CRMICRO MOSFET Design Guidelines

Introduction

CRMICRO MOSFETs are designed to provide excellent performance across a wide range of power conversion applications. This document provides guidance on implementing these devices in practical circuits to achieve optimal performance and reliability.

MOSFET Selection Process

Selecting the right MOSFET for your application requires careful consideration of several factors:

Voltage Rating

The voltage rating should be chosen with sufficient margin above the maximum expected drain-to-source voltage in the application. As a general rule, select a device with a voltage rating at least 150% of the maximum expected voltage. For applications with significant voltage transients, consider even higher margins.

Current Rating

The current rating must account for the maximum DC and RMS currents in your application. Pay attention to both continuous current handling capability and the device's SOA (Safe Operating Area) for pulsed current conditions.

On-Resistance (RDS(on))

On-resistance directly affects conduction losses and system efficiency. Lower RDS(on) reduces losses but may increase die size and cost. Balance RDS(on) with switching losses based on your operating conditions.

Technology Considerations

CRMICRO offers several MOSFET technologies optimized for different applications:

• SGT (Shielded Gate Trench): Optimal for high-frequency applications with best RDS(on) × Qg figure of merit
• SJ (Super Junction): Best for high-voltage applications (600V+) with lowest RDS(on)
• Trench: Good balance for general-purpose applications

Thermal Management

Proper thermal management is critical for reliable operation and achieving projected performance:

Power Dissipation Calculation

Total power dissipation includes conduction and switching losses:

P_total = P_cond + P_switch

Conduction losses:

P_cond = I_RMS² × R_DS(on) × duty cycle

Switching losses depend on operating conditions and device characteristics. See device datasheets for specific information.

Thermal Resistance

The thermal resistance from junction to ambient (R_θJA) is a critical parameter for thermal design:

T_J = T_A + P_total × R_θJA

Ensure that the junction temperature (T_J) remains below the maximum rating specified in the device datasheet.

PCB Layout for Thermal Management

Effective thermal management through the PCB requires:

• Adequate copper area connected to the device's thermal pad
• Thermal vias to transfer heat to inner and back-side copper layers
• Multi-layer PCBs for better heat spreading
• Consideration of the final application's cooling method (natural convection, forced air, etc.)

Gate Drive Design

Proper gate drive design is essential for optimal MOSFET performance:

Gate Drive Voltage

CRMICRO MOSFETs typically require a gate drive voltage of 10V to 15V for full enhancement. Avoid exceeding the maximum gate-to-source voltage rating (usually ±20V).

Gate Drive Current

The gate driver must be capable of sourcing and sinking sufficient current to charge and discharge the gate capacitance quickly, reducing switching losses. The required current depends on the switching frequency and gate charge (Qg).

Gate Resistance

External gate resistance (R_G) affects switching speed and losses:

• Lower R_G results in faster switching but may cause ringing
• Higher R_G reduces ringing but increases switching losses
• Optimize R_G based on your specific application requirements

Gate Drive Circuits

For high-performance applications, consider dedicated gate driver ICs that provide:

• Separate source and sink drive capabilities
• Protection features like desaturation detection
• Galvanic isolation if required
• Adjustable drive strength

Layout Guidelines

Proper PCB layout is crucial for achieving the expected performance and maintaining system stability:

Minimize Parasitic Inductance

Parasitic inductances in the power loop can cause voltage overshoots and ringing:

• Keep high-current loops as small as possible
• Use multiple vias for high-current connections
• Place decoupling capacitors close to the MOSFET
• Utilize copper planes for power return paths when possible

Gate Drive Loop

The gate drive loop should also be minimized to reduce ringing and EMI:

• Keep gate drive components close to the MOSFET
• Use tight routing for gate signals
• Separate gate drive and power loops to prevent coupling

Protection Considerations

Implementing protection circuits is important for reliable operation:

Overcurrent Protection

Implement current limiting or shut-off to protect the MOSFET during fault conditions. This can be achieved through:

• Sense resistor with comparator
• Active current monitoring in the controller
• Desaturation detection (for hard switching applications)

Overtemperature Protection

Monitor device temperature and implement thermal shutdown if safe operating limits are exceeded.

Overvoltage Protection

Protect against voltage transients that may exceed the device rating using snubber circuits or active clamps.

CRMICRO MOSFET Product Series

CRMICRO offers several MOSFET series optimized for different applications:

SGT Series

CRMICRO's Shielded Gate Trench MOSFETs offer the best balance of on-resistance and gate charge for high-frequency applications. Ideal for LLC resonant converters and high-efficiency adapters.

SJ Series

Super Junction MOSFETs provide the lowest on-resistance for high-voltage applications, making them ideal for PFC boost circuits and high-voltage adapters.

Trench Series

Standard trench MOSFETs offer a good balance of performance and cost for general-purpose power conversion applications.

Design Tools and Resources

CRMICRO provides design tools and resources to assist with your implementation:

• SPICE models for simulation of CRMICRO devices
• Reference designs for common applications
• Application notes covering specific implementation topics
• Design calculators for common parameters
• Technical support from our application engineering team

Testing and Validation

Proper testing ensures design reliability:

Electrical Testing

• Static electrical tests at room and elevated temperatures
• Dynamic switching performance validation
• Efficiency measurements across operating range

Thermal Validation

• Temperature measurements under worst-case conditions
• Thermal imaging to identify hot spots
• Power cycling for long-term reliability assessment