10W isolated buck converter requires no optocoupler

The secondary output, it said, is suitable for powering isolated IGBT, SiC or GaN gate drivers.

The IC is called L6983i, and it can operated from inputs across 3.5 – 38V input-voltage range.

It has internal main and synchronous rectifier mosfets, capable of sourcing and sinking up to 4.5A.

Operation can be set at frequencies between 200kHz and 1MHz, or be synchronised to an external source.

To get the isolated output, a secondary winding is needed on the primary buck inductor. In operation, power is transferred to the isolated secondary after the buck fet turns off.

Operation is constant-frequency peak-current mode. “Therefore, the intersection between the error amplifier output and the sensed inductor current generates the PWM control signal to drive the power switch,” according to the company. “The device operates in forced PWM control, allowing negative currents to flow in the synchronous mosfet, hence transferring energy to the secondary coil during the off-time.”

The output voltage of the primary is regulated, but the output of the secondary varies depending on several factors.

“The input voltage value influences the peak current in the secondary winding, hence determining the amount of energy delivered to the secondary output and in turn the isolated output voltage value” said ST. “The secondary output line regulation is mainly affected by the transformer and in particular by its leakage inductance. Keeping constant the leakage inductance, a higher input voltage causes the isolated output voltage to slightly increase.”

Schottky diode forward voltage, output current, and the resistance of the secondary coil also have an effect.

Shut-down can be to as low as 2µA, and functions include: adjustable soft-start, internal loop compensation, selectable spectrum-spreading for EMC reduction, a power good indicator, over-current protections, thermal shut-down and a separated bias voltage pin to improve efficiency when a suitable separate supply is available.

Packaging is 3 x 3mm QFN16, and STEVAL-L6983IV1 is the matching evaluation board.

Applications are foreseen in industrial automation, small traction systems, charging stations and isolated safety equipment.

The L6983i product page can be found here

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