Electrical safety of optical isolators in conditions of possible failures

Broadcom's optical isolation gate drivers are widely used to drive IGBTs in applications such as solar inverters, motor control systems, and more. Optical isolation is a proven and reliable technology for providing isolation between the power IGBT and control circuits. In addition, optical isolation reduces the influence of common mode noise (CMR) on the control signal and prevents erroneous opening / closing of the IGBT.

To ensure operability and maintain the integrity of the isolation barrier, a voltage exceeding the nominal value should be avoided at the optical isolation. However, it is quite difficult to arrange when a fault occurs due to a short circuit of the IGBT. This article describes the effect of possible unprotected IGBT failures on the isolation barrier of Broadcom's optically isolated gate drivers.

Optical isolation and isolator structure

Optical isolation in Broadcom components provides a high level of isolation with a three-layer isolation barrier with an overall thickness exceeding that of other technologies. The three layers of the insulating barrier are a silicon-polyimide film-silicon structure (Figure 1).  

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Components that use alternative isolation technologies (inductive and capacitive isolation) have a much lower isolation barrier (less than 17 microns) and will not be able to provide an adequate level of safety in the event of one of the IGBT failure modes described in this article.

References ACPL-337J 4.0 Amp Gate Drive Optocoupler with Integrated (VCE) Desaturation Detection, Active Miller Clamping, Fault and UVLO Status Feedback, Broadcom, AV02-4390EN.