TI's internal inductive transformer achieves high-level power transmission
TI Solutions, 400V/800V System Active Discharge

As the market continues to adopt high-voltage battery packs, and the need for isolation switches throughout vehicles increases, it is time to take a look at insulation technology.

Alex Triano, an engineer on the solid-state relay team at Texas Instruments (TI), said in a presentation titled “TI’s Solutions for the Electric Vehicle and Metaverse Era” at the e4ds webinar on the 12th, “Isolated switches are used throughout EVs to solve problems including pre-charging, active discharging, and insulation resistance monitoring of high-voltage capacitors.”

When the vehicle is started, the high voltage capacitors within the battery management system must be pre-charged. Similarly, when the vehicle is turned off, systems such as the traction inverter or onboard charger must safely discharge the HV capacitors.

Frequent monitoring of the high-voltage rail during vehicle operation is performed within the battery management system, particularly monitoring insulation resistance to ensure that the high voltage is isolated from the metal chassis ground. All of these cases require a reliable insulation solution.

Next, Triano engineers introduced TI's isolation technology.

Electromechanical relays (EMR) use air or a mixture of certain gases as a dielectric. The strength of ERMs ranges from 1 to 20 V rms per micron. This in turn means that these solutions must be physically large to achieve sufficient insulation against high voltages.

The advantage of these relays is that they have low resistance, as they basically create a metal-to-metal connection. The disadvantage is speed and wear due to the mechanical basis. They can take milliseconds to turn on or off, and must be kept within design limits, such as vibration and magnetic immunity.

Most relays specify a maximum operating temperature of 85C, and these solutions are often expensive due to the cost of materials and assembly.

Photo and opto-isolation use higher dielectrics. They provide strong EMI performance, but have the disadvantage that the LEDs can suffer from photodegradation or partial discharge failures, which can wear out over time. In these small devices, the amount of power delivered is limited because the light energy is limited. Most photo-relays are rated for up to 85C operation.

One of the isolation technologies used by TI is the internal inductive transformer. It uses a laminate or polyimide that provides high dielectric strength. This technology can also achieve high speed and high level power transmission. The downside is that the IC must be designed to limit EMI emissions.

Technologies such as Faraday shields or spread spectrum modulation are used to ensure that EMI performance meets automotive standards. These transformers can operate at high temperatures such as 125C and are cost effective.

The insulation technology used in solid state relays (SSRs), including automotive isolation drivers and switches released on the 10th, is capacitor insulation. This technology uses silicon dioxide, which provides the highest insulation strength. It can achieve high speed and low power consumption. EMI emissions must be limited within the IC development, and overall, it limits capacitive power transmission.

TI uses this technology in isolation switches and drivers to create complete isolated SSR solutions. A major advantage of this solution is that the components do not move, so they do not wear out over time, providing improved reliability.

Triano engineers also explained how solid-state relays work in applications.

When the system is turned on, there are HV positive and negative contactors that are connected to the high voltage DC link capacitors. Failure to manage the inrush current can result in damage such as fitting or welding of the machine high voltage contactors.

One solution is to use a pre-charge circuit with a power resistor to limit the current during startup. The TPSI3050-Q1 and TPSI3052-Q1 are isolated switch drivers that form a solid-state relay solution for pre-charge switches. Both products provide 5kV RMS reinforced isolation, are automotive qualified up to 125C, and integrate their own isolated bias supply.

These products can drive various types of external switches such as silicon controlled rectifiers, MOSFETs, IGBTs or silicon carbide FETs. Additionally, 10V and 15V gate drive options are available, providing optimized efficiency for solution cost.

The next application introduced is active discharge. When the vehicle is powered off or a collision occurs, the high voltage capacitors must be discharged to a safe level within seconds. Solid state relays can be used to connect or disconnect high power pull-down resistors. The TPSI3050-Q1 and TPSI3052-Q1 provide active discharge solutions within 400V or 800V systems.