From headlamps and brake lights to fans and solenoids, modern vehicles rely on dozens of electronically switched loads in their automotive designs. When one of these loads silently disconnects, the consequences can be serious. However, open load detection is the technology that can catch these failures before they become a safety problem.

With Infineon's automotive smart power switch families, open load detection is no longer a manual diagnostic task. It's built in, autonomous, and compliant with automotive requirements — ready to use from the start.

What is an open load condition?

An open load condition occurs when the load becomes disconnected from its power source. In automotive systems, that's typically the battery voltage (VBAT). When the circuit breaks, current can no longer reach the load. The result is a complete loss of functionality.

There are three primary reasons why open loads occur:

  • Wire harness failures where broken wires or lost ground connections caused by vibration, heat cycling, or mechanical stress
  • Connector failures caused by disconnected pins or corroded contacts, particularly common in harsh environments exposed to moisture and temperature extremes
  • Load failures where the load itself breaks internally; a classic example is a burnt bulb filament that snaps the circuit even though all wiring remains intact

Each of these failure modes looks identical from a circuit perspective: no current, no output. 

Why must open load conditions be detected?

In most cases, open load detection isn’t only good engineering practice, it is a requirement – whether for legal, functional safety or diagnostic:

Legal requirements:

Dashboard signaling for load failures is mandatory in most countries. If a direction indicator lamp (turn signal) fails, the driver must be alerted. Without open load detection, that warning signal never fires, resulting in regulatory non-compliance and reducing operational safety because critical load-disconnection faults may remain undetected.

Functional safety requirements:

In ISO 26262-compliant systems, an undetected open load can lead to a safety goal violation. When a fault is caught immediately, the system can respond appropriately, whether that means switching to a fallback mode or raising a Diagnostic Trouble Code (DTC).

Diagnosis requirements:

Effective automotive diagnostics dramatically reduce workshop time. Instead of a technician manually probing the entire system, an smart switch that flags the exact faulty circuit can save on labor time.

Two core approaches to open load detection

The right detection method depends on the state of the high-side switch. There are two distinct scenarios:

ON-state detection: The channel is active, VDS is low. With current flowing through the circuit, monitoring the load current is the most direct way to spot a missing load.

OFF-state detection: The channel is deactivated, VDS is high. Here, the output voltage becomes the key diagnostic signal. We strongly recommend using at least a pull-up resistor to polarize the output, without it, reliable OFF-state detection is not possible.

When the switch is on and current should be flowing, there are three implementation options:

  • External current sensors (e.g., shunt resistors): Suited to high-current applications using discrete MOSFETs and gate drivers. A current amplifier evaluates the signal, but this approach requires higher implementation effort and more PCB space.
  • Integrated current sense: Smart high-side switches with a dedicated sense pin (IS) mirror a scaled version of the load current directly to the microcontroller. No external amplifier needed as everything is in one package, and implementation complexity drops significantly.
  • Integrated open-load comparator: The most autonomous option. An internal comparator compares the sense signal against a threshold and raises a fault flag automatically, without burdening the MCU with analog evaluation. The entire diagnostic loop – current mirror → IS pin → comparator → fault response – happens inside the switch.

With the channel off, output voltage monitoring takes over. There are two ways to implement this:

Voltage divider + microcontroller ADC: A pull-up transistor and current-limiting resistor attempt to pull the output toward VBAT during a diagnostic test window. A resistor divider scales the output voltage into the ADC range. If no load is connected, the output floats high, so the MCU reads a high voltage and flags an open load. If a load is present, the output stays near GND. A protection resistor shields the ADC input from transient voltages.

Integrated VDS comparator - Most devices in the Infineon PROFET™ +2 12V and SPOC™ product families integrate a VDS comparator directly. When VOUT exceeds a defined threshold, typically VS − 1.8 V for PROFET™ +2, a fault flag is raised automatically.

When the switch is on and current should be flowing, there are three implementation options:

  • External current sensors (e.g., shunt resistors): Suited to high-current applications using discrete MOSFETs and gate drivers. A current amplifier evaluates the signal, but this approach requires higher implementation effort and more PCB space.
  • Integrated current sense: Smart high-side switches with a dedicated sense pin (IS) mirror a scaled version of the load current directly to the microcontroller. No external amplifier needed as everything is in one package, and implementation complexity drops significantly.
  • Integrated open-load comparator: The most autonomous option. An internal comparator compares the sense signal against a threshold and raises a fault flag automatically, without burdening the MCU with analog evaluation. The entire diagnostic loop – current mirror → IS pin → comparator → fault response – happens inside the switch.

With the channel off, output voltage monitoring takes over. There are two ways to implement this:

Voltage divider + microcontroller ADC: A pull-up transistor and current-limiting resistor attempt to pull the output toward VBAT during a diagnostic test window. A resistor divider scales the output voltage into the ADC range. If no load is connected, the output floats high, so the MCU reads a high voltage and flags an open load. If a load is present, the output stays near GND. A protection resistor shields the ADC input from transient voltages.

Integrated VDS comparator - Most devices in the Infineon PROFET™ +2 12V and SPOC™ product families integrate a VDS comparator directly. When VOUT exceeds a defined threshold, typically VS − 1.8 V for PROFET™ +2, a fault flag is raised automatically.

The pull-down resistor: the detail that matters

Alone, a pull-up resistor can detect an open load, but it cannot distinguish it from a short to battery. Both conditions result in VOUT ≈ VS. To differentiate them, a pull-down resistor must also be used. Diagnosis then runs in two steps:

·       Pull-up active → VOUT high = open load detected

·       Pull-up inactive → VOUT still high = short to battery confirmed

The newer SPOC™ Smart Guard from Infineon takes this one step further: it integrates both pull-up and pull-down resistors on-chip, eliminating external components and simplifying PCB design considerably.

Deepen your knowledge with online training

Want to learn more about open load detection? Infineon's online training course "How to detect open loads with high-side switches" walks you through the concepts covered in this article. From the fundamentals of open load conditions to step-by-step resistor sizing calculations, including interactive visuals and real-world examples. It's the fastest way to get hands-on confidence before you start your design.

Enroll here!

Why choose Infineon for open load detection?

Infineon's PROFET™ and SPOC™ smart high-side switch families are engineered specifically for the demands of automotive diagnostics:

  • Integrated current sense with a scaled sense output for ON-state monitoring
  • Built-in VDS comparator for automatic OFF-state open load detection (PROFET™ +2 and most SPOC™ devices)
  • Fault reporting via SPI or dedicated diagnostic pins
  • SPOC™ Smart Guard 12V: integrated pull-up and pull-down resistors — the most compact open load detection solution available

Ready to design your next open load detection circuit? Explore Infineon's full portfolio of automotive smart high-side switches and find the right device for your application.