The safe and reliable powering of solenoids and actuators over a vehicle’s life is a critical requirement. In applications such as engine control, transmission control and braking systems, this can run to 100’s of millions of cycles.
In general, there are four topologies from which engineers can choose when it comes to driving solenoids and actuators. The most appropriate topology will depend on specific application requirements including performance, efficiency, component count and available PCB space.
Solenoid current waveform
All topologies use the peak and hold concept whereby a higher (peak) current is used initially to activate the mechanical solenoid, followed by a drop in current (hold) for general operation. The first option is the boost topology, which boosts battery voltage to 60 V for operation. While this topology is the most energy-efficient, it also requires the greatest number of devices in terms of components, with protection diodes and additional MOSFETs inside the DC-DC conversion circuit.
An option that requires fewer components is a free-wheel diode topology. This controls the solenoid current in the same way to a boost design but at 14 V. At turn-off, energy dissipation takes place though the free-wheeling diodes. The inherent efficiency of the diodes, therefore, dictates the efficiency of the overall system.
A third method for controlling inductive loads can be found in the form of active clamp methodology. This uses a similar number of components as the free-wheel diode design but, in this case, at turn-off a Zener diode will turn the low-side MOSFET back on and the energy will be dissipated through the MOSFET. In this particular design careful consideration needs to be given to the safe operating area (SOA) of the low-side MOSFET to ensure that the component is not damaged when it is turned back on.
The final option is a repetitive avalanche design that makes use of the low-side MOSFET’s ability to work repetitively under avalanche. Avalanche occurs when the BDVSS of the MOSFET is exceeded and forces the MOSFET into breakdown. At turnoff in the repetitive avalanche circuit the energy in the inductive load is dissipated through the low Side MOSFET. This approach simplifies design as it requires fewer components than the other three schemes. In addition, it also offers the fastest switch off time of all four topologies (in order of speed we have repetitive avalanche, boost, active clamp and free-wheel). This latter point is important as a faster turn-off time not only improves the accurate control of components such as solenoids and relays, but it also can help to extend the life-time reliability of these electromechanical devices.
Until recently, engineers have not always had the option to deploy repetitive avalanche topologies as the choice of suitable MOSFETs was limited to devices based on older, planar semiconductor technologies rather than higher performance and more efficient trench structures. The latest automotive-qualified MOSFETs from Nexperia have been designed to address this challenge.
The Repetitive Avalanche MOSFET portfolio in dual N-channel devices are the first MOSFETs based on a rugged, high-performance trench silicon structure to offer guaranteed repetitive avalanche performance tested to one billion avalanche events. As well as the space and BoM (bill of material) advantages inherent to repetitive avalanche schemes, the integration of two dies into a compact package contributes further to reduced board area and system reliability.
Fully automotive qualified to AEC-Q101 at 175 °C, the new MOSFETs are available in 40 V and 60 V options with typical RDS(ON) ratings from 12.5 mΩ to 55 mΩ. All of the devices are supplied in the company’s space-saving LFPAK56 (Power-SO8) copper-clip package technology. As well as being highly robust and reliable, this package features gull wing leads that support improved manufacturability including compatibility with automated optical inspection (AOI).