Measuring Reverse Current During an Ideal Diode Source Collapse

An ideal diode path can block steady reverse current and still allow a short transient when its input collapses. The controller needs to detect the reversal and remove gate charge from the external MOSFET. Evaluate the transferred charge and output disturbance when a source fails, rather than checking only a static reverse voltage.

Identify the controlled path

The TI LM74700-Q1 data sheet describes forward voltage regulation and fast gate turnoff for reverse current blocking. Its timing and drive specifications apply under stated conditions with an external MOSFET that forms part of the response.

Draw the MOSFET body diode, gate connection, controller supply, input source, and output capacitance. Include other paths through protection devices or adjacent supplies that could carry reverse current independently of the controlled transistor.

Define a realistic collapse event

Specify whether the input is disconnected, actively pulled down, or shorted through a known impedance in the required test. These events create different voltage slopes and currents. An unplugged source that leaves its capacitor floating is not equivalent to a low impedance source fault.

Use a controlled engineering fixture and bounded energy consistent with component ratings. Characterize its switching waveform before attributing every observed spike to the ideal diode circuit.

Capture current and gate response together

Measure path current with sufficient bandwidth and a known polarity reference. Capture input, output, and gate-to-source voltage on the same time axis. Determine when current reverses, when gate discharge begins, and when the reverse path stops conducting.

Integrate the reverse current over the event to estimate charge removed from the output. For a hypothetical triangular reverse pulse with a 4 A peak and 2 microsecond duration, the transferred charge is about 4 microcoulombs. The resulting output drop also depends on effective capacitance and the continuing load.

Check MOSFET and operating variations

Repeat at relevant input voltage, load, and temperature. If substituting the external MOSFET, compare gate charge and the measured response in addition to on resistance. A lower resistance part can have a different turnoff burden.

Check controller behavior when its own supply falls and during restoration of the failed source. Keep startup charging and reverse blocking acceptance separate so a change does not improve one while degrading the other.

Save the fault fixture, current polarity, waveforms, reverse charge, and output minimum. A useful result establishes how much energy or charge can flow backward during the required event. Zero steady reverse leakage and a fast headline response time alone do not quantify the transient experienced by the assembled redundant power system.