SLYT857 August 2024 TPS1200-Q1 , TPS1211-Q1
The transition of vehicle architectures from domain- to zone-based is significantly changing automotive power distribution, with semiconductor switch-based solutions (see Figure 1) replacing the traditional melting fuses used for wire harness protection. These solutions offer benefits such as less variability in fuse-time currents, which can then potentially reduce the cable diameter, weight and cost of the wire harness. Semiconductor switches are also resettable remotely, which means that the fuses do not have to be easily accessible, giving designers the ability to place the fuses in locations that can reduce cable lengths from the power source to the load.
The system design challenges when using semiconductor switches as smart fuse devices include lowering the quiescent current when the switch is in the on state, as well as turning on outputs powering large capacitive loads typically seen at the load (the electronic control unit [ECU] input). ECUs have an input capacitance ranging from 47µF to 5mF and startup time considerations (fast charging time <1ms, medium charging time <10ms, slow charging time <50ms) based on the ECU type and number of ECUs connected together on each Power Distribution Box (PDB) output. Charging these ECU input capacitors through the metal-oxide semiconductor field-effect transistor (MOSFET) switch within the ECU startup time is one of the primary system design challenges of a zone-based architecture.
In this article, we’ll discuss various techniques to address the challenge of driving capacitive loads using high-side switch controllers.
In this method, placing the capacitor (C) between gate-GND, the slew rate of the gate and the output voltage limits the inrush current. The circuit configuration with output voltage slew-rate control is shown in Figure 3.
Equation 1 and Equation 2 calculate the inrush current and power dissipation at startup as:
Because the MOSFET is operating in a saturation region, the inrush current should be low enough to keep the power dissipation within its safe operating area (SOA) during startup. MOSFETs can handle more energy (1/2 COUTVIN2) when their power dissipation is reduced and spread over longer durations. Thus, the inrush interval needs to stretch out over a longer period of time to support higher capacitive loads.
This method is suitable for slow charging requirements (for example, 5mF and 50ms), but the design must always include a trade-off between COUT, the FET SOA, the charging time and the operating temperature. For example, charging 5mF to 12V takes 40ms with an inrush current limit of 1.5A using TI’s high-side, switching controller, the TPS1211-Q1 as gate driver. Reference [1] iterates a procedure on how to check the FET SOA during startup using this method, while reference [2] is an online tool for estimating the SOA margin for a specific MOSFET.