For the lowest practical power loss, use a high-side N-channel MOSFET controlled by an ideal-diode or reverse-polarity-protection controller. Its conduction loss is mainly the MOSFET’s on-resistance rather than a diode’s forward-voltage drop. It is not literally lossless: the MOSFET, controller and switching behavior all consume some power.
For a simpler, lower-current design, a P-channel MOSFET is often a good compromise. A Schottky diode is the simplest option when its voltage drop and heat are acceptable. If current must also be prevented from flowing backward from the load, check the topology carefully; back-to-back MOSFETs may be necessary.
First decide what the circuit must protect against
Reverse-polarity protection (RPP) prevents damage when the input supply is connected with the wrong polarity. It is not the same as several other protections that may be needed in a power input:
- Reverse-current blocking (RCB) stops current flowing from the load back toward the source, for example when the input is disconnected or the output is held up by another supply.
- Reverse-voltage protection is a broader description that can include reverse battery connections and negative input transients.
- Overvoltage protection (OVP) responds to a positive input voltage above the permitted range.
- Surge and load-dump protection handles high-energy transients, such as those that can occur in automotive or industrial wiring.
- Overcurrent protection limits or interrupts excessive current, including a short circuit.
- Inrush-current control limits the current used to charge downstream capacitors at startup.
A single MOSFET may protect against a reversed battery without providing all these functions. TI distinguishes reverse-polarity controllers from ideal-diode controllers: the former may protect input polarity without blocking reverse current, while ideal-diode designs are intended to manage reverse current as well. See TI’s ideal-diode controller application note.
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- 5Pcs Anti Reverse Connection Power Protection Board For Battery Charging Ideal Diode Module Prevent Reverse Polarity
- Input voltage: DC3-30V
- Output voltage: the difference with the input voltage, maximum 0.2V
- Output current: 4A, MAX, peak current maximum 6A
- Dimensions: length 24mm, width 16mm
Compare the loss before choosing a circuit
A series diode loses power continuously while current flows. Estimate its dissipation as P = I × VF, where I is load current and VF is the diode’s forward drop. At 10 A and 0.5 V, that is about 5 W; at 1 A and 0.4 V, it is about 0.4 W. The actual forward drop varies with diode type, current and temperature. Even a 0.6–0.7 V drop can be a substantial fraction of a low-voltage supply, as noted in this Analog Devices design note.
A conducting MOSFET instead has an approximate drop of VDROP = I × RDS(on) and conduction loss of P = I² × RDS(on). For example, a 5 mΩ MOSFET at 10 A drops about 50 mV and dissipates about 0.5 W before temperature-related resistance increase. These are calculated examples, not measured results.
| Topology | Approximate conduction loss | Reverse-current blocking | Typical fit |
|---|---|---|---|
| Silicon diode | I × 0.6–1.0 V |
Yes, in its blocking direction | Very simple, low-current protection where drop is acceptable |
| Schottky diode | I × 0.25–0.7 V, application-dependent |
Yes, in its blocking direction | Simple low-voltage, low-to-moderate-current designs |
| P-channel MOSFET | I² × RDS(on) |
Topology-dependent; not guaranteed in both directions | Simple, lower-current high-side protection |
| N-channel MOSFET with controller | I² × RDS(on), plus controller and gate-drive losses |
Controller/topology-dependent | Low-loss higher-current power paths |
| Back-to-back N-channel MOSFETs with controller | Approximately I² × (RDS(on),1 + RDS(on),2), plus controller losses |
Yes, when designed and controlled for bidirectional blocking | Power paths that need isolation in both directions |
| Integrated ideal-diode IC | Depends on the internal FET and operating conditions | Device-dependent | Compact designs within the IC’s current and thermal limits |
| eFuse or protection switch | Device-dependent | Device-dependent | Inputs needing functions beyond polarity protection |
Use the MOSFET’s maximum on-resistance at the gate voltage the circuit will actually provide. A low resistance quoted at 10 V gate drive does not establish the resistance at 2.5 V or 4.5 V. Resistance also rises as the MOSFET heats, so calculate with an appropriate hot value and verify that the PCB can remove the heat. At low load current, controller quiescent current may matter more than channel loss; at high current, the MOSFET’s I²R loss usually deserves close attention.
Choose the topology that fits the job
Series diode: simplest, not most efficient
Put a diode in series with the positive input. It is easy to understand, needs no gate-drive circuitry and blocks current in its reverse direction. It is a sensible choice when current is small and the voltage drop is acceptable, or when simplicity is more important than efficiency. At moderate or high current, account for heat and lost supply voltage. A Schottky diode can also have significant reverse leakage, so check its ratings and operating conditions.
P-channel MOSFET: a compact low-loss compromise
A common high-side arrangement places a P-channel MOSFET in series with the positive rail, with its body diode oriented to allow correctly polarized input power to start the circuit. The resulting gate-to-source voltage turns the MOSFET on for normal operation. With reversed input polarity, the body diode is reverse-biased and the MOSFET should remain off, provided the circuit is designed for the specified voltage range.
This approach can reduce loss substantially compared with a diode while keeping the component count low. Its limits are the P-channel MOSFET’s on-resistance, gate-voltage rating and operating behavior. At high current, its resistance and thermal performance can be less attractive than an N-channel solution. Check the maximum absolute gate-to-source voltage; a resistor and zener clamp may be needed to protect the gate. A simple gate network may also respond more slowly or less predictably than a dedicated controller, and one MOSFET does not automatically block every reverse-current path. TI’s comparison of ideal-diode approaches discusses these trade-offs.
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N-channel MOSFET with a controller: best fit for very low loss at higher current
An N-channel MOSFET can offer lower on-resistance for a given device size, but high-side use requires a gate drive above the source. A suitable controller provides that drive, turns the MOSFET on for low-loss forward conduction and controls it when the input is reversed or current tries to flow backward. The exact functions vary by controller, so confirm reverse-current behavior in its data sheet and application circuit.
This architecture is generally the strongest choice when current is high, the allowed voltage drop is small, the input may sag, or controlled reverse-current behavior matters. Examples of relevant controller families include TI’s LM74500-Q1, LM74720-Q1 and LM74930-Q1. They are not interchangeable: compare their intended functions, external MOSFET requirements and ratings against the circuit’s needs.
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Every MOSFET has an intrinsic body diode. When the channel is off, that diode can still provide a current path in one direction. One MOSFET may therefore be enough to reject a reversed input but insufficient to isolate a load that can feed energy back toward the source.
Back-to-back MOSFETs are used when current must be blocked in both directions while the switch is off—for example, if an output capacitor is charged, another supply shares the rail, a load regenerates energy, or a battery power path needs isolation. The body diodes oppose each other, so neither direction has an unrestricted diode path through the pair when both channels are off. The controller must support the chosen arrangement. TI’s LM74930-Q1 information describes a back-to-back external-FET architecture; Analog Devices also lists protection devices using external back-to-back MOSFETs, including the MAX16128 and MAX16127.
Calculate MOSFET loss and check the thermal result
For a MOSFET, use P = I² × RDS(on) for a first-order estimate of steady conduction loss. The following are arithmetic examples only; they do not represent measured performance or a thermal guarantee.
| Current | On-resistance | Approximate drop | Approximate loss |
|---|---|---|---|
| 1 A | 20 mΩ | 20 mV | 20 mW |
| 5 A | 10 mΩ | 50 mV | 250 mW |
| 10 A | 5 mΩ | 50 mV | 500 mW |
| 20 A | 5 mΩ | 100 mV | 2 W |
For two back-to-back MOSFETs, add their on-resistances to estimate the pair’s conduction loss. For instance, a 10 A path through two 5 mΩ FETs has about 10 mΩ total resistance and roughly 1 W of conduction loss before temperature effects. Check the data-sheet resistance at the intended gate voltage and use a hot-resistance value from the manufacturer’s data when estimating worst-case dissipation. Also verify package thermal limits, PCB copper, thermal vias, ambient temperature and enclosure conditions.
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- This board uses a low RDS (on) MOSFET, and the voltage is reduced to 0.2V within the rated current range, which is beneficial to improve the power supply efficiency and improve the reliability and safety of battery charging.
- When the output of this board is connected to the normal battery polarity, the input is connected to the normal polarity, and the input is connected to the wrong polarity, it will not be charged.
- Input voltage: DC3-30V, the input voltage is within this range
- Output voltage: the difference with the input voltage, maximum 0.2V
- Output current: 4A,MAX, peak current maximum 6A
Worked example: 12 V input, 10 A continuous load
Suppose a design operates from 9–18 V, draws 10 A continuously with a 20 A peak, and permits at most 100 mV of protection-circuit drop during normal operation. At 10 A, the maximum total series resistance for that drop is 0.1 V ÷ 10 A = 10 mΩ. The corresponding conduction loss at that operating point is 10² × 0.010 = 1 W.
If the design uses two FETs in series for bidirectional blocking, their combined hot on-resistance must meet the 10 mΩ limit to hold the 100 mV drop at 10 A. An initial target of about 5 mΩ per FET or lower leaves no allowance for resistance increase with temperature, tolerance or other series resistance; check the actual hot values and revise the target accordingly. The 20 A peak needs a separate check for pulse duration, safe operating area, controller behavior and thermal conditions.
Use a practical design sequence
- Define the electrical environment. Record the normal input range, maximum continuous positive voltage, maximum reverse voltage and duration, positive surge waveform, minimum operating voltage, continuous and peak current, and whether the output may be powered externally.
- Set the loss and thermal limits. Specify the maximum normal voltage drop, acceptable dissipation and temperature rise, ambient temperature, enclosure and available PCB copper.
- Select a topology. Choose a diode for simple low-current protection when its drop is acceptable; a P-channel MOSFET for a straightforward lower-current high-side design; an N-channel MOSFET with controller for a low-loss higher-current path; or supported back-to-back FETs when off-state current must be blocked in both directions.
- Rate the MOSFET for the real conditions. Check drain-source voltage, on-resistance at actual gate drive and hot temperature, continuous and pulsed current, safe operating area, avalanche rating where relevant, gate charge, package thermal capability and body-diode behavior. Do not choose it from nominal supply voltage alone.
- Verify controller and gate protection. Check controller absolute maximum ratings during reverse connection, gate-drive amplitude, undervoltage and startup behavior, turn-on and turn-off timing, gate discharge when input power disappears, and whether a gate-source clamp or resistor is required.
- Test both static and dynamic conditions. Measure output voltage, input current, MOSFET drain-source and gate-source voltages, and controller supply during correct polarity, reverse connection, hot-plugging and recovery. Include a charged output capacitor and any possible external supply on the load side; observe any body-diode conduction and its duration.
- Add separate protection for separate threats. If the environment requires it, design and validate TVS protection, overvoltage and undervoltage shutdown, current limiting, inrush control, fusing, filtering and transient handling as distinct functions.
Account for layout and common failure modes
- Wrong resistance figure: Using a headline
RDS(on)value specified at a gate voltage the controller does not provide can understate loss and lead to partial enhancement or overheating. - Body-diode path: A MOSFET that is off is not necessarily an open circuit. Trace its body-diode direction in the schematic and confirm whether the load can drive current through it.
- Gate overstress: Adequate drain-source voltage rating does not prove that the gate oxide is safe. Check
|VGS|under normal input, reverse input and transient conditions. - Controller mismatch: Confirm that the controller is intended for the selected MOSFET arrangement and that its own pins remain within their absolute maximum ratings during reverse connection.
- Low-side ground lift: A low-side protection switch can cause the load’s ground reference to move during switching or current transients. That can disrupt signal references, communications, shields or chassis-ground relationships; TI discusses this concern in its power-path protection overview.
- Static test only: Slow bench-supply tests may miss hot-plug transients created by wiring inductance, input and output capacitors or startup timing.
- Hidden backfeed: A charger or second supply attached to the output can create a reverse path that was absent in a basic polarity test.
- Unsupported compliance claim: An automotive-qualified IC or controller does not make an arbitrary circuit compliant with an automotive standard. Complete-system performance depends on the MOSFETs, TVS network, layout, harness, waveform and test conditions.
Automotive and industrial inputs need more than reverse-polarity protection
In an automotive or long-cable installation, assess reverse battery, jump-start voltage, cold-crank minimum voltage, load dump, inductive switching, harness transients, electromagnetic compatibility and overcurrent protection. Depending on the application, the design may also need a TVS diode, fuse coordination, input filtering, gate clamping, undervoltage lockout, overvoltage shutdown, current limiting or thermal protection. A reverse-polarity MOSFET does not automatically protect downstream electronics from excessive positive voltage or surge energy.
TI’s TIDA-00992 reference design describes a reverse-polarity automotive design for 12 V, 24 V and 48 V systems and cites ISO 7637-2 and ISO 16750-2 compliance for that reference design—not for every circuit using the same controller. TI’s TVS-less reverse-battery discussion is likewise an application-specific design approach, not a blanket assurance for different vehicles, harnesses or transient conditions.
Match the approach to the application
| Application need | Starting topology | Key check |
|---|---|---|
| Hobby or low-current board; simplicity is the priority | Series diode or P-channel MOSFET | Acceptable voltage drop and heat; gate protection for a MOSFET |
| Battery-powered product where wasted voltage matters | P-channel MOSFET for modest current; controller-driven N-channel FET as current or performance needs grow | Hot on-resistance, quiescent current and minimum input voltage |
| Industrial or automotive higher-current input | Controller-driven high-side N-channel MOSFET | Full input/transient range, gate ratings, thermal design and system-level testing |
| Power ORing, external output supply or regenerative load | Ideal-diode controller with back-to-back MOSFETs when isolation is required | Reverse-current behavior and both FETs’ combined hot resistance |
| Compact low-current rail needing an integrated solution | Integrated ideal-diode IC | Device-specific current, thermal and reverse-input limits |
| Need for current limit, fault reporting or controlled startup | eFuse, protection switch or hot-swap controller with the required polarity protection | Verify each required protection function; do not assume the product provides all of them |
For component research, useful examples include TI’s LM74500-Q1 for external N-channel reverse-polarity protection, the LM74720-Q1 ideal-diode controller family and LM74930-Q1 for a more involved back-to-back-FET protection architecture. Analog Devices lists the LTC4376 as an integrated 7 A ideal diode with a 15 mΩ internal MOSFET and reverse-input protection; the listed application rating and device limits should be checked against the current data sheet and the intended thermal conditions. These are examples, not universal recommendations.
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