What is a Power MOSFET?

• Power MOSFET: High-power version of conventional MOSFET

• Designed to handle high voltages (up to several kV) and currents (hundreds of amperes)

• Combines advantages of MOSFETs with power handling capability

• Voltage-controlled device (unlike BJT which is current-controlled)

• Dominant device in low to medium voltage power electronics

Key Applications:

• Switch-mode power supplies (SMPS)

• Motor drives and inverters

• DC-DC converters

• Automotive electronics

• LED drivers

MOSFET Basics - Three Terminal Device

Three terminals:

• Gate (G): Control terminal (analogous to base of BJT)

• Drain (D): Output terminal (analogous to collector of BJT)

• Source (S): Reference terminal (analogous to emitter of BJT)

Key Characteristics:

• Current flow from drain to source controlled by gate-source voltage

• High input impedance (M\(\Omega\))

• Unidirectional current flow when ON

• Can block voltage in only one direction when OFF

Power MOSFET Structure

Basic Structure:

• n+ source region

• p-type substrate (body)

• n- drain region (lightly doped)

• Metal gate electrode

• Insulating oxide layer (SiO\(_2\))

Operation Principle:

• Gate voltage creates electric field

• Field attracts/repels charge carriers

• Forms conducting channel

• Controls current flow

Types of Power MOSFETs

1. Based on Channel Type:

• N-channel MOSFET: More common, electrons are majority carriers

• P-channel MOSFET: Holes are majority carriers, slower switching

2. Based on Operation Mode:

• Enhancement Mode: Normally OFF, requires positive \(V_{GS}\) to turn ON

• Depletion Mode: Normally ON, requires negative \(V_{GS}\) to turn OFF

3. Based on Construction:

• VDMOS: Vertical Double-diffused MOS

• UMOS: U-groove MOS structure

• Trench MOSFET: Vertical trench gate structure

Note: Power MOSFETs are typically n-channel enhancement mode devices

MOSFET Symbols and Body Diode

Symbol Convention:

• N-channel: Arrow points inward (toward channel)

• P-channel: Arrow points outward (away from channel)

• Broken line: Enhancement mode (normally OFF)

• Solid line: Depletion mode (rarely used)

Body Diode (Intrinsic Diode):

• Parasitic diode between drain and source

• Provides reverse current path

• Useful in bridge circuits

• Eliminates need for external freewheeling diode

MOSFET Operation - Turn-ON Process

For N-channel Enhancement Mode MOSFET:

1. \(V_{GS} < V_{th}\): Device OFF

   • No conducting channel exists

   • Only leakage current flows (\(I_D \approx 0\))

2. \(V_{GS} = V_{th}\): Threshold condition

   • Threshold voltage (\(V_{th}\)): Typically 2-4V for power MOSFETs

   • Weak inversion begins

3. \(V_{GS} > V_{th}\): Channel formation

   • Electric field attracts electrons to surface

   • Forms n-type conducting channel

   • Current flows from drain to source

   • Higher \(V_{GS}\) \(\to\) wider channel \(\to\) more current

4. \(V_{GS} = 10-15V\): Full enhancement

   • Maximum channel conductivity

   • Minimum ON-resistance achieved

Operating Regions

1. Cut-off Region:

• \(V_{GS} < V_{th}\)

• \(I_D = 0\) (switch OFF)

2. Ohmic/Linear Region:

• \(V_{DS} < (V_{GS} - V_{th})\)

• Acts like voltage-controlled resistor

• Used for switching (ON-state)

3. Saturation/Active Region:

• \(V_{DS} > (V_{GS} - V_{th})\)

• Acts like current source

• Used for amplification

Equivalent circuits during turning ON of a MOSFET

Transfer Characteristics

Transfer Characteristic: \(I_D\) vs \(V_{GS}\) relationship

Key Parameters:

• Threshold voltage (\(V_{th}\)): 2-4V typically

• Transconductance (\(g_m\)):

  \[g_m = \frac{\partial I_D}{\partial V_{GS}}\]

Square Law (in saturation):

\[I_D = K(V_{GS} - V_{th})^2\]

where K is the transconductance parameter

ON-State Parameters

ON-state resistance (\(R_{DS(ON)}\)):

• Drain-source resistance when MOSFET is fully ON

• Most important parameter for device selection

• Determines conduction losses: \(P_{cond} = I_D^2 \times R_{DS(ON)}\)

• Specified at particular \(V_{GS}\) (typically 10V) and temperature ( \(25^{\circ}C\))

Gate Drive Voltage Guidelines:

• \(V_{GS} = 10V\): Usually sufficient for most applications

• \(V_{GS} = 15V\): Recommended for minimum \(R_{DS(ON)}\)

• \(V_{GS(max)} = \pm 20V\): Absolute maximum (oxide breakdown limit)

Temperature Effects:

• \(R_{DS(ON)}\) increases with temperature (\(\sim 0.6\%/^{\circ}C\))

• \(V_{th}\) decreases with temperature (\(\sim -2mV/^{\circ}\)C)

• Positive temperature coefficient helps in paralleling

Breakdown Characteristics

Voltage Ratings:

• Drain-source breakdown (\(BV_{DSS}\)):

  • Maximum \(V_{DS}\) when device is OFF

  • Range: 50V to several kV

  • Choose 2-3 times operating voltage

  • Avalanche breakdown in drain region

• Gate-source breakdown (\(BV_{GSS}\)):

  • Maximum \(V_{GS}\) without oxide damage

  • Typically \(\pm 20V\) to \(\pm 30V\)

  • Critical: Permanent damage if exceeded

  • Very thin oxide layer ( 100 nm)

Safe Operating Area (SOA):

• Defines safe current-voltage operating limits

• Limited by: current, voltage, power, and thermal constraints

• Advantage: No secondary breakdown (unlike BJTs)

Parasitic Capacitances

Main Capacitances:

• \(C_{GS}\): Gate-source capacitance

• \(C_{GD}\): Gate-drain capacitance (Miller)

• \(C_{DS}\): Drain-source capacitance

Datasheet Parameters:

• \(C_{ISS} = C_{GS} + C_{GD}\) (Input)

• \(C_{OSS} = C_{DS} + C_{GD}\) (Output)

• \(C_{RSS} = C_{GD}\) (Reverse transfer)

Impact:

• Determine switching speeds

• \(C_{GD}\) creates Miller effect

• All are voltage-dependent

Body Diode Characteristics

Body Diode Properties:

• Inherent p-n junction between drain and source

• Always present in power MOSFETs

• Forward voltage drop: 0.7-1.2V (similar to regular diode)

• Reverse recovery time: typically slow (\(100~\mathrm{ns} - 1~\mathrm{\mu s}\))

Advantages:

• Provides freewheeling path for inductive loads

• Eliminates need for external diode in many circuits

• Enables bidirectional current flow

• Useful in bridge circuits and motor drives

Disadvantages:

• Slow reverse recovery can cause losses

• Higher forward drop compared to dedicated diodes

• May require external fast diode in high-frequency applications

Switching Behavior Overview

Why Study Switching?

• Power MOSFETs used primarily as switches

• Switching losses significant at high frequencies

• Switching speed determines maximum operating frequency

• Critical for gate drive circuit design

MOSFET Switching Advantages:

• Majority carrier device: No stored charge

• No storage time: Unlike BJTs

• Fast switching: Limited only by capacitances

• Voltage controlled: No continuous gate current

Switching Limitations:

• Parasitic capacitances must be charged/discharged

• Miller effect during transitions

• Gate drive capability requirements

Switching Time Definitions

Turn-ON Times:

• \(t_{d(on)}\): Turn-on delay time

• \(t_r\): Current rise time (10% to 90%)

• \(t_{on} = t_{d(on)} + t_r\): Total turn-on time

Turn-OFF Times:

• \(t_{d(off)}\): Turn-off delay time

• \(t_f\): Current fall time (90% to 10%)

• \(t_{off} = t_{d(off)} + t_f\): Total turn-off time

Factors Affecting Switching Speed:

• Gate drive current capability

• Parasitic capacitances (especially \(C_{GD}\))

• Gate resistance

• Load characteristics

Miller Effect and Gate Charge

Miller Effect:

• Caused by gate-drain capacitance (\(C_{GD}\))

• Creates plateau in gate voltage during switching

• Slows down switching transitions

• Duration: \(t_{miller} = \frac{C_{GD} \times \Delta V_{DS}}{I_{gate}}\)

Gate Charge (\(Q_G\)):

• Total charge needed to turn MOSFET ON

• More practical parameter than capacitances

• Independent of gate drive voltage (approximately)

• \(Q_G = Q_{GS} + Q_{GD}\) (gate-source + gate-drain charge)

Switching Time with Constant Current Drive:

Gate Drive Circuit Requirements

Why Proper Gate Drive is Critical:

• MOSFET performance depends on gate drive quality

• Poor gate drive \(\to\) slow switching \(\to\) high losses

• Must provide sufficient voltage and current

Key Requirements:

• Voltage: 10-15V for full enhancement

• Current capability: To charge gate capacitances quickly

• Low impedance: Minimize gate resistance

• Fast rise/fall times: For high-frequency operation

• Isolation: For high-side switches

• Protection: Against gate overvoltage

Gate Resistance Guidelines:

• Lower \(R_G\): Faster switching, higher peak currents, more EMI

• Higher \(R_G\): Slower switching, reduced EMI, prevents oscillations

• Typical range: \(10-100~\Omega\)

Gate Drive Circuit Types

1. Direct Drive:

• From logic gates or microcontrollers

• Only for small MOSFETs with low gate charge

• Limited current capability

2. Bipolar Transistor Drivers:

• Push-pull configuration (NPN + PNP)

• Higher current capability

• Discrete component approach

3. Dedicated Gate Driver ICs:

• Optimized for MOSFET driving

• Features: dead-time, shoot-through protection, UVLO

• Examples: IR2110, TC4427, MCP1416

4. Transformer-Coupled Drivers:

• Provides galvanic isolation

• Good for high-voltage applications

• More complex circuit

Types of Power Losses

1. Conduction Losses:

2. Switching Losses:

3. Gate Drive Losses:

4. Body Diode Conduction Losses:

Thermal Management

Thermal Resistance Model:

Heat Sink Design:

where:

• \(T_j\): Junction temperature (max \(150-175^{\circ}C\))

• \(T_a\): Ambient temperature

• \(\theta_{jc}\): Junction-to-case thermal resistance

• \(\theta_{cs}\): Case-to-sink thermal resistance

• \(\theta_{sa}\): Sink-to-ambient thermal resistance

Thermal Management Tips:

• Use adequate heat sink area

• Apply thermal interface material properly

• Consider forced air cooling for high power

• Derate current at high temperatures

Paralleling Power MOSFETs

Why Parallel MOSFETs?

• Increase current capability

• Reduce ON-resistance

• Better thermal distribution

• Cost-effective solution

• Enhanced reliability

MOSFET vs BJT - Comprehensive Comparison

MOSFET Selection Criteria

1. Voltage Rating (\(V_{DSS}\)):

• Choose 2-3 times maximum operating voltage

• Account for voltage spikes and transients

• Consider derating with temperature

2. Current Rating (\(I_D\)):

• Based on RMS current (for resistive loads)

• Consider peak current capability

• Account for thermal limitations

3. ON-Resistance (\(R_{DS(ON)}\)):

• Lower resistance = lower conduction losses

• Specified at \(V_{GS} = 10V\), \(T = 25^{\circ}C\)

• Consider temperature rise effect

4. Switching Parameters:

• Gate charge (\(Q_G\)) for switching losses

• Gate capacitances for drive circuit design

• Rise/fall times for frequency capability

Applications of Power MOSFETs

MOSFET Protection

Protection Requirements:

• Overcurrent: Excessive drain current

• Overvoltage: \(V_{DS}\) exceeding \(BV_{DSS}\)

• Gate overvoltage: \(V_{GS}\) exceeding \(\pm 20V\)

• Thermal: Junction temperature exceeding limits

• dv/dt: Fast voltage transitions causing unwanted turn-on

Protection Methods:

• Snubber circuits: RC or RCD networks

• Gate clamping: Zener diodes across gate-source

• Current sensing: Shunt resistors or current transformers

• Thermal shutdown: Temperature monitoring

Layout Considerations:

• Minimize parasitic inductances

• Keep gate drive traces short

• Use ground planes effectively

Practical Design Considerations

Gate Drive Design:

• Use separate turn-on and turn-off gate resistors if needed

• Consider gate drive power supply decoupling

• Implement dead-time in bridge circuits

• Use negative gate drive voltage for better turn-off

PCB Layout Guidelines:

• Keep power loop area minimal

• Use thick traces for high current paths

• Place gate drive close to MOSFET

• Use via stitching for thermal management

Common Design Pitfalls:

• Inadequate gate drive current

• Poor thermal design

• Ignoring parasitic inductances

• Insufficient protection circuitry

Advanced MOSFET Technologies

1. SiC (Silicon Carbide) MOSFETs:

• Higher voltage capability (\(>1200~\mathrm{V}\))

• Lower ON-resistance

• Higher temperature operation (\(200^{\circ}C\))

• Faster switching speeds

• Higher cost but improving efficiency

2. GaN (Gallium Nitride) HEMTs:

• Ultra-fast switching (sub-nanosecond)

• Very low gate charge

• High frequency capability (MHz range)

• No body diode (can be advantage or disadvantage)

3. Superjunction MOSFETs:

• Overcome silicon limit of \(R_{DS(ON)}\) vs voltage rating

• Better figure of merit

• Complex manufacturing process

Summary

Key Takeaways:

• Power MOSFETs are voltage-controlled, majority carrier devices

• Excellent for switching applications due to fast switching

• Three main operating regions: cut-off, ohmic, and saturation

• ON-resistance is the most critical parameter for power applications

• Proper gate drive is essential for optimal performance

• Natural current sharing makes paralleling straightforward

Advantages:

• High input impedance, low drive power

• Fast switching, no storage time

• Positive temperature coefficient, no thermal runaway

• Easy to parallel, robust operation

Applications: Dominant in low-to-medium voltage power electronics, especially where high switching frequency is required.

Future Trends

Technology Evolution:

• Wide bandgap semiconductors (SiC, GaN) gaining market share

• Integration with smart gate drivers

• Advanced packaging for better thermal performance

• Higher voltage silicon MOSFETs with superjunction technology

Application Trends:

• Electric vehicles driving demand for high-efficiency devices

• Renewable energy systems requiring robust switching

• Data center power supplies demanding higher efficiency

• Wireless power transfer applications

The future belongs to wide bandgap semiconductors, but silicon MOSFETs will remain important for cost-sensitive applications.