Table of Contents
An ideal diode controller drives a MOSFET so the power path conducts forward with much less voltage loss than a conventional diode and turns off when current tries to reverse. That function supports redundant-supply ORing, reverse-battery protection, and source selection, but the controller is only half of the design. The external MOSFET, wiring inductance, output capacitance, and competing supply determine the actual stress and switchover.
Begin by writing the direction of every permitted and prohibited current. “Reverse protection” is too vague to select a circuit: it may mean a reversed input connector, an energized output feeding an absent input, a second supply back-driving the first, or complete bidirectional disconnection.
Define Which Direction Must Be Blocked
Draw the input source, output capacitor, load, charger or converter, and every parallel supply. For each system state, mark the expected current direction and the voltage that may remain on either side after one source disappears.
TI’s Basics of Ideal Diodes distinguishes reverse-polarity protection from reverse-current blocking. A controller intended only to protect against an input wired backward may not prevent output energy from flowing back toward the input. An ideal-diode controller is selected when both low forward loss and reverse-current control are required under its stated conditions.
In a redundant pair, also define source priority. Highest-voltage ORing, fixed priority, seamless switchover, and complete source isolation are different behaviors. If two supplies regulate near the same voltage, their tolerance and the controller’s forward-regulation or reverse threshold can determine whether they share current, oscillate, or hand off cleanly.
Choose Single-FET or Back-to-Back Control
An N-channel MOSFET contains an intrinsic body diode. In an ideal-diode arrangement, that diode initially provides a forward path; the controller then enhances the MOSFET so current flows through its lower-resistance channel. When the output rises above the input, the controller must remove gate drive before excessive reverse current develops.
A single FET cannot block both directions when its channel is off because the body diode remains. Back-to-back MOSFETs place the body diodes in opposite directions and allow a compatible controller to disconnect the path in both directions. That configuration adds resistance, gate charge, cost, and thermal loss, so use it only when the operating-state table requires it.
Analog Devices’ PowerPath primer explains ideal-diode, prioritizer, integrated-FET, external-FET, and back-to-back arrangements. Apply the topology to the fault table rather than selecting it from the application’s nominal voltage alone.
Rate the Controller and MOSFET for the Real Transient
The controller input range must cover steady voltage, cold crank or brownout where relevant, startup, negative input, and positive surge. The MOSFET needs drain-to-source voltage margin for those events plus inductive overshoot caused by interrupting current in wiring and PCB loops. An input TVS may reduce stress, but only after its clamping behavior and layout are included in the circuit model.
Check controller gate-drive capability against MOSFET gate rating and the RDS(on) test voltage. A MOSFET advertised at a low resistance with 10 V gate drive may perform very differently when the controller can provide only a lower overdrive under minimum-input conditions.
The MAX16171 datasheet lists MOSFET voltage rating, RDS(on), gate capacitance, peak dissipation, and average dissipation among its external-FET selection factors. Those are general decision dimensions; numeric limits must come from the selected controller and MOSFET pair.
Review MOSFET safe operating area if the controller ramps the gate, limits current, or can hold the FET partly on during source handoff. A pulsed current headline without the corresponding VDS, time, and temperature is not an SOA assessment.
Calculate Loss and Thermal Rise
Once fully enhanced, approximate MOSFET conduction loss as P = I² × RDS(on). Use resistance at expected junction temperature and gate drive, not only the 25°C typical headline. For back-to-back series MOSFETs, include both devices.
Compare that loss with the diode it replaces, but keep conditions consistent. A low-loss path can still overheat if package thermal resistance, copper area, enclosure temperature, or neighboring components are unfavorable. Parallel MOSFETs can lower effective resistance for high current, yet they add gate charge, space, and current-sharing/layout considerations.
Controller quiescent current may matter for an always-connected battery input. Check it in normal, shutdown, reverse, and fault states. A “zero-IQ” or “low-IQ” label may apply only to a particular mode and voltage range.
Candidates in the TI and Analog Devices catalogs should therefore be compared as controller-plus-FET solutions. Preserve the MOSFET manufacturer code and PCB footprint with the controller approval rather than leaving the FET as an unspecified low-resistance substitute.
Check Reverse Turn-Off and Supply Switchover
When an input collapses or shorts, output capacitance and the remaining supply can drive a rapid reverse current. The controller’s reverse threshold, amplifier or comparator architecture, gate-sink current, MOSFET charge, and loop inductance determine the peak and duration. A slower high-capacitance MOSFET may reduce conduction loss in steady state while allowing more reverse energy before it turns off.
At light load, a poorly matched threshold and power-path impedance can cause repeated on/off behavior near zero differential voltage. During redundant-source handoff, observe output droop, reverse current into the failing source, current drawn from the surviving source, and any gate oscillation. Test with source tolerances that place the two inputs close together, not only with a large artificial separation.
An integrated eFuse fault-control path may add programmable inrush and overload response. Do not assume those functions exist in an ideal-diode controller. If the system requires both, select an integrated combination whose limits are documented or coordinate separate devices so their thresholds and timers do not fight.
For a live-insertion design built around an external pass FET, the hot-swap controller selection process covers load-capacitance charging, linear-mode SOA, fault timing, and retry behavior that an ORing function alone does not establish.
Qualify with a Fault-Oriented Matrix
Test normal turn-on at minimum and maximum input, reverse input, output pre-bias, input removal, input short while operating, reconnect, two-source handoff, and repeated events. Use production-equivalent cables, connectors, capacitance, protection components, MOSFETs, layout, and load. Capture input/output voltage, current in each source, gate voltage, controller status, and temperature.
For an automotive or industrial design, add the transients, environmental conditions, and pass criteria from the applicable system requirements. Component automotive qualification or a voltage rating does not replace those board-level tests.
Record the permitted source arrangement, controller code, MOSFET code, transient network, layout revision, and test evidence. If the circuit protects a battery-powered controller, connect the decision to the broader automotive BMS semiconductor review. A sourcing alternate needs equivalent behavior in forward conduction, reverse blocking, gate control, transient survival, and thermal operation—not merely the same number of pins.
Frequently Asked Questions (FAQ)
Does an ideal diode controller provide reverse-polarity protection?
Some ideal diode controllers support both reverse-input protection and reverse-current blocking, but the exact behavior depends on controller supply range, MOSFET orientation, and gate-control architecture. Confirm each required direction and condition in the datasheet.
When are back-to-back MOSFETs needed with an ideal diode controller?
A single MOSFET's body diode still creates an uncontrolled path in one direction when its channel is off. Use a controller that drives back-to-back MOSFETs when the system must disconnect or block current in both directions.
Will an ideal diode controller limit a short-circuit current?
Not necessarily. Its core task is low-loss forward conduction and fast reverse-current blocking. Use a controller with documented hot-swap or current-limit functions, or add separate protection, when short-circuit energy must be controlled.
