Introduction
This circuit is a high-efficiency smart diode controller designed to provide reliable reverse-polarity protection while minimizing power losses. It uses an LM74670-Q1 controller IC in combination with an external N-channel power MOSFET IRF7478 to emulate an ideal diode with an extremely low forward voltage drop, zero quiescent current (IQ), and low IC leakage current.
In this design, the LM74670-Q1 and MOSFET are connected in series with the battery supply, replacing conventional Schottky diodes or P-channel MOSFETs (PFETs) commonly used in reverse-polarity protection circuits. The N-channel MOSFET offers low on-state resistance (RDS(on)), significantly reducing conduction losses and heat generation compared with conventional diode-based solutions.
The LM74670-Q1 provides the gate drive for the external N-channel MOSFET and incorporates a fast-response comparator to pull down the MOSFET gate when reverse polarity is detected. This helps protect the connected circuitry against reverse-polarity conditions while maintaining efficient power delivery during normal operation.
With its low power dissipation, zero-quiescent-current operation, and efficient MOSFET-based design, this circuit is well suited for battery-powered equipment, automotive electronics, and other applications requiring reliable reverse-polarity protection with minimal energy loss.
Features
- Supply 12V to 24V DC
- Load Current Up to 4Amps
- Screw Terminal for Power Input and Output
- PCB Dimensions 42.39 x 23.97mm
Schematic
Connections
- CN1: Pin 1 = + DC Output, Pin 2 = GND
- CN2: Pin 1 = + Battery 12V/24V , Pin 2 = GND
Part List
| NO. | QNTY. | REF. | DESC. | MANUFACTURER | SUPPLIER | SUPPLIER PART NO |
|---|---|---|---|---|---|---|
| 1 | 2 | CN1,CN2 | 2 PIN SCREW TERMINAL PITCH 5.08MM | PHOENIX | DIGIKEY | 277-1247-ND |
| 2 | 1 | C1 | 1uF/35V CERAMIC SMD SIZE 0805 | YAGEO/MURATA | DIGIKEY | |
| 3 | 2 | C2,C3 | 10uF/35V CERAMIC SMD SIZE 1210 | YAGEO/MURATA | DIGIKEY | |
| 4 | 1 | C4 | 4.7uF/35V CERAMIC SMD SIZE 1210 | YAGEO/MURATA | DIGIKEY | |
| 5 | 1 | C5 | 100nF/50V CERAMIC SMD SIZE 0805 | YAGEO/MURATA | DIGIKEY | |
| 6 | 3 | D1,D2,C6 | DNP | |||
| 7 | 1 | Q1 | IRF7478 SOIC8 | INFINEON | DIGIKEY | IRF7478PBF-ND |
| 8 | 1 | U1 | LM74670-Q1 | TI | DIGIKEY | 296-43171-1-ND |
MOSFET IRF7478
The IRF7478 is a high-efficiency, single N-channel HEXFET power MOSFET designed by Infineon Technologies (originally International Rectifier). Housed in a compact, surface-mount SO-8 package, this component is optimized for high-frequency DC-DC converters, motor control, and efficient power management circuits requiring low switching losses and small board footprints.
Key Specifications:
- Component Type: Single N-channel MOSFET
- Drain-Source Voltage (VDS): 60 V
- Continuous Drain Current (ID): 7.0 A (at (TA = 25oC)
- On-State Resistance (RDS(on)):
- 26 mΩ max (at VGS = 10V)
- 30 mΩ max (at VGS = 4.5V)
- Gate Threshold Voltage (VGS(th)): 1.0 V to 3.0 V
- Total Gate Charge (Qg): 21 nC (typical)
- Power Dissipation (PD): 2.5 W (at TA = 25oC)
- Package Style: SO-8 (surface-mount)
TVS Diodes
The TVS+ and TVS- are not required for the LM74670-Q1. However, they are typically used to clamp the positive and negative voltage surges respectively. The output capacitors are recommended to protect the immediate output voltage collapse as a result of line disturbance.
LM74670-Q1 Controller
The LM74670-Q1 is a zero Iq controller that is combined with an external N-channel MOSFET to replace diode in a rectifier. The voltage across the MOSFET source and drain is constantly monitored by theLM74670-Q1 Anode and Cathode pins. An internal charge pump is used to provide the GATE drive for the external MOSFET. The forward conduction is through the MOSFET 98% of the time. The forward conduction is through the MOSFET body diode for 2% of time when energy is stored in an external charge pump capacitor Vcap in circuit. This stored energy is used to drive the gate of MOSFET. The voltage drops and power losses depend on the RDS(on) of MOSFETs used to replace the rectifier diodes. The LM74670-Q1 has no ground reference which makes it identical to a diode.
PCB View
