Designing for Power Savings

March 1, 2021

The migration from traditional vehicle development to electric mobility and more efficient ICE options did not happen overnight, but the growth in OEM and supplier innovations continues to shift the technological landscape at a rapid pace. That means the automotive industry is now in a race to bring the best vehicles to buyers as soon as possible.

Range is a key challenge in e-mobility, so design teams are under tremendous pressure to save every amp possible. One resulting trend is the replacement of standard solenoid and solenoid valves with enhanced components using latching technology. This technology enables the solenoid to maintain either an open or closed state indefinitely without drawing power, providing greater efficiency. It delivers important benefits, especially in cases where a solenoid or valve operates on longer cycles. However, significant difficulties must be overcome to capitalize on the energy savings this technology offers.

First, the control module in the vehicle must be able to send one signal to actuate the solenoid and a different reverse-polarity signal to de-latch the solenoid. Second, if the solenoid loses power, it will maintain the last commanded state indefinitely rather than returning to a failsafe position like a traditional solenoid. Mitigating these issues can create unwanted complexity in the control strategy, but there are methods to overcome these obstacles and realize the desired power savings.

Latching solenoids are able to maintain their state indefinitely without drawing power

Failsafe Operation
One way to simplify the control strategy of a latching solenoid is to utilize a capacitive discharge circuit to de-latch the solenoid. TLX Technologies has developed an integrated latch release circuit for failsafe operation of latching solenoids, eliminating the need for modifications to the system control strategy. This provides a simple plug-and-play replacement for a standard solenoid while reducing the constant current draw to microamperes.

When the solenoid is energized to pull in, a capacitor is charged in parallel. Power is then removed from the solenoid coil while being maintained to the capacitor. Even though the capacitor requires a constant voltage signal to maintain charge, it draws nearly zero current. When the power source is removed from the capacitor, the capacitor discharges, therefore providing the reverse-polarity pulse needed to de-latch the solenoid. This means that the latching solenoid is controlled, simply, on the system level, the same way a traditional solenoid is controlled: power on to pull in, power off to return. Only the power consumption profile is significantly altered. For example, consider a solenoid that takes 12W of power to pull in. A traditional solenoid will also require 12W, or in the case of peak-and-hold control, perhaps 3W to hold it in the open position. Using the latch release circuit, 12W will be required to pull in, but almost no power to maintain that position indefinitely, thus notably decreasing energy consumption.

Failsafe Package
Our latch release circuit can be used in place of a standard solenoid to cut constant current draw

This latch release circuit is not completely free of challenges. One downside is that the circuit, although lightweight, does add size when integrated into the solenoid. However, this technology could also be integrated at the ECM or BCM level to optimize total system packaging. The transient power signature of the capacitor also creates a challenge. Due to the very low resistance of a depleted capacitor, there is a particularly large inrush current associated with charging a capacitor. In one such application, an inrush current of approximately 24A was observed. Although this inrush current is only realized for a couple of milliseconds, it is likely that circuit protection or isolation would be needed to protect other electrical systems on the vehicle.

Eliminating parasitic energy consumption is crucial for improving energy efficiency and range in new vehicles. The latch release circuit with failsafe for latching solenoids is a highly innovative way to keep control schemes simple while eliminating the constant power draw seen with traditionally driven systems. Although this solution does increase the size of the component, the power savings provide a significant return on investment, particularly for applications with a longer cycle time.

This article was originally published by Engine + Powertrain Technology International in March 2021