The 12 V systems and high-voltage (HV) systems in an EV have different isolation and grounding requirements. They work together sometimes but must be galvanically isolated to ensure safe and reliable operation. In addition, the Federal Motor Vehicle Safety Standard (FMVSS) 305 specification in the US details the isolation resistance that must be maintained from HV systems to chassis ground on the 12.
The FMVSS 305 specification requires that at least 500 Ω/V of isolation resistance be maintained from HV systems to chassis ground on the 12 V side. This FAQ begins by briefly reviewing the range of definitions of HV, looks at the grounding and isolation requirements for 12 V and HV systems in an EV, and closes with a brief overview of the technical requirements of FMVSS 305.
The definition of HV is not fixed and depends on technical and application factors. Generally, HV is defined based on the relative danger of electrical shock and the possibility of producing a spark or arc in the air. Application context also matters. In the case of the electric utility grid, the American National Standards Institute (ANSI) defines HV as voltages from 115 to 230 kV in a 60 Hz system. The definition of HV is quite different for EVs. HV is typically defined in automotive systems as voltages from 30 to 1,000 Vac or 60 to 1,500 Vdc.
Using that definition, 12, 24, and even 48 Vdc automotive systems are low voltage (LV).
For most EVs, the components that handle HV include:
- The battery pack
- The motor drive inverter
- The motor/generator unit (MGU)
- Certain dc/dc converters
- The on-board battery charger (OBC)
- Typically, the electric air conditioning compressor
Some components like the dc/dc converter, power electronics controller, moor drive inverter, and certain sensors can straddle between the HV and LV sections (Figure 1).
The 12 V section uses the vehicle chassis for the ground. The HV section is isolated from the chassis, and power is distributed using a two-wire system of positive and negative cables. It’s not technically grounded. The HV components are mechanically attached to the chassis but electrically isolated or ‘floating.’
It’s different for EV chargers. Grounding is an essential aspect of EV charger designs. It involves connecting the charger’s metal components to a conductor connected to the Earth. Proper grounding provides safety from electric shocks by redirecting electric current in the event of short circuits. It also improves charger reliability.
Isolation and insulation
Depending on the required isolation level, various types of isolation are needed in an EV. Isolation refers to the separation between two voltage levels, like HV and LV systems in an EV, while insulation refers to the material or medium that provides the separation. Basic and reinforced isolation are commonly found in EV systems. Basic insolation offers a single isolation layer commonly found in the LV section. Reinforced isolation uses double insulation; it provides a higher level of protection and is located in the HV section and systems that link the HV and LV sections.
The auxiliary power module (APM) is an HV-to-LV dc/dc converter and an excellent example of using reinforced galvanic isolation in an EV. Galvanic isolation is found in the power train and the control section of this dc/dc converter. The power train uses a power transformer to provide the needed isolation level.
There’s also isolation required between the control signals and the power switches in the APM. Many APM designs employ isolated gate drive ICs based on capacitive isolation (Figure 2). Optocouplers or inductive isolation based on gate-drive transformers can also provide isolation for control signals.
Isolation leakage monitoring
Safety is a critical need in the HV sections of EVs. The FMVSS 305 specification requires that at least 500 Ω/V of isolation resistance be maintained from HV systems to chassis ground. To that end, isolation leakage monitoring circuits continuously measure the isolation between the HV section and the vehicle’s chassis ground.
There are several possible points of failure in isolation between the HV and LV sections, including degradation of wiring harnesses, devices being subjected to high temperatures or high peak electrical stresses, and so on. The number and locations of monitoring points required vary with the vehicle architecture.
Isolation of the isolation leakage monitoring circuitry components is an important aspect of system design. The monitoring circuitry must connect between the HV side being monitored and the LV side where the control exists. If excessive leakage currents are detected, appropriate action must be taken. Depending on the system requirements, that can include disconnecting the malfunctioning components or sections using galvanically isolated solid-state relays or isolated electromagnetic relays.
Proper design of the grounding and isolation systems in the LV and HV sections in an EV is vital to ensure the safety of vehicle occupants and service personnel and the reliable operation of the vehicle. There are a range of systems with different requirements for grounding and isolation. In the HV section, it’s necessary to continuously monitor the integrity of the isolation leakage to enable responses in case of a failure or reduced integrity of the isolation systems.
- Automotive High-Voltage and Isolation Leakage Measurements Reference Design, Texas Instruments
- How do all-electric cars work, US Department of Energy
- Hybrid & Electric Vehicle High Voltage Isolation Fault Systems, Vehicle Service Pros
- Isolation in Electric Vehicle Systems, Skyworks
- Why Do Fully Electric Vehicles Still Have a 12V Battery in Them?, Midtronics
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