“This demonstration circuit is an active rectifier with reverse protection for batteries in automotive applications. The project is designed for 5A load current. Two clamping diodes, D1 and D2, are used on the board to protect the IC from overvoltage spikes at the input. The input supply range is 12V to 24V. The active rectifier controller chip LT8672 controls an external N-channel MOSFET (Q1) to form an ideal diode. The GATE amplifier senses across DRAIN and SOURCE and drives the gate of the MOSFET to regulate the forward voltage to 20mV. As the load current increases, GATE is driven higher until a point is reached where the MOSFET is fully on. If the load current is reduced, the GATE amplifier drives the MOSFET gate lower to maintain a 20mV drop. If the voltage VDRAIN is reduced to a point where a forward drop of 20mV cannot be supported, the GATE amplifier drives the MOSFET off.
During fast SOURCE–DRAIN transients such as fast varying input (SOURCE) signals where the regulating 20mV loop is too slow, fast pull-up (FPU) and fast pull-down (FPD) current paths turn on and off the external MOSFET quickly. This rectifies the input signal the same way a diode would do but with much less power dissipation. The SOURCE and GATE pins are protected against reverse input voltages of up to –40V. GATE is pulled to SOURCE when SOURCE goes negative, turning off the MOSFET and isolating DRAIN from the negative input. The gate voltage for the external MOSFET is provided by the auxiliary boost regulator, which regulates its output AUX to 11V above DRAIN. D2 Input power LED, D4 output power LED, CN1 input , CN2 output.
Active rectification is a technique for improving the efficiency of rectification by replacing diodes with actively controlled switches like MOSFET-based circuits, whereas normal semiconductor diodes have a roughly fixed voltage drop of around 0.5-1 volts, active rectifiers behave as resistances, and can have an arbitrarily low voltage drop. Active rectification has many applications. It is frequently used for arrays of photovoltaic panels to avoid reverse current flow that can cause overheating with partial shading while giving minimum power loss. The constant voltage drop of a standard p-n junction diode is typically between 0.7 V and 1.7 V, causing significant power loss in the diode. Electric power depends on current and voltage: the power loss rises proportionally to both current and voltage.
The LT8672 is an active rectifier controller for reverse input protection. It drives an external N-channel MOSFET to replace a power Schottky diode. It’s very low quiescent current and fast transient response meet the tough requirements in automotive applications where AC input signals of up to 100kHz are present. These signals are rectified with minimum power dissipation on the external FET, simplifying thermal management on the PCB. With a drop of only 20mV, the LT8672 solution eases the minimum input voltage requirement during cold crank and start-stop, allowing simpler and more efficient circuits. If the input power source fails or is shorted, a fast turn-off minimizes reverse current transients. An available shutdown mode reduces the quiescent current to 3.5μA. An integrated auxiliary boost regulator provides the required boost voltage to turn the external FET fully on. A power good pin signals when the external FET is ready to take load current.”