ST Introduces 5 W Isolated Buck Converters for Automotive and Industrial Applications


STMicroelectronics has launched two new devices, the A6986I and L6986I that operate from an input range of 4 to 38 volts and provide a non-isolated primary voltage and an isolated secondary voltage. They don’t require optocoupler or ancillary components. The secondary side voltages are governed by the turn ratio of the external isolation transformer. The primary side voltage can be adjusted by the choice of eternal resistors.

To regulate the primary output voltage, these units operate in forced-PWM mode with peak-current-mode control and to provide better reliability. For better reliability, the devices feature a programmable soft-start. Pulse-by-pulse current sensing makes for primary-side constant-current protection. Oscillations caused by the transformer leakage inductance are filtered by a preset current-sense blanking time of 300 ns.

These units also provide thermal shutdown and over-voltage protection. They have external pins for setting the switching frequency and for synchronization.

Operation of the Isolated Buck Converters

Both of the isolated buck (Iso-buck) converters can be thought of as consisting of three subsections. 

These subsections are the primary side, the isolation transformer, and the secondary side:

  • The Primary Side: The primary side is depicted in blue. It serves as the regulation loop of the peak current mode, constant frequency control architecture that serves to regulate the primary voltage (VOUT_pri). It is the intersection between the error amplifier output and the sensed inductor current that generates the PWM control signal to drive the power switch of the primary side. This serves to define the duty cycle, thus regulating the primary output voltage.

With the MLF pin set to ground, the units operate in forced PWM control. This allows negative currents to flow in the synchronous MOSFET. As such, during off-time energy can flow to the transformer’s secondary coil.

  • The Isolation Transformer: The isolation transformer, depicted in grey, provides isolation between the primary side and secondary side. As stated, its turns ratio largely determines the isolated output voltage.
  • The Secondary Side: The secondary side is depicted in green. It serves to generate the secondary side voltage, with the D1 providing rectification. Filtering is provided by an LC filter consisting of the transformer’s secondary winding and capacitor COUT_sec.

Applications for the A6986I Automotive Converter

The 100% duty cycle and the wide range of the A6986I enable it to meet the  load-dump and cold-crank specifications of automotive systems, making them suitable for:

  • Hybrid/Electric-Vehicle (H/EV) traction drives.
  • On-Board Chargers (OBC).

Applications for the L6986I Industrial Converter

The industrial-grade L6986I will find application in:

  • Welders.
  • Motor drives.
  • Uninterruptible power supplies (UPS).
  • Solar power conditioning systems.


Both units operate over a junction temperature range of -40 to +150 degrees C. The automotive A6986I is AEC-Q100 Grade-1 qualified.

Getting to Market Faster

ST offers two evaluation boards to help designers more quickly develop single and dual output converters.