Hybrid and electric vehicle (EV) powertrains carry high-voltage circuits, often rated at 400 volts (V) and rising to 800 V on newer platforms. In this case, a single connector that separates while the circuit is still live can create an arc across the gap, injuring anyone nearby. A high-voltage interlock loop (HVIL) serves as protection against loose or disconnected connectors.
This article discusses how the loop detects a disconnected or improperly mated connector. It also covers how it ties into the battery management system and the contactors.
What is a high-voltage interlock loop?
HVIL is a low-voltage loop, wired in series through every high-voltage connector in the vehicle. This loop runs independently of the high-voltage power path itself. When a connector loosens, disconnects, or gets damaged, the loop opens. A detector within this loop notes the open-loop condition and accordingly signals the vehicle’s control unit.
Figure 1. A high-voltage connector pair with the high-voltage terminals and the low-voltage interlock terminals labeled inside the same housing. (Image: Machines, MDPI)
Figure 1 illustrates the idea of an HVIL at the connector level. We can observe the high-voltage and low-voltage terminals sitting together inside the same connector housing. Mating the connector closes both paths at once. That is why the interlock loop can ride along on the same connector as the high-voltage circuit. Disconnecting it also opens both paths. That open low-voltage path is what the HVIL detector picks up, and it responds by opening a power relay.
The importance of HVIL extends beyond on-road travel to the vehicle’s entire life cycle. It means that even during technician assembly or repair, and during any scheduled maintenance, the HVIL keeps an eye on any connector that may be working loose to protect personnel.
How is HVIL wired through an actual EV’s high-voltage system?
A technician working on EV diagnostics sees HVIL as a single circuit that passes through each high-voltage component of the EV. Figure 2 shows this arrangement in an internal vehicle diagnostic guide. We can again see that the high-voltage components of the EV, such as the dc-dc converter, the drive inverter, the onboard charger, and the high-voltage junction box, are all connected in a closed loop with HVIL in each stage.
Figure 2. An interlock system on an actual EV’s high-voltage system, from the battery to the drive inverter and onboard charger. (Image: Tesla)
The BMS drives a small constant current through this loop and measures the voltage drop across four known resistance points. In this particular case, 0.02 A through 240 Ω yields a nominal drop of 4.8 V. Any measured value outside this expected range signals an HVIL failure to the BMS.
The vehicle state changes according to the HVIL signal. In Standby, Support, or Charge mode, the BMS reports an HVIL failure as a FAULT, and the contactors open or stay open. In Drive mode, the BMS reports the failure only as a WARNING, and the contactors stay closed. This is to avoid a sudden loss of propulsion while driving.
How does HVIL detect a fault in the loop?
Engineers generally prefer to keep circuitry simple even for high-voltage applications. Here, two voltage signals, HVIL-Send and HVIL-Return, do the actual sorting inside an HVIL circuit. A window comparator checks each one against an upper and a lower threshold, and the combination of where both signals land classifies the loop’s state.
Figure 3. Voltage thresholds a window comparator uses to sort the interlock loop into a closed, open, short-to-battery, or short-to-ground state. (Image: Texas Instruments)
Figure 3 comes from a reference design built around a 10 mA constant current through an expected 200 Ω load across four high-voltage connectors in series. Under normal operation, the system drives the analog HVIL-Send signal above the upper threshold, while the loop’s internal voltage drop keeps the returning HVIL-Return signal within the upper and lower threshold levels.
If a fault occurs, it shifts these signals into a distinct combination of threshold zones, which allows the comparator gates to instantly isolate and identify the exact failure mode.
How does the BMS use HVIL status to open or close the contactors?
On startup, the BMS clears a startup inhibit flag and issues a PWM signal to begin the HVIL check. This is shown in Figure 4 as a step-by-step process. The interlock status steps through three states in order: undetermined, invalid, and finally OK, and only that last state confirms the loop closed successfully.
Figure 4. The interlock status stepping from undetermined to OK, followed by the contactors moving from open through precharge to closed as the battery management system validates the loop. (Image: Electricity, MDPI)
The BMS will not issue an HVContactor-Close command until the HVIL status reaches OK. Precharge contactors close first. Precharge voltage then climbs toward 95% of the source voltage, 47.5 V on this system’s 50 V bus, in roughly 100 ms. The main contactors close next, as the status moves from open through precharge to closed.
The same mechanism runs in reverse. An HVContactor-Open command can arrive manually from a technician or automatically from a detected fault. Either way, the BMS cuts the contact coil supply, and the status moves back from closed to open.
Summary
Confirm that your HVIL connector design couples the high-voltage and low-voltage terminals in a single housing. If you’re validating an HVIL design, checking the loop’s threshold logic is the first thing to do. Watch the 95% threshold and the roughly 100 ms it takes to reach it, since that timing keeps a contactor closure safe from inrush events.
References
- Design, Prototyping, and Integration of Battery Modules for Electric Vehicles and Energy Storage Systems, Electricity, MDPI
- Automotive High-Voltage Interlock Loop (HVIL) Reference Design, Texas Instruments
- Hybridization of a Backhoe Loader: Electric Drive System Design, Machines, MDPI
- Tech Note: HVIL Diagnostic Guide, Tesla
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Filed Under: FAQs, Featured Contributions, Power Electronics