Reviewing Negative Inductor Current in Forced PWM Buck Operation

A synchronous buck converter operating at light load may show inductor current crossing below zero. Whether this is expected depends on its control mode and protection behavior. Review the selected mode before changing the inductor or treating every negative excursion as a measurement error.

Use the device’s mode definition

Mode names are not fully interchangeable between manufacturers. Read how the selected converter controls the low side switch near zero current, how it handles reverse current, and whether it skips pulses. The Analog Devices LTC3302 data sheet distinguishes operation that disallows negative inductor current from forced continuous operation, which includes a reverse current limit.

Record the actual mode pin or register state on the board. An assembly option, floating control pin, or firmware setting can make the bench unit behave differently from the schematic annotation.

Separate average and instantaneous current

The average inductor current can be small and positive while its ripple crosses zero. As a conceptual example, a triangular waveform averaging 0.1 A with 0.6 A peak to peak ripple spans approximately minus 0.2 A to plus 0.4 A. This calculation describes a waveform; it is not a prediction for every converter operating mode.

Confirm probe zero and polarity before interpreting the result. Compare the inductor current with the switch node waveform and load current. A small negative offset in the measurement chain can otherwise imitate shallow reverse current.

Trace returned energy

Examine where energy can go when the converter sinks output current or when an external source raises the output. Review input capacitance, upstream source behavior, and any blocking elements. A bench supply that cannot sink current may respond differently from the source used in the final product.

Do not assume that a converter capable of negative inductor current is approved to absorb arbitrary externally supplied output power. Check its documented limits and operating conditions. Forced switching behavior, reverse current protection, and output overvoltage handling are separate aspects of the design.

Compare modes under relevant conditions

Measure light load efficiency, output ripple, current direction, and response to unloading in the intended operating modes. Keep input source and load wiring consistent. Include startup and externally prebiased output conditions only within the device’s specified operating envelope.

Save the mode configuration and synchronized waveforms with the result. A useful conclusion states whether the observed current agrees with the selected control mode and whether the surrounding system can handle the associated energy flow. It does not declare negative current universally desirable or universally faulty.