Synchronous Buck Converter – A Practical Guide

A synchronous buck converter is a type of DC-DC converter used to efficiently step down voltage. Unlike a traditional buck converter that relies on a diode for freewheeling current, a synchronous buck converter replaces the diode with a MOSFET, reducing power loss and improving efficiency. In this guide, we'll break down how it works, key design considerations, and why N-Channel MOSFETs are preferred for high-side switching.

1. How a Synchronous Buck Converter Works

A synchronous buck converter has the following main components:

• High-Side MOSFET (Q1) – Acts as a switch to control the voltage.
• Low-Side MOSFET (Q2) – Replaces the traditional diode to reduce power loss.
• Inductor (L) – Stores energy and helps maintain continuous current flow.
• Output Capacitor (C) – Smooths voltage ripple and stabilizes the output.
• Gate Driver & Controller – Controls the switching of Q1 and Q2.

Basic Operation:

1. High-Side MOSFET (Q1) Turns ON: Current flows from the input through Q1 and the inductor, delivering power to the load while storing energy in the inductor.
2. Q1 Turns OFF, Low-Side MOSFET (Q2) Turns ON: The inductor continues supplying current to the load through Q2, improving efficiency.
3. This Switching Cycle Repeats: Alternating between Q1 and Q2 ensures a stable output voltage.

Example Calculation:

Let's say we need to design a 12V to 6V synchronous buck converter supplying 5A of current. Given a switching frequency of 500 kHz and an inductor ripple current of 30% of the load current, we can determine the inductor value:

L=(Vin?Vout)×D/ ΔIL×fsL

Where:

• Duty Cycle (D) = Vout / Vin = 6V / 12V = 0.5
• Inductor Ripple Current (ΔIL) = 30% × 5A = 1.5A
• Switching Frequency (fs) = 500kHz

L=(12V?6V)×0.51.5A×500kHz=4μHL

So, a 4μH inductor would be a good choice.

2. Design Considerations

To get the best efficiency and performance, here are some crucial aspects to consider:

(A) Dead Time Management

• There must be a short delay between turning OFF Q1 and turning ON Q2 to prevent shoot-through current, which can cause significant power loss and overheating.

(B) Proper Gate Drive Circuitry

• The high-side MOSFET needs a gate voltage higher than VIN, which is typically achieved using a bootstrap circuit.

(C) Inductor Selection

• A higher inductor value reduces ripple but can slow transient response.
• A lower inductor value improves response time but increases current ripple and EMI.

(D) Capacitor Selection

• Low-ESR capacitors (like ceramic capacitors) help reduce output voltage ripple and ensure stable operation.

3. Choosing MOSFETs

Selecting the right MOSFETs is critical to an efficient design. Here’s what to look for:

(A) High-Side MOSFET (Q1):

• Low R_DS(on) – Reduces conduction losses.
• V_GS(th) – Should match the gate driver voltage.
• Low Gate Charge (Q_G) – Ensures faster switching.
• V_DSS – Must be ≥ 20% higher than VIN to handle voltage spikes.

(B) Low-Side MOSFET (Q2):

• Low R_DS(on) – Minimizes conduction loss.
• Fast Switching Capability – Improves efficiency.

4. Why Use N-Channel MOSFETs for High-Side Switching?

Advantages of N-Channel MOSFETs:

Lower R_DS(on) → Higher Efficiency

Better Electron Mobility → Faster Switching

Easier to Drive with Bootstrap Circuit

Available in High Voltage Ratings

Why Not P-Channel MOSFETs?

Higher R_DS(on) → More Power Loss

Complex Gate Drive (Requires pulling gate below VIN)

Limited High-Voltage Availability

Example:

If we use an N-Channel MOSFET for a 12V to 6V buck converter, the required gate voltage is:

VGate=Vin+VGS(th)

If V_GS(th) is 5V, then the driver must supply 12V + 5V = 17V, which is achieved using a bootstrap circuit.

N-Channel MOSFETs are the preferred choice due to their efficiency and ease of drive.

5. Additional Design Considerations

(A) Choosing the Right Switching Frequency

• Higher frequency: Smaller inductors, but increased switching loss.
• Lower frequency: Reduced switching loss, but requires larger components.

(B) Thermal Management

• Proper heat sinking and PCB layout are crucial to prevent excessive heat buildup.

(C) PCB Layout

• Keep high-current paths short to minimize resistance and EMI.
• Place bypass capacitors close to MOSFETs to avoid voltage spikes.

Conclusion

A synchronous buck converter is a powerful and efficient way to step down voltage in DC-DC applications. By carefully selecting MOSFETs, gate drivers, inductors, and capacitors, you can optimize efficiency and performance.

Using N-Channel MOSFETs for high-side switching, combined with good thermal management and PCB layout, ensures a reliable and high-performance design.