MOSFET Parasitics and Their Equivalent Model: Complete Guide for Power Electronics Engineers
MOSFET Parasitics and Their Equivalent Model
In power electronics, an ideal MOSFET is often represented as a perfect switch. However, real MOSFETs contain several unwanted electrical elements called parasitics. These parasitic elements significantly affect switching performance, efficiency, EMI, voltage overshoot, ringing, and thermal behavior.
As switching frequencies increase into hundreds of kilohertz and several megahertz, understanding MOSFET parasitics becomes essential for converter design, PCB layout optimization, gate driver design, and switching loss analysis.
What are MOSFET Parasitics?
Parasitics are unwanted electrical parameters naturally present inside a MOSFET due to its physical structure and packaging.
Major MOSFET parasitics include:
- Gate-to-Source Capacitance (CGS)
- Gate-to-Drain Capacitance (CGD)
- Drain-to-Source Capacitance (CDS)
- Gate Resistance (RG)
- Drain Inductance (LD)
- Source Inductance (LS)
- Package Resistance
- Body Diode
- Body Diode Reverse Recovery Charge
Complete MOSFET Equivalent Parasitic Model
Drain
●
│
LD
│
●─────────┐
│ │
CDS │
│ │
│ Body
│ Diode
│ │
│ │
Gate ●──RG──●────CGD─────────┘
│
CGS
│
│
LS
│
Source
This equivalent model is commonly used for switching analysis and SPICE simulation.
1. Gate-to-Source Capacitance (CGS)
CGS exists between the MOSFET gate and source terminals.
It is formed by:
- Gate Oxide
- Semiconductor Structure
Functions:
- Determines gate charging time
- Affects turn-on speed
- Influences gate driver requirements
Large CGS requires:
- Higher gate current
- Stronger gate driver
2. Gate-to-Drain Capacitance (CGD)
CGD is commonly called the Miller Capacitance.
This is one of the most important parasitic capacitances in MOSFET operation.
It creates:
- Miller Effect
- False Turn-On
- Switching Delay
- Gate Voltage Disturbances
During switching:
i = CGD × dv/dt
Fast voltage transitions generate current through CGD.
3. Drain-to-Source Capacitance (CDS)
CDS appears between drain and source terminals.
Manufacturers usually specify:
Coss = CDS + CGD
Effects:
- Switching Losses
- Resonance
- Voltage Overshoot
- ZVS Performance
Relationship Between Datasheet Capacitances
Manufacturers usually specify:
Ciss = CGS + CGD
Coss = CDS + CGD
Crss = CGD
4. Gate Resistance (RG)
Every MOSFET contains internal gate resistance.
This resistance originates from:
- Gate Metallization
- Bond Wires
- Semiconductor Structure
Effects:
- Controls switching speed
- Influences gate oscillations
- Affects EMI performance
5. Drain Inductance (LD)
Drain inductance comes from:
- Package Leads
- Bond Wires
- PCB Connections
It contributes to:
- Voltage Overshoot
- Current Ringing
- Switching Stress
Voltage generated:
V = LD × (di/dt)
6. Source Inductance (LS)
Source inductance is extremely important in high-speed switching.
It creates:
- Gate Voltage Distortion
- False Turn-Off
- Switching Oscillation
LS is often called:
Common Source Inductance (CSI)
Why Common Source Inductance is Dangerous
When switching current changes:
VLS = LS × (di/dt)
This voltage appears in series with the gate drive.
Effects:
- Slower switching
- False triggering
- Gate ringing
- Increased switching losses
7. MOSFET Body Diode
Every power MOSFET contains an intrinsic body diode.
The body diode is formed naturally by:
- P-N Junction Structure
It allows reverse current flow.
Applications:
- Synchronous Buck Converters
- Half-Bridge Circuits
- Full-Bridge Inverters
8. Reverse Recovery Charge (Qrr)
The body diode stores charge when conducting.
During reverse blocking:
- Stored charge must be removed.
- Additional current flows.
- Switching loss increases.
This stored charge is called:
Qrr
Parasitic Capacitance Model During Switching
CGD
Gate ●──||──────Drain
CGS
Gate ●──||──────Source
CDS
Drain●──||──────Source
These capacitances dominate switching behavior.
Miller Plateau Explained
During MOSFET turn-on:
- Gate voltage initially rises.
- Current begins increasing.
- Drain voltage starts falling.
- CGD becomes active.
A flat region appears in gate voltage called:
Miller Plateau
This region determines switching speed and switching loss.
Package Parasitics
MOSFET packages contribute:
- Lead Inductance
- Lead Resistance
- Bond Wire Inductance
Common packages:
- TO-220
- TO-247
- D2PAK
- DFN
- QFN
Modern packages aim to minimize parasitics.
Parasitics in Si MOSFET vs SiC MOSFET vs GaN HEMT
| Parameter | Si MOSFET | SiC MOSFET | GaN HEMT |
|---|---|---|---|
| CGD | Moderate | Low | Very Low |
| Qrr | High | Low | Nearly Zero |
| Switching Speed | Moderate | High | Very High |
| Parasitic Sensitivity | Moderate | High | Extremely High |
Impact of MOSFET Parasitics on Converter Performance
MOSFET parasitics affect:
- Switching Loss
- Efficiency
- Thermal Performance
- EMI
- Voltage Overshoot
- Current Ringing
- Gate Driver Design
How to Reduce Parasitic Effects
Use Short PCB Traces
Short traces reduce:
- Loop Inductance
- Voltage Overshoot
Use Kelvin Source Connection
Kelvin source minimizes:
- Common Source Inductance
- Gate Oscillation
Optimize Gate Resistance
Proper gate resistance helps:
- Reduce Ringing
- Control EMI
- Improve Stability
Place Decoupling Capacitors Nearby
Input capacitors should be placed very close to:
- MOSFET Drain
- MOSFET Source
Use Proper PCB Layout
Minimize:
- Power Loop Area
- Gate Loop Area
SPICE Model Including MOSFET Parasitics
A practical SPICE model generally contains:
RG CGS CGD CDS LD LS RD RS Body Diode
These parameters are available in manufacturer SPICE models and are critical for accurate switching simulations.
Applications Where MOSFET Parasitics Become Critical
- GaN Converters
- SiC Inverters
- EV Chargers
- Solar Inverters
- Data Center VRMs
- High-Frequency DC-DC Converters
- Motor Drives
Frequently Asked Questions (FAQs)
What is the most important MOSFET parasitic?
For switching performance, CGD (Miller capacitance) and common source inductance are often the most critical parasitic elements.
Why does a MOSFET contain a body diode?
The body diode is naturally formed by the MOSFET semiconductor structure and cannot be removed in conventional silicon MOSFETs.
What causes voltage overshoot?
Voltage overshoot is mainly caused by parasitic inductance interacting with fast current transitions.
Why is common source inductance harmful?
It creates unwanted voltage that disturbs gate drive signals and switching performance.
Why are GaN devices more sensitive to parasitics?
Because GaN devices switch extremely fast, even small parasitic inductances and capacitances can significantly affect performance.
Key Takeaways
- Real MOSFETs contain parasitic capacitances, inductances, resistances, and body diodes.
- CGS, CGD, and CDS dominate switching behavior.
- Common source inductance significantly affects gate drive performance.
- Parasitics cause overshoot, ringing, EMI, and switching losses.
- Proper PCB layout is essential for minimizing parasitic effects.
- Accurate SPICE simulations require inclusion of parasitic elements.
Conclusion
MOSFET parasitics are unavoidable, but understanding them is essential for designing efficient and reliable power electronic systems. As switching frequencies continue to increase and advanced devices such as SiC MOSFETs and GaN HEMTs become more common, parasitic effects play an increasingly important role in converter performance.
Engineers who understand MOSFET equivalent models, Miller capacitance, common source inductance, body diode behavior, and package parasitics can significantly improve switching performance, reduce losses, minimize EMI, and build more robust power conversion systems.
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