperature from eventually reaching the threshold for entering the thermal shutdown cycle.
With its built-in protection, the self-protecting MOSFET offers a cost-effective solution for switching various automotive loads. Its internal features enhance system reliability, while the compact size of Diodes’ SOT23 package reduces space and costs compared to competitive alternatives.
In summary, these innovations from Diodes ensure that automotive systems remain robust and efficient under extreme conditions, addressing challenges unique to the automotive sector while keeping costs manageable.
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