Automotive low side output switch architecture suitable for 8 to 48 volt buses and beyond

A novel switch architecture caters to extended battery range coupled with robust protection and diagnostic strategies.

For automotive control units, the smart output switches operating contactor coils, lamps, solenoids, PTC (positive temperature coefficient) heater coils and the like form an indispensable “last mile” interface to the external world. Legacy solutions available from global automotive semiconductor companies suffer from the following weaknesses:

  • No off-the-shelf power switch IC is currently available beyond 24 volts, whereas upcoming vehicle architectures are envisioning 48 VDC or higher buses to reduce the weight of associated copper wiring.
  • They consume a minimum of two interface CPU pins per switch; one for the command output and another for the status input.
  • “Short to power bus” protection is based on current limiting followed by a thermal trip, in the process often exceeding the absolute maximum junction temperature prior to the actual tripping event.
  • The on-chip power device often has limited overvoltage and inductive transient capabilities, requiring external protection components.

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The circuit in Figure 1 illustrates a solution that resolves all of these issues.


Figure 1 This design idea is primarily intended to depict a novel switch architecture that can cater to extended battery range coupled with robust protection and diagnostic strategies. It also uses only a single CPU pin-per-switch, as opposed to two CPU pins for legacy architectures. The load coil can be typically rated as a 48V DC, 122 mA contactor coil used for EV/HEV circuits for a 48 volt bus. A typical 12 volt automotive relay coil may draw 95 mA, while a 24 volt version can draw approximately 68 mA.

In this example circuit:

  • Q1 = MMBTA 92-q
  • Q2 = 2N2222
  • Q3 = MMBTA 42-q
  • D1,D2 = 1N4004
  • D3 = 1N4148
  • R1 = 47K ohm
  • R2,R3 = 47K ohm
  • R4 = 2.2 ohm
  • R5 = 10K ohm
  • R6 = 1K ohm
  • C1 = C2 = 0.1 microfarad, 25 volts

The selection of components, resistor values etc. may need to be tailored to address the expected range of application loads, along with CPU and other logic voltage/current characteristics.

The heart of the circuit is the Q1-plus-Q3 latch. Q2 acts as constant current limit that prevents momentary thermal runaway that would otherwise destroy Q1 during a “short to power bus” event. In automotive application, a service engineer or production technician might accidentally short-circuit the output point (Q1 collector) to the power bus. Automotive norms dictate that the power switch must incorporate built-in short circuit protection to cover this and other scenarios.

When the CPU wants to switch on the load, it issues a rising-edge command through the C1-plus-R1 edge trigger circuit to the Q1-plus-Q3 latch. This command sets the latch output to its active low level, thus turning on the contactor coil load. The CPU pin can now reconfigure itself as a diagnostic status input, since the Q1-plus-Q3 latch now self-sustains at the “on” state without needing any further base current assistance from the CPU. The CPU pin, now acting as a diagnostic input, reads a “logic high status”, indicating that the latch output is healthy and powering the load as intended.

In case of an accidental short to power-bus event—in other words, the load coil facing a short circuit across itself—Q3 being a PNP transistor switches off due to its base emitter circuit in effect getting short-circuited (Figure 2). This “resets” the latch by turning off Q1, also saving it from the over-current condition.


Figure 2 This short-circuit protection field performance plot of the circuit shows its near-instantaneous trip mechanism for the output switch, which resets the associated latch.

Transistor Q2 limits the momentary rise in Q1 current at the short-circuit instant to a value of around 318 mA (700 mV/2.2 ohms). This abrupt current limit saves Q1 from destruction due to a thermal runaway before Q3 transistor’s turn-off can safely place it in an “off” state. The CPU diagnostic status input pin now reads a “logic low” level indicating that the latch is turned off, due to a short circuit fault at the output. Capacitor C2 provides ESD protection, along with noise interference filtering that may accidentally operate the latch.

Whenever the CPU needs to issue a turn-off command to a latched switch which is in an “on” state, it presents a logic low level at its output interface pin. This low-level voltage turns off the latch through diode D1. Diode D6 and resistor R1 suppress the inductive flyback transient. Diode D1 may be replaced by a suitable Schottky diode to ensure a robust turn-off of the latched switch.

Unlike legacy smart switch solutions, this circuit does not depend on thermal trip-based protection, hence its junction temperature remains absolutely stable. At the time of a short circuit, there will be an instantaneous current-limited spike. The architecture unburdens software from repeatedly monitoring and turning off the smart switch for short-circuit protection (in order to ensure a safe turn-off even before a thermal trip) as may be required for a legacy switch architecture. It instead repartitions the instant short-circuit protection task to the hardware trip circuit, leaving software to monitor and detect the the short-circuit event on an as-possible basis without worry about any real-time protection deadlines.

The circuit can alternatively be implemented in integrated circuit (IC) form, at least to a degree. A power switch element may alternatively be located at and connected to the IC from the outside if it can’t be packaged within the chip, which can still integrate the remainder of the low current driving, protection and diagnostic logic.

Vishwas Vaidya is a graduate of the Indian Institute of Technology in Delhi, India. Currently, he is self-employed as an engineering consultant and industry faculty member in the field of embedded systems for global automotive clients and high-repute academic institutions. Vishwas’ articles and research reports have appeared in many worldwide engineering publications.

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