Avago engineers optical isolators for electrified vehicles

  • 25-Jun-2012 11:38 EDT

Figure 1: The battery array feeds its output to the power inverter that is part of the electric motor drivetrain. Optical isolators provide high-voltage isolation between the inverter and the gate-drive inverter control and between the sensors and monitoring circuits.

The transition from gasoline-fueled vehicles to gasoline-electric hybrids or all-electric vehicles presents many design challenges for the circuits and subsystems employed in the vehicle due to transients, extreme temperatures, and many other factors. In many of the drive-by-wire systems planned for future vehicles, as well as today’s hybrid and full-electric vehicles, the system electronics must perform flawlessly and maintain immunity to electrical noise, large transients, static discharges, and other challenges.

From the battery array to various subsystems such as the drivetrain or entertainment systems, optical isolators deliver a solid solution that provides high electrical isolation and high noise immunity. The battery array supplies power to the motor drive subsystem, which typically includes a power inverter, various sensors, and isolation optocouplers (see Figure 1). The inverter typically switches voltages of more than 300 V, and thus this portion of the system must also contain lots of isolation to prevent any high voltages from reaching the low-voltage circuits.

Protect the inverter

In the design of the power inverter, fault protection and feedback from the motor are key portions of the system design. DESAT (desaturation) fault-protection techniques have been in use for many years; however, the complex circuit nature and high associated cost marginalized its use to a very small market segment. Today’s ability to integrate DESAT functionality into the optocoupler that also provides signal isolation now enables a single-package low-cost solution that opens the market for medium- and low-power inverters in industrial applications.

The power stage of a typical three-phase inverter is susceptible to several types of failures, most of which are potentially destructive to the power IGBTs (insulated gate bipolar transistors). These failure modes can be grouped into four basic categories: phase and/or rail supply short circuits due to user misconnect or bad wiring, control-signal failures due to noise or computational errors, overload conditions induced by the load, and component failures in the gate-drive circuitry.

Under any of these fault conditions, the current through the IGBTs can increase rapidly, causing excessive heating. The IGBTs become damaged when the current load approaches the saturation current of the device, and the collector-to-emitter voltage rises above the saturation-voltage level. To prevent damage to the drive, fault protection must be implemented to reduce or turn off the overcurrents during a fault condition.

In a three-phase switching power conversion application, a typical intelligent power module (IPM) incorporates seven gate drivers that drive six IGBTs—three high-side and three low-side—switching the three-phase power, and an additional IGBT for the braking function, to protect phase running from over- or under-voltage on the dc bus (see Figure 2).

In these systems, high-reliability optocouplers that provide the necessary isolation between the inverter MCU (microcontroller unit) and the high-voltage devices should be used. Digital optocouplers such as the ACPL-K43T/M43T/M46T or similar devices from other vendors are typically employed in IPM-based inverters, and isolated-gate drivers such as the ACPL-312T/38JT are typically applied to drive the discrete IGBTs or MOSFETs (metal-oxide-semiconductor field-effect transistors) (see Figure 3).

ACPL-312T/38JT devices contain an AlGaAs LED that is optically coupled to an integrated circuit with a power output stage (Figure 3, middle and right). Devices such as the ACPL-38JT are easy-to-use, intelligent gate drivers that include features such as user-configurable inputs, integrated VCE detection, under-voltage lockout (UVLO), “soft” IGBT turn-off, and isolated fault feedback to provide maximum design flexibility and circuit protection. The outputs (VOUT and FAULT) of the ACPL-38JT are controlled by the combination of VIN, UVLO, and a detected IGBT DESAT condition.

Use DESAT to detect faults

The fault-detection method adopted in the ACPL-38JT monitors the saturation (collector) voltage of the IGBT and triggers a local fault shutdown sequence if the collector voltage exceeds a predetermined threshold. A small gate-discharge device slowly reduces the high short-circuit IGBT current to prevent damaging voltage spikes. Before the dissipated energy can reach destructive levels, the IGBT is shut off. During the off state of the IGBT, the fault-detect circuitry is simply disabled to prevent false "fault" signals.

When the DESAT input detects a fault condition (voltage on the terminal exceeds 7 V), the IGBT gate voltage (VOUT) is slowly lowered. Next, the FAULT output goes Low, and that indicates to the host microcontroller that a fault condition exists. The microcontroller would then take the appropriate action to prevent further damage.

By directly measuring the collector voltage, the ACPL-38JT limits the power dissipation in the IGBT even with insufficient gate-drive voltage. Another more subtle advantage of the desaturation detection method is that power dissipation in the IGBT is monitored, while the current-sense method relies on a preset current threshold to predict the safe limit of operation. Therefore, an overly conservative overcurrent threshold is not needed to protect the IGBT.

A high-speed internal optical link minimizes the propagation delays between the microcontroller and the IGBT while allowing the two systems to operate at very large common-mode voltage differences that are common in industrial motor drives and other power-switching applications. The optocoupler’s output circuit provides local protection for the IGBT to prevent damage during overcurrents, and a second optical link provides a fully isolated fault-status feedback signal for the microcontroller. A built in “watchdog” circuit monitors the power-stage supply voltage to prevent IGBT damage caused by insufficient gate-drive voltages.

The forward optical signal path, as indicated by LED1, transmits the gate control signal (Figure 3, right). The return optical signal path, as indicated by LED2, transmits the fault status feedback signal. Both optical channels are completely controlled by the input and output circuits in the optocoupler, respectively, making the internal isolation boundary transparent to the microcontroller.

When an IGBT fault is detected, the output detector IC (integrated circuit) immediately begins a “soft” shutdown sequence, reducing the IGBT current to zero in a controlled manner to avoid potential IGBT damage from inductive over-voltages. Simultaneously, this fault status is transmitted back to the input buffer IC via LED2, where the fault latch disables the gate control input and the active low fault output alerts the microcontroller.

IGBTs typically require gate voltages of 15 V to achieve their rated VCE(ON) voltage. At gate voltages below 13 V typically, their on-voltage increases dramatically—especially at higher currents. At very-low gate voltages (below 10 V), the IGBT may operate in the linear region and quickly overheat. The UVLO function causes the output to be clamped whenever insufficient operating supply (VCC2) is applied. Once VCC2 exceeds VUVLO+ (the positive-going UVLO threshold), the UVLO clamp is released to allow the device output to turn on in response to input signals.

As VCC2 is increased from 0 V (at some level below VUVLO+), first the DESAT protection circuitry becomes active. As VCC2 is further increased (above VUVLO+), the UVLO clamp is released. Before the time the UVLO clamp is released, the DESAT protection is already active. Therefore, the UVLO and DESAT fault detection features work together to provide seamless protection regardless of supply voltage (VCC2).

Roy Tan, Development Engineer for Isolation Products, Avago Technologies, wrote this article for Automotive Engineering.

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