Ideal Diode Design Considerations

Introduction

In power electronics and circuit protection applications, ideal diodes—also known as active diodes or smart diodes—play a crucial role by improving the performance of conventional diode circuits. Unlike standard diodes (e.g., silicon, Schottky), ideal diodes are typically implemented using active circuitry that replicates the function of a perfect diode: zero forward voltage drop and infinite reverse impedance.

This article explores the basic principles behind ideal diodes, important design considerations, and popular integrated circuit (IC) examples.

What is an Ideal Diode?

An ideal diode is a theoretical component that:

• Conducts current perfectly in the forward direction (with 0V forward voltage drop),

• Blocks current completely in the reverse direction (with infinite reverse resistance),

• Switches instantly between ON and OFF states.

In reality, no passive component behaves this way. Even low-drop diodes like Schottky diodes have forward voltage drops (typically ~0.3V–0.5V) and leakage in the reverse direction. Ideal diode controllers or active diode circuits overcome these limitations using MOSFETs and control logic.

Why Use Ideal Diodes?

• Using ideal diodes (especially as ICs) provides multiple benefits:

• Higher Efficiency: Reduced power loss due to near-zero forward drop.

• Improved Thermal Performance: Less heat generation.

• Faster Response: Active control enables quicker turn-on/off.

• Reverse Polarity Protection: Automatically blocks incorrect input voltage.

• OR-ing Power Supplies: Smooth transitions between multiple power sources.

Core Design Principles

Controller + MOSFET Architecture

Most ideal diode ICs consist of:

• A power MOSFET (external or internal),

• A control circuit that monitors voltage polarity,

• A gate driver that controls the MOSFET conduction.

Forward conduction occurs when the anode voltage exceeds the cathode, prompting the controller to enhance the MOSFET. Reverse blocking is achieved by turning off the MOSFET when the polarity is reversed.

Voltage Drop Monitoring

Ideal diode controllers compare the voltage across the MOSFET's drain-source (V_DS). If V_DS becomes negative (indicative of forward conduction), the controller turns on the gate, reducing resistance and mimicking an ideal diode.

Reverse Leakage Control

The controller senses reverse current and turns off the MOSFET rapidly, preventing damage or backflow into the power source.

Gate Drive Strength

A strong gate driver is essential, especially in high-current applications. Faster MOSFET switching reduces losses but can cause ringing; proper gate resistors and layout are needed.

Thermal Considerations

Though the forward voltage drop is low, the MOSFET still dissipates some power:

Efficient heat sinking and PCB copper area are critical in high-power designs.

Typical Applications

• Battery-powered devices

• Automotive reverse battery protection

• Redundant power supply OR-ing

• USB/adapter switchover

• Energy harvesting circuits

Design Example

Reverse Protection with LTC4359

• Input: 12V DC

• Load: 5A

• External MOSFET: IRFZ44N (R_DS(on) ≈ 0.025Ω)

• Forward loss: 5² × 0.025 = 0.625W (vs 5W for regular diode at 1V drop)

• Features: Fast response, <100mV forward drop, minimal reverse leakage

Integration Tips

• Place the sense lines (VIN, VOUT) as close to MOSFET terminals as possible.

• Use wide PCB traces or copper pours for current handling.

• Add TVS diodes if working in transient-heavy environments (e.g., automotive).

• Use heat sinks or thermal vias for high-current designs.

Conclusion

Ideal diode controllers provide a smart, efficient alternative to passive diodes, especially where power efficiency, protection, and rapid response are critical. Whether you're designing for automotive, industrial, or battery-operated systems, these ICs can dramatically enhance your circuit's performance and longevity.