Why MOSFETs Replace Catch Diodes in Synchronous Buck Converters

Replacing the catch diode with a low-resistance MOSFET in a buck converter significantly improves power efficiency by drastically reducing conduction losses during the off-cycle of the main switch. In traditional asynchronous buck converters, the catch diode exhibits a fixed forward voltage drop that dissipates substantial heat, especially in low-voltage and high-current applications. By utilizing a synchronous rectifier configuration—where a secondary MOSFET acts as a synchronized mechanical switch—engineers replace this fixed voltage drop with a very small, purely resistive voltage drop. This overview covers the mechanics of diode losses, the physics of MOSFET conduction, and the direct impact of synchronous rectification on power supply thermal performance.

The Problem with Catch Diodes

In a standard (asynchronous) buck converter, the catch diode—often a Schottky diode—provides a path for the inductor current when the main high-side switch turns off. While effective and simple, diodes have an inherent physical limitation: the forward voltage drop (\(V_f\)).

Even the best Schottky diodes have a forward voltage drop between 0.3V and 0.5V. The power lost as heat in the diode during conduction is calculated as:

\(P_{diode} = V_f \times I_{out} \times (1 - D)\)

Where \(I_{out}\) is the load current and \(D\) is the duty cycle. When the output voltage is very low, the main switch is off for most of the cycle, meaning the catch diode conducts the majority of the time. If a converter outputs 1V at 10A, a diode with a 0.4V drop wastes nearly 4 Watts of power, severely capping the maximum possible efficiency of the circuit.

The Low-Resistance MOSFET Advantage

A synchronous buck converter replaces this passive diode with an active, low-side MOSFET. Unlike a diode, a fully enhanced MOSFET does not have a fixed forward voltage drop. Instead, it behaves like a simple resistor when turned on. This parameter is known as the drain-to-source on-resistance (\(R_{DS(on)}\)).

Modern MOSFETs can achieve \(R_{DS(on)}\) values of just a few milliohms (mΩ). The conduction loss in the MOSFET is calculated using Ohm's law for power:

\(P_{MOSFET} = I_{out}^2 \times R_{DS(on)} \times (1 - D)\)

Using the previous example of a 10A load, a MOSFET with an \(R_{DS(on)}\) of 5mΩ (0.005 Ohms) only creates a voltage drop of 0.05V (\(10A \times 0.005\Omega\)). While the diode dissipated nearly 4 Watts of heat, the MOSFET dissipates less than 0.5 Watts under the exact same conditions.

Impact on Low-Voltage, High-Current Electronics

The transition from a catch diode to a MOSFET is essential for modern electronics, such as microprocessors, FPGAs, and GPUs, which require very low voltages (often below 1.2V) and very high currents (tens or hundreds of amps).

In these scenarios, the efficiency gains are substantial for two reasons:

1. Reduced Voltage Drop Ratio: At a 1.0V output, a 0.4V diode drop represents a 40% loss relative to the output voltage. A MOSFET's 0.05V drop represents only a 5% loss.
2. Improved Thermal Management: Less wasted power means less heat generated on the printed circuit board (PCB). This reduces or eliminates the need for bulky heatsinks and airflow, allowing for smaller, denser electronic devices.

The Trade-Off: Control Complexity

While replacing the diode with a MOSFET solves the conduction loss problem, it introduces control complexity. The high-side and low-side MOSFETs must be precisely synchronized—hence the name "synchronous" buck converter.

If both MOSFETs are on at the same time, they create a direct short circuit from the input voltage to the ground, a destructive condition known as "shoot-through." To prevent this, the controller must insert a brief "dead time" where both switches are off. During this tiny fraction of a second, the intrinsic body diode of the low-side MOSFET conducts the current, ensuring the inductor path is never broken while maintaining maximum efficiency for the rest of the switching cycle.