Nexperia — Application-Specific MOSFETs

Fig. 1: Typical application circuit for hot-swap controller with ASFET

Immediately after insertion, the gate voltage is controlled by the controller, while the ASFET operates in linear mode: it behaves like a voltage-controlled resistor to limit the inrush current, allowing the load capacitance to charge safely and avoid disturbing the backplane voltage, which is common to other parts of the system.

Once the load capacitance has safely charged, the ASFET is then turned fully on. In this mode of operation, low on-resistance is important, because it minimizes conduction losses and increases system efficiency.

So the main design consideration when selecting an ASFET for inrush current limiting is to combine low on-resistance with an enhanced safe operating area (SOA) for strong linear-mode performance.

Zero temperature coefficient

When an ASFET is turned on, two competing effects determine how its current behaves with increasing temperature. As the temperature rises, the threshold voltage falls, thereby increasing the current.

By contrast, the resistance of the silicon increases with increasing temperature, thereby reducing the current. The resulting effect is shown in the ASFET’s transfer characteristics.

The effect of the resistance increase dominates at high currents, meaning that localized heating leads to lower currents. The threshold-voltage drop dominates at low currents, meaning that localized heating lowers the threshold voltage. This condition effectively increases the current within hot cells, leading to thermal run-away.

Consequently, for a given drain-source voltage, there is a critical current below which there is positive feedback and a subsequent risk of thermal run-away. Above this critical current, there is negative feedback and thermal stability, as shown in Figure 2. This critical current is known as the zero temperature coefficient (ZTC) point.