What to consider when using self-protected MOSFETs in ruggedized electronic systems

Power devices used in automotive electronic systems must endure extremely harsh environmental conditions. Devices must withstand high voltage spikes resultant from switch-off transients and load dump faults. Ambient operating temperatures exceeding 120°C push device junction temperatures higher. The numerous connectors in wiring harnesses, in place for ease of assembly and repair, afford opportunity for intermittent electrical connection to devices. Even normal operating conditions can significantly stress devices as new loads require ever increasing power.

In order to improve system reliability and reduce warranty costs, designers incorporate fault protection circuitry with power devices, to prevent device failure and subsequent costly damage to the electronic system. Often this is accomplished via external sensors, discrete circuitry, and software, but more commonly designers utilize fully self-protected MOSFET power devices. As technology advances, these devices offer superior fault protection at lower system cost.

Figure 1 shows the general topology of a fully self-protected MOSFET. Other features commonly found on these devices include status indication, digital input, differential input, and over-voltage and under-voltage lockout. High side configurations include on-chip charge pump capability. However, the three circuit blocks common to most devices that provide most of the device protection are current limit, temperature limit, and drain to source over-voltage clamping.

Figure 1 Self-protected MOSFET topology