Generally, we see a tendency to replace electromechanical fuses and relays in the automotive board net. Especially Electronic Control Units (ECU) for Advanced Driver Assistance Systems (ADAS) functions are driving this trend. For safety reasons, ADAS functions have to be always accessible and shouldn't be disrupted by malfunctions of other nodes in the electrical system. For this reason, other loads that represent an overload or short circuit in the electrical system have to be disconnected and isolated as quickly as possible before an undervoltage on the power supply causes the ADAS ECUs to go into low voltage reset mode and stop operating. For these reasons, various semiconductor suppliers develop integrated smart high-side power switches with overcurrent detection for the electrical system with 12 V.

We anticipate this trend to multiply into domains with higher voltages. However, for the high voltage electrical system of battery electrical vehicles, there are no smart electronic switches or eFuses available on the market yet. The smart electronic high voltage switch reference design proves the concept of an eFuse for this voltage domain.

The main semiconductor switches are made of Silicon Carbide (SiC) MOSFETs. SiC MOSFET feature very low RDSon and high switching speed. Low RDSon contributes to low power dissipation during on-state. To reduce power dissipation even more, six SiC MOSFETs have been connected in parallel. The reference design is capable to run continuously with 50 A and passive cooling only.

Every SiC MOSFET has an intrinsic body diode that conducts reverse current i.e. from drain to source. Reverse current occurs e.g. when the load generates energy, for example when a motor quickly decelerates. In order to be able to break the circuit also when the load is in generative state, the MOSFETs are connected “back to back” or anti-serial. With this topology, the switch becomes bidirectional, i.e. is able to disconnect when current flows in either direction.

Summary and Advantages:
The reference design of a smart electronic HV-switch leverages the benefits of SiC MOSFETs which feature:

• fast switching.
• low on resistance (RDSon) for low conduction losses.
• high voltage rating.
• Ultra-fast turn-off time of only 2.5µs enables the protection of other high voltage power semiconductors in the load circuit.
• Arc-free operation.

With its state-of-the-art key components, the reference design shows how challenges of electronic circuitry can be mastered:

• generation of isolated auxiliary voltages with capacitors instead of magnetic transformers.
• processing and measuring high currents in the range of 50 A with current shunt resistors, and high voltages in the range of 800 V with high impedance voltage dividers.
• how to drive the gate of a Silicon-Carbide MOSFET.
• how to bridge a galvanic isolation for linear signals with optocouplers.