Optocouplers for hybrid electric power trains

Hybrid-electric vehicles must operate reliably under harsh conditions that demand special qualities on the part of power-conditioning electronics.

Consider the electrical system for a hybrid-electric vehicle (HEV). Because it powers traction motors, the electrical system on hybrids can easily reach several hundred volts and handle power levels into the kilowatt range or higher. Hybrids typically use dc-dc converters to boost the voltage from a high-voltage battery high enough to power the traction motor and also for reducing voltages to around 14 V for ordinary vehicular electronics.

In HEVs, both ac-dc and dc-dc converters are key building blocks. For example, Prius models made after 2003 have a main battery voltage of about 200 V. Dc-dc converters step down the main battery voltage to 12 V for charging the auxiliary battery and for powering auxiliary circuits. There are additional step-down conversions to 5, 15, and 24 V for various other operations.

On plug-in hybrids, ac-dc converters take power from the utility grid for battery charging. These chargers are typically high-power converters ranging from a few hundred watts up to 2 kW with output voltage between 48 and 300 V. Because severe high voltages are present, it is imperative to add safety insulation against shock hazards. Designers must build in galvanic isolation between the primary and secondary of both ac-dc and dc-dc converters because of the presence of hazardous high voltage (above 25 Vac or 60 Vdc). The standards and regulations for HEV are still being worked out, but there are already a handful of international and national standards that apply. They include the Federal Motor Vehicle Safety Standards (FMVSS) FMVSS 305 standard which applies for vehicles that use more than 48 V for propulsion power. It is enforced by the National Highway Traffic Safety Administration (NHRSA) of the U. S. Dept. of Transportation. It’s designed to avoid fatalities and injuries during a crash caused by electrolyte spillage from propulsion batteries, intrusion of propulsion battery-system components into the occupant compartment, and electric shock. The requirement states the isolation barrier between battery and exposed conductive part should maintain 500 Ω/V before and after the crash impact.

ECE-R 100 is another regulation set forth by the United Nations Economic Commission of Europe (UNECE) and is widely adopted in the EU. Its scope covers electric road vehicles with maximum design speed exceeding 25 km/hr. It sets isolation resistance between any exposed conductive part and each polarity of the battery at 500 Ω/V minimum under normal operation and post impact.

ISO 6469-3 and IEC 61851-21 are safety standards for protection against electrical shock. The IEC document covers electrical devices and equipment and the ISO regulation deals with all other technologies. Hybrid-electrical vehicles, having both combustion engines and electrical motors, come under both standards. According to ISO 6469-3 insulation resistance should exceed 5k Ω/V for double/reinforced insulation. In addition, insulation should withstand a dielectric breakdown test at two times the constantly increasing working voltage (3,250 Vrms or 3,750 kVrms) for 1 min, at a frequency between 50 and 60 Hz.